Crystallization of allulose under reduced pressure

1 . A process for the preparation of a solid allulose material comprising crystalline allulose, the method comprising the steps of

(a) providing in an evaporating crystallizer an aqueous mother liquor containing dissolved allulose;

(b) maintaining until the end of crystallization the aqueous mother liquor within the evaporating crystallizer at a crystallization temperature within a range of from 20 to 55° C.;

(c) maintaining until the end of crystallization the vapor phase above the aqueous mother liquor within the evaporating crystallizer at a crystallization pressure within a range of from 40 to 200 mbar; and

(d) inducing crystallization of allulose from the aqueous mother liquor at the crystallization temperature and at the crystallization pressure in a supersaturated state thereby obtaining the solid allulose material as a precipitate, and a supernatant; wherein crystallization is induced at a supersaturation coefficient y of the aqueous mother liquor of at most 1.05, including reducing the crystallization pressure from a first crystallization pressure to a second crystallization pressure.

2 . The process according to claim 1 , wherein

the crystallization temperature is 29° C. to 55° C.; and/or 45+/−3° C.;

optionally wherein the crystallization temperature is kept essentially constant over time, until the end of crystallization; or does not change relatively by more than 2.0° C.

3 . The process according to claim 1 , wherein step (b) additionally includes maintain in the vapor phase above the aqueous mother liquor at a vapor temperature within the range of from 25 to 65° C.;

optionally, wherein the vapor phase above the aqueous mother liquor is does not change relatively by more than 2.0° C.; by not more than 1.5° C.; by not more than 1.0° C.; or by not more than 0.5°.

4 . The process according to claim 1 ,

wherein the crystallization pressure is at least 41 mbar, or at least 42 mbar, or at least 43 mbar, or at least 44 mbar, or at least 45 mbar, or at least 46 mbar; optionally wherein the crystallization pressure is at least 60 mbar, or at least 61 mbar, or at least 62 mbar, or at least 63 mbar, or at least 64 mbar, or at least 65 mbar, or at least 66 mbar; and

wherein the crystallization pressure is at most 180 mbar, or at most 160 mbar, or at most 140 mbar, or at most 120 mbar, or at most 100 mbar, or at most 90 mbar, or at most 80 mbar, or at most 70 mbar, or at most 65 mbar, or at most 64 mbar, or at most 63 mbar, or at most 62 mbar, or at most 61 mbar, or at most 60 mbar;

optionally, wherein the crystallization pressure is within the range of from 45 to 200 mbar; or 46 to 200 mbar; or 40 to 100 mbar; or 45 to 100 mbar; or 46 to 100 mbar; or 40 to 60 mbar; or 45 to 60 mbar; or 46 to 60 mbar or; 60 to 200 mbar; or 61 to 200 mbar; or 62 to 200 mbar; or 60 to 100 mbar; or 61 to 100 mbar; or 62 to 100 mbar;

optionally wherein the crystallization pressure is kept essentially constant over time until the end of crystallization; and does not change relatively by more than 20 mbar; or by not more than 15 mbar; or by not more than 10 mbar; or by not more than 5 mbar.

5 . The process according to claim 1 , wherein in step (d) crystallization is induced by means of seed crystals;

optionally wherein the seed crystals are added to the aqueous mother liquor that is contained in the evaporating crystallizer.

6 . The process according to claim 5 , wherein the number of crystals of crystalline allulose that are obtained is at most 150% of the number of seed crystals that are employed.

7 . The process according to claim 5 ,

wherein the seed crystals are added to the aqueous mother liquor before it is supplied to the evaporating crystallizer; or

optionally wherein the seed crystals are provided in form of a suspension in an alcoholic liquid, which is added to the aqueous mother liquor, optionally wherein the alcoholic liquid comprises methanol, ethanol, n-propanol, iso-propanol, tert-butanol or any combination thereof.

8 . The process according to claim 5 , wherein the seed crystals are provided in form of a suspension in an alcoholic liquid, which is added to the aqueous mother liquor, wherein the volume of the suspension is at most 1.0 vol.-% of the volume of the aqueous mother liquor that is contained in the evaporating crystallizer.

9 . The process according to claim 1 , wherein in step (d) crystallization of allulose from the aqueous mother liquor is induced at the crystallization temperature and at the crystallization pressure in a metastable zone of the supersaturated state.

10 . The process according to claim 1 , wherein in step (d) of the process according to the invention crystallization is induced at a supersaturation coefficient y of the aqueous mother liquor of at least 1.01, or at least 1.02, or at least 1.03, or at least 1.04:

optionally wherein in step (d) crystallization of allulose from the aqueous mother liquor is induced at the crystallization temperature and at the crystallization pressure in an intermediate zone of the supersaturated state or wherein in step (d) crystallization of allulose from the aqueous mother liquor is induced at the crystallization temperature and at the crystallization pressure in a labile zone of a supersaturated state.

