GB2416776A - Enhancement of the fermentability of carbohydrate substrates by chromatographic purification - Google Patents

Abstract

A process for the production of ethanol by fermentation, comprising the steps of: providing an aqueous feedstock comprising dissolved carbohydrates, dissolved salts, and dissolved amino acids; separating the feedstock by chromatography into a raffinate fraction containing a major portion of the salts and a fermentation fraction containing a major portion of the carbohydrates and of the amino acids; and fermenting the fermentation fraction to convert the carbohydrate to ethanol. Preferably, the feedstock is a sucrose-containing stream obtained as a by-product in the extraction or purification of sucrose from plant sources.

Description

24 1 6776

ENIIANCEMENT OF THE FERMENTABILITY OF CARBOHYDRATE

SUBSTRATES BY CHROMATOGRAPLIIC PURIFICATION

The present invention relates to methods for the production of ethanol by fermentation, and in particular to methods for the provision of improved fermentation feed materials.

Fithanol is gaining wide popularity as a fuel, particularly when mixed with petrol (gasoline) to form a mixture known as gasohol. Automobiles can run on gasohol containing up to about 1() volume percent ethanol without requiring engine modifications. Ethanol is also 1() widely used as a chemical solvent and as a raw material in the manufacture of drugs, plastics, lacquers, polishes, perfumes and the like.

Ethanol is derived primarily from the fermentation of mash, usually from starch and/or sugars of natural origin. Natural fermentation is capable of producing an alcohol/water product mixture containing up to about 12 volume percent ethanol. Subsequently, the ethanol is separated from the ethanol/water product mixture Separation is carried out al least initially by distillation (the ethanol/water system exhibits an a'. eotrope at 90/o ethanol).

2() It would be desirable to provide feedstocks for ethanol production from low-cost sources, wherein the feecistocks undergo fast fermentation and wherein the ease of separation of the ethanol after fermentation is enhanced.

One source of carbohydrate for ethanol fermentation is the byproducts from sugar production. Sugar is produced from both sugar beet and sugar cane.

Sugar is extracted from sugar beet by diffusion. The conventional manufacturing process starts by cleaning the sugar beet with water and slicing it into thin slices called cassettes.

The slicing machines work in a similar manner lo a kitchen grater. The cassettes they 3() produce have a "V" cross section, whicl1 ensures that the largest possible surface area is exposed to maxmse the extraction of sugar. The cassettes are mixed with hot water at around 70 C and the sugar passes from the plant cells into the surrounding water. Then the vegetable material is mechanically pressed to extract as much remaining sugar as possible.

The pressed vegetable material is USCti to produce animal feed. The liquid resulting from the clt'fusion process is dark in colour and is called raw juice.

T}IC raw juice is purified by carbonatation. This involves mixing the juice with milk of lime (aueou.s calcium hydroxide) and adding carbon dioxide. During this process, the carbon dioxide and the milk of lime combine to produce calcium carbonate, which precipitates out taking most of the impurities from the raw juice with it. T}liS precipitated lime contains Important trace elements and is used as soil improving agent. The liquid resulting from the carbonatat.ion process is pale yellow and is called thin juice.

The thin juice, while much purer than the raw juice, Is still relatively} ow h1 sugar content.

Therefore the thin juice is concentrated by a rnult-et't'ect evaporation process, where water is boiled ol'f to mciease the solici content of the juice from about 15% in thin juice to about 657O. The resulting concentrated juice, known as thick Juice, is passed through filters.

The thick luice is then purified by crystal]isation. The crysta]}isation process conventionally takes place in vacuum pans, in which the thick juice is boiled at dower temperatures under vacuum. When the juice reaches a predetermined concentration it is seeded with tiny sugar crystals, which provide the nucleus for larger crystals to form and grow. When the crystals reached the desired size, the process is stopped and the resultant mixture of sugar crystals and syrup is spun in centrifuges to separate the mixture. The sugar crystals are waslled, ciried and cooled to yield a white sugar.

Some sugar remains In the separated syrup. Thus, the crystallization step is repeated with the separated syrup to produce so-called raw sugar. The process is then repeated a third time to produce so-called final sugar and molasses. The colour of the sugar produced increases with each successive crystallization step. The raw and final sugar of the second and third step are generally not clean enough to be sold and thus they are reintroduced into the purification process by re-dissolving into the thick juice. However, reintroducing the final sugar ot'the third step is often problematic as, clue to the combination of temperature and long residence times in the crystallisaton pans, it is often intensely coloured.

