Evaporative cooling vent systems

The example is taken from a polymerization batch process and has also been referred to previously by Dahl et al. [1999] and Kosanovich et al. [1996], The dataset consists of 50 batches from which eight process variables are measured over approximately 120 time intervals. From this set of batches, two quality variables on the final product were also available. Both process and quality variables are listed in Table 10.7. The reactor in this chemical process (see Figure 10.26) converts the aqueous effluent from an upstream evaporator into a polymer product. The reactor consists of an autoclave and a cooling/heating system. It also has a vent to control the vapor pressure in the autoclave. The recipe specifies reactor and heat source pressure trajectories through five stages. 

Failure to remove excess reaction ECH from the system prior to condensate addition was due to the absence of cooling water flow to the ECH/water condenser. The cooling water system for the condenser had not been commissioned after the plant shutdown, and was isolated both at a manually operated valve and at a valve operated via the plant computer. The lack of condensing capacity resulted in high vapour flows to vent, with a consequent increased back-pressure on the reactor which caused a reduced evaporation rate. 

The neutralization reaction of the above equation is conducted in one or more strongly agitated reaction vessels, whether in a gypsum or in a hemihydrate mode. The system is highly exothermic and the slurry is maintained at 80-85°C for dihydrate processing, 95-100°C for hemihydrate, by evaporative or air cooling. During the reaction of phosphate rock with sulfuric acid, fluorine is evolved and must be scrubbed from the vent gas. 

The drying air is cooled in a scrubber/condenser where the water, evaporated in the spray dryer, is removed. The air is subsequently recycled to the direct gas-fired air heater. A vent will exhaust a volume of drying air equal to the volume of combustion products from the gas burner. However, most of the air is recycled in the system. 

In open recirculating systems evaporation is the major factor in heat disposal. In these evaporative systems circulating water is continuously scrubbed with air, therefore saturated with dissolved oxygen. In contrast, water in closed recirculating cooling systems usually will contain minimum dissolved oxygen, even though the systems may include vented expansion tanks. 

---