- Email: [email protected]
Energy savings in heat-integrated distillation columns
~
Pergamon ENERGY
PII:S0360-5442(96)00157-0 SAVINGS
IN HEAT-INTEGRATED COLUMNS
Energy Vol. 22, No. 6, pp. 621-625, 1997 © 1997 ElsevierScience Ltd. All rights reserved Printed in Great Britain 0360-5442/97 $17.00+ 0.00
DISTILLATION
M. NAKAIWA, *'~ K. HUANG,* M. OWA,* T. AKIYA,* T. NAKANE,* M. SATO, * and T. TAKAMATSU ~ ~Department of Chemical Systems, National Institute of Materials and Chemical Research, Tsukuba 305, Japan and ~Research Institute of Industrial Technology, Kansai University, Suita 564, Japan
(Received 11 September 1996) A b s t r a c t - - T h e heat-integrated distillation column (HIDiC) provides one of the most effective applications of heat-pump technologies to industrial processes. It reinforces a separation process and yields larger energy savings than other methods such as overhead-to-reboiler heat pumps, which involve moving heat between the hottest and coldest points in the distillation column. A simulation study of this column has been applied to the benzene-toluene system to evaluate energy consumption and the required number of stages for comparison with a conventional column. In an example, the total energy requirements were reduced about 60% below those for a conventional column. © 1997 Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
Distillation columns are the major energy consumers in the chemical and petrochemical industries. In order to reduce energy consumption, many studies have been carded out and new distillation columns have been proposed since the 1970s [1-5]. The HIDiC is one of these (Fig. 1). Mah et al [6] analyzed this column (called SRV distillation) to evaluate the loss of available energy. Takamatsu et al [7] indicated the possibility for operation without either a reboiler or a condenser as in an Ideal HIDiC
|-T [ta4milw Fig. 1. Schematic of the heat-integrated distillation column (HIDiC).
~Author for correspondence. Fax: +81-298-54-4660; e-mail: [email protected] 621 EGY 22-6-E
622
M. Nakaiwaet al
(Fig. 2). Huang et al [8] used a dynamic model and studied the control feasibility of the HIDiC. Nakanishi et al [9] developed a design method for this column, and Noda et al [10] recently carded out a fundamental experimental study. Applications to distillation in other industries have also been studied [11,12]. In this study, the amount of energy required for the separation of mixtures is estimated by using a simulation model. The possibilities for energy reduction are considered for three typical conditions. 2. OPERATIONOF THE HIDiC A distillation column is generally divided into the rectifying and stripping sections. Thermal energy has to be supplied to the stripping section and removed from the rectifying section. The bottom reboiler and top condenser are the locations for the input and removal of thermal energy, respectively. If the energy removed from the rectifying section could be reused in the stripping section or waste heat were available, then energy savings would be achieved in a distillation column. An HIDiC is constructed in such a manner that the rectifying and stripping sections are separated because a compressor and a throttling valve are installed between them as shown in Fig. 1. The manipulation is completed by exchanging heat between the two sections. To provide the temperatures necessary to serve as driving forces for heat transfers from the rectifying to the stripping section, the former must be operated at a higher pressure than the latter. In the rectifying section, the total vapor and liquid flows decrease towards the top. The total vapor and liquid flows in the stripping section are reduced towards the bottom. In other words, the amount of reflux is changed from one stage to the others by condensation or evaporation. 3. ESTIMATIONOF THE REQUIREDENERGY As stated, in an HIDiC the rectifying section is operated at a higher temperature than the stripping section by compressing vapor from the stripping section. The total vapor and liquid flow rates decrease gradually with heat exchange towards the top in the rectifying section and towards the bottom in the stripping section. Consequently, this system may be operated at lower external reflux ratio than the minimum reflux ratio required in conventional distillation. The energy required for separation is obtained by calculating the loads on the reboiler and condenser from the external reflux ratio RMXNthe ratio of reflux to the bottom product-flow rate R'M]N, and the reflux ratio RMAXfor the compressor at the bottom
Fig. 2. Schematic of the ideal HIDiC.
Energy savings in heat-integrated distillation columns
623
stage of the rectifying section, based on the operating pressures in the two sections. The minimum value of the compressor load is obtained from the minimum RMAX, i.e. Qc = DA (RMI N + 1),
(1)
QB = BA (RMIN
(2)
--
1),
W = DA (RMA x + 1),
(3)
where A is the latent heat and w the compression work per unit flow rate; w depends on the composition of the flow through the compressor and on the pressure difference between the two sections. The possibilities for energy reduction fall into three categories, depending on the relation between the q value of the feed flow and (XD - Z F ) / ( X D -- X B ) . For case (I), q > (XD -- ZF)/(XD -- XB) and the condenser load may be reduced to zero. The reboiler load is calculated from Eq. (2) and the following relation: RMIN =
q - (XD -- XB)/(XD --
(4)
ZF).
