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4.5. SIMULATION OF UNIT OPERATIONS
4.5.1. Reactors
User-defined components are created in Aspen Plus using a ‘‘user-defined compo- nent wizard.’’ The minimum required information is the molecular weight and normal boiling point. The program also allows the designer to enter molecular structure, specific gravity, enthalpy, and Gibbs energy of formation, ideal gas heat capacity, and Antoine vapor pressure coefficients, but for complex molecules usually only the molecular structure is known.
It is often necessary to add user components to complete a simulation model. The design engineer should always be cautious when interpreting simulation results for models that include user components. Phase equilibrium predictions for flashes, decanters, extraction, distillation, and crystallization operations should be carefully checked against laboratory data to ensure that the model is correctly predicting the component distribution between the phases. If the fit is poor, the binary inter- action parameters in the phase equilibrium model can be tuned to improve the prediction.
Table 4.2. Unit Operation Models in Aspen Plus1and UniSim DesignTM
Unit Operation Aspen Plus Models UniSim Design Models
Stream mixing Mixer Mixer
Component splitter Sep, Sep2 Component Splitter
Decanter Decanter 3-Phase Separator
Flash Flash2, Flash3 Separator, 3-Phase Separator
Piping components
Piping Pipe, Pipeline Pipe Segment, Compressible Gas Pipe
Valves & fittings Valve Valve, Tee, Relief Valve
Hydrocyclone HyCyc Hydrocyclone
Reactors
Conversion reactor RStoic Conversion Reactor
Equilibrium reactor REquil Equilibrium Reactor
Gibbs reactor RGibbs Gibbs Reactor
Yield reactor RYield Yield-shift Reactor
CSTR RCSTR Continuous Stirred Tank Reactor
Plug-flow reactor RPlug Plug-flow Reactor
Columns
Shortcut distillation DSTWU, Distl, SCFrac Shortcut column
Rigorous distillation RadFrac, MultiFrac Distillation, 3-Phase Distillation
Liquid-liquid extraction Extract Liquid-Liquid Extractor
Absorption and stripping RadFrac Absorber, Refluxed Absorber, Reboiled Absorber Fractionation
Rate-based distillation Batch distillation
PetroFrac RATEFRACTM BatchFrac
3 Stripper Crude, 4 Stripper Crude, Vacuum Resid Column, FCCU Main Fractionator
Heat transfer equipment
Heater or cooler Heater Heater, Cooler
Heat exchanger HeatX, HxFlux, Hetran, HTRI-Xist Heat Exchanger
Air cooler Aerotran Air Cooler
Fired heater Heater Fired Heater
Multi-stream exchanger MheatX LNG Exchanger
Rotating equipment
Compressor Compr, MCompr Compressor
Turbine Compr, MCompr Expander
Pump, hydraulic turbine Pump Pump
Solids handling
Size reduction Crusher
Size selection Screen Screen
Crystallizer Crystallizer Crystallizer, Precipitation
Neutralization Neutralizer
Solids washing SWash
Filter Fabfl, CFuge, Filter Rotary Vacuum Filter
Cyclone HyCyc, Cyclone Hydrocyclone, Cyclone
Solids decanting CCD Simple Solid Separator
Solids transport Conveyor
Secondary recovery ESP, Fabfl, VScrub Baghouse Filter
User models User, User2, User3 User Unit Op
Conversion Reactor (Stoichiometric Reactor)
A conversion reactor requires a reaction stoichiometry and an extent of reaction, which is usually specified as an extent of conversion of a limiting reagent. No reaction kinetics information is needed, so it can be used when the kinetics are unknown (which is often the case in the early stages of design) or when the reaction is known to proceed to full conversion. Conversion reactors can handle multiple reactions, but care is needed in specifying the order in which they are solved if they use the same limiting reagent.
Equilibrium Reactor
An equilibrium reactor finds the equilibrium product distribution for a specified set of stoichiometric reactions. Phase equilibrium is also solved. The engineer can enter the outlet temperature and pressure and let the reactor model calculate the duty needed to reach that condition, or else enter a heat duty and let the model predict the outlet conditions from an energy balance.
An equilibrium reactor solves only the equations specified, so it is useful in situations in which one or more reactions equilibrate rapidly, while other reactions proceed much more slowly. An example is the steam reforming of methane to hydrogen. In this process, the water-gas-shift reaction between water and carbon monoxide equilibrates rapidly at temperatures above 4508C, while methane conver- sion requires catalysis even at temperatures above 8008C. This process chemistry is explored in Example 4.2.
