Aspen Plus Simulation

Aspen Plus Simulation

Chuen Chan

Department of Chemical Engineering The University of Queensland

Bachelor of Engineering, Chemical

27 October 2000

1E406 Individual Inquiry Aspen Plus™ SimulationAcknowledgments

I would like to take the opportunity to thank the following people for supporting thisindividual inquiry.

v Ms Caroline Crosthwaite for giving me the chance to do this topic as well as hervaluable comments and guidance.

v Dr Daniel Thomas from Queensland Alumina for giving me important operatingvariables for the manufacture of alumina.

v The following fellow students who has also provided me with important operatingvariables (who worked in Queensland Alumina as part of their industrial experience)and have helped me with the simulation:• Jeff Foley• Brent Merrick• Sarah Nicholson• Tony Scott• Ewan McPherson• Loh Hong Siang• Pranesh Khatri• Ngai Ghi• Awang Abu Bakar• Justin Thamboo

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1E406 Individual Inquiry Aspen Plus™ SimulationAbstract

Computer-aided process design programs, often referred to as process simulators,flowsheet simulators, or flowsheeting packages, are widely used in process design.Aspen Plus™ by Aspen Technology is one of the major process simulators that arewidely used in chemical process industries today. It specialises on steady-state analysis.The department of Chemical Engineering, University of Queensland, uses Aspen as partof its second year course teaching syllabus. It is used in the subject 1E202, ProcessSystems Analysis, to introduce students to computer-aided process design software, aswell as to meet some of the subject’s goals.

Therefore, the aim of this project is to look at two processes in detail and to develop asimplified Aspen simulation and create scenarios, which will be use as teaching toolsfor future 1E202 students.

The chosen two chemical processes are the manufacture of vinyl chloride monomersand alumina. Literature reviews were done first to fully understand the above twoprocesses. A base case design was generated and a generic block diagram with the massand energy balance for each process was done (Both as an Aspen block diagram and aProcess Flow Diagram). They were then implemented into Aspen software itself.Finally, it is converted to teaching tools in the form of tutorial/assignments, written inaccordance to subject 1E202 teaching goals.

Both processes were successfully implemented into Aspen software and converted intoteaching tools that illustrates process or structure analysis. Recommendations for futurework include the expansion of the two processes to produce aluminum and polyvinylchloride resins.

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1E406 Individual Inquiry Aspen Plus™ SimulationTable of Contents

Acknowledgments……………………………………………………………………….iAbstract……………………………………………………………………………….…iiTable of Contents……………………………………………………………………….iiiList of Figures……………………………………………………………………………vList of Tables……………………………………………………………………………viList of Appendices……………………………………………………………………...vii1.Introduction............................................................................................................11.1.Objective..........................................................................................................12.Plan of study...........................................................................................................13.Process 1 – The Manufacture of Vinyl Chloride Monomers....................................23.1

Background......................................................................................................2

3.2Process description...........................................................................................23.2.1Chlorination...............................................................................................33.2.2Evaporator.................................................................................................33.2.3Pyrolysis....................................................................................................33.2.4.Quencher...................................................................................................33.2.5.Condenser..................................................................................................33.2.6.Distillation.................................................................................................33.3Aspen model diagram.......................................................................................43.3.1Direct Chlorination (B1 to B3)...................................................................43.3.2Evaporation (B4 to B6)..............................................................................53.3.3Pyrolysis (B7 to B9)..................................................................................53.3.4Distillation (B10 and B12).........................................................................63.43.5

Mass and Energy Balance.................................................................................6Optimisation of the process..............................................................................7

4.Process 2 – The Manufacture of Alumina from Bauxite..........................................74.1Background......................................................................................................74.2Process Description..........................................................................................84.2.1Bauxite Preparation....................................................................................84.2.2Digestion...................................................................................................84.2.3Clarification...............................................................................................94.2.4Precipitation.............................................................................................104.2.5Calcination...............................................................................................114.3Aspen model diagram.....................................................................................124.3.1Digestion (B1 to B4)................................................................................13

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1E406 Individual Inquiry Aspen Plus™ Simulation4.3.2

4.3.34.3.44.3..44.5

Clarification (B6 to B8)...........................................................................14Precipitation (B9 to B11).........................................................................15Calcination (B12 and B13).......................................................................15Heat recovery system (B14 and B15).......................................................16

Mass and Energy Balance...............................................................................16Optimisation of the process............................................................................16

5.Conclusion and Recommendations........................................................................175.1.The Manufacture of Vinyl Chloride Monomers..............................................175.2

The Manufacture of Alumina..........................................................................18

6.References............................................................................................................19

List of Figures

Figure 1: Manufacture of Vinyl Chloride Monomer………………………….………2Figure 2: Aspen model block diagram for manufacture of

Vinyl Chloride Monomer…………………………………………….……..4Figure 3: Block Diagram of Alumina Manufacturing Process……………………….8Figure 4: Aspen model block diagram for manufacture of Alumina………………...12

List of Tables

Table 1: Amount of feed rate…………………………………………………………….4Table 2: Composition of Bauxite……………………………………………………….12Table 3: Amount of feed rate…………………………………………………………...13Table 4: Smelting grade alumina……………………………………………………….13

List of Appendices

Appendix A – Process Flow Diagrams

Appendix B – Teaching tools – Tutorials/AssignmentsAppendix C – Teaching tools – Sample Answers

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1E406 Individual Inquiry Aspen Plus™ Simulation1. Introduction

Aspen Plus™ by Aspen Technology has been used to simulate steady state chemicalprocesses. It has been included in the second year subject 1E202, Process SystemsAnalysis, as part of the teaching syllabus by the chemical engineering department.It is used to meet the following subject goals [1]:

1. Be able to implement, simulate and interpret a process flowsheet using the AspenPlus simulator.

2. Be able to list key operating and design parameters and equipment selection criteriafor a limited set of unit operations and processes.

