CHEN 4460 – Process Synthesis, Simulation and Optimization

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lab Lecture No. 2 – Sequencing of Separation Trains

September 18, 2012

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

Sequencing Separation Trains

Example 1: Four Components

The feed to a separation process consists of the following species:

Species Number Species

1 Ethane

2 Propane

3 1-Butene

4 n-Butane

Example 1: Four Components

• It is desired to separate this mixture into essentially pure species. The use of two types of separators is to be explored:– Ordinary distillation– Extractive distillation with furfural

Separator Type

I II

1 1

2 2

3 4

4 3

5Speci

es

nu

mb

er

• The separation orderings are:

Example 1: Four Components

• Determine the number of possible separation sequences.

• What splits would you forbid so as to reduce greatly the number of possible sequences?

Example 1: Solution

• To determine the number of possible sequences , use Eqs. (8.9) and (A):

• Combining Eqs. (8.9) and (A), the number of possible sequences is:

)!1(!

)]!1(2[ 1

PP

PTN PT

s

4058!3!4

!62

)!14(!4

)]!14(2[2 314

(A)

(8.9)

Example 1: Solution

• For ordinary distillation, the approximate relative volatilities, between adjacent species are as follows at 150 F:

Adjacent binary pair at 150 oF:

C2 /C3 2.6

C3 /1-C4= 2.45

1-C4= /nC4 1.18

Example 1: Solution

Example 1: Solution

From Fig 5.3, forbid use of extractive distillation when >2 for ordinary distillation.

Thus, forbid the use of extractive distillation for the splits:

C2/C3 C3/1-C4= C3/nC4 (=2.45x1.18=2.89)

Thus, the only splits to be considered for extractive distillation are those involving nC4/1-C4

= and 1-C4

=/furfural C2

C3

1-C4=

n-C4

C3

1-C4=

n-C4

1-C4

n-C4=

Example 1: Solution

C2

C3

1-C4=

n-C4

C3

1-C4=

n-C4

n-C4

1-C4=

Furfural1-C4

=

Furfural

C2

C3

n-C4=

1-C4

Furfural

C2

C3

n-C4

1-C4=

Furfural

1-C4=

Furfural

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.

Example 2: Three Components

• A mixture of benzene, toluene and biphenyl needs to be separated. The boiling points of the three components are: 80.1 C, 110.8 C, and 254.9 C, respectively.

• Suggest possible separation sequences for this mixture. Select the most appropriate sequence for each of the two cases in the table below.

• Simulate the two sequences you suggest using Aspen Plus, to produce each of the three components at 99% purity.Case Benzene Toluene Biphenyl

1 10% 10% 80%

2 10% 60% 30%

Example 2: Solution

• Case 1: Direct SequenceTotal cooling: 10.7 MM kcal/hr

Total heating: 30.0 MM kcal/hr

Example 2: Solution

• Case 1: Indirect Sequence Total cooling: 4.4 MM kcal/hr

(Direct sequence: 10.7 MM kcal/hr)

Total heating: 27.0 MM kcal/hr

(Direct sequence: 30 MM kcal/hr)

Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed.

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

Example 2: Solution

• Case 2: Direct SequenceTotal cooling: 1055 MM kcal/hr

Total heating: 1064 MM kcal/hr

Example 2: Solution

• Case 2: Indirect Sequence Total cooling: 14.5 MM kcal/hr

(Direct sequence: 1055 MM kcal/hr)

Total heating: 25 MM kcal/hr

(Direct sequence: 1064 MM kcal/hr)

Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed.

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