5940
Ind. Eng. Chem. Res. 2003, 42, 5940-5945
RESEARCH NOTES Thermodynamic Analysis of Thermally Coupled Distillation Sequences Olga A. Flores,† J. Carlos Ca´ rdenas,† Salvador Herna´ ndez,*,† and Vicente Rico-Ramı´rez‡ Facultad de Quı´mica, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, Guanajuato 36050, Me´ xico, and Departamento de Ingenierı´a Quı´mica, Instituto Tecnolo´ gico de Celaya, Av. Tecnolo´ gico y Garcı´a Cubas s/n, Celaya, Guanajuato 38010, Me´ xico
Thermodynamic efficiency calculations have been performed for the separation of ternary and quaternary mixtures of hydrocarbons in both conventional and thermally coupled distillation sequences. When ternary mixtures were considered, energy savings achieved in the thermally coupled distillation sequences were between 10 and 30% in comparison to the two conventional distillation sequences. Regarding thermodynamic efficiency, thermally coupled distillation sequences presented the highest values in almost all of the cases considered. When the analysis was extended to the separation of quaternary mixtures, the thermally coupled distillation sequences show acceptable energy savings and the thermodynamic efficiency of a thermally coupled distillation sequence linked to a side stripper and a side rectifier was better than that obtained in the conventional distillation sequence; however, for the thermally coupled distillation with prefractionator (Petlyuk-type column), the second law efficiency was low when compared to that of the conventional distillation sequence for the separation of a quaternary mixture with a low content of intermediate components. 1. Introduction Distillation remains as one of the most widespread used separation methods despite its high energy demands on the reboilers. Nevertheless, the trend in process systems design is to include separation schemes that can provide significant energy savings; one alternative is the use of complex distillation sequences such as thermally coupled distillation sequences (TCDSs), which can offer energy savings around 30% for the separation of multicomponent mixtures in contrast to conventional distillation trains widely used in the chemical industry.1-5 Even though the Petlyuk column, the most important thermally coupled distillation scheme, was introduced 50 years ago,6 its use had been limited because of potential control problems.7,8 Recently, however, motivated by energy and capital savings, industrial implementations of TCDS schemes in companies such as BASF have been reported.9 Furthermore, it has been extensively shown that control in TCDS is not a more difficult task than that in distillation trains based on conventional distillation columns.10-13 As a result, current research efforts are focused on the design of new TCDSs for the separation of mixtures of more than three components14-17 and the control properties of such integrated schemes of separation.18,19 With respect to thermodynamic efficiencies of TCDS for ternary mixtures, Agrawal and Fidkowski20 have * To whom correspondence should be addressed. Tel.: (+52473) 732-0006 ext. 8142. Fax: (+52-473) 732-0006 ext. 8139. E-mail:
[email protected]. † Universidad de Guanajuato. ‡ Instituto Tecnolo´gico de Celaya.
recently reported that TCDSs do not necessarily present higher efficiencies than those of conventional distillation sequences and that the outcome of the comparison depends on the mixture and the composition. In this work, we study the energy consumption and the thermodynamic efficiencies of TCDS with side columns. Ternary and quaternary mixtures of hydro-
Figure 1. Conventional direct distillation sequence for ternary mixtures (DS).
10.1021/ie034011n CCC: $25.00 © 2003 American Chemical Society Published on Web 10/08/2003
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5941
Figure 2. Conventional indirect distillation sequence for ternary mixtures (IS). Figure 4. Indirect TCDS for ternary mixtures (TCDS-I).
plained in the work of Seader and Henley.21 Here we present those equations.