3. Be able to determine the economic potential of a process and to identify theoperational variables that affect the economics.

4. Have an appreciation of the impact of operational, technical, economic, safety andenvironmental issues on process goals.

1.1. Objective

This project, therefore, is to look at two chemical processes, namely, the manufacture ofVinyl Chloride and Alumina, in detail, to develop simplified Aspen simulations andcreate scenarios to use as teaching tools for future 1E202 students.

2. Plan of study

The following describes the methodology of the project:1. Literature review

An understanding of each of the processes is of upmost importance. Hence, looking upfor references, which describes the processes, current technologies, thermodynamics,reactions etc, is required. From here, a base-case design is initiated.2. Setting Generic Block Diagram

This involves in setting up the structure of the processes and creating the Aspen modeldiagram. Degrees of freedom analysis, mass and energy balances are also included.

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1E406 Individual Inquiry Aspen Plus™ Simulation3. Familiarisation with the new Aspen Plus software

As the current version (version 10.1) has just been installed in the department and isconsidered new, some time has to be allocated for the user to be familiar with thesoftware.

4. Implementation of processes into Aspen

This is to simulate the processes into Aspen software. This includes the components aswell as each unit specifications.

5. Writing documentation for teaching tool

Finally, to convert the processes into teaching tools according to the subject goals asmentioned previously. Examples of the outline would be like goals of the system, massand energy balances, sensitivity analysis etc.

3. Process 1 – The Manufacture of Vinyl Chloride Monomers

3.1 Background

The manufacture of vinyl chloride, a monomer intermediate for production of polyvinylchloride, is an important plastic that is widely used for wire and cables, paper and textilecoatings, and other domestic uses [2]. It is considered as a commodity chemical that isproduced continuously throughout the world. Vinyl chloride is an extremely toxicsubstance, and therefore, industrial plants that manufacture it or process it must bedesigned carefully to satisfy governmental health and safety regulations [3].

3.2 Process description

In addition to data from chemistry laboratory, two patents (Benedict, 1960;B.F.Goodrich Co., 1963) play a key role in process synthesis [3].Shown below is the block diagram of the process

C2H4Cl2

HClChlorinationEvaporatorPyrolysisQuencherCondenserDistillationC2H3Cl

Figure 1: Manufacture of Vinyl Chloride Monomer

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1E406 Individual Inquiry Aspen Plus™ Simulation3.2.1 Chlorination

Ethylene, stored as gas in large cylindrical vessels (at 1000 psia and 70ºF [3]) andchlorine, stored as a liquid (at 150 psia and 70ºF [3]) is fed into a cylindrical reactionvessel at 363K and 1.5 atm [3] with ferric chloride as a catalyst. The following reactiontakes place:

C2H4+Cl2→C2H4Cl2

Experimental data has indicated that at the above reactor conditions, 98% of theethylene are converted to 1,2-dichloroethane [3].

3.2.2 Evaporator

This unit performs the temperature-and phase-change operations in the form of a largekettle, with tube bundle inserted across the bottom. Saturated steam is used as a heatingmedium whereby the dichloroethane liquid is heated to its boiling point and vaporised.The large vapor space is provided to enable liquid droplets, entrained in the vapor, tocoalesce and drop back into the liquid pool, that is, to disengage from the vapor whichproceeds to the pyrolysis furnace.

3.2.3 Pyrolysis

This unit performs two operations: It preheats the vapor to its reaction temperature,773K [3] and it carries out the pyrolysis reaction.C2H4Cl2→C2H3Cl+HCl

The dichloroethane intermediate is converted to vinyl chloride by thermal cracking.60% of conversion is assumed as claimed by the patent [3].

3.2.4. Quencher

A quench tank is designed to rapidly quench the pyrolysis effluent to avoid carbondeposition in a heat exchanger. Cold liquid (principally dichloroethane) is showeredover the hot gases, cooling them to their new dew point, 443K [3].

3.2.5. Condenser

To produce a saturated liquid at 279K, the phase-change operation is carried out by acondenser that transfers heat to a refrigerant [3].

3.2.6. Distillation

This consists of two distillation columns whereby the components are being separated.The first column separates the HCl from other components while the second column

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1E406 Individual Inquiry Aspen Plus™ Simulationseparates the vinyl chloride from the unreacted dichloroethane. The unreacteddichloroethane is then recycled back for optimisation purposes.

3.3 Aspen model diagram

An attempt has been made to try to simulate the above vinyl chloride monomermanufacturing process in Aspen simulation software. Shown below is the Aspen modelblock diagram.

C2H4Cl2B1MixerHClB2RstoicB3MixerB4HeaterB5PumpB6HeaterB7RstoicB8HeaterB9HeaterB10SepC2H3ClB11SepB12FsplitPurgeFig 2: Aspen model block diagram for manufacture of Vinyl Chloride MonomerThe simulation is based on a petrochemical complex on the Gulf Coast that produces800 million pounds per year (∼37.85 tons/hr) due to the risen demand for vinyl chloridemonomer [3]. It uses the following feed rate:Table 1: Amount of feed rate [3]ComponentChlorineEthylene

Flow rate (tons/hr)51.4420.37

The main product will be vinyl chloride monomer while the by-product is hydrogenchloride. Both have a purity of 99.9%.

Below gives the description, assumptions and basis for each block.

3.3.1 Direct Chlorination (B1 to B3)

B1 (Mixer)

Ø This block simulates the mixing of raw materials, namely, chlorine and ethylene.B2 (Rstoic)

Ø This block simulates the chlorination reactor. The unit is to be operated at a

temperature of 363K and a pressure of 1.5atm for reasons mentioned earlier

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1E406 Individual Inquiry Aspen Plus™ SimulationØ Assumptions – only the following reactions takes place:

1. C2H4+Cl2→C2H4Cl2

⇒ Chlorination

Ø It is assumed that 98% of C2H4 has been converted to C2H4Cl2 [3].

