Reactive absorption in chemical process industry: A review on current activities

Chemical Engineering Journal 213 (2012) 371–391

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Review

Reactive absorption in chemical process industry: A review on current activities

Ömer Yildirim a, Anton A. Kiss b, Nicole Hüser a, Katharina Leßmann a, Eugeny Y. Kenig a,⇑a University of Paderborn, Faculty of Mechanical Engineering, Fluid Process Engineering, Pohlweg 55, D-33098 Paderborn, Germanyb AkzoNobel - Research, Development & Innovation, Process Technology ECG, Zutphenseweg 10, 7418 AJ Deventer, The Netherlands

h i g h l i g h t s

" A reactive absorption review covering industrial processes and research activities." Role of reactive absorption as a core environmental protection process and a key separation method is highlighted." A major application of reactive absorption is removal of CO2, H2S, SOx and NOx." Another major application is industrial production of basic chemicals such as nitric acid and sulphuric acid.

a r t i c l e i n f o

Article history:Received 13 June 2012Received in revised form 24 September 2012Accepted 25 September 2012Available online 23 October 2012

Keywords:Reactive absorptionGas treatmentCO2/H2S/SOx/NOx removalNitric acidSulphuric acidIndustrial applicationsResearch activities

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.09.121

Abbreviations: ABB, Asea Brown Boveri; AEEA, N-Babcock and Wilcox; CCS, carbon capture and storagisopropanolamine; DMEA, N,N-dimethylethanolamineN-ethyl ethanolamine; EFMA, European Fertilizer Mdesulphurisation; HEA, high efficiency absorption; HEpetroleum gas; LLB, Lurgi Lentjes Bischoff; MDEA, metNTU, number of transfer units; NGCC, natural gas comOff-gas Treating Process; TEA, triethanolamine; TGT,⇑ Corresponding author. Tel.: +49 (0)5251 60 2408;

E-mail addresses: [email protected] (E.Y. KenigURL: http://mb.uni-paderborn.de/fvt

a b s t r a c t

Reactive absorption (RA) is a unit operation comprising the absorption of gases in liquid solutions withsimultaneous chemical reactions within a single apparatus. The role of RA as a core environmental pro-tection process has grown up significantly, and nowadays, this technology belongs to the most importantseparation methods in the chemical process industry, among others, for gas treatment and purification,removal of harmful substances, as well as for the production of basic chemicals, e.g. sulphuric and nitricacid.

This article provides a comprehensive review on current RA applications covering both industrial pro-cesses and research activities.

� 2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722. Reactive absorption applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

2.1. Removal of CO2 and/or H2S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
2.1.1. Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
2.1.1.1. CO2 removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3752.1.1.2. H2S removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3772.1.1.3. Removal of CO2–H2S mixtures and other impurities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

2.1.2. Research activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3772.1.2.1. Improvement of solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

ll rights reserved.

(2-aminoethyl)ethanolamine; aMDEA, activated methyldiethanolamine; AMP, 2-amino-2-methyl-1-propanol; B&W,e; DEA, diethanolamine; DEEA, N,N-diethyl ethanolamine; DETA, diethylenetriamine; DGA, diglycol amine; DIPA, di-; DMP, N,N0-dimethylpiperazine; DTI, Department of Trade and Industry; EDTA, ethylenediaminetetraacetic acid; EEA,anufactures Association; EOP, electrical output penalty; EPA, US Environmental Protection Agency; FGD, flue gasTP, height equivalent to a theoretical plate; HTU, height of transfer units; LNG, liquefied natural gas; LPG, liquefied

hyldiethanolamine; MEA, monoethanolamine; MHI, Mitsubishi Heavy Industries; MMEA, N-methylmonoethanolamine;bined cycle; OCFE, orthogonal collocation on finite elements; PZ, piperazine; RA, reactive absorption; SCOT, Shell Claustail gas treatment; TMEDA, N,N,N0 ,N0-tetramethylethylendiamine.

fax: +49 (0)5251 60 2183.), [email protected] (A.A. Kiss).

http://dx.doi.org/10.1016/j.cej.2012.09.121

mailto:[email protected]

mailto:[email protected]

http://mb.uni-paderborn.de/fvt

http://dx.doi.org/10.1016/j.cej.2012.09.121

http://www.sciencedirect.com/science/journal/13858947

http://www.elsevier.com/locate/cej

372 Ö. Yildirim et al. / Chemical Engineering Journal 213 (2012) 371–391

2.1.2.2. Improved integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3792.1.2.3. Cement industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

2.2. NOx removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
2.2.1. Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3792.2.2. Research activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
2.3. Nitric acid production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
2.4. Desulphurisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
2.5. Sulphuric acid production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
3. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

Table 1Industrial applications of absorption [2].

The gas industry Gas dehydrationRemoval of CO2 and H2SSelective absorption of H2S

Refineries Hydrocarbon absorbers for lean oil etc.H2S absorbersVarious types of stripping columnsSour water strippers

The petrochemical industry Synthesis gas processingGas saturationEthylene oxide absorptionAcrylonitrile absorption

The chemical industry Synthesis gas processing (CO2 removal)Chlorine dryingHCl and ammonia absorptionAbsorption of nitrous gases

The cellulose industry Sulphur dioxide absorptionChlorine dioxide absorptionFlue gas scrubbing with sulphur recovery

Food processing Stripping various components producingodoursProcessing fatty acidsHexane absorption and stripping

The metal and packagingindustries

Absorption of triethylamine (in foundries)Absorption of lube and cooling oilsAbsorption and recovery of solventvapours

Exhaust air scrubbing Removal of acid components (wet- anddry-scrubbing of SOx and NOx)Removal of base componentsRemoval and recovery of organic solvents

Wastewater/sewage treatmentand pollution control

Air stripping of chlorinated hydrocarbonsDesorption and recovery of ammoniaEffluent neutralizationDeaeration of seawater

1. Introduction

The current EU legislation imposes tighter restrictions aimed atreduction of the impact of process industries on the environment.Consequently, substantial research work focuses on developingsustainable processes allowing efficient handling of waste and pro-duction of chemicals. Process industries intensively work with gasstreams. Both natural and industrial gases are processed, for in-stance in gasification of different fuel sources (coal, oil and naturalgas) or in diverse manufacturing processes of chemical industry.

Before using gas streams as chemical feedstock or releasingthem into atmosphere, it is usually necessary to change their com-position by removing one or several components. Basically, thereexist five methods suitable for gaseous component capturing,namely, absorption, adsorption, permeation through a membrane,chemical conversion to another compound, and condensation [1].From these operations, absorption is undoubtedly the most impor-tant one. During absorption process, a gas to be treated is broughtin contact with a liquid stream and gas components are transferredinto the liquid (solvent) phase in which they are soluble. Importantgas purification applications are listed in Table 1 [2].

Depending on the mechanism binding the gas compounds insolvent, one can distinguish between physical and chemical (orreactive) absorption [3,4]. Physical absorption of gas or gas mixturecomponents in a liquid solvent comprises mass transfer at the gas–liquid interface and mass transport within the phases. It dependson the gas solubility and the operating conditions (e.g. pressureand temperature). A classic example of physical absorption of agas into a liquid is the absorption of carbon dioxide (CO2) intowater (H2O) – usual in the beverage industry. Chemical absorption,also known as reactive absorption (RA), is based on a chemicalreaction between the absorbed substances and the liquid phase.It largely depends on the stoichiometry of the reaction, concentra-tions of the reactants and mass transfer rates.

This paper focuses on RA operations. They may be carried out indifferent units, with a wide spectrum of phase flow types andinteractions (see Table 2). In most cases, RA is carried out in plateor packed columns. In plate columns, the phases flow counter-cur-rently, whereas on each given plate, a cross-flow is usually estab-lished. In packed columns, a liquid flows along the packingsurface, whereas a gas stream occupies the rest of the free volume.There exist numerous plate and packing designs. In [1], guidelinesare given how to select the most appropriate internals for specificRA applications – depending on the Hatta number. RA may alsoproceed in spray contactors, the simplest type of an absorption col-umn. The desired contact area is achieved by dispersing the fluid insmall droplets.

Optimal design of RA processes requires adequate models cov-ering mass and heat transfer, reaction kinetics and column hydro-dynamics. Different possibilities to model RA units are presentedby Fig. 1. Large-scale applications are modelled by sub-dividingcolumns into smaller segments, so-called stages. Each stage corre-sponds to a single tray or to a segment of packed columns. Theequilibrium stage model assumes that the gas and liquid streamsleaving a stage are in thermodynamic equilibrium. Due to its sim-plicity, this model (first published in [5]) has been used in the pastdecades for a variety of applications, especially for non-reactivesystems. Yet, in real absorption processes, the thermodynamicequilibrium is usually not attained within a stage. For this reason,

Table 2Reactive absorption units [3].

Unit group criterion Unit type

Both phases in a continuous form � Packed columns� Thin film contactors� Wetted-wall columns� Contactors with flat surface� Laminar jet absorber� Disc (sphere) columns

A disperse gas phase and a continuous liquidphase

� Plate columns� Plate columns with packing� Bubble columns� Packed bubble columns� Mechanically agitatedcolumns� Jet absorbers

A disperse liquid phase and a continuous gasphase

� Spray columns� Venturi scrubber

Ö. Yildirim et al. / Chemical Engineering Journal 213 (2012) 371–391 373

tray efficiencies or HETP (height equivalent to a theoretical plate)values [6] are introduced to build a link to real columns.

Absorption is usually dominated by the mass transport kinetics.Besides, in reactive processes, chemical reactions must be takeninto account. The equilibrium stage concept can hardly addressthese issues reasonably. A more theoretically justified approach isbased on using HTU/NTU values [7]. The so-called HTU/NTU methodallows determining the column height as a product of two values,HTU (Height of a Transfer Unit) and NTU (Number of TransferUnits). The HTU value depends on column load, column internalsand empirical mass transfer correlations, whereas the NTU valueis obtained by integrating the inverse of the driving force over thecolumn height. In the HTU/NTU method, chemical reactions areusually considered with the aid of enhancement factors [8–10].The enhancement factors depend on the type and order of the reac-tion and the mass transport model. The complexity of the involvedreaction scheme cannot be easily captured by a single parameter,and, thus, this method often gives inaccurate results.

The most adequate and reliable method for the description ofstaged absorption units is the rate-based approach, a method thattakes the rates of multicomponent mass and heat transfer and thechemical reaction into account directly [11]. The mass transfer be-tween the phases can be described by different theories, such as thetwo-film model or the penetration/surface renewal theory. Themostly used two-film model assumes that mass transfer resistanceis concentrated in thin films at the interface. In these films, mass

Fig. 1. Modelling approaches for react

transfer occurs by steady-state molecular diffusion, whereas thebulk phases are ideally mixed. The multicomponent diffusion in thefilms can be described by the Maxwell–Stefan equations. Chemicalreactions are considered by including reaction source terms in thebalance equations for each bulk phase and in the mass transportequation for the film. The necessary model parameters, e.g. the filmthicknesses, are determined using empirical correlations. For manyapplications, the two-film model parameters can be found in theliterature and, therefore, the method is often preferred [12].

The present article gives a review on industrial applications ofRA for gas treatment/purification and for production of basicchemicals. Among the considered systems are those containingCO2, H2S, NOx and SOx, as well as nitric and sulphuric acid. In addi-tion, for each process, the latest research activities are discussed.The fundamentals of RA are outside the scope of this article andcan be found elsewhere [3,13,14].

2. Reactive absorption applications

2.1. Removal of CO2 and/or H2S

We consider the removal of the acid gases CO2 and/or H2S in asingle section, because these components are often met in thesame gas stream and they are treated with similar solvents.

The solvent must be chosen under consideration of the follow-ing factors [15,16]:

� Required purity of cleaned gas stream for further processing orend use.� Composition of feed gas.� Utility requirements, process costs.� Corrosion and solvent degradation/solvent losses.

For the treatment of gases containing CO2 and/or H2S, aqueoussolutions of ethanolamines, carbonates as well as ammonia can beused. The application of ammonia has decreased in the past years[1,17]. However, absorption with aqueous ammonia offers manyadvantages over conventional ethanolamine processes, e.g. higherCO2 absorption efficiency, lower decomposition temperatures andlower costs [18,19]. In this review, we only consider amine and car-bonate processes, as they are mainly used for acid gas removal [1].

Amines: In general, alkanolamines have at least one hydroxylgroup and one amino group in their chemical structure. The hydro-xyl groups increase the water solubility and reduce the vapour

ive absorption (adapted from [3]).

HO CCH

H

N

MonoethanolamineHO

HO

CC

CC

HN

Diethanolamine

HO

HO

CC

CC

CCN OH

Triethanolamine

C

C

CC

CC OH

OH

NH

Diisopropanolamine

HO

HO

CC

CC

N CH3

Methyldiethanolamine

HO CCCC

H

H

NO

2 (2-aminoethony) ethanol

Fig. 2. Molecule structure of amines.

Table 3Qualitative characteristics of the amines for the removal of CO2 (adapted from [21]).

