Optimizing of three pathways for obtain DME using genetic algorithm and Aspen HYSYS
Dimethyl Ether (DME) is the simplest ether, colourless, nontoxic, no carcinogenic, noncorrosive whose boiling point is -25°C and is easily condensed above 0.5 MPa at environmental temperature. The synthesis and production of DME it’s playing an important role on the development of industry and is growing attention because of its multiple uses not only as a chemical product but also as an energy source and fuel replacement. Nowadays, this chemical is used as a replacement of chlorofluorocarbons (CFC’s) as an aerosol propellant, as an answer of accords signed on Montreal Protocol, which limits and suggest the replacements of chemical products that damage the ozone layer. Also, there are several studies where is shown that Dimethyl Ether’s physical properties are similar to those of liquefied petroleum gas (LPG) but with several advantages such as low NOx, SOx and PM emissions during combustion, hydrocarbons and carbon monoxide. Also there is neither production of soot nor smog during combustion and is preferred from other fuel alternatives like hydrogen, methanol and ethanol not only because of the simple procedure to obtain it but also for its safety, versatility and well-to-wheel performance and efficiencies. In addition, Dimethyl Ether is well used as a primary material and key intermediate for the synthesis of important chemicals and commodities such as dimethyl sulphate, methyl acetate and light olefins.
The commercial process applied to obtain DME (Turton et al.) begins with the preheating of the feed stream, then the reaction step is achieved on a packed bed reactor and the product stream is cooled leading finally to a sequence of two distillation columns allowing the separation of DME from wastewater. The most commercially used route, obtains pure methanol from syngas (which is mostly conformed of , and very often some ) by two reaction catalysed by a cooper-containing catalysts such as Cu/ZnO/Al2O3 or Cu/ZnO/Cr2O3, in these reactions, carbon monoxide reacts with water in a reverse reaction where the main product is methanol and water which reacts with carbon monoxide on a water gas shifting reaction, whose products are carbon dioxide and hydrogen which is also a reverse reaction. Then, a methanol dehydrogenation reverse reaction is catalysed using Alumina catalyst (), alumina modified with silica, phosphorus, zeolites materials so as modified H-ZSM-5 and SAPOs where methanol synthetized on the stage above interacts with the acid catalyst obtaining Dimethyl Ether and water as a sub product of the reaction. Several authors have studied and described the reaction rate by two kinetic models, which are the Langmuir-Hinshelwood model and Eley-Rideal model. The data required for the kinetic modelling has been obtained experimentally using autoclaves batch reactors, where liquid flow rate and temperature were controlled according to Hosseininejad et al. Then, liquid and vapour streams are analysed in a chromatography to know the initial rate reaction and the concentration of each product. There are some no common pathways to obtain DME, where ionic exchange membranes are used due to their “water resistant” faculty, this is because water deactivates alumina catalyst, and decreasing dimethyl ether conversion, increasing the capital cost because of the necessity of replace the deactivated catalyst. Also, catalytic distillation is being studied showing some advantages over commercially process as concentration field redistribution allows a dynamic equilibrium displacement so the reaction yield can be improved.
Having in account all the alternatives described above, some authors have considered mixing the two process stages mentioned, i.e. reaction stage and separation stage in search of process intensification, where dramatically plant size reduction is sought for improve process efficiency and a reduction of the equivalent annual operating costs (EAOC). To achieve this, catalytic distillation is suggested as an alternative route for obtaining Dimethyl-ether. Lei et al. propose two arrangements of towers where catalytic distillation is used. One of them is a fixed bed reactor and a catalytic distillation column. Also An et al. proposed a catalytic distillation column with an ordinary distillation column, as an alternative for enhance DME formation rate as the methanol reaction is an equilibrium-controlled-reaction.
For this work, three reactor and columns arrangements, (Fixed bed reactor and catalytic distillation column, a fixed bed reactor and two ordinary distillation columns and a catalytic column followed by an ordinary distillation column) are optimized having in account their performance, efficiency and economic issues. To achieve this task, each arrangements will be simulated on a chemical process simulation software, in this case Aspen HYSYS, and will be linked aided with a modification of the original library developed by the library of Olaf Trygve Berglihn on Matlab, that will be used as an intermediary (middleware) between Aspen Hysys and a genetic algorithm (Matlab) to optimize a cost function.
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