Computer simulation of Dinitrotoluene Nitration Process Master of Science in Engineering, Degree Programme in Chemical Engineering Datasimulering av Dinitrotoluen Nitreringsprocess. Moses Ruhweza Faculty of Health, Science and Technology Subject: Process Engineering Points: 30 ECTS Supervisors: Lars Nilsson and Lars Stenmark Examiner: Lars Järnström Date: 2018-01-19
88
Embed
Computer simulation of Dinitrotoluene Nitration Process1182021/FULLTEXT02.pdf · 2018. 2. 12. · Computer Simulation of Dinitrotoluene Nitration Process Moses Ruhweza Department
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Computer simulation of Dinitrotoluene Nitration Process
Master of Science in Engineering, Degree Programme in Chemical Engineering
Datasimulering av Dinitrotoluen Nitreringsprocess.
Moses Ruhweza
Faculty of Health, Science and Technology
Subject: Process Engineering
Points: 30 ECTS
Supervisors: Lars Nilsson and Lars Stenmark
Examiner: Lars Järnström
Date: 2018-01-19
II
Computer Simulation of Dinitrotoluene Nitration Process Moses Ruhweza Department of Engineering and Chemical Sciences Karlstad University
Abstract This paper presents an approach for modelling a commercial dinitrotoluene (DNT) production
process using the CHEMCAD simulation software. A validation of the model was performed based
on results of an experimental study carried out at Chematur Engineering AB, Sweden.
Important parameters such as fluid properties, temperature profile and other operating conditions for
CHEMCAD steady state model were selected so as to obtain the crude DNT yield as well as the acid
–and organic phase compositions within the same range as the reference values from the experimental
study. The results showed that the assumption of the steady state model was correct, and that acid –
and organic phase compositions were in good agreement, although with a slightly lower sulphuric acid
concentration than that observed in the experimental study.
Also, a detailed study was carried out to analyse the effects of physicochemical conditions on the
desired product yield. Both the results from the experimental study and the simulated model agree that
the effects of mixed acids or heats of mixing acids contribute significantly to the energy balance.
For the appropriateness of the thermodynamics, a NRTL model was chosen and the reactor system
was optimized by an equilibrium based approach, producing MNT in 99.8% yield and crude DNT in
99.9% yield. An 80.1/19.9 DNT isomer ratio of the main isomers was achieved and a reduction of by-
products in the crude DNT shows a good agreement between the model and the experimental study.
Keywords: Dinitrotoluene, CHEMCAD, thermodynamics, steady state model, MNT
III
Datasimulering av Dinitrotoluen Nitreringsprocess.
Moses Ruhweza Department of Engineering and Chemical Sciences Karlstad University
Sammanfattning
I denna rapport presenteras en metod för att modellera en kommersiell nitreringsprocess för
tillverkning av dinitrotoluen (DNT) med simuleringsprogrammet CHEMCAD. En validering av
modellen gjordes baserat på resultat från en experimentell studie utförd hos Chimärer Engineering AB,
Sverige.
CHEMCAD-modellen utgår från ”steady-state” drift av anläggningen. Viktiga parametrar såsom
fluidegenskaper, temperaturprofil och andra driftsbetingelser i CHEMCAD-modellen valdes för att
erhålla ett utbyte av DNT samt sammansättningar av såväl syrafas som organisk fas i god
överensstämmelse med referensvärdena från den experimentella studien.
Resultaten visade att antagandena i modellen var korrekta och sammansättningarna för syrafasen och
den organiska fasen överensstämde med data från den experimentella studien.
Det genomfördes också en detaljerad studie för att analysera effekterna av fysikalisk-kemiska
betingelser på det önskade produktutbytet. Både resultaten från den experimentella studien och data
från anläggning i drift överensstämde med den simulerade modellen avseende utspädningsvärmens
bidrag till energibalansen.
För att erhålla en lämplig beskrivning av reaktionssystemets termodynamik valdes en NRTL-modell
och reaktorsystemet optimerades, vilket gav 99,8 % utbyte av MNT och 99,9 % DNT utbyte. Ett
förhållande på 80,1 / 19,9 mellan de två huvudisomererna av DNT uppnåddes och en minskning av
biprodukter i DNT produktblandningen. Detta är två exempel på en bra överensstämmelse mellan
modellen och experimentstudien.
IV
Acknowledgements I would like to express my sincere gratitude to Chematur Engineering AB for providing me an
opportunity to do my Master’s Thesis at their company.
I sincerely thank my supervisor Lars Stenmark, Development Director at Chematur Engineering AB,
Sweden, for his constructive advice and supervision throughout this project.
I also wish to extend my thanks to Stefan Johansson, Technology Manager Nitroaromatics and
Margareta Dahl, Manager Process Design for their insightful comments and constructive suggestions
to improve the quality of this project work.
With full pleasure, I converge my heartiest thanks to the Process Department as well as all the
employees at Chematur Engineering AB for their invaluable advice and wholehearted cooperation
without which this project would not have seen the light of day.
Lastly, I would like to thank Professor Lars Nilsson for his exemplary guidance and monitoring
throughout this project.