11 . The process according to claim 1 , wherein in step (d) of the process according to the invention crystallization is induced at a supersaturation coefficient y of the aqueous mother liquor of at most 1.04, or at most 1.03, or at most 1.02, or at most 1.01, optionally wherein the supersaturated state is achieved by increasing the allulose concentration as a consequence of evaporation under reduced pressure.

12 . The process according to claim 1 ,

wherein step (d) comprises

adding a suspension of seed crystals in an aqueous non-alcoholic liquid to the aqueous mother liquor;

maintaining the vapor phase above the aqueous mother liquor within the evaporating crystallizer at the crystallization pressure until the end of crystallization; wherein the crystallization pressure is kept essentially constant over time, and does not change relatively by more than 20 mbar; and/or

wherein step (d) comprises the substeps of

(d-1) evaporating water from the aqueous mother liquor thereby increasing the concentration of dissolved allulose until spontaneous crystallization occurs and/or inducing crystallization by means of seed crystals;

(d-2) promoting crystal growth by adding further aqueous mother liquor; and

(d-3) withdrawing precipitate from the evaporating crystallizer,

optionally wherein the crystallization temperature at all substeps is kept essentially constant over time until the end of crystallization; and/or

wherein the aqueous mother liquor provided in step (a) has a content of dry matter of at least 80 wt.-%; relative to the total weight of the aqueous mother liquor; and/or

wherein the aqueous mother liquor provided in step (a) has an allulose content of at least 90 wt.-%; relative to the total content of dry matter that is contained in the aqueous mother liquor; and/or

wherein the aqueous mother liquor provided in step (a) has a fructose content of at most 10 wt; relative to the total content of dry matter that is contained in the aqueous mother liquor.

13 . The process according to claim 1 , wherein the aqueous mother liquor provided in step (a) has a density within the range of from 1.36 to 1.46 g cm −3 .

14 . The process according to claim 1 ,

wherein the aqueous mother liquor provided in step (a) has a fructose content of at least 0.1 wt.-%;

relative to the total content of dry matter that is contained in the aqueous mother liquor and/or wherein the aqueous mother liquor provided in step (a) has a fructose content of at least 0.1 wt.-%;

relative to the total weight of the aqueous mother liquor optionally wherein the aqueous mother liquor provided in step (a) has a fructose content of at most 10 wt.-%; relative to the total weight of the aqueous mother liquor;

optionally wherein the aqueous mother liquor provided in step (a) has a content of allulose of at least 80 wt.-%; relative to the total weight of the aqueous mother liquor.

15 . The process according to claim 1 , wherein the at least a portion of the supernatant is recycled to step (a);

optionally wherein the process comprises the additional step of (e) separating at least a portion of the precipitate from at least a portion of the supernatant, optionally wherein step (e) involves centrifugation.

16 . The process according to claim 1 , wherein the process is further characterized by one or any combinations of the following:

the supernatant obtained in step (d) has a content of dry matter of at most 78 wt.-%; relative to the total weight of the supernatant; and/or

the process comprises the additional step of (e) separating at least a portion of the precipitate from at least a portion of the supernatant, optionally wherein step (e) involves centrifugation; and/or

the process comprises the additional step of (f) drying the solid allulose material; and/or

the aqueous mother liquor at the crystallization temperature has a viscosity of at most 2000 mPa-s measured by means of a rotary viscosimeter at a speed of 100 rpm; and/or

the viscosity of the aqueous mother liquor within the evaporating crystallizer does not change relatively by more than 100 mPa-s; and/or

the process is operated as a batch process, the process is operated as a semi-continuous process, the process is operated as a continuous process, or the process is operated as a fed-batch process; and/or

a precipitate and/or a supernatant is continuously or semi-continuously withdrawn from the evaporating crystallizer; and/or

withdrawn solvent and/or supernatant and/or precipitate is substituted by fresh aqueous mother liquor that is supplied to the evaporating crystallizer; and/or

the process comprises the stages of: (i) at least partially filling the evaporating crystallizer with the aqueous mother liquor, (ii) performing evaporating crystallization, (iii) supplying fresh aqueous mother liquor to the evaporating crystallizer, (iv) performing evaporating crystallization, and (v) optionally, emptying of the evaporating crystallizer; optionally wherein at stage (iii) the fresh aqueous mother liquor is supplied to the evaporating crystallizer when a predetermined condition has been achieved in the course of stage (ii), wherein said predetermined condition is selected from the group consisting of a viscosity of the aqueous mother liquor within the evaporating crystallizer, a density of the aqueous mother liquor within the evaporating crystallizer, a dry substance mass of the aqueous mother liquor within the evaporating crystallizer, a particle size of the precipitate that is withdrawn from the evaporating crystallizer, a torque at a stirred stirring the aqueous mother liquor within the evaporating crystallizer, a volume of the aqueous mother liquor within the evaporating crystallizer, a yield of the precipitate that has been withdrawn from the evaporating crystallizer a volume or weight of water that has been evaporated and withdrawn from the evaporating crystallizer; an evaporation rate, a supersaturation coefficient y of the aqueous mother liquor within the evaporating crystallizer, a crystallization pressure, and a crystallization temperature; and/or