Amines

Primary Secondary TertiaryMEA DEA TEA, MDEA

High Low

High Low

High Low

High Low

Low High


pressure of the alkanolamine. The amino group supplies the alka-linity in aqueous solutions, which is necessary for the absorptionof acid gases.

Alkanolamines can be subdivided into three groups according totheir chemical structure: (1) primary amines, (2) secondary amines,and (3) tertiary amines. Primary amines have a nitrogen atom withtwo hydrogen atoms directly attached to it. Secondary amines haveone hydrogen atom attached to the nitrogen atom and tertiaryamines have no hydrogen atoms directly attached to the nitrogenatom. The chemical formulas of the most widespread amines,MEA, DEA, TEA, DIPA, MDEA and DGA, are shown in Fig. 2. Otheramines and amine based solvents are discussed in [20]. In CO2/H2Sabsorption with primary amines, following reactions take place [1]:

Ionisation of water : H2O$ Hþ þ OH� ð1Þ

Ionisation of dissolved H2S : H2S$ Hþ þ SH� ð2Þ

Hydrolysis and ionisation of dissolved CO2 :

CO2 þH2O$ HCO�3 þHþ ð3Þ

Table 4Major properties for the most common amines (adapted from [22]) .

Property MEA MDE

Chemical formula C2H7NO C5H1

Molecular weight (g/mol) 61.08 119.Density (kg/m3) 1012 1038Boiling point (K) 443 243Vapour pressure, 293 K (kPa) 0.0085 0.001Vapour pressure, 393 K (kPa) 15.9 n/aSolubility at 293 K Soluble SoluPseudo 1st order rate constant at 298 K (m3/kmol/s) 7000 3.5Activation energy (kJ/mol) 46.7 44.3Absorption capacity (mol CO2/mol amine) 0.5 1.0

Protonation of alkanolamine : RNH2 þHþ $ RNHþ3 ð4Þ

Carbamate formation : RNH2 þ CO2 $ RNHCOO� þHþ ð5Þ

Table 3 shows some characteristics of primary, secondary andtertiary amines used in the gas treatment. These properties areimportant for choosing the most appropriate solvent for the gastreatment process in consideration. The enthalpies of reactionand evaporation decrease from primary to tertiary amines. Theseenthalpies are directly related to the energy required for solventregeneration. Therefore the regeneration of tertiary amines re-quires lower energy consumption than of primary amines. Thereaction rate of primary amines is higher than of tertiary amines.Therefore high solvent circulation rates are necessary, when ter-tiary amines are applied. Primary amines are more corrosive thansecondary or tertiary amines. The corrosivity influences the choiceof equipment material and the efficiency of the absorption processby limiting the working capacity [21]. The high corrosivity of pri-mary amines and their reaction products increase the investmentcosts, since corrosion resistant materials are necessary. The loadingdescribes the capability of one mole solvent to absorb one moleacid gas components. The loading increases from primary to ter-tiary amines. This implies higher amine concentrations of primaryamines for comparable removal results with tertiary amines. Somequantitative information for the most common amines can befound in Table 4 [22].

Carbonates: The processes are based on: (1) application of hotpotassium carbonate, (2) absorption by ambient temperature so-dium or potassium carbonate solutions with vacuum regeneration,and (3) absorption into solutions containing free caustics at ambi-ent temperature. In this article, only high temperature absorptionis considered, because this technology is applied for both, CO2

A AEEA DETA AMP PZ

3NO2 C4H12N2O C4H13N3 C4H11NO C4H10N2

16 104.15 103.17 89.14 86.141029 955 934 1100513 207 438 420

3 0.00015 0.02 0.1347 0.10660.969 n/a n/a 41.66

ble Soluble Soluble Soluble 14%wt12100 49740 681 53700n/a n/a 41.7 351.0 1.0 1.0 1.0

Table 5CO2 removal application in major industrial processes (adapted from [24]).

Process Common cleanuptargets

Hydrogen manufacture <0.1% CO2

Ammonia manufacture <16 ppm CO2

Natural gas purificationPipeline gas <1% CO2

LNG feedstock <50 ppm CO2

Synthesis gas for chemical production (H2/CO) <500 ppm CO2

Coal gasification �500 ppm CO2

Ethylene manufacture (steam cracker gastreating)

�1 ppm CO2

Power plantsNGCC power plant <0.5% CO2


and H2S removal, while processes at ambient temperatures are notsuitable for CO2 removal. The other processes are described in [1].

During acid gas removal, H2S reacts with the hydroxyl group:

H2Sþ OH� $ HS� þH2O ð6Þ

This reaction proceeds extremely rapidly. The reaction of CO2

with alkaline solutions usually occurs slower than the reaction ofH2S. CO2 absorption is accompanied by the two following simulta-neous chemical reactions [1].

Mechanism 1. Formation of HCO�3 via the reaction of CO2 with ahydroxyl group:

CO2 þ OH� $ HCO�3 ðfastÞ ð7Þ
Coal fired power plant <1.5% CO2
Table 6Desired quality of treated syngas for various downstream applications (adapted from[15]).

HCO�3 þ OH� $ CO2�3 þH2O ðinstantaneousÞ ð8Þ

Mechanism 2. Reaction of CO2 with water and dissociation ofHCO�3 :

CO2 þH2O$ H2CO3 ðslowÞ ð9Þ
Downstream use Power plants Hydroprocessing Chemical production
Sulphur (wppm) 10–15 <1 <0.01–1CO2 (vol.%) – <0.1 0.05–2.0CO – <50 wppm H2/CO control

as per requirement

H2CO3 þ OH� $ HCO�3 þH2O ðinstantaneousÞ ð10Þ

Which mechanism mainly takes place depends on the pH-valueof the solution. At pH-values higher than 8, the 1st mechanism pre-dominates, otherwise the 2nd mechanism determines the process.In industrial applications, the pH-value is mostly higher than 8[23].

2.1.1. Industrial applicationsThe typical flow sheet of a RA process for gas cleaning includes

an absorption column to perform the removal of the compoundsfrom the feed gas. The outlet gas leaves the column nearly free ofunwanted compounds (clean gas). This step is followed by a strip-ping column, in which the solvent is recovered (Fig. 3).

In our article, the industrial RA applications are subdividedaccording to the captured components. In some cases, CO2 is thetarget component, as in the fossil fuel combustion. In other pro-cesses, it is necessary to capture H2S selectively (e.g. in the tailgas treating). However, in the most applications, both CO2 andH2S are captured along with some other components, e.g. COS,HCN, CS2.

Table 5 shows examples of specifications for the CO2 removalfor different gases depending on their further processing [24]. InRef. [15], the purity requirements for syngas in various down-stream applications are listed (Table 6).

Fig. 3. Process flow diagram of an absorption/desorption process.

2.1.1.1. CO2 removal. According to [25], CO2 is mainly produced bygasification, reforming or in power plants. More than 97% belongsto the fossil fuel combustion [26]. Possible sources are coal, oil, nat-ural gas and biomass. The applications of absorption processes areeither post-combustion, as in power plants, or pre-combustion, asin the reforming or gasification of natural feed stocks. These appli-cations are illustrated in Fig. 4 [27]. Depending on the type of thepower plant, the concentration of CO2 in the gas phase can varyfrom 3 to 16 mol%. In Table 7, typical compositions of exhaustgases from coal-fired and gas-fired power plants are listed. Besides,CO2 removal occurs in ammonia and hydrogen production plants.

The choice of solvent depends on different given conditions, e.g.feed gas composition and requirements concerning clean gasspecifications [1,15]. Table 8 lists the CO2 capturing processesand their licensors. Additionally, this table gives information aboutthe industrial applications, the used solvents and the numbers ofinstallations. The information is taken from the Gas ProcessesHandbook [28].

A typical flow sheet of a RA process consists of an absorptioncolumn that captures CO2 from the feed gas (acid or sour gas) byan amine solvent, thus leaving the outlet gas free of CO2 (cleangas), followed by a stripping column that performs the CO2 desorp-tion and the recovery of the amine used as recyclable solvent(Fig. 3). Reactive absorption with MEA is an established technologywhich is used since the seventies. The Econamine FG process (pur-chased by Fluor Daniel) is one of the most widely used processes inindustry using MEA. There are more than 24 existing plants usingthe Econamine FG process, each with a total capacity of 6–1000 t absorbed CO2/day. It uses a 30 wt% MEA solution with cor-rosion inhibitors. The process can recover 85–95% of the CO2 in fluegases and produces CO2 with a purity of more than 99% – depend-ing on parameters such as the CO2 content of the flue gas, theheight of the absorption column, the liquid-to-gas stream ratioand the regeneration efficiency (CO2 loading of the lean solvent)[25,29].

Alternatively, non-inhibited MEA solutions can be used for CO2

absorption. The so-called ‘‘ABB Lummus Crest-technology’’ appliessolutions with low MEA concentration of 15–20 wt%; thus,the addition of inhibitors against corrosion is not required. The

CO2

compression

CO2

separation

CO2

separationPowerplant

Powerplant

Gasification

Reforming

Shift

Air separation

Powerplant

2N

2 2N /O2H

2CO

2CO

2H2CO /H

2CO /H

2CO

2O

Air

Coa

l, O

il,

Nat

ural

Gas

,

Biom

ass

Oxy-combustion

Pre-combustion

Post-combustion Separation by absorption

2 2N /O

Fig. 4. CO2 origin and process flow (adapted from [27]).

Table 7Typical exhaust gas composition (adapted from [29]).

N2 (mol%) CO2 (mol%) O2 (mol%) Water vapour (mol%) NOx (mol%) SO2 (mol%) Ash (mol%)

Coal-fired powerplant 70–75 12–16 3–4 6–7 400 ppm 150 ppm Up to 30 ppmGas-fired powerplant 70–75 3–5 10–12 7–10 <50 ppm <10 ppm –

Table 8Reactive absorption processes for the capturing of CO2 (adapted from [28]).

Process Licensor Solvent Installations

Amine Guard FS UOP LLC Ucarsol (MDEA-based) � 500+ units worldwide� Mostly treating natural gas, ammonia syngas and hydro-

gen streamsBenfield UOP LLC Hot potassium

carbonate� 700+ units worldwide� 65+ treat natural gas� 200+ treat ammonia syngas� 110 in hydrogen plants� Other installations: in SNG, partial oxidation, coal gasifica-

tion and petrochemical applicationsKerr-McGee/ABB Lummus Global

Absortion/stripping technologyRandall Gas Technologies, ABBLummus Global Inc.

MEA � 4 on coal-fired boiler flue gases� 2 produce gaseous chemical-grade CO2

� 2 produce food-grade liquidFluor Econamine FG Plus process Fluor Enterprises, Inc. MEA > 30% � 24 units worldwide

� Recover CO2 from low-pressure, O2 containing streamsLRS 10 – CO2 removal Advantica Ltd. DEA promoted

potassium carbonate� 30 plants worldwide� Mainly retrofits in the ammonia, hydrogen, natural gas

and chemicals industries


total capacity of all absorption plants operated world-wide variesin the range 180–720 t CO2/day. Depending on the inlet SOx con-centration, a preliminary desulphurisation step may be required.In the ‘‘ABB Lummus Crest-technology’’, the acceptable SOx con-centration is maximum 50 ppm. However, the application ofMEA-based processes is limited due to the corrosive salts formedby MEA and SOx present in process streams. Other disadvantagesare relatively high enthalpies of reaction and evaporation. This re-sults in high energy requirements for the recovery of the solvent.

The Dow Chemical Company developed MDEA-based solventsoffered under the trade name UCARSOL. These solvents can beapplied for CO2 removal from ammonia syngas as well as for CO2

and H2S removal from natural gas. If required, selective H2Sremoval can be accomplished by a solvent from the UCARSOLfamily [30]. This solvent is applied in the Amine Guard FS processfor CO2 removal from ammonia syngas. There are more than 500units worldwide using this technology in treating natural gas,ammonia syngas and hydrogen streams.

Some companies use sterically hindered amines for the removalof CO2. These amines are developed as alternatives for MEA (e.g.less corrosive or higher regeneration rate) [1]. By the reaction ofhindered amines with CO2, higher loadings (1 mol/mol amine)than by MEA (0.5 mol/mol amine) can be reached. Furthermore,the energy required for solvent regeneration with hindered aminesis much lower than, e.g., with MEA. Therefore, hindered amines canreduce both capital and utility costs in the separation of carbondioxide. Kansai Electric Power Company and Mitsubishi HeavyIndustries have developed and have been using a hindered aminecalled KS1. It is now used at a fertiliser plant in Malaysia [25]. An-other hindered amine is 2-amino-1-methyl-1-propanol (AMP)[20].

CO2 removal is also carried out by hot potassium carbonate thatis used in many plants (e.g. ammonia plants, natural gas treating).The most often applied technology is UOP’s Benfield process withmore than 700 units worldwide [28]. In this process, diethanola-mine (DEA) is used as an activator. The Catacarb process (Eickmey-

Table 9Reactive absorption processes for the capturing of H2S (adapted from [28]).


