Applications of Microfiltration and Ultrafiltration in the Sugar Industry

Microfiltration and ultrafiltration retain microparticles that do not form a filter cake, but remain as solutes in the filtrate. The separation performance depends on the size and shape of the micropores on the membrane.

Microfiltration membranes are typically uniform porous membranes with tortuous pores. Their retention characteristics are usually expressed directly by the measured average pore size. Their pore size distribution is relatively wide, ranging from 0.02 to 10 μm, and their membrane thickness from 50 to 250 μm.

Ultrafiltration membranes consist of two layers: a surface-active layer and a support layer. The surface-active layer is very thin, 0.1 to 1.5 μm thick, with micropores of 1 to 20 nm in size. The pore size is relatively uniform and orderly arranged. The support layer is 200 to 250 μm thick, supporting the surface-active layer and providing sufficient strength to withstand pressure. The support layer is porous, has large pore sizes, and low flow resistance.

The pore size of an ultrafiltration membrane is usually defined by the molecular weight of the substances it retains. The molecular weight cutoff is the molecular weight of the substance that can be retained by 90% of its components. This determination is performed using typical spherical molecules of known molecular weight, such as glucose, sucrose, bacitracin, myoglobin, pepsin, and globulin, as reference materials. Commercial ultrafiltration membranes have molecular weight cutoffs ranging from 3 million to 500,000, categorized into several levels. The relationship between molecular weight cutoff and average pore size is shown in the table below.

The molecular weight cutoff mentioned above refers to spherical molecules, i.e., molecules with symmetrical dimensions. Many organic polymers are elongated with small diameters, allowing them to pass through smaller micropores.

Microfiltration and ultrafiltration membrane equipment is manufactured using a variety of materials and product types. The most commonly used are high-molecular-weight organic polymers such as cellulose acetate, cellulose nitrate, mixed cellulose esters, polytetrafluoroethylene, polyvinyl chloride, polystyrene, and later-developed polysulfone and polysulfonamide. Foreign manufacturers of filter membranes include brands such as MF-Milipore, Fluoropore, Mitex, Polyvic, and CelotateSelectron, each type offering multiple products with different pore sizes (manufacturers include Dupond, Dow, Rhone-Pouleng, and Schleicher). Domestic products are also developing; multiple manufacturers in Shanghai, Beijing, Dalian, Suzhou, Wuxi, and other locations offer filter membranes with various pore sizes. Organic polymer membranes are typically planar (or plate-and-frame) or manufactured as spiral plates or hollow fiber structures (long bundles with an inner diameter of 0.5–1.5 mm). Microporous tubes are also made from high-molecular-weight polyethylene using a sintering process.

In recent years, the manufacture of membranes using inorganic materials has seen significant development, commonly employing materials such as metals, metal oxides, ceramics, and glass. For example, microporous membranes are formed on the surface of porous ceramics using components such as Al2O3, TiO2, and ZrO3, typically in a tubular multi-channel (7-19 channels) form. Alternatively, they can be made by sintering materials such as stainless steel powder, offering advantages such as heat resistance, good chemical stability, and high mechanical strength.

Microfiltration and ultrafiltration are now widely used in many fields, particularly in water purification, wine purification, removal of bacteria and various particles from liquids, and recovery of useful substances or collection of harmful substances from factory wastewater. The potential application of microfiltration and ultrafiltration in the sugar industry has long been noted and studied, with considerable research in recent years.

Membrane filtration typically employs a transverse (tangential) flow mode, where the liquid being treated flows over the filter surface in a direction parallel to the filter surface. Most of the liquid passes through the filter membrane to become the purified liquid, while a small portion does not permeate and, along with the retained solutes (usually impurities), becomes the concentrate and is discharged later. This working method differs from conventional filtration methods, also known as "one-end closed" filtration, where substances that cannot pass through the filter layer accumulate on the filter surface to form a filter cake. Compared to this, the crossflow method has a larger amount of residue that does not pass through the filter medium, but it can operate continuously and has a higher filtration speed (due to less accumulation of retained substances). However, conventional filtration methods can also be used when removing suspended particles using microfiltration membranes.

The ratio of feed volume to concentrate volume is represented by the symbol VCF. It is a very important parameter in membrane filtration. A high VCF value means a higher degree of impurity concentration and a smaller residual liquid volume, but the membrane's filtration performance will decrease (due to more impurity accumulation).

Saska in the United States conducted several studies. They first used a ceramic membrane filter (Rhone-Poulene product, molecular weight cutoff 300,000, with 19 channels) to treat clarified juice from a sugar factory at 90°C for 20 hours. The initial permeate flow rate was 250–300 L/m²·h, gradually decreasing to 200–150 L/m²·h. The initial pressure was low, stabilizing at 0.4 MPa after 2 hours. The liquid flow rate was 6.5 cm/min. The permeate flow rate was also significantly affected by the VCF value. Multiple experiments showed that when the VCF was 2–3, the permeate flow rate was 200–300 L/m²·h; when the VCF was 8–10, the permeate flow rate decreased to 100–200 L/m²·h. Furthermore, the turbidity of the processed sugar juice also had a significant impact; higher turbidity resulted in faster membrane fouling, meaning a faster decrease in filtration speed during use. After filtration, the sugar juice's hyaluronic acid remained unchanged, and the color value decreased only slightly (from 11600 to 11400 IU), but the turbidity decreased from 240 NTU to 1.0, with 99.6% of the turbidity removed. In addition, 46% of the starch and 77% of the dextran were removed.

