Energy Conservation Using Membrane Separation Technology

Transcription

1 Energy Conservation Using Membrane Separation Technology M. M. Khan 1, M. D. Eqbal *1, M. Danish 1 & M Yusuf Ansari 2 1 Department of Chemical Engineering, 2 Department of Petroleum Studies Z.H. College of Engineering & Technology Aligarh Muslim University, Aligarh Abstract Energy is most vital aspect of any chemical industry. Many chemical processes are highly energy intensive such as distillation processes. Saving even small amount of energy consumption of any process may leads to large profit. A lot of attempts are being made to save energy by different technologies; however, membrane separation technology offers the possibility in large scale reduction of energy. The membrane technology is not only eco-friendly but also reduces the energy and expenses. One of the major reasons behind the great popularity of this membrane technology is that it saves energy in chemical separation processes and petroleum refining applications. New approach in separation technology such as the hybrid of distillation column with membrane separation is very promising with respect to energy conservation point of view. Demerits of distillation column can be eliminated using this hybrid technology. In this paper attempts has been made to discuss the promising option for conservation of energy using membrane separation technology. I. INTRODUCTION Separation processes are very common in chemical and petrochemical industry. It is one of the most energy intensive as well as capital intensive industrial processes. About 40-70% of the capital is invested in the separation process. Distillation is the most common separation process used in almost all the chemical and petroleum industry. It has been reported that more than half of the total energy consumption is accounted under distillation. Membrane separation technology is the emerging technology which offers an alternative path for the various separation processes which can reduce energy consumption significantly. The membrane separation process along with the distillation process is called hybrid separation process, are found to be more sustainable and economical. One of the main advantages of the membrane separation is that it is not based on the vapor liquid equilibrium. In addition to it the separation which takes place trough the membrane is due the driving force which is generated through the lowering the partial pressure.however it is very selective. II. ENERGY CONSUMPTION IN SEPARATION PROCESSES As we know that the separation process is very energy intensive process. Since more than 40% of the total energy consumption is used in separation process and 90% of this energy is used in distillation process. An attempt to reduce this energy consumption has a great impact on the industry economy and sustainability. III. ENERGY CONSERVATION IN SEPARATION PROCESSES Due to the fast depletion of resources and scarcity of energy there is a great need to conserve it. In this paper, the methods are being discussed to conserve the energy in separation process especially distillation processes is under the target due to its unavoidable popularity. For example in petrochemical industry the membrane separation saves the purpose of reducing the energy consumption to 70% in the production of various chemicals such as ethanol, ethylene glycol, acetic acid and ethyl acetate. Thus installing a membrane separation unit, with the distillation column is called hybrid distillation column reduces the reflux ratio thus enabling the large reduction in energy utilization. IV. DESCRIPTION OF MEMBRANE SEPARATION PROCESS Membrane separation process is very simple process to understand. In this process membrane acts as semi permeable barrier between two liquid phases, or a liquid and a gas phase or two gas phases and separation occurs due to different sorption and diffusion behavior. When feed is passed through the membrane the permeate passes through the membrane and retentate is blocked. 53

2 V. HYBRID DISTILLATION COLUMN (HDC) Membrane technology had appeared in the late 1960s till then its application is growing day by day. Membrane distillation is one of the emerging technologies of membrane engineering which is used as the separation process.hybrid distillation column is basically a distillation column along with the membrane separation plant attached to it. In this process the vapor molecule are transported through the membrane due to difference in vapor pressure. VI. ADVANTAGES OF HYBRID MEMBRANE DISTILLATION COLUMN Low operating temperature Low hydrostatic pressure Capable to utilize low grade waste Use the alternative energy sources such as geothermal, solar energy can be used Can be used with the working distillation unit by the addition of membrane separate unit Can be used in different configuration to separate closed boiling mixture, azeotropic mixture and ternary mixture. Achieved a better, cheaper, easier, enhance separation. an important role in the energy profile of the whole process. For example, multiple choices are available for placing the membrane unit in hybrid membrane distillation column such as (i) before the feed fed in to distillation column, (ii) at the top of the column, (iii) at the bottom of the column, and (iv)at the intermediate point of column. The process of hybrid distillation column is quite simple, feed is fed in distillation column and the process is same as conventional, however when the membrane separation unit get combined with distillation column, it affects the entire separation process. The reflux ratio is reduced causing the reduction in number of stages distillation column. As a result the total heat duty of the entire process become lesser and it leads to the reduction in energy consumption. In short membrane separation unit reduces the load on distillation column. VIII. SEQUENCING OF HYBRID DISTILLATION MEMBRANE SEPARATION UNIT In the first sequence, Error! Reference source not found., the membrane separation unit is connected with the distillation column through the distillate stream. As the distillate stream enter to the membrane with less purity the permeates gives the maximum pure distillate while the retentate is sent back to the column below the reflux stage since it does not matches with the reflux composition purity. This sequence ultimately decreases the reflux ratio which decreases the load on the column and hence the energy consumption is decreased. VII. PROCESS DESCRIPTION OF HYBRID SEPARATION Hybrid separation system is a concept in which two different separation processes get combined to give a better separation with the objective of efficient energy reduction. In particular, the combination of the distillation column with the membrane separation unit can be a good example of hybrid separation system. In this type of system, sequencing of separation units plays 54

