Handbook of Solvents - George Wypych - ChemTech - Ventech!

1554 Isao Kimura
Figure 22.2.12. Flow diagram of energy recovery process.
Savings of energy
The net calorific values for acetone, methanol and IPA is 6,962, 5,238 and 7,513
kcal/kg, respectively, and the simple mean value of their mixture is 6,600 kcal/kg. Let us assume
that solvents are generated from a process at a maximum rate of 160 kg/h. The total
heat generation of the VOC oxidation unit is as follows:
6,600 x 160 = 1,056 x 10 3 kcal/h
Heavy oil = 10,200 kcal/kg; specific gravity of heavy oil = 0.86
The saved energy is as follows:
1,056 x 10 3 / 10,200 = 103.5 kg/h
103.5 / 0.86 = 120 l/h [1]
If no solvent is supplied, the required heavy oil is 170 l/h [2]
If solvent is supplied at max. rate, the required heavy oil is only 50 l/h.
[1] - [2] = 50 l/h
Thus, energy saving performance is 71% ([1]/[2] x 100 = 71 %)
Figure 22.2.12 shows the process flow diagram and Figure 22.2.13 shows an industrial
application of the energy recovery unit.
The VOC laden gas from a painting plant is supplied through a filter and is blown to a
heat exchanger and then to succeeding units. The fan drive motors are controlled by inverters
to adjust the flow rate. In the heat exchanger the feed is preheated with a waste heat
boiler’s off gas, at a temperature of 350°C, to 60 - 100°C before it is fed to a VOC oxidation
unit. In the oxidation unit, heavy oil and VOC are mixed and burnt. At approximately
800°C, VOC is burnt out with yield of more than 97 %. The off gas at 800°C is fed to a waste
heat boiler where it generates 1,300 kg/h steam at 8 MPa. The heat transfer gas at 350°C is
used for the preheating the feed gas.