Energy balance:
∑
(nh + Q + Ws) -
out of system
∑
(nh + Q + Ws) ) 0 (1)
∑
(ns + Q/Ts) ) ∆Sirr (2)
in to system
Entropy balance:
∑
(ns + Q/Ts) -
out of system
in to system
Availability balance:
[ ( ) ] ∑ [ ( ) ] T0
∑ nb + Q 1 - T in to system
+ Ws -
s
nb + Q 1 -
Figure 3. Direct TCDS for ternary mixtures (TCDS-D).
out of system
T0 Ts
+ Ws ) LW (3)
carbons have been considered. For the case of ternary mixtures, the two conventional distillation sequences shown in Figures 1 and 2 and the two TCDSs indicated in Figures 3 and 4 were analyzed. In the case of the separation of quaternary mixtures, one conventional distillation sequence (Figure 5) and two TCDSs (Figures 6 and 7) were considered.
Minimum work of separation:
2. Thermodynamic Efficiencies
where b ) h - T0s is the availability function, LW ) T0∆Sirr represents the lost work in the system, and η is the thermodynamic efficiency. Second law efficiencies for all of the schemes considered can be computed from enthalpies, entropies, and optimal heat consumptions in the reboilers; the values of those properties can be
When continuous-flow and steady-state-flow systems, such as those shown in Figures 1-7, are analyzed under the laws of thermodynamics, thermodynamic efficiencies can be computed through standard expression as ex-
Wmin )
∑
out of system
nb -
∑
nb
(4)
in to system
Second law efficiency: η)
Wmin LW + Wmin
(5)
5942 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
Figure 6. TCDS for quaternary mixtures (TCDS-SS/SR). Figure 5. Conventional distillation sequence for quaternary mixtures (CDS).
obtained through the use of generalized rigorous process simulators, such as Aspen Plus 11.1. 3. Energy-Efficient Designs The design and optimization strategies for conventional distillation sequences involving the separation of ternary (Figures 1 and 2) and quaternary (Figure 5) mixtures are well-known. The energy-efficient design methods for TCDS-D (Figure 3) and TCDS-I (Figure 4) for the separation of ternary mixtures are described in the work of Herna´ndez and Jime´nez.4 Basically, preliminary designs of the TCDS options are obtained from the conventional distillation sequences shown in Figures 1 and 2. For example, the design of TCDS-D (Figure 3) is obtained by using a thermal link in the vapor phase in the conventional direct distillation sequence (Figure 1), which eliminates the reboiler in the column C-1. The vapor flow VF is changed until the minimum energy demand in the reboiler of the column C-2 of TCDS-D is obtained. The energy-efficient design of TCDS-I (Figure 4) is obtained directly from the conventional indirect distillation sequence (Figure 2) by removing the condenser of the column C-1 and introducing a thermal coupling in the liquid phase. This liquid recycle stream (LF) is varied until the minimum energy requirement for TCDS-I is obtained. For quaternary mixtures,17 the design and optimization of TCDS-SS/SR (Figure 6) and TCDS-PR (Figure 7) are clearly more complicated because of the higher number of recycle streams (LF and VF). The design
Figure 7. TCDS for quaternary mixtures (TCDS-PR).