For case (II), q = (XD - Z F ) / ( X D -- X B ) , and both the condenser and reboiler load may be set equal to zero. The ideal HIDiC (see Fig. 2) may be operated in this condition. For case (III), q < ( X o - ZF)/(XD -- X a ) a n d the reboiler load may be reduced to zero. The condenser load is calculated from Eq. (1) and the relation RMIN = (XD -- ZF)I(zF
--
XB) -- q(XD
-- XB)/(ZF --
XB).
Compression work may be allowed for by using q' instead of q, where q' = q - Q,,,/(AF).
4.
(6)
SIMULATION STUDY
A benzene-toluene mixture was adopted to carry out the simulation study for evaluation of a conventional column and of the HIDiC. The feed conditions and separation specifications are summarized in Table 1. For the purpose of our simulation, the model of a distillation column with equilibrium stages and operated at a specified pressure with zero pressure drop through stages is assumed. For heat transfer, the product of an overall heat-transfer coefficient and a heat-transfer area U A was assumed to be given by the following equation: Q = UAAT,
(7)
where AT is the temperature difference between the paired stages. U A was assumed to be constant for all thermally coupled stages. A tridiagonal matrix method [13] was used to carry out the simulations. Thermodynamic properties of binary mixtures were computed using the Soave-Redlich-Kwong equation of state [14]. Conventional columns are operated at a pressure of 0.1013 MPa. Table 1. Feed conditions, products specifications and operating pressures. Mixture
Benzene/toluene
Components: zF
0.5 0.98
XD
0.02
xa F q value Condenser Operating pressure
0.1 kmol/s (I): 1.0, (II): 0.5, (III): 0.0 Total condenser Rectifying section at 0.2026 MPa Stripping section at 0.1013 MPa
624
M. Nakaiwa et al Table 2. Calculation results.
Example
(1)
(II)
(Ill)
Distillation method
Conventional HIDiC
Conventional HIDiC
Conventional HIDiC
Minimum reflux ratios (-)
1.40
1.77
2.37
1.51 0.0 RMAX= 1.66 62 8.0 1,730 1,480 320 RMA x = RMI N =
Reflux ratio (-) Ideal stages AU (kW/K) Reboiler (kW) Condenser (kW) Compressor (kW) (efficiencty = 0.8)
1.54 28 -4,000 4,000 --
1.95 24 -3,210 4,800 --
1.88 RMIN= 0.0 RMAX= 2.07 76 12.3 100 1,480 380 RMA x =
2.61 22 -2,700 5,950 --
2.45 RMIN= 1.0 RMAX= 2.70 50 12.7 20 3,000 460 RMA x =
Table 2 shows simulation results for the conventional columns and the HIDiCs. RMAx in the HIDiC is I. 1 times larger than the theoretical minimum reflux ratio of a conventional column. The condenser load in Table 2 includes the latent heat of condensation for the distillate, so that the condenser load is almost zero for vapor top products in cases (I) and (II). Takamatsu et al [6] pointed out that case (II) may be realized in any distillation by adjusting the feed thermal condition. All examples indicate a substantial energy reduction for the HIDiC, but the number o f stages is increased compared to a conventional column. In case (II), the total energy requirements were reduced by about 60% below those for the conventional column. In these calculations, the compression load was tripled and converted into heat energy. Although an infinite number o f stages is necessary for the theoretical minimum reflux condition (minimum energy requirement) in a conventional column, the HIDiC can operate with energy requirements less than that o f the minimum value for a conventional column. REFERENCES 1. King, C. J., Separation Processes, 2nd ed., McGraw-Hill, New York, 1980. 2. Lueprasitsakul, V., Hasebe, S., Hashimoto, I. and Takamatsu, T., Journal of Chemical Engineering Japan, 1990, 23, 580. 3. Nakaiwa, M., Owa, M., Akiya, T., Lueprasitsakul, V. and Takamatsu, T., Kagaku Kogaku Ronbunshu, 1988, 14, 63. 4. Takamatsu, T., Lueprasitsakul, V. and Nakaiwa, M., Journal of Chemical Engineering Japan, 1988, 21, 595. 5. Tomisaka, Y., Tanaka, Y. and Nakanishi, E., Kagaku Kogaku Ronbunshu, 1991, 18, 24. 6. Mah, R. S. H., Nicholas, J. J. and Wodnik, R. S., Journal of the American Institute of Chemical Engineering, 1977, 23, 651. 7. Takamatsu, T., Nakaiwa, M. and Nakanishi, T., Kagaku Kogaku Ronbunshu, 1996, 22, 985. 8. Huang, K., Naikaiwa, M., Akiya, T., Aso, K. and Takamatsu, T., Journal of Chemical Engineering Japan, 1996, 29, 344. 9. Nakanishi, T., Aso, K., Noda, H., Nakaiwa, M. and Takamatsu, T. Asia-Pacific Conference on Sustainable Energy and Environmental Technology (APCSEET'96), paper No. 75, Singapore 19-21 June 1996. 10. Noda, H., Aso, K., Kobayashi, N., Nakaiwa, M. and Takamatsu, T., paper No. 76, in [9]. 11. Nakaiwa, M., Owa, M., Akiya, T., Nakane, T. and Sato, M., llth International Symposium on Alcohol Fuels, paper No. 12, Sun City, South Africa, 14-17 April 1996. 12. Nakaiwa, M., Akiya, T., Owa, M. and Tanaka, Y., Energy Conversions and Management, 1996, 37, 295. 13. Wang, J. G. and Henke, G. E., Hydrocarbon Processing, 1966, 45, 155. 14. Soave, G., Chemical Engineering Science, 1972, 27, 1197. NOMENCLATURE A = Heat-transfer area (m 2) B = Flow rate at the bottom (kmol/s) D = Flow rate of distillate (kmol/s) F = Flow rate of feed (kmol/s) Q = Heat transferred (kW) Qa = Reboiler duty (kW) Qc = Condenser duty (kW)
q = q value q' = q value given in Eq. (6) R M A x = Reflux ratio at the bottom stage of the rectifying section RM~N= External reflux ratio R~tN = The ratio of reflux to flow rate at the bottom
Energy savings in heat-integrated distillation columns U = Overall heat transfer coefficient (kW/(m2K)) W = Compression work (kW) w = Compression work per unit flow rate (kW/kmol)
xa = xD= ZF = AT = h=
Mole fraction of bottom product Mole fraction of distillate Mole fraction of feed Temperature difference (K) Latent heat (kJ/kmol)
625
Pergamon ENERGY
PII:S0360-5442(96)00157-0 SAVINGS
IN HEAT-INTEGRATED COLUMNS
Energy Vol. 22, No. 6, pp. 621-625, 1997 © 1997 ElsevierScience Ltd. All rights reserved Printed in Great Britain 0360-5442/97 $17.00+ 0.00
DISTILLATION
M. NAKAIWA, *'~ K. HUANG,* M. OWA,* T. AKIYA,* T. NAKANE,* M. SATO, * and T. TAKAMATSU ~ ~Department of Chemical Systems, National Institute of Materials and Chemical Research, Tsukuba 305, Japan and ~Research Institute of Industrial Technology, Kansai University, Suita 564, Japan
(Received 11 September 1996) A b s t r a c t - - T h e heat-integrated distillation column (HIDiC) provides one of the most effective applications of heat-pump technologies to industrial processes. It reinforces a separation process and yields larger energy savings than other methods such as overhead-to-reboiler heat pumps, which involve moving heat between the hottest and coldest points in the distillation column. A simulation study of this column has been applied to the benzene-toluene system to evaluate energy consumption and the required number of stages for comparison with a conventional column. In an example, the total energy requirements were reduced about 60% below those for a conventional column. © 1997 Elsevier Science Ltd. All rights reserved.
1. INTRODUCTION
Distillation columns are the major energy consumers in the chemical and petrochemical industries. In order to reduce energy consumption, many studies have been carded out and new distillation columns have been proposed since the 1970s [1-5]. The HIDiC is one of these (Fig. 1). Mah et al [6] analyzed this column (called SRV distillation) to evaluate the loss of available energy. Takamatsu et al [7] indicated the possibility for operation without either a reboiler or a condenser as in an Ideal HIDiC
|-T [ta4milw Fig. 1. Schematic of the heat-integrated distillation column (HIDiC).
~Author for correspondence. Fax: +81-298-54-4660; e-mail: [email protected] 621 EGY 22-6-E
622
M. Nakaiwaet al
(Fig. 2). Huang et al [8] used a dynamic model and studied the control feasibility of the HIDiC. Nakanishi et al [9] developed a design method for this column, and Noda et al [10] recently carded out a fundamental experimental study. Applications to distillation in other industries have also been studied [11,12]. In this study, the amount of energy required for the separation of mixtures is estimated by using a simulation model. The possibilities for energy reduction are considered for three typical conditions. 2. OPERATIONOF THE HIDiC A distillation column is generally divided into the rectifying and stripping sections. Thermal energy has to be supplied to the stripping section and removed from the rectifying section. The bottom reboiler and top condenser are the locations for the input and removal of thermal energy, respectively. If the energy removed from the rectifying section could be reused in the stripping section or waste heat were available, then energy savings would be achieved in a distillation column. An HIDiC is constructed in such a manner that the rectifying and stripping sections are separated because a compressor and a throttling valve are installed between them as shown in Fig. 1. The manipulation is completed by exchanging heat between the two sections. To provide the temperatures necessary to serve as driving forces for heat transfers from the rectifying to the stripping section, the former must be operated at a higher pressure than the latter. In the rectifying section, the total vapor and liquid flows decrease towards the top. The total vapor and liquid flows in the stripping section are reduced towards the bottom. In other words, the amount of reflux is changed from one stage to the others by condensation or evaporation. 3. ESTIMATIONOF THE REQUIREDENERGY As stated, in an HIDiC the rectifying section is operated at a higher temperature than the stripping section by compressing vapor from the stripping section. The total vapor and liquid flow rates decrease gradually with heat exchange towards the top in the rectifying section and towards the bottom in the stripping section. Consequently, this system may be operated at lower external reflux ratio than the minimum reflux ratio required in conventional distillation. The energy required for separation is obtained by calculating the loads on the reboiler and condenser from the external reflux ratio RMXNthe ratio of reflux to the bottom product-flow rate R'M]N, and the reflux ratio RMAXfor the compressor at the bottom
Fig. 2. Schematic of the ideal HIDiC.
Energy savings in heat-integrated distillation columns
623
stage of the rectifying section, based on the operating pressures in the two sections. The minimum value of the compressor load is obtained from the minimum RMAX, i.e. Qc = DA (RMI N + 1),
(1)
QB = BA (RMIN
(2)
--
1),
W = DA (RMA x + 1),
(3)
where A is the latent heat and w the compression work per unit flow rate; w depends on the composition of the flow through the compressor and on the pressure difference between the two sections. The possibilities for energy reduction fall into three categories, depending on the relation between the q value of the feed flow and (XD - Z F ) / ( X D -- X B ) . For case (I), q > (XD -- ZF)/(XD -- XB) and the condenser load may be reduced to zero. The reboiler load is calculated from Eq. (2) and the following relation: RMIN =
q - (XD -- XB)/(XD --
(4)
ZF).
For case (II), q = (XD - Z F ) / ( X D -- X B ) , and both the condenser and reboiler load may be set equal to zero. The ideal HIDiC (see Fig. 2) may be operated in this condition. For case (III), q < ( X o - ZF)/(XD -- X a ) a n d the reboiler load may be reduced to zero. The condenser load is calculated from Eq. (1) and the relation RMIN = (XD -- ZF)I(zF
--
XB) -- q(XD
-- XB)/(ZF --
XB).
Compression work may be allowed for by using q' instead of q, where q' = q - Q,,,/(AF).
4.
(6)
SIMULATION STUDY
A benzene-toluene mixture was adopted to carry out the simulation study for evaluation of a conventional column and of the HIDiC. The feed conditions and separation specifications are summarized in Table 1. For the purpose of our simulation, the model of a distillation column with equilibrium stages and operated at a specified pressure with zero pressure drop through stages is assumed. For heat transfer, the product of an overall heat-transfer coefficient and a heat-transfer area U A was assumed to be given by the following equation: Q = UAAT,
(7)
where AT is the temperature difference between the paired stages. U A was assumed to be constant for all thermally coupled stages. A tridiagonal matrix method [13] was used to carry out the simulations. Thermodynamic properties of binary mixtures were computed using the Soave-Redlich-Kwong equation of state [14]. Conventional columns are operated at a pressure of 0.1013 MPa. Table 1. Feed conditions, products specifications and operating pressures. Mixture
Benzene/toluene
Components: zF
0.5 0.98
XD
0.02
xa F q value Condenser Operating pressure
0.1 kmol/s (I): 1.0, (II): 0.5, (III): 0.0 Total condenser Rectifying section at 0.2026 MPa Stripping section at 0.1013 MPa
624
M. Nakaiwa et al Table 2. Calculation results.
Example
(1)
(II)
(Ill)
Distillation method
Conventional HIDiC
Conventional HIDiC
Conventional HIDiC
Minimum reflux ratios (-)
1.40
1.77
2.37
1.51 0.0 RMAX= 1.66 62 8.0 1,730 1,480 320 RMA x = RMI N =
Reflux ratio (-) Ideal stages AU (kW/K) Reboiler (kW) Condenser (kW) Compressor (kW) (efficiencty = 0.8)
1.54 28 -4,000 4,000 --
1.95 24 -3,210 4,800 --
1.88 RMIN= 0.0 RMAX= 2.07 76 12.3 100 1,480 380 RMA x =
2.61 22 -2,700 5,950 --
2.45 RMIN= 1.0 RMAX= 2.70 50 12.7 20 3,000 460 RMA x =
Table 2 shows simulation results for the conventional columns and the HIDiCs. RMAx in the HIDiC is I. 1 times larger than the theoretical minimum reflux ratio of a conventional column. The condenser load in Table 2 includes the latent heat of condensation for the distillate, so that the condenser load is almost zero for vapor top products in cases (I) and (II). Takamatsu et al [6] pointed out that case (II) may be realized in any distillation by adjusting the feed thermal condition. All examples indicate a substantial energy reduction for the HIDiC, but the number o f stages is increased compared to a conventional column. In case (II), the total energy requirements were reduced by about 60% below those for the conventional column. In these calculations, the compression load was tripled and converted into heat energy. Although an infinite number o f stages is necessary for the theoretical minimum reflux condition (minimum energy requirement) in a conventional column, the HIDiC can operate with energy requirements less than that o f the minimum value for a conventional column. REFERENCES 1. King, C. J., Separation Processes, 2nd ed., McGraw-Hill, New York, 1980. 2. Lueprasitsakul, V., Hasebe, S., Hashimoto, I. and Takamatsu, T., Journal of Chemical Engineering Japan, 1990, 23, 580. 3. Nakaiwa, M., Owa, M., Akiya, T., Lueprasitsakul, V. and Takamatsu, T., Kagaku Kogaku Ronbunshu, 1988, 14, 63. 4. Takamatsu, T., Lueprasitsakul, V. and Nakaiwa, M., Journal of Chemical Engineering Japan, 1988, 21, 595. 5. Tomisaka, Y., Tanaka, Y. and Nakanishi, E., Kagaku Kogaku Ronbunshu, 1991, 18, 24. 6. Mah, R. S. H., Nicholas, J. J. and Wodnik, R. S., Journal of the American Institute of Chemical Engineering, 1977, 23, 651. 7. Takamatsu, T., Nakaiwa, M. and Nakanishi, T., Kagaku Kogaku Ronbunshu, 1996, 22, 985. 8. Huang, K., Naikaiwa, M., Akiya, T., Aso, K. and Takamatsu, T., Journal of Chemical Engineering Japan, 1996, 29, 344. 9. Nakanishi, T., Aso, K., Noda, H., Nakaiwa, M. and Takamatsu, T. Asia-Pacific Conference on Sustainable Energy and Environmental Technology (APCSEET'96), paper No. 75, Singapore 19-21 June 1996. 10. Noda, H., Aso, K., Kobayashi, N., Nakaiwa, M. and Takamatsu, T., paper No. 76, in [9]. 11. Nakaiwa, M., Owa, M., Akiya, T., Nakane, T. and Sato, M., llth International Symposium on Alcohol Fuels, paper No. 12, Sun City, South Africa, 14-17 April 1996. 12. Nakaiwa, M., Akiya, T., Owa, M. and Tanaka, Y., Energy Conversions and Management, 1996, 37, 295. 13. Wang, J. G. and Henke, G. E., Hydrocarbon Processing, 1966, 45, 155. 14. Soave, G., Chemical Engineering Science, 1972, 27, 1197. NOMENCLATURE A = Heat-transfer area (m 2) B = Flow rate at the bottom (kmol/s) D = Flow rate of distillate (kmol/s) F = Flow rate of feed (kmol/s) Q = Heat transferred (kW) Qa = Reboiler duty (kW) Qc = Condenser duty (kW)
q = q value q' = q value given in Eq. (6) R M A x = Reflux ratio at the bottom stage of the rectifying section RM~N= External reflux ratio R~tN = The ratio of reflux to flow rate at the bottom
Energy savings in heat-integrated distillation columns U = Overall heat transfer coefficient (kW/(m2K)) W = Compression work (kW) w = Compression work per unit flow rate (kW/kmol)
xa = xD= ZF = AT = h=
Mole fraction of bottom product Mole fraction of distillate Mole fraction of feed Temperature difference (K) Latent heat (kJ/kmol)
625