Acetone Process Energy Recovery by Means of Energy Analysis

Transcription

1 Chemical Engineering Letters: Modeling, Simulation and Control 3 (2018) Chemical Engineering Letters: Modeling, Simulation and Control Journal Homepage: Acetone Process Energy Recovery by Means of Energy Analysis E. kianfar 1,*, M. Salimi 2, m. faghih 3, J. baghbani 4 1,3,4 Ph.D. Student, Department of Chemical Engineering, Arak Branch, Islamic Azad University, Arak, Iran 2 Ph.D. Department of Chemical Engineering, Arak Branch, Islamic Azad University, Arak, Iran ARTICLE INFO ABSTRACT Article History: Received 2 February 2018 Received in revised form 2 March 2018 Accepted 4 May 2018 Available online 7 May 2018 Keywords: Energy analysis, Thermal convertor, Thermal network modification, Energy recovery Dehydration of isopropyl alcohol is a method of acetone production which is endothermic. Thus isopropyl 88% w/w soluble in water is preheated in a thermal convertor up to 102 degrees centigrade and is inserted into R-401 reactor. Reactor s internal temperature is kept in optimum temperature 349 degrees centigrade using H-401 oven. Exerting flow of R-401 reactor is cooled using two water refrigerants and water flow 5 degrees centigrade to facilitate separation. In this paper, energy recovery of acetone production process is analyzed. The aim of this study is to recover valuable sources of energy and modify thermal convertors network in this process. According to energy analysis, hot exerting flow of reactor contains valuable amounts of energy and grate potential to preheat feed without using vapor. On the other hand, by transferring the energy of hot exerting flow of R-401 reactor, the temperature of this flow will reduce 44 degrees centigrade which lowers the thermal convertors after reactor. Of the results of this study, we can mention kilograms daily vapor storage and a daily kilograms decrease in cooling fluid and thermal convertors. 1. INTRODUCTION Due to energy crisis and rapid increase in oil price in world market in early 1970s, industrialized western countries which were major oil and natural gas importers, conducted vast studies to develop new technologies to decrease the energy consumed in a chemical reaction to reduce investments and dependence on oil exporter countries which leaded to pinch technology as a tools to design optimized thermal network exchanger [1]In late 1970s, Linn Hoff and Vredeveld studied a thermodynamic method to reduce energy consumption in thermal network exchangers and introduce concepts such as combination curves as a useful tool to recycle energy [2]. At the course of time, pinch technology developed dramatically, so that it was used not only in thermal network exchangers but also in distillation towers, furnaces, evaporators, turbines, reactors. Of course this technology faced problems including pressure drop limitations in modification of existing system, complexity of the unit, piping investment, safety issues etc. In the early 1990s, limitation of pressure drop was resolved and in the mid-1990s * Corresponding author: E. kianfar Ph.D. Student, Department of Chemical Engineering, Arak Branch, Islamic Azad University, Arak, Iran 16

2 problems which made this technology unpractical and non-economic was solved using decomposition theory [3].Tools to analyze pinch combination curves and combination curves are comprehensive. Pinch technology warrants minimum investment in the designing of thermal convertor networks. Exergy analysis (fig.1) makes use of first and second thermodynamic laws to calculate and distinguish optimized and non-optimized exergy flow in the system. But unfortunately, this method does not provide practical practices to avoid exergy waste. On the other hand, pinch technology is a general method in designing process which is able to aim most possible modifications before final designing and implantation. The drawback of this method would be clear when used in power generation system. Consequently, a new method has been developed to conquer drawbacks of the two above methods which is name combining analysis of pinch and exergy (fig.2) [4-6].Thus, in designing power generation and consumption systems pinch technology is not practical by itself and some other tools are needed to strengthen it. As a result, combining analysis of pinch and exergy method is really helpful. In this method, exergy combination curves (ECC) and Exergy general combination curves (EGCC) is used in system analysis. This way, by changing temperature axis to Carnot factor (ηc=1- T0/T), exergy combination curve is drawn and then exergy general combination is drawn based on that. The area between two combination curves will be equal to exergy drop in thermal exchanger s network [7-9]. 2. ACETONE PROCESS EXPLANATION Fig.1. Combination pinch and exergy curves (4) An azeotrope solution of water and isopropyl alcohol (88% w/w) is sent to arbitrage V-401 tank. In this tank potential vapors are separated from feed and a homogenous liquid feed is provided. Exerting fluid is sent from V- 401 tank to P-401 pump where the pressure of azeotrope feed increase up to 230 kpa. Exerting flow of pump is 23 degrees centigrade and definite pressure.due to reaction phase, this flow must be vaporized and then enter R-401 reactor. Azeotrope feed reaches 102 degrees centigrade in exposure to low pressure vapor in E-401 convertor with flow of 1280 kg/h and enters reactor vaporized. Hot exerting product of reactor includes water, hydrogen, acetone and not reacted alcohol with 349 degrees centigrade and is sent to E-402 and E-403 thermal convertors (Figure 2).In convertors mentioned above, hot exerting flow of reactor is cooled to 20 degrees centigrade in thermal exchange with cooling water and cool water 5 degrees centigrade and enters separation and purification stage [10]. Separation stage exactly starts from V-402 diphasic separator. In this separator light gases, hydrogen in particular, is separated from water, acetone and not reacted alcohol and set to hydrogen purification unit as off gas [11]. The remaining liquid in V-402 separator is raw acetone which will be purified in T-402 and T-403 distillation towers (Fig.3). Fig.2. schematic of synthetic part of acetone production process 17