strategy for those complex schemes starts from the stage sections of a conventional distillation sequence (Figure 5) with three distillation columns. Such sections are
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5943 Table 1. Mixtures Analyzed mixture type ternary
components
M1
n-pentane (A) n-hexane (B) n-heptane (C) n-butane isopentane n-pentane isobutane n-butane n-hexane n-pentane (A) n-hexane (B) n-heptane (C) n-octane (D)
M2 M3 quaternary
Table 2. Feed Compositions (kmol/h)
mixture
M4
then properly connected and optimized for the minimum energy consumption; the interconnecting streams are used as search variables on the optimization procedure. For instance, for TCDS-SS/SR (Figure 6), the design method uses the separation sections of a conventional distillation sequence (Figure 5), on which distillation column C-1 performs the split AB/CD (sections 1 and 2) and two additional columns (C-2 and C-3) perform the binary separations A/B (sections 3 and 4) and C/D (sections 5 and 6), respectively. Hence, for the preliminary design of the TCDS-SS/SR scheme with a side stripper and a side rectifier, the sequence includes a main column by adding the stages of (1) the conventional distillation column that performs the split AB/ CD (sections 1 and 2), (2) the rectifying section of the column for the separation A/B (section 3), and (3) the stripping section of the column that performs the split C/D (section 6). This main column is then thermally coupled through one liquid and one vapor interconnecting streams to the stripping stages of the column for the separation A/B (section 4) and the rectifying section of the column that separates the binary mixture C/D (section 5), respectively. Finally, the flows of the two interconnecting streams are varied until the minimum energy consumption for the TCDS option is obtained. The procedure for TCDS-PR is similar to the one described here; the difference is that a new section (section 7, Figure 7) is required to achieve the split B/C, which is not considered in the conventional distillation sequence of Figure 5. Sections 1 and 2 in the conventional distillation column C-1 of Figure 5 constitute the prefractionar column C-1 in TCDS-PR. The distillation column C-2 in TCDS-PR can be constructed by interconnecting the stripping section (section 4) of the conventional distillation column C-2 to the rectifying zone (section 5) of the conventional distillation sequence of Figure 5 using the stages in section 7 of Figure 7. The stages required in section 7 are equivalent to a total reflux column for the split B/C. The resulting design of TCDS-PR (Figure 7) is optimized for minimum energy consumption using VF and LF as search variables. 4. Cases of Study Energy consumptions and thermodynamic efficiencies for the separation of ternary and quaternary mixtures were calculated. Table 1 shows the mixtures considered, and Table 2 presents the sets of compositions analyzed. It is well-known that the energy savings obtained in the TCDSs for ternary separations depend strongly on the amount of the intermediate component. For that reason, we have considered cases with a low or high content of the intermediate components for both types of mixtures.
mixture type ternary quaternary
feed
components
F1 F2 F3 F4
18.16/9.08/18.16 13.62/18.16/13.62 13.62/9.08/9.08/13.62 9.08/13.62/13.62/9.08
Table 3. Energy Savings between Conventional and Complex Sequences mixture M1
M1
M2
M2
M3
M3
M4
feed F1
F2
F1
F2
F1
F2
F3 F4
distillation sequence
total heat duty (kW)
DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I CDS TCDS-SS/SR TCDS-PR CDS TCDS-SS/SR TCDS-PR
1129.6 697.1 864.2 635.1 935.6 836.9 962.1 790.9 2133.5 1590.2 2493.0 1543.3 2834.5 1942.6 2270.0 1957.9 1015.8 859.2 1064.1 923.7 1089.8 981.7 1218.5 1031.2 966.0 773.0 851.0 1300.0 1078.0 1007.0
% of energy savings 38 26 10.5 17.7 25.5 38 31.5 13.7 15.5 13 10 15 20 22
The ternary mixtures were separated using the distillation schemes of Figures 1-4, and the separation of the quaternary mixtures was performed using the conventional distillation sequence of Figure 5 as well as the complex distillation sequences of Figures 6 and 7. The TCDS options considered in this work for quaternary mixtures represent TCDS options with side columns and a Petlyuk-type system. The last type of TCDS sequence can be implemented in a divided-wall distillation column. Pressures in the columns were set in order to guarantee the use of cooling water in the condensers. 5. Results The design and optimization methods have been previously reported in the works of Herna´ndez and Jime´nez4,5 and Blancarte-Palacios et al.17 For that reason, we only present the trends observed in the optimum energy demands. When ternary mixtures were analyzed, in general the TCDSs presented energy savings in the range between 10 and 38%, in contrast to conventional distillation (Table 3). These results are in agreement with the classical result of energy savings around 30% widely reported. Two clear trends for the TCDSs were obtained: (1) for both types of feed compositions, the TCDSs always required less energy consumption to achieve the separation; (2) energy savings were higher in the feed composition with a low content of the intermediate component.
