Antecedentes internacionales

In Problem SM.5, you simulated the compression and vaporization of the two fresh reactant streams (pure toluene S1 and pure methanol S4) from ambient conditions of 25°C and 1 atm to a saturated-vapor state at 460 kPa. The feed to the reactor also contains recycled material from the toluene column C1 and the methanol column C3, as depicted in the following block flowsheet:

methanol recycle

where the red-dashed ovals ^SM.1, ^SM.2, etc. in the above flowsheet represent the focus of each styrene monomer problem in this chapter.

The two recycle streams (S26 and S21) must be mixed with the fresh reactant stream (S7) and then heated before entering the reactor, creating a recycle loop in the process. In Problem SM.6 you are to simulate the recycle loop. Click here to view a mathematical model and mathematical algorithm for this flowsheet simulation. The conceptual model for mixing and heating the reactor feed is as follows:

S9

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:

SM6_465, SM6_480, SM6_495, SM6_510, SM6_525, or SM6_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.

Your HSYSY process flow diagram (PFD) contains the compression, vaporization, and mixing of the two fresh reactants, as well as the rest of the process unit operations from Reactor R1 to distillation Columns C1 and C3. A set operator labeled MeOH has been added to the PFD to set automatically the total molar flow of the pure methanol stream (S4) using the specified total molar flow of the pure toluene stream (S1).

The liquid distillate streams (S17 and S23) from the two distillation columns have been compressed and heated to saturated vapors at 460 kPa, in preparation for their recycle back to the reactor. The hydrogen fuel stream (S13), the two vent streams (S17v and S23v), and the wastewater stream (S18) have been further processed to exit the flowsheet at 1 atm.

The recycle loop is currently missing in your HYSYS PFD; that is, mixing the fresh reactants (S7) with the recycled reactants (S21 and S26) to produce the mixed reactants (S8) and then heating those mixed reactants to form the reactor feed (S9). The process state of Stream S9 is calculated based on assuming the process state of Stream S10, which you did in Problem SM.1 for the reactor inlet stream. For valid closure of the material and energy balances, the process state of Stream S9 exiting the fired heater FH1 must be identical to that of Stream S10 entering the adiabatic reactor R1; that is, their pressure, molar flow rate, molar enthalpy, and component mole fractions must agree within set tolerances. To close the recycle loop, you will manually execute the iterative method of successive substitutions for two iterations and then use the HYSYS Recycle operator to finish the iteration process. Proceed as follows to close the recycle loop:

1. Use the HYSYS drag Zoom button to expand the PFD view from Mixer M2 to Reactor R1 and from recycle Streams S26 and S21.

2. Add a mixer operator labeled M2 into the flowsheet to combine the fresh reactant stream with the two recycle streams. The inlet streams to the mixer are S26, S7, and S21, in that order. The outlet stream is labeled S8.

3. Add a heater operator labeled FH1 to increase the temperature of Stream S8 to the reactor inlet temperature that was assigned to you. The heater used to reach such a high temperature in Stream S9 is called a fired heater, basically a furnace that burns natural gas. Use the HYSYS heater with a pressure drop as indicated in the “Flowsheet Design Variables” section of

Chapter 1. The resulting outlet pressure must be 400 kPa. You must specify the temperature of Stream S9 using the reactor inlet temperature that was assigned to you.

4. Open the Workbook window and view Streams S1 and S4. Hide Stream S8 and place Stream S9 before S10 in the Material Streams, Compositions, and Component Flows pages.

5. Compare the temperature, pressure, molar flow rate, and molar enthalpy of Streams S9 and S10 using the Material Streams page in the Workbook window. Compare the component mole fractions of these two streams using the Compositions page.

The temperature and pressure of these two streams are exactly the same, because you specified them to be those values. However, the molar flow rate, molar enthalpy, and component mole fractions of these two streams do not match.

6. Assume a new process state for Stream S10 by copying the conditions of S9 into S10 using the Define from Other Stream… button in the property window of Stream S10.

Resetting the process state of Stream S10 causes HYSYS to recalculate all of the process units and produce a different calculated S9. The process states of S9 and S10 should now be closer.

Chapter 4 Flowsheet Development Exercises – Problem SM.6 Page 4-21

This iterative technique of successive substitutions is used to find that process state of the reactor feed stream that closes the material and energy balances for the flowsheet recycle.

7. Copy the conditions of S9 into S10 again to assume a new process state for the reactor feed stream. The molar flow rate, molar enthalpy, and component mole fractions should be even closer for these two material streams.

You could continue this process, manually performing the iterations of successive substitutions until the process state of S9 is identical to that of S10. However, HYSYS provides the Recycle operator to automate the iteration process.

8. Open the pre-placed recycle operator RCY and connect Stream S9 as the input and Stream S10 as the output to this operator, in order to automate the iteration process.