Lurgi tail-gas treatment process (LTGT) Lurgi Oel-Gas-ChemieGmbH

MDEA � Six LTGT units for processing Claus tail gases in operation or underdesign

Fluor hydrogenation/amine Claus tail gas treatingprocess

Fluor Enterprises, Inc. – � Three plants engineered or constructed

HCR (High Claus Ratio) SIIRTEC NIGI – � First commercial plant started in November 1988� Since then 10 plants under construction

0,01

0,1

1

10

3010 100 1000 10000

0,1 1 5 10 100 1000

Hot potBenfieldDGA

SulfinolBenfield-Hi-pure

Chemical solvents

Physicalandchemicalsolvents

Amisol

ZnO

ZnO

Rectisol

RectisolPurisol SpasolvSelexol

SulfinolMDEADEA

Molecular sieves

Parti

al p

ress

ure

ofH

2S/C

OS

and

CO

2in

gas

feed

, ba

r

Sulfur in product gas, ppm (mL/m³)

CO2 content, ppm (mL/m³)

Fig. 5. Selection of appropriate gas purification process for simultaneous H2S/COSand CO2 removal (adapted from [21]).


er and Associates) also uses hot potassium carbonate, which isapplied for the removing of CO2 from ethylene oxide recycle gas.Exxon Flexsorb HP process uses a hindered amine activator,whereas the Giammarco-Vetrocoke uses an organic activator forthe removal of CO2 [31]. In Ref. [32], it is reported that the ADIPand ADIP-X process can be applied for the bulk and deep removalof CO2 from gas streams.

2.1.1.2. H2S removal. Hydrogen sulphide is a colourless, toxic gas,that can be found in biogas, petroleum and natural gas [33]. Gasescontaining H2S are called sour gases [1], while the removal of H2Sfrom the gas is called sweetening. Most often, hydrogen sulphidemust be removed because of its high corrosivity and due to envi-ronmental reasons [31,32].

Coke oven gas from coke plants also contains H2S. For thereduction of SO2 emissions and to fulfil technological and safetyrequirements, the desulphurisation of coke oven gas is necessary.The H2S content of coke oven gas is limited to 500 mg=m3

n [34].Another example for H2S removal can be found in the Claus pro-

cess that produces elemental sulphur from H2S containing gases.The tail gas leaving this process consists of sulphur vapour andSO2, which is treated using the so-called tail gas treatment (TGT)process. The product of this process is used as a feed stream inthe Claus process. The TGT process consists of a reaction step form-ing H2S from the mentioned tail gas components and an amine-based absorption step to bring the H2S concentration to the valuethat is required for the Claus process [33]. The Shell’s SCOT processis a TGT process that is often applied.

H2S absorption can also be used in hydrodesulphurisation. Inthis process, sulphur compounds are removed from mineral oilproducts by hydrogenation. Table 9 shows suitable processes forselective H2S removal. The listed processes can recover sulphurfrom a gas stream up to 99%. The Lurgi Tail-Gas Treatment (LTGT)process is a wet-scrubbing process for the treatment of the tail gasfrom the Claus process. Nearly six LTGT units are in operation. TheFluor hydrogenation process and the High Claus Ratio process arealso applied for Claus tail gas treatment for nearly complete sul-phur recovery [28].

2.1.1.3. Removal of CO2–H2S mixtures and other impurities. Togetherwith CO2 and H2S, raw gases often contain other impurities, e.g.mercury, COS, CS2 and organic sulphur compounds. For manyindustrial applications, it is necessary to remove both CO2 andH2S and possibly other undesirable compounds for economic andecological reasons [35]. Natural gases, refinery gases and synthesisgases contain high amounts of CO2 and H2S [32,36–38]. A high CO2

content in natural gas reduces the heating value of the gas. There-fore, CO2 has to be removed from natural gas for increasing itsquality. H2S must be removed, because both this component andthe products of its reaction are highly corrosive [15]. Levels of lessthan 1% for CO2 and 4 ppm for H2S must be achieved for preventingcorrosion of the equipment and to fulfil fuel gas requirements[36,39]. Syngas contains acidic compounds, such as H2S, COS andCO2. The aqueous solutions of these compounds are corrosiveand also must be removed for the avoidance of corrosion of processequipment [15].

In gas liquefaction processes, acid gas treatment includes theremoval of CO2 and sulphur compounds (H2S, COS and mercap-tans). This is applied in both LNG (liquefied natural gas) [33] andin LPG (liquefied petroleum gas) production [40].

As mentioned earlier, for choosing the appropriate absorptionprocesses for a given separation problem, the knowledge of thegas inlet as well as outlet concentrations is necessary. This is evenmore complex, when more than one component is to be removed.For the selection of the appropriate removal process for CO2 andsulphur containing gases, a schematic representation is given in[21] (see Fig. 5).

Table 10 contains above mentioned processes for the removal ofCO2 and H2S containing gases. MEA, MDEA, DEA and DIPA are themost common amines for simultaneous CO2 and H2S removal,due to their low cost, high reactivity and easy regeneration. Thedisadvantages, e.g. corrosiveness, can be overcome by the additionof corrosion inhibitors [1].

The most widely used technologies for simultaneous CO2 andH2S removal are the Benfield process based on hot potassium car-bonate (over 700 units worldwide), the Amine Guard FS processapplying the MDEA-based solvent UCARSOL (�500 units) and theADIP process using DIPA or MDEA (over 400 units). The aMDEAprocess, developed and licensed by BASF, is one of the few technol-ogies in which additional mercaptan removal is possible.

2.1.2. Research activitiesCurrent research activities concerning RA for the removal of CO2

and/or H2S containing gases are mostly addressed to carbon cap-ture and storage (CCS). When integrating amine scrubbers into a

Table 10Reactive absorption processes for the capturing of CO2/H2S and other components (adapted from [28]).


Resulf CB&I TPA MDEA or formulated MDEA � 45 Resulf units� 3 Resulf-10 units� 2 Resulf-MM units� Purification of sulphur recovery unit (SRU) tail gas for incineration

ADIP Shell Global SolutionsInternational B.V.

DIPA or MDEA � 400+ ADIP units in operation or under construction� Removal from natural gas, refinery gases and synthesis gases� Removal of H2S, CO2, COS from liquefied petroleum gas or natural gas

liquidsADIP-X Shell Global Solutions

International B.V.MDEA + additive � Applied in one natural gas application

� Removes products from natural gas, refinery gas and synthesis gas, H2S canbe reduced to low-sulphur levels

Advanced amines Prosernat-IFP GroupTechnologies

High load DEA, selective MDEA,activated MDEA

� 120+ units� Amine based process to sweeten natural gases� Cover all types of acid gas removal applications

aMDEA process BASF AG MDEA � 200+ plants operating� 30+ under design or construction, mostly treating synthesis gas, natural

gas and hydrogen streamsAmine Guard FS UOP LLC Ucarsol (MDEA-based solvent) � 500+ units worldwide

� Mostly treating natural gas, ammonia syngas and hydrogen streamsBenfield UOP LLC Hot potassium carbonate � 700+ units worldwide

� 65+ treat natural gas� 200+ treat ammonia syngas� 110 in hydrogen plants� Other applications: in SNG, partial oxidation, coal gasification and petro-

chemical applicationsFluor improved

EconamineFluor Enterprises, Inc. DGA � 55+ Econamine plants

� 7 improved Econamine plants


power plant, the energy requirements will increase enormously[17]. However, Rochelle [41] claims that amine scrubbing willprobably be the dominant technology for CO2 capture from coal-fired power plants in 2030. Hence, there is a huge potential foroptimisation which will require significant research work.

2.1.2.1. Improvement of solvents. The main room for improvementof the industrially used processes are related to the solvents.According to Davidson [42] and Wang et al. [43], the ideal chemicalsolvent possesses a high reactivity with respect to CO2, low regen-eration cost requirements, high absorption capacity, high thermalstability and reduced thermal degradation, a low environmentalimpact and low costs. According to Veawab et al. [44], 70–80% ofthe operating costs arise due to the regeneration of the solvent.Additionally, the most commonly used, amine based, solventMEA has several disadvantages like a low carbon dioxide loadingcapacity, solvent degradation in case that SO2 and O2 are presentin the flue gas, high equipment corrosion rate and high energy con-sumption [43].

Lepaumier et al. [45,46] studied the degradation in the presenceof CO2 and oxidative degradation of several amines to find newpromising solvents. It can be seen that some amines are more sta-ble than MEA. For example, MDEA (N-methyldiethanolamine),AMP (2-amino-2-methylpropan-1-ol), DMP (N,N0-dimethylpipera-zine) and TMEDA (N,N,N0,N0-tetramethylethylendiamine) are morestable in the presence of CO2; furthermore, DMEA (N,N-dimethy-lethanolamine) and AMP reveal less oxidative degradation.

Different types of amines and their mixtures were comparedexperimentally and numerically by Dubois and Thomas [47]. TheCO2 absorption efficiencies increase when mixing different typesof amines: tertiary or sterically hindered amines with an activator(e.g. piperazine (PZ)) or a primary amine (e.g. MEA). For example,the absorption efficiency of an aqueous mixture of MDEA (30%)and PZ (12.5%) is about 15 times higher than the absorption effi-ciency of a 30% aqueous MDEA solution [47].

Vaidya and Kenig [48] studied the reaction kinetics of aqueoussolutions containing N-ethylethanolamine (EEA) and N,N-diethy-lethanolamine (DEEA), which are potentially attractive, as they

can be prepared from renewable resources. It was found, that theCO2 absorption rate was enhanced, when even a small amount ofEEA was added to an aqueous DEEA solution. Further, they studiedthe kinetics of the reaction of CO2 with N-methylmonoethanol-amine (MMEA) and N,N-dimethylethanolamine (DMEA) [49] andacceleration of the CO2 reaction with DEEA in aqueous solutionsby PZ [50]. It was found that the CO2 absorption rate could be sub-stantially enhanced by adding just a small amount of PZ.

Solvents containing diamines have a good potential for CO2 cap-ture, because of the presence of one or more primary or secondaryamino groups. The kinetics of CO2 reaction with diamine N-(2-ami-noethyl)ethanolamine (AEEA) was investigated by Bindwal et al.[51].

Parallel to its use as an amine blend activator, concentrated PZis also applied as a solvent; besides, PZ-promoted potassium car-bonate (K2CO3) solutions are also in use. These applications repre-sent separate research areas. For instance, Freeman et al. [52] madeexperiments to measure the oxidative and thermal degradation,the CO2 solubility, the CO2 mass transfer rates in PZ and the vola-tility of aqueous PZ solutions. In addition, the energy requirementwas estimated using the simulation tool Aspen Plus�. The experi-ments revealed that oxidative degradation of PZ is slower than thatof MEA. Thermal degradation is insignificant for temperatures ofup to 150 �C. Therefore, high-pressure strippers are allowed to im-prove the energy performance. The mass transfer measurementshave shown that the rate of absorption into PZ is higher than thatinto MEA. The simulations of a simple stripper section indicatedthat the equivalent work required for the stripping of a PZ solutionis 10–20% lower than for MEA. Furthermore, Bishnoi and Rochelle[53] made experiments to measure reaction kinetics, mass transferand solubility, Chen and Rochelle [54] studied derivatives of aque-ous piperazine solutions, and Plaza and Rochelle [55] demon-strated the benefits of PZ in a 1-month pilot plant campaign.

Oexmann et al. [56] analysed the post-combustion CO2-captureprocess using a solution of aqueous K2CO3 promoted by PZ withthe simulation tool ASPEN Plus� (equilibrium stage model). Itwas shown that the use of a mixture of 2.5 m K2CO3 and 2.5 mPZ may significantly reduce the required heat duty for solvent

Fig. 6. Primary sources of NOx emission in global scenario (adapted from [71]).


regeneration in comparison to MEA. When attaining a CO2-capturerate of 90%, the heat duty for this process is as low as 2.4 GJ/t CO2

compared to 3.3 GJ/t CO2 (for MEA). By integrating this CO2-cap-ture and a CO2-compression process into a coal-fired power plant,the overall energy requirement will reduce from 0.342 kW h el/kg CO2 to 0.288 kW h el/kg CO2. In contrast to this preliminaryevaluations Oexmann and Kather [57] found that the K2CO3 pro-cess is better in terms of heat duty, but, considering higher opera-tional costs, due to the high price of PZ, the overall costs for CO2

removal are 18% higher than for MEA. More information on thistechnology can be found in Cullinane and Rochelle [58].

Further research is done on ammonia. The chilled ammonia pro-cess was already patented in 2006 by Eli Gal and is still under re-search. Darde et al. [59] performed thermodynamic analysis of thesystem. The study showed that the chilled ammonia process al-lowed for a significant reduction of the energy consumption inthe desorber compared to the energy consumption of the processbased on amines.

