Optimum Design Of Gas Membrane Separation And Its Cascade Coupling Processes

Gas membrane separation has been widely used in industry. During the early period, it was merely used to separate the single binary mixtures; but in the recent years, its applications have been extended to the complex systems, e.g., a large group of various multicomponent mixtures. In consequence, some new challenges have emerged, e.g., the reliable prediction of membrane separation ability, the accurate simulation of nonideal multicomponent membrane separation, the shortcut design of collaborative multi-technology processes, and even the custom design of contaminant removal unit. In this thesis, a few targeted researches have been done, i.e., the correlation approach for predicting polymeric membrane selectivity, the finite element model for membrane process simulation, the thermodynamic model for analyzing energy loss along with mass transfer, the circle absorption for removing styrene deeply, and the shortcut design method for designing cascade coupling separation sequences. These attempts are expected to promote the development of membrane technology, including integrating it with traditional separation technologies and extending its applications in the newly developing fields.To predict membrane separation ability for newly arisen mixtures, the correlations between gas critical properties and mass transport parameters in polymer membranes were established on the basis of "solution-diffusion" mechanism. Solubility selectivity mainly depends on the difference in critical temperature, and diffusivity selectivity would be observably affected by both critical volume and polymer morphology. In comparison^with rubbery materials, glassy polymers are3orders of magnitude more selective in diffusivity. The correlations can shorten the period and reduce the expense for matching membrane with mixture separation, screening materials for membrane preparation. As a practical example, glassy polyimide membrane was selected by this method for TFE recovery. According to the experimental data, the selectivity αAir/TFE>23and permeation rate JTFE=0.1GPU, which is in convert with the predicted result. The integrative process of membrane unit and distillation was simulated with UniSim Design. As a result,91.5%of TFE in the vent gas can be recovered, and the specific consumption is0.08USD kg-1TFE. Such performance is superior to conventional acetone absorption, which handled with an energy consumption of0.19USD·kg-1TFE. In addition, the membrane system did not need volatile absorbent, so that it would not pollute the product of TFE.The numerical solution approach of general membrane modules, including hollow fiber and spiral wound modules, were established by finite element method on UniSim Design platform. Membrane modules were supposed to be the mass transfer units composed of finite membrane cells. Two basic membrane.dll units were used to construct a trial-and-error iteration process in software to simulate permeation process in a cell with multiple undefined streams. General nonideal factors, i.e., concentration polarization, variable permeation parameters and pressure drop, were thoroughly taken into account in simulation. The experimental data and simulation result of CO2membrane separation indicated that the relative error of finite element analysis is less than3.0%, which is close to the performance of classical differential method, and much smaller than the error (10.5%) of logarithmic average method. Besides, finite element method supplied a way to analyze the local permeation situation. For instance, the one-stage two-step CH4purification process designed after analyzing CH4concentration variation in modules can improve CH4recovery ratio from77%to95%, while energy efficiency and process capacity (0.466kWh·Nm-3CNG;0.81Nm3·m-2·h-1) are comparable with the general one-stage unit.To analyze energy dissipation in gas membrane separation and direct the optimal design of multi-stage separation systems, nonequilibrium thermodynamic model based on mass transfer characteristics was proposed. Mass transfer in membrane module was supposed as a two-step process, i.e.,Ⅰ) selective trans-membrane permeation and Ⅱ) the mix of local permeated gases differing in composition. Membrane selectivity, feed composition and pressure ratio governed the efficiency of Step Ⅰ, while permeation stage cut would affect the efficiency of Step Ⅱ. For the recycle membrane cascade systems, the partial modification of framework was suggested after thermodynamic analysis finding the stages in which work is wasted seriously. According to the retrofit of the4-stage CO2separation process, system energy efficiency was improved by5.9%, total compression duty was reduced by5.5%, membrane area was reduced by5.4%, and then construction investment was expected to be reduced by3.5%. For membrane system requiring high permeation stage cut, the multi-step framework handling with stepwise upgrade in feed/permeate pressure ratio was proposed after analyzing the effects of feed composition by thermodynamic model. According to the modification of C3H6recovery system, with gradient pressure ratio (pF/Pp=5.2～44.4), system energy efficiency could be improved from12.4%to13.7%, that means a promotion extent of10.6%.To sweeten the dehydrogenation vent gas before H2recovery, multi-stage cycle absorption with fresh ethylbenzene as absorbent was designed. Active styrene was deeply removed from the vent gas and then highly enriched in the absorbent for recovery via this operation. General absorption/desorption system, limited by absorbent regeneration degree, cannot deeply remove styrene. In order to avoid the limitation, fresh ethylbenzene was selected as absorbent, because it is styrene-free, supplied abundantly and steadily, moreover, the mixture after absorption can be processed by existing styrene refining plants. Better than the general cycle-free absorption in which styrene can only be concentrated to1.99mol%,5-stage cycle absorption tower could enrich styrene up to18.65mol%. By this way the energy used for styrene further purification would be reduced by73%. On this foundation, the combinational system of cycle absorption. condensation and membrane unit was designed for hydrogen recovery. The economic benefit was dominated by membrane permeation stage cut. It is resulted that the optimum stage cut is about0.84and the corresponding compressor output pressure is about1.85MPa. According to the process simulation results, this system can recover93%of styrene,86%of other aromatic hydrocarbons and90%of hydrogen in the feed.For shortcut design of cascade coupling multi-technology separation sequences, graphical design method based on triangular-coordinate system and vector analysis was established. This method could solve the problem that a large group of various multicomponent mixtures should be fractionated to multiple products, e.g., the separation tasks for multipurpose use of refinery gases. Refinery gases including dozens of species were simplified as the ternary mixture of fuel gas, H2and condensable hydrocarbons. In this manner, the combinatorial explosion of possible sequences was avoided. Besides, the dominant ranges of general separation technologies were demarcated in triangular-coordinate with target recovery ratio and specific consumption as the criterion. This partition map was applied to select the superior separation technologies, merge these similar off gases, and also properly arrange the feed input position to avoid serious work loss caused by mixing streams with large composition difference. According to the design for multipurpose use of aromatic isomerization vent gas, the integrative cascade separation system can recover more than85%of H2and97mol%of condensable hydrocarbons in the feed, with specific consumption equal to0.227kWh·Nm-3feed. Better than the patent process of MTR in USA, a well-known corporation in membrane technology, such a customized design resulted in an energy saving of21%. In addition, a280kt/a integration process has been successfully launched in2008for ZRCC of Sinopec, in which13streams of feed were processed together, and more than95%of H2and94%of condensable hydrocarbons have been recovered with the specific consumption reduced to0.099kWh·Nm-3feed.