This process provides for the treatment of spent pulping liquors in a fluidized bed reactor for the purpose of recovering the pulping chemicals and simultaneously complete the pollution free destruction of the unwanted organic pulping residue by thermal oxidation.

The chemicals, recovered as pelletized salt cake, will be sold or recycled to the digesters depending upon the type of cooking process used.

Weak liquor, as received from the pulp mill, is collected in weak liquor storage to provide buffer capacity for a few days in the pulp mill operation. The weak liquor is evaporated in multiple effect evaporators and pumped to a concentrated liquor storage tank. The liquor concentration for injection into the fluidized bed is adjusted to the level needed for autogenous oxidation depending upon the pulping yield and heating value of the spent liquor. Some soda cooking liquors contain enough volatiles to make autogenous combustion impractical (liquor concentration becomes too high). In these cases either auxiliary fuel or heat recovered from the hot exhaust gas is used to maintain the fluidized bed temperature.

Weak liquor is also used as the scrubbing medium makeup in the venturi scrubber, where it can be concentrated to varying degrees, depending upon the temperatures of the flue gases, before being passed on to the multiple effect evaporators. The venturi scrubber used as the primary scrubber operates as a direct contact type evaporator. Alternately, it can be utilized to concentrate liquor from the multiple effect evaporators to final reactor feed concentration. This mode of operation is usually chosen when spent liquors tend to foam or scale because of process characteristics.

Concentrated liquor is pumped to a specially designed feed gun, which sprays the liquor with the aid of compressed air, into the upper free gas space of the fluidized bed reactor, commonly referred to as freeboard. In the freeboard, water is evaporated from the concentrated liquor and leaves the reactor with the combustion gases. Evaporation of water from feed liquor helps keep the gas temperature in the reactor freeboard near 1,200 degrees F. Thus the freeboard area is operating as a spray dryer.

Additional water is usually sprayed into the freeboard to insure that the freeboard temperature is maintained well below the ash melting point (which might otherwise cause excessive ash build up to the reactor wall).

The partial dried liquor is injected directly into the bed at the bottom through feed guns with compressed air. This method increases the residence time of volatiles in the fluidized bed and consequently provides more burning of volatiles in the bed and less in the freeboard. This feed injection method reduces the amount of supplementary heat required for the fluidized bed when operating on pulping liquor from soda cooks.

In this system, the product, salt cake, will be contaminated only by inorganics dissolved out of the pulped wood and by possible contamination of the cooking chemicals themselves.

The temperature of the fluidized bed is kept at a temperature between 1,200 and 1,350 degrees F by the oxidation of organic substances depending upon the particular process. Temperature variations within the bed can be controlled to only +10 degrees F. The granular bed product is withdrawn through the side of the reactor from the area of the bed at a rate equal to the rate of accumulation of inorganics from the oxidation of spent liquor.

A variable speed screw conveyor (metering screw) controls the rate of product discharge. The product passes through a water cooled screw conveyor before entering a bucket elevator for transport to a storage silo or direct discharge into transportation facilities. As an alternate to the metering screw and cooling screw a less costly fluid bed cooler and associated controls can be used.

The fluidized bed material is kept in a fluidized state by the total flow of combustion air, which enters the bed through nozzles in an "orifice plate". The orifice plate distributes the flow of air uniformly and also supports the weight of the bed. The space in the reactor below the orifice plate is referred to as the windbox, and it assists in the distribution of the fluidizing air flowing in from the fluidizing air blower through the start-up air preheater.

Agglomeration:
Very small inorganic particles are continuously fused to pellets making up the fluidized bed. This process is called agglomeration and the rate of this agglomeration depends upon 1) the chemical composition of the inorganics which in turn determines the space velocity of gases in the fluidized bed and 2) the reactor and orifice plate design.

Despite the effects of design and particularly the minute particle size of inorganics created by the single freeboard feed gun, a diminishing of the smaller fractions of particles usually occurs in the fluidized bed, which may have an adverse effect upon proper fluidization. To counteract this, injection of particle fines (dust) may be required at times in order to assure proper particle size distribution, which in turn is fundamental to complete and reliable fluidization of any type of bed material.

The required amount of particle fines can be made available for this purpose from either a cyclone mounted in the hot exhaust gas ducting or a pellet grinder. The cyclone system recovers particle fines from the reactor discharge gasses while the pellet grinder system takes pellets from the pellets storage silo, grinds them to produce particle fines less than 30 mesh (Tyler). The dust from either a cyclone or pellet grinder system is injected with steam or compressed air into the fluidized bed in order to shift the particle size distribution of the bed to the smaller ranges. A small fines storage bin will provide the necessary surge capacity in order to avoid delay in the availability of fines.

The hot gases discharged from the reactor go either to a heat recovery exchanger, a waste heat boiler or directly to the venturi scrubber. In the venturi scrubber direct contact of the hot exhaust gases with weak liquor cools the gas while at the same time evaporating water from the weak liquor and removing remaining dust loading from the exhaust gases.

The separator section of the venturi scrubber system and the secondary scrubber serve essentially to remove any weak liquor which may be carried over in minute droplet form. The final exhaust gas discharged to atmosphere is clear to the point where it can meet existing air pollution abatement rules and regulations. It will be odorless and will only be visible as a white plume of water vapor.

Start-up time and procedure:
Start-up of the system is accomplished by preheating the fluidized air in a gas or oil fired air preheater to approximately 1,000 degrees F. Once the bed temperature has reached this temperature, granular charcoal is injected into the fluidized bed. Combustion of charcoal will raise the bed temperature to 1,200 degrees F, at which point the introduction of spent liquor may begin. Total start-up time from a cold start will require about 24 hours. Temperature will drop about 50 degrees per day; consequently, a start-up after a short period of down time can be made with a system which is still hot.

Operation and Maintenance:
This chemical recovery system can be operated by one or two people depending upon the sophistication of the controls incorporated. One person can easily operate the system when provisions are made for handling routine maintenance on a quick response basis, relieving the operator of the necessity of leaving the control room for more than 15 minutes at a time. Start-up and shut-down is accomplished manually. However, provisions are made for an automatic shut-down sequence in the event of a major failure.

Safety:
The system is intrinsically safe from explosion hazards when operating autogeneously. The only high pressure vessel used in this system would be the steam boiler drums and tubes. Oil and particularly gas fired air preheaters are equipped with all necessary flame safety controls.