Membrane glucose demudding by a decanter-membrane process: Challenges and benefits

Dextrose syrups are produced from starch by liquefaction combined with saccharification. In the conventional production process, rotary vacuum filters (RVFs) with a diatomaceous earth (kieselguhr) coating as filter aid are often used for removal of the so-called mud phase after the liquefaction and saccharification step. This mud phase consists of insoluble impurities and suspended solids such as proteins, fats and enzymes. The use of these RVFs is commonly associated with high investment and operating costs, e.g. high costs for kieselguhr and for the disposal of kieselguhr combined with sugar losses in the kieselguhr. Furthermore, the RVFs introduce a high complexity to the process combined with safety issues for the operators. Alternatively, closed systems with microfiltration (MF) and ultrafiltration (UF) have been proposed either as stand-alone units or in combination with high-speed separators (HSS) or decanters for concentration of the mud fraction and purification of the sweeteners (Lancrenon et al 1994, Singh and Cheryan, 1997, Singh and Cheryan, 1998). Apart from replacing the RVFs, these approaches eliminate the need for filter aid. Furthermore, downstream processing costs are also reduced since the membrane processes are removing more colour and turbidity from the dextrose syrups compared to the RVFs. The state-of-the-art concepts in the industry are either (1) stand-alone units with polymeric or inorganic membranes as open channel modules – tubular or plate-and-frame or (2) a process combination of HSS with polymeric spiral wound modules or decanters with inorganic tubular modules. Figure 1: Membrane – decanter concept for starch demuddingDespite their inherited advantages over RVFs, the electrical power consumption of these concepts with either open-channel membrane modules or with HSS as an initial step is commonly higher than for RVFs alone. To overcome this, a novel concept has been developed combining a decanter with spiral wound membranes. The key to this concept is the optimisation of the decanter performance to provide a feed stream which can be handled by a spiral wound module and thus provide a more robust and cost-reduced alternative to the HSS concept. The basic concept of the process is shown in Figure 1.The initial test and development of this concept started independently on the Alfa Laval decanter and membrane sites. The first membrane tests for demudding were carried out by Alfa Laval Business Centre Membranes – previously DDS Filtration/Danish Separation Systems (DSS) during the 1990ies. In these tests, flat sheet GR40PP membranes (polysulphone, Molecular Weight Cut-Off (MWCO) 100 kDalton) and FS40PP membranes (fluoro polymer, MWCO 100 kDalton) were tested in an M39 open channel plate-and-frame module. From these tests it was concluded that it was possible to use the GR40PP and FS40PP membranes in the M39 module for demudding of wheat-based and corn-based syrups. However, the use of open-channel plate-and-frame modules was associated with high investment and operating costs. Independent of the membrane development, the Alfa Laval Product Centre Decanter from 2001 and onwards tested decanters for demudding of wheat-starch based syrups. These tests demonstrated that a two-phase decanter can remove up to 99% of the mud but the colour removal was inferior compared to the RVFs. On this background, the idea to use the combination of a two phase decanter with spiral wound modules emerged. In 2008, the first combined test of a two-phase decanter with a spiral wound membrane unit was conducted at a producer of wheat-starch based syrup. The tests were conducted on a high maltose syrup and glucose DE45 using a full-scale STNX438 decanter and pilot scale membrane unit with a 4” spiral wound module. During the test, it was concluded that the decanter can remove up to 99% of the mud and achieve a relatively dry mud phase with low sugar loss, while the membrane unit polished the syrups exceeding the quality achieved by the RVFs on site. In Figure 2, a representative result from these tests is shown. Furthermore, it was confirmed that the spiral wound module could handle the feed from the decanter and could be cleaned with a simple alkaline cleaning. Figure 2: Results from the combined membrane – decanter testThe first two lines based on this new demudding concept have now been operating on industrial scale since the beginning of 2011 at a producer of corn-based syrup in Europe. The first of these lines combines an STNX 938 decanter with an Alfa Laval Membrane-UF 10 membrane unit with sequential cleaning and is demudding 25 – 35 m3/hour of glucose DE95. The second line consists of an STNX 438 decanter with an Alfa Laval Membrane-UF 10 membrane unit with sequential cleaning and is demudding 15 – 20 m3/hour of glucose DE40. Both lines are working as expected from the results obtained during the development of the concept. Generally, this presentation will show the successful development of the new concept for demudding of starch-based sweeteners from initial tests to the first successful installation of the concept on industrial scale.

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