V
Table of Contents 1. INTRODUCTION .................................................................................................................... 1
2.3. IMPURITIES ................................................................................................................................... 8 2.4. EFFECTS OF PHYSICOCHEMICAL FACTORS ON NITRATION OF TOLUENE ........................................................ 9
2.4.1. Effect of temperature ................................................................................................................. 9 2.4.2. Effect of mixed acids ............................................................................................................... 10 2.4.3. Effect of Spent Acid ................................................................................................................ 10 2.4.4. Effect of nitro compounds solubility ............................................................................................. 11
3. PROCESS SIMULATION ........................................................................................................ 11
3.1. IMPORTANCE OF SIMULATION ........................................................................................................... 12 3.2. SELECTION OF A THERMODYNAMIC METHOD ....................................................................................... 13 3.3. ANALYSIS OF THE DNT NITRATION PROCESS ........................................................................................ 14
3.8.1. Defining a Hydrocarbon pseudo-component .................................................................... 25 3.8.2. Estimating by the UNIFAC method .................................................................................... 25 3.8.3. Estimating by the Joback method ..................................................................................... 25
8.1. APPENDIX A................................................................................................................................... 70 8.2. APPENDIX B ................................................................................................................................... 74 8.3. APPENDIX C ................................................................................................................................... 76 8.4. APPENDIX D .................................................................................................................................. 79
VII
List of Figures
FIGURE 1: THREE ISOMERS FORMED BY NITRATION OF TOLUENE. PERCENTAGES ARE EXAMPLES OF VALUES TYPICAL
FOR ISOTHERMAL CONDITIONS. ............................................................................................................................. 4
FIGURE 2:FORMATION OF NITRONIUM ION (THE POWERFUL ELEECTROPHILE). .............................................................. 5
FIGURE 3: MECHANISM OF THE ELECTROPHILIC ATTACK ON THE AROMATIC SYSTEM .................................................... 6
FIGURE 4: THE SIX ISOMERS OF DNT (8). ....................................................................................................................... 7
FIGURE 5: THIS FIGURE SHOWS THE OXIDATION BY-PRODUCTS FORMED DURING THE DNT NITRATION PROCESS (6). ... 9
FIGURE 6: A PROCESS SYNTHESIS PROBLEM (14,15) . ..................................................................................................... 13
FIGURE 7: THE FIGURE SHOWS THE PROCESS FLOWSHEET FOR THE DNT NITRATION PROCESS. ................................... 14
FIGURE 8: THIS FIGURE SHOWS A SET-UP OF MIXING TWO LIQUIDS. .............................................................................. 17
FIGURE 9: WATER MOLECULES SURROUNDING A SULFATE ION. .................................................................................... 18
FIGURE 10: THE GRAPH OF THE ELECTRIC POTENTIAL FOR CHARGE Q RELATIVE TO THE WATER MOLECULES AT R. .... 19
FIGURE 11: THE POTENTIAL ENERGY FOR A CHARGE Q AT DISTANCE R ........................................................................ 20
FIGURE 12: A SET-UP SHOWING MIXING OF TWO LIQUIDS. ............................................................................................ 21
FIGURE 13: THIS FIGURE SHOWS A DIALOG BOX FOR THE PROPERTY ESTIMATION OF NEW COMPONENTS. ................... 24
FIGURE 14: MOLECULAR STRUCTURE OF TOLUENE (42). ............................................................................................... 26
FIGURE 15: CHEMCAD UNITOPS THAT CALCULATE FLOW AS A FUNCTION OF PRESSURE. ........................................... 34
FIGURE 16: A SET-UP SHOWING THE EFFICIENCY OF A HEAT ENGINE. .......................................................................... 37
FIGURE 17: THE EQUILIBRIUM REACTOR IN CHEMCAD .............................................................................................. 41
VIII
List of tables
TABLE 1: ISOMER CONTENT OF DINITROTOLUENE .......................................................................................................... 6
TABLE 2: RELATIONSHIP BETWEEN THE COMPOSITION OF NITROTOLUENE INTERMEDIATES AND TEMPERATURE. ....... 10
TABLE 3: SOLUBILITY OF DNT IN SULPHURIC ACID. MODIFIED FROM (11). ................................................................... 11
TABLE 4: COMPONENTS DEFINED IN CHEMCAD TO REPRESENT DNT NITRATION PROCESS. ..................................... 23
TABLE 5: PROPERTIES OF THE THREE VERSIONS OF TOLUENE CREATED WITH DIFFERENT METHODS. .......................... 27
TABLE 6: THIS TABLE SHOWS THE NUMBER OF EACH MOLECULAR GROUP WHICH OCCUR WITHIN THE STRUCTURE BEING
TABLE 26: THIS TABLE SHOWS A COMPARISON OF THE CRITICAL PROPERTIES OF THE NEW COMPONENTS WITH VALUES
(SEE APPENDIX C). .............................................................................................................................................. 61
1
1. Introduction This master’s thesis concerns the development of a simulation model and optimization technology for
industrial DNT nitration process. All of the simulation/analysis was done using CHEMCAD, version
7.1. I chose CHEMCAD because it enables one to quickly build fundamental steady-state models of
chemical process. The work was done at Chematur Engineering AB.
1.1. Background Computer simulation of dinitrotoluene (DNT) production facility is the main subject of this master’s
thesis. A study regarding the use of CHEMCAD for designing and analysing process equipment and
benefits of process simulation, is also a focus in this thesis.
Nitration of aromatic hydrocarbons such us benzene and toluene has been extensively studied, mainly
due to its industrial importance in the manufacturing of organic synthetic compounds, and its role in
the development of our present understanding of organic reactions, particularly electrophilic
substitution (1–3).
Commercially, the nitration of toluene is mostly performed to produce toluene diisocyanate (TDI) via
DNT and toluenediamine (TDA).
In a DNT production facility, nitration of toluene is performed in two stages with the production of
nitrotoluene intermediates in the first stage which is known as mononitration and the production of
DNT in the second stage, also known as dinitration.
With a sharp rise in market demand for chemicals such as TDI, many companies have implemented
process simulation technology in order to maximize production capacity, improve safety and
environmental management among others (4).
An example of a company that can provide such an excellent process technology service to their
customers is Chematur Engineering AB (CEAB).
Chematur engineering AB has its headquarters in Karlskoga, Sweden, where it was founded by Alfred
Nobel in the late 19th century. The company has an extensive experience (tracing back to the days of
Alfred Nobel and his achievements) in modelling and simulation of chemical plants. This in turn has
made CEAB the global provider of excellent technology and therefore a forerunner in providing
engineering expertise.
In a recent work, CEAB did a research and developed their existing technology of producing toluene
diisocyanate (TDI) by optimizing their proprietary pump nitration process for continuous production
of Dinitrotoluene (DNT).
2
The DNT production was done in pilot scale experiments using a bench scale pump nitration unit
installed at their Technology Centre.
This master thesis presents a computer simulation model of CEABs pump nitration process for
continuous production of DNT.
1.2. Purpose
The aim of this master’s thesis: The purpose of this master’s thesis was to build a simulation model for the production of DNT using
CHEMCAD. A steady-state model is used as a basis for the simulation, and it is desired to see how
well this model predicts the process characteristics (e.g. flowrates, compositions, nitration temperature,
properties, equipment sizes, etc.) of the DNT nitration process compared to an experimental study.
1.3. Limitations
Although the master’s thesis has reached its aims and was completed within the limits of the
assignment’s due date, there were some unavoidable limitations. The vast majority of the literature was
over 50 years old and some related secondary sources cited in those literature were difficult to locate
and retrieve. As a result, those secondary sources couldn’t be entered in the reference list, but they are
all cited in the body of the paper. Also, during the assembly of the model it became apparent that some
process units could not be directly modelled. For instance, the use of an output stream that goes
directly to the recycled stream in the process could not be done since CHEMCAD does not allow
multiple streams to be sent directly into other process units. To solve this situation a stream divider
was added to the model so the separation unit sent its output to the stream splitter, which was then
the input for the decanter.
1.4. Methodology Several methods to build a simulation model for the production of DNT (for determining the
solvability of the process system) were considered: Hand calculations, spreadsheet and CHEMCAD.
Hand calculations were only used to solve easy problems and the method was not used for complex
problems due to the number of calculations required and the need to re-work the entire process if
design conditions were changed.
The advantage of using a spreadsheet is that it is fairly easy to update changes in it. However, it is time
consuming to set-up and it can be difficult to add or change some steps.
3
The designing of the simulation model was divided into smaller activities: obtaining information,
backbone assembly, workarounds, error checking and updates. The information gathered for use
included, among others, physical properties of chemicals, experimental records and process diagrams
(PFD and P&ID). Once the information was gathered, an agreement was reached on how to build the
model.
The backbone of the simulation model was reviewed by my supervisor and experienced process
engineers from CEAB’s process department
1.5. Organization of the report This report consists of six chapters which will cover the designing of a simulation model for the DNT
nitration process. Here is an overview of each presented chapter:
Chapter One: presents the introduction of the thesis. This chapter also discusses the purpose
of the study, the methodology of the study and its limitations.
