In a broad sense, the industry uses the method of electrolyzing saturated NaCl solution to produce NaOH, Cl2 and H2, and uses them as raw materials to produce a series of chemical products, which is called the chlor-alkali industry. In a narrow sense, the chlor-alkali industry mainly includes two major products: caustic soda and polyvinyl chloride.
Chlor-alkali products are of many types and highly correlated. Its downstream products reach thousands of varieties and have high economic extension value. It is widely used in important industries such as agriculture, petrochemicals, light industry, textiles, building materials, electricity, metallurgy, national defense and military industry.
Chlor-alkali production principle and process:
There are two methods for producing caustic soda: ion membrane method and diaphragm method. Ion exchange membrane alkali production technology is a clean and energy-saving electrolytic alkali production technology that appeared in the mid-1970s. It is the main method for producing high-purity alkali and is widely used in the world.
Currently, most chlor-alkali is produced by ion exchange membrane method, The principle of ion exchange membrane electrolysis of brine:
In the electrolytic cell, the anode chamber and the cathode chamber are separated by a cation exchange membrane. Unlike asbestos fleece membrane, the cation exchange membrane has selective permeability. It only allows Na + to pass through with a small amount of water molecules, and other ions are difficult to pass through. During electrolysis, a strictly refined NaCl solution is injected into the anode chamber from the bottom of the electrolytic cell, and water is injected into the cathode chamber. In the anode chamber, Cl – discharges to generate Cl2, which is released from the top of the electrolytic cell. At the same time, Na + flows to the cathode chamber with a small amount of water molecules through the cation exchange membrane. In the cathode chamber, H + discharges to generate H2, which is also released from the top of the electrolytic cell. However, the remaining OH – cannot move to the anode chamber due to the barrier of the cation exchange membrane, so it gradually accumulates in the cathode chamber to form a NaOH solution.
Because the cation exchange membrane can prevent Cl – from passing through, the NaOH solution generated in the cathode chamber contains very little NaCl impurities. The product produced by this method is more concentrated and purer than the product produced by diaphragm electrolysis, and the energy consumption is also low.
By electrolyzing salt water solution, three substances are produced at the same time: NaOH solution, chlorine, and hydrogen. The NaOH solution is separated and concentrated to obtain solid NaOH. The chlorine and hydrogen are input into the next process through the chlorine compressor and hydrogen compressor to make HCl or hydrochloric acid, which can be used as raw materials for the production of polyvinyl chloride (PVC). According to different process requirements, chlorine can also be directly compressed into liquid chlorine. Shanghai Kante Vacuum’s hydrogen compressor and chlorine compressor are essential main equipments in the chlor-alkali industry.
Hydrochloric acid electrolysis technology
In the process of using chlorine, such as in the chlorination of methane, the phosgenation of amines to produce isocyanates, and the dehydrochlorination of chloroalkanes to olefins, a large amount of hydrogen chloride gas is generated while obtaining the product. If it cannot be fully utilized, it will directly affect the normal production. In order to solve this kind of problem, the use of hydrochloric acid electrolysis to produce chlorine and hydrogen is a very effective solution.
At present, hydrochloric acid electrolysis technology is relatively mature and has reached industrial production. The most advanced ion membrane oxygen cathode technology (ODC) has the advantage of low power consumption. The power consumption of producing one ton of chlorine is only about 1150kwh, which has considerable advantages over traditional salt electrolysis in terms of energy consumption. Its disadvantages are: due to its high corrosiveness, it has very high requirements for process equipment and materials, and the one-time investment is relatively large.
Brine oxygen cathode electrolysis (ODC) technology
Brine oxygen cathode electrolysis (ODC) technology can save about 30% of electricity compared to traditional ion membrane electrolysis technology, and will indirectly reduce the same CO2 emissions. It is an environmentally friendly and energy-saving production process and a milestone in reducing electricity and CO2 emissions.
The anode of the oxygen cathode ion membrane electrolyzer is exactly the same as the traditional ion membrane electrolyzer, but an oxygen permeable layer is added to the cathode chamber. The cell produces chlorine on the anode side; caustic soda on the cathode side, but the hydrogen precipitated at the cathode reacts with the external oxygen to generate water, so no hydrogen is released on the cathode side, which reduces the cathode discharge potential, generally reduces the cell voltage by 0.85~1.00V, and reduces the DC power consumption per ton of alkali by 700~800 kwh.
At present, the industrial plant of Bayer Company in Germany with an annual capacity of about 20,000 tons was put into operation in the first half of 2011, and it is expected that the technology will soon mature. For chlor-alkali enterprises that have no demand for hydrogen products or can properly balance them, the use of oxygen cathode cell technology can achieve the goals of energy saving and consumption reduction, low carbon emission reduction and increased economic benefits.
The low-purity chlorine generated during the start-up and shutdown of the electrolysis process, the chlorine generated during accident treatment, and the waste chlorine and hydrochloric acid gas generated in each process are all treated by the waste gas treatment process, and low-concentration alkali solution is used for absorption. In the treatment of chlorine, Shanghai Canter Vacuum’s LRC series high-performance dry chlorine compressors and wet chlorine compressors are used for compression, hydrogen electrolysis reaction, production of hydrochloric acid, and high-purity chlorine for downstream products.
