US2258467A - Method and apparatus for the recovery of heat and chemicals from black liquor - Google Patents

Description

Patented Oct. 7, 1941 METHOD AND APPARATUS FOR THE RECOV- ERY OF HEAT AND CHEMICALS FROM BLACK LIQUOR Vincent P. Owens, Plainfield, N. J., assignor to Combustion Engineering Company, Inc., New York, N. Y., a corporation of Delaware Application October 14, 1939, Serial No. 299,517

3 Claims.

This invention relates to a self-sustaining method of and apparatus for the recovery of heat and chemicals from black liquor obtained in the preparation of pulp from wood by the caustic soda or sulphate process and in general includes an evaporator for removing water from the black liquor to increase its density, a recovery furnace for the recovery of chemicals and a waste heat boiler associated with the furnace.

The p-rimary object of my invention is the provision of improvements whereby the recovery unit is maintained self-sustaining despite varying operating conditions.

How the foregoing, together with such other objects and advantages as will hereinafter appear or are incident to my invention, are realized, is illustrated in preferred form in the accompanying drawing, wherein the figure is a more or less diagrammatic sectional elevational View of a recovery system embodying my invention.

'Referring to the drawing black liquor received from a suitable source, such as multiple effect evaporators customarily employed in association with recovery units, is led by means of a pipe 2 to a head or storage tank 3, the liquor thus in 'r troduced normally having a given density of approximately 45% of solids. The liquor is led from the tank 3 by means of a pipe 4 to an evaporator 5 where the evaporation of water takes place by virtue of hot gases led thereinto as by means of a conduit or connection 6 leading from the oiltake of the waste heat boiler 8 to the inlet 9 of the evaporator 5.

High density liquor under pressure of a pump 5a and heated by a heater 5b is led from the evaporator to the nozzles l of the recovery furnace by means of pipes |2, this high density liquor being sprayed into the furnace by means of the nozzles, the liquor being fed to thenozzles at a pressure of at least substantially 50 pounds and at a temperature of at least substantially 230, as disclosed in the application of Fay Harry Rosencrants and Alexander Leopold Hamm, Serial No. 193,398, led March 2, 1938.

The furnace has its two side walls |3 (only one of which appears in the drawing) and its front wall i4 lined with closely spaced upright stream evaporating tubes i5 connected at their lower ends into bottom headers I6 and at their upper ends into upper headers which are con- "l nected into the circulation of the boiler 8 as by means of upcomers I8. The rear wall I9 of the furnace is lined with similar closely spaced upright steam evaporating tubes 2E) connected at,

their lower ends into a lower header I6 yand at their upper ends into the bottom drum 2| of the boiler 8.

` Preheated air in amounts sufficient for combustion is introduced into the furnace as by means of a plurality of air inlets 22. The furnace is provided with a refractory lined bottom 23 and the recovered chemicals may be withdrawn through a spout 24.

The waste heat boiler 8 above referred to comprises in general the lower drum 2| above mentioned, an upper drum 25, a front bank of upright boiler tubes 2B connecting the drums 2| and 25, a rear bank of upright boiler tubes 21 connecting said drums, a plurality of widely spaced up-right boiler tubes 28 connecting the lower drum 2| with an upper drum 29 spaced horizontally from the upper drum 25, and circulation tubes 39 and 3| connecting the upper drums 25 and 29. A superheater 32 is located in the space between the bank of boiler tubes andthe boiler tubes 28. t

A baffle 33 located at the front portion of the front bank of boiler tubes 2S extends upwardly from the lower drum 2| to a point somewhat below the upper drum 25 and a baffle 34 located at the rear portion of said front bank of boiler tubes extends downwardly from the upper drum 25 to a point somewhat above the lower drum 2|. Laterally extending baille plates 35 extend from the baille 3d to the front tubes of the rear bank of boiler tubes 2l and a rear bai-lle 35 is provided at the rear of said rear bank of boiler tubes. This rear baille terminates at its upper end at a point below the upper sheathing wall 3l, thus providing the offtake 1 above referred to. Rear sheathing 33 is provided to form the conduit or connection 6 leading from the oitake 'i to the inlet 9 of the evaporator 5.

I he baling referred to provides a plurality of passages for the gases, it being noted that the hot gases enter at the top of the fro-nt bank for downward flow thereover, and then pass in upward flow over the rear bank for exit through the oiftake l from whence they are led through the conduit il and inlet 9 into the evaporator.

The temperature of the waste gases entering the evaporator after passing through the gas passages in normal ilow and temperature is such that liquor introduced into the evaporator at a given density of of solids will be increased in the evaporator to a density of approximately to '70% of solids for introduction into the furnace so as to maintain a drying and burning rate having a ratio such that the unit will be selisustaining, as will further appear.

In order to prevent the furnace from becoming wet and inoperative in the event that the waste liquor being supplied to the evaporator drops below the aforesaid given density of 45% of solids, as for example, would be the case if there was an unbalancing of the multiple effect evaporators usually associated with the system, I have provided means for supplying gases of higher temperature to the evaporator whereby this lower density liquor is increased to the high value above mentioned which is necessary for self-sustaining operation of the unit while at the same time avoiding operating difficulties in the steam generating equipment, as will further appear.

This temperature control means comprises a bypass opening 39 provided in the upper portion of the baille 34 and an adjustable damper 40 controlling said opening. When the damper lil] is closed as shown in the drawing the hot gases enter at the top of the rst pass and flow downwardly over the tubes of the rst bank 2S, then pass to the bottom of the second pass and ow upwardly over the tubes of the second bank 2l and finally iiow through the upper oiftalre 1 into the conduit 6 where they flow downwardly for introduction into the evaporator 5 through the evaporator gas inlet 9. An opening 4I is provided at the lower portion of the rear baffle 35 through which some of the waste gases may flow into the lower portion.of the conduit 6, as indicated by the arrow 42.

