An Engineering Approach to the Synthesis of Integrated Distillation Schemes for Systems Involving Difficult Separations

AIChE Spring Meeting and Global Congress on Process Safety
April 4, 2012 - 12:00am

An Engineering Approach to the Synthesis of Integrated Distillation Schemes for Systems Involving Difficult Separations

D. Jantes Jaramillo, G.T. Polley

Dept. of Chemical Engineering, University of Guanajuato, Mexico

The separations of iso-pentane and n-pentane and of iso-butane and n-butane are examples of what may be termed a difficult separation. They each not only require a large number of distillation stages but operation at elevated pressure if cooling water is to be used in the overhead condenser.

A standard problem that included both of these separations was proposed by Heaven (1969). This problem has been studied by Rathore et al (1974),  Andrecovich & Westerberg (1982), Morari & Faith (1980), Meszaros & Fonyo (1986), Rajah & Polley (1995), Sobocam & Glavic (2002) and others. The details of the separation are given in Table 1.

Table 1.  Heaven's Problem

Component

Mole Fraction

Molal Flow (kmol/hr)

A: Propane

0.05

45.36

B: i-butane

0.15

136.08

C: n-butane

0.25

226.80

D: i-pentane

0.20

181.46

E: n-pentane

0.35

317.52

Total

1.00

907.20

Given the attention directed at this problem, the obvious questions are why look at it again? The answer is that all of the work published to date appears to have involved the examination of all of the possible separation schemes. Consideration of the engineering needs ahead of synthesis reduces this number from 14 to 2. Given this situation there is no need for sophisticated software to solve this problem. Standard flow-sheeting packages can be used.

In solving this problem the reflux ratios are initially set at 1.33 times the minimum value in order to reduce the column height for the difficult separations. In final refinement of the design the more common criterion of 1.1 times minimum value can be investigated.

Engineering Heuristics

Step 1. Identify the most difficult separation.

Step 2. Identify any other difficult separation.

Step 3. Identify Possible Schemes

Step 4. Identify Opportunities for Thermal Integration

Table 4. Summary Options

Scheme

Column

Pressure

Heat Load

No. Plates

1

ABC/DE

20

8.16

46

A/BC

16.0

2.60

33

B/C

6.5

9.04

76

D/E

2

20.1

91

Q SAVING

-7.78

Summations

32.12

246

2

A/BCDE

15.6

4.25

34

BC/DE

20

8.12

46

B/C

6.5

9.04

76

D/E

2

20.1

91

Q SAVING

-7.8

Summations

33.71

247

Comparison with Result from the Literature

Step 5.  Examination of Assumptions and Refinement of Design

Conclusions

In this paper an examination of how existing engineering tools can be used to identify thermally integrated distillation schemes for problems involving difficult separations.

The presence of difficult separations greatly reduces the number of separation schemes that can be considered to be viable.

References

Heaven D.L.  M.S. Thesis, University of California, Berkeley, 1969

Andrecovich M.J. & Westerberg A.W. A simple synthesis method based on utility bounding for heat integrated distillation sequences', AIChEJ, 1985,31(3), 363-375

Sobocan G. & Glavic P. A simple method for systematic synthesis of thermally integrated distillation sequences, Chem. Eng. J, 2002,89,155-172

Meszaros I. & Fonyo Z. A new bounding strategy for synthsizing distillation schemes with energy integration, Comp. & Chem. Engng. 1986,10(6),545-550

Morari M. & FaithD.C. The synthesis of distillation trains with heat integration, AIChEJ, 1980, 26,916-928

Rathore R.N.S., Van Kormer K.A. & Powers G.J. Synthesis strategies for multi-component separation systems with energy integration, AIChEJ, 1974,20,491-501

Rajah W. & Polley G.T. Synthesis of practical distillation schemes, Trans.I.Chem.E. 1995, 73A,953-964

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