CHE 4181

Simulation Assignment #4

Work both problems below using Aspen Plus. Save the files for each problem in Canvas. Print your answers to each question below

1. Consider a distillation column with a feed stream at 110 F and 40 psia entering above stage 15. The feed flow includes 12lb-mole/hr propane, 18 lb-mole/hr n-butane, 21lb-mole/hr n-pentane, and 29lb-mole/hr n-hexane. The column has a total condenser with distillate draw and akettle reboiler with a bottoms draw. In addition a liquid side stream is taken off the column on stage 10 primarily to purify the butane. The column which has 32stages, operates at a pressure of 20psia in the condenser and 25 psia in the reboiler.

Using the NRTL property method and the Radfrac model initially specify the molar reflux ratio as 20 and the distillate and side draws based on a mass balance assuming that we will want the column to have pure propane in the distillate and pure butane in the side product.

A) Based on these specs what are the purities (mole fraction) of propane in the distillate and butane in the side draw?

xC3in distillate ______

xC4 in side draw ______

B) Design the column tonow give a at least 0.995 molar purity of propane in the distillate and at least 0.995 molar purity butane in the side stream by adding the purity design specs and varying the molar reflux for the side purity and the distillate draw for the distillate purity.

What is the distillate flow rate (lbmole/hr)? ______

What is the molar reflux ratio required to achieve the desired purities?______

What is the stage 3 temperature (F) ?______

What is thereboiler duty (Btu/hr)? ______

C) Now investigate the effect of lowering the system pressure on this column by repeating part B and completing the table below. At each condenser pressure specify the reboiler pressure as 5 psia greater than the condenser pressure.Note: The case for the first row is done in part B.

Condenser Pressure (psia) / Reflux Ratio / Distillate Rate (lbmole/hr) / Condenser Temperature (F) / Condenser Duty (Btu/hr)
30
25
20

D) Now investigate the effect of changing the feed stage (Above stage) at a condenser pressure of 20psia by repeating part B and completing the table below. Note: The case for the second row is done in part B.

Feed Stage / Reflux Ratio / Reboiler Duty (Btu/hr) / Condenser Temperature (F) / Condenser Duty (Btu/hr)
13
15
17

2. A company is interested in producing methanol from hydrogen and carbon dioxide. You are to design a reaction and separation sequence that will result in a methanol purity of 99.5%. The feed to the system will be 10000 kmoles/hr in a 3:1 H2 to CO2 molar ratio at a pressure of 40 atm and an inlet temperature of 150 C. The following competing equilibrium reactions will be present in the reactor which is operated adiabatically with a 4 atm pressure drop across the bed.

CO2 + 3 H2↔ CH3OH + H2O

CO2 + H2↔ CO + H2O

Assume the contact time in the bed is sufficient for the reactions to reach equilibrium. The vapor phase exit stream is to be cooled to 85 C where the pressure drop across the cooler is 2 atm. The stream is then sent to a control valve where the pressure is reduced to 1.5 atm before sending it into a flash separator.

The condensed phase from the separator is sent to a 20 stage distillation column with the feed entering on stage 10. In the distillation column the reboiler operates at a pressure of 1.5 atm and the condenser operates at a pressure of 1 atm. A partial condenser (Pertial Vapor Liquid) should be used in the column since non-condensable gases will be present. You may specify the condenser temperature as 55 C. The NRTL property method should be used for the simulation.

Your objective is to maximize the production rate of the methanol while still being able to ensure that 99.5% purity spec is not sacrificed. The purity can be achieved by varying the reflux ratio however the reflux should not be less than 1 or exceed 50.

For initial guesses in your specification of the column use a reflux ratio of 5, and a guessed distillate rate. By looking at the methanol molar flow in the feed to the column you have an idea what the Distillate should be although the Distillate Rate Specification in RADFRAC is for the total of the liquid and vapor draws.

To maximize production gradually increase your draw off rate from an initial guess that converges. Note: Once you hit the constraints (i.e. doesn’t converge) you will need to go back to a stable caseand possibly proceed with a smaller increment to get the results. Try to get the total distillate draw off rate to within the closest 0.1 kmole/hr flow rate.

A) Give your results in the first row of the Table below for the base case reactor feed temperature of 150 C.

B) Redo the entire procedure and give your results in the second row of the Table below for the reactor feed temperature of 125 C.

C) Redo the entire procedure and give your results in the third row of the Table below for the reactorfeed temperature of 100 C.

Reactor / Product Cooler / Distillation Column
Temperature
(Centigrade) / CO2Conversion / Duty (GJ/hr) / Total Distillate Rate (kmole/hr) / Reflux
Ratio / Reboiler Duty (GJ/hr)
150
125
100