In normal operation when the black liquor introduced into the evaporator is of a given density of approximately 45% of solids the damper 40 is in its closed position to provide gas flow as just described and with this condition of liquor introduction and waste gas flow the furnace will be self-sustaining.

If for any reason the density of the liquor being introduced into the evaporator should drop to a lower density than said given density and if the waste gas now to the evaporator should remain as just described, then the density of the liquor leaving the evaporator would also be lowered. This lower density liquor when introduced into the furnace because of its carrying larger quantities of water than would permit of self-sustaining operation of the furnace, would require additional heat in the furnace to evaporate the additional water and to obtain this heat auxiliary fuel would have to be introduced into the furnace, and this would materially increase the cost of operation. If the heat were not added the furnace would become wet and inoperative.

In order to control the evaporation in the evaporator I have provided the aforesaid bypass means whereby the temperature of the gases entering the evaporator may be increased, it being noted that when the damper 40 is opened boiler heating surface is short circuited and hot entering gases pass directly through the bypass 39 across the upper portion of the boiler tubes to and through the offtake 1 and into the conduit 6 leading to the evaporator. Thus an instantaneous increase in the temperature of the gases entering the evaporator is effected to give the required control which will keep the furnace supplied with liquor of the density necessary for self -sustaining operation.

The system may thus be operated continuously as a self-sustaining one because by the control afforded through the medium of the adjustable bypass means I obtain a flexibility of control to raise and lower the temperature of gases to the evaporator which enables me to maintain a supply of black liquor to the furnace of constant density even though the density of the liquor supplied to the evaporator varies.

The importance of the foregoing will be seen from the following. When the highly concentrated heated black liquor leaves the nozzles further concentration takes place due to the reduction in pressure which results in the flashing of water content of the black liquor into steam. Due to this and to the fact that the black liquor is delivered to the nozzles at an elevated temperature the gravitating particles of black liquor are dried -to substantial dryness before lodging and collecting on the bottom to a suflicient degree to sustain combustion there without the aid of other fuel. There the solids are brought to incandescence in a reducing atmosphere (insufficient air for complete combustion being passed through the bed of particles as by means of preheated air inlets 22a), chemical is converted for recovery and flows out in molten form and the volatile combustibles rise and are consumed in the combustion space above the bed furnishing the additional heat required for drying the gravitating black liquor particles. The installation is thus self-sustaining when the liquor supplied to the evaporator is of normal density despite the fact that the combustion of the combustible volatiles and the drying of the gravitating black liquor particles is done in a space defined by steam evaporating tubes subject to radiant heat. In order to maintain the furnace self-sustaining under varying conditions occurring in the black liquor treating portion of the system which would result in the liquor supplied to the evaporator being below normal density, it is absolutely necessary to meet such conditions, and I do this by means of the control above described. It will also be seen that I am enabled to do this without incurring operating difficulties in the steam generating system because the slagging tubes and superheater tubes abstract sufficient heat from the gases to cause condensation of the major portion of the volatilized chemicals and precipitation of the same in solid form. Due to agglomeration the solid particles reach a size suicient to gravitate into the combustion chamber proper directly therebelow.

Thus the system operates to minimize carry over into the convection bank, thus preventing clogging of the tubes of the convection bank in directly by-passing hot gases from the waste gas space to the evaporator, to obtain superheated steam, to reduce slagging difficulties, to return a maximum of recoverable chemical to the combustion chamber and to be self-sustaining despite varying operating conditions.

I claim:

1. The self-sustaining method of recovering chemicals from black liquor which comprises normally supplying liquor of given low density to a waste heat evaporator; increasing the density of said black liquor in said evaporator to a given high density by normally delivering waste gases from a recovery furnace and boiler unit into said evaporator after said waste gases have passed over and among the boiler surfaces of said unit; delivering said high density liquor into said recovery furnace and boiler unit at a given rate; and maintaining the density of the liquor leaving said evaporator and being delivered into said furnace and boiler unit constant when the liquor supplied to the evaporator drops below said given low density by controllably by-passing hotfwaste gases in said boiler and furnace unit to increase the temperature of the waste gases delivered into said evaporator.

2. The self-sustaining method of recovering chemicals from black liquor which comprises normally supplying black liquor at a given density of approximately 45% of solids to a waste heat evaporator; normally delivering waste gases from a recovery furnace and boiler unit after they have passed over and among the boiler surfaces of said unit into said evaporator at a temperature to raise the density of said liquor to approximately 65 to 70% of solids; delivering said high density liquor into said recovery furnace and boiler unit at a given rate whereby the furnace will be self-sustaining; and maintaining the density of the liquor leaving said evaporator and being delivered into the furnace constant at the aforesaid high density when the liquor supplied to the evaporator drops below said given low density' by controllably by-passing hot waste gases in said furnace and boiler unit to increase the temperature of the waste gases delivered into said evaporator.

3. The self-sustaining method of recovering chemicals from black liquor which comprises normally supplying black liquor of given low density to a waste heat evaporator; increasing the density of said black liquor in said evaporator to a given high density by normally deliv-V ering Waste gases from a recovery furnace and boiler unit into said evaporator after said waste gases have passed over and among the boiler surfaces of said unit; delivering said high density liquor into said recovery furnace and boiler unit at a given rate; chilling slag forming particles in the gases in advance of said boiler; and maintaining the density of the liquor leaving said evaporator and being delivered into said furnace and boiler unit constant when the liquor supplied to the evaporator drops below said given low density by controllably by-passing hot Waste gasesV in said furnace and boiler unit to increase

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