System and process for recovering heat from weak black liquor in a wood pulping process

1. A method of cooling and recovering heat from weak black liquor produced in a pulp mill, comprising:

digesting fibers in a digester to form weak black liquor;

directing the weak black liquor through a series of tubes that form a part of a spray film evaporator;

cooling and recovering heat from the weak black liquor passing through the spray film evaporator by:

i. directing a feed to the spray film evaporator;

ii. spraying the feed onto the tubes of the spray film evaporator and vaporizing the feed to produce a vapor; and

iii. directing the vapor from the spray film evaporator and wherein the heat associated with the vapor can be utilized as a heat source for other processes in the pulp mill.

2. The method of claim 1 including directing the vapor produced by the spray film evaporator to a multi-effect evaporation system.

3. The method of claim 2 including feeding at least a portion of the vapor produced by the spray film evaporator to an intermediate effect of the multi-effect evaporation system.

4. The method of claim 2 including directing weak black liquor from the spray film evaporator to an intermediate effect of the multi-effect evaporation system and from the intermediate effect directing the weak black liquor rearwardly through one or more effects and then reversing the direction of the weak black liquor by directing the weak black liquor forwardly through one or more effects.

5. The method of claim 2 wherein the multi-effect evaporator produces a series of condensates and the method includes directing one or more, but not all, of the condensates from the multi-effect evaporation system to the spray film evaporator and vaporizing at least a portion of the condensates.

6. The method of claim 1 including the reducing the temperature of the weak black liquor in the spray film evaporator at least 20° F.

7. A method of treating weak black liquor produced in a pulp mill, comprising:

digesting fibers in a digester to form weak black liquor;

directing the weak black liquor through a series of tubes that form a part of a spray film evaporator;

cooling the weak black liquor in the spray film evaporator;

after cooling the weak black liquor, directing the weak black liquor to a multi-effect evaporation system and concentrating the weak black liquor therein to produce a concentrated black liquor and condensate;

directing the concentrated black liquor to a recovery boiler;

recovering heat from the weak black liquor by spraying condensate produced by the multi-effect evaporation system onto outside surfaces of the tubes and utilizing heat recovered from the weak black liquor to heat and vaporize the condensate which produces a vapor; and

directing the vapor to the multi-effect evaporation system and utilizing the heat associated with the vapor to concentrate the weak black liquor in the multi-effect evaporation system.

8. The method of claim 7 including feeding at least a portion of the vapor produced by the spray film evaporator to an intermediate effect of the multi-effect evaporation system.

9. The method of claim 7 including feeding the weak black liquor from the spray film evaporator into an intermediate effect and therefrom rearwardly through one or more effects and thereafter reversing the direction of the weak black liquor and directing the weak black liquor forwardly through internal or external heat exchangers and then through one or more effects located on a forward side of the effect initially receiving the weak black liquor.

10. The method of claim 7 wherein the multiple effect evaporation system produces a series of condensates and the method includes combining a series, but not all, of the condensates and directing the combined condensates to the spray film evaporator and vaporizing at least a portion of the combined condensates.

11. The method of claim 7 further including:

directing the vapor produced by the spray film evaporator to an intermediate effect of the multi-effect evaporator system; and

directing the weak black liquor cooled by the spray film evaporator to one or more feed flash tanks of the multi-effect evaporation system thereafter to the last effect, and thereafter directing the weak black liquor in a forward direction through the multi-effect evaporation system.

12. The method of claim 11 wherein the multi-effect evaporation system produces a series of condensates, and the method includes combining less than all of the condensates and directing the combined condensates to the spray film evaporator and evaporating at least a portion of the combined condensates.

13. The method of claim 11 wherein the multi-effect evaporation system produces a series of condensates, and wherein the method includes combining two or more condensates produced by the multi-effect evaporation system and directing the combined condensates to the spray film evaporator, and wherein the combined condensates directed to the spray film evaporator is less contaminated than other condensates produced by the spray film evaporator and more contaminated than still other condensates.

14. The method of claim 7 wherein the spray film evaporator includes a housing and a bundle of tubes contained in the housing and wherein cooling the weak black liquor includes directing the weak black liquor through the bundle of tubes and cooling the weak black liquor by spraying the condensate onto the outside surfaces of the bundle of tubes contained within the housing.