In some simulation programs, the equilibrium reactor model requires the designer to specify both liquid and vapor phase products, even though one of the streams may be calculated to have zero flow. If the real reactor has a single outlet, then the two product streams in the model should be mixed back together.
Gibbs Reactor
The Gibbs reactor solves the full reaction (and optionally phase) equilibrium of all species in the component list by minimization of the Gibbs free energy, subject to the constraint of the feed mass balance. A Gibbs reactor can be specified with restrictions such as a temperature approach to equilibrium or a fixed conversion of one species.
The Gibbs reactor is very useful when modeling a system that is known to come to equilibrium, in particular high-temperature processes involving simple molecules. It is less useful when complex molecules are present, as these usually have high Gibbs energy of formation; consequently, very low concentrations of these species are predicted unless the number of components in the model is very restricted.
The designer must specify the components carefully when using a Gibbs reactor in the model, as the Gibbs reactor can solve only for specified components. If a component that is actually formed is not listed in the component set, then the Gibbs reactor results will be meaningless. Furthermore, if some of the species have high Gibbs free energy, their concentrations may not be properly predicted by the model.
An example is aromatic hydrocarbon compounds such as benzene, toluene, and xylenes, which have Gibbs free energy of formation greater than zero. If these
species are in a model component set that also contains hydrogen and carbon, then a Gibbs reactor will predict that only carbon and hydrogen are formed. Although hydrogen and coke are indeed the final equilibrium products, the aromatic hydro- carbons are kinetically stable and there are many processes that convert aromatic hydrocarbon compounds without significant coke yields. In this situation, the de- signer must either omit carbon from the component list or else use an equilibrium reactor in the model.
Continuous Stirred Tank Reactor (CSTR)
The CSTR is a model of the conventional well-mixed reactor. It can be used when a model of the reaction kinetics is available and the reactor is believed to be well mixed;
i.e., the conditions everywhere in the reactor are the same as the outlet conditions. By specifying forward and reverse reactions, the CSTR model can model equilibrium and rate-based reactions simultaneously. The main drawback of using the CSTR model is that a detailed understanding of kinetics is necessary if byproducts are to be predicted properly.
Plug-Flow Reactor (PFR)
A plug-flow reactor models the conventional plug-flow behavior, assuming radial mixing but no axial dispersion. The reaction kinetics must be specified, and the model has the same limitations as the CSTR model.
Most of the simulators allow heat input or removal from a plug-flow reactor. Heat transfer can be with a constant wall temperature (as encountered in a fired tube, steam-jacketed pipe, or immersed coil) or with counter-current flow of a utility stream (as in a heat exchanger tube or jacketed pipe with cooling water).
Yield-Shift Reactor
The yield-shift reactor overcomes some of the drawbacks of the other reactor models by allowing the designer to specify a yield pattern. Yield-shift reactors can be used when there is no model of the kinetics, but some laboratory or pilot plant data are available, from which a yield correlation can be established.
Yield-shift reactors are particularly useful when modeling streams that contain pseudocomponents, solids with a particle size distribution, or processes that form small amounts of many byproducts. These can all be described easily in yield corre- lations but can be difficult to model with the other reactor types.
The main difficulty in using the yield-shift reactor is in establishing the yield correlation. If a single point—for example, from a patent—is all that is available, then entering the yield distribution is straightforward. If, on the other hand, the purpose is to optimize the reactor conditions, then a substantial set of data must be collected to build a model that accurately predicts yields over a wide enough range of conditions. If different catalysts can be used, the underlying reaction mechanism may be different for each, and each will require its own yield model. The development of yield models can be an expensive process and is often not undertaken until corporate management has been satisfied that the process is likely to be economically attractive.
Modeling Real Reactors
Industrial reactors are usually more complex than the simple simulator library models.
Real reactors usually involve multiple phases and have strong mass transfer, heat transfer, and mixing effects. The residence time distributions of real reactors can be determined by tracer studies and seldom exactly match the simple CSTR or PFR models.
Sometimes a combination of library models can be used to model the reaction system. For example, a conversion reactor can be used to establish the conversion of main feeds, followed by an equilibrium reactor that establishes an equilibrium distri- bution among specified products. Similarly, reactors with complex mixing patterns can be modeled as networks of CSTR and PFR models, as described in Section 1.9.10 and illustrated in Figure 1.19.
