chapter 3c - selection methods ii

8/18/2019 Chapter 3c - Selection Methods II

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EP426Chemical Process Design and Optimization

Chapter 3b - Separation train synthesis.Development of a separation process

Part 2


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Sequence Notation

AB/C

A/B

[AB/C]; [A/B]


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Sequence Notation


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Identifying the Best Sequences

1. Remove thermally unstable, corrosive, or chemically reactivecomponents early in the sequence.

2. Remove final products one-by-one as distillates (the direct sequence).

3. Sequence separation points to remove, early in the sequence, thosecomponents of greatest molar

percentage in the feed.

4. Sequence separation points in the order of decreasing relative volatilityso that the most difficult splits are made in the absence of othercomponents. (low volatility at last)

5. Sequence separation points to leave last those separations that givethe highest purity products.

6. Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column. The reboiler duty is notexcessive.

HEURISTICS of favourable separation sequence


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Exercise 1: Determine number of possiblesequence.


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Number of possible sequence: 14(5 components)

14


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Exercise 2: Use heuristics to determine a goodsequence of ordinary distillation units.


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Solution:

Sequence based on heuristic 4


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Quantitative method for evaluatingdistillation sequences

Minimum vapour flow

How: Use Underwood method to estimate minimumvapor and liquid flows

Why: To screen for different alternatives based oncost estimation.

Vapor flow method is a viable estimate of costbecause:

1. Major component contributing to the required columndiameter & trays capital cost

2. Reflects utility costs (e.g., utilities use in reboiler +condenser)


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Quantitative method for evaluatingdistillation sequences

Minimum vapour flow

How: Use Underwood method to estimate minimumvapor and liquid flows


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Example 1


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Step 1: Material Balance


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Step 2: Determine Alpha


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Step 3: Define Sequence


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Step 4: Determine Vmin for each sequence

= 6.4

= 8.9

= 10.7

= 5.5


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Answer


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Homework 1

Q. Calculate the total vapor flow produced in the sequence assuming actual to min.

reflux ratio of 1.2:1


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Challenge

Repeat Homework 1

Q1. Evaluate the total internal vapour flow for direct sequence

configuration.

Q2. Evaluate the total internal vapour flow for any distillation

sequencing of your own choosing.

Q3. Compare those three configurations. Which configurations is

the most favourable and why? Describe the disadvantage(s) of

this method for distillation sequencing?


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Homework 2

Each component is to be separated with a specification of 99 %. Assume

that the non key component will not distribute but end up either at the topor bottom depending on their relative volatility compared to the keycomponent. Determine the best sequence(s).

Determine the best sequence(s).


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For each separation, CA is estimated assuming 99 mol % recovery of

light key in distillate and 99 mol % recovery of heavy key in bottoms.The following steps are followed:

1. Set distillate and bottoms column pressures

2. Estimate number of stages and reflux ratio (e.g., using HYSYS.Plant“Shortcut Column”).

3. Select tray spacing (typically 2 ft.) and calculate column height, H.4. Compute tower diameter, D (using Fair correlation for flooding

velocity, or HYSYS Tray Sizing Utility).

5. Estimate installed cost of towerSize and cost ancillary equipment(condenser, reboiler, reflux drum). Sum total capital investment,

CTCI.6. Compute annual cost of heating and cooling utilities (COS).

7. Compute CA assuming ROI (typically r = 0.2). CA = COS + r CTCI

Estimating Annualized Cost, CA


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Selecting sequence based onAnnualized Cost C

A

Species

Propane A

1-Butene B

n-Butane C

trans -2-Butene D

cis -2-Butene E

n-Pentane F

Example: Assuming 99 mol % recovery of product A,B,C,F


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1

st

Branch of Sequences

Sequence Cost, $/yr

1-5-16-28 900,200

1-5-17-29 872,400

1-6-18 1,127,400

1-7-19-30 878,000

1-7-20 1,095,600

Species

Propane A

1-Butene Bn-Butane C

trans -2-Butene D

cis -2-Butene E

n-Pentane F


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2

nd

Branch of Sequences

Sequence Cost, $/yr

2-(8,9-21) 888,200

2-(8,10-22) 860,400

Species

Propane A


trans -2-Butene D

cis -2-Butene E

n-Pentane F


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Sequence Cost, $/yr

3-11-23-31 878,200

3-11-24 1,095,700

3-12-(25,26) 867,400

3-13-27 1,080,100

Species

Propane A


trans -2-Butene D

cis -2-Butene E

n-Pentane F

3

rd

Branch of Sequences


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4

th

Branch of Sequences

Sequence Cost, $/yr

4-14-15 1,115,200

Species

Propane A


trans -2-Butene D

cis -2-Butene E

n-Pentane F


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Answer

Sequence Cost, $/yr

1-5-16-28 900,200

1-5-17-29 872,400

1-6-18 1,127,400

1-7-19-30 878,000

1-7-20 1,095,600

Sequence Cost, $/yr

2-(8,9-21) 888,200

2-(8,10-22) 860,400

Sequence Cost, $/yr

3-11-23-31 878,200

3-11-24 1,095,700

3-12-(25,26) 867,400

3-13-27 1,080,100

Sequence Cost, $/yr

4-14-15 1,115,200


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Homework 3


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Example: Determine Rmin

Appendix


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Calculate using Underwood Eqn.(Assume sat. liq and therefore q = 1)

Step 1: Determine the root of Underwood equation value

Appendix

A di


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Calculate using Underwood Eqn.(Assume sat. liq and therefore q = 1)

Step 2: Using the calculated root of the Underwood equation,

and determine Rmin

Appendix


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EndNext: MidtermDETSAR HALL (12:45 PM to 1:45PM) 2 Questions (Chapter 1 to 3)

Next Lesson: Chapter 4 - Optimisation

chapter 3c - selection methods ii

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