Thermal effects, water evaporation

Heat Requirement of the Process. Heat is required for vaporization in the extractive distillation column, and for the reconcentration of magnesium nitrate solution. Overall thermal effects caused by the magnesium nitrate cancel out, and the heat demand for the complete process depends on the amount of water being removed, the reflux ratio employed, and the terminal (condenser) conditions in distillation and evaporation. The composition and temperature of the mixed feed to the still influence the relative heat demands of the evaporation and distillation sections. For the concentration of 60 wt% HNO3 to 99.5 wt% HNO3 using a still reflux ratio of 3 1, a still pressure of 760 mm Hg, and an evaporator pressure of 100 mm Hg, the theoretical overall heat requirement is 1,034 kcal/kg HNO3. 

That the evaporator could be made as efficient as economically justifiable. Thermal efficiency of an evaporator is increased by multiple-effect operation, by recompression of the vapor, by a combination of these, and by a number of other design features. While sea water evaporators had rarely been made with more than three effects, commercial evaporators of six and seven effects are common and ten-effect evaporators have been used. 

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

The condensation of water vapor has two simultaneous effects on the evaporation of ammonia, a thermal effect and a dilution effect. In the thermal effect, latent heat is released by the condensing water vapor, which tends to raise the droplet temperature and therefore enhance the evaporation of ammonia. In the dilution effect, the concentration of ammonia in the droplet decreases with increasing water vapor condensation. For a large dilution, the mole fraction of ammonia and the activity coefficient are small, reducing the evaporation rate of ammonia. 

Figure 27.4 shows the radius of an ammonia droplet as a function of time for different relative humidities. Initially, the thermal effect of water vapor condensation is dominant and the rate of evaporation of ammonia is increased with increasing relative humidity. Later, however, the rate of evaporation of ammonia is slowed down due to the dilution efect. 

The last term 5y accounts for film evaporation under both dynamic and thermal effects [58]. Since the impinging liquid spray is multicomponent (water and urea in our case), species tracking must be considered in the film. A set Ns—1 of conservation equations must be solved, where Ns in the number of chemical components considered ... 

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