It is known to use molasses from the crysta]lisation of sugar in either the cane or the beet processes as a feedstock for subsequent fermentation processes. The molasses may undergo further treatments before fermentation is carried Out. For example, JP-A-1168275 describes treating molasses by dilution, heating, acljustng the pll with a mineral acid, and separation of a precipitated impurity traction, prior to carrying out i'ennentaton on the molasses. EP-A-0756()11 describes carrying out a chromatographic step on molasses to separate it into a sugar-rich fraction and a low-sugar fraction, and then mixing the low- sugar fraction with further molasses to produce a fermentation feedstock. GB-A-2152057 ciescrhes a feedstock for the production of glutamic acid. The feedstock is produced frown 1() molasses by the steps of inverting the molasses, followed by passing the inverted molasses through an ion exchange column to obtain a sugarcontaining eluate fraction suitable for use as the feedstock.

The present invention provides a process for the production of ethanol by fermentation, comprising the steps of: providing an aqueous feedstock comprising dissolved carbohydrates, dissolved salts, and ds.solved amino acids; separating the feedstock by chromatography into a raffinate fraction containing a major portion of the salts and a fermentation fraction containing a major portion of the carbohydrates and of the amino acids; and fennenting the l'ermentation fraction to convert the carbohydrate to ethanol.

Suitably, the aqueous feedstock is a by-product of sugar manufacture. That is to say, a by- product of the extraction and purification of sucrose from natural sources such as sugar cane and sugar beet. Preferably, the feedstock is a by-product of the extraction and purification of sucrose from sugar beet. In such cases the aqueous feedstock frequently comprises dissolved betaine, and suitably (but not necessarily) the chromatographic separating step further produces a betame traction containing a major portion of the betaine.

The feedstock may be a thick juice as hereinbet'ore described, or it may be a liquor fraction from the first or second crystallization stages, or it may be a molasses from a third crystallization stage. These feeclstocks contain, respectively, increasing amounts of coloured organic impurities and salts, which are thought to interfere with the production of ethanol by fermentation, and with the separation of ethanol after fennentaton.

Accordingly, the chromatography conditions are preferably selected such that these impurities are substantially absent from the second fraction produced in the chromatographic separation step.

Preferably, the feedstock used in the process of the present invention is the liquor from the second stage of sucrose crystallization from thick juice, in particular thick beet juice. In these embodiments, the process of the present invention further comprises the.stcps of: (i) crystallismg a sucrose thick juice to yield white sugar and syrup; (it) crystallising the said syrup to yelcl raw sugar and a liquor; wherein the liquor is used (optionally with dilution or concentration) as the aqueous feedstock for the chromatographic separation.

It is unusual to use only two crystallization stages in the production of crystalline sucrose.

However, has been found that by limiting the number of successive crystallization steps of the thick juice to two, preferably with recycle oi the sugar from step (ii), the colour of the white sugar obtained by the first crystallization step Is improved. This is a further benefit of the processes according to the present invention.

In certain preferreci processes according to the present invention, the feedstock has the following composition: total dissolved solids from about 2() to about 70 wt %, preferably from about 40 to about 60wt %; dissolved carbohydrate from about 50 to about 90 wt% on dry substance, preferably from about 55% to about 75 wt % on dry substance; dissolved salts (or ash) from about 5 to about 25wt.% on dry substance, preferably from about 10% to about 2()wt.% on dry substance; nitrogenous compouncls: from about 2 to about 15 wt.% on dry substance, preferably from about 10% to about 12wt.%; and other organic impurities: from about 1 Vito to about I Owl.% on dry substance, preferably front about 5% to about wt.' on dry substance.

The dissolved carbohydrate preferably comprises a dissolved monosaccharide or clisaccharide. Suitably, at least 90% of the dissolved carbohydrate is monosaccharide or disaccharide, and preferably it is sucrose. A particularly preferred feed.stock is the liquor obtained from the second crystallization step carried out on a sugar beet thick juice. The carbohydrate content of the feedstock is measured by HPLC using an Aminex 87K column (BioRad Laborator-ie.s Inc) with retractive index detection s The dissolved salts present in the feedstock are detrimental to the fermentation of sucrose into ethanol, and removal of these salts in the chromatography step allows more rapid fermentation to higher ethanol concentrations. The wt.% salt (ash) values herein are as measured after heating to 600 C in air for 4 hours. Approximate values may also be to determined by measuring conductivity. The conductirnetric salt measureTnerlt compares the conductivity of the solution with a standard salt solution (potassium chloride). This gives a good approximation of the total ash content.

