The Development of Carboxylic Acid Separation by Nanofiltration Membrane for Carboxylate Platform Using Lignocellulosic Biomass

Open Access
Xiong, Boya
Graduate Program:
Agricultural and Biological Engineering
Master of Engineering
Document Type:
Master Thesis
Date of Defense:
June 06, 2014
Committee Members:
  • Thomas Lehman Richard, Thesis Advisor
  • Manish Kumar, Thesis Advisor
  • John Michael Regan, Thesis Advisor
  • Ali Demirci, Thesis Advisor
  • Virendra Puri, Thesis Advisor
  • Nanofiltration
  • Lignocellulosic biomass
  • Mixed carboxylic acids
  • Carboxylate platform
  • Acid separation
  • Biorefinery.
The carboxylate platform utilizes acidogenic digestion, converting lignocellulosic biomass anaerobically into short and medium chain carboxylic acids, which serve as reactive intermediates for downstream biofuel production. As a mixed-culture, non-sterile process, acidogenic digestion may provide a low-cost strategy for high substrate conversion rates and high product yield coefficients from lignocellulosic biomass. However, carboxylic acids in high concentrations are toxic to microorganisms, and can rapidly build up to inhibitory levels. For this process to achieve high conversion rates at an economical biomass loading rate, there is a need to separate and recover carboxylic acids during acidogenic digestion. Very limited research has been done to test different acid recovery technologies that can be applied to this carboxylate bioconversion platform. In this context, the goals of this study were to 1) identify the correlation between solid loading rate and acid yield 2) separate carboxylic acids from acidogenic digestion liquor using nanofiltration (Achilli et al.) and 3) integrate this NF acid removal process into a batch digestion system, with system performance evaluated by the acid yield. The biomass substrate for the digestion studies was mainly hot water pretreated willow wood. The effects of pH and feed pressure on the acid and sugar rejection were investigated with 10 day old willow digestion liquor using two commercialized NF membranes, GE Desal-DK and Desal-DL membrane. In general, NF membrane achieved 0% to 40% rejection of carboxylic acid with the exception of butyric acid (>99% rejection), and > 90% rejection of sugars. The high rejection of butyric acid may be due to butyric acid’s intermolecular interaction with the complex compounds in the digestion liquor. The lactic and acetic acid rejection decreased with pH which was correlated with the degree of dissociation of the acid, while the sugar rejection was not changed significantly by pH. Raising the feed pressure increased the permeate flux and slightly increased sugar rejection but unfortunately also increased the acid rejection. It is concluded that low pH and low pressure are the favored operational parameters to separate lactic and acetic acid. To integrate the processes of separation and digestion, the pretreated willow was digested and the acids were separated from the liquor intermittently by short term NF. After separation the retentate was recycled back into the digestion reactor with water added to maintain the original solids loading rate. The integrated digestion system was able to remove 86.8% of total acid produced and lowered the acid concentration by 87% compared to a control digestion system without acid removal. The acid yield was also enhanced by separation, but not by a statistically significant amount. This study provides preliminary engineering design data to accelerate the development of a robust and scalable digestion and separation process for the carboxylate platform of lignocellulosic biofuel production.