Application of Titanium Spiral Plate Heat Exchanger in Epoxy Plant 1. Preface The wastewater from the epichlorohydrin plant contains a large number of impurity particles. The main impurity components are Ca(OH)₂ and CaCl₂, which are extremely prone to scaling. The temperature of the wastewater is approximately 85°C, which belongs to low-grade thermal energy and is not easy to be recycled and utilized. However, if it is not recycled and treated, on the one hand, it will cause a waste of energy; on the other hand, due to the relatively high temperature of the wastewater, discharging it into the water treatment plant will also damage the ecological environment of the microorganisms in the sewage treatment system. Therefore, in the energy-saving renovation of the cyclization system of the epichlorohydrin plant, it is considered to add a heat exchanger before the saponification tower. The aqueous solution of dichloropropanol (DCH) entering the saponification tower absorbs the waste heat of the wastewater in the heat exchanger. The main component concentrations of the aqueous DCH solution are: DCH 4; 2%, HCl; 2%, and it is highly corrosive. It is planned to reduce the temperature of the wastewater from 85°C to about 72°C and increase the temperature of the aqueous DCH solution from 40°C to 55°C. 2. Model Selection For this renovation, there are mainly four key issues to be faced in the selection of the heat exchanger: Firstly, the working medium on the wastewater side is prone to scaling; Secondly, since it is an equipment renovation, the available space is relatively small; The third issue is the low quality of the heat source; The fourth issue is that the aqueous DCH solution is highly corrosive, and attention should be paid to preventing corrosion and leakage. This requires the heat exchanger to have good anti-scaling ability and high heat transfer efficiency, and its structure should be compact, with a large heat transfer area per unit volume and reliable sealing. Commonly used heat exchanger types include shell-and-tube type, plate type, and spiral plate type. The shell-and-tube heat exchanger has the worst economic efficiency. Its sealing is the most reliable, but its anti-fouling performance is average, and it cannot meet the design requirements in terms of reliability; The plate heat exchanger has the best economic efficiency, but its sealing is not reliable enough and its anti-fouling performance is average; The spiral plate heat exchanger has the strongest anti-fouling ability, reliable sealing, and also has good heat transfer efficiency and a high heat transfer area per unit volume. Therefore, it has the highest reliability and good economic efficiency. 3. Pickling Process Although the spiral plate heat exchanger has been selected in the design considering the scaling tendency of the working medium, due to the strong scaling characteristics of the working medium and the influence of uneven flow, etc., the equipment must be cleaned and descaled after running for a period of time. There are mainly three methods for cleaning the spiral plate: water washing, pickling, and steam flushing. Since the aqueous DCH solution contains 1; 2% of HCl, which can react with the main scaling component Ca(OH)₂ to form CaCl₂ that is easily soluble in water, and it does not affect the reaction of the aqueous DCH solution entering the saponification tower in the next step. Therefore, in this design, the aqueous DCH solution is used to pickle the wastewater side of the heat exchanger. In the actual renovation, two titanium spiral plate heat exchangers, A and B, are equipped, with one in operation and the other as a standby. When the heat transfer effect of heat exchanger A obviously decreases after running for a period of time, the system is switched to B. At the same time, the aqueous DCH solution is introduced to the wastewater side of heat exchanger A for pickling, and the pickled aqueous DCH solution is introduced into the saponification tower. A bypass specifically designed for pickling is installed on the DCH pipeline. Pickling can be achieved by switching valves, and the process is simple and easy to implement. Usually, pickling for 30 minutes once a week can achieve satisfactory results. Since there is no consumption of additional acid solution, the cost of pickling is almost zero. 4. Conclusion There are few precedents for the design of heat exchangers for working media that are highly corrosive, prone to scaling, and of low-grade thermal fluid. The success of this renovation provides an example for the design of spiral plate heat exchangers operating under harsh working conditions and also fully demonstrates the superiority of spiral plate heat exchangers. The unique pickling process in this design also makes the heat exchanger using wastewater as the working medium have satisfactory operability, thus ensuring the reliability and economic efficiency of the heat exchanger.