CHEN 4460 – Process Synthesis, Simulation and Optimization

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 5 – Sequencing Ordinary Distillation ColumnsSeptember 18, 2012

Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

Sequencing Distillation Columns

Lecture 5 – Objectives

Understand how distillation columns are sequenced and how to apply heuristics to narrow the search for a near-optimal sequence.

Be able to apply systematic methods to determine an optimal sequence of distillation-type separations.

Sequencing OD Columns• Use a sequence of ordinary distillation (OD)

columns to separate a multicomponent mixture provided: in each column is > 1.05. The reboiler duty is not excessive. The tower pressure does not cause the mixture to

approach the TC of the mixture. Column pressure drop is tolerable, particularly if

operation is under vacuum. The overhead vapor can be at least partially condensed

at the column pressure to provide reflux without excessive refrigeration requirements.

The bottoms temperature for the tower pressure is not so high that chemical decomposition occurs.

Azeotropes do not prevent the desired separation.

Pressure/Condenser Algorithm

Number of Sequences for OD• Number of different sequences of P –1 ordinary

distillation (OD) columns, NS, to produce P products:

)!1(!)]!1(2[

PPPNs (8.9)

 P # of Separators  Ns2 1 13 2 24 3 55 4 146 5 427 6 1328 7 429

Example: 4 Components

Example: 4 Components

Best Sequence using Heuristics• The following guidelines are often used to reduce

the number of OD sequences that need to be studied in detail: Remove thermally unstable, corrosive, or chemically

reactive components early in the sequence. Remove final products one-by-one as distillates (the

direct sequence). Sequence separation points to remove, early in the

sequence, those components of greatest molar percentage in the feed.

Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of other components.

Sequence separation points to leave last those separations that give the highest purity products.

Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column. The reboiler duty should not be excessive.

Class Exercise

Design a sequence of ordinary distillation columns to meet the given specifications.

Exercise – Possible Solution

Guided by Heuristic 4, the first column in position to separate the key components with the

greatest SF.

Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of other components.

Exercise – Possible Solution

= 3.6 = 2.8

= 1.5

= 1.35

Complex Columns• In some cases, complex rather than simple

distillation columns should be considered when developing a separation sequence.

Ref: Tedder and Rudd (1978)

Regions of Optimality• As shown below, optimal regions for the various

configurations depend on the feed composition and the ease-of-separation index (ESI):

ESI = AB/ BC

ESI 1.6 ESI 1.6

Sequencing V-L Separation• When simple distillation is not practical for all

separators in a multicomponent mixture separation system, other types of separators must be employed and the order of volatility or other separation index may be different for each type.

• If they are all two-product separators and if T equals the number of different types, then the number of possible sequences is now given by:

• For example, if P = 3, and ordinary distillation, extractive distillation with either solvent I or solvent II, and LL extraction with solvent III are to be considered, T = 4, and applying Eqns (8.9) and (A) gives 32 possible sequences (for ordinary distillation alone, NS = 2).

(A)sPT

s NTN 1

Example: Butenes Recovery

• For T = 2 (OD and ED), and P = 4, NS = 40.• However, since 1-Butene must also be separated

(why?), P = 5, and NS = 224.• Clearly, it would be helpful to reduce the number of

sequences that need to be analyzed. • Need to eliminate infeasible separations, and

enforce OD for separations with acceptable volatilities.

3.31196.336.1Fn-Pentane4.02161.43.7Ecis-2-Butene4.12155.40.9Dtrans-2-Butene3.73152.0-0.5Cn-Butane3.94146.4-6.3B1-Butene4.1797.7-42.1APropane

Pc, (MPa)Tc (C)b.pt.(C)Species

3.31196.336.1Fn-Pentane4.02161.43.7Ecis-2-Butene4.12155.40.9Dtrans-2-Butene3.73152.0-0.5Cn-Butane3.94146.4-6.3B1-Butene4.1797.7-42.1APropane

Pc, (MPa)Tc (C)b.pt.(C)Species

PropaneButaneButenePentane

1-Butene and 2-Butene are structurally very different, whereas, the optical isomers are much closer related and are difficult to separate

by distillation

Example: Butenes Recovery

• Splits A/B and E/F should be by OD only ( 2.5)

• Split C/D is infeasible by OD ( = 1.03). Split B/C is feasible, but an alternative may be more attractive.

• Use of 96% furfural as a solvent for ED increases volatilities of paraffins to olefins, causing a reversal in volatility between 1-Butene and n-Butane, altering separation order to ACBDEF, and giving C/B = 1.17. Also, split (C/D)II with = 1.7, should be used instead of OD.

• Thus, splits to be considered, with all others forbidden, are: (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II

Adjacent Binary Pair ij at 65.5 oC

Propane/1-Butene (A/B) 2.45

1-Butene/n-Butane (B/C) 1.18

n-Butane/trans-2-Butene (C/D) 1.03

cis-2-Butene/n-Pentane (E/F) 2.50

Estimating Annualized Cost• 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: Set distillate and bottoms column pressures using Estimate number of stages and reflux ratio by WUG

method (e.g., using Aspen Plus “DSTWU Column”) Select tray spacing (typically 2 ft.) and calculate column

height, H Compute tower diameter, D (using Fair correlation for

flooding velocity, or Aspen Plus Tray Sizing Utility) Estimate installed cost of tower (e.g. Peters &

Timmerhaus) Size and cost ancillary equipment (condenser, reboiler,

reflux drum). Sum total capital investment, CTCI

Compute annual cost of heating and cooling utilities (COS)

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

Butenes Recovery – 1st Branch

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

SpeciesPropane A1-Butene Bn-Butane C

trans-2-Butene Dcis-2-Butene En-Pentane F

(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II

Butenes Recovery – 2nd Branch

SpeciesPropane A1-Butene Bn-Butane C

trans-2-Butene Dcis-2-Butene En-Pentane F

Sequence Cost, $/yr

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

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

(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II

Butenes Recovery – 3rd Branch

SpeciesPropane A1-Butene Bn-Butane C

trans-2-Butene Dcis-2-Butene En-Pentane F

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

(A/B…)I, (…E/F)I, (…B/C…)I,(A/C…)I , (…C/B…)II, and (…C/D…)II

Butenes Recovery – 4th Branch

SpeciesPropane A1-Butene Bn-Butane C

trans-2-Butene Dcis-2-Butene En-Pentane F

Sequence Cost, $/yr

4-14-15 1,115,200

(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II

Example: Butenes Recovery• Lowest Cost Sequence

Sequence Cost, $/yr2-(8,10-22) 860,400

Example: Butenes Recovery

Summary – Sequencing

Understand how distillation columns are sequenced and how to apply heuristics to narrow the search for a near-optimal sequence.

Be able to apply systematic B&B methods to determine an optimal sequence of distillation-type separations.

On completion of this part, you should:

Other Business• Homework

– SSLW: 8.1, 8.2, 8.3– Due Tuesday September 25

• Next Lecture – September 25– Review of Non-Ideal Thermodynamics (SSLW 223-230)