Contaminating Gases and its Effects
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 conducive for electronics. High humidity, corrosive gases and particulates contaminants are characteristic of the pulp mill environment endangering the operation of virtually all electronic devices.
Creation of contaminating gases
The digestion process is the primary generator of contaminating gases, chiefly acid gases such as 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 wastewater treatment 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.
Location of Electronic Process Controls - Pulp Side of Process
Wood-yard Area
While this area is not a generator of corrosive gases and thus has a lesser contamination problem, it is still in such close proximity to the contaminant generation areas, that blow-over from such areas can easily contaminate the wood-yard area too.
Digester Area
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.
Paper Machine Area
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
Warehousing/Shipping
This area of the mill will have a motor control center and a control room.
By-Products Side of Process
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.
Evaporators/Recovery Boiler/Caustic Area
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.
Sewage Treatment Sludge Dewatering Plant
Typically there would an operator control room and one or more motor control centers for the sewage treatment sludge dewatering plant.
Total paper and Pulp Mill Requirements
From the above mentioned locations within a pulp and paper mill, there will be 25 to 30 control rooms, motor control rooms/ distributed control systems and rack rooms, where industrial air filtration systems 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.
Controlling Gaseous Pollutants infiltrating in the Control/MCC rooms from outside air
Two types of solutions are available for industrial air filtration and elimination of contaminants, depending on the size and frequency of pedestrian traffic of the room.
They are as follows:
In the case of rooms with the very low movement of personnel coming in and out, the 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 the 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 Gas Phase Filtration Systems 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.
Note:
All large pulp and paper plants have limekilns inside their facilities. Care should be taken for designing the clean air pressurization equipment for rooms located near the limekilns due to abnormally high presence of suspended lime dust. Amount of pressurization air is to be increased for such rooms to the extent of 10 air changes per hour or more so as to create higher positive pressure inside the rooms. This would prevent the carryover of suspended fine dust to a large extent.
Unlike Control/Computer/Electronic Rack rooms, most of the MCC rooms are NOT air-conditioned. Generally, such rooms generate a lot of heat due to the presence of electrical appliances and contacts. The usual practice is to provide exhaust fans at ceiling level to exhaust out the room air and bring in outside air to extract the heat.
However, this method of cooling is extremely detrimental since undesirable outside contaminated air is used to cool the room, which, in fact accelerates the corrosion of electrical controls. For some of such critical locations, where extraction of heat, along with corrosion control is an important parameter, higher quantity of pressurization air (say 20 to 30 air changes per hour) is to be introduced. Since, protection of such rooms have to be carried out by providing cleaned pressurized air, it is desirable to provide air leakage ports, again at the room ceiling level. By this, the MCC room shall not only be protected from corrosion, but also enough air would be thrown out of the room to extract the generated heat. Needless to say, the consumption of chemical media for such locations would be on a higher side, but at the expense of not having any corrosion related breakdowns.
Prevent Corrosion Damage in Paper and Pulp Industry – A Case Study… Click here.
Other important design requirements
Room Air-conditioning
- Less than 50% Relative Humidity with less than 6% change per hour.
- 21-23°C, with less than 1°C change.
Room Construction & Integrity
- Well Sealed Room (not leak-proof – a small amount of leakage is desired and is a requirement).
- Provision of airlock entryways, with door/windows provided with weather-stripping.
