Separation of saccharides

In many commercial enterprises, sugar is utilized to a great extent for its sweetening properties. It is used in the sweetening of foods, for the manufacture of syrups and confectionery items, in preserves and jams, as a chemical intermediate for detergents, emulsifying agent and other sucrose derivatives such as plasticizers, resins, glues, etc. The usual derivation of sugar is from cane sugar and sugar beets. It is obtained by crushing and extracting the sugar from the cane with water or extracting the sugar from the sugar beet with water followed by evaporation and purifying with lime, absorbent carbon and/or various liquids. The chief component of this type of sugar is sucrose, while other sugars may contain other polysaccharides such as dextrose and levulose (fructose). Other polysaccharides which possess sweetening properties include glucose, maltose, etc. The various polysaccharides possess varying degrees of sweetness, especially when in pure form and not contaminated by any reversion products.

One source of glucose which possesses a relatively high degree of sweetness and which, in turn, may be converted to fructose, the latter possessing an even greater degree of sweetness, is a starch. As is well known, starch is present in many naturally-occurring plants including corn, potatoes, rice, tapioca, wheat, etc. Heretofore, it has been known to treat starch with an enzyme such as amyloglucosidase to obtain glucose. However, the treatment heretofore provided entailed a relatively long residence time in order to obtain a glucose syrup which contained about 94% glucose. The relatively long residence time which has heretofore been required restricts the throughput of glucose and results in the appearance of reversion products which impart a bitter taste to the glucose, thus negating the sweetening property of the compound as well as requiring further treatment in order to remove the offending product. One such reversion product which imparts a bitter taste comprises isomaltose.

In addition to the reversion products which are formed during the reaction period, some of the polysaccharide remains unconverted. In order to obtain the desired mono- or disaccharide in a relatively pure form, it is necessary to separate these unconverted polysaccharides from the desired reaction product. In addition, if a free enzyme has been used to effect the desired conversion, it must also be separated from the reaction product. As will hereinafter be shown in greater detail, it has now been found that the desired separation process may be effected by separating the reaction mixture comprising monosaccharides, disaccharides, unconverted polysaccharides, and, in some instances, enzyme over an ultra-filtration membrane at a relatively high separation efficiency whereby a purification of the desired sugars is effected.

This invention relates to a process for the separation of mono- or disaccharides from polysaccharides. More specifically, the invention is concerned with the separation of a mono- or disaccharide from unreacted polysaccharides and, in some instances, conversion agents such as enzymes in which the ultra-filtration membrane employed in the separation will possess a high separation efficiency while effecting the separation reaction at a relatively high flux.

Syrups which are high in glucose or maltose concentration comprise desirable chemical entities. By utilizing these syrups which are high in glucose or maltose content for sweetening processes such as in baking goods, candies or in the case of maltose, in culture media or even as a sweetener or nutrient, it is possible to utilize lesser amounts of these syrups, thereby concomitantly reducing the overall cost of the finished product.

The US-A-3 668 007 discloses a process for the separation of a mono- or disaccharide from polysaccharides by passing a mixture of monosaccharide, disaccharide and polysaccharides over an ultra-filtration membrane at ultra-filtration conditions, and recovering the mono- or disaccharide. The membrane disclosed by the said patent is fundamentally different from that one according to the invention both in chemical composition and type. The chemical composition is that of a linear aromatic polyamide and not of a cross-linked polyurethane. The type is a self-supporting asymmetric hollow fiber and not a composite membrane.

The US-A-4 005 012 shows a process for desalination of saline water with a reverse osmosis membrane prepared by forming an ultra-thin film on a microporous support by reacting an amine modified polyepihalohydrin with a polyfunctional agent such as a diisocyanate. But desalination is not comparable with separation of saccharides, i.e. separation of organic molecules having different molecular sizes.

It is therefore the object of this invention to provide a separation process utilizing an ultra-filtration membrane capable of effecting the separation of mono- or disaccharides from polysaccharides while employing a relatively high flux with a concurrent high separation efficiency.

