1996: Customized aMDEA Process for Acid Gas Removal

Customized aMDEA Process for AcidGas Removal

The operational experience with aMDEA gas scrubbing units shows that they contribute substantiallyto both the reliability and the economy of the ammonia plant.

R. Hugo, H. Meissner, and R. WelkerBASF AG, Ludwigshafen, Germany

Introduction

The CO2 removal system represents a majorenergy consumer in an ammonia plant. Anenergy-efficient gas scrubbing process is thus

sine qua non for economic production. On the otherhand, the economics of the ammonia plant also requirehigh on-stream times. Each extra plant shutdown dueto a failure of the CO2 removal unit causes a consider-able loss of production and adversely affects plantsafety. For these reasons, the selection of BASF'saMDEA process can make a substantial contributionto both the reliability and the economy of the wholeammonia plant.

Assuming that U.S. ammonia producers are alreadywell acquainted with aMDEA, this article will brieflyrecapitulate the applications and process features,which will subsequently be illustrated by a detailedconsideration of two examples of retrofitting existingCO2 removal units (with all the inherent system con-straints this task implies.

Process History

The development of the aMDEA process was startedin the late 1960s. The first aMDEA unit went on-stream in 1971 with the commissioning of the No. HIammonia plant in Ludwigshafen, Germany. In the fol-lowing years, eight more BASF plants were fitted withaMDEA gas scrubbing units.

The operational experience with these aMDEA unitsprovided the incentive to license the process from1982 onward. To date, the aMDEA process has beenoperated successfully in a total of 66 reference plantsworldwide, with some 20 additional units currentlybeing under design or construction.

Applications

The aMDEA process is suitable for a wide range ofapplications (Figure 1). Besides removing CO2 fromammonia synthesis gas, aMDEA units can be used topurify CO/H2 synthesis gas, to sweeten natural gas,

AMMONIA TECHNICAL MANUAL 330 1997

and for speciality applications such as purification ofblast furnace gases.

40% of all aMDEA reference units are to be foundin ammonia plants; more than half of these were inturn originally operated with alternative solvents andconverted to aMDEA to solve various operating prob-lems encountered with the former solvents.

Process Features

The aMDEA solvent systems are aqueous solutionsof the effectively nonvolatile methyldiethanolamineplus a small amount of an activator to enhance theCO2 absorption rate (Figure 2).

Generic MDEA reacts with water and CO2 to yieldthe corresponding protonated species and bicarbonate(Figure 3). The overall rate of conversion is very low.The absorption can be accelerated by the fast reactionbetween CO2 and the activator, a secondary amine,which together form a carbamate. The carbamate inturn reacts with the bulk solvent (aqueous MDEA)transferring its CO2 and thereby being regenerated forfurther reaction. The activator therefore behaves in asimilar manner to a homogeneous liquid catalyst withno net consumption, but rather several reaction-regen-eration cycles along the length of the absorber.

The high loading capacity of MDEA results in lowsolvent circulation rates, while the activator keeps theabsorber height to a minimum. The solvent regenera-tion can be carried out to a large extent simply byflashing the aMDEA solution.

An extra degree of flexibility is achieved by varyingthe activator concentration (Figure 4). This has theeffect of tuning solvent behavior to either a more"chemical" or a more "physical" character. The highlyactivated MDEA 06 has more of a chemical solventnature, that is, good absorption efficiency but energyintensive regeneration, while the weakly activatedMDEA 01 has only a moderate absorption efficiencybut benefits from an energy efficient regeneration,similar to that for a physical solvent.

Typical application criteria for CO2 removal fromammonia synthesis gas are:

• aMDEA types 02 thru 04 in a standard two-stageunit, characterized by low energy consumption values,high gas purities, and good recovery rates (Figure 5);

• aMDEA types 04 thru 06 in a standard single-stageunit, characterized by reduced investment costs andvery low CO2 slippage in the treated gas (Figure 6).

The number of aMDEA solvent systems combinedwith a variety of appropriate process configurationsensure a design tailor-made for a given application, beit a revamp or a new plant.

Retrofit of Acid Gas Removal Units

For the "grassroots" design of a new plant, the sol-vent, the configuration, and the process parameterscan be customized to meet all production and siterequirements. For a revamp, however, the designneeds considerable process flexibility to conform to agiven application. Many additional constraints resultfrom the equipment already installed and the integra-tion of the unit in the ammonia plant. The followingexamples illustrate such situations.

The first example involves typical conditions for theU.S. reference plants, all of which entailed conver-sions of ammonia synthesis gas scrubbing units toaMDEA (Figure 7). The second example describes therevamp of a hot potassium plant in Australia.

