# (204d) Optimal Operation Strategy for Batch Vacuum Evaporation Process By Iterative Dynamic Programming

- Conference: AIChE Annual Meeting
- Year: 2013
- Proceeding: 2013 AIChE Annual Meeting
- Group: Computing and Systems Technology Division
- Session:
- Time:
Monday, November 4, 2013 - 3:15pm-5:45pm

A vacuum evaporation technique is widely

used in the food, bio, pharmacy, and semiconductor industries. The vacuum

evaporation processes concentrate liquid at lower temperature by lowering the

boiling point of the liquid in the vacuum condition. By operating at lower

temperature, the vacuum evaporation process prevents the thermal decomposition

of the liquid and alleviates the corrosion problems of the materials. However,

the vacuum evaporation process lowers the dew point of the generated steam as

well as the boiling point of the liquid. Because the lowered dew point of the

steam reduces the mean temperature difference in the condenser, the driving

force for the heat transfer is reduced in the low pressure condition. For this

reason, the condenser size is dramatically increased or the evaporation rate is

limited in the general vacuum evaporation process [1,2]. This is a very important issue in the some processes of handling

the corrosive liquid such as a strong acid. Because the corrosion-resistant

materials are very expensive, the condenser becomes the most important unit.

Generally, the vacuum evaporation process is performed in the batch mode. Because

the steam generation rate and the mean temperature difference are changed in

the batch operating mode, the condenser area requirement also varies depending

on the operating time. The condenser must be designed to meet the maximum

condenser area requirement among the whole operation time. The maximum condenser

area requirement is determined by the depressurizing curve and the heating

curve. In this research, we suggest an optimal operation strategy to minimize

the condenser area in a batch vacuum evaporation process for sulfuric acid

purification. This process concentrates dilute sulfuric acid drained from the

semiconductor wafer cleaning. We established the discrete-time dynamic model of

the sulfuric acid purification process using pilot plant data. The objective

function is to minimize the maximum condenser area requirement among the whole

operation time. The manipulated variables are operating pressure and heat

supply according to the operation time. Because this problem is highly

nonlinear and dynamic programing, we developed an iterative dynamic programming

algorithm to solve it. By using this characteristic of the minimizing the

maximum problem, this problem is converted to maximizing the mean temperature difference

and eliminating the peak point of condenser area requirement. Through the

optimal operation strategy, the system was depressurized as slowly as possible

within a range that the temperature never exceed the maximum temperature limit

(190°C). Simultaneously, the heater duty was

controlled to generate the optimal amount of the steam in response to the

operating pressure change. That means large amount of the steam is generated at

high pressure condition and small amount of the steam is generated at low

pressure condition. To validate the effect of it, the optimal operation was

compared with a conventional operation which is depressurizing quickly and supplying

constant heat duty as shown in figure 1. As a result, this optimal operation

strategy showed the great reduction of the condenser area and the operation

time. In case of the operation time is fixed to 100 min, the condenser area was

reduced from 11.6 m^{2} to 2.9 m^{2}. In case of the condenser

area is fixed to 10 m^{2}, the operation time was reduced from 117 min

to 54 min.

Figure1. The simulation result of optimal

operation when operation time is 100 min.

1. Mao, W., H. Ma, and B. Wang, *Performance of batch vacuum distillation
process with promoters on coke-plant wastewater treatment*. Chemical Engineering Journal, 2010.

**160**(1): p. 232-238.

2. Hiroshi Ogata, and Norio Tanaka, *Reduction of Waste in Semiconductor
Manufacturing Plant, Sulfuric Acid Recycling Technology*. Oki

Technical Review 160, 1998. Vol.63: p. 41-44.