Comparing Membrane & Distillation on an Equivalent Basis

Comparing Membrane & Distillation on an Equivalent Basis
Adrian Chavez Velasco, Rakesh Agrawal
Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN

Among the separation technologies portfolio, distillation continues to be the preferable method for majority of the separations in the Chemical and Petrochemical plants. Just in the U.S. alone, it is estimated that about 90-95 % of all separations in chemical plants and refineries are performed by means of distillation columns [1]. Although distillation is widely used, it is frequently perceived to have a low energy efficiency, whereas in contrast, membrane separations are classified as a highly energy efficient processes which could potentially replace distillations on a wide scale [2], [3] [4].

However, this conclusion is not well supported since it has been mainly derived from comparisons made only with respect to conventional distillation, ignoring other operation modes that could be more energy efficient. Apparently, there is a widespread confusion in considering distillation as an absolute heat driven device, which sometimes requires a high amount of thermal energy input at the reboiler. The comparison between work driven membranes and heat driven distillations often leads to inaccurate conclusions. In reality, it is feasible to operate distillation solely with work input without requiring any external heat [5]. Our aim is to compare solely work driven distillations with membranes to avoid comparing two different forms of energy with each other.

The current research work centers on the development of a consistent procedure to compare the energy performance between membranes and distillation. This methodology comprises the use of rigorous simulation, and the optimization of both processes as an attempt to evaluate them at their best operating conditions. The proposed method was applied to the analysis of two important separation cases; p-xylene/o-xylene and propane/propylene. The results showed that the use of membranes with demonstrated permselectivity values does not always yield to the highest energy efficiency. Indeed, for the studied mixtures, the operation of distillation with work input consumes about an order of magnitude less energy than membranes for the case of p-x/o-xylene and ~27% less energy for the separation of propane/propylene under the conditions encountered at industrial scale (high recovery and purity). Membranes certainly offer some advantages over distillation in terms of energy efficiency, but for the analyzed cases, it only happens when the recovery is moderate or when the inlet is quite enriched in the most permeable component.

The extension of the developed methodology for the separation of other mixtures would enable researchers and practitioners to correctly identify under which conditions the use of either membranes or distillation would bring into the highest energy savings among them, thus contributing to design and operation of efficient separation processes.

Bibliography

[1] U.S. Department of Energy, "Materials Research for Separation Technologies: Energy and Emission Reduction Opportunities," 2005.
[2] D. S. Sholl and R. P. Lively, "Seven chemical separations to change the world," Nature, vol. 532, pp. 435-437, 2016.
[3] J. W. Koros, "Evolving Beyond the Thermal Age of Separation Processes: Membranes Can Lead the Way," AIChE Journal, vol. 50, no. 10, pp. 2326-2334, 2004.
[4] R. P. Lively and D. S. Sholl, "From water to organics in membrane separations," Nature Materials, vol. 16, pp. 276-279, 2017.
[5] A. A. Shenvi, D. M. Herron and R. Agrawal, "Energy Efficiency Limitations of the Conventional Heat Integrated Distillation Column (HIDiC) Configuration for Binary Distillation," Ind. Eng. Chem. Res., vol. 50, pp. 119-130, 2011.