WO2001090422A2

WO2001090422A2 - A method for the recovery of sucrose from molasses

Info

Publication number: WO2001090422A2
Application number: PCT/IL2001/000451
Authority: WO
Inventors: Asher Vitner; Tal Reuveny
Original assignee: Tate & Lyle Public Limited Company
Priority date: 2000-05-22
Filing date: 2001-05-21
Publication date: 2001-11-29
Also published as: AU2001260572A1; IL136284A0; WO2001090422A3

Abstract

The invention provides a method for recovery of sucrose from molasses comprising combining molasses with a methanolic solution whereby there is formed a single liquid phase which retains carbohydrates from said molasses in solution, the solution being super saturated with respect to sucrose, separating non-carbohydrate matter which precipitates, inducing sucrose crystallization by seeding the super-saturated solution with sucrose; and recovering formed sucrose crystals.

Description

A METHOD FOR THE RECOVERY OF SUCROSE FROM MOLASSES

The present invention relates to a method for the recovery of sucrose from molasses.

Sugar, which is virtually pure sucrose, is recovered by exhaustive crystallization from aqueous solutions originating in sugar-beets or sugar-cane. Molasses (as defined in Cane Sugar Handbook, 12^th Edition published by John Wiley, page 566) "...is the liquid residue from which no more sugar can -be removed economically" by further crystallization.

The recovery of sucrose from molasses presents considerable commercial interest. The handbook quoted above states (on page 400: "The sucrose in blackstrap is, by definition, not recoverable by ordinary means. Several processes have been suggested for the recovery of sucrose from final molasses". It then proceeds to describe, on pages 400-407, a number of processes to this end which use a variety of separation technologies in various combinations. Ultra filtration by membranes, chromatography, chemical conversion (e.g. to sacharate followed by regeneration), ion-exchange are listed i.a. These processes are generally onerous which restricts their application to special situations, related mainly to sugar beet molasses such as recovery of a valuable by-product betaine or the possibility to command a high price for the sugar recovered by virtue of governmental regulations. The more plentiful molasses derived from cane "blackstrap molasses" are by and large left untreated for sucrose recovery and fetch a very low price for their sugar content.

It is the purpose of the present invention to provide for a simple and economically efficient process to recover sucrose from molasses that is particularly applicable to blackstrap molasses and generally to impure, sucrose-rich syrups that form in the conventional processes of sugar manufacture. "Molasses" is used in the present context as a generic name to all impure, sucrose-rich syrups whatever the episodic, commercial designation of such syrup.

At the core of the invention resides the surprising discovery that molasses can be diluted by methanol without precipitation of carbohydrates taking place.

Such precipitation appears to be mandated by known solubilities and would be inferred from known art as detailed further below. The water soluble carbohydrates present in molasses consist practically only of sucrose, glucose and fructose, the latter two being traditionally encompassed under the name of Invert. Table 1 presents typical compositions of molasses as such. These compositions are also recalculated for the sucrose-invert-water (S I W) they contain as forming 100% thereby deriving reference compositions of pure syrups that contain only these three components.

Table 1 .

S=Sucrose; l=lnvert; W=Water

Samples of the molasses syrups listed in Table 1 were mixed with pure methanol in a ratio of 1.5 methanol to 1 molasses, centrifuged to obtain complete clarity and the clear liquor sampled and analyzed one hour after centrifugation. The single liquid phase contained all the water originally present in the molasses, all of the added methanol and solutes originally contained in the molasses that did not precipitate. The Invert under these conditions is completely soluble. Hence, the ratio of I to S in solution compared to l/S in the molasses prior to dilution indicates how much S precipitated; the ratio of I to NCH (non-carbohydrates) compared to l/NCH in molasses indicates the extent of NCH precipitation. Table 2 below clearly indicates that all of the sucrose was maintained in solution while part of the non-carbohydrates (NCH) was precipitated. Table 2. Methanol/Molasses=1.5

MeOH=methanol; MSS=molasses; NCH=non -carbohydrates

A similar series of dilutions was done with a ratio of methanol/molasses=4/1 which lowered the content of water in the liquid phase fourfold. Such lowering might be expected to further decrease the solubility of sucrose. In fact, as shown in Table 2.1 all of the sucrose stayed in solution.

