The computerized process control boom that occurred in the cellulose fiber (wood pulp) processing industry in the 1970s and 1980s was focused on increased plant productivity. This was achieved through increased production with a simultaneous workforce reduction. It's not uncommon to find pulp mills with production rates that far surpass nameplate capacities and only a fraction of the original number of employees. In most cases, increased plant throughput was unanimously cheered, whereas the workforce reductions generated bad feelings and fostered resistance to further automation projects. Unfortunately, this stigma persists to this day, even though job elimination through automation has become rare in pulp manufacturing. Automation projects now focus almost exclusively on efficiency, quality and safety. Process automation has become the good guy as it helps our fellow employees get home safe, save money for the company, and reduce our environmental footprint.
This article presents both a human and technical perspective on a recent process control project at a Weyerhaeuser Co. pulp mill in Oglethorpe, Georgia. It was a project with excellent economic payback and numerous "soft” benefits for employees responsible for operating that unit. This was a complex project that demanded that we develop it with the operators, rather than just for them.
Weyerhauser's Fling River Mill in Oglethorpe, Georgia, produces more than 350,000 tons per year of fluff pulp and specialty cellulose fiber products for a diverse group of customers. The cellulose fiber is recovered from wood chips in a large continuous digester, which is essentially a heated and pressurized plug flow reactor that dissolves lignin in caustic cooking liquor. Dissolved chemicals and lignin are separated from the fiber and recycled. The recovered lignin also serves as fuel in our chemical recovery furnace, and greatly reduces our demand for non-renewable energy sources. The tan-colored pulp is then bleached to remove residual lignin and to whiten the fibers to our customers' specifications.
Elemental chlorine was the dominant bleaching chemical in the cellulose fiber industry until the late 1980s, when the industry starting replacing it with chlorine dioxide. Chlorine dioxide is a very effective and environmentally favorable bleaching agent, but it's also unstable, and must be stored as a weak aqueous solution. This makes it necessary to manufacture and store chlorine dioxide solution at the mill site. The dominant chlorine dioxide generation processes reduce sodium chlorate to chlorine dioxide in a strong acid solution with methanol as a reducing agent. The desired reaction is:
3 NaClO3 + 2 H2SO4 + 0.85 CH3OH g 3 ClO2 + Na3H(SO4)2 + 2 H2O + 0.06 CH3OH + 0.52 HCOOH + 0.27 CO2
However, there are several side reactions, one of which can be summarized as:
3 NaClO3 + 2 H2SO4 + 1.5 CH3OH g 1.5 ClO2 + 0.75 Cl2 + Na3H(SO4)2 + 4.5 H2O + 1.5 CO2
Notice that in the desired reaction, every atom of chlorine is used to produce one molecule of chlorine dioxide. However, only half of the chlorine atoms in the side reaction produce chlorine dioxide with the remainder being consumed to produce elemental chlorine gas.
The ratio of chlorine dioxide molecules produced per molecule of sodium chlorate consumed is known as the conversion efficiency. Conversion efficiency is maximized by restricting generator acid and chlorate concentrations to a very narrow range. This task is made difficult by the lack of an on-line liquor strength sensor or analyzer. The only tools plant operators have to control reaction conditions are lab tests, look-up tables and their own experience and insight. In addition to controlling chemical concentrations, the operators also have to manage the concentration of suspended salt crystals, generator level, temperature and production rate. They must remain vigilant and keep an eye on secondary variables, such as generator vacuum and the ratio of steam to production rate. If any of these factors are allowed to drift too far from their targets, the chlorine dioxide can explosively decompose into chlorine gas and oxygen in an event euphemistically referred to as a "puff.” These puffs lift a heavy pressure relief plate on top of the generator and produce an intimidating boom when the plate slams back down. The equipment is designed to handle these events, and they aren't an exceptional hazard to people or equipment, but they do shut down production briefly, and the vibration has been known to damage pipes. These decomposition events can be caused by many factors, including high temperature, foam, contaminants in the water supply or chemical feeds, and even ultraviolet light.
This entire, complex interactive process is typically controlled manually by one operator. Our economic driver on this project was an increase in conversion efficiency, but we were well aware of the fact that this process is touchy and unforgiving and not popular among the operators. A significant secondary benefit of automation was to provide our coworkers with the tools needed to make this a less complex and more manageable process. Our hope was that this job will become more attractive and transferable to new recruits as our experienced operators begin to retire.