Chapter Two: covers the scientific literature review and relevant information used to
accomplish the project work.
Chapter Three: this chapter explains the details of the simulation development and selected
methodology used for the process.
Chapter Four: presents the results predicted by CHEMCAD
Chapter Five: presents a discussion of the process simulation
Chapter Six: discusses the conclusion and future work to improve this study.
4
2. Nitration Theory
2.1. Nitration Nitration has a long history of industrial application and an extensive research on its mechanisms (5) .
Today, nitration is the main reaction used to synthesize one of the most important and largest groups
of industrial chemicals, namely aromatic nitro compounds.
A typical industrial nitration is mainly carried out by a mixed-acid reaction of concentrated sulfuric
acid and nitric acid (6). The reaction generates nitronium ions (NO2+) which are added onto aromatic
substrates via electrophilic substitution. In this way, benzene, toluene and phenol are converted into
the simplest of all aromatic nitro compounds, namely, nitrobenzene, nitrotoluenes, and nitrophenols
(3,5,7).
2.2. Nitration of Toluene
2.2.1. Mononitration Toluene undergoes nitration on reaction with a mixture of concentrated sulfuric acid and concentrated
nitric acid. In this way, three different isomers of mononitrotoluene (MNT) are formed (8). See Figure
1 for details.
Figure 1: Three isomers formed by nitration of toluene. Percentages are examples of values typical for isothermal conditions.
5
2.2.1.1. Generation of the electrophile To initiate this reaction, we need to form a powerful electrophile, namely the nitronium ion (NO2
+).
𝐻𝑁𝑂3 + 2𝐻2𝑆𝑂4 ⇋ 𝑁𝑂2+ + 2𝐻𝑆𝑂4
− + 𝐻3𝑂+
The first step of this reaction is protonation of the OH-group and the reason is to make it a good
leaving group so that we can generate a powerful electrophile see Figure 2. After protonation, the
negative oxygen forms a double bond and expels water (second step), giving a powerful electrophile,
which is, as said before, the nitronium ion (3,5,8) . The sulphuric acid acts as a catalyst in this reaction.
Figure 2:Formation of nitronium ion (the powerful eleectrophile).
2.2.1.2. Electrophilic Attack on Aromatic System The third step (Figure 3) is that the nitronium ion attacks the aromatic ring of toluene, giving an
unstable intermediate (arenium ion). The nitro (-NO2) group can now be added on the ortho position.
It should be noted that when toluene undergoes electrophilic substitution, most of the substitution
takes place at its ortho and para positions, because the methyl group on toluene is an ortho-para
director (3).
The last step (re-aromatization) is to remove the hydrogen atom and this is done by using hydrogen
sulphate (HSO4- ) or excess water in the mixture. The hydrogen sulphate grabs the hydrogen and
reforms a double bond, giving ortho-nitrotoluene. All the three isomers are formed but the nitration
proceeds with predominant formation of the ortho isomer, but the para and meta product is formed
as well (3,8).
6
Figure 3: Mechanism of the electrophilic attack on the aromatic system
2.2.2. Dinitration The second nitration step, dinitration, takes place in the same way as mononitration but it’s more
difficult to achieve because of steric hindrance and deactivation of the aromatic ring by the nitro group.
As a result, a higher temperature and a higher sulfuric acid concentration is required (8).Several isomers
are formed of which 2,4- and 2,6-DNT are the most important, see Figure 4 and Table 1.
Table 1: Isomer content of dinitrotoluene
Isomers Organic product
(Wt.-%)
2,4-DNT 76.1
2,6-DNT 19.8
3,4-DNT 2.25
2,3-DNT 1.23
2,5-DNT 0.54
3,5-DNT 0.08
7
Figure 4: The six isomers of DNT (8).
8
2.3. Impurities
In the industrial production of DNT, impurities are formed and the formation rates of these impurities
are significantly affected by the initial reaction conditions, including nitration temperature and-, initial
sulfuric and nitric acid concentration in the mixed acid (5,8).
The presence of the methyl group in toluene makes it easier to be oxidized to nitrocresols. According
to (9), nitration of toluene to MNT generates on average about 0.7 wt% nitrocresols, which are mainly
dinitro-para and ortho-cresol (80% 2,6-dinitro-p-cresol). Benzoic acid products, nitrogen dioxide
(NO2) and, carbondioxide (CO2) are also formed due to the oxidative power of the acid (9,10).
Oxidative degradation of nitrocresols leads to the formation of nitrous acid, mainly during dinitration.
9
Figure 5: This figure shows the oxidation by-products formed during the DNT nitration process (6).
It is important to minimize the formation of by-products during nitration of toluene as this causes a
reduction in yield (6).
2.4. Effects of physicochemical factors on nitration of toluene
2.4.1. Effect of temperature The nitration temperature is a crucial parameter as it influences the yield of the mononitro isomers.
The nitration temperature also influences the reaction rate but at a considerably lower degree (8).
10
Pictet’s study, as cited in (11), showed that when nitrating toluene with a mixture of nitric and sulfuric
acids at lower temperatures, relatively more para-nitrotoluene could be obtained than at higher
temperatures. A similar study by Orlova, also as cited in (11), concluded that a lower nitration
temperature causes an increase of the para-nitrotoluene content while the meta-nitrotoluene and the
ortho-nitrotoluene content decreases Table 2.
Table 2: Relationship between the composition of nitrotoluene intermediates and temperature.
Temperature
(°𝑪)
Composition of the product
Ortho-isomer Para-isomer Meta-isomer
30 56,9 39,9 3,2
60 57,5 38,5 4,0
For safety reasons and for the purity of the product, it is of great importance to keep the nitration
temperature as low as possible and constant. Using too high temperature causes the reaction to proceed
violently and by-products, especially oxidation products, are easily formed (11). Therefore, the nitration
temperature should not exceed 40 ℃ during mononitration and 70 ℃ during dinitration, since above
this “safety” limit both the methyl group and the aromatic nucleus are attacked oxidatively, leading to
an increased formation of by-products, including nitrocresols and nitrophenols (11) .
2.4.2. Effect of mixed acids The composition of the mixed acid depends on the compound being nitrated and the number of nitro
groups to be introduced. For instance, if more nitro groups are to be added (e.g., during dinitration),
then the acid concentration should be higher.
The ratio of the nitric acid, sulfuric acid and water should be chosen wisely. Otherwise nitration of
toluene might be incomplete. Since water is formed during nitration and it dilutes the mixed acid, the
amount of sulfuric acid must be chosen in such a way that it binds up all the water formed (11).
It is preferable to use a very slight excess of nitric acid (e.g., 1-2% in both nitration stages), to avoid
oxidation processes (11).
Higher concentration of sulfuric acid increases the rate of the reaction by increasing the concentration
of the electrophile, nitronium ion (3).
2.4.3. Effect of Spent Acid The acid leaving the dinitration stage is re-used for the nitration of toluene to mononitrotluene. This
in turn affects the nitration reaction and the formation of the nitrotoluene intermediates (11).