Table 2.1 Methanol/Molasses = 4, 1hr and 120hrs after the addition of MeOH

For the sake of comparison, synthetic solutions were prepared of pure sucrose and Invert (taken as 1 :1 mixture of glucose and fructose) in water in the ranges that obtained in the various molasses as listed in Table 1. Each of these was mixed with 1.5 and 4.0 methanol to 1.0 of synthetic syrup and checked for precipitation. Remarkably, while (as shown by Tables 2 & 2.1) no carbohydrates precipitated from molasses immediately nor after 120 hours, plentiful sucrose precipitated from the comparable synthetic solutions virtually immediately. Further observations established that at room temperature as well as at 5°C dilution of molasses by methanol in the ranges of 1-to-4 parts of methanol to 1 part of molasses results in solutions that are stable for at least one day and generally for several days. However, by maintaining such solutions for longer periods of time below 10°C, preferably below 5°C, eventually slow crystallization starts. Once such spontaneous crystallization starts, it continues until the amount of sucrose present in super-saturation precipitates as fine crystals.

The rate and duration of crystallization varies with the extent of dilution by methanol, temperature and the particular molasses. In all cases, as illustrated by examples further below, sucrose is the only carbohydrate that precipitates - all of the Invert being retained in solution.

Thus, according to the present invention, there is now provided a method for recovery of sucrose from molasses comprising: a) combining molasses with a methanolic solution whereby there is formed a single liquid phase which retains carbohydrates from said molasses in solution, said solution being super saturated with respect to sucrose; b) separating non-carbohydrate matter which precipitates; c) inducing sucrose crystallization by seeding said super-saturated solution with sucrose; and d) recovering formed sucrose crystals.

More particularly, sucrose is recovered from molasses by a preferred process according to the present invention, consisting of the following steps:

1. Diluting the molasses by methanol in the ratio of 0.5-to-10 methanol to 1 molasses, preferably 1.2-to-3 to 1 ;

2. Separating any solids that precipitate by a conventional technology of Solids/Liquid separation;

3. Washing the solids with distilled methanol from step 6 below to recover any sucrose retained in the precipitate and recycling the remaining methanolic solution to step 1 ; 4. Seeding the clear liquor from step 2 with sucrose crystals from step 5 and allowing time for crystallization to proceed to completion;

5. Separating the precipitated sucrose and washing it with methanol similarly to step 3;

6. Distilling the separated liquor from step 5 to recover methanol and a molasses product that contains as carbohydrates all of the Invert in the feed molasses and the sucrose that did not crystallize as well as the impurities that were not removed as solid precipitate in step 2; and

7. Dissolving the sucrose from step 5 in water and stripping all residual methanol by distillation to obtain a sucrose syrup that can be refined to high purity sugar or used as such.

As will be obvious to a practitioner, the key steps of the process consist of using methanol for dilution and for the removal of precipitated impurities and then using seeding for crystallization. Other operations are auxiliary and admit numerous technological choices to suit best each particular case.

"A Process For Treating A Sucrose Syrup" PCT/GB97/03542 describes a process for treating molasses by alkanols and other oxygenated compounds of 3 carbon atoms or higher. It claims the splitting of molasses into two products by virtue of the formation of two liquid phases, one which is higher in sucrose content and one that is lower than the original molasses. Dilution by virtue of super-saturation is not mentioned.