3 Liquid exerting fluid in V-402 separator including water, acetone and not reacted alcohol is sent to T-402 tower where 99.9 % molar acetone is recycled. Product of T-402 distillation tower which is obtained from partial condenser is acetone with 99.9% molar purity which will be sent to storage tanks at 61 degrees centigrade [11].The product in the bottom of T-402 tower consisting of water and not reacted alcohol is sent to T-403 tower to recover alcohol and send it to the beginning of the process. T-403 tower (isopropyl alcohol tower) consists of 19 sieve trays and complete condenser. Product at top of T-403 tower is 88% w/w isopropyl alcohol in water at 83 degrees centigrade which will be sent to V-401 tank and mixed with feed. T-401 tower waste is water which will be sent to E-408 thermal convertor to later be sent to water purification unit after cooling [12]. Fig.3. schematic of separation and purification part of acetone 3. SINITIC OF REACTIONS IN REACTOR R-401 Acetone formation reaction from isopropyl alcohol is endothermic and standard reaction enthalpy is 62.9 kj/mole. Sinitic of reaction is controlled in the presence of catalyst and it can be showed by approximate concentration of isopropyl alcohol (fig.4). Fig.4. Sinitic of acetone production from isopropyl alcohol reaction 4. THERMAL MODEL NETWORK ANALYSIS OF ACETONE PROCESS As mentioned in explanation of the process, reactor temperature is high since reaction is endothermic which is provided with hot fluid supplied by H-401 oven. Entering feed changes phase in low pressure of E-401 convertor and enters R-401 reactor. Since hydrogen, as one of the products, is so light, exerting fluid of R-401 reactor should be cooled as much as possible to separation temperature of hydrogen. Cooling is conducted in two stages with cool water (30 degrees centigrade) and cooling fluid which enters E-403 convertor at 5 degrees centigrade. Highest energy consumption takes place in convertor before (E-401) reactor and two converters after the (E-402 and E-403) reactors. The process was simulated in Aspen plus software for energy analysis and thermal information was extracted (table 1). 18