11
One method of adding the acid from dinitration to mononitration stage is by mixing the spent acid
with concentrated nitric acid and sulfuric acid.
A disadvantage of this method is an increment of temperature, mainly due to the heat of dilution of
the mixed acid (11).
2.4.4. Effect of nitro compounds solubility The solubility of nitro compounds is an important factor in the nitration process.
The more easily the organic phase dissolves in the acid phase, the higher the reaction rate, and the
degree of nitration that can be obtained in a given time.
For example, nitrobenzene and trinitrotoluene (TNT) dissolve easily in concentrated sulfuric acid.
However, TNT dissolves with difficulty in mixed acids but its solubility might be high when the
content of nitric acid falls to a few percent, as in the spent acid (11).
The solubility data for DNT in sulphuric acid of various concentrations are tabulated in Table 3.
Table 3: Solubility of DNT in sulphuric acid. Modified from (11).
Concentration
% H2SO4
Solubility of DNT in sulphuric acid
g DNT/ 100 g sulphuric acid
40°𝐶 50°𝐶 70°𝐶
80 - 2.5 3.8
83.6 3.6 4.7 6.3
88.7 10.0 12.8 -
90 - 16.8 20.0
3. Process simulation In this modern age of powerful computers, the role of process simulation in the chemical industry has
grown immensely, and with good reason.
Process simulation is a computer presentation of a real-world process plant or system by a
mathematical model which is then solved to obtain information about the performance of the process
(12).
12
3.1. Importance of simulation Process simulation allows engineers to model processes in extreme detail without having to spend the
time, manpower and money for physically testing the design in a real-world industrial environment.
For instance, consider being asked to design a distillation column to produce a mixture of benzene
and toluene into an overhead product containing 95% benzene and a bottom product containing 90%
toluene. This process can be designed by hand calculations (e.g., calculating the condenser and reboiler
duties, mass and energy balances and estimating tray efficiencies) or by physically building a pilot plant
of the process. However, when the design conditions are changed (e.g., 95% toluene and a feed rate
of 850kg/h instead of 750kg/h), it takes time and becomes costly to test the potential designs.
With the help of commercial process simulators (e.g., Aspen Plus, Aspen HYSYS and CHEMCAD)
however, a tremendous amount of time and money can be saved.
Therefore, in the ever more competitive world of processing and manufacturing, process engineering
services are no longer complete without the presence of process simulators (13).
Process simulators are extensively being used as powerful tools to increase, among others, the
production capacity, profits of a company, competitiveness and to reduce the build-up time for new
manufacturing process. It can also be used to reduce the capital equipment costs by optimizing the
process.
Chemical engineers use process simulation tools among others Aspen Plus and CHEMCAD to design
complex process plants such as large-scale process and manufacturing industries, where energy use is
measured in megawatts, costs and profits are measured in hundreds of millions of dollars and materials
measured in thousands of tons.
Another benefit of process simulation tools is that it allows chemical engineers to predict capital cost
expenditures, evaluate optimization options and determine the overall effects of potential process
changes in one area. Furthermore, chemical engineers rely on process simulation to answer what-if
questions asked by operational staff or management of the process plant. All in all, chemical engineers
use process simulation tools to accurately predict the outputs of the process when the process inputs
and outputs are given.
13
Figure 6: A process synthesis problem (14,15) .
3.2. Selection of a thermodynamic method Just like the foundation of a building, the methods used for estimating thermodynamic and transport
properties determine the perfect conditions of a chemical process simulation. These days, the process
industry or engineers rely on using simulators to perform their computations, and all commercial
simulators today are equipped with a countless of property packages with property estimation methods
such as NRTL, SRK, UNIQUAC, and many more. It is of great importance to know which property
package is appropriate for one’s process computation. The objective of this section is to provide some
accurate and deep understanding into the performance of those property packages and selection of a
proper method to represent the various physical and chemical phenomena under a given set of
operating conditions where mononitration and dinitration occurs. Another aim is to enable the reader
to make the correct selection of property method.
CHEMCAD has several property packages that each consist of different computational methods that
are used to estimate thermodynamic and transport properties (16).
What kind of thermodynamic and transport properties are of interest in process simulation?
If we take a look at the pump nitration process which is Chematur’s process for production of DNT,
we find that the process involves separating, moving of fluids, vaporizing etc. (5,8).
To select the correct property method, the pump nitration process was analysed and the properties
required to execute some computations were identified (Figure 7, appendix D).
14
Figure 7: The figure shows the process flowsheet for the DNT nitration process.
3.3. Analysis of the DNT nitration process
3.3.1. Pump A pump increases the pressure of a liquid stream by adding work to it. Like most pumps, a centrifugal
pump converts input power to kinetic energy of the fluid by accelerating fluid in an impeller (17,18).
Therefore, the required properties include heat capacities, liquid density and pressure.
3.3.2. Separator In the design of the separator, it was necessary to understand how the chemical components partition
themselves between the two liquid phases and to set up the mass relationships of the phases. Also, it
was important to understand how much liquid and vapour are produced at the operating temperatures
and pressures. This means that the separation process required the properties of vapour and liquid
densities, enthalpies and pressures (17,18).
3.3.3. Heat Exchangers Heat exchangers in the pump nitration process allow the fluids to be cooled. The properties required
to represent the cooling process are: liquid vapour pressure, heat of vaporisation, liquid heat capacities,
densities, more (19).
3.3.4. Reactors The reactor allows the reactants to undergo a chemical reaction. Hence, the required properties include
heat of formation, heat of reaction, enthalpies, densities etc. (20).
15
3.4. Thermodynamic models CHEMCAD has over 50 phase equilibrium-K (K-value) models and about twelve enthalpy models
stored in its library. In other words, the phase equilibrium-K and enthalpy models are methods used
for the prediction of vapour-liquid or vapour-liquid-liquid phase equilibrium (called the phase
equilibrium-K) and the heat balance, which is called the enthalpy-H (16,21,22).
It should be noted that a selection of an inappropriate property method, leads to convergence
problems and erroneous results. Therefore, it was fundamental to consider, among others:
The process species and compositions.
The phases involved in the system.
Temperature and pressure operating ranges.
Nature of the fluids.
To understand why a particular property method is prefered in any process simulation, you must
understand some thermodynamic relationships among others:
Fugacity
Activity
Equilibrium
Enthalpy
3.4.1. Fugacity
Fugacity is the tendency of a substance to prefer one phase (liquid, solid and gas) over another. In
other words, fugacity is sort of or acts as a correction factor of pressure in real systems (23,24).
Fugacity can be estimated or determined from gases that are closer to reality than an ideal gas. Real
gases behave differently from ideal gases. An ideal gas is made up of molecules whose only interactions
are elastic collisions. Therefore, these molecules have no intermolecular forces between them contrary
to real gases. As a result, the ideal gas law may not hold for most gases and vapours encountered in
reality (25,26).
To solve this problem and accurately calculate chemical equilibria for real gases, pressure is replaced
by fugacity. So, fugacity is related to how non-ideal a gas behave and it is derived from equation of
states (e.g. Van der Waals, NRTL, UNIQUAC, and SRK) or other expressions that can describe non-
ideal systems.
