The purification of corn steep liquor as a fermentation feedstock by ultrafiltration

Abstract

THE OBJECTIVE of this study was to devise a purification process, using ultrafiltration membranes as the core technology, for the preparation of corn steep liquor (CSL) as a fermentation feedstock. This process inherently required the development of a pretreatment system for the ultrafiltration membranes for the removal of suspended solids and high fouling material from corn steep liquor. The ultrafiltration membrane system was required for the separation and removal of colloidal solids from corn steep liquor, and to fractionate and separate out unwanted proteins, to render the feedstock suitable for sterilisation and subsequent fermentation. THE CONCEPT of membrane technology was investigated in order to find a more practical alternative for what was deemed to be a difficult process problem. In particular, various pretreatment technologies were investigated to form a compact and robust process package. THE CORN STEEP LIQUOR, a by-product of the corn wet milling process, was obtained from African Products, Germiston, in the form of a concentrated slurry directly from an evaporator system. A diagnostic of the feedstock was carried out and from this information, it was decided that three pretreatment options would be investigated. The first option was the pH treatment of the corn liquor, by the addition of ammonia which induced the precipitation of solids. This was followed by liquid-solid separation, and the clarified liquor was fed to the membrane system. The second option looked at the separation of suspended solids from the liquor by the use of broth conditioning additives and separation of the solids by a decanter centrifuge. The third option investigated was the use of a gyratory screening system for the removal of all solids greater than 100 μ in size. IN THE pH TREATMENT of CSL, the process is effected by the addition of base to pH 7. The technology involves neutralisation of CSL in a mixing system, under predetermined conditions of temperature, agitation and rate of addition, followed by subsequent liquid-solid separation. Trials were conducted on a pilot plant to test the process. Initial trials, conducted on a small scale pilot filter press, proved to be successful for this application. A suspended solids removal of up to 98% was achieved. The average suspended solids in the filtrate was found to range between 0.1 to 0.25 %. Tests were also conducted on a hired “state of art” filtration plant under various conditions. A diaphragm membrane press was found to provide the best performance. Protein recoveries of above 95 % at fluxes of 35 L/m2h at temperatures above 50 °C, and an incremental application of feed pressure was most suited for the process. The removal of the colloidal solids by the above-mentioned process was found to improve the quality of sterilisation. A reduction of more than 90 % in coagulated solids was achieved. v i IT WAS OBSERVED that the separation of suspended solids from CSL is enhanced by the use of coagulation and flocculation. Although not commonly used for this purpose, it was felt that a decanter centrifuge was well suited for the subsequent separation of the flocs from the clarified liquor. This work describes the results of the trials with such a device and the impact of broth conditioning on the efficiency of the separation. Trials have been conducted using an Alfa-Laval Model NX210 decanter, which was not specifically built for the work and therefore imposed several limitations on its performance. Despite these shortcomings, preliminary trials proved to be successful in achieving the separation objective. Tests were conducted using five different batches of CSL. With a maximum suspended solids loading of 4.3 % and a feed rate of 700 L/h, a solids recovery of 90 % was achieved. The clarified liquor contained residual solids between 0.5 and 0.8 %. The sludge had a solids concentration that ranged between 43 % and 65 %. COAGULATION AND FLOCCULATION dosages were kept within the limits of the laboratory evaluations. Flocculant dosages were controlled between 100 and 200 ppm, with the coagulant operating at higher dosages of between 400 and 2000 ppm. The only controllable parameter on the machine itself was the scroll differential speed. The best performance in terms of the cake dryness and centrate clarity was obtained at the lowest scroll differential speed of 4 rpm. THE USE OF GYRATORY SCREENS entailed passing the raw liquor through a set of two screens. The technology involves the use of a gyratory mechanism, which aids in the cleaning of the screens during continuous operation. Trials have been conducted on a pilot plant to test the system. Since the unit used was designed specifically for quick on-site screening exercises, it did not possess the added flexibility and robustness of a properly designed full scale unit. This imposed some limitations on its performance. However, despite these shortcomings, the trials conducted on the pilot plant proved to be successful in meeting the outlined objectives. A NUMBER OF TRIALS were performed on various batches of CSL. There was considerable batch to batch variation in the suspended solids content of the CSL and this was found to ultimately affect the throughput of the screening process. The feed suspended solids varied between 10 and 18 %. The highest throughput achieved was 400 L/h at a feed suspended solids loading of 14.5 %. It was found that temperature made a significant impact on the separation. The loss of heat in the feed stream caused excessive coagulation to occur thus increasing the suspended solids loading and lowering the throughput. The total solids in the sludge stream varied between 45 and 77 %. Protein loss in the sludge stream was around 1 %. Careful attention had to be given to the handling of the sludge stream. This stream displayed rheological characteristics typical of a non-Newtonian thixotropic fluid. The 100 μm screen operated best vi i when prior separation was done using a 180 to 200 μm screen. This reduced the solids loading on the tighter screen and increased the throughput by 10 to 15 %. The self cleaning mechanism also performed more efficiently under these conditions. THE SELECTED OPTION was then based on the influence the operation had on the ultrafiltration membranes, sterilisation of the product prior to fermentation and ultimately the fermentation performance. Subsequent testing of the pretreatment options were performed on an ultrafiltration membrane test cell. The product from the gyratory screens were found to produce the best overall results, where the highest fluxes and least amount of fouling occurred on the membranes tested. ONCE THE PRETREATMENT OPTION was decided, the development of the membane ultrafiltration system was then pursued. Trials were conducted on a laboratory scale, in a membrane test cell, to determine the preliminary screening of the membrane type, fouling effects and fluxes. It was found that polyvinylidene and polyacrylonitrile membranes produced the best overall fluxes of 11.25 and 10.96 L/m2h respectively. These membranes produced permeate protein concentrations of 121 and 115 g/L respectively. Sterilisation tests conducted on the permeate streams produced also showed that these two membranes had the lowest suspended solids concentrations. FERMENTABILITY tests conducted, showed that the ultrafiltered CSL, from these two membranes, produced increased cell counts and protein utilisation along with an increased product yield. Approximately 42 g/L of biomass was generated with lysine yields of 46 g/L. Further testwork revealed the non-Newtonian nature of CSL and its inherent viscosity effects. BENCH-SCALE testwork was conducted for various membrane configurations. With tubular membranes and hollow fibre membranes, average fluxes of 6.23 and 4.5 L/m2h were achieved respectively. Spiral wound membranes were found to be more consistent in their performance, with average fluxes of around 6.25 L/m2h. For the spiral wound membranes, it was found that the Desal-2 mesh spacer with a 80 mil thickness was most appropriate for the duty. PILOT PLANT testwork was conducted to scale-up the membrane system and to eliminate possible risks associated with the technology. The pilot plant studies showed up a number of principle design variables which needed careful attention. The flaws in the piloting system were subsequently rectified and this helped to improve the overall performance of the system.