(696f) Attainability and Energy Consumption Studies of Membrane Processes

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 plus®. 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.