16
For two-phases of a species to be in equilibrium, the pressure, temperature and the chemical potential
must be equal in both phases. Similarly, for pure species co-existing as liquid and vapour, if they are to
be in equilibrium, then the temperature, pressure and fugacity in both phases must be the same (27).
3.4.2. Activity Activity is a ratio of fugacity to the fugacity of the standard state of a material (pure component,
mixture or solution) at the same temperature and pressure (28,29). The variation of the activity of
component (activity coefficient) with temperature and composition is important in thermodynamic
process because it used to determine the Gibbs energy of mixing of a component, which in turn is
used to determine the equilibrium state of any chemical reaction.
3.4.3. Equilibrium At equilibrium, all thermodynamic properties such us free energy (U), Helmholtz free energy (A),
Gibbs free energy (G), amongst others., are minimized (30). To minimize the free energy functions,
i.e., A, U, and G we need to have a method for determining vapour-liquid/liquid-liquid
equilibrium(31).
17
3.5. Enthalpy changes upon mixing When two liquids for example sulphuric acid and water are mixed, the resulting enthalpy is not
necessarily the sum of the pure component enthalpies since the unlike interactions between molecules
is most likely different than the like interactions. In other words, if the H2SO4 – H2O interactions are
stronger than the H2SO4 – H2SO4 and H2O - H2O interactions, then the mixing process will be
exothermic. The task of this section is to study the change in enthalpy that occurs when mixing occurs.
The change of enthalpy upon mixing two liquid streams is shown in Figure 8
Figure 8: This figure shows a set-up of mixing two liquids.
Two different inlet streams with moles of liquid 1 (n1) are mixed with moles of liquid 2 (n2) in a mixer
and the resulting mixture leaves the process unit at a temperature T3. The energy balance for this
process can be defined as follows:
𝑞 = (𝑛1 + 𝑛2)ℎ3 − 𝑛1 ℎ1 − 𝑛2ℎ2 (1)
Heat of mixing may either be positive or negative. If it is positive then the reaction is endothermic
(meaning heat absorbed because the mixture has a higher enthalpy than the pure component) and if
its negative then the reaction is exothermic (heat given off because the mixture has a lower enthalpy
than the pure component).
From the conclusion above, you can tell from different types of molecular interactions in case heats
of mixing will contribute significantly to the energy balance. Nonideal mixtures have a fairly large heat
of mixing. For instance, the mixing of sulphuric acid and water produces so much heat because the
energy level of the system goes down and it releases heat.
18
In details, let us assume a very concentrated solution like 18 moles of sulfuric acid in one litre of
solution. The small amount of the water molecules will surround the sulphate ions very quickly since
there are so many sulphate ions. Therefore, the initial energy that is given off is enormous and of
course that energy will raise the temperature, causing the solution to boil vigorously.
Figure 9: Water molecules surrounding a sulfate ion.
The positive ends of water molecule attract themselves to the negative ions (sulfate ions) in the
solution, resulting to a lower energy state. So, the enthalpy of dilution is a negative quantity i.e., it is an
exothermic reaction which expels heat (energy is taken away from the ions and expelled to the
solution). Figure 9, shows one way to look at why that happens.
The negative charge has an electrical field around it and so that means that the electrical potential V
around the negative charge increases as the positive charge gets closer to it (34).
𝑉 =
𝑘𝑄
𝑟
(2)
where k is Coulomb’s constant
19
Figure 10: The graph of the electric potential for charge Q relative to the water molecules at r.
The electric potential energy for a charge q at r is then given as:
𝑈 =
𝑘𝑄𝑞
𝑟
(3)
20
Figure 11: The potential energy for a charge q at distance r
Notice that the charge q is positive which means that the electrical potential energy goes down as the
charge gets closer. In other words, the potential energy gets expelled and turns into heat when the
positive charge comes closer.
Again, in a mixture (e.g., water and an acid solution) the enthalpy of dilution is negative as a result of
a lower energy state.
3.6. Heats of dilution of mixed acids 2 kg of pure water at 21.1 ℃ was mixed adiabatically with 1 kg of 80% wt.-% sulphuric acid solution
at 21.1℃.
21
Figure 12: A set-up showing mixing of two liquids.
The mass is conserved and if we assume that there is no accumulation in the mixing unit then the:
Total mass balance: 𝑚3 = 3𝑘𝑔
To find the composition at the outlet stream, a balance equation was used based on sulphuric acid.
Notice that the balance equation can be used on either sulphuric acid or water.
𝑥𝑆𝐴𝑚1 = 𝑥3𝑆𝐴𝑚3 → (0.8 ∗ 1) = (𝑥3𝑆𝐴 ∗ 3) (4)
𝑥3𝑆𝐴 = 0.27 → 𝑥3𝑊 = 0.73 (5)
To find the outlet temperature of the mixture, an energy balance equation is needed.
Recall that for an adiabatic process 𝑑𝑞 = 𝑑𝐻 = 0
If a change in enthalpy is zero, that implies that the inlet enthalpy is the same as the outlet enthalpy
(32).
𝐻𝐼𝑁 = 𝐻𝑂𝑈𝑇 (6)
22
So, as seen earlier from equation (1) the specific enthalpy of the acid mixture can be solved from
𝑚1ℎ1 + 𝑚2ℎ2 = 𝑚3ℎ3 (7)
Since the inlet streams are both at 21.1 ℃, we should be able to use an enthalpy concentration diagram
to find the specific enthalpy of these solutions at a specific temperature and concentration.
3.6.1. Enthalpy concentration diagram Reference 55 shows the specific enthalpy of the solution in units of kJ/kg as a function of mass fraction
of sulphuric acid. The specific enthalpy of the solution is shown for several isotherms (each curve
represents different temperatures). In this case, both the inlet streams are mixed at a temperature of
21.1℃. The second stream is pure water, so the mass fraction of sulphuric acid is zero and the
isotherm is 21.1 ℃. The specific enthalpy can then be found by tracing the curve up to when the x-
axis is equal to zero. By visual inspection, this is approximately ℎ2 = 99 𝑘𝐽/𝑘𝑔. The first stream is at
80 wt.-% and also at 21.1 ℃. The 21.1 ℃ isotherm can also be traced up to where the x-axis is equal
to 80 wt.-%. This gives approximately ℎ1 = −240 𝑘𝐽/𝑘𝑔, also by visual inspection. The values can
then be substituted in equation (24) and the specific enthalpy of the mixture ℎ3 = −14 𝑘𝐽/𝑘𝑔 .
Therefore, the temperature at which sulphuric acid solution of 𝑥3𝑆𝐴 = 0.27 mass fraction equal to
ℎ3 = −14 𝑘𝐽/𝑘𝑔 can be found at the intersection of a horizontal line from ℎ3 = −14 𝑘𝐽/𝑘𝑔 , and
a vertical line from 𝑥3𝑆𝐴 = 0.27. By visual inspection the outlet temperature is 48 ℃ since it is in-
between 37.80 ℃ 𝑎𝑛𝑑 65.60 ℃ .
