Fouling Evaporation

Pulp and Paper Mill Waste Streams Five conditions were chosen for the enrichment of the original waste-activated sludge. Three foul condensate effluents from the pulp mill kraft chemical recovery process were selected based on their high COD (1) the combined evaporator condensates (3200 tank foul condensate or FC 3200), (2) blow heat accumulator overflow foul condensate (FC BHAO), and (3) evaporator foul condensate (FC EVAP). The remaining two media used for enrichment consisted of primary clarifier effluent (primary out abbreviated as l°OUT ) supplemented with methanol, and waste-activated sludge supplemented with methanol (WAS-only). 

Speed control for the compressor of an evaporation system with linear characteristics is shown in Figure 19-11. Inlet guide vane control permits greater capacity turndown end allowence for evaporator fouling before the surge condition is reeched. 

Evaporating problems (refrigeration systems), 235-241 drown tubes in refrigerant, 237-238 increasing plant throughput, 237-239 evaporator fouling, 239-240 refrigerant condenser problems,... 

Work in connection with desahnation of seawater has shown that specially modified surfaces can have a profound effect on heat-transfer coefficients in evaporators. Figure 11-26 (Alexander and Hoffman, Oak Ridge National Laboratory TM-2203) compares overall coefficients for some of these surfaces when boiling fresh water in 0.051-m (2-in) tubes 2.44-m (8-ft) long at atmospheric pressure in both upflow and downflow. The area basis used was the nominal outside area. Tube 20 was a smooth 0.0016-m- (0.062-in-) wall aluminum brass tube that had accumulated about 6 years of fouhng in seawater service and exhibited a fouling resistance of about (2.6)(10 ) (m s K)/ J [0.00015 (fF -h-°F)/Btu]. Tube 23 was a clean aluminum tube with 20 spiral corrugations of 0.0032-m (lA-in) radius on a 0.254-m (10 -in)... 

Circffiation and heat transfer in this type of evaporator are strongly affected by the liquid level. Highest heat-transfer coefficients are achieved when the level, as indicated by an external gauge glass, is only about halfway up the tubes. Shght reductions in level below the optimum result in incomplete wetting of the tube walls with a consequent increased tendency to foul and a rapid reduction in capacity. When this type of evaporator is used with a liquid that can deposit salt or scale, it is customary to operate with the liquid level appreciably higher than the optimum and usually appreciably above the top tube sheet. 

Evaporators—These usually utilize a fin-tube design. Spirally finned tubes of 1.25 in to 2 in outer diameter (OD) with three to six fins per ineh are eommon. In the ease of unfired designs, earbon steel eonstruetion ean be used and boilers ean run dry. As heavier fuels are used, a smaller number of fins per ineh should be utilized to avoid fouling problems. 

Evaporation, Heat exchange, condensing vapours U.sually not May become necessary when processing thermally sensitive materials or if fouling of heat-transfer surfaces is possible. 

In forced circulation evaporators the liquid is pumped through the tubes. They are suitable for use with materials which tend to foul the heat transfer surfaces, and where crystallisation can occur in the evaporator. 

Evaporator type Very viscous > 1000 Medium viscosity < 1000 max Low viscosity < 100 Foaming Scaling or fouling Crystals produced Solids in suspension ... 

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