Amino acid salts are of great interest for CO2 capture. They arepresent in the environment in a natural way and, therefore, envi-ronmentally friendly. In addition, they have the same functionalgroups as alkanolamines, and their reactivity and CO2 absorptioncapacity are comparable to aqueous alkanolamines [60]. A studyon amino acid salt systems is published in [60]. Potassium sarco-sine showed a better absorption rate than MEA, but the energyrequirement for regeneration was higher. Aronu et al. [61] studiedthe performance of amino acid salts formed from the neutralisa-tion by an organic base. It was shown that these amino acid saltshad a better CO2 absorption potential than amino acid salts formedfrom neutralisation by an inorganic base. Vaidya et al. [62] investi-gated the kinetics of carbon dioxide removal by aqueous alkalineamino acid salts and found that the CO2 absorption rate was en-hanced, when even a small amount of potassium glycinate wasadded to an aqueous DEEA solution.

According to Wang et al. [43], another promising solvent is amixture of alkanolamines and enzymes. The latter can catalysevery fast reactions at ambient conditions requiring only small sol-vent amounts.

A new interesting possibility is the use of demixing solvents asin the DMXTM process proposed by IFP Energies Nouvelles [63]. Inthis process, a liquid solvent is used that splits onto two separateliquid phases after the heat exchanger between absorber and strip-per, thus building a water-rich phase, with a very high carbon diox-ide loading, and an amine-rich phase. Consequently, the amine-rich phase does not need to be sent to the stripper and the energyrequirements for solvent regeneration is lowered. Process simula-tions showed that the reboiler energy can be reduced from3.7 GJ/ton CO2 typical for a standard MEA process down to 2.3 GJ/ton CO2 for the DMXTM process.

2.1.2.2. Improved integration. Capturing CO2 from flue gas streamsrequires cost-effective capture technology and energy-efficientintegration of the capture unit into the power plant. According toPfaff et al. [64], currently such processes are characterised by largeheat requirements for the regeneration of the solvent and auxiliarypower for pumps, blowers and the final compression of the sepa-rated CO2. Due to these energy demands, the net efficiency of theoverall power plant process decreases to about 9–13%.

Pfaff et al. [64] analysed the integration of a CO2 capture processinto a greenfield hard-coal-fired power plant using the simulationsoftware EBSILON Professional� from STEAG Energy Services. Thisstudy shows that the integration of waste heat from the CO2 cap-ture unit into the water-steam-cycle of the power plant will min-imise the overall net efficiency loss. More specifically, the wasteheat from the desorber overhead condenser and from the CO2 com-pressor can be used.

Further studies on process integration of coal-fired power-plants were done by Stankewitz and Fahlenkamp [65] and Luc-quiaud and Gibbins [66]. In [65], heat sources and sinks were quan-tified with the help of simulations (Aspen Custom Modeler� fromAspenTech and EBSILON Professional� from STEAG Energy Ser-vices). Recovering of waste heat can significantly reduce the lossof efficiency. Lucquiaud and Gibbins [66] proposed a method toestimate electrical output penalty (EOP) values for CO2 captureand compression systems that are integrated within the powerplant. They designed a tool to assess solvent-based capture tech-nology integrated into the power cycle of new-build coal-firedplants. The integration of supercritical coal-fired power plants withhigh temperature (580 �C) and pressure (23 MPa) were studied byRomeo et al. [67,68] and Aroonwilas and Veawab [69].

2.1.2.3. Cement industry. Carbon capture plays also an importantrole in cement industry. Contrary to common flue gases, here,the gases contain relatively large amount of CO2. A recent theoret-ical study in [70] considers a process flow diagram for the integra-tion of the carbon capture process in the cement production plant.In the absorption step, aqueous MEA is applied. It could be shown,that each tonne CO2 that can be avoided in a 1 Mt/y European ce-ment plant would cost 107€. The corresponding cost for a 3 Mt/yAsian plant would be 59€. Furthermore, these costs can be reducedif other solvents are applied and the process is integrated into theadjacent power plants.

2.2. NOx removal

Nitrogen oxides are gases with high reactivity containing vary-ing amounts of nitrogen and oxygen. Most of the NOx emissions arein the form of NO that is then oxidised to NO2 in the atmosphere.All combustion processes are sources of NOx. During combustion,some N2 is converted to NO at high temperatures and in the pres-ence of oxygen; the higher the combustion temperature is, themore NOx is produced. Emissions of NOx remained more or lessconstant in the past decades, with the major sources being thecombustion in industrial facilities, electric power generation,non-road engines and on-road vehicles. Fig. 6 shows the primarysources of nitrogen oxides. Remarkable, the transportation sectorproduces the highest amount of NOx (61%), while the total emis-sion of industrial activities is about 32%, according to Chunget al. [71].

The applied NOx removal processes can be classified as dry andwet processes. Dry processes are catalytic or non-catalytic denitri-fication processes, while the scrubbing of nitrogen oxides by wateror aqueous nitric acid belongs to the wet methods [71,72]. The wetmethods belong to the class of RA processes [73].

2.2.1. Industrial applicationsThe industrial NOx treatment is carried out by capturing NOx in

water or aqueous nitric acid. In addition to gas cleaning, the sameRA process is also used for the manufacturing of nitric acid. The lat-ter technology is addressed in Section 2.3. Absorption of nitrous


gases is a highly complex process, due to the interaction of severalcomponents and chemical reactions in both the liquid and gasphases. During the absorption of nitrogen oxides, the followingreactions may occur [73,74]:

2NOþ O2 ! 2NO2 ðgas phaseÞ ð11Þ

2NO2 ! N2O4 ðgas phaseÞ ð12Þ

NOþ NO2 ! N2O3 ðgas phaseÞ ð13Þ

NOþ NO2 þH2O! 2HNO2 ðgas phaseÞ ð14Þ

3NO2 þH2O! 2HNO3 þ NO ðgas phaseÞ ð15Þ

2NO2 þH2O! HNO3 þHNO2 ðliquid phaseÞ ð16Þ

N2O3 þH2O! 2HNO2 ðliquid phaseÞ ð17Þ

N2O4 þH2O! HNO3 þHNO2 ðliquid phaseÞ ð18Þ

3HNO2 ! HNO3 þH2Oþ 2NO ðliquid phaseÞ ð19Þ

Note that absorption of NOx can further be enhanced by theaddition of hydrogen peroxide (H2O2), according to the followingreaction mechanism:

2NOðlÞ þ 3H2O2ðlÞ ! 2HNO3ðlÞ þ 2H2OðlÞ ð20Þ

HNO2ðlÞ þH2O2ðlÞ ! HNO3ðlÞ þH2OðlÞ ð21Þ

Hydrogen peroxide reacts with HNO2 and NO from the liquidphase, and, as a consequence, it enhances the formation of nitricacid [75]. The NOx gas is charged with a certain amount of H2O2,and the gas mixture reacts on a solid catalyst to hydrogen nitratethat is either withdrawn for further use or converted into nitricacid by condensation or by water scrubbing. This process, patentedby Degussa (now Evonik Industries), allows the removal of NOx

(even at low concentrations, below 2000 ppm) with high efficien-cies above 90% [76], depending on the NO2 concentration in totalNOx – this aspect being discussed in [77]. Note that flue gasesand tail gases may have widely varying NO2/NOx-ratios and corre-spondingly varying abatement efficiencies.

Several basic approaches are typically used to reduce the tail-gas NOx levels: improved RA, chemical scrubbing, adsorption andcatalytic tail-gas reduction [78]. Intense research and developmenteffort was invested in these methods during the past decades. Inwhat follows, we focus only on the methods related to RA.

The efficiency of absorption depends mainly on the operatingpressure, temperature and the number of stages. The temperatureof the gas between stages is especially important, because it gov-erns the progress of oxidation, the limiting stage for the entireabsorption process [78]. In existing plants, the following improve-ment options can be used:

� Expand the absorption volume: The advantage of this option isthat it does not require any new technology. The absorption vol-ume is added in the form of a second tower – new absorptiontower designs with very few stages have been devised. How-ever, large additional volumes only result in small reductionsof tail-gas NOx levels, since the oxidation of NO to NO2 proceedsvery slowly when the NOx concentration is low. The drawback isthat added absorption volume in stainless steel is expensive[78].� Lower the absorption temperature: The use of cooling energy in

the absorption process significantly accelerates the oxidationof NO to NO2. The disadvantage here is that the required refrig-eration equipment and piping means further investment. An

alternative aid to reduce the NOx level is to cool down thenitrous gas, so that more N2O4 is formed than NO2. The N2O4

is then scrubbed with nitric acid at 0 �C; in this way it can bestripped out and converted to nitric acid (N2O4 + ½O2 + H2O ?2HNO3) in an absorption reactor operated in the range of 60–80 �C. This technique is actually similar to that in concentratednitric acid production. However, the method is more suitablefor investment in a new plant than for the expansion/upgradingof an existing plant [78].

Chemical scrubbing is a generic technology by which severalcontaminants are removed from the gas stream by means of ascrubbing liquid reacting with them. The chemical scrubbing ofNOx out of tail gas is covered in a number of patents and scientificpublications [78]. It is worth noting that all the scrubbing methodshave problems related to the scrub liquor: e.g. cost, regeneration,quantity, and environmental impact. These problems are alwayseasier to manage, if the NOx removal is part of an integrated chem-ical plant, what is typically the case in practice. The following scrubliquors have been proposed so far [78]: aqueous suspension ofmagnesium carbonate and magnesium hydroxide [79], solutionof vanadium in nitric acid [78], ammonium sulphide and bisul-phide [80], milk of lime [81], ammonia [78], hydrogen peroxide[82], and urea [78].

At industrial scale, ammonia scrubbing is used in the US byGoodpasture (Texas) who has developed a safe process based onscrubbing of the tail gas with ammonium nitrate solution. The ni-trite formation is suppressed by aeration and the presence of freeacid. The tail gas is then led through a scrub liquor containingammonia (pH 7.5–8.5). The scrubbing product is a 30–50% ammo-nium nitrate solution, while the tail gas has a residual level of�200 ppm NOx [78].

The hydrogen peroxide scrubbing method is based on the fol-lowing overall reactions:

NOþ NO2 þ 2H2O2 ! 2HNO3 þH2O ð22Þ

2NO2 þH2O2 ! 2HNO3 ð23Þ

The reactions are carried out on sieve trays or in packed towerswith recirculation of the hydrogen peroxide solution. The benefit ofthis scrubbing process is that the reaction time is very fast. How-ever, the drawback is that the hydrogen peroxide scrub liquor israther expensive.

Another process was developed by Norsk Hydro in Norway,with the advantage that urea is readily available and quite inex-pensive [78]. The process is carried out at 50 �C using a solutioncontaining 20% urea and 10% free nitric acid for scrubbing tailgas to remove NOx. By doing so, both nitrogen oxides are selec-tively reduced to N2 by urea that decomposes to yield nitrogenand carbon dioxide. The resulting stack gas trail is colourless andheavily loaded with water vapour.

2.2.2. Research activitiesThe latest research activities were dedicated to aqueous alka-

line solutions, the main reason being the possibility to producevaluable substances, such as nitric acid and its salts – nitratesand nitrites [83]. Pradhan and Joshi [84] used an aqueous NaOHsolution as a solvent in a tray column, in which sodium nitrite isproduced selectively. This process can also be improved by theaddition of H2O2 and the use of structured packing as column inter-nals [75]. Other alkaline solutions were analysed and a model wasdeveloped by Patwardhan and Joshi [83], who incorporated theHNO2 decomposition, as well as the neutralisation by alkali. Thisled to a unified model for NOx absorption, which can be used forpredicting process rates and selectivity, for various gas composi-tions and liquid-phase pH-values [83].


BioDeNOx is a novel process – developed and tested at Wagen-ingen University and TU Delft [85,86] – for the removal of NOx fromindustrial flue gas, in which the reactive absorption of NO in aque-ous solutions of Fe-II(EDTA) plays a key role resulting in the forma-tion of a nitrosyl complex Fe-II(EDTA)(NO). Basically, the BioDeNOx

process combines the principles of wet absorption of NO into anaqueous Fe-II(EDTA) solution with biological regeneration of thescrubber liquor in a bioreactor. Note that the oxygen present inthe flue gas will also be absorbed and oxidise Fe-II(EDTA). This isin fact an undesired reaction, since the resulting Fe-III(EDTA) doesnot react with NO.

Winkelman et al. [87] developed a rate-based model for thesimultaneous RA of NO and O2 in aqueous Fe-II(EDTA) solutionsin a counter-current packed column, and evaluated the effect ofprocess conditions on the absorber performance (e.g. NO removalefficiency and selectivity). The absorber performance was particu-larly dependent on the operating temperature, while lower tem-peratures are in favour of both the NO removal efficiency andselectivity [87]. Notably, over-designing the absorber may resultin decreasing performance, because, at certain process conditions,desorption of NO may occur resulting in lower NO removal effi-ciencies at increasing column lengths.

More recently, Kenig and Seferlis [88] also demonstrated theability of using rate-based models in a compact form – by usingthe orthogonal collocation on finite elements (OCFE) – to accu-rately predict the steady-state and dynamic behaviour of industrialand experimental columns for NOx absorption. OCFE formulationreduces the overall size (total number of equations) of the RA pro-cess model, while preserving its structure and accuracy. A compre-hensive description of the OCFE method is given by Dalaouti andSeferlis [89,90].