the crystallization pressure is kept essentially constant over time until the end of crystallization and wherein the crystallization temperature is regulated to keep the evaporation rate essentially constant; and/or

the crystallization temperature is kept essentially constant over time until the end of crystallization and wherein the crystallization pressure is regulated to keep the evaporation rate essentially constant; and/or

the crystallization pressure is kept essentially constant over time until the end of crystallization and wherein the crystallization temperature is regulated to keep the supersaturation coefficient y essentially constant; and/or

the crystallization temperature is kept essentially constant over time until the end of crystallization and wherein the crystallization pressure is regulated to keep the supersaturation coefficient y essentially constant; and/or

withdrawn solvent and/or supernatant and/or precipitate is substituted by fresh aqueous mother liquor that is supplied to the evaporating crystallizer;

wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated; and wherein the supersaturation coefficient y is determined and wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated according to the determined supersaturation coefficient y to keep the supersaturation coefficient y essentially constant; and/or

the process is free of any step of crystallization under atmospheric pressure; and/or

the process is free of any step of cooling crystallization; and/or

the solid allulose material comprises crystalline allulose optionally together with amorphous allulose; and/or

the solid allulose material has a degree of crystallinity of at least 90%; and/or

the solid allulose material consists essentially of crystalline allulose; and/or

the solid allulose material consists of particles having an average particle size d50 (X Fe max ) of at least 120 μm; and/or

the solid allulose material consists of particles having an average particle size d50 (X Fe max ) of at most 330 μm; and/or

the solid allulose material consists of particles having an average particle size d50 (X Fe max ) within the range of 75 to 500 μm; and/or

the solid allulose material has a residual content of alcohol of at most 5 ppm.

17 . The process according to claim 1 , wherein solvent is continuously or semi-continuously withdrawn from the evaporating crystallizer by evaporation;

wherein solvent is withdrawn from the evaporating crystallizer with an evaporation rate:

(i) of at least 5.0 g/h, or at least 7.0 g/h, or at least 9.0 g/h, or at least 11 g/h, or at least 13 g/h, or at least 15 g/h, or at least 17 g/h; and/or

(ii) of at most 50 g/h, at most 48 g/h, at most 46 g/h, at most 44 g/h, at most 42 g/h, at most 40 g/h, at most 38 g/h, at most 36 g/h, at most 34 g/h, at most 32 g/h, or at most 30 g/h, or at most 28 g/h, or at most 26 g/h, or at most 24 g/h, or at most 22 g/h, or at most 20 g/h; and/or

(iii) within the range of 11±2.0 g/h, or 13±4.0 g/h, or 13±2.0 g/h, 15±6.0 g/h, or 15±4.0 g/h, or 15±2.0 g/h, or 17±8.0 g/h, 17±6.0 g/h, or 17±4.0 g/h, or 17±2.0 g/h, or 19±10 g/h, or 19±8.0 g/h, 19±6.0 g/h, or 19±4.0 g/h, or 19±2.0 g/h;

optionally wherein the process is operated as a semi-continuous process, a continuous process, or a fed-batch process.

18 . The process according to claim 1 , wherein withdrawn solvent and/or supernatant and/or precipitate is substituted by fresh aqueous mother liquor that is supplied to the evaporating crystallizer;

wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is controlled or regulated;

optionally, wherein the viscosity of the aqueous mother liquor within the evaporating crystallizer is measured and wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated according to the measured viscosity to keep the viscosity of the aqueous mother liquor within the evaporating crystallizer essentially constant;

optionally, wherein the density of the aqueous mother liquor within the evaporating crystallizer is measured and wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated according to the measured density to keep the density of the aqueous mother liquor within the evaporating crystallizer essentially constant;

optionally, wherein the dry substance mass of the aqueous mother liquor within the evaporating crystallizer is measured and wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated according to the measured dry substance mass to keep the dry substance mass of the aqueous mother liquor within the evaporating crystallizer essentially constant;

optionally, wherein the particle size of the precipitate that is withdrawn from the evaporating crystallizer is measured and wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated according to the measured particle size to keep the particle size of the precipitate essentially constant;

optionally, wherein the aqueous mother liquor is stirred with a stirrer within the evaporating crystallizer and the torque is measured at the stirrer, and wherein the supply of fresh aqueous mother liquor to the evaporating crystallizer is regulated according to the measured torque to keep the torque at the stirrer essentially constant.

19 . A solid allulose material obtainable or obtained by the process of claim 1 ,

optionally, wherein the solid allulose material has a residual total amount of alcohol of at most 5 ppm.

20 . The process of claim 1 , wherein the first crystallization pressure is within a range of 50 to 200 mbar and the second crystallization pressure is within a range of 40 to 100 mbar; optionally wherein the first crystallization pressure is within the range of 80±20 mbar and the second crystallization pressure is within the range of 50±10 mbar.