Ø The slightly above atmospheric pressure is to prevent leakages into the reactor

[3].B3 (Heater)

Ø This block simulates a condenser unit whereby the outlet of the above reactor is

condensed by cooling water at 298K before being introduced to the pumpdownstream [3].

3.3.2 Evaporation (B4 to B6)

B4 (Mixer)

Ø This allows the mixing of the recycle of unreacted dichloroethane from the

second distillation column.B5 (Pump)

Ø Pumping action is simulated for this block whereby pressure is increased to

26atm. At this pressure, the downstream pyrolysis reaction boils HCl at 273K,and much less costly refrigerant could be used [3].B6 (Heater)

Ø The evaporator unit is simulated for this block.

Ø This unit performs the temperature and phase change operations. Temperature is

increased to 515K using saturated steam as the heating source [3].

3.3.3 Pyrolysis (B7 to B9)

B7 (Rstoic)

Ø This block simulates the pyrolysis furnace whereby the following reaction takes

place:

1. C2H4Cl2→C2H3Cl+HCl

⇒ Pyrolysis reaction

Ø The dichloroethane is converted to vinyl chloride by thermal cracking, which

occurs spontaneously at 737K with conversions of 65% [3].

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1E406 Individual Inquiry Aspen Plus™ SimulationB8 (Heater)

Ø The quencher unit is simulated in this block whereby the outlet gas from the

pyrolysis furnace is cooled to 443K to prevent carbon deposition in heatexchangers [3].B9 (Heater)

Ø Another condenser unit whereby phase-change operations takes place. A

refrigerant is required for the operation to produce a saturated liquid at 279K [3].

3.3.4 Distillation (B10 and B12)

B10 (Sep)

Ø The first distillation column is simulated here whereby hydrogen chloride is first

separated from the rest of the components.B11 (Sep)

Ø This is the second distillation column where the separation of vinyl chloride and

the remaining components takes place.B12 (Fsplit)

Ø To prevent an accumulation of unreacted dichloroethane, a purge stream is

introduced into the system. The amount of purge is to be kept a minium tooptimise the process.

3.4 Mass and Energy Balance

A Process Flow Diagram has been done for the above process. See Appendix A fordetails.

The following abbreviations have been used for the equipment:Ø R – ReactorØ CN – CondensersØ P – PumpØ Q – QuencherØ FN – Furnace

Ø HX – Evaporator (Heat Exchanger)Ø CO – Columns

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1E406 Individual Inquiry Aspen Plus™ Simulation3.5 Optimisation of the process

The temperature of the chlorination reactor is a very important operating variable as itcontrols the amount of ethylene being converted to dichloroethane, which in turn,affects the amount of vinyl chloride produced [3].

The temperature of the pyrolysis furnace is another vital variable as it affects theconversions from dichloroethane to vinyl chloride [3].

A quencher unit was required as the rate of carbon deposition was high. However, if ona pilot-plant testing, the measurement of the rate is found to be otherwise (low), itwould be possible to implement a design with a feed/product heat exchanger [3]. Thismay in turn affect the energy balance of the system.

The above can then be used in teaching tools whereby an analysis of differentconversion rates (for chlorination reactor and pyrolysis furnace) on feed and recyclerates can be investigated. Another useful teaching tool will be to introduce the feedpreheater to the process and analyse the savings that can be made.

4. Process 2 – The Manufacture of Alumina from Bauxite

4.1 Background

Aluminum is considered to be the most abundant metallic element in the earth’s crustand it’s the third most common element around. It is not found in its elemental form butalways tenaciously held in many compounds, most of which contain oxygen or silica, orboth. The most important aluminous ore for the manufacture of aluminium is bauxite. Itconsists of several hydrous aluminium oxide phases [gibbsite (Al(OH)3), boehmite(AlOOH) and diaspore (Al2O3 H2O)] as well as normal impurities such as Fe2O3, SiO2and TiO2. The difference in composition between the minerals lies in differentgeographical locations. [4]

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1E406 Individual Inquiry Aspen Plus™ Simulation4.2 Process Description

The process is based on the Gladstone Alumina refinery. Shown below is the blockdiagram of the process

NaOHCaOWaterRawBauxiteBauxitePreparationDigestionClarificationPrecipitationCalcinationAluminaFigure 3: Block Diagram of Alumina Manufacturing Process

4.2.1 Bauxite Preparation

The bauxite entering the refinery must be uniform and sufficiently fine so that theextraction of the Al2O3 and other operations are successful [5]. In most modern plants,the bauxite is mixed with a portion of the process solution and is ground as a slurry toprevent a dusty working environment. Rod mills and ball mills are used mostfrequently. The ground slurry is then passed over screens or through cyclones, with thefiner particles (less than 0.15 cm) progressing and the course ones being returned tomills.

An innovation recently adopted is to hold ground slurry for several hours in agitatedtanks. [6] These tanks are operated so that plant feed is uniformly blended. Sometimes,bauxites are dried to improve handling or washed to remove clay.

4.2.2 Digestion

In digestion, all of the Al2O3 in the bauxite must be extracted. It works by the principlethat gibbsite and boehmite are soluble in sodium hydroxide (NaOH), the solubilitybeing temperature, concentration of NaOH and bauxite characteristic dependent [5].Most of the impurities in bauxite are not soluble in NaOH, except reactive silica (SiO2),which forms a quite insoluble compound during processing. The following describes thetwo important reactions occurring during this stage.Reaction 1 – Digestion

Al(OH)3+Na++OH−⇔Al(OH)4+Na+

−

AlOOH+Na++OH−+H2O⇔Al(OH)4+Na+

−

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1E406 Individual Inquiry Aspen Plus™ SimulationIt was found that the solubility of gibbsite is higher than boehmite [6]; hence theconditions for digesting a boehmitic bauxite must be more severe. The temperatureand/or NaOH concentration must be increased. For diaspore bauxites, CaO is charged toallow digestions at lower temperature.

