Effect of membrane and process characteristics on cost and energy usage for separating alcohol-water mixtures using a hybrid vapor stripping-vapor permeation process

Abstract

BACKGROUND Alcohols, including ethanol and butanol, are receiving increased attention as renewable liquid biofuels. Alcohol concentrations may be low in a biological process due to product inhibition and, for non‐starch feedstocks, limited substrate concentrations. The result is high separation energy demand by conventional distillation scenarios, despite favorable vapor–liquid equilibrium and, for butanol, partial miscibility with water. A hybrid vapor stripping–vapor permeation process, termed membrane assisted vapor stripping (MAVS), incorporating a fractional condensation step was found to be at least 65% more energy efficient than conventional distillation approaches. The effect of process design, component performance, and capacity changes on the energy usage and processing cost of MAVS systems for separating ethanol, 1‐butanol, and acetone/butanol/ethanol (ABE) mixtures from water was studied. RESULTS For the recovery of 1‐butanol from a 1 wt% aqueous solution, the 99.5 wt% 1‐butanol product contained 7.0 times as much heating value energy as the MAVS process required to recover and dry it. The calculated cost to perform this separation was 0.126 US$ kg−1‐product (0.102 US$ L−1) for a heating value cost of 3.69 US$ GJ−1, far below the current values for crude oil and conventionally‐produced ethanol. Energy (electricity, natural gas) was 23% of this cost. The largest capital cost item was the compressor on the overhead vapor stream from the stripping column. Capital costs for membranes/modules was the 6th highest cost category, representing only 4% of the capital cost. A 10‐fold increase in membrane cost caused the cost of production to increase 38%. CONCLUSION Hybrid MAVS processes are an energy‐ and cost‐efficient means to recover alcohols from water. Despite recent fluctuations, fossil fuel costs are projected to increase. Thus processes utilizing mechanical energy to recapture and transfer thermal energy, including MAVS, should have a greater cost advantage in the future. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Prospects and challenges for the recovery of 2‐butanol produced by vacuum fermentation – a techno‐economic analysis

“…Other issues, such as membrane selectivity, thermal stability, and cleaning downtime, also affect process performance, and can lead to significantly increased production costs [59]. Even though the results herein presented for 2-butanol recovery systems are still far from the desired requirements, defined as 10% of its combustion energy (3.6 MJ/kg [23]), they are within the ranges reported for its isomer 1-butanol.…”

“…Most recently, membrane‐assisted vapor‐stripping systems were found to be 65% more energy efficient than conventional distillation, but the stripping rate in these processes can be limited by the membrane flux, causing solvents to circulate back to the fermentor . Other issues, such as membrane selectivity, thermal stability, and cleaning downtime, also affect process performance, and can lead to significantly increased production costs . Even though the results herein presented for 2‐butanol recovery systems are still far from the desired requirements, defined as 10% of its combustion energy (3.6 MJ/kg ), they are within the ranges reported for its isomer 1‐butanol.…”

Review: membrane materials for the removal of water from industrial solvents by pervaporation and vapor permeation

“…Higher temperatures translate into higher partial pressure driving forces which, in turn, result in lower membrane areas required. Finally, for some PV/VP systems, the cost of the membrane system can be a minor component of the overall capital and operating costs, especially if compressors, vacuum pumps, large heat exchanger areas, and – if a hybrid process – distillation columns are involved . All factors considered, the higher cost per unit area of inorganic membranes may not translate into a significantly higher total system cost, if at all …”

“…Finally, for some PV/VP systems, the cost of the membrane system can be a minor component of the overall capital and operating costs, especially if compressors, vacuum pumps, large heat exchanger areas, and -if a hybrid process -distillation columns are involved. 224,225 All factors considered, the higher cost per unit area of inorganic membranes may not translate into a significantly higher total system cost, if at all. 226 T-type and chabazite zeolites As noted above, NaA-type membranes can be degraded by dealumination when contacted with feed fluids exhibiting a high water activity or acidic condition.…”

Development and optimization of a novel process of double‐effect distillation with vapor recompression for bioethanol recovery and vapor permeation for bioethanol dehydration

“…Purchase cost of VP membrane modules (VP‐101 and VP‐102) is estimated as $250 m −2 of membrane area (i.e. membrane cost of $200 m −2 and membrane housing cost of $50 m −2 ) with a bare module factor of 2 at Chemical Engineering Plant Cost Index (CEPCI) of 571.2 . Membrane performance is assumed same within its expected life of a conservative 3 years, after which the membrane sheets are to be replaced with new ones but membrane module units remain the same.…”