The nitrogenous compounds in the feedstock may include, but are not limited to, betaine, l 5 amino acids and nucleosicies. The term "nitrogenous compounds" does not include inorganic nitrates car ammonia. The principal amino acids present in the f'eedstock are Lysine, Alanine, Glycine, Valine, Serine, Leucine, Isoleucine, and Tyrosine.. They are Important nutrients for the fermentation of the carbohydrates to ethanol, and it is an advantage of the present invention that the chromatography conditions are selected to substantially retain these nutrients, and preferably also other nutrients such as nucleosides (uridine, adenosine, guanosine), in the t'ermentaticn fraction.

The other organic impurities in the feedstock include in particular higher molecular weight compounds and colour impurities, such as those found in molasses. These are preferably removed into the raffinate fraction.

Detailed discussion of the components of the byproduct streams from sugar manufacture that can be used in the practice of the present invention may be found for example in Sugar Technology - Beet and Cane Sugar Manufacture, Van der Poel, Schiweck & Schwartz, Bartens 19'38, isbn 387040-065-x., the entire content calf which is incorporateci herein by reference.

As already noted, the fermentation fraction contains a major fraction of the carbohydrate and amino acids. By major fraction is meant more than 50% of these components originally present in the feedstock, preferably more than 75% of these components present in the feedstock. The concentration of these components in the fermentation fraction is preferably at least double the concentration of the components in the f'eedstock, more preferably at least three times the concentration of the components in the feedstock.

Conversely, the concentration of the salts and colour components in the fermentation l'raction preferably is less than 50/o, more preferably less than 25'Yo of the concentration of these components in the f'eedstock.

For example, the fermentation fraction may have the following composition: total dissolved solids from 15 to 30 wt %, preferably 20-25 wt%; dissolved carbohydrate from about 85 to about 98 wt. on dry solids (DS), preferably from about 92 to about 96 wt.o on DS; dissolved salts for ash1 from about 0.5 to about 5 wt.% on DS, preferably from about 0.5 to about 2 wt.% on DS; amino acids from about ().5 to about 5 wt.% on DS, preferably from about I to about 3 wt.% on DS; other organic impurities from about 0.5 to about 3 wt.% on INS, preferably from about 1 to about 2 wt.% on DS.

The application chromatography to the separation of sugar containing streams is described 2() in a number of patent applications, including EP-A-0054544, EP-A-() 3455 11, WO96/10650, WO94/17213, and WO97/45185, the entire contents of which are incorporated herein by reference and will not be discussed furthers The chromatographic separation may be carried out on an ion exchange resin (cationic or anionic), a size exclusion resin, an at'finity chromatography bed. Preferably, the separation is carried out on a monovalent cation exchange resin (i.e. an anionic resin having a monovalent cation or 11+ initially hound thereto). Suitable resins are discussed in the above references.

The chromatographic separation step may conveniently be carried out by a simple batch method, or by a simulated moving bed (SMB) method. The simulated moving bed method 3() may he sequential or continuous or comprise a combination of a continuous and a sequential method. In a continuous simulated moving bed process, all fluid streams typically flow continuously. 'I'he continuous simulated moving bed process has been disclosed for example in US-A- 2985589. The streams are: the supply of teed solution and eluent, the circulating of the liquid mixture, and the withdrawal of products. The flow rate for these flows may be adjusted in accordance with the separation goals (yield, purity, capacity). Typically 8 to 20 partial packed beds are combined into a loop. The e]uenl and feed supply and product withdrawal points are shit'ted cyclically in the downstream direction In chr-'matographic bed. On account of the supply ol'eluent and feed solution, the withdrawal of products, and the flow through the chromatographic bed, a dry solids profile is firmed hi the chromatographic bed. Constituents having a lower migration rate in the chromatographic material are concentrated in the back slope of the separation profile, i.e. dry solids profile, while constituents having a higher migration rate are concentrated in the 1() front slope. The points of introduction of the feed solution and eluent and the withdrawal points of the product or products are shifted cyclically at substantially the same rate at which the dry solids profile moves in the chromatographic bed. The eluent and feed supply and product withdrawal points are shifted cyclically by using feed and product valves located along the chromatographic bed, typically at the upstream and downstream end ol' each partial packed bed. It' product fractions of very high purity are desired short cycle times and multiple partial packed beds must be employed, in which case the apparatus has the requisite valves and feed and withdrawal equipment.

