(399b) New Vapor Compression Alternatives in Thermally Coupled Distillation

New
Vapor Compression Alternatives in Thermally Coupled Distillation

José A. Caballero*; Miguel A. Navarro; Juan .A.
Reyes-Labarta

*Department
of Chemical Engineering. University of Alicante. Ap. Correos 99. 03080
Alicante. Spain

Abstract

The costs of chemical process are often dominated by
the separation costs. Between the different separation techniques, distillation
is the most important and commonly used in all chemical and petrochemical
industries. Distillation handles more than 90%
of separations and and this trend seems unlikely to change in the near
future. The capital investment for these
distillation systems was estimated in 8x10⁹ US$ [1]. Soave & Feliu [2] calculated that distillation accounts
about the 3% of the total US energy consumption which is equivalent to 2.87x10¹⁸
J or 54 million tons of crude oil.

A renew interest
in Thermally Coupled Distillation (TCD) appeared in, say the last 15-20 years,
due the important potential savings in energy: typical values around 10% to 50
has been reported [3-5]
when compared with conventional distillation sequences. Although it has been
proved that fully thermally coupled systems are the arrangements that requires
the minimum energy in a sequence of columns [6] it is possible to identify situations in
which some column sections are operating far away the optimal conditions. For
the sake of simplicity, but without losing generality, we will focus on the
special case of a three component Petlyuk configuration or its
thermodynamically equivalent Divided Wall Column (DWC). It will be evident that
the extension to other thermally coupled configurations with more complex
arrangements is straightforward.

The simulation of
a DWC or its thermodynamically equivalent Petlyuk configuration can be carried
out by decomposing it in the three separation tasks that form the sequence (See
Figure 1) - each one of these separation tasks can be considered a distillation
column- If we optimize each one of these columns independently, when we put
together columns 2 and 3 (referred to Figure 1) it is evident that we have to
modify the liquid or vapor flows in column 2 or column 3 to satisfy the mass balance
-See Figure 1b-. (Usually it is better to perform a simultaneous optimization
of all the system ?included column 1- to get the optimal performance of the configuration).
But the adjustment of flows produces that some sections work in suboptimal conditions
(at least when compared to the individual separations tasks). If the dominant
column is column 3 we have to increase flows in column 2 and then condenser
duty increases. If the dominant column is column 2 the flow adjustment in
column 3 produces an increase in the reboiler duty.

Figure 1. Generation by decomposition in its basic separation tasks of a
Petlyuk configuration (c) and the equivalent Divided Wall Column (d).

Instead of
adjusting the flows, it is possible to introduce a heat exchanger (reboiler or
condenser), but the extra heat added/removed is approximately equal to the
extra heat if the flows are adjusted, and it is necessary to include an extra
heat exchanger. Alternatively, it is possible to withdraw the extra
vapor/liquid flow and use this stream as utility in the rest of the plant, what
not always is feasible. An interesting alternative, presented in this paper,
consists of implementing a vapor compression cycle using this extra
vapor/liquid stream as shown in Figure 2.

Figure 2. Vapor compression and integration with the column reboiler (left)
or condenser (right).

Some of the
characteristics of these new arrangements are the following:

1. In Petlyuk/DWCs is usually not economically attractive to
implement a vapor compression cycle between condenser and reboiler due to the
large difference of temperatures (there is at least one component with an
intermediate boiling point) and therefore large compression ratios. But with
this arrangement the difference of temperatures is smaller and then the
alternative could be economically attractive.

2. Both the heat duties in reboiler and condenser are reduced: the
first one is due to the heat integration in the vapor compression; the other
due to the reduction of internal vapor and liquid flows in the corresponding
section.

3. There is a tradeoff between the savings in energy consumption in
reboilers and condensers and, the investment and operation of the new
equipment, mainly the compressor.

Examples

We present here
two examples. Corresponding to each one of the options presented in Figure 2.
The first one consists in the separation of a mixture of Benzene, Phenol and
o-Cresol (Data and main results are showed in Tables 1 and 2). All simulations
were carried out in Aspen-Hysys using SRK equation of state and default values.
Cost correlations are from Turton et al [7], and updated to 2012 cost. In this example
it is necessary increase the boilup ratio (column 3 in Figure 1) to avoid a
very large number of stages in the column, which produces large energy
consumptions in the reboiler (this is an intrinsic problem of this system
independently of the column sequence).

It is interesting
remark that the savings in energy in the reboiler are greater than 80%. There
is also a similar reduction in the energy consumption in the condenser. The
investment is amortized during the first year of operation. After the
amortization the savings in utilities cost is around 2.9 MM?/year (~ 76% in
reduction in cost of utilities).

Table 1. Data and
main results for the example 1 without compression cycle.

Table 2. . Data
and main results for the example 1 with compression cycle.

*Referred to new
equipment.

The second example corresponds to a mixture
of ethylene, ethane and propane. Here the main cost is due to the necessity of
refrigeration in the condenser, and therefore the implementation of a reverse
cycle (expansion previous compression) is of interest (second case in Figure
2). Tables 3 and 4 shows main data and results

In this example the energy savings in
condenser is around 39%. The investment is amortized in less than two years. After
this time the benefit is around 2.8 MM?/year.

Table 3. Data and
main results for the example 2 without compression cycle.

Table 4. . Data
and main results for the example 2 with compression cycle.

Acknowledgements

The authors wish to
acknowledge support from the Spanish Ministry of Science and Innovation under
project (CTQ2009-14420-C02).

References

1.            Humphrey,
J., Separation processes: playing a critical role. Chemical Engineering
Progress, 1995. 91(10): p. 43-54.

2.            Soave,
G. and J.A. Feliu, Saving energy in distillation towers by feed splitting.
Applied Thermal Engineering, 2002. 22(8): p. 889.

3.            Caballero,
J.A. and I.E. Grossmann, Structural considerations and modeling in the
synthesis of heat-integrated-thermally coupled distillation sequences.
Industrial & Engineering Chemistry Research, 2006. 45(25): p.
8454-8474.

4.            Fidkowski,
Z.T. and R. Agrawal, Multicomponent thermally coupled systems of
distillation columns at minimum reflux. AIChE Journal, 2001. 47(12):
p. 2713-2724.

5.            Rudd,
H., Thermal Coupling For Energy Efficiency. Chemical Engineer-London,
1992(525): p. S14-S15.

6.            Halvorsen,
I.J. and S. Skogestad, Minimum energy consumption in multicomponent
distillation. 3. More than three products and generalized Petlyuk arrangements.
Industrial & Engineering Chemistry Research, 2003. 42(3): p.
616-629.

7.            Turton, R., et al., Analysis Synthesis
and Design of Chemical Processes2003, New York: McGraw-Hill.