3 so it is joined with the distillate while the retentate is pushed back into the column below that intermediate point. In this case also the load on the distillation column is minimized, causing the reduction in the reflux ratio which leads to the decrease in the energy consumption. The fourth possible configuration, Error! Reference source not found., is basically the type of third configuration in which membrane is connected in between the distillation column and feed is taken from intermediate point. If the purity of permeate does not matches with the distillate then it is send back to the column between reflux and intermediate point and the retentate is given below the intermediate point. In the second sequence, Error! Reference source not found., the membrane is put in the feed stream both output streams that are permeate and retentate from the membrane enter the distillation column. Since the permeate is much pure than the retentate so it lies on the top of column while retentate is given at the bottom of the column and feed enters in the middle. In the third configuration, Error! Reference source not found., the membrane is placed anywhere in the intermediate points of the column. From that inter mediate point a stream is fed to the membrane, the composition of the permeate matches with the distillate IX. ENERGY CONSERVATION IN HYBRID DISTILLATION COLUMN The energy demand in distillation column is depend upon the reflux ratio and given by the following equation Energy demand=(r+1)*heat of vaporization (Eq 1) If we replace this conventional distillation column completely by the membrane separation unit we need only to supply the heat of vaporization which is the only energy required for the separation. Thus it saves huge amount of energy however, it cannot be possible due to need for high performance of membrane and large investment cost. So there is needed to connect the membrane separation unit with the distillation tower which is also effective enough to reduce the large amount of energy required in conventional distillation process. It has been observed that in order to separate the mixture of water and acetic acid in equal mass fraction to get 99% distillate and 99% at bottom with reflux ratio 55

4 as 5 is about kcal/hr, Error! Reference source not found.. If this conventional distillation is completely replaced by the pervaporation for the same feed and distillate the energy is required is only Kcal/hr for the membrane of selectivity 400, Error! Reference source not found.. This energy is about 6 times lesser than the energy supplying in conventional distillation. But when we have hybrid system that is distillation membrane system the energy required for the same operation is about Kcal/hr, Error! Reference source not found.. This case study is performed by Masahiko Matsukata et al (2011). the reboiler. Hybrid technology gives better separation result when distillation is carried out at cryogenic temperature such as in case of ethylene and propylene, energy requirement is very high thus creating a high load on the condenser. Hence hybridization of the membrane with distillation column is quite satisfactory in reducing the energy consumption especially when the key component of the distillation goes to extreme end. XI. REFERENCES [1] Ellen S. Lenz, Jose A. Caballero, Ignacio E. Grossmann, Majid Keyvani, Design of hybrid distillation-vapor membrane separation system. [2] I-Lung Chien. (2012),Opportunities for energy saving in azeotropioc separation processes. Process system [3] Masahiko Matsukata, Eiichi Kikuchi, Ken-ichi Sawamura.(2011), Review on prospects for energy saving in distillation process with micro porous membranes. Membrane separation and technology,vol.14 [4] M. S.EI-Bourawi, Z. Ding, R.Ma, M. Khayet. (2006). A framework for better understanding membrane distillation separation process. Membrane Science 285., 4-29 [5] P. Kresis, A. Gorak. (2006).Process analysis of hybrid separation process, Combination of distillation and pervaporation. Chemical engineering research and design,87(a7), [6] TorbjcrnPettersen, Kristian M. Lien. (1995). Design of hybrid distillation and vapor permeation processes. Membrane Science 99., [7] Wolfgang Stephan a, Richard D. Noble a, Carl A. Koval.(1995). Design methodology for a membrane/distillation column hybrid process. Membrane Science 99., [8] Yellaiah Naidu, Ranjan K. Malik. (2011). A generalized methodology for optimal configurations of hybrid distillation pervaporation processes. chemical engineering research and design 8 9., X. CONCLUSION The hybrid membrane distillation is very useful, as conventional distillation become very expensive, when the relative volatility is closed to one due to the load on 56

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