Our article entitled Energy efficient design of membranes processes by use of entropy production minimization has just been accepted for publication in Computers & Chemical Engineering. In the paper, we discuss how to develop more energy efficient membrane separation processes by use of entropy production minimization. For our example, which is a unit for separation of CO2 from syngas, minimization of the entropy production is equivalent to minimization of the work requirement, e.g. due to re-compression.
In the paper, we use optimal control theory to minimize the total entropy production of a membrane unit for separation of CO2 from natural gas, by control of the partial and total pressures on the permeate side. We find that with the control of the permeate partial pressures, the total entropy production can be significantly reduced (circa 38%), while the reduction is lower with the control of the total permeate pressure only (circa 6.4%). The continuous optimal results can serve as ideal limits for the practical design. We find that a threestep permeate pressure that approximates the optimum reduces the entropy production by 5.3%. This corresponds to a reduction of the compressor power of 3.8%, when the permeate gas is re-compressed to be further processed. This concept is illustrated in the figure below. We expect the results and findings in our work to be of general validity, also for other membrane separation processes.
The work was carried out in collaboration with Elisa Magnanelli and Signe Kjelstrup, both from the Department of Chemistry at NTNU, and Eivind Johannessen from Equinor (previously Statoil)