For this project, a ‘sweetening process’, whereby the Al2O3 concentration is kept lowenough to dissolve is introduced as the monohydrate grade bauxite contains a mixture ofgibbsite and boehmite. The monohydrate grade is digested under relatively mildconditions, producing an intermediate Al2O3 concentration. In a separate vessel, thetrihydrate grade is added to the flow from the first digest to raise the Al2O3concentration to the desired level. This allows moderate digest conditions.Reaction 2 – CausticizationCaO+H2O→Ca(OH)2

Ca(OH)2+Na2CO3→CaCO3+2NaOH

The reason for this reaction is primarily to make up the NaOH losses. Lime (CaO) isadded to the digest. Na2CO3 is formed by decomposition in the digest of organicmaterial from the bauxite and CO2 adsorbed into process solutions from the atmosphere[5].

4.2.3 Clarification

The next major step in the process is to separate the solid residue from the sodiumaluminate solution. This separation must be as complete as possible as any remainingsolids will cause contamination to the product.

The slurry is fed into the center of thickening tanks (also called settlers), each 40 m indiameter. The velocity of the slurry becomes very slow as the solution flows radiallyacross the thickener. The solids, having a higher specific gravity than the solution, sinkto the bottom of the thickener. The fine solids behave as a relatively stable colloidalsuspension, hence they settle slowly if not treated further. Flocculants are added toimprove the clarity of the thickener overflow, the solids content of the underflow, aswell as the settling rate of the solids.

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1E406 Individual Inquiry Aspen Plus™ SimulationThe particle size distribution of the residue solids is usually bimodal. The term sand isused to describe coarse fraction over 100 µm in diameter whereas the rest of the solidsare finer than 10µm [5]. The settler underflow first has sand removed in sand traps,which is recycled to bauxite grinding. With the residue concentrated in the thickenerunderflow, the next task is to wash it with fresh water so that it can be discarded withinenvironmental standards. This is done in countercurrent washing thickeners similar indesign to those used for settling. Fresh water is use to recover the soda and aluminacontent in the residue before being pumped to large disposal dams. This result inminimising the total costs of the soluble salts lost due to incomplete washing and thecost of evaporating the dilution introduced.

The final stage of clarification is polish filtration of the overflow settler solution. Thisstage is to remove the few particles of solids remaining to protect product purity. Heavycotton cloth provided the early filter medium, but it was easily damaged and slowlyattacked by the caustic solution. Polypropylene is the current fabric choice and isunaffected by process conditions [6]. The plant uses Kelly-type constant pressure filters.On occasions, small quantities of Al(OH)3 or DSP will partially blind the cloth. Hence,lime is added to produce a filter aid – tricalcium aluminate (3CaO⋅Al2O3⋅6H2O) topromote the formation of a porous, rigid filter cake.

4.2.4 Precipitation

With all the solids removed, the liquor leaving the filter area contains alumina in clearsupersaturated solution. This filtered solution must be cooled before precipitation. Thisis done by flash evaporation whereby a steam jet pump is used to remove non-condensable gases from the system to create vacuum. Steam is given off and is used toheat spent liquor (from classifiers) returning to digestion.

Precipitation of crystals is used to recover dissolved alumina from the liquor. Thereaction is the reverse of the first digestion reaction:Al(OH)4+Na+→Al(OH)3+Na++OH−

−

The above reaction is done in rows of precipitation tanks that are seeded with Al(OH)3to promote crystal growth.

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1E406 Individual Inquiry Aspen Plus™ SimulationThe first objective in precipitation is to produce Al(OH)3 that, when calcined, meets theproduct specification. The second objective is to obtain high yield from each volume ofthe solution. Care must be taken to achieve this as the number of new particlesgenerated in precipitation must equal the number of particles leaving as product for thesystem to remain balanced. This requires balancing nucleation, agglomeration, growth,and particle breakage, so a combination of science and art has developed [5].

In the plant, the precipitation tank is agitated, with a holding time each of about 3 hours.During the 25 – 30 hours pass through precipitation, alumina of various crystal sizes isproduced. The entry temperature and the temperature across the row, seed rate andcaustic concentration are control variables used to achieve the required particle sizedistribution in the product [6]. Consequently the process moves slowly from being toofine to too coarse and back again. The aim is to minimise the frequency and amplitudeof the changes.

Slurry leaving the precipitators is led to three stage ‘gravity’ classification tanks to beseparated into three size ranges. The primary classifiers collect the coarse fraction,which becomes the product. The intermediate and fine crystals from the secondary andtertiary classifiers are washed and returned to the precipitation tanks as seed.

The spent caustic liquor, which is essentially free from solids, from the overflow of thetertiary classifier is returned through an evaporation stage. This liquor will be heatedand reconcentrated, to be recycled back to dissolve more alumina in the digesters. Freshcaustic soda is added to the stream to make up for process losses.

4.2.5 Calcination

The final operation in production of alumina is calcination. The temperature of Al(OH)3is raised above 1380 K [6] resulting in the reaction:2Al(OH)3→Al2O3+3H2O

The product from the primary classifier is washed on horizontal-table vacuum filters toremove process liquor. The quality of the wash water is of some concern because suchimpurities as calcium and magnesium can be adsorbed on the surface of the product.

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1E406 Individual Inquiry Aspen Plus™ SimulationThe resulting filter cake is then fed to a series of calcining units to remove both freemoisture and chemically-combined water. The calcining units are made up of rotarykilns each 100 m long and 4 m in diameter. They are mounted on bearings and rotateabout an axis inclined at a small angle to the horizontal. Damp Al(OH)3 from the filtersenters the upper end and slowly tumbles toward the lower end, travelling against astream of hot gas formed by the combustion of natural gas at the discharge end.Cooling the alumina is first carried out in rotary or satellite coolers, which preheat thesecondary combustion air for kilns; and then in fluidised-bed coolers for further cooling.The final product – pure alumina is discharged on to conveyor belts, which carry it tostorage buildings where it is stockpiled for shipment.