In the sequential simulated moving bed process, some of the fluid streams do not flow 2() continuously. The streams are: the supply of feed solution and elueut, the circulating of the liquid mixture, and the withdrawal of products (eluting phase; two to four or more products). The flow rate and the volumes of the different feeds and product tractions may be adjusted in accordance with the separation goals. The process commonly comprises three basic phases: feeding, elusion and circulation. luring the feeding phase, a feed solution, and possibly also an eluent during a simultaneous eluting phase, is introduced into predetermined partial packed beds, and simultaneously a product fraction or fractions are withdrawn. During the eluting phase, eluent is introduced into a predetermined partial packed bed or predetermhled partial packed beds, and during these phases two, three or even four product fractions are withdrawn. During the circulating phase, no feed solution 3() or eluent is supplied to the parka] packed beds and no products are withdrawn.

The method of the present invention may comprise either continuous or sequential, or a combination of a continuous and a sequential process. Suitably, the SM13 method comprises two consecutive SMB loops as described in EP-A-0764219. Suitably, the SMB method Is adapted to produce three fractions as described in EP-A-0345511 and EP-A- ()764219. In particularly preferred processes, the l'eedstock is a byproduct of the production of sucrose from sugar beet, and the SMB chromatographic separation divides the feedstock into a fermentation fraction containing the sucrose and amino acids, a raffhate fraction containing the salts and colour impurities, and a betaine fraction. SMB has advantages that include continuous or quasi-continuous operation and minimal dilution of the product streams by eluant. Preferably, the total eluant added in the chromatographic separation is less than about 50% of the volume of the feedstock, more preferably less than about 15% of the f'eedstock. . The chromatography thereby has the additional advantage of concentrathg the sugars and nutrients such as amino acids and nucleosdes in the fermentation traction.

Specific embodiments of the present invention will now be described further, by way ol' example, with reference to the accompanying drawings, in which: Fig. I shows a block diagram of the chromatographic separation process used in an embodiment of the invention; and Fig. 2 shows a graph of the measured concentrations of various constituents against chromatography fraction number for separation of a beet sugar liquor on an ionic chromatography resin in the potassium form.

F.xample I sugar beet thick juice was prepared as described above. The juice was subjected to a two-step crystallization, with recycle of the crystalline sucrose from the second crystallization to the first crystallization stage. This gave a very white sugar from the first crystallization, and a liquor from the second crystallization containing residual sucrose and the various non-sucrose impurities including amino acids, salts, colour impurities and Detains. The sugar content of the liquor was about 75% on dry solids basis, and the total solids concentration was about 70-80% The liquor was fed to the two-stage chromatographic separation process shown schematically in Fig. I. The liquor F is fed initially to a homogenlir.er l where it is diluted with a recycled dilute fraction 9,1() from the separators. The dilute fraction 9, 10 consists mainly ol' water, together with some salts and sucrose to a total solids content of about 2()'. The diluted feed stream 4, The sugar content of the liquor was about 75% on dry solids basis, and the total solids concentration was about 50-60%, is then Ted to a SMIB chromatographic separation.systein 2 constructed in accordance with EP-A-0345511 and comprising six columns t'lled with an a strongly acidic ion-exchange resin such as PUROLITE PCR 651 (available from the Purolite Company).

The output from the clrc>malographic separator 2 Is shown schematically in l;ig. 2. This shows the composition of samples I to 28 of a complete SMB cycle, as measured at point A shown schematically in Fig.l. Samples 1- 3 and 27-28 are the raffinate fraction 6, consisting mainly of the colour Impurities and salts. Samples 4-7 and 17-2() are complex 1 5 mixtures that are recycled through a loop 11 internally within the SMB separation.

Samples 8-1() are dilute fraction lo (about 10% total dry solids), and are recycled to the homogenizer I as diluent for the liquor F. Samples 11- 16 are the fermentation fraction 5 containing the major part of the sucrose. It has now been found that important nutrients including amino acids and nucleosides (Not shown in Fig. 2) run with the sucrose on the chromatography column, and are concentrated in the fermentation fraction 5. This reduces the amount of nutrients that need to be added iT't the fermentation step.

