Drying fuel alcohols and natural gas with biosorbents based on agricultural by-products
Formulate high performance biosorbents from agricultural by-products for drying fuel alcohols and natural gas in a pressure swing adsorption (PSA) process.
Gas dehydration is an important component of energy production and distribution. Prior to distribution, natural gas must be dehydrated to control corrosion and prevent formation of solid hydrocarbon-water hydrates. In biofuel production, current processing techniques through fermentation generate low concentration alcohols (5-12% ethanol mixed with water and additional organics), which are then similarly dehydrated to achieve fuel-grade alcohol (greater than 99%). Current dehydration methods typically have concerns, such as pollution, high energy consumption, and high processing costs. Dr. Catherine Niu, with the Department of Chemical and Biological Engineering at the University of Saskatchewan, and her research team have undertaken a multi-year, multi-objective research project developing, analyzing, and testing the use of biomaterials (e.g., flax shives, canola meal, and oat hulls) as a bioadsorbent to dry natural gas and bio-alcohols.
The overall objective of this research project was to develop and test high performance biosorbents from agricultural by-products for drying natural gas and fuel alcohols (bio-alcohols) in a pressure swing adsorption process. The first study evaluated the use of flax shives as a biosorbent in natural gas dehydration in a pressure swing adsorption system as well as the development of a pressure swing adsorption system for future related projects. The second study evaluated the use of canola meal after protein extraction in drying butanol, and the third study evaluated the use of oat hulls in drying butanol in a similar system as a comparison. The fourth study in this project presents a comparative economic analysis of three dehydration processes to investigate the feasibility of using biosorbents in an industrial facility.
In natural gas production, it is necessary to dehydrate gases before distribution through pipelines to reduce corrosion and prevent formation of solid hydrocarbon-water hydrates. Current dehydration methods have high energy consumption, poor efficiency, and create environmental pollutants. To mitigate these elements, it may be possible to use high-cellulose biomaterials as a cost-effective biosorbent in pressure swing systems. A pressure swing system changes gases from high pressure to low pressure (vacuum) to move materials through a molecular sieve, potentially composed of biomaterials, to dehydrate natural gas or other bio-fuels. Pressure swing systems have low operating temperatures (typically lower than 60°C) and the ability to quickly and easily change pressures within the chamber, making them a plausible low-energy and low-cost component in the dehydration process. This first study focused on characterizing flax as a potential biosorbent, but also on the development of the pressure swing absorption system for use in future studies. Flax had higher water adsorption capacity (0.9g water /g flax) than most commercial adsorbents and demonstrated stable performance after 70 hydration/dehydration cycles. Flax was also stable up to 200°C, making it a plausible biosorbent material in natural gas dehydration.
The next study in this project evaluated the use of canola meal after protein extraction as a biosorbent in dehydrating butanol. Using the previously developed pressure swing system, Niu and colleagues studied the effects of various operating parameters (temperature, pressure, feed butanol composition, feed butanol flow rate, and canola meal particle size) on the effectiveness of canola meal as a biosorbent in the dehydration process. Canola meal was dried, sieved, and pelleted, then characterized using various analytical techniques to determine physical and chemical properties. They then modeled parameters to determine optimal conditions for testing to determine the effect of these parameters on water uptake, butanol uptake, water selectivity, butanol recovery, and maximum effluent butanol concentration. In their initial experiments, Niu and colleagues achieved greater than 97 v/v% butanol with biosorbent water uptake of 0.21g water/g canola meal. Using modeled optimal conditions, they achieved 90% butanol recovery and a final effective butanol concentration of 99.2 v/v% (fuel grade bio-alcohol). Reusibility of the canola meal as a biosorbent was also tested, where they cycled through the dehydration process 16 times, and found no deterioration in biosorbent quality. Overall, this project reports the capability to dry butanol from 55 to 99 v/v% using canola meal after protein extraction as a biosorbent material.
Niu and colleagues performed similar tests drying butanol with oat hulls as a comparison. They used similar techniques to characterize physical and chemical properties of oat hulls to model ideal process conditions, and then used pressure swing adsorbtion system to test the suitability of oat hulls as a biosorbent. Water adsorption of oat hulls was slightly lower than that of canola meal for drying lower grade butanol (55% v/v), but higher for drying higher grade butanol (95% v/v), meaning that canola meal biosorbent may have better performance for dehydrating lower-concentration bio-butanols and oat hulls may be better suited as a biosorbent to dehydrate higher-concentration bio-butanols. Using oat hulls, the highest adsorption capacity was 0.132 g biosorbent/g water and was capable of achieving 99.0% butanol content as an output.
The final study of this project presented a techno-economic analysis on three dehydration processes (tetraethyl-glycol system, temperature swing adsorption, and pressure swing adsorption) to investigate if pressure swing adsorption using biosorbents would be a feasible industrial solution for drying natural gas or bio-alcohols. To do this, they simulated a model using available industrial data and data collected from previously performed analyses to simulate operating conditions on these three systems. Result highlights include capital, operating, and recovery costs for each system. Their analysis shows that the pressure swing adsorption technique had the lowest capital cost ($2.45 million USD), lowest annual operating cost ($956,000 USD/year), and lowest gas emissions (<0.1 kg/h), when compared to temperature swing adsorption ($4.44 million USD, $1,350,851 USD/year, and 88.99 kg/h) and tetraethyl-glycol systems ($30.7 million USD, $982,821 USD/year, and 9.78 kg/h).
Additionally, the pressure swing system has fewer pieces of equipment, higher potential for automation, as well as lower and safer operating temperatures. Dehydrated gas recovery between canola meal or oat hull pressure swing adsorption systems were comparable (both above 99.9%), but overall cost for the oat hull biosorbent system was slightly higher as the water adsorption capacity of oat hulls is slightly higher than that of canola. Further research is required at the pilot scale to verify this economic analysis.
From this project, Niu and colleagues have developed various novel technologies with practical implications for canola producers. They have formulated and developed new technologies in biosorbents originating from canola waste products. Additionally, they have established processes with high efficiency and low cost for drying biofuel alcohols and natural gas. They present technologies to reduce the processing costs in renewable bioenergy, and the commercialization of by-products (canola meal, oat hulls, flax shives) could potentially increase revenue and reduce by-products left in, or requiring burning, in the field.