Conversion from an amine-based solvent

The U.S. references are all ammonia plants with acapacity in the range 650-1,500 mtpd, and all have abackground similar to that of the unit portrayed in thefollowing example. The gas scrubbing unit comprisesa single-stage absorption and stripper regeneration(see Figure 6) and was originally operated withmonoethanolamine (ME A).

This unit has been converted to another amine basedsystem in the context of a capacity increase. The alter-native of achieving a higher plant capacity by anincrease of the MEA concentration was ruled out bythe operational experience regarding the corrosivity ofthe MEA solution, especially at higher concentration.

Understandably, the corrosivity of the solution wasinvestigated extensively. Following the swap, solventanalyses showed no untoward heavy metal content inthe solution for the first year. Thereafter, some weightloss was observed on the corrosion coupons and sub-sequently a steep increase in the concentration of


Figure 1. Application of BASF's aMDEA-process. Figure 2. Activated MDEA solvent system.

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Figure 7. Plant locations. Figure 8. Damaged tubes from the fourexchangers.


heavy metals (Fe, Cr, Ni) in the solution took place.Corrosion problems occurred especially in the solventheat exchangers (2 parallel stacks, each of 2 individualexchangers) and in the regenerator (duplicate paralleltray columns) along with several additional shutdownsand the need to replace corroded equipment. Thehopes placed in this second solvent thus proved to beunfounded, as the unit still suffered from leaks due tocorrosion.

At this point, a straight solution swap to aMDEAwas carried out without any plant modifications. In arapid turnaround, the previous solvent was drained off,the unit cleaned, filled with aMDEA solution andrestarted.

Equipment inspection after draining off the previ-ous solvent

No Corrosion was detected in the absorber column.The stripper columns showed corrosion in the bot-

tom sections and between the trays. The corrosion wasof a local nature, being characterized by cavities of 2in. (5 cm) diameter and up to 2/5 in. (1 cm) depth froma height of approximately 6.5 ft (2 m) up to the loca-tion of the packing support grid. The material aroundthe cavities was friable and could easily be removedmechnically. The material within the manhole showedsimilar corrosion effects.

The stripper reboiler heads were damaged with thepitting corrosion characteristic of hot CO2/water vaporattack.

The four solvent heat exchangers are fitted withstainless (SS) tubing and carbon steel (CS) shells.Although the expansion valve is located downstreamthe solvent heat exchangers, degassing of CO2 of therich solution most probably occurs, which results in atwo-phase flow through the exchangers, giving rise tosevere corrosive attack: In the exchanger heads (pre-dominantly at the hot end) pitting corrosion wasobserved over the entire CS material (about 2 mmdepth) as well as at the welds (4 mm depth). Eventhough the tubes were of SS, about 10% leaked.Corrosion was confined to within the tubes, as theouter surface still appeared to be in good shape. TheCS elements used to support the tube bundles wereextensively corroded. Figure 8 depicts the damaged

tubes from the four exchangers.The shell of the lean solution cooler (solvent on

shellside) also exhibited wear-and-tear, probably as aresult of erosive corrosion.

Special attention must be paid to the feed device of

Table 1. Heavy Metal Content of the aMDEASolution

Months ofOperation

71113162023262933

Cr(wppm)

711232145

Ni(wppm)

111121468

Fe(wppm)

754765564

the regenerator when converting a chemical solvent toaMDEA: about 30-50% of the CO2 is released fromthe rich solution by depressurization. The degassingtherefore starts just downstream the pressure reliefvalve and continues in the upper section of the regen-erator. In a tray column flashing takes place on the toptray. The flashing solution, however, shall not enterthe downcomer, as degassing in the downcomer canresult in flooding of the column. In case the regenera-tor is fitted with packing, the existing distributor mustbe adapted for the operation with a flashing feed. Atany rate, the disengagement zone above the feeddevice must be sufficiently high hi order to avoid solu-tion entrainment.

Precleaning and repair work

The system was cleaned section by section: firstopening manholes and bottom drains of columns andtanks, then removing orifice meters in the liquid linesafter closing off the taps hi impulse pressure line con-nections beforehand and finally flushing the equip-


Figure 9. Corrosion rates in bottom of stripper. Figure 10. Revamp of hot-pot unit in Australia.

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11. aMDEA compared to hot potassium carbonate systems.


ment with water using firehoses and/or water jets. Theheat exchanger tube bundles were removed andcleaned with a high pressure water jet. Particularly thetubing in the solvent heat exchangers showed a firmdeposit layer inside the tubes, which could neverthe-less be removed with a caustic solution jet. The outersurface of these tubes was covered with a grayish-brown layer (most probably FeCO3) after cleaning,which could also be removed easily.

