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Chapter 7

Basics of Process Simulation With SimSci PRO/II Chien Hwa Chong Taylor’s University, Subang Jaya, Malaysia

This chapter aims to provide a step-by-step guide in simulating an integrated process flowsheet using SimSci PRO/II (Schneider Electric, 2015). The concept of simulation is based on sequential modular approach and follows the onion model for flowsheet synthesis (see Chapter 1 for details). The case study on n-octane production (Example 1.1) is used for illustration throughout the chapter.

7.1 EXAMPLE ON N-OCTANE PRODUCTION A simple example that involves the production of n-octane (C8H18) is demonstrated (Foo et al., 2005), with detailed descriptions given in Chapter 1. The basic simulation setup involving the registration of components, thermodynamic model, and reaction stoichiometry is to be carried out in the Component Selection window, Thermodynamic window, and Reaction Component window of PRO/II, respectively, while the other steps are carried out in the flowsheet. The individual steps are discussed in the following subsections.

7.2 STAGE 1: BASIC SIMULATION SETUP 7.2.1 Units A user can select different “Units of Measure” such as English-Set1, MetricSet1, and SI-Set1 prior to start of a new process flowsheet (refer detailed steps for change of units in Fig. 7.1).

7.2.2 Component Selection User is required to define the components, viz., nitrogen, ethylene, i-butane, n-butane, and n-octane using the component selection from the “Input” windows features (refer detailed steps for component selection in Fig. 7.2). Chemical Engineering Process Simulation. http://dx.doi.org/10.1016/B978-0-12-803782-9.00007-8 Copyright © 2017 Elsevier Inc. All rights reserved.

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FIGURE 7.1 Units of measure of the PRO/II.

FIGURE 7.2 Component selection from the system or user-generated databank.

7.2.3 Thermodynamics Method Thermodynamic methods are used to simulate the physical behavior of component system by calculating several physical properties (refer detailed steps for thermodynamic data in Fig. 7.3). For this process, PengeRobinson model1 is to be used.

1. See Chapter 3 for detailed discussion on thermodynamic selection.

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FIGURE 7.3 Selection of property calculation system using the SimSci-thermodynamic data.

7.3 STAGE 2: MODELING OF REACTOR To select the equipment, a user can choose unit operations from the right side column of the flowsheet. A list of unit operation models are classified as general, pressure change, column, reactors, heat exchanger, solid, batch, utilities, user-aided, classic, and miscellaneous categories. For this case, a conversion reactor is used for demonstration. The user is required to create an input stream, S1, and an output stream, S2, after selecting the conversion reactor, R1, from the right panel (refer detailed steps Fig. 7.4).

FIGURE 7.4 Construction of flowsheet topology on a process flow diagram.

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TABLE 7.1 Specifications for Process Feed Stream (Foo et al., 2005) Components

Flowrate (kg-mol/h)

Condition

Ethylene (C2H6)

20

i-Butane (i-C4H10)

10

T: 93 C

Nitrogen (N2)

0.1

P: 20 psia

n-Butane (n-C4H10)

0.5

FIGURE 7.5 Defining feed stream properties.

We then proceed to define reactor feed stream properties. Component flowrates, temperature (T), and pressure (P) of the stream are given in Table 7.1. Detailed steps for this step are shown in Fig. 7.5. For the conversion reactor, the user is required to key in the reaction data. Double-click on the reactor column to key in thermal specification, extent of reaction, and reactor data (see Fig. 7.6 for detailed steps). Reaction stoichiometry and fractional conversion must be supplied for a conversion reactor simulation. It can be defined in the Reaction Data Sets window. We proceed to define pressure drop and extent of reaction for the reactor (see detailed steps in Fig. 7.7). On the completion of specifying the reactor, the user can proceed to execute the simulation (Fig. 7.8) and examine the simulation results (Fig. 7.9).

7.4 STAGE 3: MODELING OF SEPARATION UNITS A flash drum is used to separate the reactor effluent stream (S2) into vapor top (S3) and liquid bottom (S4) streams. Pressure drop and temperature of the

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FIGURE 7.6 Specifying reaction data for the process.

FIGURE 7.7 Specifying pressure drop and extend of reaction for the reactor.

FIGURE 7.8 Simulation report of the conversion reactor.

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FIGURE 7.9 Displaying component molar flowrates on the process flow diagram.

FIGURE 7.10

Adding a flash model for the primary separation process.

flash column are set at 2.0 psi and 93 C C, respectively. Detailed steps for adding a flash model are shown in Fig. 7.10. The next step is to simulate a rigorous distillation model. The column is used to simulate any vapor/liquid separation process. When a column condenser checkbox is selected, it is always designated as tray number 1, meanwhile the column reboiler is always designated as the highest numbered tray in the model (refer detailed steps for launching a rigorous distillation model in Fig. 7.11). Data needed to simulate a rigorous distillation model are shown in Table 7.2. In PRO/II simulator, either a feed or a heat duty on the top and bottom trays must be specified. In addition, the pressure must be defined. The user is required to select suitable algorithm for a rigorous distillation simulation. Next, the feeds and products phase, tray, and rate are set. There are some commonly used algorithms for modeling of distillation columns in PRO/II, viz., the inside-Out, Sure, Chemdist, Enhanced IO, Electrolytic, and

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FIGURE 7.11 Launch a rigorous distillation model in PRO/II.