DEACON technology
The HCl catalytic oxidation method for preparing chlorine was first proposed by Deacon in 1875, so it is called Deacon technology. Its stoichiometric formula can be expressed as:
HCl(g)+1/4O2(g)←→1/2H2O(g)+1/2Cl2(g)+6.8kcal
The traditional Deacon process is carried out in a single-stage reactor, using CuCl2 as a catalyst and the reaction temperature is 430-475℃. However, in practical applications, this method has the following engineering problems:
(1) Limited by the reaction equilibrium, the conversion rate of HCl is low, less than 80%; increasing the temperature is conducive to increasing the reaction rate, but it will reduce the conversion rate; although lowering the temperature can increase the conversion rate of the reaction, it reduces the reaction rate.
(2) Due to the low conversion rate, the unreacted HCl combines with the possible condensed H2O to form hydrochloric acid, which brings serious equipment corrosion problems. At the same time, the condensed H2O increases the viscosity of the catalyst and reduces the fluidity of the catalyst.
(3) The active component CuCl2 is easy to volatilize during high temperature, resulting in catalyst loss.
Based on the Deacon process, many researchers have conducted extensive and in-depth research and proposed many improvement plans, mainly in two aspects:
(1) Improvement of catalysts to solve the problem of catalyst loss;
(2) Improvement and development of the reaction process to make the conversion rate of HCl close to 100%, and also solve the problem of equipment corrosion.
Since the oxidation reaction of hydrogen chloride is an exothermic reaction, from the perspective of reducing the thermal degradation of the catalyst and ensuring the stability and operability of the process operation, it is very important to suppress excessive hot spots in the catalyst filling layer. Excessive hot spots can also cause the reaction to run away, and hydrogen chloride and chlorine can also cause high-temperature gas corrosion problems of equipment materials.
The introduction of a tubular fixed bed reactor, this reactor is characterized by high hydrogen chloride conversion rate; large processing capacity, the production capacity of a single reactor can reach 400,000 tons/year; reaction heat can be converted into steam recovery, reducing system energy consumption; and the entire process is environmentally friendly, with only a small amount of wastewater discharged.
The economic advantages of the DEACON process are:
(1)The catalyst used has high catalytic efficiency and long life, and the catalyst cost is low;
(2)Through the hydrochloric acid circulation system, almost all hydrogen chloride raw materials can be converted into chlorine, and the raw material utilization rate is high;
(3) Compared with the electrolysis method, the equipment investment and operating costs are relatively low.
The biggest advantage of DEACON technology is that it has extremely low energy consumption. It only consumes power electricity. For every ton of chlorine produced, the power electricity consumed is only about 180 kwh, which is far lower than the energy consumption of the electrolysis method. It is a low-carbon and economical process technology route with great development prospects.
The process flow of chlorine treatment generally includes cooling, drying and dehydration, purification and compression and transportation of chlorine. The main corrosive media in the chlorine treatment process are wet chlorine, chlorine water, chloride and dilute sulfuric acid. The material requirements for equipment are extremely strict, such as waste chlorine absorption tower, HCl exhaust scrubber, dry chlorine compressor, wet chlorine compressor and other main equipment.
In the chlor-alkali industry, sealing issues have always been a major concern. Once the leakage occurs, it will cause serious problems.
The EVP team once met a customer from the European chlor-alkali industry. The main chlor-alkali products are: chlorine (Cl2), solid and liquid sodium hydroxide (NaOH), calcium chloride, calcium hypochlorite (based on sodium and calcium processes), hydrochloric acid (HCl) and sodium hypochlorite (NaClO). There are 4+2 Garo chlorine compressor closed units on site.
The working conditions as follow:
| Specification | Case 01 | Case 02 |
| Gas | Dry Chlorine gas | Dry Chlorine gas |
| Average molecular mass (kg/kmol) | 69.9 | 69.9 |
| Total volume at inlet conditions (m3/h) | 230 | 450 |
| Quantity of compressor (Pcs.) | 4 | 2 |
| Inlet total absolute pressure (kPa) | 76 | 76 |
| Inlet temperature (֯C) | 20 | 20 |
| Outlet total absolute pressure (kPa) | 350 | 350 |
| Outlet temperature (֯C) | 34 | 34 |
| Differential pressure (kPa) | 274 | 274 |
| Compression ratio | 4.6 | 4.6 |
| Class of service | continues | continues |
| Mist content (ppm) | 200 | 200 |
Customer feedback: “At present, our production process unit uses GARO-liquid ring compressors for chlorine, which uses H2SO4 acid as the liquid ring. However, the leakage of H2SO4 from the compressor gland can cause serious problems, affecting the foundation of the compressor room structure and the spread of the acid. Since sulfuric acid is highly corrosive, this has caused us serious concern.”
The EVP team selected LRC-3 and LRC-6 titanium material + double-end imported mechanical seal liquid ring compressors for chlorine compressors. This type of compressor is an independently developed and produced high-pressure, high-pumping speed, low-consumption liquid ring compressor, up to 8BarG., and parallel connection can reach 15BarG. High-pressure liquid ring compressor.
For more information about the chlor-alkali industry and the selection of compressor vacuum pumps, please consult Shanghai EVP Vacuum Technology Co., Ltd.
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