4 Table.1. Information about thermal convertors Exchanger Name E-401 E-402 E-403 Exchanger Type HEATER COOLER COOLER Q(MJ/h) ENERGY ANALYSIS To analyze energy and then optimize acetone unit, first we will define goal. The goal is to reduce vapor and cooling fluid consumption. To achieve this goal, a scenario will be suggested according to desired energy amount in every thermal convertor (table 1). First we will calculate the enthalpy of R-401 reactor hot exerting flow: H R-401 Outlet = CPT Where, CP = Mass Flow Cp Cp = kj/kg C CP = = kj/h C H R-401 Outlet = CP T = = kj/h = MJ/h Since energy of R-401 reactor exerting flow is much higher than energy transmitted in E-401 convertor, i.e. H R- 401 Outlet > E Need@E-401, it can be used instead of vapor in this convertor. This scenario was simulated in software and results were positive. Now, the maximum energy that should be transmitted from R-401 reactor to feed flow should be calculated. Maximum energy transmission happens when hot flow can preheat feed flow up to 349 degrees centigrade (T Inlet = Toutlet) which is of course out of reach and current thermal convertor cannot provide desired level for this much energy transmission.as a result, a temperature should be selected as optimum and calculations be performed based on that. In this process optimum temperature is a temperature that reduce the temperature of R-401 reactor exerting flow so that there will be no need to use cooling water in E-402 convertor or its amount decreases. For this purpose, some temperature was predicted for feed preheating and the amount of energy needed to get feed temperature to the predicted temperature and the temperature of exerting hot flow of convertor after heat exchange with feed was calculated. First prediction is 125 degrees centigrade: E Need@E-401 = H Feed Inlet H Feed Outlet E Need@E-401 = (CP T) Outlet (CP T) Inlet E Need@E-401 = ( ) ( ) = kj/h where H=m.cP. T = CPT & CP = Cp Mass Flow Since the energy needed to get feed from 33 degrees centigrade to 125 degrees centigrade, is much less than reactor exerting fluid enthalpy, thus it is possible to use hot flow instead of vapor and the only limitation is the second goal, i.e. reducing the temperature of hot flow to less than 55 degrees centigrade and reduction of cooling water consumption in E-402 convertor. To achieve that, we can calculate the temperature of hot flow after heat exchange with feed flow using energy balance of E-401 convertor: (H2-H1) Feed = (H2-H1) Reac Outlet Left side of above relation is the energy needed to increase the temperature of feed to predicted temperature (E Need@E-401) so we ll have: E Need@E-401= (H2-H1) Reac Outlet Using above relation, the temperature of hot flow after heat exchange with feed flow will be degrees centigrade which is far away from determined temperature for cooling R-401 reactor exerting flow (45 degrees centigrade) and a new prediction will be needed. The second prediction will be 190 degrees centigrade and calculation will be repeated: E Need@E-401 = H Feed Inlet H Feed Outlet where: H= CPT & CP = Cp Mass Flow 19

5 E = (CP T) Outlet (CP T) Inlet E Need@E-401 = ( ) ( ) = kj/h The calculated energy needed show that at this temperature (predicted temperature of 190 degrees centigrade), the enthalpy of R-401 reactor exerting flow will be many times as much as E Need, thus the temperature of hot flow after heat exchange with feed flow is calculated again. The same energy balance will be used as the first stage: E Need@E-401= (H2-H1) Reac Outlet Calculations show that if feed flow is preheated by hot flow exerting R-401 reactor up to 190 degrees centigrade, the temperature of exerting flow of this reactor will become 44 degrees centigrade. At this temperature there is no need to use cooling water flow in E-402 thermal convertor and this convertor may be omitted. Finally, results of energy analysis were transferred to software and acetone process was simulated again (fig.5). 6. CONCLUSION Fig.5 simulation based on energy analysis results In this paper acetone process was energy analyzed. First energy needed in thermal convertors was calculated by software and reported. Then energy enthalpy of hot flow exerting R-401 reactor to be used a replaced with vapor in E-401 convertor was calculated. Calculations showed that the enthalpy of this reaction is much more than the energy needed to increase feed temperature from 33 degrees centigrade to predicted temperatures (125 degrees centigrade and 190 degrees centigrade), thus using exerting flow instead of water is possible. But fulfilling second goal depends on the temperature of exerting flow after exchanging heat with feed. For this, in every stage of temperature prediction, the temperature of feed flow after exchange was calculated using energy balance and it was found that if the temperature of exerting fluid is increased up to 190 degrees centigrade, besides savings in vapor consumption, hot flow temperature decreases down to 44 degrees centigrade. Given that in current procedure, water with flow of kilograms per hour is used to cool exerting flow of R- 401 reactor to 45 degrees centigrade, it can be said that by using this method, which is based on energy analysis, this much fluid can be stored which is economically very valuable. By decreasing the temperature of exerting flow of reactor to 44 degrees centigrade after energy exchange with feed flow, E-402 thermal convertor is no longer needed and this convertor would be omitted from convertor s network. Energy analysis results in acetone production unit saves kilograms of vapor decreases kilograms of cooling fluid daily and reduces thermal convertors needed. REFERENCES [1] Linnhoff, B. A. A. U. O. M. M. E. (1994). Use pinch analysis to knock down capital costs and emissions. Chemical Engineering Progress;(United States), 90(8). [2] Linnhoff, B., Townsend, D. W., Boland, D., Hewitt, G. F., Thomas, B. E. A., Guy, A. R., & Marsland, R. H. (1982). User Guide on Process Integration for the Efficient Use of Energy, IChemE, Rugby, UK, There is no corresponding record for this reference. [3] March, L. (1998). Introduction to pinch technology. Targeting House, Gadbrook Park, Northwich, Cheshire, CW9 7UZ, England. [4] March, L. (1998). Introduction to pinch technology. Targeting House, Gadbrook Park, Northwich, Cheshire, 20

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