ChE 312 Process Synthesis & Design Tutorial 4: Separators – Part 1 Objective: The purpose of this tutorial is to give you practice in selecting a property method, using component splitter, choosing a distillation column operating pressure, and using shortcut distillation design in HYSYS. It will help to attempt Q1 before coming to the tute class. Multicomponent distillation problem In the synthesis of alcohols from carbon monoxide and hydrogen, a mixture of alcohols results. After initial distillation steps to remove methanol and heavy alcohols, a stream of 100 lbmol/hr of saturated liquid at 20 psia with the following composition remains: Component ethanol n-propanol isobutanol n-butanol
mol% 25 50 10 15
This mixture is to be separated into three desired product streams according to the following specifications: 1. Ethanol rich stream: 98 mol% ethanol with a recovery of at least 98%. 2. N-propanol rich stream: containing mostly n-propanol and virtually all the remaining ethanol, but no more than 2% of the isobutanol in the feed mixture. 3. Butanols rich stream: containing all the n-butanol, at least 98% of the isobutanol in the feed, and no more than 1% of the n-propanol in the feed mixture. Two distillation columns, operating at moderate pressures, are used for the separation: DIS1
FEED
DIS2 (ethanol rich)
D1
D2
BOT2 (n-propanol rich) BOT1 (butanols rich)
We’ll attack this problem in several stages: 1. Translating the product specifications above into split fractions used in HYSYS. 2. Choosing a property method using the Property Method Selection Assistant. 3. Establishing appropriate pressures at which to operate columns D1 and D2 by using simple component separators to perform flash calculations. 4. Using the column pressures found in 2 to do a shortcut design. 5. Then later, in Tutorial 5, rigorously simulating the column designs using the different distillation columns available in HYSYS ( can also customize your column in HYSYS, if needed), then refining the columns, if necessary, so that they satisfy the specifications. 1
Calculating split fractions (try this before the tute!) 1. Component splitters may be used to mimic the behaviour of distillation columns for preliminary work. We’ll specify the split fractions for each component in the distillate stream of both units. The split fraction of component i in the distillate is: (flowrate of i desired in distillate) / (flowrate of i in feed stream) Q1: By using the specifications on page 1 and selected balances, complete the molar flowrates table below, then complete the split fraction table. Molar flowrates (lbmol/hr) Component FEED ethanol
25
n-propanol
50
isobutanol
10
n-butanol
15
Total
100
DIS1
BOT1
DIS2
BOT2
(butanols rich)
(ethanol rich)
(n-propanol rich)
25
0
49.5
0.5
24.5
.5
0.5
49
9.8 0 74.7
0
15 25.3
.2
0 25
0 49.7
Split fractions (using table above) Component Column D1 Column D2 (DIS1/FEED) (DIS2/DIS1) 1 .98 ethanol 0.0101 n-propanol 0.99 0 isobutanol 0.02 0 0 n-butanol Choosing a property method 2. Use the Launch Property Wizard on Fluid Package Selection Page to suggest an appropriate property method for this separation problem. Note that there are two other property selection systems available in the software with the course textbook. Q2: What property method(s) would you recommend? Record of logic / discussions (optional):
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Choosing column operating pressure and condenser type An algorithm (ULO1!) for choosing the operating pressure and condenser type for distillation columns is given in SSL2 Fig. 7.9 / SSL3 Fig. 8.9.
1 3. Build an HYSYS flowsheet as shown on page 1 using Component Splitter units for the distillation columns. Enter the component split fractions for D1 and D2 from the second table on page 2. Please use the NRTL property method. Later we’ll compare the NRTL method with your answer to Q1. We would like to operate the columns a little above atmospheric pressure (to avoid vacuum operation on one hand and needlessly high pressure on the other), and also to use cooling water, if possible, in the condensers. Let’s nominate an operating pressure of 20 psi absolute for both columns. In this plant, cooling water is able to totally condense a stream if the stream has a bubble point of 120°F (49°C) or higher. You need to force the Component Splitter units to perform a bubble point calculation on the distillate streams at a pressure of 20 psi by specifying a vapor fraction of zero for both DIS 1 and DIS 2:
Run the simulation and also save the flowsheet. Q3: What are the calculated bubble points? Can cooling water be used in total condensers for these columns? DIS1: Using cooling water ok? DIS2: Using cooling water ok?
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Performing shortcut column design 4. Adapt your current flowsheet, or create a new one if you wish. Replace the Component Splitter blocks with Short Cut Distillation blocks Use the Help system to read briefly about Short Cut Distillation’s capabilities and methods. Set up both columns as follows: Total condenser. Hint : Top Product Phase is Liquid Condenser and reboiler pressures of 20 psi (a simplification) For column D1 use n-propanol as the light key and isobutanol as the heavy key. For column D2 use ethanol as the light key and n-propanol as the heavy key. Hint: Use results from previous simulation for mole fractions of light and heavy components. Reflux ratio = 1.3×Rmin. Hint: Rmin will be calculated once streams are connected and remaining specifications in Design- Parameters Section are given. Use this value to calculate reflux ratio. Save, or Save As, the simulation, then run it. Verify that your results are the same as below.
Q4: Complete the following table summarising shortcut designs. DSTWU recommendations Number of stages (rounded up) Feed stage (to nearest integer) Distillate flowrate in lbmol/hr (to 1 dp) Reflux ratio (to 2 dp)
Column D1
Column D2
If you did not get the right results, what troubleshooting strategies could you try? We will use the results from Q4 next week.
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5. Try changing the property method to the one you selected in Q1, then re-run the simulation. Do the shortcut designs change much? That is, how much does the choice of VLE method actually matter?
Q5: What could you do to gain confidence in your choice of VLE method?
Getting ready for next tute 15. Change the property method back to NRTL, re-run the simulation and save it.
Please keep this simulation in a place where you can access it next week. Part 2 of the tutorial will continue using the simulation you have created.
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