The clarified juice filtered through a ceramic membrane was then filtered again using a G30 desalination membrane (molecular weight cutoff 2500), repeated three times. The original clarified juice had a color value of 18000 IU, which decreased to 2000 IU after membrane filtration. The color value of sugar A obtained from boiling the original sugar juice was 3000 IU, meaning that the decolorization effect of membrane filtration exceeded that of a single crystallization. This implies that membrane filtration may replace the traditional recrystallization process of first boiling raw sugar and then refining it. The color value of sugar A obtained from boiling the clarified juice filtered through the membrane was 250 IU, which decreased to 100 IU after washing. This type of membrane filtration has a VCF value between 1 and 7, operates at a pressure of 1.4 MPa, and has a permeate flow rate of approximately 40 L/m².h, significantly lower than that of ceramic membranes.

He also experimented with diluting intermediate molasses from a refined sugar factory and filtering it using a ceramic membrane (molecular weight cutoff 300000). The feed concentration was 31 Bx, the filtrate concentration was 28.5 Bx, the feed turbidity was 330 NTU, and the filtrate turbidity was 30 NTU, a reduction of 91%. The feed color value was 21000 IU, and the filtrate color value was 18000 IU, a reduction of 14%. This treatment removes most of the high molecular weight substances and suspended particles, resulting in minimal contamination of the resin during subsequent use. It is an excellent pretreatment method for ion exchange and ion repulsion methods.

The pre- and post-membrane filtered sugar solutions were concentrated to high concentrations, and their viscosities were measured. At 80 Bx, the viscosity of the filtered sugar solution was 500 cp, while the unfiltered solution was 1000 cp, a reduction of approximately half. At 84 Bx, the viscosities were 1700 and 2700 cp, respectively, with the filtered solution being about 37% lower. The viscosity reduction of the membrane-filtered sugar solution is approximately equivalent to a 2 Bx reduction in sugar concentration, thus increasing the crystallization rate of boiled sugar by 3–5%.

The filtration rate of this treatment is also affected by the original clarity and turbidity of the material. For example, at a VCF of 5.0, the permeate flow rate of a sugar solution with an initial turbidity of 11 NTU is approximately 64 L/m², while that of a sugar solution with a turbidity of 80 NTU is only approximately 45 L/m²·h.

The residue from membrane filtration contains a large amount of sugar and concentrated impurities, requiring dilution with water before membrane filtration to recover the sugar (similar to washing the mud layer with water in general filtration).

In Madsen's experiment, beet extract was neutralized with a small amount of lime and filtered to remove insoluble matter. It was then filtered using a polysulfone ultrafiltration membrane GR61 with a molecular weight cutoff of 20,000 at 60–80 °C and 0.4–0.6 MPa. The filtrate purity was 91.5–94.5%, the color value was 1000–2500 IU, and the decolorization rate was 95%. The permeate flow rate was 45 L/m²·h. Another method involved treating syrup using a GR61P ultrafiltration membrane at 80℃ and 0.42 MPa. The syrup concentration was 65°Bx, and the filtration flow rate was 9 L/m²·h. This removed 30-50% of the pigment and increased the syrup purity by 1-1.5%. Ultrafiltration membranes with lower molecular weight cutoffs showed higher decolorization effects.

Chen Shan conducted a detailed study on the effects of ultrafiltration membranes on different types of sugarcane juice from sugar mills. The ultrafiltration membranes were made of polyethersulfone (PEK) with molecular weight cutoffs of 10,000, 20,000, and 70,000, respectively. The treated sugarcane juices included mixed juice, neutralized juice, and mixed clarified juice (a mixture of clarified juice and filtrate) from a sulfite process sugar mill. Nine experiments were conducted for each type, and the results were very similar. After treatment with ultrafiltration membranes, the color value of the mixed juice decreased by 30-38%, the turbidity decreased by 40-57%, and the purity increased by 0.5-0.8. For the mixed juice, the color value decreased by 81-84%, the turbidity decreased by 96-97%, and the purity increased by 1.6-1.9. Comparing ultrafiltration membranes with different molecular weight cutoffs, the 10,000 molecular weight membrane was slightly more effective than the 70,000 molecular weight membrane, resulting in slightly higher filtrate quality. The color value and turbidity of the filtrate after mixed juice treatment were lower than those after treating the clear juice. Ultrafiltration is quite effective at removing pigments and turbidity, but the increase in juice purity is not significant. This is because it removes relatively few inorganic substances, most of which are low molecular weight, and these are the main factors affecting juice purity.

The filtration rate of clear juice treated with ultrafiltration membranes is higher than that of mixed and neutralized juice, with approximately 30 liters of filtrate per square meter of membrane per hour. Such a filtration rate is difficult to meet production needs. To achieve a filtration capacity of 20 t/h, approximately 600 m² of ultrafiltration membranes would be required. This presents significant challenges in terms of both investment and management.

Ultrafiltration membranes are susceptible to fouling during use, leading to a rapid decline in performance. The large amount of residue left after filtration necessitates additional processes and equipment for sugar recovery. These issues require further research and solutions.

The sugar industry in Guangdong and Guangxi has researched and applied domestically produced PA microporous tube filters. These are cylindrical containers containing multiple microporous filter tubes. Liquid enters from the outside through the micropores and flows out through the tubes. The filter tubes are sintered polyethylene microporous tubes, 25 mm in diameter, available in various models, and can trap particles with diameters ranging from 0.1 to 10 μm. Tests have been conducted filtering clarified juice (a mixture of clarified and filtered juice) from sulfite process sugar factories, syrup from evaporators, and reconstituted syrup from sugar refineries (with activated carbon added). All tests showed that most of the insoluble matter in the sugar solution was removed, with a slight decrease in color value and a slight increase in purity. The filtration speed is relatively high when used for filtering clear juices, reaching 13 kg/min·m² for continuous use for 8 hours. After filtration, backwashing with hot water and compressed air can remove sediment. When used for filtering syrups, the filtration speed is lower, and it is more prone to clogging.