The corroded equipment surfaces were washed offwith a water jet. Corrosion cavities were then hollow-grinded, polished and repaired with a corrosion protec-tion layer (0.6-1 mm thickness) applied by a thermalmetal spraying procedure. The bottom section of thestripper columns was covered with metal dust from thespraying procedure, and the trays and the column basehad to be cleaned with a vacuum cleaner.

Cleaning procedure prior to startup

The two stripper columns were flooded with con-densate and then drained. This procedure was repeatedonce more with samples being taken to check for theabsence of residual suspended particles in filtrationtests.

Afterwards, the whole unit was filled with conden-sate, heated up to 176-194°F (80-90°C) and circulatedfor 4 h, with about 10% of the flow rate passingthrough the mechanical filter. The condensate analysisafter 4 h circulation showed no foam activity or sus-pended particles, and the sample had a yellowish-brown coloration. The condensate circulation proce-dure was repeated, with the sample showing no decol-oration as a result.

The cleaning efforts can be reduced or enhanceddepending on the very situation and the history of eachplant. In this case, for instance, the system was notflushed by circulating caustic solution or diluteMDEA solution as usually recommended, especiallyfor the removal of grease or other residues such as cor-rosion inhibitors, rust, and so on. The caustic flushingshould, however, be employed when considerable con-struction work has been carried out, if new equipmenthas been installed or if the entire plant is new.Startup

The system was checked for leakages and put undernitrogen pressure. The aMDEA premix was introducedfrom tank containers into the system and diluted withwater down to 45wt.% amine concentration. The sol-vent loop was closed, defoamer was added, and thecirculating solvent was heated up to the design condi-tions. The gas throughput was set to 75% of the designcapacity. Within 3 days, a capacity of 108% could beachieved and the conditions (energy, solvent, flowrate) had been optimized to meet the design CO2 slipof 20 vppm.

Operating experience after the swap

The corrosion attack associated with the previoussolvent is clearly indicated by a material loss from thetest coupons in the mm per year range (Figure 9).After the revamp to aMDEA, the corrosion was com-pletely eliminated, without needing to add any corro-sion inhibitor. In contrast to the previous solution, theheavy metal content of the aMDEA solution (as tabu-lated in Table 1) was kept virtually constant in the lowppm range.

Other amine-based systems (such as MEA, DEA,DIPA) have also been successfully converted toaMDEA with considerable energy savings achievedand a complete eradication of earlier corrosion prob-lems. The demands on the flexibility of the solvent,however, are considerably higher if the solvent beingreplaced is of a completely different nature to anamine solution, as in the following example.

Conversion of a hot potassium carbonate unit

The second example deals with the very first revampof a hot-potassium carbonate unit to aMDEA. The 720mtpd ammonia plant in Australia comprised a two-stage hot-pot gas scrubbing unit. Toxic arsenic saltswere used both for activation and corrosion inhibition.In recent years, sections of the pipework and packedbeds had to be replaced owing to the corrosivity ofthe hot-pot solution.

The main problems, however, were escalating costsfor disposal of the arsenic-containing purge stream andthe presence of arsenic in the plant wastewater.

The process configuration and the operating condi-


lions of this Australian hot-pot unit (Figure 10) aresignificantly different from a standard two-stageaMDEA unit (see Figure 5):

• No lean/semilean solvent heat exchanger.• Ratio semilean to lean solvent flow rate 4.5 vs. 6-7

for aMDEA.• Lean solvent temperature of 169°F (76°C) vs.

122°F (50°C) for aMDEA.• Semilean solvent temperature of 234°F (112°C) vs.

167-185°F (75-85°C) for aMDEA.

Table 2. Three Consecutive Flushing Steps UsingNaOH Solution

NaOH(wt.%)

4.03.91.7

Circulation00

886

Max. Arsenic(wppm)

1,90016060

• Low pressure flash fitted with packing of about 3times the height as for aMDEA.

• Density of hot-pot solution approximately 25%higher compared to aMDEA.

Generally, the main challenge for the solvent con-version of hot-pot units arises from the considerablylower temperature level in the absorber and the loweraMDEA density. Both factors typically lead to bottle-necks due to a lack of cooling capacity (most units donot even have a semilean solution cooler!) and inade-quate capacity in the solvent pumps.

Process modifications

For the above reasons, a straight solution swap (asdescribed in the section on conversion from an amine-based solvent) is not usually feasible. The followingmodifications had to be carried out for this Australianrevamp assignment.

