Discussion on the chlorine metering method in chlor-alkali plants

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Author: Zeng Jiangping (Yanhua Chemical Plant, Jianghan Oilfield Branch of Sinopec, Qianjiang, Hubei). When reproducing this article, please acknowledge the copyright of the author. Abstract: This paper explores the metering methods for liquid and gaseous chlorine in chlor-alkali plants. By analyzing the properties of chlorine and the usage of flow meters, it presents selection criteria for metering instruments under various process conditions, which provides useful guidance for chlorine metering in chlor-alkali plants of similar types. Brief description of the chlorine production and treatment process: The brine sent from the salt production section of the salt and nitrate plant is refined before being fed into the ion-exchange membrane electrolysis section, where it is electrolyzed to produce caustic soda, chlorine, and hydrogen. The wet chlorine produced in the electrolysis section is sent through pipelines to a chlorine treatment unit, where it is dried and pressurized to yield dry gaseous chlorine. This dried chlorine is used as an intermediate material and fed into downstream facilities such as hydrochloric acid synthesis plants, bleaching powder factories, and strong chlorine product production units. As a product for external sale, chlorine is converted into liquid chlorine using a chlorine liquefaction unit; this liquid chlorine is then delivered to customers via pipelines, liquid chlorine cylinders, or liquid chlorine tankers. As a key raw material, chlorine flow rate monitoring is of great significance for process control and cost accounting. 2 Selection of chlorine measurement method: Due to its special properties, it is difficult to measure the flow rate of chlorine. Over the years, we have conducted numerous studies and experiments on this issue, and have essentially developed a relatively effective method for measuring chlorine gas. Below, the methods for measuring chlorine gas under various process conditions in chlor-alkali plants are discussed separately. 2.1 Measurement methods for gaseous chlorine. Chlorine is a highly corrosive gas with a density of 3.21 kg/m3, and the operating pressure is generally between 0.1 and 0.3 MPa. Due to the high corrosiveness of chlorine gas, there are strict requirements regarding the materials used for measuring instruments. For the metering of gaseous chlorine, we used orifice plate flow meters and vortex flow meters. 2.1.1 Orifice plate flow meters are used to measure chlorine flow rate. In the hydrochloric acid synthesis plant, chlorine and hydrogen must be fed into the synthesis furnace in a specific flow ratio to burn and produce hydrogen chloride gas. In this context, chlorine is primarily used for intermediate control; therefore, high precision in measuring the chlorine flow rate is not required, and orifice plate flow meters can be used to measure it. Since chlorine is highly corrosive to most metals, it is important to choose the right material for pressure tap plates: these can be made from 316L, Hastelloy C, or tantalum. Considering the low moisture content in chlorine gas and the cost of fabricating orifice plates, we have chosen 316L material for manufacturing the orifice plates. 2.1.2 Vortex street flowmeters are used to measure chlorine flow rate. A vortex flow meter is a type of fluid vibration flow meter that uses the principle of fluid oscillation to measure flow rate. One of the raw materials used by the bleaching powder plant and the hypochlorite production department at Jianghan Oilfield Salt Chemical Industry Complex is gaseous chlorine, and the measurement of chlorine is carried out using Rosemount vortex flowmeters from the 8800D series in the United States. The main reasons are: (1) Faults such as internal leakage in the isolation valves of the chlorine supply pipes cause the slurry in the reaction vessel to flow back into the chlorine pipeline. Under such process conditions, orifice flow meters and rotameter cannot be used to measure the chlorine flow rate; on the other hand, vortex flow meters have good fluid passage properties, and the backflow of slurry does not accumulate inside the meter, thus avoiding measurement errors ； (2) Since the diameter of the chlorine pipeline is 50 mm, using a mass flow meter for measurement results in high costs and poor cost-effectiveness. It can be seen that choosing a vortex flow meter as the solution for measuring chlorine flow is the optimal choice. Under these process conditions, the key points for selecting a vortex flowmeter are: (1) material ； (2) Requirements for the straight pipe sections before and after the flow meter ； (3) Installation location of the temperature and pressure compensation instrument. By following the above steps, a relatively accurate and stable flow rate value can be measured. Based on over 10 years of use, the results are quite good. 2.2 Discussion on liquid chlorine metering methods. The chlorine gas that has been dried is then sent to a chlorine liquefaction unit where it is compressed to produce liquid chlorine. Liquid chlorine is a yellow-green, transparent liquid. Liquid chlorine is transferred from storage tanks to tank cars using liquid chlorine transfer pumps, and it is also delivered to end-users through pipelines or filled into steel cylinders. In all these cases, measurement of the liquid chlorine is necessary, with different methods being used for each case: mass flow meters are used for measuring the amount of liquid chlorine filled into tank cars, mass flow meters are also used for pipeline delivery, while electronic scales are employed for filling steel cylinders. 