Automation in monitoring gases and particulates in the pulp and paper industry

In all major industries increasing importance is placed on the use of instruments that analyse process streams effectively and efficiently for long periods of practically unattended operation. This helps the process operator to keep an inventory of the materials leaving the process, and check the operating efficiency. In the field of pollution, it helps to confirm that the process is operated within the confines of usually stringent government regulations, in the form of set emission limits. The use of continuous or automated instruments usually cuts back on manpower requirements, while rigid maintenance becomes necessary because of the complexity of operating some of these sophisticated pieces of equipment. The Canadian pulp and paper industry is required to monitor the pollution it generates of water, air and land. This paper deals with what this industry has done, and is trying to do, in automating the monitoring of pollutant gases and particulate matter. Brief kraft pulping process The principal wood-chip pulping process is called kraft (see figure 1). Here, an aqueous solution of sodium sulphide (Na.S) and hydroxide is used to break down wood-chips, at high temperature and pressure, into fibres which are then washed, bleached and made into paper. The spent reagent, referred to as 'black liquor', now containing unused chemicals and dissolved lignin from the wood, is then concentrated and burnt in the chemical-recovery furnace, the lignin providing useful heat. The process steps of pulping, black liquor oxidation (to reduce pollution) and liquor transportation, oxidize the sodium sul-phide to thiosulphate and sulphate. The oxidized chemicals are then reduced in the reducing zone of the recovery furnace back to NazS to be reutilized in the pulping step. The hot gases leaving the furnace and containing some hydrogen sulphide, sulphur dioxide, carbon dioxide, water vapour, carbon monoxide and oxygen are often contacted with black liquor in a direct-contact evaporator in order to preconcentrate the liquor prior to burning. The hot gases strip the black liquor of its organic mercaptans, sulphides ,and disulphides which are the major pollutants carried to the atmosphere via the stack. In certain kraft processes, direct contact between the hot gases and black liquor is avoided and the final concentration of the black liquor is achieved in multiple-effect evaporators. The major gaseous pollutant in such cases is sulphur dioxide (SO2) which causes 'acid rain', a source of great concern to environmentalists. The stack gases also include particulate matter consisting 1982 …