In a standard paper production line, wood chips are cooked with chemicals in aqueous solution, the precise composition of the cooking chemicals depending upon the desired resultant chemical mixture. The resulting chemical mixture, sometimes referred to as "stock" is composed of wood pulp and liquid containing residual chemical and dissolved woody materials, the liquid portion being commonly referred to in the art as "red or black liquor." Separation of the pulp from the black liquor is normally carried out in a washing operation after which the wash liquor is evaporated to recover the chemicals contained therein.
The most common type of washer system includes a rotary vacuum drum onto which the stock is spread. The drum is perforated, and a vacuum maintained inside causes the separation of the liquid from the pulp. The pulp assumes the form of a pulp mat which is still impregnated with chemicals and organics. A shower washer is usually positioned above the drum and typically extends axially along the drum and directs water at and through the pulp mat to remove these substances. A typical washing installation may include several washer drums in sequence with wash water being flowed against the flow of pulp movement so that the final washing stage uses clean water. Subsequent washing stages may be required if the washed pulp is to be bleached.
The liquor effluent from the washers comprises water, spent cooking solvents, and organic materials. The cooking solvents consist of sulfates (SO.sub.2) held in an aqueous solution, methanol/ODTP, and other toxic contaminants. In addition to the liquid contaminants, airborne contaminants are emitted from the washing process due to the high heat and steam introduced during the washing process.
As is customary in the industry, a hood is placed over the drum and washers to prevent steam from being lost and to protect workers from the washing chemicals. Access portals to the interior of the washing areas are incorporated in the hood structure to enable workers to clean the washing screens and provide general maintenance to the interior of the washer.
Current hood technology employed in the pulp and kraft mill industries consists mainly of loose fitting metal covers with downward extending resilient flaps made of neoprene. While present hood technology may adequately protect workers from the hazardous liquid emissions of the washing operation, contemporary hoods do little or nothing to reduce exhaust gases which contain airborne chemical and toxic contaminants. Also, present hoods do not provide a means for establishing a negative hood pressure within the wash area to control out-gassing of contaminants and process evaporation.
As part of current environmental pollution controls, pulp mills incinerate toxic off-gases evacuated from their washing operations so that they can comply with EPA regulations. This incineration process necessitates that pulp mills incur additional manufacturing expense in the form of fuel consumption to incinerate the toxic gases. However, the EPA is moving toward more stringent regulations for paper mills; partly to encourage oxygen delignification (OD) implementation, which significantly reduces water usage and the emission of chlorinated pollutants to the atmosphere during the bleaching operation. Present gas incineration strategies will not provide sufficient pollution reductions to meet the new EPA standards. Furthermore, the published EPA goal for kraft mills is a "closed mill" design with zero water discharge. In order for kraft mills to meet the new EPA regulations soon be to promulgated, current hood technology must be improved to enclose pulp washers and contain the steam and vapor emissions. An additional benefit to the implementation of new hood technology will be to significantly reduce or eliminate the fuel costs for off-gassing incineration, as well as avoidance of EPA fines.
Therefore, there is a great need in the pulp mill industry for an improved hood enclosure that will permit maintenance access and the passage of various types of conduits into the interior of the washing unit while minimizing the emission of toxic substances into the environment. The hood must also be economical and resistant to the caustic chemicals used in washing systems.