For this project, the composition of the bauxite used is as follows:Table 2: Composition of Bauxite [7]ConstituentMonohydrate Grade (wt %)Al2O355Available Al2O350Fe2O312SiO25TiO23Other (mainly H2O)25

Trihydrate Grade (wt %)

5044174326

For the monohydrate grade, of the available Al2O3, 40% is from gibbsite and 10% isfrom boehmite. Whereas for the trihydrate grade, all available Al2O3 consists ofgibbsite.

4.3 Aspen model diagram

NaOHB14MixerB15HeatxCa(OH)2TrihydrateGradeBauxiteB9FlashMonohydrateGrade BauxiteB1MixerB2RstoicB3MixerB4RstoicB5FlashB6SepB7SepB10RstoicRigid FilterCakeB11SepB12HeaterB13HeaterAluminaProductCaOCO2B8MixMudwasteFigure 4: Aspen model block diagram for manufacture of Alumina Done by Chuen Chaniii

1E406 Individual Inquiry Aspen Plus™ SimulationThe simulation is based on Gladstone Alumina refinery with the following feed rate :Table 3: Amount of feed rate [7]Component

Monohydrate grade bauxiteTrihydrate grade bauxiteCaOCa(OH)2NaOH

Flow rate (tons/hr)725902014.773.52

The product will be a smelting-grade alumina with the following specification:Table 4: Smelting grade alumina [6]Item

Particle sizeFe2O3SiO2CaOTiO2

Normal range10 – 200 µm0.01 – 0.04 wt%0.01 – 0.03 wt%0.02 – 0.08 wt%0.002 – 0.005 wt%

Many of the simulated units are simplified. For example, only one flash unit is beingsimulated while realistically, there should be a series of flash vessels. Assumptions andbasis also has to be included for the simulation. Below gives the description,assumptions and basis for each block.

4.3.1 Digestion (B1 to B4)

B1 (Mixer)

Ø This is used to simulate part of the bauxite preparation whereby the raw feed ismixed together with the caustic and slaked lime to form a slurry.

Ø Assumptions – the bauxite has been pre-grinded in compartment rod/ball millsto allow better solid and liquid contact during digestion.B2 (Rstoic)

Ø This block simulates the digestion units. The units are operated at a temperatureof 573K and a pressure of 3500kPa [7].

Ø Assumptions – only the following reactions takes place:1. 2NaOH+Al2O3→2NaAlO2+H2O 2. CO2+2NaOH→Na2CO3+H2O

⇒ Digestion⇒ Carbonation

Ø Digestion – this reaction has been described earlier. For this simulation, it is

assumed that 97% of Al2O3 has been converted to NaAlO2 [7].

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1E406 Individual Inquiry Aspen Plus™ SimulationØ Carbonation – this reaction occurs naturally with the carbon dioxide from the

atmosphere [6]. This reduces the effectiveness of the liquor (caustic liquor) todissolve alumina.B3 (Mixer)

Ø This allows the addition of Ca(OH)2 which will be used for the later reactions in

the digesters.B4 (Rstoic)

Ø This block again, simulates the digestion units whereby only causticization

reaction occurs.

1. Na2CO3+Ca(OH)2→CaCO3+2NaOH

⇒ Causticization

Ø Caustization – as mentioned previously, the above reaction is to regenerate

NaOH and CaCO3 will be removed later. It is assumed that all Na2CO3 has beenreacted [7].B5 (Flash2)

Ø Flashing units are simulated in this block. Flash tanks are used for heat recovery

purposes.

Ø As the second year students may not fully understand a flash unit, a short

description of the flash unit will be included in the teaching tool. See AppendixB.

Ø The simulation is done with an outlet temperature of 383K and outlet pressure of

250kPa [6].

4.3.2 Clarification (B6 to B8)

B6 (Sep)

Ø This block is used to simulate settlers.

Ø All unreacted raw feed (Al2O3, Fe2O3, SiO2 and TiO2 and CaCO3) is considered

as solids.

Ø This simulation is run with an underflow containing 25% solids [8].

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1E406 Individual Inquiry Aspen Plus™ SimulationB7 (Sep)

Ø This block is used to simulate Kelly-type constant pressure filters whereby all

remaining solids are separated and the liquid leaving the filter area containsalumina in a clear supersaturated liquid.B8 (Mixer)

Ø This simulates the washer units. The underflow from the previous settler is

mixed together with water from the heat recovery sections. The product is calledmudwaste.

4.3.3 Precipitation (B9 to B11)

B9 (Flash2)

Ø The resultant supersaturated liquid needs to be cooled before precipitation. This

is done in vacuum flash vessels, which is simulated by this block.Ø The target outlet temperature is 345K [6].B10 (Rstoic)

Ø This block is used to simulate precipitators. The following ‘crystallization’reaction takes place:

1. 2NaAlO2+H2O→Al2O3+2NaOH

⇒ Precipitation

Ø It is assumed that all NaAlO2 has been crystallized to Al2O3.B11 (Sep)

Ø Slurry leaving the precipitators is being separated in 3 classifiers, namely,primary, secondary and tertiary. But for simplicity, 1 single unit is beingsimulated.

Ø It is simulated on an overflow concentration of about 3 g/L of alumina.

Ø The resultant overflow liquid is termed as spent liquor and is heated beforebeing recycle back to the digesters.

4.3.4 Calcination (B12 and B13)

B12 (Heater)

Ø Calcination equipment (rotary kilns) is simulated for this block. The underflowfrom the primary classifier (B10) is heated to temperatures above 1373K toremove water [7].

