Issues to be noted in the production of liquid chlorine via high-temperature and high-pressure methods

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This post was last edited by sunjl1981 on 2013-1-6 at 22:13. Issues to be noted in the production of liquid chlorine using high-temperature and high-pressure methods. Liquid chlorine is the product obtained by liquefying chlorine gas, and it represents a major chlorine-consuming product in chlor-alkali industries. Compared to chlorine gas, liquid chlorine is easier to store and transport. It is primarily used in the production of organic chlorine products such as chloromethane, chlorinated paraffins, perchloroethylene, and chlorothalonil, as well as a wide range of inorganic chlorine products including pesticides, bleaching powder, ferric chloride, and titanium tetrachloride. It is also used for disinfecting urban tap water. The common methods for liquefying chlorine to produce liquid chlorine include the low-temperature and low-pressure method, the medium-temperature and medium-pressure method, and the high-temperature and high-pressure method. The low-temperature and low-pressure method is further divided into the ammonia-calcium chloride method and the Freon method. In China, the ammonia-calcium chloride low-temperature and low-pressure method was commonly used for the early production of liquid chlorine. Since this method uses calcium chloride brine as a coolant for secondary heat transfer, it results in high energy consumption, large equipment sizes, and severe corrosion; as a result, it has been gradually replaced by the Freon direct-cooling method. Currently, some enterprises in China still employ the low-temperature and low-pressure process. In recent years, thanks to the rapid advancement of chlor-alkali production technology, the output of caustic soda increased from 6.939 million tons in 2001 to 16.791 million tons in 2006, more than doubling over a period of 4 years; the output of liquid chlorine also rose significantly. Some chlor-alkali companies have introduced or developed their own more advanced liquid chlorine production processes using the medium-temperature and medium-pressure method, as well as the high-temperature and high-pressure method. 1 High-temperature and high-pressure process: The production process for liquid chlorine using this method can be summarized as follows: Chlorine gas, which has a absolute pressure of around 0.2 MPa after drying, passes through a buffer tank before entering a liquid ring compressor unit filled with concentrated sulfuric acid as the lubricant. Here, the chlorine gas is compressed to around 1 MPa. After the acid mist is removed using an acid mist collector, the gas enters a liquefier where it is liquefied into liquid chlorine through heat exchange with circulating water. The resulting liquid chlorine then goes into a storage tank; after being pressurized using tank pressure or a pump, it is packaged and sold. The tail chlorine from the gas-liquid separator is returned to the three-in-one furnace to synthesize hydrochloric acid. A schematic of the production process is shown in Figure 1 (omitted). 2 Characteristics of the process flow and problems encountered during operation As can be seen from the process flow diagram for the liquid chlorine production using high temperature and high pressure, this process is simple and requires less space; it is significantly more energy-efficient than the low temperature and low pressure process using ammonia and calcium chloride (with energy savings of over 50% relative to the latter process). Therefore, most newly built chlor-alkali plants in recent years have adopted this method. However, some enterprises that adopted the high-temperature and high-pressure process for liquid chlorine production encountered certain problems in the early stages of operation, including repeated corrosion and penetration of the \"S\"-shaped coupling between the main unit and the sulfuric acid separator ； The liquefier has leaked multiple times ； The packaging speed of liquid chlorine is slow ； Sometimes, at the beginning of operation, the temperature rise in high-pressure units is significant, reaching around 70°C ； There have been cases of incorrect liquid chlorine tank level readings; in one company, the level gauge indicated zero while the actual liquid level in the tank was 0.8 meters. There have also been instances where concentrated sulfuric acid entered the liquefier through capture devices, resulting in the product containing acid. The occurrence of these problems led to multiple shutdowns for maintenance, and the frequent shutdowns further exacerbated equipment corrosion. Given the above circumstances, some have argued that the high-temperature and high-pressure method is not suitable for liquid chlorine production, or that this process is not yet mature. This increases the difficulty of promoting the liquid chlorine production process using high-temperature and high-pressure methods. 