Chemical recovery systems

Melville, S., and Williams, T., Application of Economic Optimization to the Chemical Recovery System of a Kraft Pulping Process, Laboratory for Applied Industrial Control, Report 107. Purdue University, West Lafayette, IN, 1977. 

While the Kraft process initially had two distinct disadvantages, it produced a dark pulp and required chemical recovery, the pulp is stronger and has better dimensional stability than in the old soda process. Chemical recovery systems rapidly became quite advanced and most advanced mills are now aiming for 98-99% chemical recovery. 

A feature of the kraft process vital to continued success is its integral, well-tested chemical recovery system. The digestion liquor for each batch of chips to be pulped is mainly obtained from the chemicals recovered from the spent liquor of previous digestions, and has approximately the composition given in Table 15.8. For kraft pulping it is usual to specify all of the components of the digestion liquor on a NaiO equivalent basis. This puts all the active constituents on the same sodium ion content basis. Thus, the actual concentration of sodium hydroxide present for a 73 g/F, NaiO equivalent is given by Eq. 15.12. 

NazCOs 18.2 0.29 Negligible. Presence incidental as a result of chemical recovery system... 

Albrecht WN. 1980. Technical assistance report TA80-48. Chemical Recovery Systems, Inc., Elyria, OH. Cincinnati, OH U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health. NIOSH-HE/TA-80-48. NTIS PB81-167926. 

Among the various pulping chemical recovery systems are the Stora (123), Sivola (124), and Tampella (125) processes, which have been demonstrated commercially. The first of the process steps given above can be accomplished with sodium sulfate or sulfite by smelting under reducing conditions, which is the oldest method for producing sodium sulfide (126). 

The US pulp and paper industry operates over 550 facilities which employ over 200,000 people. Total shipments are 60 billion with an additional 80 billion in converted products. Several proeesses eontribute to the emission of solvents. These include chemical pulping kraft process (terpenes, alcohols, methanol, acetone, chloroform), bleaching (acetone, dichloromethane, chloroform, methyl ethyl ketone, carbon disulfide, chloromethane, and trichloroethane), wastewater treatment (terpenes, alcohols, methanol, acetone, chloroform and methyl ethyl ketone), and evaporators in chemical recovery systems (alcohols and terpenes). 

The overall wastewater characteristics from wood pulping processes may vary seasonally because of the changes in characteristics of wood and variations in the temperature of the water. The volume and characteristics of the process wastewater depend upon the degree of water reuse, chemical recovery systems, and the type and quality of paper involved. 

Figure 20-1 Simplified overview of a typical kraft process showing chemical pulping, oxygen delignification, pulp bleaching, and chemical recovery systems. Important mixing operations are indicated by the NAMF mixing icon.

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