The pulp and paper industry has been at the forefront of inducting computer process control equipment into its operations. In this highly competitive, energy & chemical intensive industry, it has been necessary to maximize operating efficiencies and productivity, thus they have become increasingly dependent upon sophisticated electronic process control systems.
Although the need for computerization is great in the pulp and paper making process, the operating conditions are not accommodating for electronics. High humidities, corrosive gases and corrosion-causing particulates characteristic of the pulp mill environment endanger the operation of virtually all electronic devices.
The digestion process is the primary generator of contaminating gases, chiefly hydrogen sulphide, and also sulphur dioxide, carbon monoxide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide. Also, turpines are given off by the black liquor. It is generally accepted that hydrogen sulphide levels of 1 to 5 PPM typically exist throughout a pulp and paper mill, depending upon the wind directions on a particular day.
If a mill has a bleach plant, then chlorine and chlorine dioxide will be present in the bleach plant and paper machine areas. The level in these areas will typically be in the 1 PPM range.
Other areas, such as boiler house, recovery boiler section and waste water treatment facilities would have their share of sulphur dioxide, NOx, chlorine, and hydrogen sulphide contamination.
In order to keep computerized process control equipment from experiencing corrosion, the hydrogen sulphide level must be brought down to 3 PPB (parts per billion) and chlorine down to 1 PPB levels.
There are motor control centers (MCC) for the slasher deck, debarking area, screen room and chipper. Additionally, there is a control room, which controls all these various aspects of the wood-yard operation. While this area is not a generator of corrosive gases, and thus has a lesser contamination problem, it is still in such a close proximity to the contaminant generation areas, that blow-over from such areas can easily contaminate the wood-yard area too.
There will typically be a control room and one or two motor control centers in this area, controlling the digesters, and the screen and washing areas.
The wet end of the paper machine will have a motor control center and a control room. The dryers will generally have two or three motor control centers and the re-winder will have a motor control center. In some of the high technology process areas, such as the paper machine, a rack room will be attached to the control room.
This area of the mill will have a motor control center and a control room.
Power Boiler/Turbine Generator Area – the power boiler is controlled by a control room and typically has one or two motor control centers. The turbine generator also will have a motor control center. In addition, the electrostatic precipitator/bag filter, which removes the emissions from the power boiler, will also have a motor control center.
Where the black liquor off the digesting process runs through the evaporators, there will be a motor control center. The recovery boiler will have a motor control center or two, and be controlled by a control room. The caustic area, composed of dissolving tanks and a lime-kiln, will generally have a motor control center and a control room.
Typically there would an operator control room and one or more motor control centers for the sewage treatment sludge dewatering plant.
From the above mentioned locations within a pulp and paper mill, there will be 25 to 30 control rooms, motor control rooms and rack rooms, where air purification equipment might be required. Moreover, for each additional paper machine, there will be another 7 to 8 motor control centers to be protected.
Of course, this all depends on the complexity of the mill process and the degree of computerization of the individual processes.
For controlling gaseous contaminants infiltrating in the Control/MCC rooms from outside air
Two types of solutions are available for control and elimination of contaminants, depending on the size and frequency of pedestrian traffic of the room. They are as follows:
In case of rooms with very low movement of personnel coming in and out, only pressurization with chemically cleaned air is sufficient.
Provide from 3 to 6 air changes per hour, to attain approximately 2.5 to 5 mm WC positive pressure inside the room. By this method, there will be a net outflow of clean air from inside the room to outside atmosphere only, thereby eliminating the leakage of outside contaminated air into the room.
Mostly, all centralized Computer/Control rooms, Rack rooms and MCC locations would require this type of air purification.
Rooms with high pedestrian traffic, such as, Operator Control rooms would require air purification by re-circulation mode in addition to air pressurization. This is due to the absorption of contaminants on clothes and body surface of plant personnel while attending to duties at the different process locations, which are generally highly contaminated. Such absorbed gases would immediately de-sorb upon entry to a relatively cleaner area.
For such areas, it is advisable to provide re-circulation type air purifiers working independently, in addition to the pressurization equipment. The equipment should be designed to handle 6 to 10 air treatment cycles or air changes per hour, wherein the inside room air is continuously cleaned of contaminants that are being carried in due to movement of personnel. The pressurization equipment here, should provide 3 to 6 air changes per hour, as already explained above.