3 Issues to Consider in the Production of Liquid Chlorine via High-Pressure Methods: By carefully analyzing the problems that arise in the high-temperature and high-pressure process for producing liquid chlorine, and by taking appropriate measures during design and construction to ensure accurate selection of materials, reliable construction quality, and strict control over process parameters during production management, these issues can be avoided or overcome. 3.1 In terms of production operations: (1) Eliminate the fear of “high pressure” and sulfuric acid. Most companies originally used the low-temperature and low-pressure method to produce liquid chlorine; when they were suddenly exposed to sulfuric acid and high pressure, they felt fearful psychologically, which sometimes led to missing the best opportunity to address the problems, and in some cases even resulted in serious consequences due to improper handling methods. (2) Strictly control the sulfuric acid concentration. Sulfuric acid used in the high-pressure process serves two purposes: one is to function as the liquid ring of the compressor ； Second, there are two drying steps for the chlorine gas that has been treated with hydrogen chloride. If moisture enters the liquefier, it will react with sulfuric acid and chlorine, corroding the liquefier, shortening its service life and causing leaks. The mechanism and driving force for sulfuric acid to absorb water, as well as its relationship with the vapor pressure above it, temperature, and the moisture content of chlorine gas, are shown in Table 1. Table 1 y-x data table for sulfuric acid at 50°C Mass fraction of sulfuric acid/% Vapor pressure of water/Pa X/% Y/% Mass fraction of water in chlorine/% 950.773 2.27 0.00079 0.00029 0.78514 37.69 0.0079 0.00198 5.7998 0.49 0.035 0.00908 0.102.64 157.65 0.105 0.026575 295.9266 44.70 0.299 0.076 0.70689.16 170 0.706 0.179 Note: X = Mole percentage of H2O/H2SO4 in the liquid phase ； Y- Mole percentage of H2O/C12 in the gas phase. As can be seen from Table 1, it is essential to maintain a sulfuric acid mass fraction of at least 95%; this not only ensures that the chlorine gas output by the chlorine compressor meets the specified moisture content requirements but also helps to ensure an effective drying effect of the chlorine gas coming from previous processing steps. (3) Strengthen employee training, improve staff quality, and carry out operations carefully in strict accordance with the operating procedures. 3.2 Selection of Equipment and Components (1) Selection and control of sealing fluid. Since concentrated sulfuric acid produced by the ore method contains slag and other impurities, it can easily erode and scour the balance tubes as it circulates between the separator and the main unit, thereby shortening their lifespan and leading to leaks. This not only requires shutdown for repairs but also causes corrosion of the equipment due to improper handling of the sulfuric acid. Therefore, it is essential to use concentrated sulfuric acid produced by the vapor absorption method, with a mass fraction of over 98%. (2) Selection of acid mist collectors. The amount of acid mist that enters the liquefier determines, to a certain extent, its lifespan. Given that most of the acid mist particles are smaller than 10/μm, low-speed and highly efficient demisting cylinders are used in the design to ensure an adequate filtering area; this allows the average velocity of the gas as it passes through the glass fiber layer to be kept below 0.025 m/s, thereby ensuring that the mass fraction of sulfuric acid after demisting is below 25×10-6, or even undetectable. (3) Select an appropriate sulfuric acid circulation pump, sulfuric acid cooler, and cooling water volume. The heat generated during the compression of chlorine is removed by concentrated sulfuric acid. Chlorine is continuously compressed and cooled inside the compressor, and all the heat produced during this process is taken away by the concentrated sulfuric acid, thereby maintaining a roughly constant temperature of chlorine at both the inlet and outlet of the chlorine compressor. Therefore, it is important to select the sulfuric acid circulation pump and sulfuric acid cooler appropriately, as well as to design the cooling water temperature and flow rate reasonably. (4) Sulfuric acid separators, acid mist collectors, liquefiers, etc. should be purchased in accordance with the design requirements. (5) Choose measurement and control instruments with high sensitivity and accuracy. (6) Regarding packaging speed. With pressure packaging in the tank, the initial packaging time is 50 mird per bottle, and it gradually decreases thereafter. This is because as the liquid chlorine in the tank decreases, the tank pressure drops, causing the packaging speed to slow down. If packaging is carried out while producing, the packaging speed will increase, but this will affect the stable operation of the chlorine compressor due to changes in the pressure in the storage tank. Therefore, to increase the packaging speed, a liquid chlorine shielded pump can be installed between the liquid chlorine storage tank and the packaging station; this allows for continuous packaging, with one bottle being packaged in about 25 minutes. Liquid chlorine shielded pumps are already widely used in the industry. # + + hcbbs