Attainability and energy consumption studies of membrane processes
Ali Alshehri, Zhiping Lai*
Advanced Membrane and Porous Materials Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Saudi Arabia
After more than 50 years intensive studies in membrane materials using modern technologies, the performance of existing membranes has been dramatically improved. As a result, membrane technology has been successfully commercialized in many challenging separation processes such as seawater desalination, natural gas separation, CO2 capture, and recovery of hydrocarbons, etc. With more and more experiences accumulated from these commercial membrane processes, conceptual design of a membrane process becomes feasible. We built a membrane module embedded in a commercial chemical engineering software, Aspen plusŪ. The membrane module included two membrane models. One is the simplest well mixed model where both feed and permeate sides are considered well-mixed. This model can be solved analytically, so it can be used to study the most important membrane process parameters. The other model is hollow fiber in counter flow configuration. This model requires complicated numerical solution but the results are more close to real membrane processes. Using this tool we can perform process simulation of not only membrane processes but also hybrid systems that combine membrane process with other separation processes such as distillation. Two important results shown in Figure 1 are obtained from the simulation results of a single-stage membrane process: (1) membrane selectivity S and pressure ratio g are the two control parameters and both exhibit a minimum value in order to meet a separation task that is defined by product purity and recovery ratio; (2) the minimum energy consumption is a monotonic function of the membrane selectivity. A minimum membrane selectivity is required in order to compete with a distillation process.
Figure 1: Left, the attainability of single stage membrane process is defined by selectivity and pressure ratio; right, the minimum energy assumption decreases monotonically with selectivity.
For multi-stage cascade membrane processes, the attainability is determined not only by selectivity and pressure ratio, but also by the recycle ratio which is defined as the ratio of flow rate of the stream recycled from one stage back to the previous stage to the flow rate of the feed. When the recycle ratio is 0, multi-stage membrane processes will decay to a single-stage membrane process; while if the recycle ratio increases to infinity, the minimum selectivity and pressure ratio of a n-stage membrane process have the following relationships with that of a single-stage membrane process: , . The minimum energy consumption of the multiple stage membrane processes also decreases monotonically with membrane selectivity.