Methanol is thus quite unique compared to its homologues in diluting molasses to form a single liquid phase by virtue of maintaining sucrose in super-saturation that is stable for all process purposes. Ethanol (EtOH), the closest homologue of methanol does not exhibit super-saturation behavior. Table 3. Comparative dilution by methanol and ethanol

As shown by Table 3 above, with ethanol, in the range of dilutions in which super-saturation would be desirable so that solid non-carbohydrates can be separated, two liquid phases form, one of which is a sucrose-rich, viscous aqueous phase. This obviously renders solids separation difficult and negates crystallization via the super-saturation route.

However, it was surprisingly found that provided that 0.5 parts or more of methanol were present for each part of molasses, ethanol could be used for diluting molasses with super-saturation being maintained. It was further found that methanol/ethanol mixtures when used to dilute molasses, separate solid impurities and subsequently recover sucrose by seeding with sucrose and crystallization, provides for advantageous adjustments in crystallization rates and of sucrose recovery.

Other oxygenated organic compounds that are fully miscible with methanol and are non-solvents for sucrose, such as C₃ and C₄ alkanols, can be used in admixture with methanol to some advantage.

In the most general terms, the process consists of diluting molasses by methanol, and optionally, by an additional organic solvent that is compatible with methanol, so as to form a single liquid phase which retains all of the carbohydrates in solution, this solution being super-saturated with respect to sucrose; separating such non-carbohydrates matter that precipitates; inducing sucrose crystallization by seeding with sucrose and recovering the sucrose crystals; distilling the solvent for recycle and recovering all remaining carbohydrates in a by-product - Invert-rich molasses (IRM).

It will be realized that the molasses, both prior to solvent mixing and after, can be treated with reagents, such as those including acid or alkali moieties, to remove cation and anion impurities as solid precipitates.

Furthermore, variations in the desired proportions of components can be achieved by stage-wise mixing ancl variation of, e.g., the preferred methanol/ethanol ratios in the various stages of the process.

Changes in temperature and/or pH allows for further control of the process.

Similarly distillation can be used for disproportionation.

For further clarification of the invention reference is made to Figure 1 which presents the process in three sections: A, B and C, wherein each of these is described below.

While the invention will now be described in connection with certain preferred embodiments in the following examples and with reference to the attached figure, so that aspects thereof may be more fully understood and appreciated, it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the scope of the invention as defined by the appended claims. Thus, the following examples which include preferred embodiments will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purposes of illustrative discussion of preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of fomnuiation procedures as well as of the principles and conceptual aspects of the invention.

EXAMPLES

Example 1

Section A. In this section molasses is diluted by solvent and the non-carbohydrates (NCH) which precipitate are separated.

Molasses (1) and a methanol containing solution (7) which is recycled from section C, are mixed thoroughly so as to obtain a uniform mixture. This can be done in a single vessel provided with strong agitation or in two or more agitated vessels in series. The viscosities of solvent and of molasses differ greatly. Precautions should be taken to prevent incomplete mixing. A useful configuration for efficient mixing consists of using two agitated vessels in series with (1) and (7) fed into the 1^st vessel and the mixture formed flowing into the second vessel, the mixture flowing out of the second vessel being partly recycled into the 1^st vessel. This scheme (which is well known per se in engineering practice) mitigates viscosity differences as well as fluctuations in the composition of the molasses feed (1). The methanol-diluted molasses is subjected to solids/liquid separation. Conventional techniques such as centrifugation and filtration are suitable. The solids that separate are dark colored and generally contain 50% to

14* = The precipitation was done at 60 °C

The solids are washed with part of methanol-containing solution (7) and the wash liquor rejoins the main stream (7) that dilutes (1) prior to solids/liquid separation. Section B. in this section sucrose is crystallized and recovered.

The diluted and clarified molasses stream (3), which is super-saturated with respect to sucrose, is mixed with recycled sucrose crystals that induce further crystallization. This takes place in crystallizers that are gently agitated vessels. Solvent evaporation is not required which provides for simplicity of operation. The sucrose that crystallizes is partly recycled and the rest separated, washed by fresh solvent, dissolved in water and the residual solvent recovered. These operations are similar to those of Section A. Schematically, (4) represents the recovered sucrose syrup, (5) the recycle of sucrose crystals. The sucrose-depleted mother liquor (6) is sent to Section C.