TABLE 7.2 Specification for a Rigorous Distillation Model Specification Pressure

15 psia (top)

Number of trays

10

Reflux ratio

10

Feed tray

4

Overheads rate, kgmol/h

0.45

Bottom rate, kgmol/h

0.73

Ethylene flowrate, kgmol/h

0.359

Condenser type

Partial

20 psia (bottom)

Eldist algorithm, with the default being inside-out algorithm (PRO/II 9.3.2, Reference Manual, 2015). This method is effective and fast in solving vapor/ liquid staged column models. For this case, the “Chemdist” algorithm is selected. Table 7.3 shows suitable systems for different algorithms. The “Eldist” is recommended for electrolyte and equilibrium electrolytic reactions. The “LLEX” can be used for liquideliquid extractor; chemical reaction; and kinetic, equilibrium (nonelectrolyte), and conversion. For nonpower law kinetics, the “Chemdist” algorithm is more suitable. For hydrocarbon system where water is present, the “Sure” algorithm is recommended. The Sure and Chemdist algorithms also can be used to model vapor-liquid-liquid

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TABLE 7.3 Algorithm for Specific Systems (PRO/II 9.3.2, Reference Manual 2015, Schneider Electric) System

Algorithm

Electrolyte

Eldist

Equilibrium electrolytic reactions

Eldist

Liquideliquid extractor

LLEX

Chemical reaction

Chemdist, LLEX, RATERFRAC

Kinetic, equilibrium (nonelectrolyte) and conversion reactors

Chemdist (nonideal chemical system)da NewtoneRaphson method, LLEX

Kinetic and equilibrium (nonelectrolyte)

RATERFRAC

Nonpower law kinetics

Chemdist

Hydrocarbon system where water is present Refinery and chemical systems

Sure

FIGURE 7.12

Specifying algorithm and pressure profile for the distillation column.

equilibrium systems. Detailed steps for launching and specifying algorithm of a rigorous distillation model with pressure profile are given in Fig. 7.12. Next, the condenser type is set to “Partial” (see Fig. 7.13). For feeds and products specification, refer to Fig. 7.14 to specify feed tray and flowrates for S5 and S6.

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FIGURE 7.13 Select type of condenser for the rigorous distillation column.

FIGURE 7.14 Specifying feed tray and rates of overhead and bottoms for the distillation column.

Next, the user is required to provide two specifications for the distillation model. It is suggested to use the specific component flowrates and reflux ratio for distillation column simulation. Detailed steps for specifying the variables for the column are shown in Fig. 7.15.

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FIGURE 7.15 Column specification and variables for the distillation column.

7.5 STAGE 4: MODELING OF RECYCLE SYSTEMS In PRO/II software, recycle techniques are divided into three main categories, viz., thermal recycle, mass and energy balance recycles, and purge/makeup systems. The thermal recycle technique is used for heat exchanger networks where only the stream temperature changes. To use this method, users need to specify the output conditions. As long as all data stay within the specified tolerance level, the recycle loop will converge. Recycle convergence in PRO/II consists of direct-substitution, Wegstein acceleration technique, and Broyden acceleration (Fig. 7.16). The user can select the Problem Recycle Convergence and Acceleration Options from the

FIGURE 7.16 Steps to modify recycle convergence for the process.

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FIGURE 7.17 Splitter specification for recycle process.

input button from the main window. The direct-substitution method used the last computed values for the tear streams for the next trial solution of the recycle loop; hence, it is relatively slower as compared to other methods.2 For the n-octane case, excessive ethylene is recycled back to the reactor. Prior to recycle, impurities can be purged using a splitter. Remember, the splitter unit must have two or more product streams and it requires specification in terms of rate of a component, recovery of a fraction of the total feed, etc. It is only for identical component composition separation. The splitter calculation is based on the selected outlet pressure parameter and stream specification data. In this case study, a splitter is added to the flowsheet to remove impurities. 95% of all components with flowrate of 1.2 kgmol/h are recycled. Fig. 7.17 shows information required and detailed steps in simulating a component splitter. To reduce the energy losses, temperature and pressure of the recycle stream are required to be adjusted following the feed stream, S1. Fig. 7.18 shows detailed steps for simulating the compressor and cooler. To recycle stream, S10, the user is required to reroute stream, S1. Fig. 7.19 shows steps in setting “Breakpoints” prior to create a recycle loop. The reroute command is a specific feature in PRO/II used to recalculate an unobstructed path and correct any problems caused by moving a connection or a unit operation icon and the “Breakpoints” command allows the user to make changes in the input data (PRO/II 9.3.2, Reference Manual, 2015). Fig. 7.20 shows detailed steps in creating a “Breakpoints” for the reactor, R1. 2. See Chapter 4 for more in-depth discussion of this topic.