The HYSYS recycle operator continually iterates on the process state of these two streams to close the recycle loop within set tolerances for the pressure, molar flow rate, molar enthalpy, and component mole fractions. It uses an enhanced version of successive substitutions called the Wegstein method. When the recycle operator has converged, you will get a green

converged message at the bottom of the recycle property window.

9. Compare the temperature, pressure, molar flow rate, and molar enthalpy of Streams S9 and S10 using the Worksheet/Condition page in the Recycle: RCY window. Compare the component mole fractions of these two streams using the Worksheet/Composition page. The process states of Streams S9 and S10 should now be within set tolerances. Thus, the material and energy balances for the recycle portion of the chemical process flowsheet have been closed.

10. Select the Parameters/Numerical page in the Recycle: RCY window and examine the iteration count; that is, the number of iterations that were required to close the material and energy balances for the flowsheet. Since you did two iterations manually, note that you need to add two to the RCY iteration count.

What is the number of total iterations required to close the material and energy balances for the recycle loop in the styrene monomer flowsheet? Click here to download a Word file, add your initials to its name, and complete all of the questions contained within.

The Parameters/Variables page within a Recycle operator (like RCY) controls the tolerance tightness of certain process variables for a material stream involved in the iteration process to close the material and energy balances of a recycle loop. Based on information provided through the HYSYS Help menu, the following table presents some of those variables and their tolerances used in Recycle Operator RCY:

Stream

Variable Recycle RCY

Sensitivity Internal

Tolerance Tolerance

Type Actual

Tolerance

P - pressure 10 0.01 kPa absolute 0.1 kPa

n - molar flow rate 1×10^-4 0.001 kgmol/s relative 1×10^-7

ˆH - molar enthalpy 1 1.00 kJ/s absolute 1.00 kJ/s

xj - each mole fraction 1 0.0001 absolute 0.0001 Subscript “j” is for each chemical component in a mixture of nc chemical compounds.

The actual tolerance equals the recycle sensitivity times the internal tolerance.

When a recycle operator is iterating on a material stream, the process variables that uniquely defined the state of that material stream are the pressure, molar flow rate, molar enthalpy, and nc-component mole

fractions, whether that stream is a mixture of two or more chemical components or just a mixture of one component (a.k.a. pure compound). The tolerance checks for these variables are given as follows:

Stream Variable Tolerance Check Explanation

P - pressure abs

(

)

n - molar flow rate ^{in out}

in

abs n n n

 − 

 

 

 

 ^{≤ 1×10}

-7 must match to within 7 digits

ˆH - molar enthalpy ^abs

(

)

xj - each mole fraction abs

(

)

10, 000for each j-th component in mixture Subscript “in” means Stream S9 and subscript “out” means Stream S10 for recycle operator RCY.

When these (3+nc) process variables have converged, all other variables associated with the material stream will have converged too, like vapor fraction, temperature, and molar entropy. Click here to learn more about the general iteration process on a recycle loop in a chemical process flowsheet (i.e., a PFD).

Once HYSYS has closed the material and energy balances, the styrene monomer production rate in Stream S24 must be checked to see if it meets the desired rate given in the “Flowsheet General

Assumptions” section of Chapter 1 (i.e., 288.5022 kgmol/h or250,000 mt/yr). Open the Workbook window, select the Component Flows page, and check the styrene monomer flow rate in Stream S24. Since it is not met, you are to conduct a manual trial-and-error iteration. Type a new value for the pure toluene flow rate in Stream S1, hit the <Enter> key then hit the <Enter> key again, allow HYSYS to complete the re-calculations, observe the styrene monomer flow rate in Stream S24, and stop your manual iteration when the desired styrene flow is met. After completing the iteration on the toluene flow rate in Stream S1, you must check the design constraints given in the “Flowsheet Design Specifications” section of Chapter 1 to see if they have been satisfied. Open the property windows of Columns C1 and C3 and inspect the

“Specifications” area in the Design/Monitor page. All design specifications should have been nearly met.

What is the equimolar flow rate of pure toluene (S1) and pure methanol (S4) into the flowsheet for a production rate of 250,000 mt/yr of styrene monomer? What are the molar flow rates for toluene and methanol in S10, the stream entering Reactor R1? What are the reflux ratios for Columns C3 and C1?

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 S1, S4, S9, S10, and S24. Thus, all other streams 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 pure toluene (S1), reactor inlet (S10), styrene monomer (S24), and ethylbenzene (S24) 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? Although equimolar flow rates of pure toluene and pure methanol enter the flowsheet, the molar ratio of toluene to methanol in Stream S10 that enters Reactor R1 is not equimolar.

What material stream quantity that enters the flowsheet would you vary to get the toluene-to-methanol ratio in Stream S10 to be equimolar?

Chapter 4 Flowsheet Development Exercises – Problem SM.7 Page 4-23