5944 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 Table 4. Thermodynamic Efficiencies of Conventional and Complex Sequences mixture
feed
M1
F1
M1
F2
M2
F1
M2
F2
M3
F1
M3
F2
M4
F3 F4
distillation sequence
η (%)
DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I DS TCDS-D IS TCDS-I CDS TCDS-SS/SR TCDS-PR CDS TCDS-SS/SR TCDS-PR
13 21 21 28 15 16 19 22 7 11 7 11 5.5 9 8 9 17.5 17.6 26 29 18 15 22.5 25.5 20 22.5 10.5 18 17 20
For the quaternary mixture analyzed, the TCDS-SS/ SR option presented energy savings of 20% (Table 3) in comparison to the conventional distillation sequence; in addition, the two TCDS options required less energy to complete the separation. The second law efficiencies calculated for all of the sequences for the separation of ternary mixtures show that, except for one case of study out of the 24 cases indicated in Table 4, the introduction of thermal links increased the thermodynamic efficiency. These increments were not superior to 8% because the conventional distillation sequences and the corresponding TCDSs have the same levels of temperatures in the product streams; the key difference is the introduction of thermal links. The optimization of these thermal links causes significant energy savings and improves the values of the second law efficiencies. At this stage, it is important to say that the second law efficiency calculated for the TCDS-D option is higher than that obtained for the distillation sequence, despite the fact that in the distillation sequence the levels of temperature are lower than those in TCDS-D, which uses the energy at the highest level (column C-2). This, of course, is compensated with the significant energy savings achieved through the use of the thermal links. This problem is not present in the indirect distillation sequence and TCDS-I options, which use energy at the same levels of temperature. The thermodynamic efficiencies computed for the quaternary mixture with a low content of intermediate components (Table 4) indicate that TCDS-SS/SR is the most efficient scheme, but its increment (2.5%) is significantly lower than the one obtained in the case of ternary mixtures. An important result is obtained for the second law efficiency of TCDS-PR (Petlyuk-type column); the corresponding efficiency is nearly half of the efficiencies of the other two distillation sequences
for quaternary mixtures. The reason is that the energy needed for the quaternary separation in TCDS-PR is supplied to the highest temperature level. On the other hand, TCDS-SS/SR can use energy at intermediate levels of temperature. This result is similar to that reported in the work of Agrawal and Fidkowski20 for the Petlyuk system for the separation of ternary mixtures. For the case of feed composition with a high content of intermediate components (F4), TCDS-PR presented a thermodynamic efficiency very similar to that obtained in the conventional distillation sequence. 6. Conclusions Energy consumptions and second law efficiencies were obtained for the separation of ternary and quaternary mixtures of hydrocarbons for conventional and complex distillation arrangements. When ternary mixtures were analyzed, TCDSs presented energy savings up to 38% in contrast to the conventional direct and indirect distillation sequences. Regarding thermodynamic efficiency, in most of the cases, the introduction of thermal links increased its value, but this increment was not superior to 8%; such an increment is caused by the reduction in the energy consumption through the use of thermal links. For quaternary mixtures of hydrocarbon, again important energy savings were observed, but when the TCDS is structurally similar to the Petlyuk system and the mixture to be separated has a low content of intermediate components, the thermodynamic efficiency is lower than that obtained in the conventional distillation sequence because all of the energy required to distill the quaternary mixture is supplied at the highest level of temperature. This last result is in agreement with that reported by Agrawal and Fidkowski20 for the separation of ideal ternary mixtures. Acknowledgment Financial support from CONCyTEG, Guanajuato, Me´xico, is gratefully acknowledged. Nomenclature C-1 ) distillation column C-2 ) distillation column C-3 ) distillation column h ) molar enthalpy LF ) interconnecting liquid flow n ) mole flow Q ) heat S ) molar entropy T0 ) temperature of the surroundings Ts ) temperature of the system VF ) interconnecting vapor flow Wmin ) minimum work for the separation Ws ) shaft work η ) second law efficiency
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Received for review July 18, 2003 Revised manuscript received September 10, 2003 Accepted September 29, 2003 IE034011N