2.3. Nitric acid production

Nitric acid – also known as aqua fortis or azotic acid – is an inor-ganic acid of major industrial importance. Due to its properties as avery strong acid, a powerful nitration and oxidising agent, nitricacid is crucial in the production of many chemicals, e.g., pharma-ceuticals, dyes, synthetic fibres, insecticides, and fungicides [91].Remarkable, nitric acid is mostly used in the production of ammo-nium nitrate for the fertiliser industry – about 70% of the globalproduction amounting 54.5 million tons in 2010 [92]. The worldproduction of NH4NO3 is largely concentrated in two regions, theformer USSR (24.5%) and Western Europe (21.7%), although USA,China and Central Europe are also large producers [93].

Nitrogen oxides (NOx) are formed not only as waste gases inmany industrial processes, but also during the oxidation of ammo-nia in the nitric acid production process. From the eco-efficiencyviewpoint, it is beneficial to combine the removal of NOx fromwaste gases with the production of nitric acid by NOx absorptionin water or diluted nitric acid. Overall, this is a very complex pro-cess involving chemical reactions in both gas and liquid phase. Ageneral approach to the description of such processes was gradu-ally developed in [73,74,88,94].

2.3.1. Industrial applicationsNitric acid is met in nature only in the form of nitrate salts, and

sodium nitrate was used as feedstock when the large-scale produc-tion of nitric acid started. Only in the beginning of the 20th cen-tury, when the sodium nitrate reserves were considered toapproach exhaustion, other processes were developed to replacenitrogen from nitrates with atmospheric nitrogen. The three tech-niques used industrially were [78]:

1. Direct processes to produce NO by a reaction of atmosphericnitrogen with oxygen at temperatures over 2000 �C. The directcombustion of air in an electric arc – developed by Birkelandand Eyde (see [78]) – was abandoned due to its poor energy effi-ciency. Other direct processes developed later – such as thermalNO synthesis with fossil fuels or in nuclear reactors – did notgain widespread acceptance.

2. Production of ammonia by hydrolysis of calcium cyanamideunder pressure – a process of only transient value, until theHaber–Bosch process became available.

3. Production of ammonia from nitrogen and hydrogen, by theHaber–Bosch process – a process used nowadays to produceammonia feedstock for nitric acid production.

The crucial step in the nitric acid production is the catalyticcombustion of ammonia, a process developed by Ostwald andimplemented at industrial scale in 1906 at Gerthe in Germany[78]. The Ostwald process involves three key chemical steps:

� Catalytic oxidation of ammonia with atmospheric oxygen toyield nitrogen monoxide.� Oxidation of NO product to nitrogen dioxide or dinitrogen

tetroxide.� Absorption of the nitrogen oxides to yield nitric acid.

The following exothermic reactions occur in the above men-tioned process steps [91]:

4NH3ðgÞ þ 5O2ðgÞ ! 4NOþ 6H2OðgÞ DHR ¼ �226 kJ=mol ð24Þ

2NOðgÞ þ O2ðgÞ ! 2NO2ðgÞ DHR ¼ �114 kJ=mol ð25Þ

3NO2ðgÞ þH2OðlÞ ! 2HNO3ðaqÞ þ NOðgÞDHR ¼ �117 kJ=mol ð26Þ

4NO2ðgÞ þ 2H2OðlÞ þ O2ðgÞ ! 4HNO3ðaqÞ DHR ¼ �63 kJ=molðonly when air is presentÞ ð27Þ

Note that the oxidation of nitrogen monoxide is a rare exampleof a homogeneous third-order gas-phase reaction, while theabsorption of NOx is a heterogeneous reaction [78]. The nitric acidproduction processes are characterised by the way in which thethree steps mentioned above are implemented. In single-pressureprocesses, the ammonia combustion and NOx absorption take placeat the same working pressure (in the range of 1–14 bar, cf. Ta-ble 11) – with very few plants currently employing only relativelylow pressure (1–2.2 bar) for both oxidation and absorption steps.In dual-pressure (split-pressure) processes, the absorptionpressure (6.5–13 bar) is higher than the oxidation pressure(1–6.5 bar, Table 11). The modern plants feature oxidation at4–6 bar and absorption at 9–14 bar, while older plants stillemploy atmospheric combustion and medium-pressure absorption[95].

Most new plants work with the dual pressure process – illus-trated in Fig. 7 [95]. An air–NH3 mixture, with a composition ratioof 9:1 is catalytically converted mainly to NO that is afterwardsoxidised and reacts with residual O2 to form NO2 and NO2-dimer.The mixture of NO2/dimer is introduced into the bottom of theabsorption tower, in counter-current with de-ionised processwater. Another air stream is introduced into the column to re-oxi-dise the NO that is formed in reaction. The nitric acid is withdrawnfrom the bottom of the column. Oxidation takes place in the freespace between the trays, while absorption occurs on the trays –usually sieve or bubble cap trays [95].

High-strength nitric acid (98–99%) is produced by dehydrationfollowed by bleaching, condensation and absorption in a weak

Filtration

Air compression

Heating

Filtration

Power

Heat

Evaporation

Filtration

Ammonia

Mixing

Air

Catalytic reactor

Heat recovery

Heat recovery

CoolerCondenser

Absorption

Boiled feed water

Cold tail gas

Cooling water

NOx compression

Heat recoveryCooler

Condenser

Power

Cooling water

Cooling waterProcess water

Nitric acid production

NOx abatement

Expander

Steam turbine

Superheated steam

Hot tail gas

PowerAtmosphere

Hot tail gas

Reducing agent

Steam export

Fig. 7. Nitric acid production – dual pressure plant (adapted from [95]).

Dehydrating Column

Bleacher

Condenser

Strong Nitric Acid

Absorption Column

Cooling Water

Weak Nitric Acid

H2SO4

50-70% HNO3

HNO3, NO2, O2

O2, NO

Air

Fig. 8. Flow diagram of high-strength nitric acid production from weak nitric acid (adapted from [95]).


Table 11Pressures of single and dual pressure processes [95].

Single pressure process (bar) Dual pressure process

Oxidation pressure 1–14 1–6.5Absorption pressure 1–14 6.5–13


nitric acid, while commercial grade nitric acid solutions (52% and68%) are produced via the Ostwald process [95]. Weak nitric acid(30–70%) is produced by catalytic oxidation of ammonia, followedby condensation and absorption in water. In the high-strength ni-tric acid process shown in Fig. 8, an acid concentration of 98–99% isobtained using extractive distillation. Weak nitric acid is intro-duced into a distillation column with a dehydrating agent (e.g. sul-phuric acid 60%). Both acids are fed to the top of the column, atatmospheric pressure, in counter-current to the ascending vapours.The vapour leaving the column is concentrated nitric acid (99%)that passes to a bleacher and then to a condenser. The concen-trated nitric acid is obtained in the condenser, while the by-prod-ucts (O2 and NO) are introduced into the absorption column thatproduces weak nitric acid.

The design of the nitric absorption columns requires the indi-vidual tailoring of each tray (typically bubble cap or sieve trays)for its cooling requirements and the gas volume rates needed foroxidation of the nitric oxide. A good summary of absorber designaspects is available in the book of Keleti [96]. Capacities of around1800 t/d HNO3 were achieved in a single RA column of up to 6 m indiameter, 80 m high, and containing as many as 30–50 trays. Forabatement reasons, single absorption columns reach tail gas NOx

compositions of about 200 ppm, although the economic optimumfor absorber efficiency would lead to higher tail gas NOx concentra-tions (1500–2500 ppm). Various methods are used to reduce tailgas NOx concentrations to an acceptable level for discharge to theatmosphere (200–230 ppm), the most commonly employed beingthe extended absorption and (non-)selective catalytic reduction(SCR and NSCR). Extended absorption uses an additional columnfor oxidation and reaction of nitrogen oxides with water to formacid. Relatively few trays are required, but large oxidation volumes– often employing refrigeration to promote the oxidation and tominimise the column size. This method is most effective for highpressure (HP) absorption in which abatement to less than200 ppm NOx can be achieved in a single HP column [91].

Table 12 provides an overview of the main producers of nitricacid in the US and Western Europe – the capacities being givenin thousands of metric tons per year on a 100% HNO3 basis, andthe market share as percent of the regional market [93]. Remark-able, on a country-by-country basis, the Western European

Table 12Main producers of nitric acid in the US and Europe.

Company and location Capacity (thousands ofmetric tons)

Regional market share(% in 2010)

CF Industries (incl. Terra) 2635 30.7 (US)PCS Nitrogen 1565 18.2 (US)LSB Industries 1240 14.5 (US)Dyno Nobel, Inc. 655 7.6 (US)Invista Inc. 470 5.5 (US)Yara Internat. (incl. 50%

GrowHow UK)6760 45.8 (West EU)

BASF (incl. 50% Prod. Eng.Chim. Rhin)

2500 16.9 (West EU)

Grande Paroisse (+Prod.Eng. Chim. Rhin)

1290 8.7 (West EU)

Fertiberia S.I. (incl. Adubosdu Portugal)

925 6.3 (West EU)

OCI Nitrogen (formerlyDSM Agro B.V.)

755 5.1 (West EU)

production of nitric acid is dominated by five countries accountingfor 73.3% of total nitric acid production in 2010: Germany 17.7%,France 17.0%, Netherlands 16.4%, Belgium 12.0%, and United King-dom 10.2%.

2.3.2. Research activitiesThe current research activities are primarily driven by the main

pollution problem in the nitric acid manufacture, namely the abate-ment of NOx in the tail gases. There are only few options to reducethe emissions to acceptable levels: to capture the NOx and convert itto additional nitric acid or nitrate–nitrite salts, or to render the NOx

harmless by converting it to non-polluting compounds. A largenumber of processes have been developed and patented to reducethe emissions, they can be classified into three main categories:absorption, adsorption and catalytic reduction [93]. The absorptionabatement deals with the optimisation of the existing absorptionsystem, or modifications aimed at increasing the absorption capac-ity. Tail gases pass through an absorber containing either water oran aqueous solution of ammonia, urea or NaOH. When water isused, the resulting weak acid is recycled leading to slightly in-creased nitric acid yields (1–3% higher). When other absorbentsare used, the recovered NOx is usually consumed in the productionof nitrogen solutions for fertiliser use. If NaOH is used, pure sodiumnitrite and sodium nitrate may be recovered [93].

The need to make the nitric acid processes more efficient, whilemeeting the much stricter environmental regulations (e.g.200 ppm in EU, or 1.5 kg NOx per metric ton of nitric acid inUSA), led to new advances in understanding the absorption chem-istry. One such development is the High Efficiency Absorption(HEA) technology developed by Rhône-Poulenc, which can reducethe size of an absorber column by up to 35%. HEA is applicablewhen the gas-phase NOx concentration is low (<8000 ppm), andthe rate-limiting step for nitric acid formation is typically thegas-phase oxidation of NO. By directly oxidising nitrous acid inthe liquid phase to nitric acid, the decomposition of nitrous acidis avoided and the large oxidation volumes for NO2 regenerationfrom NO are greatly reduced [91].

Much effort has been put into the development of advancedrate-based models of NOx absorption. Here, bulk reactions and filmreactions have to be considered in both phases. A comprehensiveanalysis of such models including their implementation in AspenCustom Modeller (AspenTech) and validation by comparison withexperimental data for a pilot scale packed column and an industrialsieve tray column is given in [73,88,97].

More recently, catalysts were developed for the selective reduc-tion of NOx to N2 at relatively low temperatures, using NH3 asreducing agent. Although additional NH3 is consumed, this processmay provide control of emissions in an efficient way, due to lowoverall costs [93].

2.4. Desulphurisation

Sulphur compounds (e.g. SOx) are produced in several industrialprocesses, such as combustion of organic fuels or reforming of oil.In the following sections, various ‘‘Flue Gas Desulphurisation’’(FGD) processes are presented, and the current industrial and aca-demic research activities are summarised.

2.4.1. Industrial applicationsThere is a wide range of industrial processes for removal of sul-

phuric oxides from gases. The applications can be classified intowet processes (limeston gypsum, sea-water washing, ammoniascrubbing, Wellman-Lord), semi-dry (circulating fluidised bed,spray dry, duct spray dry) and dry processes (furnace sorbent injec-tion, sodium bicarbonate injection) [98,99]. Out of these processes,only the wet processes are performed by RA.

Flue gasPrecipitator

Fly ashID Fan

By-pass damper

Booster fan Reheater

Absorber

Flue

HydrocyclonesWaste-watertreatment

Water discharge

Sludge

Gypsum

AirSlurrytank

Limestonemills

Limestonestore

Fig. 9. Schematic of a limestone gypsum FGD process (adapted from [98]).

Flue gas

By-passdamper

Flue

Inlet damper

Booster fan

Outletdamper

ReheaterAbsorber

Packing

Sea water fromcondenser

Return to sea

Oxidation air

Fig. 10. The seawater washing process (adapted from [98]).