The process according to the invention for the separation of a mono- or dissaccharide from polysaccharide by passing a mixture of monosaccharide, disaccharide and polysaccharides over an ultra-filtration membrane at ultra-filtration conditions, and recovering the mono- or disaccharide is characterized in that an ultra-filtration membrane is used which comprises a cross-linked polyurethane prepared by cross-linking a polyamine of the group of amine-modified polyepihalohydrins, polyethyleneimine, polyvinylamine, diethylenetriamine, dipropylenetriamine, triethylenetetramine, tripropylenetetramine, tributylenetetramine, tetraethylenepentamine and pentaethylenehexamine, with a cross-linking agent of the group of aliphatic or aromatic diisocyanates, composited on a support.

A specific embodiment of this invention is found in a process for the separation of a mono- or disaccharide from polysaccharides which comprises passing a mixture of glucose or maltose and starch over an ultra-filtration membrane comprising an amine-modified epichlorohydrin composited on polysulfone at a temperature in the range of from ambient to 70°C and a pressure in the range of from 20 to 1000 pounds per square inch (140-7000 kPa), and recovering the glucose or maltose.

The supports which may be employed will comprise various polymeric compositions such as, but not limited to, polysulfones, polyesters, polycarbonates, polyolefins such as polyethylene, polypropylen, polyvinyl chloride, etc.

Examples of polyamine compounds which may be cross-linked will include amine-modified polyepihalohydrins such as epichlorhydrin, epibromohydrin, 1,2 - epoxy - 4 - chlorobutane, 2,3 - epoxy - 4 - chlorobutane, 1,2 - epoxy - 5 - chloropentane, 2,3 - epoxy - 5 - chloropentane, 1,2 - epoxy - 4 - bromobutane, 2,3 - epoxy - 4 - bromobutane, 1,2 - epoxy - 5 - bromopentane, 2,3 - epoxy - 5 - bromopentane, polyethyleneimine, polyvinylamine, diethylenetriamine, dipropylenetriamine, triethylenetetramine, tripropylenetetramine, tributylenetetramine, tetraethylenepentamine or pentaethylenehexamine.

The cross-linking agent which may be employed to react with the amine may comprise ethylene diisocyanate, propylene diisocyanate, benzene diisocyanate, toluene diisocyanate, naphthalene diisocyanate, or methylene bis(4 - phenylisocyanate).

In one embodiment, the ultra-filtration membranes which are used in the process of this invention may be prepared by coating a support of the type hereinbefore set forth with an aqueous solution of the polyamine. The polyamine will be present in the aqueous solution in an amount in the range of from 0.4% to 1.2% by weight. After coating the support with the polyamine, the excess solution is drained and the coated support is allowed to stand for a period in the range of about 0.1 to about 5 minutes. Following this, the cross-linking agent in an organic solvent such as hexane, heptane, cyclohexane, methylcyclopentane, benzene, toluene, etc. is then added to the coated support to effect the desired cross-linkage. The cross-linking agent will be present in the organic solvent in an amount in the range of from 0.16% to 0.5% by weight. After allowing the coated support to remain in the cross-linking solution for a period ranging from about 0.1 to about 5 minutes, the organic solution containing the cross-linking agent is drained off from the support, the support is then washed with a hydrocarbon solvent such as those enumerated above, drained and prepared for use. The preparation of the ultra-filtration membrane is preferably effected at ambient temperature and atmospheric pressure.

It is also contemplated that the membrane which is used in the present invention may be prepared in a continuous manner. When such a type of operation is employed, a support of the type hereinbefore set forth is continuously passed through an aqueous bath of the polyamine for a predetermined period fo time and is continuously withdrawn. The coated support is then continuously passed through an organic bath containing the cross-linking agent, again for a predetermined period of time whereby the polyamine is cross-linked to form the desired ultra-filtration membrane. After passage through the organic solution containing the cross-linking agent, the membrane system is continuously withdrawn and may, if so desired, be washed with a hydrocarbon solvent to remove any excess cross-linking agent and/or unreacted polyamine.

