(118d) An Engineering Approach to the Synthesis of Integrated Distillation Schemes for Systems Involving Difficult Separations

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

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

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