The limestone gypsum process (see Fig. 9) is the most commonFGD process, which is applied in different variants for over30 years. This process is well understood and used by many com-panies. Most commonly an open spray tower is applied in whichthe flue gas is flowing upwards. Marsulex, ABB, Lurgi Lentjes Bisc-hoff (LLB), Babcock Borsig, Kawasaki and IHI operate and sell thistechnology. Babcock and Wilcox (B&W) and Babcock Hitachi havea very similar design. In this variant, a tray is located in the bottomof the gas treatment zone. Mitsubishi Heavy Industries (MHI) usesa different design, without an open spray tower. In this case, a layerof packing is used for an effective gas/liquid surface [98].

Flue gas can be desulphurised using seawater that has a naturalalkalinity. This is a relatively new process (see Fig. 10) and it is pro-vided by two suppliers only, ABB and LLB. However it is expandingvery quickly, due to major advantages. ABB has already built 21plants [98]. Contrary to other FGD processes, no solid componentsare produced. Moreover, the plant design is relatively simple. How-ever, the technology is limited to coasted sites, due to the sea-water requirements.

The ammonia scrubbing technology (see Fig. 11) is similar tothe limestone–gypsum process. Here, ammonia is used as theabsorbent and reacts with SO2. As a result, ammonium sulphate

Flue gas

Precipitator

Fly ashID Fan

By-pass damper

Booster fan Reheater

Absorber

Flue

Water

Hydrocyclone

Bleed

Air

Ammoniastorage tank

Compactor

Dryer Bleed

Purge

Pre-scrubber

Centrifuge

Fig. 11. The ammonia scrubbing process (adapted from [98]).

Flue gasPrecipitator

Fly ashID Fan

By-pass damper

Booster fanReheater

Absorber

Flue

TankTank

Water

Regenerated liquor

SulphatesolidsEDTA

Sodium carbonate orhydroxide make-up

Sodium carbonate /hydroxideBlowdown

Condenser

Fig. 12. The Wellman-Lord process (adapted from [98]).


is produced that can be used, for instance, in the fertiliser industry[98]. This technology has been applied for over 50 years; currentlythere are mainly two suppliers, LLB and Marsulex [98,99].

The Wellman-Lord process (see Fig. 12) uses an aqueous sodiumsulphite solution for the removal of SO2. In this process, valuableproducts can be obtained, for instance, elementary sulphur, sul-phuric acid or liquid SO2. The Wellman-Lord process has more than40 implementations in Japan, USA and Germany. The main advan-tage of this process is that it does not need significant amounts ofsorbent. Moreover, there is little waste produced during the pro-cess [98].

Note that some processes mentioned here use scrub liquors (e.g.milk of lime, limestone) with a suspension of solid particles, thusinvolving a quite complex gas–liquid–solid system. A phenomeno-

logical explanation for such three-phase systems is given else-where [100–104].

2.4.2. Research activitiesIn the past decades, the research has focused on the wet pro-

cesses analysis. The limestone technology and the Wellman-Lordprocess are well-known, and many applications are available.Newer technologies are the ammonia or seawater scrubbing, theyare still in the investigation stage.

A method presented by Ipek et al. [105] uses waste waterammonia for the absorption of SO2. In this study, the behaviourof SO2 is analysed experimentally. An investigation of the temper-ature impact on the SO2 removal can be found in [106].

Fig. 13. General diagram of sulphuric acid production (adapted from [112]).

Air FeedwaterSulphur

Main blower

Waste heatboilerSulphur burner StreamDryer

Inter-mediateabsorber

Oleumabsorber Heat

exchanger

Heatexchanger

Heatexchanger

Heatexchanger

Converter bed 1

Converter bed 2

Converter bed 3

Converter bed 4

Water

Finalabsorber Heat exchanger

Stack

H2SO4 96-98% Oleum 20-37%

Fig. 14. Sulphuric acid plant (double absorption) based on sulphur combustion (adapted from [112]).


Seawater absorption is analysed by several research groups. Thesolubility of SO2 in seawater was investigated in [107,108]. Mea-surements were carried out using a laboratory batch reactor at1 bar and 298.15 K. In a later study, the impact of different pres-sures and temperatures was investigated [109]. Rodriguez-Sevillaet al. [108,109] developed models for the determination of solubil-ity and activity coefficients. Using Aspen Plus�, various influencingparameters, e.g., pH value or SO2 concentration in the feed stream,were analysed by Zhihua and Zhihong [110].

2.5. Sulphuric acid production

Sulphuric acid is the worldwide most produced chemical sub-stance. The yearly production amounts up to 150 million tonnes.North America, Japan and Western Europe produce nearly the half

of the total worldwide production. Sulphuric acid is mostly used asan ingredient, the fertiliser industry being its largest single con-sumer, mostly for manufacturing phosphate fertilisers. Also the(petro-)chemical and the oil refining industries make use of sul-phuric acid, e.g., as an acidic dehydrating agent [111].

2.5.1. Industrial applicationsA general process diagram for the sulphuric acid production is

shown in Fig. 13, the main steps being the SO2 formation and con-version to SO3, followed by the RA of SO3 to yield H2SO4. Note that,actually, there are various types of plants, but only two types pro-duce sulphuric acid using RA methods [112]. The two major pro-cesses used for the sulphuric acid production are the leadchamber process and the current contact process [113]. Remark-able, the contact process remained practically unchanged from

FurnaceBoiler

Reactor

HX3 H2SO4

Abs2

HX1

FEHE1 FEHE2

H2SO4

5

4

1

2

3

5

4

1

2

3

HX4

H2SO4

Abs1

H2SO4

HX2

S (lq)

AirSteam

Air

SO2

SO2

50°

430°

180°

434°

70°

88°

65° 180°

105°

65°

432°

242°

316°

445°

450°

610°

50°

539°

420°

70°

Fig. 15. Flowsheet of a modern sulphuric acid production plant [114].

Table 13List of plants build after 1990 in Western Europe (adapted from [112]).

Location Process type Costs Company Capacity t H2SO4.d-1 Year Emission level

Hamburg, Germany Cu Smelter acid (5–8.4% SO2) 1M EUR Norddeutsche Affinerie 918t.d�1 1991 < 800mg SO2.Nm�3

Helsingborg, Sweden Sulphur burning (17.5% SO2) 36M EUR Kemira Kemi 1,000t.d�1 1992 < 0.9kg SO2.t�1 H2SO4

Harjavalta, Finland Copper and nickel based smelter acid (7–12% SO2) 33M EUR Outokumpu extension 2,430t.d�1 1995 < 4,500t SO2.y�1

Tessenderlo, Belgium Sulphur burner (11.5% SO2) n/a Tessenderlo Chemie 1,000t.d�1 1992 300ppm SO2

Leuna, Germany Sulphur burner n/a Domo n/a 1996 99.9% conversion rateHuelva, Spain Cu Smelter acid (5–10.2% SO2) 39M EUR Atlantic Copper 1,270t.d�1 1996 >99.6%Ludwigs-hafen, Germany Sulphur burner n/a BASF 900t.d�1 n/a 0.65kg SO2.t�1 H2SO4

Le Havre, France Sulphur burner (11.5% SO2) n/a Millennium 800t.d�1 1992 2.6kg SO2.t�1 H2SO4

Huelva, Spain n/a n/a Fertiberia 2,400t.d�1 2000 n/aWorms, Germany Spent acid regeneration; H2SO4 (NH4)2 SO4 53M EUR Rhöm GmbH 500t.d�1 1994 n/a


its introduction in the late 1800s, until the seventies when the dou-ble absorption process was introduced in order to reduce the SOx

emissions. The double absorption process is shown in Fig. 14, whilethe process flowsheet is given in Fig. 15 [114]. However, the doubleabsorption part only reduced the SOx emissions – but it did notchange significantly the nature of the process or the equipmentused. Nowadays, all modern plants use the double absorption tech-nique that can lead to SO3 conversion rates up to 99.85%, a signif-icant improvement over single absorption processes (up to 98 %only) [114]. Table 13 lists the new plants in Western Europe.Rhone-Poulenc group has already built five plants producing3600 t sulphuric acid per day.

Two fundamental steps are present in the production of SO3

and absorption of SO3. SO3 is produced by catalytical oxidationof sulphur dioxide (Eq. 28). Possible sources for producing sul-phur dioxide are sulphur burning, pyrites roasting, and smelting,sulphuric acid regeneration and some other processes [112]. The

main reaction in the absorber is highly exothermic, as shown byEq. 29:

SO2ðgÞ þ 1=2O2ðgÞ¢ SO3ðgÞ DHR ¼ �96:2 kJ=mol ð28Þ

SO3ðgÞ þH2OðlÞ¢ H2SO4ðlÞ DHR ¼ �132 kJ=mol ð29Þ

The simplified flowsheet of such an industrial process consistsof a sulphur burner, a multi-pass converter (catalytic reactor),economisers, heat exchangers (HX), feed-effluent heat exchangers(FEHE) and SO3 absorption towers (Abs) – as illustrated in Fig. 15[114]. SO2 conversion is further improved and tail gas emissionsare reduced by an intermediate SO3 absorption step (Abs1) inwhich the production of sulphuric acid takes place. This absorptionstep is placed after the fourth bed of catalyst in order to change thegas composition by removing most of the SO3 and thus shifting theconversion equilibrium to higher values. The absorption of SO3 isfinalised in the second absorber (Abs2) which also purifies the

335

355

375

395

415

435

455

0 0.2 0.4 0.6 0.8 1Column length / [-]

Tem

pera

ture

/ [K

]

Abs1_Temp_GasAbs1_Temp_Acid

Bottom Top

335

355

375

395

415

435

455

0 0.2 0.4 0.6 0.8 1Column length / [-]

Tem

pera

ture

/ [K

]

Abs2_Temp_Gas

Abs2_Temp_Acid

Bottom Top

0 0.2 0.4 0.6 0.8 1Column length / [-]

0

0.02

0.04

0.06

0.08

0.1

Mol

ar fr

actio

n / [

-]

Abs1_SO3

Abs1_H2O

Bottom Top

0

0.02

0.04

0.06

0.08

0.1

Mol

ar fr

actio

n / [

-]

0 0.2 0.4 0.6 0.8 1Column length / [-]

Abs2_SO3

Abs2_H2O

Bottom Top

Fig. 16. Temperature (above) and composition profiles (below) along the SO3 absorption columns.


outlet gas emissions. Fig. 16 shows typical temperature and com-position profiles along the two counter-currently operated absorb-ers [114]. SO3 concentration is reduced from 9.81% to 0.01% in theintermediate absorber, and from 0.44% to only ppm levels in the fi-nal step. Note that the dimensionless column length is in fact theratio between the real and the total absorber lengths.

The SO3 absorption towers are packed with ceramic saddles orstructured packing, for realising a maximum possible contact area,and are typically carbon steel vessels lined with acid proof brickand mortar. As of early nineties, all metal towers – with no bricklining – were built from high silicon stainless steel alloys, such asSX, Saramet, or ZeCor. Well-designed absorption columns operateat very high absorption efficiencies (>99.5%), typically over 99.8%or even over 99.9%. Teflon or glass packed fibre bed mist elimina-tors are typically used in the interpass and final absorption towers,where high efficiency collection of the acid mist is critical [113].

2.5.2. Research activitiesSulphuric acid production is a mature industry and most of the

developments in production techniques were made in the last cen-tury. Hence, there seems to be little scope for improvement [112].Not surprisingly, the research activities are very scarce, as sulphu-

ric acid has been produced for many decades at the highest worldproduction rate of any chemical.

The main developments are in reducing (accidental) pollution.For example, there are improvements in the materials used inthe construction of the plants or their design, such as the use ofdouble shell vessels [112]. Currently, the need is to optimise sec-tions in the different process stages depending on site require-ments and local conditions [112,114]. Other developments arecarried out with respect to energy recovery from primary energyand demisters with very high efficiency.

Some studies are related to the presence of NOx in acid plantsthat can appear through a number of mechanisms in the upstreamoperations – mainly by oxidation of nitrogen from air (thermalNOx), as nitrogen containing impurities in the feed (feed NOx), oras the organic bound nitrogen present in some fuels during thecombustion step (fuel NOx). Usually, nitrogen oxides have to be de-stroyed and any remaining SO2 has to be stripped with air in orderto improve the product quality and avoid emission problems forthe customer [112]. Depending on the plant design, the concentra-tion of NOx in the feed gas is limited to 5–15 ppm in order to pro-duce an acid that would require no additional treatment.

Wet sulphuric acid plants that directly convert hydrogen sul-phide to sulphuric acid were also proposed as a way to reduce


the effort, energy and capital input [113]. More recently, UhdeGmbH presented the development of a new type of wet sulphuricacid plant (Emission Free Sulphuric Acid Plant) covering the keyidea of the process, the development stages of lab scale experi-ments up to the realisation of the individual process steps in a mo-bile pilot plant. This technology is aimed to replace sulphuric acidplants with backup combustion system and to reduce the SO2

emissions from coke oven plants in order to comply with legisla-tion [115].