• At full capacity, the suction pressure of the leansolution pump (when operated with aMDEA) wouldhave fallen below the required limit. To provide suffi-cient head for the less dense aMDEA, a lean/rich sol-

vent heat exchanger was installed to cool the leansolution from the stripper bottom and the lean solutionpump was operated at higher speeds. A peculiarity ofthe solvent heat exchanger (in contrast to thelean/semilean solvent heat exchanger of the standarddesign (see Figure 5)) is the shellside flow of CO2-loaded rich solution and the tubeside flow of the leansolution. This arrangement was chosen in order to takeinto account the high ratio of rich to lean solution inthe operation of the existing unit. CO2 is thus releasedby heating up the rich solution in the exchanger. Alean solution air cooler instead of a solvent heatexchanger was not a practical alternative due to thehigher space requirements, and an additional watercooler was ruled out by the increased water consumption.

• The duty of the existing lean solution cooler wasincreased by providing cooling water at a lower tem-perature. The lean solution temperature could bereduced from 169°F (76°C) to 158°F (70°C) in thisway, thus giving rise to a higher solution loadingcapacity and a reduced solvent flow rate. By thismeans, the aMDEA design was rendered compatiblewith the capacity limits of the existing pumps.

• The hot-pot unit was operated at a process gas tem-perature of 433°F (223°C). As such conditions arebeyond the operational experience of aMDEA refer-ence units, it was decided to install a process gas cool-er upstream of the regenerator gas reboiler to reducethe absorber inlet gas temperature down to 329°F (165°C). A gas inlet temperature of 329°F is still adequateto operate the regenerator properly since the energyconsumption is much less for aMDEA. The new heatexchanger is part of a reformer feedstock saturatorscheme, thus providing energy recovery from the feedgas.

9 The lean absorber section was fitted withpolypropylene packing, which was severely deformed.The two PP packed beds have now been replaced by 1in. S.S. TP304 rings.

• Parts of the piping showed corrosion and werereplaced by SS TP304 lines.

« About 80% of the released CO2 was previouslyvented to the atmosphere at 194°F (90°C).

Only 20% of the CO2 was cooled down to 113°F (45°C) condensed and processed up to foodgrade quality.An additional CO2 cooler, reflux drum, and reflux


Table 3. Comparison of Operating Data for the Hot-Pot Solution

Hot-Pot Solution aMDEA Solution

Feed gas

CO2 slipLean solutionSemilean solutionAbsorber bottomProcess gas reboilerSteam reboilerSpecific energy consumption

185°F (85°C)429 psia (29.6 bara)18 vol.% CO2

<500 vppm169°F (76°C)234°F(112°C)226°F (108°C)in operationin operation49.9 mbtu/lb mol CO2

185°F429 psia18 vol.% CO2

<500 vppm158°F(70°C)180°F (82°C)203°F (95°C)in operationnot in use32.7 mbtu/lb mol CO2

pump were installed parallel to the existing reflux cir-cuit in order to provide a moderate temperature of theCO2 off-gas stream, thus limiting the vapor phase sol-vent losses.

Cleaning of the Unit

The unit had been operated with a hot-pot solutionsince 1969 using arsenic salts as corrosion inhibitorand activator. It was therefore to be expected that con-siderable amounts of precipitated corrosion products(deposit of iron/arsenic complex) were deposited onequipment surfaces. There were some concernsregarding the fact that aMDEA is known to dissolvesuch deposits quite well, which might have increasedfoaming susceptibility and carryover of the solutionfrom the regenerator into the condensate reboiler.

The cleaning procedure was carried out as follows:« Three consecutive flushing steps using NaOH

solution at 194°F (90°C), each of which was brokenoff after the arsenic content had attained its maximumare shown in Table 2.

• Two consecutive water flushing steps were at194°F (90°C); the water analysis after the secondflushing step gave 60 wppm iron, 7 wppm arsenic, 80wppm NaOH and 100 wppm solids and the sampleexhibited a dark brownish color. The solids were iden-

tified as very fine rust particles (probably iron/arseniccomplex) in the size range 5-50 urn; the foam test withaMDEA indicated a significant foaming tendency.

• A further water flushing step was not carried out inorder to limit the amount of arsenic contaminatedflushing water. It was decided to allow for the higherfoaming tendency by running the mechanical filter (5-10 u,m cartridges) at its maximum throughput from thevery beginning of the solvent circulation and employ-ing an appropriate defoamer dosage rate.