While, as heretofore set forth, the preparation of the ultra-filtration membrane system is preferably effected at ambient temperature and atmospheric pressure, it is also contemplated that the membrane system, after preparation, may be subjected to an annealing or heat-curing step in which the membrane system is subjected to an elevated temperature which may range from about 60° to about 100°C for a predetermined period which may range from about 1 to about 4 hours. As will hereinafter be shown in greater detail in the examples at the end of the specification, this treatment will not affect the activity or separation efficiency of the membrane when used in the separation of mono- and disaccharides from polysaccharides.

The process of this invention may be effected in any suitable manner and may comprise either a batch or continuous type operation. For example, when a batch type operation is employed, the feedstock which comprises the reaction mixture resulting from the treatment of a polysaccharide with an appropriate enzyme such as alpha-amylase, beta-amylase, amyloglucosidase, and other alpha-glucosidases, cellulase, etc. and which contains monosaccharides, disaccharides, undigested polysaccharides, and, in some instances, the enzyme itself, in the event that a soluble enzyme has been used, is placed in an appropriate apparatus such as a cell which contains the membrane at the bottom thereof. The apparatus is then pressurized to a predetermined operating pressure which may be in the range of from abouit 10 to about 2000 psi (70-14,000 kPa) or more to force the permeate through the ultra-filtration membrane. In addition, the apparatus is also maintained at a predetermined operating temperature which may vary over a relatively wide range, i.e., from subambient (about 5°C) up to about 80°C or more. The permeate which comprises a mono- or disaccharide is recovered, while the retentate may also be collected for further treatment with additional enzyme in order to convert the remaining polysaccharide to the desired mono- or disaccharide, or further hydrolysis.

It is also contemplated within the scope of this invention that the process may be effected in a continuous manner of operation. When such a type of operation is employed, the feedstock which is similar to that hereinbefore described is continuously charged to an apparatus which is provided with an ultra-filtration membrane of the type hereinbefore set forth. The desired apparatus is maintained at predetermined operating conditions of temperature and pressure. After passage through the membrane, the permeate, which contains the monosaccharide or disaccharide such as glucose or maltose, is continuously recovered and passed to storage, while the retentate may also be recovered and recycled back to a conversion zone for further treatment to prepare more of the desired product.

The following examples are given for purposes of illustrating the process of the present invention.

Example 1

In this example, a membrane system was prepared by coating a polysulfone support with an aqueous solution containing 0.60% of amine-modified polyepichlorohydrin at ambient temperature and atmospheric pressure. The solution was allowed to remain on the support for a period of 1.0 minute following which the excess solution was drained. Thereafter, the coated polysulfone support was treated with a 0.25 wt% toluene diisocyanate dissolved in an organic solvent comprising n-heptane. After allowing the coated polysulfone support to be treated with this organic solution for a period of 1.0 minute, the excess solution was drained, the composite membrane was washed with n-heptane and air-dried for a period of 5.0 minutes.

The composite membrane prepared according to the above paragraph was then placed at the bottom of a cell measuring 43 mmx100 mm. Following this, 50 ml of a glucose solution possessing 30% dissolved solids and containing 17.8% of DP₉+, 0.2% DP₃, 3.0% DP₂ and 79.0% of glucose was placed in the cell. The cell was pressured to about 50 psi (3,3 bar), and the flux rate of the permeate which passes through the membrane was measured. The flux rate of this test was 9.1x10³ ml/min/cm^2. Both the permeate and retentate were analyzed and it was found that the permeate containing only 0.4% of DP₉+, 0.3% of DP3, 3.2% of DP₂ and 91.1% of glucose. In addition, the retentate was found to contain 32.4% of DP₉+, 0.4% of DP₃, 2.8% of DP₂, 62.2% glucose and 0.1% each of DP₇, DP_s, DP₅ and DP₄.