The found values by hand calculation are quite consistent with CHEMCAD values. The predicted
temperature by CHEMCAD was 48.90 ℃ (less than 2 % error). Many solutions with different
compositions of mixed acids were modelled with CHEMCAD and the overall values agree with
literature values.
23
3.7. Component specification In CHEMCAD, process engineers often use the word component when they talk about chemicals.
This section will illustrate how to create or add a new component into the databases of CHEMCAD
and the different steps as part of creating the new component.
The components present in feed streams and possible products are defined in the components
specification menu and are listed in Table 4.
Table 4: Components defined in CHEMCAD to represent DNT nitration process.
3.11.6. Separators The separator is a rigorous separation model that combines input streams and separates the results
into two output streams of different composition and thermal conditions.
By specifying split fractions and split flow rates for each component, almost any kind of separation
can be performed by the CHEMCAD separation model. The CHEMCAD separator provides various
output temperature specifications for the product streams that include bubble point, subcooled, dew
point and superheated conditions. This module can be used to design a separator, such as setting apart
a pure component from a mixture or separating the components from a process stream.
The component separator is useful when trying to model steady state conditions of unusual separation
equipment and conditions. The heat balance is made by setting the UnitOp duty equal to the difference
between the inlet and the outlet streams. However, the outlet flow systems of the separators do not
bear any resemblance to the actual plant separators. In the actual plant the recycled acid is separated
off first whereas in the simulated model the recycled acid and the spent acid are separated in the last
step. Notice that; the recycle acid in the actual plant is separated from the two-phase liquid system
consisting of crude organics and excess spent acid. In other words, the separators perform a liquid-
liquid extraction (LLE).
Liquid-liquid extraction is a separation technology that separates chemical components based on their
relative solubility in two immiscible liquids. The fluids are fed into the separator, where the phases are
immiscible or slightly miscible with each other. A formation of a dispersion occurs and one of the
liquids is dispersed as droplets in the other liquid.
At the interface or between the dispersed phase (droplets) and the surrounding liquid, a mass transfer
occurs. When the relative densities of the liquids are different, the droplets accumulate below or above
the surrounding liquid, and this in turn leads to a subsequently separation of the two liquids.
The process of simulating separation in CHEMCAD is straightforward once the user knows the
compositions of the outlet to a separation or has a fully specified feed.
49
4. Results The results from the experimental study and available industrial data were used to validate the
simulated model. Both the available industrial data (given in parenthesis) and the experimental data are
complementary.
The abbreviation “n.d.” in the tables below stand for “not detected”. In other words, the values weren’t
measured or considered in the experimental study.
4.1. Spent acid from nitration As it can be seen in Table 13, the composition of the spent acid has a good agreement between the
experimental study and the predicted data from CHEMCAD. The predicted composition of the spent
acid has a concentration of approximately 69.0 wt.-% sulphuric acid, 1.5 wt.-% nitric acid and 1. 0 wt.-
% organic compounds (MNT/DNT). Although the concentration of the sulphuric acid is slightly
lower than the observed experimental value of 70.40 wt.-%, the predicted values still indicate that the
simulated model does not differ significantly from the real-world process.
The composition of the spent acid is the same as the composition of the recycled stream but the flow
rates are different. The flow rate of the spent acid in the simulated model was 21723 kg/hr at 40 C
and at a pressure of 3 bars.
The spent acid is treated in a separate process to a final concentration of approximately 93 wt.-%
sulphuric acid. As a result, 116.720 tons of sulphuric acid per year is recovered for recirculation to
the process (Table 14). The main organic content of the spent acid, MNT/DNT, are also recovered
and reused in the process (Table 15).
50
Table 13: Experimental results versus simulation predictions for Spent Acid.
a The DNT nitration process was conducted without the recovered acid mixture(NAR) and recovered DNT (Rec DNT).
bDNT nitration process conducted with NAR and
Rec DNT.
Table 14: This table shows the composition of the Recovered Sulphuric Acid.
Recovered Sulfuric Acid
Composition:
H2SO4 93.0 wt.-%
HNO3 0.5 wt.-%
MNT/DNT 0.2 wt.-%
Flow rate 14590 kg/hr
Temperature 40 C
Pressure 300 kPa
Table 15: This table shows the Recovered DNT.
Recovered DNT
Composition
DNT (mainly) + MNT saturated with water
Flow rate 221.1 kg/hr
Temperature 50 C
Pressure 300 kPa
Spent Acid Composition
Component Experimental study Simulated model
(%)a (%)b
H2SO4 70.40 69.0
HNO3 0.70 1.50
HNO2 1.70 0.00
Organics 1.00 0.98
Water 27.30 29.02
51
4.2. The MNT Content In Table 16, the MNT composition in the experimental study was reported as 89.60 % whereas the
predicted value from the simulation model was 92.70 wt.-%.
The MNT contains 0.02 wt.-% cresols and phenols (other organics) which is reasonable good
compared to the expected value of 0.05 wt.-%. Also, the MNT contains 1.44 wt.-% acid content which
is similar to the expected traces in the actual plant of the DNT nitration process.
An increase of cresols and phenols favours the formation of effluent gas from nitration. As it can be
observed in Table 16, the simulated model doesn’t indicate a complete conversion of toluene. The
unreacted toluene in the mononitration stage helps in correcting the Delta T.
The production rate of the MNT was 1600 kg/hr at 40 C.
Table 16: The composition of the acid phase in the organic phase and MNT yield
Crude MNT Composition
Component Experimental study Simulated model
(%) (%)
MNT 89.60 92.70
DNT 7.80 0.00
Toluene 2.30 (6) 6.00
H2SO4 n.d (1.2) 1.00
HNO3 n.d (0.6) 0.14
HNO2 n.d (0.2) 0.02
Water n.d (0.3) 0,29
Other Organics 0.19 (0.05) 0.02
52
4.3. Acid to Mononitration Stage The composition of the acid stream flowing to the mononitration circuit is the same as the composition
of the stream recycled back to the reactor system in CCT2. The rate of the acid to the mononitration
circuit was 17200 kg/hr. at 70 C. The acid stream contained approximately 0.2 wt.-% organic phase
and the concentration of the sulphuric acid was approximately 80 wt.-%, see Table 17.
The simulation model accounted for the recovered nitric/sulfuric acid mixture as well as the recovered
DNT which in turn caused the sulphuric acid concentration and the nitric acid concentration in the
mononitration stage to decrease and increase respectively. The sulphuric acid concentration decreased
to about 68 wt.-% whereas the nitric acid increased to about 3 wt.-%. To maintain the acid
concentration in the mononitration stage, an increase of 1-3 wt.-% of the sulphuric acid concentration
in the dinitration stage is required. The low content of the nitrous acid is due to a high fractional
conversion of its dissociation to nitric oxide, nitrogen dioxide and water.
Table 17: The composition of the organic phase in the acid phase.