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FIGURE 7.18 A compressor and a cooler simulation steps for the process.

FIGURE 7.19 Reroute the breakpoints stream for recycle purposes.

Furthermore, the user is required to add on a mixer, M1, prior to connecting to the reactor, R1 (see detailed steps in Fig. 7.21) to connect the feed and recycle stream. Fig. 7.22 shows the steps in simulating heat-integrated flowsheet. In the final steps, user can add on a worksheet to the process flow diagram (Fig. 7.23).

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FIGURE 7.20 Specifying the breakpoints placement before the unit.

FIGURE 7.21

Steps in completing a recycle loop on process flow diagram.

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FIGURE 7.22 A complete integrated flowsheet with heat integration.

FIGURE 7.23 Integrated flowsheet with a properties table.

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7.6 CONCLUSION A four-stage integrated process flowsheet was developed using SimSci PRO/II to produce n-octane (C8H18). All basic simulation setup starts from Component Selection window, Thermodynamic window, Reaction Component window of PRO/II followed by simulation in flowsheet. To recycle valuable raw materials, “reroute” and “breakpoints” functions are needed. It is advisable to limit 100 iterations in column simulation. No specification is required for recycle stream for PRO/II.

EXERCISES 1. Simulate the following separation process using data provided in Tables E1 and E2. A single-state expansion of a light hydrocarbon stream. Firstly, feed stream is cooled through a feed chiller (X-1) and any liquid is removed with a vertical separator (F-2). The vapor is expanded (E-4) and liquids are separated using a second vertical separator (F-3).

TABLE E1 Specifications for Process Feed Stream Components

Flowrate (lb-mol/h)

Condition

Methane

6100

T: 90 F

Ethane

500

P: 994.7 psia

Propane

200

N-pentane

100

N-hexane

70

TABLE E2 Equipment Specifications Equipment

Tag

Specification

Feed chiller (Model: Exchanger 1)

X-1

Temperature out ¼ 35 F Delta pressure ¼ 10 psi

Two Vertical separators (Model: Flash 1)

F-2 F-4

Adiabatic flash Delta pressure ¼ 0

Expander

E-3

Pressure out ¼ 275 psia Efficiency ¼ 0.80

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FIGURE E1

Process flow diagram for the hydrodealkylation process (Seider et al., 2004).

2. (Fig. E1) A hydrodealkylation process is adapted from Seider et al. (2004), where toluene is converted to benzene with a side reaction to produce biphenyl (see PFD in Fig. E1). Process feed stream (Table E3) is preheated in a furnace to 500 C along with a recycle stream, before being fed to the reactor. The stream properties are shown in Table E3. The reactor operates with high conversion rate (70%), with effluent temperature of 650 C and pressure drop of 5.0 psi. The reaction stoichiometries are given as follows. Main reaction (R-1): C7 H8 þ H2 / C6 H6 þ CH4

TABLE E3 Specification for Process Feed Stream Components

Flowrate (kg-mol/h)

Condition

H2 (hydrogen)

1200

T ¼ 25 C

CH4 (methane)

1500

P ¼ 600 psia

C6 H6 (benzene)

0

C7 H8 (toluene)

120

C12 H10 (biphenyl)

0

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Side reaction (R-2): 2C6 H6 / C12 H10 þ H2 Effluent of the reactor is sent to a Flash unit, where hydrogen and methane (top stream) are separated from toluene, benzene, and biphenyl (bottom stream). The bottom stream will be sent to further purification steps. However, for brevity, the purification steps are approximated using a component splitter (using Splitter model in SimSci PRO/II), where toluene product is withdrawn, while other components are mixed with reactor effluent and recycled to the Flash unit. For the Splitter model, 90% of toluene is set to recycle to the Flash unit, while benzene and biphenyl are withdrawn as product stream. The latter will undergo further purification (excluded from consideration). A simple heat exchanger network can be designed to recover energy from feed stream to the Flash unit, to minimize energy consumption of the furnace. Recommended thermodynamic package for this system is SoaveeRedlicheKwong equation of state.

REFERENCES Foo, D.C.Y., Manan, Z.A., Selvan, M., McGuire, M.L., October, 2005. Integrate process simulation and process synthesis. Chemical Engineering Progress 101 (10), 25e29. Schneider Electric, 2015. SimSci PRO/II 9.3.2, Reference Manual 2015. SimSci, USA. Seider, W.D., Seader, J.D., Lewin, D.R., 2004. Product and Process Design Principles. Synthesis, Analysis, and Evaluation, second ed. John Wiley and Sons, Inc., pp. 136e142