In Problem SM.1, you simulated the reactor in the styrene monomer project using chemical reaction data provided by the Research and Development Department of BEEF, Inc. The Reactor R1 has only one effluent stream, the vapor Stream S11, because the flow rate of Stream noLiq equals zero. Based on physical properties such as critical temperature, critical pressure, and normal boiling temperature for each reaction component, why should the effluent leaving the reactor be only in the vapor or gas phase? To verify the stream phase, it may help to think of a phase (PT) diagram for each pure component. Physical properties of various chemical components are given in Chapter 1. Click here to download a Word file, add your initials to its name, and complete all of the questions contained within.
A global flowsheet for a chemical process depicts basically the raw materials entering the
flowsheet and the product, byproducts and wastes leaving the flowsheet. Chapter 1 gives a block flowsheet for the chemical process of converting toluene and methanol to styrene monomer. A global flowsheet for the styrene monomer chemical process is shown below.
Raw Materials Reactor Effluent Separation Sequence Recycle Reactants
Off Gas Byproduct
Pure Product
Wastewater
This global flowsheet shows the reactor producing an effluent stream (containing toluene, methanol, styrene monomer, ethylbenzene, water, and hydrogen), which must be separated to purify the product. The reactor effluent goes through a separation sequence in which the off gas (hydrogen), byproduct (ethylbenzene), pure product (styrene monomer) and waste water are isolated. Unreacted raw materials are also separated in the sequence. The reactants are then recycled back to the reactor. The number of process exit streams—off gas, byproduct, etc.—determines the number of separation units required in the sequence. As a rule of thumb (i.e., a heuristic rule), the number of required separation units is one less than the number of chemical components in the effluent stream, if each chemical component in that stream is to be separated into a pure stream for that component..
The first step in the design of a separation sequence is to decide what the first separation unit is.
Some different types of separation are phase splitting, distillation, and extraction. Phase splits are usually the cheapest method of separation, because they only involve the cooling of the material process stream.
Think of your separator funnel in organic chemistry labs to visualize a phase split. Therefore, if possible, a phase splitter is the first separation unit in the separation sequence.
If the reactor effluent is in the vapor state, it must be cooled to allow a phase separation to take place. In the styrene monomer project, the reactor effluent is cooled to allow the formation of three distinct phases, the vapor phase and two immiscible liquid phases. The two liquids are an organic phase and an aqueous phase. To predict which components each phase contains, remember that “like dissolves like.”
The organic phase contains the organics of toluene, ethylbenzene, and styrene monomer. The methanol partitions between both the organic phase and the aqueous phase. A three-phase separator is also known as
a decanter. Appendix J provides a mathematical description of how a decanter is modeled. Also, Appendix G describes how a cooling operation functions.
You now know that the reactor effluent must be cooled from a vapor phase to a temperature at which three phases exist. You must determine to what temperature the stream is cooled to produce the phase separation. The cooling is typically carried out by a heat exchanger in which the effluent is cooled by a water stream. The cooling water is not directly mixed with the effluent. Rather, the two streams exchange heat thru a metal barrier so that the hot effluent is cooled while the cold water is heated. To design a heat exchanger that cools the hot stream the entire way to the temperature of the cold stream is unfeasible, because such a heat exchanger would be infinitely large in area. Applying a heuristic rule, the hot stream is cooled to within 5 to 10°C of the initial cold stream temperature. As given in the “Flowsheet Economic Analysis” section of Chapter 1, the cooling water is supplied at 31°C. Thus, the reactor effluent is cooled to about 38°C. At 38°C three phases may exist, and they could be separated in a decanter.
The next step in the styrene monomer project is to simulate the first separation unit. Within Aspen HYSYS, you are to simulate the effluent cooling and three-phase separator. The conceptual model for the reactor, cooler, and decanter is as follows:
S15 S13
R1 S14
S10 S11 S12
QE3
E3 F3
Using the above stream and equipment labels, you are to complete your HYSYS simulation as follows:
• Click one of the following web links to download the starter HYSYS file for your assigned reactor inlet temperature and then save it with your initials in its name to a team folder:
SM2_465, SM2_480, SM2_495, SM2_510, SM2_525, or SM2_540.