Table 5. Examples of crystallization

(In the following table, the example numbers correspond to those of Table 4, numbers in () relate to streams in the accompanying figure )

15* The pH of the solution was adjusted to 4 with H₂SO

16* The molasses was dehydrated to remove 5% of its water content prior to the addition of solvent. This Table illustrates the scope for optimization for each particular molasses that the present invention provides. Section C. In this section the solvent is recovered and recycled and an Invert-rich molasses (IRM) obtained.

Recovery of the methanol present in the methanolic solution (7) is by distillation. A column fed by live, low-pressure steam can be used to achieve complete stripping of the solvent economically. The remaining syrup (8) contains all of the invert and the fraction of sucrose that was not recovered in Section B. it constitutes an Invert-rich molasses (IRM) that is much lighter in color than the feed molasses - a feature of considerable value in most applications. Further, an IRM will be, as a rule, significantly richer in carbohydrates than the molasses from which it is derived - another desirable feature illustrated by the examples in Table 6.

Table 6. Compositions of Invert-rich molasses (IRM)

Naturally, the foregoing description comprises only the main operations for each section. Many variations of detail are possible to select from so as to optimize each particular case. Thus the NCH precipitated in Section A may be prepared according to their final destination as feed, fertilizer or fuel; the feed to crystallization (3) may be decolorized by active carbon so as to obtain lighter colored products (4) and (8) etc. The examples detailed in the foregoing tables were restricted to the main operations as described in sections A, B and C above. Example 2 a. Preparation of mother liquor

1300 gr molasses, 990gr ethanol and 830gr methanol and 100 gr sucrose seed crystals were shaken in a 2500 gr flask, at pH 6.5, for 24 hours. The obtained solution was filtered to yield mother liquor and sucrose crystals. Mother liquor was obtained by filtration of the solution. b. Precipitation of solids

The following steps were done at 55°C.

The mother liquor obtained in (1) and 25gr K₂SO₄ were added to 400 gr Molasses.

260gr methanol and 260gr ethanol were added drop-wise t the obtained solution. A precipitate was formed and filtered.

After filtration, 5gr of H₂SO₄ and 50gr of ethanol were added to the solution. The newly formed precipitate was filtered. c. Crystallization of sucrose

100 gr of sucrose seed crystals and the clear solution obtained in step b were shaken at 30°C for 10 hours. The obtained solution was filtered to yield 198gr sucrose crystals (production of 98gr sucrose), of low color and >98% purity.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. A method for recovery of sucrose from molasses comprising: a) combining molasses with a methanolic solution whereby there is formed a single liquid phase which retains carbohydrates from said molasses in solution, said solution being super saturated with respect to sucrose; b) separating non-carbohydrate matter which precipitates; c) inducing sucrose crystallization by seeding said super-saturated solution with sucrose; and d) recovering formed sucrcse crystals.

2. A method according to claim 1, wherein the ratio of methanol to molasses within said supersaturated solution formed in step a is between about 0.5:1 and 10:1.

3. A method according to claim 1 , wherein the ratio of methanol to molasses within said supersaturated solution formed in step a is between about 1.2:1 and 3:1.

4. A method according to claim 2, wherein said methanolic solution further comprises at least one oxygenated organic compound which is fully miscible with methanol and which is a non-solvent for sucrose.

5. A method according to claim 2, wherein said methanolic solution further comprises ethanol.

6. A method according to claim 5, wherein said compound is selected from a C₃ and a C₄ alkanol. ,

7. A method according to claim 4 wherein said compound is acetone.

8. A method according to claim 1 wherein carbohydrates in the supersaturated solution remaining after step b are separated using methods known per se.

9. A method according to claim 1 wherein carbohydrates in the mother solution obtained after the crystallization step c are separated using methods known per se.