3. Concluding remarks

This article presents a comprehensive review on current appli-cations of reactive absorption (RA), covering both industrial pro-cesses and the corresponding research activities. A majorapplication of RA is the removal of CO2 and/or H2S from variousindustrial streams. As these components often appear in the gasstreams simultaneously, their removal is considered in the samesection using the same absorption technologies – mainly basedon aqueous amine or alkaline solutions. In some applications,CO2 and H2S have to be removed selectively. Selective CO2 absorp-tion can be found in the fossil fuel combustion or in differentproduction processes, as in ammonia and hydrogen productionplants. Selective H2S removal is mainly applied in the coke ovengas purification, in hydro-desulphurisation and in tail gas treat-ment of the Claus process. In many applications, CO2 and H2S areremoved simultaneously with other components, such as HCN,CS2 or COS – by applying amines that are non-selective towardsjust one of the components. The main bottleneck in gas sweeteningby RA is the high energy requirement – most of it being used forthe regeneration of the solvent. Therefore, significant researcheffort is being made towards the development of new solvents,such as amine blends or hindered amines.

Nitrogen oxides can be removed using RA in water or aqueousacid solutions. The same RA process is also applied for the produc-tion of nitric acid – which can be further optimised by adding H2O2.In the nitric acid manufacturing, nitrogen dioxide is produced fromammonia which reacts with water in a RA column.

Reactive absorption is also applied for the desulphurization (i.e.sulphur oxides removal). The most common process is the lime-stone gypsum process. Ammonia and seawater are also used as sol-vents in industry, but these processes are rather in theinvestigation stage. Another important industrial application ofRA is in the production of sulphuric acid. Plants producing sulphu-ric acid employ three key steps: SO2 synthesis by, e.g., burning ofsulphur, followed by SO2 oxidation to SO3 that is afterward con-verted to sulphuric acid in absorption columns. The major targethere is to achieve very high SO3 conversion, in order to reducethe SOx emissions and to achieve high energy recovery.

In future, RA is expected to gain increasing importance for car-bon capture and storage or conversion. Furthermore, RA can be ap-plied for the removal of gases produced in the purification ofbiogas. For the design and optimisation or RA processes, advancedrate-based models are recommended.

References

[1] A.L. Kohl, R.B. Nielsen, Gas Purification, fifth ed., Elsevier, 1997.[2] P. Chattopadhyay, Absorption and Stripping, Asian Books Private Limited,

2007.[3] E.Y. Kenig, A. Górak, Reactive absorption, in: K. Sundmacher, A. Kienle, A.

Seidel-Morgenstern (Eds.), Integrated Chemical Processes, Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim, Germany, 2005.

[4] G. Astarita, D.W. Savage, A. Bisio, Gas Treating with Chemical Solvents, Wiley,New York, 1983.

[5] E. Sorel, La Rectification de l’alcool, par Ernest Sorel, Gauthier-Villars et fils,1894.

[6] W.A. Peters, The efficiency and capacity of fractionating columns, Ind. Eng.Chem. 14 (1922) 476–479.

[7] T.H. Chilton, A.P. Colburn, Mass transfer (absorption) coefficients predictionfrom data on heat transfer and fluid friction, Ind. Eng. Chem. 26 (1934) 1183–1187.

[8] P.V. Danckwerts, Gas–Liquid Reactions, McGraw-Hill, New York, 1970.[9] G. Astarita, Mass Transfer with Chemical Reaction, Elsevier, Amsterdam,

1967.[10] J.C. Charpentier, Mass-transfer rates in gas–liquid absorbers and reactors, in:

Advances in Chemical Engineering, Academic Press, New York, 1981, pp. 1–133.

[11] E.Y. Kenig, Complementary modelling of fluid separation processes, Chem.Eng. Res. Des. 86 (2008) 1059–1072.

[12] E.Y. Kenig, L. Kucka, A. Górak, Rigorous modeling of reactive absorptionprocesses, Chem. Eng. Technol. 26 (2003) 631–646.

[13] L.K. Doraiswamy, M.M. Sharma, Heterogeneous Reactions: Analysis,Examples and Reactor Design, John Wiley and Sons, New York, 1984.

[14] R. Zarzycki, A. Chacuk, Absorption: Fundamentals and Applications,Pergamon Press, Oxford, 1993.

[15] P. Mondal, G.S. Dang, M.O. Garg, Syngas production through gasification andcleanup for downstream applications – recent developments, Fuel Process.Technol. 92 (2011) 1395–1410.

[16] M. Bergel, I. Tierno, Sweetening technologies: a look at the whole picture, in:24th World Gas Conference, Argentina, 2009.

[17] R.J. Notz, I. Tönnies, N. McCann, G. Scheffknecht, H. Hasse, CO2 capture forfossil fuel-fired power plants, Chem. Eng. Technol. 34 (2011) 163–172.

[18] H. Bai, A.C. Yeh, Removal of CO2 greenhouse gas by ammonia scrubbing, Ind.Eng. Chem. Res. 36 (1997) 2490–2493.

[19] A.C. Yeh, H. Bai, Comparison of ammonia and monoethanolamine solvents toreduce CO2 greenhouse gas emissions, Sci. Total Environ. 228 (1999) 121–133.

[20] P.D. Vaidya, E.Y. Kenig, CO2-alkanolamine reaction kinetics: a review of recentstudies, Chem. Eng. Technol. 30 (2007) 1467–1474.

[21] H. Hiller, R. Reimert, F. Marschner, H.-J. Renner, W. Boll, E. Supp, M. Brejc, W.Liebner, G. Schaub, G. Hochgesand, C. Higman, P. Kalteier, W.-D. Müller, M.Kriebel, H. Schlichting, H. Tanz, H.-M. Stönner, H. Klein, W. Hilsebein, V.Gronemann, U. Zwiefelhofer, J. Albrecht, C.J. Cowper, H.E. Driesen, Gasproduction, in: Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCHVerlag GmbH & Co. KGaA, 2000.

[22] C.-H. Yu, C.-H. Huang, C.-S. Tan, A review of CO2 capture by absorption andadsorption, Aerosol Air Qual. Res. 12 (2012) 745–769.

[23] G. Astarita, D.W. Savage, J.M. Longo, Promotion of CO2 mass transfer incarbonate solutions, Chem. Eng. Sci. 36 (1981) 581–588.

[24] A. Aboudheir, G. McIntyre, Industrial Design and Optimization of CO2

Capture, Dehydration, and Compression Facilities, http://bit.ly/SZWvr0.[25] D. Chapel, C.L. Mariz, J. Ernest, Recovery of CO2 from flue gases: commercial

trends, in: Canadian Society of Chemical Engineers Annual Meeting,Saskatchewan, Canada, 1999.

[26] M. Jensen, M. Musich, J. Ruby, E. Steadman, J. Harju, Carbon Separationand Capture. <http://www.undeerc.org/PCOR/newsandpubs/pdf/CarbonSeparationCapture.pdf> (10.05.12).

[27] L. Faramarzi, Post-Combustion Capture of CO2 from Fossil Fuelled PowerPlants, PhD Thesis, DTU Chemical Engineering, 2010.

[28] Hydrocarbon Processing, Gas processes Handbook, 2004. <http://www.aimsgt.com/technicalpapers/GasProcessesHandbook_2004.pdf> (10.05.12).

[29] J. Rolker, W. Arlt, Abtrennung von Kohlendioxid aus Rauchgasen mittelsAbsoprtion, Chem. Ing. Tech. 78 (2006) 416–424.

[30] DOW, UCARSOL AP 814 Solvent for CO2 Removal. <http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0043/0901b80380043401.pdf?filepath=gastreating/pdfs/noreg/170-01420.pdf&fromPage=GetDoc>(10.05.12).

[31] A. Aboudheir, P. Tontiwachwuthikul, R. Idem, Rigorous model for predictingthe behavior of CO2 absorption into AMP in packed-bed absorption columns,Ind. Eng. Chem. Res. 45 (2006) 2553–2557.

[32] J.B. Rajani, Treating Technologies of Shell Global Solutions for Natural Gasand Refinery Gas Streams. <http://www.ripi.ir/congress12/treating%20technologies.pdf> (10.05.12).

[33] J.M. Klinkenbijl, M.L. Dillon, E.C. Heyman, Gas Pre-treatment and Their Impacton Liquefaction Processes. <http://www.ipt.ntnu.no/~jsg/undervisning/naturgass/dokumenter/GasPreTreatment.pdf> (10.05.12).

[34] W. Seyfferth, R. Dittmer, H. Thielert, Generic Methods for Coke Oven GasDesulphurisation. <http://www.coke-oven-managers.org/PDFs/2004/Seyfferth.pdf> (10.05.12).

[35] R. Faiz, M. Al-Marzouqi, Insights on natural gas purification: simultaneousabsorption of CO2 and H2S using membrane contactors, Sep. Purif. Technol. 76(2011) 351–361.

[36] B. Mandal, S.S. Bandyopadhyay, Simultaneous absorption of CO2 and H2S intoaqueous blends of N-methyldiethanolamine and diethanolamine, Environ.Sci. Technol. 40 (2006) 6076–6084.

[37] S. Mokhatab, P. Meyer, Selecting best technology lineup for designing gasprocessing units, in: GPA Europe Sour Gas Processing Conference, Sitges,Spain, 2009, pp. 1–21.

[38] J.A. Bullin, J.C. Polasek, J.W. Holmes, Optimization of New and ExistingAmine Gas Sweetening Plants using Computer Simulation, http://bitly.com/UD7oLN.

http://www.undeerc.org/PCOR/newsandpubs/pdf/CarbonSeparationCapture.pdf

http://www.undeerc.org/PCOR/newsandpubs/pdf/CarbonSeparationCapture.pdf

http://www.aimsgt.com/technicalpapers/GasProcessesHandbook_2004.pdf

http://www.aimsgt.com/technicalpapers/GasProcessesHandbook_2004.pdf

http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0043/0901b80380043401.pdf?filepath=gastreating/pdfs/noreg/170-01420.pdf&fromPage=GetDoc



http://www.ripi.ir/congress12/treating%20technologies.pdf

http://www.ripi.ir/congress12/treating%20technologies.pdf

http://www.ipt.ntnu.no/

http://www.ipt.ntnu.no/

http://www.coke-oven-managers.org/PDFs/2004/Seyfferth.pdf

http://www.coke-oven-managers.org/PDFs/2004/Seyfferth.pdf


[39] B.P. Mandal, A.K. Biswas, S.S. Bandyopadhyay, Selective absorption of H2Sfrom gas streams containing H2S and CO2 into aqueous solutions of N-methyldiethanolamine and 2-amino-2-methyl-1-propanol, Sep. Purif.Technol. 35 (2004) 191–202.

[40] J.W. Holmes, M.L. Spears, J.A. Bullin, Sweetening LPG’s with Amines. <http://www.bre.com/portals/0/technicalarticles/Sweetening%20LPG%27s%20with%20Amines.pdf> (10.05.12).

[41] G.T. Rochelle, Amine scrubbing for CO2 capture, Science 325 (2009) 1652–1654.

[42] R.M. Davidson, Post-combustion Carbon Capture from Coal Fired Plants:Solvent Scrubbing, International Energy Agency Coal Research, 2007.

[43] M. Wang, A. Lawal, P. Stephenson, J. Sidders, C. Ramshaw, Post-combustionCO2 capture with chemical absorption: a state-of-the-art review, Chem. Eng.Res. Des. 89 (2011) 1609–1624.

[44] A. Veawab, A. Aroonwilas, A. Chakma, P. Tontiwachwuthikul, Solventformulation for CO2 separation from flue gas streams, in: First NationalConference on Carbon Sequestration, Washington, DC, USA, 2001.

[45] H. Lepaumier, D. Picq, P.-L. Carrette, New amines for CO2 capture. I.Mechanisms of amine degradation in the presence of CO2, Ind. Eng. Chem.Res. 48 (2009) 9061–9067.

[46] H. Lepaumier, D. Picq, P.-L. Carrette, New amines for CO2 capture. II. Oxidativedegradation mechanisms, Ind. Eng. Chem. Res. 48 (2009) 9068–9075.

[47] L. Dubois, D. Thomas, Carbon dioxide absorption into aqueous amine basedsolvents: modeling and absorption tests, Energy Proc. 4 (2011) 1353–1360.

[48] P.D. Vaidya, E.Y. Kenig, Absorption of CO2 into aqueous blends ofalkanolamines prepared from renewable resources, Chem. Eng. Sci. 62(2007) 7344–7350.

[49] G.N. Patil, P.D. Vaidya, E.Y. Kenig, Reaction kinetics of CO2 in aqueous methyl-and dimethylmonoethanolamine solutions, Ind. Eng. Chem. Res. 51 (2012)1592–1600.

[50] P.D. Vaidya, E.Y. Kenig, Acceleration of CO2 reaction with N,N-diethylethanolamine in aqueous solutions by piperazine, Ind. Eng. Chem.Res. 47 (2008) 34–38.