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1E406 Individual Inquiry Aspen Plus™ SimulationB13 (Heater)

Ø The product is then cooled in a series of equipment (initially in rotary coolersand then, in fluidised-bed coolers) to room temperature.

4.3.5 Heat recovery system (B14 and B15)

B14 (Mixer)

Ø This is used to simulate a mixer whereby fresh NaOH solution is added to thecaustic liquor recycled from the classifier (B10).B15 (Heatx)

Ø The flash heat generated from flasher units (B5) is used to preheat incomingcaustic liquor in tubular heat exchangers. Condensate from this heat exchangersis used to wash the mudwaste (B22).

4.4 Mass and Energy Balance

A Process Flow Diagram has been done for the above process. See Appendix A fordetails.

The following abbreviations are used for the equipment:Ø CR – CrusherØ V – Vessels

Ø R – Digesters (Reactors)Ø S – Settlers and ClarifiersØ F – Filter

Ø HX – Heat exchangersØ PR – PrecipitatorØ CL – CalcineØ M – MixerØ C - Cooler

4.5 Optimisation of the process

The critical operating unit for the process is the temperature and pressure of the digesterunits. As different operating values will have different conversions for Al2O3, this willaffect the final yield of the product.

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1E406 Individual Inquiry Aspen Plus™ SimulationFrom the simulation, another important variable is the flash temperatures and pressures.At the moment, the flash temperature and pressure used are 383K and 250kParespectively. A change in either of the variables will affect the composition of the vaporand the liquid streams, namely, the water and NaAlO2 concentrations. It would be idealto have as low concentration of NaAlO2 at the vapor stream. When increasing thetemperature, it was found that water concentration has been increased in the vaporstream, but NaAlO2 concentration did not changed much. An increase in pressure willhowever result in both concentration decreases in the liquid stream.

Classifier’s overflow composition can also be changed to control the amount of NaOHreturn to the digesters. This will in turn affect the amount of NaOH recycled back to thedigesters for reaction.

The above has been converted to a teaching tool based on structure and analysis of thewhole process. Due to complex nature of the process, no sensitivity analysis has beenincluded.

5. Conclusion and Recommendations

It has been shown that it is possible to simulate processes using Aspen software. To beas realistic as possible, operating values for each unit is required. Otherwise, heuristicassumptions have to be made. Both processes have been simplified and scenarios wereintroduced into the teaching tools.

5.1. The Manufacture of Vinyl Chloride Monomers

Recommendations for further work on the above is as follows:

Ø An expansion from the process to include manufacture of polyvinyl chloride resinsfrom the monomers.

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1E406 Individual Inquiry Aspen Plus™ Simulation5.2The Manufacture of Alumina

Recommendations for further work on the above is as follows:

Ø To simulate the above process as a solids handling process. This will then includethe pre-treatment process of crushing and particle size distribution (PSD) will haveto be specified. This will make the simulation more realistic.

Ø Where possible, use the actual unit for simulation. For example, use a filter insteadof a separator to simulate filtration process. Another example would be the use ofcrystalliser instead of a reactor to simulate precipitation process.Ø Impurities such as Na2O should be considered.

Ø Another further challenge would be to simulate from the product alumina toaluminium by the Hall-Héroult process [6].

Ø It would be seem that the block diagram is too complicated for 2nd Year students.Hence, one proposed strategy is to ‘break’ the process to 4 major processes andultimately integrate them into one whole process.

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1E406 Individual Inquiry Aspen Plus™ Simulation6. References

[1]. Caroline Crosthwaite, 1E202 Process systems analysis study guide,

Department of Chemical Engineering UQ, 2000, pages 4 and 5.

[2]. Lyle F.Albright, Processes for Major Addition-Type Plastics and their

Monomers, McGraw Hill, 1974, pages 177, 191, 213 table 6-1.[3]. Warren et al., Process Design Principles – Synthesis, Analysis, and

Evaluation, Wiley & Sons, 1999, pages 45 to 55.

[4]. John J. McKetta, Encyclopedia of Chemical Processing and Design, Vol 3,

Marcel Dekker Inc. 1976, pages 57, 58

[5]. Wolfgang et al., Ulmann's Encyclopedia of Industrial Chemistry, Vol A1, 5th

edition, VCH Publishing, 1985, pages 568, 570, 572, 574, 579.

[6]. A.R. Burkin, Production of Aluminium and Alumina, Wiley & Sons, 1987,

pages 15, 16, 19, 21, 25 to 27, 33, 37.

[7]. Queensland Alumina Limited Company brochure – The Gladstone Alumina

Refinery, Queensland Alumina Ltd

[8]. Stanley M.Walas, Chemical Process Equipment – Selection and Design,

Butterworth, 1990, page 320 table 11.9.

[9]. Nist Chemistry Webbook, http://webbook.nist.gov/[10]. Engineering and Applied Science Knowledge, http://knovel.com/[11]. Aspen Plus™ Version 10.1 help file.

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1E406 Individual Inquiry Aspen Plus™ SimulationAppendix B

Teaching Tools – Assignments/Tutorial

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1E406 Individual Inquiry Aspen Plus™ SimulationThe Manufacture of Vinyl Chloride Monomers (VCM)

The manufacture of vinyl chloride, a monomer intermediate for production of polyvinylchloride, is an important plastic that is widely used for wire and cables, paper and textilecoatings, and other domestic uses. It is considered as a commodity chemical that isproduced continuously throughout the world. Vinyl chloride is an extremely toxicsubstance, and therefore, industrial plants that manufacture it or process it must bedesigned carefully to satisfy governmental health and safety regulations.Build and use Aspen simulations to answer the following questions.

1. A proposed method of manufacturing vinyl chloride monomers is to thermally crackdichloroethane from chlorination of ethylene.