Acid Composition
Component Experimental study Simulated model
(%) (%)
MNT n.d 0.20
DNT n.d <0.01
H2SO4 78.60 79.58
HNO3 1.30 1.92
HNO2 1.50 <0.01
Water 18.80 18.32
53
4.4. Crude DNT With the operating conditions, the simulation shows that annually 175.000 tons of Crude DNT with a
cresol/phenol content of 0.03 wt.-% can be produced. The crude DNT contains a MNT content of
approximately 0.1 wt.-% and a TNT content of 0.03 wt.-% which is less than what is obtained in the
industrial plant.
The acid content in the crude DNT is approximately 1.2 wt.-%, and it has therefore to be further
treated. As a result, 32.000 tons of nitric/sulfuric acid mixture (50 wt.-% acid content) per year is
accessible for recirculation in the process, Table 18. The isomer distribution of 2,4-DNT and 2,6-DNT
is 80.49 wt.-% and 19.51 wt.-% respectively. This ratio is similar to the values (80.1-and 19.9 wt.-%)
obtained from the experimental study.
The flow rate of crude DNT was 21989 kg/hr. at 70 C.
Table 18: This table shows the composition of the recovered nitric/sulphuric mixture
Nitric/Sulfuric acid mixture
Composition
HNO3/ H2SO4 50 wt.-%
HNO2 0.2 wt.-%
Water Balance
Flow rate 4000 kg/h
Temperature 40 C
Pressure 300 kPa
54
Table 19: A comparison of the composition of Crude DNT with the results obtained from the
simulated model and available industrial data (in parenthesis) or experimental data.
Crude DNT Composition
Component Experimental study Simulated model
(%) (%)
DNT 99.64 98.64
MNT 0.22 0.09
TNT 0.14 0.03
H2SO4 n.d (0.22) 0.22
HNO3 n.d (0.26) 0.36
HNO2 n.d (0.10) <0.01
Water n.d (0.51) 0.63
O.org (cresol/phenol) <0.01(0.05) 0.03
55
4.5. Energy balance The energy balance error is approximately 4%.
Table 20: The overall energy balance of the DNT nitration process.
Overall Energy Balance kW
Input Output
Feed Streams -57675.5
Product Streams -69495.1
Total Heating 0
Total Cooling -10515.7
Power Added 189.3
Power Generated 0
Heat of Reaction -3976
Total -71978 -69495.1
56
4.6. Mass balance As observed from Table 21, the overall mass balance has a relative percent error of 0.001%.
Table 21: This table shows the overall mass balance of the DNT nitration process.
Overall Mass Balance
kmol/h
kg/h
Input Output Input Output
Toluene 119.382 0 11000 0.001
o-Nitrotoluene 0.508 0.956 69.645 131.143
p-Nitrotoluene 0.335 0.68 45.898 93.218
m-Nitrotoluene 0.028 0.063 3.839 8.623
2,4-Dinitrotolue 0.535 92.313 97.473 16813.55
2,6-Dinitrotolue 0.155 22.38 28.144 4076.115
3,4-Dinitrotolue 0.016 2.327 2.857 423.77
2,3-Dinitrotoluene 0.004 1.483 0.686 270.125
2,5-Dinitrotolue 0.001 0.543 0.102 98.952
3,5-Dinitrotolue 0 0.044 0 7.972
Sulfuric Acid 142.753 152.2 14001.05 14927.65
Nitric Acid 242.028 6.423 15250.9 404.705
HNO2 0.186 0.011 8.763 0.533
Water 178.26 357.638 3211.359 6442.84
Nitric Oxide 0 0.002 0 0.048
Nitrogen Dioxide 0 0.047 0 2.162
Carbon Monoxide 0 0.012 0 0.323
Carbon Dioxide 0 0.018 0 0.8
Nitrogen 0 0.091 0 2.54
Oxygen 0 0.1 0 3.196
Nitrocresol 0 0.023 0 3.582
Trinitrotoluene 0 0.028 0 6.298
DNOC 0.005 0.002 0.99 0.351
NBA 0 0.007 0 0.725
Cresol 0 0.02 0 2.212
Total 684.195 637.409 43721.7 43721.43
57
4.7. Heat of dilution The heat of dilution of mixing acid solutions agrees with literature values. As seen in the tables
below, the relative percent error is between 2-5% which is acceptable.
𝜎𝑒𝑟 =|𝑥𝑛 − 𝑥𝑠𝑚|
𝑥𝑠𝑚× 100%
Where:
Xn value from the numerical calculation,
Xsm value from the simulated model.
The diagram of enthalpy versus weight percent was used for the sulphuric acid and water solution
whereas for the nitric acid and water solution numerical calculations and available literature data was
used to analyse the results.
Table 22: Sulphuric acid mixed with water.
Stream Name SA Feed Water Feed Outlet (SA 50)
Temp C 1 1 112
Pres bar 1 1 1
Total kmol/h 0.0102 0.0555 0.0657
Total kg/h 1 1 2
Flow rates in kg/h
Sulfuric acid 1 0 1
Water 0 1 1
Nitic acid 0 0 0
58
Table 23: Sulphuric acid mixed with 50 wt.-% sulphuric acid.
Stream Name SA Feed SA50 Feed Outlet (SA 75)
Temp C 1 1 75
Pres bar 1 1 1
Total kmol/h 0.0102 0.0329 0.0430
Total kg/h 1 1 2
Flow rates in kg/h
Sulfuric acid 1 0.5 1.5
Water 0 0.5 0.5
Nitic acid 0 0 0
With hand calculations, the temperature increase for the case in Table 24 was found to be 58 C.
This means that the predicted temperature of the model (60 C) has an acceptable percent error of
4.7%. The heat of dilution predicted by CHEMCAD is 422 kJ/kg.
�̇�𝐼𝑁∆𝐻 = �̇�𝑂𝑈𝑇𝐶𝑃∆𝑇
∆𝑇 =1𝑘𝑔/ℎ𝑟(0 − 403.2)𝑘𝐽/𝑘𝑔
2𝑘𝑔/ℎ𝑟 ∗3.4607𝑘𝐽
𝑘𝑔℃
= 58 ℃
59
Table 24: Nitric acid mixed with water.
Stream Name NA Feed Water Feed Outlet (NA 50)
Temp C 1 1 60
Pres bar 1 1 1
Total kmol/h 0.0159 0.0555 0.0714
Total kg/h 1 1 2
Cp kJ/kg 1.7558 4.2248 3.4607
Flow rates in kg/h
Sulfuric acid 0 0 0
Water 0 1 1
Nitic acid 1 0 1
With hand calculations, the temperature increase for the case in Table 25 was found to be 53 C.
This means that the predicted temperature of the model has an acceptable percent error of 2.2%.