[51] A.B. Bindwal, P.D. Vaidya, E.Y. Kenig, Kinetics of carbon dioxide removal byaqueous diamines, Chem. Eng. J. 169 (2011) 144–150.

[52] S.A. Freeman, R. Dugas, D.H. Van Wagener, T. Nguyen, G.T. Rochelle, Carbondioxide capture with concentrated, aqueous piperazine, Int. J. Greenh. GasControl 4 (2010) 119–124.

[53] S. Bishnoi, G.T. Rochelle, Absorption of carbon dioxide into aqueouspiperazine: reaction kinetics, mass transfer and solubility, Chem. Eng. Sci.55 (2000) 5531–5543.

[54] X. Chen, G.T. Rochelle, Aqueous piperazine derivatives for CO2 capture:accurate screening by a wetted wall column, Chem. Eng. Res. Des. 89 (2011)1693–1710.

[55] J.M. Plaza, G.T. Rochelle, Modeling pilot plant results for CO2 capture byaqueous piperazine, Energy Proc. 4 (2011) 1593–1600.

[56] J. Oexmann, C. Hensel, A. Kather, Post-combustion CO2-capture from coal-fired power plants: preliminary evaluation of an integrated chemicalabsorption process with piperazine-promoted potassium carbonate, Int. J.Greenh. Gas Control 2 (2008) 539–552.

[57] J. Oexmann, A. Kather, Post-combustion CO2 capture in coal-fired powerplants: comparison of integrated chemical absorption processes withpiperazine promoted potassium carbonate and MEA, Energy Proc. 1 (2009)799–806.

[58] J.T. Cullinane, G.T. Rochelle, Carbon dioxide absorption with aqueouspotassium carbonate promoted by piperazine, Chem. Eng. Sci. 59 (2004)3619–3630.

[59] V. Darde, K. Thomsen, W.J.M. van Well, E.H. Stenby, Chilled ammonia processfor CO2 capture, Energy Proc. 1 (2009) 1035–1042.

[60] H. Knuutila, U.E. Aronu, H.M. Kvamsdal, A. Chikukwa, Post combustion CO2

capture with an amino acid salt, Energy Proc. 4 (2011) 1550–1557.[61] U.E. Aronu, H.F. Svendsen, K.A. Hoff, Investigation of amine amino acid salts

for carbon dioxide absorption, Int. J. Greenh. Gas Control 4 (2010) 771–775.[62] P.D. Vaidya, P. Konduru, M. Vaidyanathan, E.Y. Kenig, Kinetics of carbon

dioxide removal by aqueous alkaline amino acid salts, Ind. Eng. Chem. Res. 49(2010) 11067–11072.

[63] L. Raynal, P. Alix, P.A. Bouillon, A. Gomez, M. Le Febvre De Nailly, M. Jacquin, J.Kittel, A. Di Lella, P. Mougin, J. Trapy, The DMX™ process: an original solutionfor lowering the cost of post-combustion carbon capture, Energy Proc. 4(2011) 779–786.

[64] I. Pfaff, J. Oexmann, A. Kather, Integration studies of post-combustion CO2-capture process by wet chemical absorption into coal-fired power plant, in: 4thInternational Conference on Clean Coal Technologies, Dresden, Germany, 2009.

[65] C. Stankewitz, H. Fahlenkamp, Integration of a subsequent CO2 scrubbing inthe coal-fired power plant process, Energy Proc. 4 (2011) 1395–1402.

[66] M. Lucquiaud, J. Gibbins, On the integration of CO2 capture with coal-firedpower plants: a methodology to assess and optimise solvent-based post-combustion capture systems, Chem. Eng. Res. Des. 89 (2011) 1553–1571.

[67] L.M. Romeo, I. Bolea, J.M. Escosa, Integration of power plant and aminescrubbing to reduce CO2 capture costs, Appl. Therm. Eng. 28 (2008) 1039–1046.

[68] L.M. Romeo, S. Espatolero, I. Bolea, Designing a supercritical steam cycle tointegrate the energy requirements of CO2 amine scrubbing, Int. J. Greenh. GasControl 2 (2008) 563–570.

[69] A. Aroonwilas, A. Veawab, Integration of CO2 capture unit using single- andblended-amines into supercritical coal-fired power plants: Implications for

emission and energy management, Int. J. Greenh. Gas Control 1 (2007) 143–150.

[70] D.J. Barker, S.A. Turner, P.A. Napier-Moore, M. Clark, J.E. Davison, CO2 capturein the cement industry, Energy Proc. 1 (2009) 87–94.

[71] S.J. Chung, K.C. Pillai, I.S. Moon, A sustainable environmentally friendly NOx

removal process using Ag(II)/Ag(I)-mediated electrochemical oxidation, Sep.Purif. Technol. 65 (2009) 156–163.

[72] J.A. Patwardhan, M.P. Pradhan, J.B. Joshi, Simulation of NOx gas absorptionunder adiabatic condition and comparison with plant data, Chem. Eng. Sci. 57(2002) 4831–4844.

[73] B. Hüpen, E.Y. Kenig, Rigorous modelling of NOx absorption in tray and packedcolumns, Chem. Eng. Sci. 60 (2005) 6462–6471.

[74] E.Y. Kenig, U. Wiesner, A. Górak, Modeling of reactive absorption using theMaxwell–Stefan equations, Ind. Eng. Chem. Res. 36 (1997) 4325–4334.

[75] J.L. de Paiva, G.C. Kachan, Absorption of nitrogen oxides in aqueous solutionsin a structured packing pilot column, Chem. Eng. Process. 43 (2004) 941–948.

[76] A.G. Degussa, US 5112587, 1992.[77] D. Thomas, J. Vanderschuren, The absorption–oxidation of NOx with

hydrogen peroxide for the treatment of tail gases, Chem. Eng. Sci. 51 (1996)2649–2654.

[78] M. Thiemann, E. Scheibler, K.W. Wiegand, Nitric acid, nitrous acid, andnitrogen oxides, in: Ullmann’s Encyclopedia Of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.

[79] Osterreichische Stickstoffwerke, US 3 034 853, 1962.[80] Office National Industriel de L’azote, US 3 329 478, 1967.[81] Potasse et Engrais Chimiques, US 3 348 914, 1967.[82] J.C. Adrian, J. Verilhac, A process for reduction of NOx content in flue gas, in:

2nd International Conference on Control of Gaseous Sulphur and NitrogenCompound Emissions, University Salford, 1976.

[83] J.A. Patwardhan, J.B. Joshi, Unified model for NOx absorption in aqueousalkaline and dilute acidic solutions, AIChE J. 49 (2003) 2728–2748.

[84] M.P. Pradhan, J.B. Joshi, Absorption of NOx gases in plate column: selectivemanufacture of sodium nitrite, Chem. Eng. Sci. 55 (2000) 1269–1282.

[85] P. van der Maas, Chemically Enhanced Biological NOx Removal from FlueGases: Nitric Oxide and Ferric EDTA Reduction in BioDeNOx Reactors,Wageningen Universiteit, 2005.

[86] R. Kumaraswamy, U. Van Dongen, J.G. Kuenen, W. Abma, M.C.M. VanLoosdrecht, G. Muyzer, Characterization of microbial communitiesremoving nitrogen oxides from flue gas: the BioDeNOx process, Appl.Environ. Microbiol. 71 (2005) 6345–6352.

[87] J.G.M. Winkelman, F. Gambardella, H.J. Heeres, A rate based reactor model forBiodeNOx absorber units, Chem. Eng. J. 133 (2007) 165–172.

[88] E.Y. Kenig, P. Seferlis, Modeling reactive absorption, Chem. Eng. Prog. 105(2009) 65–73.

[89] N. Dalaouti, P. Seferlis, Design sensitivity of reactive absorption units forimproved dynamic performance and cleaner production: the NOx removalprocess, J. Clean. Prod. 13 (2005) 1461–1470.

[90] N. Dalaouti, P. Seferlis, A unified modeling framework for the optimal designand dynamic simulation of staged reactive separation processes, Comput.Chem. Eng. 30 (2006) 1264–1277.

[91] S.I. Clarke, W.J. Mazzafro, Nitric acid, in: Kirk-Othmer Encyclopedia ofChemical Technology, John Wiley & Sons, Inc., 2005.

[92] EPA, AP 42 Compilation of Air Pollutant Emission Factors – Nitric Acid.<http://www.epa.gov/ttn/chief/ap42/ch08/final/c08s08.pdf> (10.05.12).

[93] J. Glauser, Nitric Acid, CEH, Marketing Research Report 757.8000, 2011.[94] U. Wiesner, M. Wittig, A. Górak, Design and optimization of a nitric acid

recovery plant from nitrous waste gases, Comput. Chem. Eng. 20 (1996)S1425–S1430.

[95] EFMA, Production of nitric acid, in: Best Available Techniques for PollutionPrevention and Control in the European Fertilizer Industry, EuropeanFertilizer Manufacturers’ Association, Brussels, 2000.

[96] C. Keleti, Nitric Acid and Fertilizer Nitrates, CRC Press, 1985.[97] B. Hüpen, E.Y. Kenig, Rigorose Modellierung und Simulation von

Chemisorptionsprozessen, Chem. Ing. Tech. 77 (2005) 1792–1799.[98] DTI, Technology Status Report – Flue Gas Desulphurization (FGD)

Technologies, Department of Trade and Industry, 2000.[99] R.K. Srivastava, W. Jozewicz, C. Singer, SO2 scrubbing technologies: a review,

Environ. Prog. 20 (2001) 219–227.[100] M.P. Dudukovic, F. Larachi, P.L. Mills, Multiphase reactors – revisited, Chem.

Eng. Sci. 54 (1999) 1975–1995.[101] M.P. Dudukovic, F. Larachi, P.L. Mills, Multiphase catalytic reactors: a

perspective on current knowledge and future trends, Catal. Rev. Sci. Eng. 44(2002) 123–246.

[102] P.A. Ramachandran, R.V. Chaudhari, Three-Phase Catalytic Reactors, Gordonand Breach Science Publishers, New York, 1983.

[103] K.D.P. Nigam, A. Schumpe (Eds.), Three-phase sparged reactors, Gordon andBreach Science Publishers, Amsterdam, 1996.

[104] P. Trambouze, H. van Landeghem, J.P. Wauquier, Chemical Reactors: Design,Engineering, Operation, Gulf Publishing Company, 1988.

[105] U. Ipek, M. Ekinci, E.I. Arslan, Y. Cuci, H. Hasar, Simultaneous SO2 removal bywastewater with NH3, Water Air Soil Pollut. 196 (2009) 245–250.

[106] B. He, X. Zheng, Y. Wen, H. Tong, M. Chen, C. Chen, Temperature impact onSO2 removal efficiency by ammonia gas scrubbing, Energy Convers. Manage.44 (2003) 2175–2188.

[107] G. Al-Enezi, H. Ettouney, H. El-Dessouky, N. Fawzi, Solubility of sulfur dioxidein seawater, Ind. Eng. Chem. Res. 40 (2001) 1434–1441.

http://www.bre.com/portals/0/technicalarticles/Sweetening%20LPG%27s%20with%20Amines.pdf



http://www.epa.gov/ttn/chief/ap42/ch08/final/c08s08.pdf


[108] J. Rodriguez-Sevilla, M. Alvarez, G. Liminana, M.C. Diaz, Dilute SO2 absorptionequilibria in aqueous HCl and NaCl solutions at 298.15 K, J. Chem. Eng. Data47 (2002) 1339–1345.

[109] J. Rodriguez-Sevilla, M. Alvarez, M.C. Diaz, M.C. Marrero, Absorptionequilibria of dilute SO2 in seawater, J. Chem. Eng. Data 49 (2004) 1710–1716.

[110] Z. Zhihua, Z. Zhihong, The study of influence factor to desulphurizationefficiency in seawater, in: 2nd International Conference on Bioinformaticsand Biomedical Engineering, iCBBE 2008, Shanghai, China, 2008, pp. 3731–3733.

[111] EPA, AP 42 Compilation of Air Pollutant Emission Factors – Sulfuric Acid.<http://www.epa.gov/ttn/chief/ap42/ch08/final/c08s08.pdf> (10.05.12).

[112] EFMA, Production of sulphuric acid, in: Best Available Techniques forPollution Prevention and Control in the European Fertilizer Industry,European Fertilizer Manufacturers’ Association, Brussels, 2000.

[113] H. Müller, Sulfuric acid and sulfur trioxide, in: Ullmann’s Encyclopedia ofIndustrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.

[114] A.A. Kiss, C.S. Bildea, J. Grievink, Dynamic modeling and process optimizationof an industrial sulfuric acid plant, Chem. Eng. J. 158 (2010) 241–249.

[115] J.C. Schöneberger, H. Thielert, A new process concept for an emission freesulphuric acid plant, in: 8th European Congress of Chemical Engineering,Germany, Berlin, 2011.

http://www.epa.gov/ttn/chief/ap42/ch08/final/c08s08.pdf

Reactive absorption in chemical process industry: A review on current activities

Documents