The model uses a fresh feed containing chlorine and ethylene in a 2½:1 ratio,contaminated with carbon disulphide (CS2) in a 1:500 ratio. This fresh feed of mixtureis fed to a reactor operating at 343 – 363K and 1.5atm where the following reactiontakes place:

C2H4+Cl2→C2H4Cl2

The conversions for this reactor range from 20 to 98%, depending on the reactionconditions. The outlet of the reactor will be mixed with the recycled dichloroethane andcondensed fully to liquid phase before being pumped to an evaporator. After theevaporator, the vapor goes through the pyrolysis furnace operating at 773 – 788K wherethe dichloroethane undergoes thermal cracking to form VCM and hydrogen chloride.

C2H4Cl2→C2H3Cl+HCl

Conversion from 20 to 65% is possible depending on the furnace conditions. The hotstream leaving the furnace is quenched to reduce carbon deposition in heat exchangerand later condensed before entering the distillation column to separate the differentcomponents. Hydrogen chloride is first separated from the first column while the secondcolumn separates the VCM. A purge stream is introduced to prevent accumulation ofunreacted components. The concentration of ethane in the combined recycle plus freshfeed to the reactor must not exceed 0.02% of CS2. Production of 37.8 tons of VCM(99.9 %wt purity) per hour is required.

a) Draw the flow diagram for the above synthesis loop and the corresponding Aspenmodel diagram.

b) Present a degrees of freedom analysis for your model.

c) Show how the recycle stream flow rate and the reactor feed stream flow rate dependon the chlorinator and pyrolysis conversion rates.

d) Do an economic potential of the above at input-output level.

e) Investigate the effect of changing the purge stream split fraction on the fresh feedrate, and the composition of the stream entering the pyrolysis reactor. What is theminimum purge stream split fraction that can be used to satisfy the ethaneconstraint?

The Manufacture of Alumina from Bauxite

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1E406 Individual Inquiry Aspen Plus™ SimulationAluminum is considered to be the most abundant metallic element in the earth’s crustand it’s the third most common element around. It is not found in its elemental form butalways tenaciously held in many compounds, most of which contain oxygen or silica, orboth. The most important aluminous ore for the manufacture of aluminium is bauxite. Itconsists of several hydrous aluminium oxide phases [gibbsite (Al(OH)3), boehmite(AlOOH) and diaspore (Al2O3 H2O)] as well as normal impurities such as Fe2O3, SiO2and TiO2. The difference in composition between the minerals lies in differentgeographical locations.

The manufacture of alumina from bauxite is a very complex process but is mainlyconsists of four major steps:Ø DigestionØ ClarificationØ PrecipitationØ Calcination

For more detail description of the above, go to website www.qal.com.auThe process flow is shown on figure 1.

Process Analysis

a) Convert the above alumina process based on Queensland Alumina refinery intoAspen simulation.

b) Present the above process in a process flow diagram with mass and energy balancesand the corresponding Aspen block diagram.

c) Do a degrees of freedom analysis using the Aspen block diagram.

To simplify the analysis, make use of the following assumptions for your Aspen model:The bauxite is consists of two grades – monohydrate grade and trihydrate. Thecomposition of the two grades are as follows:ConstituentMonohydrate Grade (wt %)Trihydrate Grade (wt %)Al2O35550Available Al2O35044Fe2O31217SiO2TiO233Other (mainly H2O)2526Other feeds include NaOH, CaO and Ca(OH)2The feed rates are as followsComponentFlow rate (tons/hr)Monohydrate grade bauxite725Trihydrate grade bauxite90CaO20Ca(OH)214.7NaOH73.52Ø Conversion for the digestion reaction is 97% while the rest is 100%.

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1E406 Individual Inquiry Aspen Plus™ SimulationØ The flash unit (see bottom for description) outlet temperature is to be 383K and

outlet pressure of 250kPa while the vacuum flash unit is to be at 345K.

Ø The underflow of the clarification unit consists of 25% unreacted components.Ø All solids entering the filter will be filtered out.

Ø A single unit can be simulated for the separation units in the precipitation sectionwith an overflow Al2O3 concentration of 3g/L.

Ø The calciners are operating at 1373K and the final product is cooled to roomtemperature.Short description of flash unit

A flash unit is a vessel whereby a phase change is induced due to pressure difference, toseparate the components. For the above process, product exiting the digesters is at hightemperature and pressure. It is introduced into a lower pressure vessel whereby some ofthe liquid vaporises and some condenses. The vapor is then used for heating purposes.

Vapor

Low PressureVesselHigh PressureProductLiquid

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1E406 Individual Inquiry Aspen Plus™ SimulationFigure 1: Process Flow of Alumina Process

(Reprinted with permission from Queensland Alumina Limited)

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1E406 Individual Inquiry Aspen Plus™ SimulationAppendix C

Teaching Tools – Sample Answers

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1E406 Individual Inquiry Aspen Plus™ SimulationThe Manufacture of Vinyl Chloride Monomers (VCM)

1 (a)

Flow Diagram for the Vinyl Chloride Monomer Synthesis

HClFeedChlorinationEvaporatorPyrolysisQuencherCondenserDistillationC2H3ClPurgeAspen model diagram

HClFeedB1RstoicB2HeaterB3MixerB4PumpB5HeaterB6RstoicB7HeaterB8HeaterB9SepC2H3ClB10SepB11FsplitPurge1 (b) Degrees of Freedom Analysis

Number of Components, n = 6 (Cl2, C2H4, C2H4Cl2, C2H3Cl, HCl, CS2)Number of reactions in B1 and B7, r = 1B1:B2:B3:B4:B5:B6:B7:B8:B9:B10:B11:

Rstoic= c + 4 + r Heater

Mixer= n(c + 2)PumpHeater

Rstoic= c + 4 + rHeaterHeaterSepSepFsplit

= c + 5= c + 4= 2c + 4= c + 3= c + 4= c + 5= c + 4= c + 4= 2c + 2= 2c + 2= c + 3= 14c + 40

