Electronic Corrosion Control in Pulp and Paper Plants
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 and chemical intensive industry, it has been necessary to maximise operating efficiencies and productivity, thus they have become increasingly dependent upon sophisticated electronic process control systems.
Although the need for computerisation is great in the pulp and paper-making process, the operating conditions are not conducive for electronics. High humidity, corrosive gases and particulate 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, terpenes 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 of these gases in these areas will typically be in the 1 PPM range.
Other areas, such as boiler houses, recovery boiler sections and wastewater treatment have their share of sulphur, dioxide, NOx, chlorine, and hydrogen sulphide contamination.
In order to keep computerised 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 level.
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 centres 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 centre and a control room. The dryers will generally have two or three motor control centres (MCC) and the re-winder will have a motor control centre. In some of the high-technology process areas, more control rooms are used that make it more susceptible to corrosive gases.
Warehousing/Shipping
This area of the mill will have a motor control centre 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 centres. The turbine generator also will have a motor control centre. In addition, the electrostatic precipitator/bag filter, which removes the emissions from the power boiler, will also have a motor control centre.
Evaporators/Recovery Boiler/Caustic Area
Where the black liquor off the digestion process runs through the evaporators, there will be a motor control centre. The recovery boiler will have a motor control centre 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 centre and a control room.
Sewage Treatment Sludge Dewatering Plant
Typically, there would be an operator control room and one or more motor control centres 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 centres to be protected. Of course, this all depends on the complexity of the mill process and the degree of computerisation 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 in the room.
They are as follows:
In the case of rooms with very low movement of personnel coming in and out, only pressurisation 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 centralised 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 recirculation mode in addition to air pressurisation. This is due to the absorption of contaminants on the clothes and body surfaces of plant personnel while attending to duties at the different process locations, which are generally highly contaminated. Such absorbed gases would immediately desorb 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 pressurisation 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 the movement of personnel. The pressurisation equipment here should provide 3 to 6 air changes per hour, as already explained above.
Note:
All large pulp and paper plants have lime kilns inside their facilities. Care should be taken for designing the clean air pressurisation equipment for rooms located near the lime kilns due to the abnormally high presence of suspended lime dust. The amount of pressurisation 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 the 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, a higher quantity of pressurisation air (say 20 to 30 air changes per hour) is to be introduced. Since the protection of such rooms has to be carried out by providing cleaned pressurised 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 the higher side, but at the expense of not having any corrosion-related breakdowns.
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 and 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.
Gas Phase Filtration System – The Most Effective Way of Removing Airborne Molecular Contamination
Bry-Air Gas Phase Filtration Systems protect electronic equipment and devices from the threat of abrupt failures due to corrosive gases. It reduces downtime by removing corrosive gases through the process of adsorption and chemisorption. It is designed to maintain the ANSI/ISA-71.04-2013 standard, IEC standards and various other environmental standards followed by electronics manufacturers.
These GPF systems also neutralise odourous gases and make the environment comfortable for humans.
Bry-Air: A Step Ahead, Always
Bry-Air is the only Gas Phase Filtration Company in India having its own R&D and Testing Lab for granular media and chemical filters. Backed by world-class technology and systems, the laboratory is capable of testing the performance of impregnated/non-impregnated loose granular media and impregnated/non-impregnated air cleaning devices such as chemical filters, under one roof. The tests are conducted as per ISO/ASHRAE Standard 145.1 and BSR/ASHRAE Standard 145.2/ISO Standard 11155-2 guidelines. The core objectives of the tests are to assess the performance and efficiency of loose granular sorptive media and chemical filters in any particular application.
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