Comparative Example 1

To illustrate the ability of a composite membrane comprising a cross-linked polyurethane composited on a support to effectively separate mono- and disaccharides from polysaccharides, another experiment was performed in which a glucose feedstock and a maltose feedstock, each containing 30% dissolved solids, were passed through an ultra-filtration membrane comprising only the support, in this case polysulfone. The apparatus which was used was identical to that set forth in Example 1 above, the glucose and maltose feed being passed through the polysulfone membrane at a pressure of 20 psi (140 kPa). In the case of the glucose feed, the flux rate was 5.2x 10^-3 ml/min/cm², while the flux rate of the maltose feed was 2.7x10-³ ml/min/cm². The results of these experiments are set forth in Table I below:

Example 2

To illustrate the differences in separation efficiency and flow rates of various ultra-filtration membranes comprising a cross-linked epiamine, a series of experiments were performed in which 9 membranes were prepared in a manner similar to that set forth in Example I above. A polysulfone support was treated with an aqueous solution of the amine-modified epichlorohydrin, the amount of epiamine in the solution varying from 0.4% to 1.20%. After coating the polysulfone with these solutions, the excess solution was drained and the coated polysulfone treated with a solution of toluene diisocyanate dissolved in n-heptane, the amount of toluene diisocyanate varying from 0.16% to 0.50%. After washing the membrane with n-heptane, the membranes were then air-dried and used in a sugar separation process at ambient temperature and pressures ranging from about 20 to about 90 psi (140-620 k Pa). The results of these tests are set forth in Table II below:

It is noted from the results set forth in the above table that the composite membrane prepared from 0.75 epiamine and 0.25% toluene diisocyanate shows the highest selectivity, while the membrane prepared from 0.6% epiamine and 0.25% toluene diisocyanate showed the highest flux rate coupled with good selectivity. It is to be noted that flux rates which are acceptable in this type of separation are those which are in excess of 3x10-³ ml/min/cm² at ambient temperature and operating pressure of 50 psi (340 k Pa).

Example 3 and Comparative Example 2

To illustrate the possibility of subjecting the membrane to a heat-curing step while retaining the activity and efficiency of the membrane, a series of experiments were carried out in which, in the first case, a polysulfone support was subjected to a temperature of 80°C for a period of 2 hours. In the other experiments, a membrane system was prepared by coating the polysulfone support with an aqueous solution containing varying amounts of amine-modified polyepichlorohydrin at ambient temperature and atmospheric pressure. Following the coating, excess solution was drained and the coated polysulfone support was treated with an organic solution containing varying amounts of toluene diisocyanate. The composite membrane was washed with n-heptane, air-dried, and annealed at a temperature of 80°C for a period of 2 hours.

The composite membrane was then utilized as an ultra-filtration membrane for the separation of glucose from a solution containing glucose and polysaccharides. The separation was effected in a manner similar to that set forth in Example I above in which 50 ml of the glucose solution containing 30% by weight of dissolved solids was passed through the membrane. The results of these experiments are set forth in Table III below:

Example 4

To illustrate the fact that the separation efficiency of mono- and disaccharides from polysaccharides is not affected by the percent of dissolved solids in the feed, a series of experiments were performed in which the feed to the ultra-filtration membrane contained varying amounts of dissolved solids, said dissolved solids varying from 1.5% to 30%. The ultra-filtration membrane which was utilized for the separation was similar in nature to the system described in Example 1 above. The results of these experiments are set forth in Table IV below:

It is readily apparent from the above examples that an ultra-filtration membrane such as that described which may, if so desired, be subjected to a heat-curing treatment, will operate at a high degree of separation efficiency for the separation of mono- and disaccharides from polysaccharides. The membrane system of the present invention which comprises an amine-modified epihalohydrin cross-linked by a cross-linking agent such as toluene diisocyanate composited on a support such as polysulfone shows a greater percent of efficiency than does the polysulfone support itself.