Hybrid Membrane-Distillation Separation for Ethylene Cracking

Hybrid membrane-distillation separation for ethylene crackingAsma EtoumiThe University of Manchester2014Abstract ? PhD ThesisGas separations are often required in chemical processes, e.g. air separation,ethylene production, etc. These are often challenging and costly processes because ofthe low temperature and high pressure needed if vapour-liquid phase separations areinvolved. This thesis focuses on hybrid membrane-distillation separations as anopportunity to develop more energy-efficient separation processes.In a typical ethylene plant, recovery, the separation and purification of thecracked product are economically important. The focus of this thesis is on the ? C2splitter? which separates the desired product, ethylene, from ethane. Cryogenicdistillation, which is currently used to separate the binary ethylene-ethane mixture, isextremely expensive in terms of both capital and operating costs, especially becauseof refrigerated cooling requirements.Hybrid membrane-distillation processes are able to effectively separate lowboilingcompounds and close-boiling mixtures and to reduce energy consumption,relative to cryogenic distillation. However, hybrid membrane-distillation processespresent challenges for process modelling, design and operation. There are two majorchallenges associated with the modelling of hybrid processes for low temperatureseparations: i) the complex interaction between the process and the refrigerationsystem and ii) the large number of structural options, e.g. conventional column,membrane unit or hybrid membrane-distillation separation, where the distillationcolumn can be integrated with the membrane unit to form a sequential, parallel, ? top? or ? bottom? hybrid scheme.This thesis develops a systematic methodology to design, screen, evaluate andoptimise various design alternatives. Schemes are evaluated with respect to energyconsumption, i.e. power consumption of process and refrigeration compressors, orenergy costs. In the methodology, process options are screened first for feasibility,based on numerous simulations and sensitivity analyses. Then, the feasible optionsare evaluated in terms of energy consumption and compared to the performance of aconventional distillation column. Finally, economically viable schemes areoptimised to identify the most cost-effective heat-integrated structure and operatingconditions.The methodology applies models for multi-feed and multi-product distillationcolumns, the membrane, compressor and refrigeration system; heat recoveryopportunities are systematically captured and exploited. For the separation ofrelatively ideal mixtures, modified shortcut design methods, based on the Fenske-Underwood-Gilliland method are appropriate because they allow fast evaluationwithout needing detailed specification of column design parameters (i.e. number ofstages, feed and side draw stage locations and reflux ratio). The modificationsproposed by Suphanit (1999) for simple column design are extended to considermulti-feed and/or multi-product columns. The complex column designs based on the approximate calculations method are validated by comparison with more rigoroussimulations using Aspen HYSYS.To design the hybrid system, a reliable and robust membrane model is alsoneeded. To predict the performance of the module model, this work applies andmodifies detailed membrane model (Shindo et al., 1985) and approximate method(Naylor and Backer, 1955) to avoid the need for initial estimates of permeate puritiesand to facilitate convergence.Heat integration opportunities are considered to reduce the energy consumption ofthe system, considering interactions within the separation process and with therefrigeration system. A matrix-based approach (Farrokhpanah, 2009) is modified toassess opportunities for heat integration. The modified heat recovery modeleliminates the need to design the refrigeration cycle and uses a new simple, linearmodel that correlates the ideal (Carnot) and a more accurately predicted coefficientof performance.This work develops a framework for optimising important degrees of freedom inthe hybrid separation system, e.g. permeate pressure, stage cut, side draw molar flowrate and purity, column feed and side draw locations. Heat recovery options between:i) column feeds and products; ii) the membrane feed and products and iii) theassociated refrigeration system are considered. A deterministic and a stochasticoptimisation algorithm are applied and compared for their efficiency of solving theresulting nonlinear optimisation problem.The new approach is demonstrated for the design and optimisation of heatintegratedsequential and parallel hybrid membrane-distillation flowsheets. Casestudy results show that hybrid scheme can reduce energy cost by 11%, compared todistillation, and that parallel schemes have around 8% lower energy costs thansequential hybrid schemes. These results suggest hybrid membrane-distillationprocesses may be competitive with distillation when applied for ethylene-ethaneseparations, but that further development of suitable membranes may still be needed.