Retrofitting Distillation Columns with Membranes

Purification via distillation can be an extremely energy-intensive process. Energy requirements can be reduced by redirecting a lower-purity distillate or bottoms stream into a low-energy system, such as a membrane, for final purification.

Separation technologies cut across all manufacturing industries and account for approximately 4.5 petajoules/yr, or about 22% of all in-plant energy used in the U.S. Distillation alone accounts for half of the industrial separation energy consumed in the U.S. (1, 2). Implementing more-efficient separation techniques could contribute significantly toward meeting global energy reduction targets.

Researchers have devoted considerable effort to reducing the energy use of separations, for example by significantly modifying distillation columns (e.g., through feed splitting) (3–6) or by replacing them with membranes (2). Both of these options are worthy of consideration, but one note-worthy alternative is a hybrid system that combines distillation and membrane separation (7, 8).

Analyses of separation energy requirements show that increasing product purity at high purities — for example, from 95% to more than 99.5% — takes proportionally much larger energy input than at lower purities (e.g., increasing purity from 90% to 95%) (9, 10). This article discusses an alternative to high-purity distillation in which a lower-purity product stream from the distillation column is fed into a membrane system for finishing. Such a hybrid system allows manufacturers to retain their current distillation column and continue to generate high-purity products with less energy demand. The purchase and retrofitting of the low-energy membrane system would require less new capital expenditure than a complete replacement of the entire distillation column. This configuration can cost less to operate, require less energy input, and emit less CO₂ than using either separation system on its own.

This article discusses implementing retrofitted hybrid separation systems and uses two realistic examples to demonstrate potential cost and energy savings. In addition, the article lends insight into a critical decision that must be made when designing such a hybrid system: the product stream composition at which to switch from distillation to membrane. Also examined is the principal challenge to large-scale adoption of this approach — the availability of suitable membrane materials and systems.