Startup

The unit was filled with aMDEA premix and thewater content and column levels adjusted by addingcondensate.

The solvent circulation was commenced at about50% of the design flow rate. After 12 h of circulation,the solution had a reddish-brown color and a foam testindicated formation of very stable foam (collapse timemore than 5 min). After filtering the aMDEA sample,the color turned light yellow and the foam activity wasreduced to the normal values.

An extra dose of defoamer was introduced into thesolution loop and the circulation rate increased to 95%of the design figure: the column levels became unsta-ble and hence a further shot of defoamer was added.


The column levels remained constant and the nextfoam test showed no significant foaming tendency.

The solution circuit was heated up to the design val-ues with the regenerator steam. The CO2 cooler andreflux pumps were put into operation. The semileanand lean solvent temperatures to the absorber wereadjusted at a later stage when the solvent coolers werestarted up.

Process gas was fed through the plant at 50% capac-ity. Within 12 h, the plant throughput was increased to80% of the design figure. Process conditions werethen optimized at full capacity.

Although the process conditions of a hot-pot systemdiffer significantly from aMDEA units, the aMDEAdesign for this unit showed a good compatibility withthe existing equipment. A comparison of the operatingdata for the hot-pot solution and the aMDEA designfor 100% capacity is tabulated in Table 3.

A summary of the benefits from the 1994 solventconversion is given in Figure 11:

• The aMDEA solvent does not require the additionof any corrosion inhibitor. Hence, the costly disposalof toxic arsenic-material (inherent for the hot-pot sys-tem) is completely avoided. Even the time-consumingpassivation of the equipment prior to a startup of thehot-pot system can be omitted, thus keeping the turn-around periods to a minimum.

• Although the process conditions are not cus-tomized for the aMDEA solvent system, the energysaving is still 35% compared with the hot-pot opera-tion.

• The operator workload is lightened as the intensesupervision required for operation of the hot-pot unitis no longer necessary. Precipitation does not occurwith aMDEA; thus, heat tracing is not required. Therevamped unit is operated with virtually no liquidpurge stream, as there is no need to separate decompo-sition products or to purge the solution thereof. Theplant monitoring has been reduced to a minimum, withonly a few simple analyses remaining to be carriedout.

Operational Experience

The flexibility of the aMDEA process also offersattractive operational features.

BASF operates two ammonia plants inLudwigshafen, Germany: Both plants No. HI and No.IV, a 1,350 mtpd unit, which have come on-stream in1971 and 1982, respectively, are equipped with low-energy two-stage aMDEA systems (see Figure 5).

Following a capacity increase some years ago, thegas scrubbing unit of the No. IV plant was being oper-ated at the limits of the certain existing equipmentitems such as solution pumps and columns. A secondplant expansion of 6% was recently carried out with-out any equipment changes or capital costs beinginvolved.

The activator content in the solution was increasedslightly, thus leading to a higher absorber efficiency byshifting to more chemical solvent characteristics. Asthe activator could be added during operation, no shut-down was necessary.

Conclusion

This article deals with a specific range of aMDEAapplications, namely the conversion of solvents inexisting ammonia plant gas scrubbing units.

Units which are designed to be operated with aminebased solvents can most often be converted to aMDEAin a straight solution swap: the old solvent is drainedoff, the system is cleaned, filled with aMDEA premix,made up with condensate, started up (faute de mieuxwith the old conditions), and optimized.

The conversion of units designed for solvents with atotally different nature to amine solutions normallyrequires some equipment changes, however. Hotpotassium revamps, for instance, are characterized bya lack of adequate cooling and pump capacities.

Furthermore, the heat integration of the gas scrub-bing unit within the whole ammonia complex canexert constraints determining the operating conditionsat certain locations in the unit, such as boiler feedwa-ter heaters (T, dT, heat duties, and so on). This canturn out to be a bottleneck, for instance, due to differ-ent physical and thermal properties of the solvents.The flexibility of the aMDEA process takes account ofsuch constraints by allowing one to change the funda-mental nature of the solvent. Even if such a revampedunit cannot be operated at the optimum conditionswhen compared to new, grassroots aMDEA units,


advantages still accrue, that is, no corrosion, energy three illuminating examples of the economic efficiencyefficiency, and no purge stream disposal. and reliability of the aMDEA process, which is further

The straight solution swap in the U.S., the revamp of characterized by the cardinal virtues of being environ-the hot-pot unit in Australia, and the capacity increase mentally friendly, easy-to-operate, and very forgiving,of BASF's ammonia plant in Ludwigshafen provide


1996: Customized aMDEA Process for Acid Gas Removal

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