• Start the HYSYS software, load your preferences, and open your retrieved starter file.
• Finish the simulation for this flowsheet section as directed in the remaining paragraphs.
Account for the pressure drops through the cooler (E3) and decanter (F3) using the data in the “Flowsheet Design Variables” section of Chapter 1. The decanter is to operate adiabatically. After simulating Process Unit E3, create a Performance Table and Plot, where the independent variable is temperature and the dependent variables are pressure, heat flow, vapor fraction, vapor mass flow, light-liquid mass flow, and heavy-liquid mass flow. Include the dew-point and bubble-point temperatures in the table and plot. Under the HYSYS Home/Flowsheet Summary menu, use the Mass/Energy Balance page to view the relative imbalances for material and energy.
After completing this simulation with the initial feed flow rate, use the HYSYS Adjust operation to iterate on the total molar flow of the reactor feed in order to obtain the desired production rate of styrene monomer in Stream S14 at 288.5022 kgmol/hr. To learn how to apply the Adjust operation, follow the directions on its Connections page. Mathematically, the Adjust operator simulates the iteration construct shown below.
In this iteration construct, SM means styrene monomer, EB means ethylbenzene, CTj is the percent molar
Chapter 4 Flowsheet Development Exercises – Problem SM.2 Page 4-7
conversion of toluene to form Compound j, and ∆Pm is the pressure drop across process unit m. These data are provided in Chapter 1.
The symbol Ψi means the process state—temperature, pressure, molar flow rate, and mole fractions—of Stream i. The product (nS11×zS11,SM)represents the calculated molar flow rate of the styrene monomer in the reactor effluent Stream S14.
At what temperature in °C do two phases (vapor-liquid) start to occur on cooling? Do three phases (vapor-liquid-liquid) start to occur on cooling? On a molar basis, what fraction of Stream S12 after cooling to 38°C goes to the vapor phase of the decanter? To the organic phase? To the aqueous phase? On a mass basis, what fraction of Stream S12 after cooling to 38°C goes to the vapor phase of the decanter? To the organic phase? To the aqueous phase?
After you have solved the above problem for your assigned reactor inlet temperature, you must provide documentation in your technical journal for the PFD (with a problem number, reactor inlet temperature, your name, and date), the Workbook datasheet minus the Unit Ops datablock, and the answers to all of the above questions. The Workbook is to contain only Streams S10, S12, S13, S14, and S15. Thus, Streams S11 and noLiq are to be hidden.
After all team members have independently answered all questions in their technical journal, your team is to meet and compare the HYSYS simulation results for the different reactor inlet temperatures.
Click here to complete an Excel template file that contains a table and graph for this team portion of the assignment. Your team is to plot the inlet toluene (S10), outlet styrene monomer (S14), and outlet ethlybenzene (S14) flow rates versus the reactor inlet temperatures. What inlet temperature should the adiabatic reactor operate at, so that the production rate of styrene monomer is maximized and that of ethylbenzene is minimized?
The reactor effluent contains the unreacted raw materials of toluene and methanol, the product of styrene monomer, and the byproduct of ethylbenzene. Since perfect separation into a pure chemical component is not possible (i.e., trace amounts of the other chemical components will always exist), you want to minimize the flow rates of these four chemical components in the off-gas stream (S13) and the wastewater stream (S15), because you must pay for the raw materials and you want to sell as much product as possible.
Based on the various reactor inlet temperatures, what are the mean averages for the molar percent losses of toluene, methanol, styrene monomer, and ethylbenzene? The molar loss of a specific component equals the sum of the molar flow rates of that component in the off-gas and wastewater streams divided by the molar flow rate of that component in the reactor effluent (either Stream S11 or S12). What conclusions can your team draw about the molar losses for these four chemical components?