Finally, samples 21-26 of' the separation profile shown in Fig.2 are the betaine fraction 12.

This undergoes further purification in a second SMB separator 3 of six columns, as shown in Fig. 1 and as described in detail in EP-A-0764219, the entire content of which is incorporated herein by reference. The further separation results in a further dilute fraction 9 which is recycled to the homogenizer 1, a raffinate fraction 8 which may be combined with the rat'finate fraction 6 from the first SMB loop, and a betaine fraction 7. The combined raffinate fractions 6,8 and the final betaine fraction 7 each contain only 18-19% sue ose on dry solids basis.

The fermentation traction 5 contains about 95% sucrose on dry solids basis, and the concentration of the sucrose Is about 23g/lOOg. 'The remainder of the sohds in the fermentation fraction are mainly nutrients such as amino acids and nucleosides. This composition is very suitable for fennentaton Lo ethanol.

The fermentation fraction gives improved fermentation performance compared to conventional molasses. The lennentator1 traction contains a substantial amount of nutrients such as amino acids and nucleosides whereby the fermentation can he canted out substantially or completely without addition of nitrogenous nutrients to the fermentation fraction. Furthenmore higher gravity fermentations are possible and hence higher ethanol content can lee achieved in the fermentation broth.

The above examples have been described for the purpose of illustration only. Many other examples falling within the scope of the accompanying claims will be apparent to the skilled reader.

Claims (11)

1. A process for the production of ethanol by fermentation, comprising the steps of: providing an aqueous feedstock comprising dissolved carbohydrates, dissolved salts, and dissolved amino acids; separating the feedstock by chromatography into a ral'finate traction containing a major portion of the salts and a l'ermentaton fraction containing a major portion of the carbohydrates and of the amino acids; and fermenting the fermentation fraction to convert the carbohydrates to ethanol.

2. A process according to claim 1, wherein the aqueous feedstock is a byproduct of sugar manuf'aclure.

3. A process according to claim 1 or 2, wherein the aqueous f'eedstock contains dissolved betaine.

4. A process according to claim 3, wherein the chro,,natographic separating step further produces a betaine fraction containing a major portion of the betame.

2()

5. A process according to any preceding claim, wherein the feedstock further comprises coloured organic impurities, and a major portion of these colour impurities are concentrated in the raffinate fraction produced in the chromatographic separation step.

6. A process according to any preceding claim, wherein the feedstock has the following composition; total dissolved solids from about 20 to about 70 wt %, preferably from about 40 to 60wt %; dissolved carbohydrate from about 50 to about 90wt% on dry substance, preferably from about 55 to about 75 wt % on dry substance; 3() dissolved salts (ash) from about 5 to about 25wt.%, on dry substance, preferably from about 10 to about 20 wt.% on dry substance; and nitrogenous compounds: from about 2 to about 15 wt.o on dry substance, preferably from about 10 to about 12 wt.% on dry substance; and other organic impurities: from about I to about 1() wt.% on dry substance, preferably from about 5 to about 8 wt.% on dry substance.

7. A process according to any preceding claim, wherein the fermentation fraction has the following composition: total dissolved solids from about 15 to about 30 wt % preferably about 20 to about wl%; dissolved carbohydrate from about 85 to about 98 wt.% on dry substance, preferably from about 92 to about 96 wt.% on dry substance; dissolved salts [or ashl from about.5 to about 5 wt.% on dry substance, preferably from about 0.5 to about 2 wt.% on dry substance; amino acids from about 0.5 Lo about 5 wt.% on dry substance, preferably from about 1 lo about 3 wt.% on dry substance; and other organic impurities from about 0.5 to about 3 wt.% on dry substance, preferably from about I to about 2 wt.% on dry substance.

8. A process according to any preceding claim, wherein the chromatographic separation step is carried out by a simulated moving bed (SMB) method.

9. A process according to claim 8, wherein the LIMB method comprises two eonseeulive SMB loops.

10. A process according Lo claim Ss or 9, wherein the feedstock is a byproduct of the production of sucrose from sugar beet, and the SMB chromatographic separation divides the feedstock into a fermentation fraction containing the sucrose and amino acids, a raffinate fraction containing the salts and colour impurities, and a betaine fraction.

11. A process according to any preceding claim, wherein the fermentation is carried out substantially without addition of nitrogenous nutrients to the fermentation fraction.