The heat of dilution of 60% nitric acid is 358 kJ/kg. That is approximately 2.2 % error from a
literature value of 350.7 kJ/kg
60
∆𝑇 =1𝑘𝑔/ℎ𝑟(0 − 350.7)𝑘𝐽/𝑘𝑔
2𝑘𝑔/ℎ𝑟 ∗3.3809𝑘𝐽
𝑘𝑔℃
= 52 ℃
Table 25: Nitric acid mixed with 20 wt.-% Nitric acid
Stream Name NA Feed NA 20 Feed Outlet (NA 60)
Temp C 1 1 53
Pres bar 1 1 1
Total kmol/h 0.0159 0.0476 0.0635
Total kg/h 1 1 2
Cp kJ/kg 1.7558 3.7648 3.3809
Flow rates in kg/h
Sulfuric acid 0 0 0
Water 0 0.8 0.8
Nitic acid 1 0.2 1.2
61
4.8. Critical properties of the new components created. The critical properties of nitrocresol and NBA deviate from literature data but the critical properties
of the other components have a relative percent error of less than 1%.
Table 26: This table shows a comparison of the critical properties of the new components with values (see Appendix A).
DNOC Literature values Predicted values
Molecular weight 198.130 198.130
Critical T [C] 786.610 786.605
Critical P [bar] 49.874 49.873
Critical V [m3/kmol] 0.450 0.449
Tboil [C] 507.35 507.35
Cresol Literature values Predicted values
Molecular weight 108.140 108.140
Critical T [C] 422.000 424.400
Critical P [bar] 50.054 50.055
Critical V [m3/kmol] 0.290 0.282
Tboil [C] 190.900 191.000
Nitrocresol Literature values Predicted values
Molecular weight 153.14 153.140
Critical T [C] 810.520 978.890
Critical P [bar] 50.086 50.085
Critical V [m3/kmol] 0.370 0.368
Tboil [C] 623.680 610.520
Nitrous acid Literature values Predicted values
Molecular weight 47.013 47.013
Critical T [C] 248.160 248.159
Critical P [bar] 76.409 76.408
Critical V [m3/kmol] Not found 0.082
62
Tboil [C] 81.830 81.830
NBA Literature values Predicted values
Molecular weight 167.120 167.120
Critical T [C] 650.070 567.865
Critical P [bar] 50.514 64.204
Critical V [m3/kmol] 0.430 0.304
Tboil [C] 415.96 335.770
63
5. Discussion
In this project, the simulation of the DNT nitration process was built in CHEMCAD using an
equilibrium reactor. A good agreement between the simulation data and experimental data or available
industrial data was found. However, some differences appeared in the flow rate of product streams,
organic phase and acid phase. These differences were caused by a change in the design conditions.
Differences of the simulation and experimental compositions for spent acid, acid to mononitration
stage and crude DNT can be explained by the liquid-liquid equilibria.
In the case of the experimental study, the separation utilized differences in solubilities. The phase
separation is achieved under gravity because of a density difference between the phases. For simulation,
CHEMCAD couldn’t solve the liquid-liquid phase equilibria. The separation was based on split
fractions instead of mass transfer from one liquid phase to another, so, this fact influences the phase
equilibria.
The mass balance over the heat exchanger in the mononitration stage and dinitration stage has a small
error (0.001%), so; small differences between calculated results in the streams occur. These differences
are observed in the amount of water, sulphuric acid and nitric acid in the streams and consequently in
the energy balance. This inaccuracy is caused by a tolerance error (0.01) of the flow rate measurements.
The default mode accuracy (0.001) for flow rate measurements is better. However, a high accuracy
slows down the simulation. The DNT nitration process involves many parameters, control loops and
reactions, so, convergence is difficult to achieve with a high accuracy mode. To overcome this issue,
the error tolerance is set to 0.1% accuracy which is usually sufficient for the overall mass and energy
balances.
Recycle streams, electrolyte model, pressure drops and increased number of iterations were also some
factors that drastically slowed down the simulation.
It was observed that the recovered nitric/sulphuric acid mixture lowers the concentration of sulphuric
acid in the spent acid. To achieve a concentration of 70 wt.-% sulphuric acid, the concentration of the
recovered sulphuric acid in the dinitration circuit was increased to approximately 80%.
64
When setting up the flowsheet, a stream splitter was added after the separation units because multiple
streams couldn’t be sent out of them.
In the dinitration stage, the outgoing streams were of the same composition. However, the streams
had different flow rates. The recycled stream to the reactor system had the highest flow rate and this
meant that DNT from the separation unit could not be traced in the mononitration stage. For this
reason, the separation unit was specified to separate all (0.99 split fraction) the DNT from the process
stream. Usually, most process units (e.g., reactors, pumps and separators) do not allow multiple streams
to be sent directly into other process units. Therefore, a stream splitter is required but the flowsheet
will not bear an accurate resemblance of the actual plant.
The heat exchanger selected was easy to control and operate but the fluids in the heat exchanger are
exposed to a much smaller surface area which in turn affects the overall heat transfer coefficient and
therefore the capacity of the heat transfer. From an industrial perspective, a plate heat exchanger is the
most suitable. The plate heat exchanger has a major advantage over the fixed head in that the fluids
are exposed to a much larger surface area since it is equipped with metal plates in which the fluids are
spread out over.
The hydraulic pump flow rate in both stages are expected to be at approximately 600 m3/hr instead of
500 m3/hr as observed in the model. This error can be explained by the performance of the controller
that adjusts the flowrate in the system downstream of the pump. The controller is set to adjust the
flow rate until the temperature increase across the reactor is 10 ℃ but instead it adjusts it to 11 ℃.
Without the use of the controller, it was observed that the hydraulic pump flow rate and the
temperature increase matched the expected values accurately.
65
6. Conclusion Commercial simulation software CHEMCAD has been used for modelling the DNT nitration process.
The simulation model has been validated with experimental data and actual plant records. An
agreement between the simulation results and the experimental or actual plant records was obtained.
Therefore, CHEMCAD can be used to model the DNT nitration process. It proved that it could
describe chemical reactions and provide valuable output. If the characteristics of the feed streams (e.g.,
composition) are known and thermodynamic models are selected wisely, the results of the heat and
material balance will be accurate.
It was shown that employing stream reference and controllers were beneficial for estimating recycle
streams and therefore promoting faster convergence.
Concerning the material balance, it is best to set the recycle fraction of the stream splitter, rather than
the exact material flow rate of the recycle stream. Setting the flow rate prevents the convergence of
the recycle stream. Also, in the recycle loop it is beneficial to specify the outlet pressure instead of the
pressure increase. Otherwise, the pressure over the pump will be raised on every iteration.
An efficient time use was made for this project due to the fact that this project was carried out at
CEAB who in turn had engineers (with process expertise) that could help.
The model for the DNT nitration process and the project work was reasonable for both parties.
Therefore, it can now be easily updated for improvements of the DNT nitration process. For
equipment sizing, it is necessary to continue validating each process unit since this validation was only
based on an actual plant with a capacity of 50,000 kg/year. Also, in future the separation unit can be
corrected to account for mass transfer of liquid-liquid separation. Enhancements or updates to process
units can be done by Chemstations on customer’s request. It is therefore necessary to request for
updates.
66
7. References
1. Grob CA. Aromatic Substitution, Nitration and Halogenation, von P. B. D. de la Mare und J.
H. Ridd. Butterworths Scientific Publications, London 1959. 1. Aufl., VII, 252 S., £ 2.10.0
d. Angew Chem [Internet]. 1961 Dec 7;73(23):784–784. Available from: