Comparison of Vapor Recompression and Thermal Coupling for Energy Reduction in Binary Distillation

Transcription

1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3 (1): Scholarlink Research Institute Journals, 2012 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(1): (ISSN: ) Comparison of Vapor Recompression and Thermal Coupling for Energy Reduction in Binary Distillation M. A. Usman; T.O. Ajayi; and K.I. Famoriyo Department of Chemical Engineering, University of Lagos, Nigeria Corresponding Author: M. A. Usman Abstract This paper investigated two options to reducing energy consumption of distillation columns namely vapour recompression and thermal coupling (split tower). These methods are applied to an equimolal, saturated liquid feed mixture of methanol-water system. The system produces a saturated distillate product of 95 mol% methanol and a saturated liquid bottom of 5 mol% methanol. The energy requirements of the options investigated are computed and compared with that of a conventional single distillation column having the same system objectives. The results show that the conventional single distillation column consumes kW, vapour recompression consumes kW, saving 33.5% of the total energy requirement, while thermal coupling consumes kW, saving 11.25% of the total energy requirement. The energy servings recorded are converted into monetary values and the economic analysis computed to determine the best economically viable option. The results show that vapour recompression is the best economically viable option amongst the options investigated and worthy of investment. The time length to recover the investments on the major equipment used for each option is also computed and this is based on the anticipated annual profit for four years of plant operation Keywords: binary mixture; vapor recompression; thermal coupling; energy savings; payout time INTRODUCTION Separation processes make up 40% - 70% of capital and operating costs of chemical manufacturing. Distillation accounts for more than 60% of the total process energy for the manufacture of commodity chemical (Zhang et al., 2008). This unit operation is characterized by its high energy consumption and thermodynamically considered to be an inefficient process, as heat supplied to vaporize a liquid mixture in the reboiler is released during condensation at a lower temperature in the condenser. Competing separation processes, such as membranebased routes, adsorption and crystallization, have the potential for significant energy savings. However, the extensive replacement of distillation by such processes will not be economically attractive for many years to come. The biggest obstacle to this is the higher capital costs of alternative processes relative to distillation, particularly for high capacity applications. Also, the wholesale replacement of existing columns requires that existing equipment be scrapped, an expensive preposition. Thus, replacing distillation with other processes will probably have to wait until new plants are built and even then only if new developments lead to more cost-effective alternative processes. Consequently there have been research efforts in the last three decades that have provided choices that minimizes the energy consumption associated with the operation of 188 distillation sequences. The most remarkable results have been obtained in terms of alternative designs for the separation of ternary mixtures. Many innovative distillation columns have been developed, such as vapor recompression distillation (Brousse et al., 2008; Annakou and Mizsey, 1995; Gros et al., 1998), heat integration distillation (Huang et al., 1996a,b; Nakaiwa et al., 2000; Olujic et al., 2003; Huang et al., 2006), and thermally coupled distillation sequences (Hernandez et al., 2003; Blancarte-Palacios et al., 2003; Duc Long and Lee, 2011) to reduce the energy consumption of the distillation column. However, application of these methods has largely been to multi-component distillation due to their interesting configurations and attendant difficulty in finding optimum of the numerous sequences. Binary distillation has received very little attention except azeotropic distillation of binary mixtures such as ethanol-water (Gomez et al., 2008; Huang et al., 2006; Mussati et al., 2006; Vane and Alvarez, 2008; Kansha et al., 2009). Ajayi et al. (1999) applied three of the identified techniques in literature (Itoh et al., 1980; Stephenson and Anderson, 1980; Fredrick and Alexandra, 1985; Jones and Marvin, 1982) namely pre-fractionation distillation, double effect distillation and heat pumping to the binary distillation of methanol and water mixture. They identified the double effect distillation as the most appropriate based on energy savings and operating cost. This

2 paper applies two other options namely vapor recompression and thermal coupling to the distillation of methanol-water mixture. The two options were compared on the basis of energy saving, capital cost, operating cost and payout time with the conventional single column distillation to determine viability of the options. METHODOLOGY The feed used for this study is saturated liquid mixture of methanol-water system with a concentration of 50 mol% methanol at 1 atm. The feedstock is to be separated into 95 mol% methanol distillate and 5 mol% methanol bottoms. Design for separation of the feedstock was carried out using a conventional single column (base case), vapour recompression and thermal coupling methods. The detailed design of the column in each case was done by implementing McCabe Thiele method (Mc Cabe et al., 1993; King, 1971; Chopney, 1994; Perry and Green, 1998). The process simulation was conducted using the commercial simulator PRO/II ver.8.1(inversys). Convention Distillation The conventional single column distillation unit is shown in Figure 1 (See appendix). The major units designed are column, condenser and reboiler. Vapour Recompression The column is operated at 1 atm. pressure. The bottom products enter the condenser-reboiler at 4 bar as liquid and leave as vapour at the same pressure, then compressed from bar to 4 bar. The column is shown in Figure 2. Part of the assumptions is that there is neither superheating of the vapour nor subcooling of the liquid. Thermal Coupling (Split Tower) Here, the first distillation column operates at 5 atm. pressure and the second column operates at 1 atm. The arrangement consists of splitting the feed in the ratio 50:50 and distilling in the two distillation columns. This is shown in Figure 3.The following assumptions were made: (1) The actual reflux ratio was 1.2 times the minimum reflux ratio. (2) The tray efficiency is 100% (3) Saturated product and reflux streams were obtained (4) Equimolal overflow(5) Molar enthalpies of each stream mixture were additive of the enthalpies of the individual constituents at that temperature and pressure.(6) 80% compressor efficiency(7) 90% electric motor drive. Economic Analysis In a free market enterprise, the primary motive for investing in a business is to make profit. Thus, it is very important to consider first the economic implications of operating each of the three options investigated. In doing this, one needs to specify the design parameters of the major equipment used in each of the methods. For the distillation columns, the following need to be known: (1) The theoretical number of equilibrium stages, (2) The column height, (3) The column diameter. For the reboilers and condensers (all assumed to be heat exchangers), their respective heat transfer areas, their operating logmean temperature differences (L.M.T.D.) as well as their overall heat transfer coefficients all need to be known. The costs of all the piping and stream dividers are obtained from literature as a factor of the costs of major items of equipment. Computation of Fixed Capital Cost Computation of fixed capital cost for the three options investigated was based on the the major equipments. For the conventional single distillation column the equipments are distillation column, condenser and reboiler. The vapor recompression option has the following major equipments; Distillation column (1 atm operating pressure), condenser-reboiler, auxiliary reboiler, compressor, and desuperheater. The key equipement for thermal coupling option are; Distillation columns 1 & 2 (5 atm and I atm operating pressures respectively), reboiler 1 (for column 1), condenser-reboiler (for both columns), condenser 2 (for column 2), flash tank (for both columns). The use of desuperheater in vapour recompression and flash tank in thermal coupling option is to have a uniform bottom composition. Computation of Operating Cost The operating cost is a function of operating labour cost, raw materials cost and cost of utilities. Since the material and energy balances for the three options were based on the same amount of feed, the cost of raw materials were assumed to remain unchanged. The utilities required are: steam estimated at the cost of 6/ton and cooling water at the cost of 2.5/ton, according to Coulson and Richardson, Payout Time The payout time is a function of fixed capital investment, working capital as well as depreciation. Fixed capital startup Pay out time Profit after tax Depreciati on It is assumed that the working capital should be 15% of the fixed capital investment. To determine depreciation, a straight-line method is used where the fixed capital is divided equally into a number of parts equal to the number of years of the life of the plant. A part is recovered each year. It is assumed that the four methods should be operational for four years. RESULTS AND DISCUSSION Table 1 shows the total energy consumption of the three methods. It can be seen that our reference method which is the conventional single distillation column consumes kw of energy; the vapour recompression method consumes l528.l06 189

3 kw, saving 33.5% of the total heat requirement of conventional single column; the thermal coupling method consumes kw, saving 11.25% of the total heat requirement of conventional single column. The savings obtained from both vapor recompression and thermal coupling are significant enough to warrant further economic analysis to ascertain their viability. In comparison, the vapor recompression saves more energy than the thermal coupling method. Table 1. Energy consumption of the three methods Method Total Energy Consumed (kw) Energy Savings (%) Conventional Single Column Vapor Recompression Thermal Coupling Table 2 shows the summary of the economic analysis for the three methods. Column 1 of the table indicates the fixed capital cost of the three options. As expected the cost of the conventional single column distillation (base case) is the least. The thermal coupling requires only slightly higher fixed capital cost than the base case. In contrast, the capital cost of the vapor recompression is tremendously higher costing times the cost of the base case. Table 2. Summary of Economic Analysis of the methods Method Conventional Single Column Vapor Recompression Thermal Coupling Fixed Capital Cost ( ) Utility Cost ( ) Annual Operating Cost ( ) Payout Time (yr) Table 2 show the utility cost of the options which is directly linked to the energy consumption of the plant in terms of cooling water and steam. Expectedly the cost is in the following order; base case > thermal coupling > vapor recompression. It is significant to note that the utility cost of the base case is times higher than that of vapor recompression while that of thermal coupling is 6.84 times higher than vapor recompression. The annual operating cost which entails utility (including electricity) and raw materials for the three options are shown in Table 2. The observed order is vapor recompression > base case > thermal coupling. This indicates the significant cost of compression involved in the vapor recompression option. The payout time of the three options as seen in Table 2 show that the conventional single column has a fixed capital of ( ) and it is however expected that this investment should have been recouped at the end of 390 days (1.07yrs), the fixed capital investment for vapour recompression amounts to ( ), it is expected that the investment should have been recovered after 1058 days (2.9yrs) of plant operation; thermal coupling method has a fixed capital investment of ( ) and the investment is expected to be recovered after 890 days (2.44yrs) of plant operation provided all the plants were to be operated for four years. These values are however based on the anticipated average profit per year for the four years of the plants' operation as the main aim of the investor is to recover his fixed capital investment as quick as possible since the equipment is expected to depreciate in value with time. Both vapor recompression and thermal coupling give encouraging results in terms energy savings. However, the various economic indices have to be thoroughly considered in arriving at the better choice of the two options both in terms of its application to fresh design and retrofit of an existing one. The payout time is below 3 years for the two options and based on this and the energy savings, vapor recompression is better than thermal coupling. The design details for the three options are shown in the appendix. CONCLUSION Vapor recompression and thermal coupling options have been applied to reduce the energy consumption in the distillation of an equimolal mixture of methanol and water, and the results were compared with the conventional single column. The vapor recompression method saves 33.5 % energy while the thermal coupling method saves only %. In comparison of the options based on energy saving and payout time, vapor recompression is better than thermal coupling. However, both options are superior to the conventional single column distillation. REFERENCES Ajayi, T.O., Usman, M.A., Adeleye, O.O. (1999): Available Options for the knockdown of Energy Consumption in Distillation Columns. NSChE Journal, l8(1&2),1-12. Annakou, O., Mizsey, P. (1995): Rigorous investigation of heat pump assisted distillation. Heat Recovery Systems & CHP, 15, Blancarte-Palacios, J.L., Bautista-Valdes, M.N., Hernandez, S., Rico-Ramirez, V., Jimenez, A. (2003): Energy efficiency of thermally coupled distillation sequences for four-component mixtures. Ind Eng Chem Res., 42, Brousse, B., Claudel, B., Jallut, C. (2008): Modeling and optimization of the steady state operation of vapor recompression distillation column. Chem Eng Sci, 40,

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5 APPENDIX Parameters used in the design and Economic Analysis of the 3 options Distillation Colum Vapour Recompression Thermal Coupling The Column Column 1 Column 2 Operating pressure (atm) Plate spacing (m) Number of ideal plates: Reflux ratio: Column diameter (m) Column height (m) Vertical vessel cost ( ) Plate installation cost ( ) Total column cost ( ) Reboiler for Option 3(TC) Heat load(kw) Overall heat transfer coefficient ( W/m 2 K) 250 Log mean temperature difference (K) Heat transfer area (m 2) Purchased cost ( ) The Heat load(kw) Overall heat transfer coefficient ( W/m 2 K) Log mean temperature difference (K) Heat transfer area (m 2) Purchased cost ( ) The Reboiler Flow rate ( kmol/hr) Heat load: (kw) Operating pressure (atm) Required steam flow rate (kg/hr) Steam cost ( ) Compressor (for VR) Flash tank (for TC) Purchased cost ( ) NA PLANT OPERATION SUMMARY Fixed capital ( ) Utility cost ( ) Annual operating cost ( ) Total energy consumption (kw) Energy savings (%) Payout time: (days)

6 Cooling Water 9 1atm Distillation Column 9 Distillate Product Steam Reboiler Figure 1. Conventional Single Distillation Column Bottoms Product Reboiler 8 10 Compressor 4 Expansion Hot water 1 1atm Distillation Column 9 11 Desuperheater Figure2. Vapour Recompression Option 7 193

7 9 9 9 Steam Distillation Column 1 Distillation Column 2 3atm 1atm Reboiler Flash Distillate Figure 3. Thermal Coupling (Split Tower) Option Products 194