When a combination of library models is used to simulate a reactor, it is a good idea to group these models in a subflowsheet. The subflowsheet can be given a suitable label such as ‘‘reactor’’ that indicates that all the unit operations it contains are modeling a single piece of real equipment. This makes it less likely that someone else using the model will misinterpret it as containing additional distinct operations.
Detailed models of commercial reactors are usually written as user models. These are described in Section 4.6.
Example 4.1
When heavy oils are cracked in a catalytic or thermal cracking process, lighter hydrocarbon compounds are formed. Most cracking processes on heavy oil feeds form products with carbon numbers ranging from 2 to greater than 20. How does the equilibrium distribution of hydrocarbon compounds with five carbons (C5 com- pounds) change as the temperature of the cracking process is increased at 200 kPa?
Solution
This problem was solved using UniSim Design.
The problem asks for an equilibrium distribution, so the model should contain either a Gibbs reactor or an equilibrium reactor.
A quick glance at the component list in UniSim Design shows that there are 22 hydrocarbon species with five carbons. To model the equilibrium among these species, we also need to include hydrogen to allow for the formation of alkenes, dienes, and alkynes. Although it would be possible to enter 21 reactions and use an equilibrium reactor, it is clearly easier to use a Gibbs reactor for this analysis.
Figure 4.5 shows the Gibbs reactor model.
To specify the feed, we must enter the temperature, pressure, flow rate, and com- position. The temperature, pressure, and flow rate are entered in the stream editor window, as illustrated in Figure 4.6. The feed composition can be entered as 100% of any of the C5paraffin species, for example, normal pentane. The results from a Gibbs reactor would be the same if 100% isopentane were entered. It should be noted, however, that if a mixture of a pentane and a pentene were specified, then the overall ratio of hydrogen to carbon would be different and different results would be obtained.
A spreadsheet was also added to the model, as illustrated in Figure 4.5, to make it easier to capture and download the results. The spreadsheet was set up to import component mole fractions from the simulation, as shown in Figure 4.7. The simula- tion was then run for a range of temperatures, and after each run a new column was entered in the spreadsheet, as shown in Figure 4.8.
When the results are examined, many of the individual species are present at relatively low concentrations. It thus makes sense to group some compounds together by molecular type, for example, adding all the dienes together and adding all the alkynes (acetylenes) together.
The spreadsheet results were corrected to give the distribution of C5compounds by dividing by one minus the mole fraction of hydrogen, and then plotted to give the graph in Figure 4.9.
It can be seen from the graph that the equilibrium products at temperatures below 5008C are mainly alkanes (also known asparaffinsorsaturated hydrocarbons), with the equilibrium giving roughly a 2:1 ratio of isopentane to normal pentane. As the temperature is increased from 5008C to 6008C, there is increased formation of alkene compounds (also known asolefins). At 7008C, we see increased formation of cyclo- pentene and of dienes, and above 8008C dienes are the favored product.
Figure 4.5. Gibbs reactor model.
Of course, this is an incomplete picture, as the relative fraction of C5compounds would be expected to decrease as the temperature is raised and C5 species are cracked to lighter compounds in the C2 and C3 range. The model also did not contain carbon (coke), and so could not predict the temperature at which coke would become the preferred product. A more rigorous equilibrium model of a cracking process might include all of the possible hydrocarbon compounds up to C7or higher.
A real reactor might give a very different distribution of C5compounds from that calculated using the Gibbs reactor model. Dienes formed at high temperatures might recombine with hydrogen during cooling, giving a mixture that looked more like the equilibrium product at a lower temperature. There might also be formation of C5
compounds by condensation reactions of C2and C3species during cooling, or loss of dienes and cyclopentene due to coke formation.
Example 4.2
Hydrogen can be made by steam reforming of methane, which is a highly endothermic process:
CH4þH2O$COþ3H2 DHrxn¼2:1105kJ=kgmol Figure 4.6. Stream entry.
Steam reforming is usually carried out in fired tubular reactors, with catalyst packed inside the tubes and fuel fired on the outside of the tubes to provide the heat of reaction. The product gas mixture contains carbon dioxide and water vapor as well as carbon monoxide and hydrogen and is conventionally known assynthesis gasor syngas.
