Process for improving the hydrolysis of cellulose in high consistency systems using one or more unmixed and mixed hydrolysis reactors

1. A process for the enzymatic hydrolysis of cellulose to produce glucose from a pretreated cellulosic feedstock, the process comprising:

(i) providing an aqueous slurry of the pretreated cellulosic feedstock, which aqueous slurry has an undissolved solids content of between about 12 wt % to about 40 wt %;

(ii) introducing the aqueous slurry of the pretreated cellulosic feedstock to an unmixed hydrolysis reactor;

(iii) adding cellulase enzymes to the unmixed hydrolysis reactor before the step of introducing (step ii), during the step of introducing (step ii), or a combination thereof, wherein in the unmixed hydrolysis reactor, the cellulase enzymes hydrolyze a portion of the cellulose to produce soluble sugars and reduce the viscosity of the aqueous slurry, thereby producing a mixture of partially hydrolyzed cellulose containing soluble sugars;

(iv) continuing the hydrolysis of the cellulose to glucose by feeding the mixture of partially hydrolyzed cellulose containing soluble sugars to a mixed hydrolysis reactor that effects mixing of the mixture or a hydrolysis system comprising mixed hydrolysis reactors that effects mixing of the mixture; and

(v) withdrawing a stream comprising glucose and unreacted cellulose from the mixed hydrolysis reactor or the hydrolysis system of step (iv), wherein

the power consumption associated with said process is reduced relative to an otherwise identical process employing a mixed hydrolysis reactor in step (iii).

2. The process of claim 1 , wherein the portion of the cellulose hydrolyzed in the unmixed reactor is between about 10 wt % and about 70 wt %.

3. The process of claim 2 , wherein the portion of the cellulose hydrolyzed in the unmixed reactor is between about 25 wt % and about 70 wt %.

4. The process of claim 3 , wherein the portion of the cellulose hydrolyzed in the unmixed reactor is between about 35 wt % and about 70 wt %.

5. The process of claim 1 , wherein the cellulase enzyme contains beta-glucosidase.

6. The process of claim 1 , wherein the water content of the aqueous slurry is between about 40% and about 140% of the maximum water holding capacity of the pretreated cellulosic feedstock.

7. The process of claim 6 , wherein the water content of the aqueous slurry is between about 60% and about 120% of the maximum water holding capacity of the pretreated cellulosic feedstock.

8. The process of claim 1 , wherein the unmixed hydrolysis reactor is a batch, fed-batch or continuous reactor with a height-to-diameter ratio of about 0.2:1.0 to about 5.0:1.0.

9. The process of claim 1 , wherein the soluble sugars in the mixture of partially hydrolyzed cellulose comprise glucose, cellobiose, glucose oligomers, or a combination thereof.

10. A process for the enzymatic hydrolysis of cellulose to produce glucose from a pretreated cellulosic feedstock, the process comprising:

(i) providing an aqueous slurry of the pretreated cellulosic feedstock, which aqueous slurry has an undissolved solids content of between about 12 wt % to about 40 wt %;

(ii) introducing the aqueous slurry of the pretreated cellulosic feedstock to a system comprising a plurality of unmixed hydrolysis reactors connected in parallel, series or a combination thereof;

(iii) adding cellulase enzymes to at least one unmixed hydrolysis reactor before the step of introducing (step ii), during the step of introducing (step ii), or a combination thereof, wherein the cellulase enzymes hydrolyze a portion of the cellulose to produce soluble sugars in said at least one unmixed hydrolysis reactor and reduce the viscosity of the aqueous slurry, thereby producing a mixture of partially hydrolyzed cellulose containing soluble sugars;

(iv) continuing the hydrolysis of the cellulose to glucose by feeding the mixture of partially hydrolyzed cellulose containing soluble sugars to one or more hydrolysis reactors that effect mixing of the mixture; and

(v) withdrawing a stream comprising glucose and unreacted cellulose from the one or more hydrolysis reactors of step (iv), wherein

the power consumption associated with said process is reduced relative to an otherwise identical process employing a system of mixed hydrolysis reactors in step (iii).

11. The process of claim 10 , wherein the portion of the cellulose hydrolyzed in the unmixed reactor system is between about 10 wt % and about 70 wt %.

12. The process of claim 11 , wherein the portion of the cellulose hydrolyzed in the unmixed reactor system is between about 25 wt % and about 70 wt %.

13. The process of claim 12 , wherein the portion of the cellulose hydrolyzed in the unmixed reactor system is between about 35 wt % and about 70 wt %.

14. The process of claim 10 , wherein the cellulase enzyme contains beta-glucosidase.

15. The process of claim 10 , wherein the water content of the aqueous slurry is between about 40% and about 140% of the maximum water holding capacity of the pretreated cellulosic feedstock.

16. The process of claim 15 , wherein the water content of the aqueous slurry is between about 60% and about 120% of the maximum water holding capacity of the pretreated cellulosic feedstock.

17. The process of claim 11 , wherein the unmixed hydrolysis reactors are batch, fed-batch or continuous reactors with a height-to-diameter ratio of about 0.2:1.0 to about 5.0:1.0.

18. The process of claim 10 , wherein the soluble sugars in the mixture of partially hydrolyzed cellulose comprise glucose, cellobiose, gentiobiose, glucose oligomers, or a combination thereof.

19. A process for the enzymatic hydrolysis of cellulose to produce glucose from a pretreated cellulosic feedstock, the process comprising:

(i) hydrolyzing the pretreated cellulosic feedstock having an undissolved solids content of between about 12 wt % to about 40 wt % with cellulase enzymes in at least one unmixed hydrolysis reactor to produce a mixture comprising partially hydrolyzed cellulose, wherein during said hydrolyzing between about 10 wt % to about 70 wt % of the cellulose is hydrolyzed; and

(ii) further enzymatically hydrolyzing the mixture comprising partially hydrolyzed cellulose and soluble sugars with cellulase enzymes in at least one mixed hydrolysis reactor to produce the glucose, wherein

the power consumption associated with said process is reduced relative to an otherwise identical process employing at least one mixed hydrolysis reactor in step (i).

20. A process for producing ethanol, the process comprising:

(i) hydrolyzing a pretreated cellulosic feedstock having an undissolved solids content of between about 12 wt % to about 40 wt % with cellulase enzymes in at least one unmixed hydrolysis reactor to produce a mixture comprising partially hydrolyzed cellulose, wherein during said hydrolyzing between about 10 wt % to about 70 wt % of the cellulose is hydrolyzed;

(ii) further enzymatically hydrolyzing the mixture comprising partially hydrolyzed cellulose and soluble sugars with cellulase enzymes in at least one mixed hydrolysis reactor to produce glucose, wherein

the power consumption associated with steps (i) and (ii) is reduced relative to an otherwise identical process employing at least one mixed hydrolysis reactor in step (i); and

(iii) fermenting the glucose with a microorganism to produce a fermentation broth comprising the ethanol and then concentrating the ethanol.