(738a) Experimental Investigation of Transport Processes and Fouling in Biofuel Process Streams

EXPERIMENTAL INVESTIGATION OF TRANSPORT PROCESSES AND FOULING IN BIOFUEL PROCESS STREAMS

Anuradha Mukherjee, Ken Bell, and Rob Whiteley

Oklahoma State University, Stillwater, OK

E-mail: anuradha.mukherjee@okstate.edu, kjbell@okstate.edu, rob.whiteley@okstate.edu,

Fluid physical properties and hydrodynamics impact transport of contaminants to equipment surfaces and ultimately affect the extent and rate of deposition. Flow velocity of the process stream and concentration of the contaminant are key variables affecting mass transfer dominated deposition. Biofuel process streams are â??complexâ?? (deposits formed from multiple foulants) fouling intensive fluids that carry biological agents, dissolved solids, biomass, and other proteinaceous substances. Despite the intensity of contaminants in these streams on/in process equipment, very little information is available about the transport of these materials, and resulting fouling.
Sweet sorghum fermented broth is one such potentially fouling liquid. The use of sweet sorghum as a bioenergy feedstock for ethanol production via liquid or solid-state fermentation is motivated by several factors â?? easily accessible sugars, low inputs for crop growth, simplistic process set-up for bioethanol production, and a potential for complete crop utilization, all provide incentives for increasing interest in the crop. Areas of concern for any bioenergy feedstock are the tendency and type of equipment fouling under process conditions. Solid transport process models or fouling models, and characteristics of solids, as fouling deposits are some of the research areas that need to be addressed in order to arrive at an effective cleaning strategy and ensure economic and safe biofuel production.
Two different methods were used to study fouling â?? in the first approach fluid characterization was used as a guide to examine fouling related transport processes. Contaminant concentration is a critical factor in subsequent transport and fouling and was quantified in the first set of experiments. Apart from solid quantification, deposit examination and transport property (density and viscosity) determination studies of sweet sorghum fluid were undertaken. The second study adopted a phenomenological approach - in heat exchangers transport, deposition, and accumulation of debris obstructs heat flow and thus emphasis is on the understanding and quantifying the additional resistance to heat transfer represented by the fouling. This was achieved by measuring surface temperatures and subsequently deducing the fouling factor.
Liquid sweet sorghum fermented product, product of in-field fermentation and suitable for subsequent on-farm separation served as the bioprocess fluid in question. Leaves and plant heads were removed from the stalk before pressing. Additionally, before the juice was mixed in with the yeast for fermentation, large particles and seeds were removed using a standard mesh paint bag of pore size 124 microns. Small biomass and particulate matter (solids from the
feedstock as well as cell biomass/yeast) that canâ??t be removed with preliminary separation methods were expected in the fermented product The National Renewable Energy Laboratory procedure for total and dissolved solids in liquid biomass was employed to determine solid content in sweet sorghum fermented liquid. Standard density and viscosity measurement techniques were used for physical property determination. Sweet sorghum composition was established using high-pressure liquid chromatography (HPLC) experiments following a previously established protocol. The deposits obtained in solid quantification experiments were examined using a Scanning Electron Microscope (SEM).
The density of the fermented sweet sorghum liquid was found to be 0.9978 g/cm3 and the viscosity was determined to be 3.4 cP. Total solid loading of fermented juice was determined to be less than 3 wt%, significantly lower compared to corn-based process streams. The main soluble constituents of the sweet sorghum fermented juice are ethanol, glycerol, and acetic acid. Solids observed under the SEM formed porous, three dimensional, multi-component deposits (crystallization, particulate) from fermented sweet sorghum juice.
Initial deposition experiments were conducted with sweet sorghum fermented liquid with solid loading of 2.6 wt%. The maximum recorded fouling resistance factor was calculated as
0.122 m2 oC/kW. This was lower than 0.34 m2 oC/kW for corn steep liquor, a corn bioethanol
byproduct and major fouling concern. Overall the growth of deposits in sweet sorghum fouling can be described by Kern and Seatonâ??s fouling model. The rate of accumulation of material on the surface, is a result of simultaneous deposition, and removal as described in Kern and Seatonâ??s fouling model. Individual contributions of deposition and removal rates are yet to be determined, but are part of ongoing efforts to define transport phenomena in bioprocess fluids.