= 11(c + 2) = 11c + 22= (14c + 40) – (11c + 22) = 36

Therefore, totalInterconnecting streamsHence, remaining

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1E406 Individual Inquiry Aspen Plus™ SimulationSpecifications:FeedB1 (Rstoic)B2 (Heater)B4 (Pump)B5 (Heater)B6 (Rstoic)B7 (Heater)B8 (Heater)B9 (Sep)B10 (Sep)B11 (Fsplit)Total

= 8= 3= 2= 1= 2= 3= 2= 2= 6= 6= 1= 36

(Pressure, Temperature, all 0% except Cl2, C2H4 and C2H6(Composition))

(Pressure, Temperature, % conversion of C2H4)(Pressure, Temperature)(Pressure)

(Pressure, Temperature)

(Pressure, Temperature, % conversion of C2H4Cl2)(Pressure, Temperature)(Pressure, Temperature)

(Top stream = 100% HCl, the rest 0%)(Top stream = 100% C2H3Cl, the rest 0%)(Purge fraction)

Degrees of Freedom = Remaining – Specifications

= 36 – 36= 0Therefore, the model is solvable.

1(c) Sensitivity Analysis on Conversion rates

A sensitivity analysis was done to show the effects of different conversion rates

(chlorination and pyrolysis furnace) to the feed flow rate and recycle stream flow rate.See figures 1 and 2 for graphs.

Both graphs have shown that at low conversion rates, both feed and recycle flow ratesare increased. Whereas at high conversion rates, both feed and recycle flow rates aredecreased.

1 (d) Economic Potential at input-output level

Cl2C2H4Manufacture ofVinyl ChlorideMonomerC2H3ClHClThe following are the prices taken from Chemical Market Reporter (October 2, 2000)Current exchange rate is US$1 = A$1.

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1E406 Individual Inquiry Aspen Plus™ SimulationCost per tonComponentCl2481.95C2H41001.70C2H3Cl793.8HCl136.08

Economic potential = (ProductVCM + ByproductHCL) – (FeedCL2 + FeedC2H4)See figure 3 for results.

The graph has shown that at higher conversion rates, profit will increase whereas therewill be lower profits at lower conversion rates.1(e) Sensitivity Analysis on Split Fraction

From the above analysis, it was found that the minimum purge stream split fraction thatcan be used to satisfy the carbon disulphide constraint is 0.04. At this split fraction, thecomposition of CS2 is 0.018%.

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1E406 Individual Inquiry Aspen Plus™ SimulationThe Manufacture of Alumina from Bauxite

Aspen Block diagram

NaOHB14MixerB15HeatxCa(OH)2TrihydrateGradeBauxiteB9FlashMonohydrateGrade BauxiteB1MixerB2RstoicB3MixerB4RstoicB5FlashB6SepB7SepB10RstoicRigid FilterCakeB11SepB12HeaterB13HeaterAluminaProductCaOCO2B8MixMudwasteDegress of Freedom Analysis

Number of Components, n = 12 (Al2O3, SiO2, FeO2, TiO2, NaOH, CO2, CaO,

Ca(OH)2, NaAlO2, CaCO3, Na2CO3, H2O)

Number of reactions in B2, r = 2 while B4 and B10, r = 1B1:B2:B3:B4:B5:B6:B7:B8:B9:B10:B11:B12:B13:B14:B15:

Mixer= n(c + 2) Rstoic = c + 4 + rMixer= n(c + 2)Rstoic = c + 4 + rFlashSepSep

Mixer = n(c + 2)Flash

Rstoic = c + 4 + rSepHeaterHeater

Mixer = n(c + 2)Heatx

= 5c + 10= c + 6= 2c + 4= c + 5= c + 4= 2c + 2= 2c + 2= 3c + 6= c + 4= c + 6= 2c + 2= c + 4= c + 4= 2c + 4= 2c + 7= 27c + 69

= 21(c + 2) = 21c + 42= (27c + 69) – (21c + 42) = 99

Therefore, totalInterconnecting streamsHence, remaining

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1E406 Individual Inquiry Aspen Plus™ SimulationSpecifications:Feed 1(Monohydrate)Feed 2 (Trihydrate)Feed 3 (CaO)Feed 4 (CO2)Feed 5 (Ca(OH)2)Feed 6 (NaOH)B2 (Rstoic)B4 (Rstoic)B5 (Flash)B6 (Sep)B7 (Sep)B9 (Flash)B10 (Rstoic)B11 (Sep)B12 (Heater)B13 (Heater)B15 (Heatx)Total

Degrees of Freedom

= 9= 9= 13= 13= 13= 13= 4= 3= 2= 5= 4= 2= 3= 1= 2= 2= 1= 99

= Remaining – Specifications= 99 – 99= 0

(Pressure, Temperature, all 0% except Al2O3, SiO2, FeO2, TiO2 and H2O (Composition))(Pressure, Temperature, all 0% except Al2O3, SiO2, FeO2, TiO2 and H2O (Composition))(Pressure, Temperature, all 0% except CaO)(Pressure, Temperature, all 0% except CO2)(Pressure, Temperature, all 0% except Ca(OH)2)(Pressure, Temperature, all 0% except NaOH)(Pressure, Temperature, % conversion of both reactions)

(Pressure, Temperature, % conversion)(Pressure, Temperature)

(Bottom composition - 25 % of Al2O3, SiO2, FeO2, TiO2 and CaCO3)

(Bottom composition – all Al2O3, SiO2, FeO2, TiO2)

(Pressure, Temperature)

(Pressure, Temperature, % conversion)(Top composition – 3 g/L of Al2O3)(Pressure, Temperature)(Pressure, Temperature)

(No vapor in the cold stream)

Therefore, the model is solvable.

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