Hydrogen can also be made by partial oxidation of methane, which is an exothermic process, but yields less product per mole of methane feed:
CH4þ12O2!COþ2H2 DHrxn¼ 7:1104kJ=kgmol
When steam, oxygen, and methane are combined, heat from the partial oxidation reaction can be used to provide the heat for steam reforming. The combined process is known as autothermal reforming. Autothermal reforming has the attraction of requiring less capital investment than steam reforming (because it does not need a fired-heater reactor), but giving higher yields than partial oxidation.
The yield of hydrogen can be further increased by carrying out the water gas shift reaction:
COþH2O$CO2þH2 DHrxn¼ 4:2104kJ=kgmol Figure 4.7. Product composition spreadsheet.
Figure 4.8. Spreadsheet results.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
400.00 500.00 600.00 700.00 800.00 900.00 1000.00 Temperature (C)
Fraction
22M propane cyclopentane cyclopentene i-pentane n-pentane dienes acetylenes pentenes methyl butenes
Figure 4.9. Product distribution.
The water gas shift reaction equilibrates rapidly at temperatures above about 4508C. At high temperatures this reaction favors the formation of carbon monoxide, while at low temperatures more hydrogen is formed. When hydrogen is the desired product, the shift reaction is promoted at lower temperatures by using an excess of steam and providing a medium- or low-temperature shift catalyst.
In an autothermal reforming process, 1000 kmol/h of methane at 208C is com- pressed to 10 bar, mixed with 2500 kmol/h of saturated steam and reacted with pure oxygen to give 98% conversion of the methane. The resulting products are cooled and passed over a medium-temperature shift catalyst that gives an outlet composition corresponding to equilibrium at 3508C.
i. How much heat is required to vaporize the steam?
ii. How much oxygen is needed?
iii. What is the temperature at the exit of the autothermal reforming reactor?
iv. What is the final molar flow rate of each component of the synthesis gas?
Solution
This problem was solved using Aspen Plus. The model must simulate the high temperature reforming reaction and also the re-equilibration of the water gas shift reaction as the product gas is cooled. A Gibbs reactor can be used for the high temperature reaction, but an equilibrium reactor must be specified for the shift reactor, as only the water gas shift reaction will re-equilibrate at 3508C. Because the methane compressor supplies some heat to the feed, it should be included in the model. Since the question asks how much heat is needed to vaporize the steam, a steam boiler should also be included. The oxygen supply system can also be included, giving the model shown in Figure 4.10.
The heat duty to the reforming reactor is specified as zero. The oxygen flow rate can then be adjusted until the desired methane conversion is achieved. For 98% conver- sion, the flow rate of methane in the autothermal reactor product (stream 502) is 2% of the flow rate in the reactor feed (stream 501), i.e., 20 kmol/h. For the purpose of this example, the oxygen flow rate was adjusted manually, although a controller could have been used, as described in Section 4.8. The results are shown in Figure 4.11.
When the simulation model was run, the following values were calculated:
i. The steam heater requires 36 MW of heat input.
ii. 674 kmol/h of oxygen is needed.
iii. The temperature at the exit of the reforming reactor is 8938C.
iv. The molar flow rates at the outlet of the shift reactor (stream 504) are
H2 2504
H2O 1956
CO 68
CO2 912
CH4 20
It should be immediately apparent from the model output that the process as simulated is far from optimal. The oxygen consumption is larger than the 500 kmol/h that would have been needed for partial oxidation. The excess oxygen is needed
because the additional steam that is being fed must also be heated to the reactor outlet temperature, which requires more of the feed methane to be burned. The corollary of this result is that the hydrogen yield, at roughly 2.5 moles per mole methane, is not much better than could have been obtained with partial oxidation followed by shift, despite the large excess of steam used.
The designer has several options that could be examined to improve this process:
1. Increase heat recovery from the product gas to the feed streams to preheat the reactor feed and reduce the amount of oxygen that is needed.
2. Reduce the amount of steam fed with the methane.
3. Bypass a part of the steam from the reformer feed to the shift reactor feed, so as to obtain the benefit of driving the equilibrium in the shift reactor without the cost of providing extra heat to the reformer.
4. Reduce the conversion of methane so that a lower reactor conversion and lower outlet temperature are required.
In practice, all of these options are implemented to some extent to arrive at the optimal autothermal reforming conditions. This optimization is explored further in problem 4.13.
Figure 4.10. Autothermal reforming model.