2.2.1 The importance of flow measurement during liquid chlorine filling. Our country has strict regulations regarding liquid chlorine containers and their filling, and departments such as public security, fire protection, and work safety supervision carry out strict oversight and inspections during transportation; the penalties for overfilling are quite severe. To this end, real-time monitoring must be carried out during the filling of liquid chlorine to prevent overfilling. It can be seen from this that metering during the liquid chlorine filling process is extremely important. 2.2.2 Application of mass flow meters in the metering of liquid chlorine filling. The Salt Chemical Plant of Jianghan Oilfield has large-capacity liquid chlorine tank trucks, and during peak filling times, around 10 such tank trucks need to be filled. Through research on the liquid chlorine metering instruments used in chlor-alkali plants manufactured by other companies and technical exchanges, it has been found that the best method for accurately measuring the flow rate of liquid chlorine is to use mass flow meters. At present, due to their late start in the industry, domestic manufacturers of mass flow meters are not able to achieve accurate measurement. As a result, most chlorine-based chemical processing plants use mass flow meters produced by the German company E+H, as well as Coriolis mass flow meters from the American company Emerson Process Management, for measuring liquid chlorine. The mass flow meter model produced by E+H is 83F50, with a DN50 size and made of Hastelloy C material; its accuracy is 0.1%. Emerson’s selected model is CFM200H, also with a DN50 size and made of Hastelloy C material; its accuracy is 0.35%. A total of 4 mass flow meters have been installed at the liquid chlorine tank truck filling station of the Salt Chemical Industry Plant in Jianghan Oilfield, meeting the filling requirements, improving filling efficiency and safety, and achieving satisfactory measurement results. Thanks to the high measurement accuracy and precision of mass flow meters, a new liquid chlorine delivery pipeline has been installed in the liquid chlorine facility to supply chlorine to users in the chemical park, replacing the previous use of liquid chlorine cylinders for this purpose. The measurement of chlorine supplied through this pipeline is also carried out using mass flow meters; Emerson’s F100H model of mass flow meter is used to ensure high-precision measurement. At present, the Salt Chemical Industry Plant of Jianghan Oilfield has implemented automatic control for liquid chlorine filling in its liquid chlorine production facility. 2.2.3 Measurement of liquid chlorine cylinders. Under normal circumstances, the weighing of liquid chlorine cylinders is done using electronic floor scales, typically specialized scales designed for liquid chlorine cylinders. The re-weighing of steel cylinders is done using an electronic hook scale. The filling capacities for liquid chlorine cylinders are 500 kg and 1000 kg. We have chosen the frame scale manufactured by Mettler Toledo, which is designed specifically for weighing cylinders during filling. The advantages of this scale include: (1) it features wheel brackets for cylinders, allowing various sizes of cylinders to be placed on it, and it also makes it easy to rotate the cylinders so that the filling tubes can be connected to them ； (2) The use of 4 dedicated bridge sensors enables impact resistance and a long sensor lifespan ； (3) The surface of the weighing platform is coated with an anti-corrosion paint to ensure corrosion resistance ； (4) The weighing display unit can transmit signals to the large weighing display screen and the DCS, enabling filling workers to monitor the filling weight. Once the signals are fed into the DCS, an automatic filling system can be established, thereby ensuring safe and efficient filling operations. 3 Conclusion
Given the importance of chlorine metering instruments, determining how to select such instruments under various process conditions—so as to ensure high cost-effectiveness, safety, and accuracy, as well as stable and reliable measurements in process control—has been a continuous effort by our plant’s instrumentation and metering personnel through exploration and practice. After years of experience accumulation and verification of application results, we have developed a currently relatively economical and reasonable metering solution for chlorine measurement in chlor-alkali plants. This solution provides a reliable guarantee for both the enterprise’s process control and settlement of chlorine trade transactions. It also serves as a useful reference for the expansion and construction of new chlor-alkali plants at Jianghan Oilfield Salt Chemical Plant, offering valuable insights for similar installations.