Application of valve alloy materials in chlor-alkali industry (HY-industry technical centre)
With the introduction of new materials and the development of valve design over the years, the days when fluid handling operators were limited by the number of valve types available to control the process are long gone. These advancements, coupled with stricter fugitive emission standards, a strong focus on the health and safety of plant personnel, and the operator’s desire to achieve a lower total cost of ownership have made valve selection a science. One of the applications where this science must be put into practice is the chlor-alkali plant.
Valves that can be used in chlor-alkali plants are customized to solve some of the toughest problems facing the industry. Today’s operators must understand the advantages of each valve and choose the right valve for different parts of the process.
In the chlor-alkali plant, there are four main stages:
Each of the above stages presents its own challenges, and other downstream considerations include chlor-alkali, including the treatment of hydrochloric acid and the synthesis of sodium hypochlorite. Each of the four stages requires specific valve alloy materials. There is a wide range of valve types used in chlor-alkali plants:
All these valves face the problem of fugitive emissions, which may be caused by insufficient sealing systems, thermal cycling and corrosion of wetted valve components, especially in the case of improper media/lining combinations. Downstream leakage is another common problem, which may be due to internal corrosion, valve seat wear, and valve seat wear caused by fouling. Valves used in chlor-alkali must have characteristics and advantages that can solve these problems.
Challenges at this stage
Salt water formulations are plagued by corrosive and corrosive media. Abrasion occurs in the accumulation of solid particles and salt, and scaling may occur. The brine itself is a mixture of water and raw salt, which is highly corrosive and has a high ion concentration. Due to the high sensitivity of the membranes and diaphragms in the electrolytic cell used in this process, the saline solution of these cells must be purified. This process involves many steps including treatment, filtration and ion exchange.
In the part of the process where solids are present, wear is a problem. A perfluoroalkoxy (PFA)-lined diaphragm valve with a polytetrafluoroethylene (PTFE) diaphragm is usually used. PFA-lined butterfly valves are also commonly used in the salt water preparation process. PFA-lined plug valves and ball valves are sometimes used, especially for filtering after removing solids from the medium. Chlorine is the main product of chlor-alkali plants. This chlorine must be cooled, dried, compressed, and liquefied for storage or transportation, or sent to downstream processes. The chlorine treatment part of the operation after chlorine gas is generated in the electrolytic cell. The most common problems encountered in chlorine treatment are fugitive emissions and corrosive media. The wet chlorine coming directly from the electrolytic cell is highly corrosive and toxic. The valves often selected here are high-performance butterfly valves, severe-working plug valves made of valve alloy materials such as Monel, or zero-discharge valves such as bellows-sealed globe valves. Use concentrated sulfuric acid (H2SO4) to dry the wet chlorine. Lined ball valves or plug valves are usually used to handle the high corrosiveness of H2SO4.
Another major by-product of the chlor-alkali process is caustic soda, usually sodium hydroxide. Depending on the technology used in the electrolyzer, the concentration of caustic soda produced is different, and the degree of salt pollution is different. This will bring challenges to valve selection, because the medium is corrosive, but it may also contain solids, especially when the product is concentrated after evaporation. As the concentration increases, the salt may precipitate out of the solution (precipitate) and have a fouling effect. Therefore, nickel alloy sleeve plug valves are usually a good choice. The absence of a cavity in this valve type prevents the accumulation of flowing medium, which may cause damage to the sealing surface. A valve with a 360-degree port lip will protect the sleeve from any wear. In low-salt corrosive applications, lined ball valves are a viable option because of their full bore design and excellent corrosion resistance.
Hydrogen is another by-product produced in the chlor-alkali process. Due to the unfavorable stoichiometric ratio (the exact ratio between the air where complete combustion occurs and the combustible gas or steam) and the low molecular weight, the amount of hydrogen produced does not make it a commercially attractive derivative. Nonetheless, the hydrogen produced can be burned as a fuel or often burned with chlorine to produce hydrochloric acid for use in factories. Difficulties in hydrogen production include the high pressure required to handle storage and the need to select materials that can avoid hydrogen embrittleme
nt. High performance butterfly valve (HPBV) is a common choice for hydrogen production.
Diaphragm valves are commonly used in chlor-alkali applications because only two parts of the valve are in contact with the pipeline medium, making it resistant to corrosion and wear. The diaphragm isolates the working parts of the valve from harmful media and further prevents corrosion of any metal parts. Even if the diaphragm shows signs of wear, the linear operation of the valve can ensure sealing. It can also prevent break-in or break-out torque. Therefore, the rolled thread should not be stuck or worn, and the stroke protects the spindle from environmental influences and prevents the valve from clogging. The diaphragm valve can also achieve bagless flow, preventing the sealing surface from accumulating and facilitating complete closure.
Sleeve plug valve is another commonly used valve in chlor-alkali applications. The use of a casing plug valve with a full-circumferential in-line sealing design can provide upstream and downstream bidirectional sealing. Sleeve plug valves can also minimize valve seat wear because the port lip protects the valve seat surface in the open or closed position. Modern valve designs have two or even three independent sealing systems, so they can provide better protection against atmospheric leakage. HPBV is the first choice to ensure two-way sealing, which is especially important in any dry chlorine line, because leaks can cause moisture to leak and cause rapid corrosion of piping system components. With HPBV, leakage protection can be provided regardless of the direction of flow.
Fully lined ball valves, plug valves and butterfly valves provide unparalleled corrosion resistance for all chemicals encountered in the chlor-alkali process. When a bellows-sealed globe valve is required, use a valve whose internal parts are made of valve alloy materials stainless steel or Hastelloy to provide adequate corrosion protection. This can also help prevent wear of the multi-walled, fully flushed stainless steel bellows as the primary seal. This flushing system can also keep the bellows clean without any particles sinking into the fins of the bellows.
Another advantage commonly found in bellows-sealed globe valves is the tight seal between the valve body and the valve cover, which can further extend the service life of the valve. The superior safety sealing system in the bellows-sealed globe valve has multiple walls, gland packing, metal back seats and cover gaskets for escape emissions protection. Bellows-sealed shut-off valves can also be used to prevent thermal cycling through the tongue and groove sealing system.
Chlor-alkali is one of the most demanding fluid handling applications. A better understanding of the challenges faced by operators allows those who choose valves and valve alloy materials to weigh the advantages and features of each valve type to meet their exact needs. It’s up to the operator to ensure that they have the most suitable valves, so that the equipment can achieve the best cost performance, but also ensure the safety of personnel and the environment.