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Pulp and paper mills can present many dangerous situations, which can be significantly decreased by utilizing continuous gas monitoring.
The chemical pulping technique in the Kraft process utilizes a combination of heat and liquor (chemicals) to delignify wood and reduce it to pulp. Reduction of the wood to pulp takes place in stages, but the heart of the process is in either batch or continuous digesters. It is here that hazardous gases such as hydrogen sulfide and mercaptan are released due to the chemical reaction between the wood chips and liquors.
Pulp stock from the digesters is washed and screened and then sent through the bleaching process. Bleaching is associated with whiteness or brightness, as it is referred to in the pulp and paper industry. Normally it consists of an oxidation process, wherein oxygen is used to dissolve unwanted colored components. In pulp bleaching, oxidation is used to break down lignin molecules, but also to bleach out the dark spots created by non-cellulose components of wood, such as resins, or foreign matter in the pulping process.
Brightness is also obtained by dissolving the lignin molecule through chlorinating. Its removal makes the remaining cellulose fibers appear white to the eye. Bleach chemicals used in this process are chlorine and chlorine dioxide most frequently, and oxygen, peroxide and ozone used as alternatives. Sulfur dioxide is also a concern in the primary bleaching process.
In Kraft mills, the chemicals used are recycled for use throughout the mill. The recovery process is integrally connected to the boilers and power production. All chemicals are recovered through the burning of black liquor (liquor which has already been through the digestive process) in the recovery boiler. Heat released by the oxidation of liquor is used to produce steam for use throughout the plant. Monitoring of hydrogen sulfide gas should be a priority in these locations.
The AMC-210 series of toxic gas sensor/transmitters are ideally suited for the rigors of pulp & paper applications, in fact they are currently in use in mills all over North America. With a linear 4-20 mA output, the AMC-210 series can be connected to any Armstrong monitor, including the AMC-1400 or AMC-1800, or to existing mill instrumentation.