Chlorine Industry - Analyzers
Chlor-alkali refers to the production of Chlorine (Cl2), sodium hydroxide/caustic soda (NaOH), and hydrogen gas (H2) from the electrolysis of saltwater. Both chlorine and hydrogen gases are used for a multitude of different operations in the chemical industry. Sodium hydroxide is ubiquitous in the chemical and oil/gas industries, with applications ranging from sulfur removal from low grade crude oil, to the production of food and soap.
The Cl2 produced from the Chlor-alkali process is often used in the creation of polymers, the most common of which is PVC. Producing sales quality PVC from raw materials involves several steps. Initially, ethylene dichloride (EDC) must be produced from the feed stocks of ethylene and chlorine. The EDC is then thermally cracked to produce vinyl chloride (VCM), which is the chemical precursor to the PVC polymer. The VCM must be purified and dosed with a polymerization inhibitor to prevent spontaneous polymerization. Below, we will examine each stage of the process and the analysis requirements.
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The Cl2 gas that comes after electrolysis is typically referred to as Wet Chlorine. Its chlorine concentration at this point should be close to 90%, with percent level water and some impurities completing the balance. The chlorine concentration is measured here to monitor the efficiency of the electrolysis process.
NCl3 is an impurity formed in the electrolytic cell if ammonium ions or organic nitrogen is present in the original brine. NCl3 is unstable and capable of accelerated decomposition at concentrations above 3%. This reaction is strongly exothermic and has caused several explosions in various chlor-alkali processes. The level of NCl3 is monitored to ensure the safety of the plant and personnel.
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The effluent from the cooling tower is still typically referred to as Wet Chlorine. Although most of the moisture is removed in the cooling tower, the gas still contains more than 30 ppm moisture, making the gas much more corrosive. The Cl2 concentration at this measurement point should be close to 100%. The chlorine concentration is measured here to monitor the efficiency of the cooling tower. The concentration of NCl3 is also monitored here to avoid a dangerous exothermic reaction.
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The third measurement point for the Cl2 is after the sulfuric acid drying tower. This gas is typically referred to as dry chlorine. Again, the Cl2 concentration will be close to 100%. The Cl2 is monitored at this location to confirm proper function of the sulfuric acid drying tower. NCl3 is measured simultaneously to ensure that it does not exceed safe limits.
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The residual chlorine stream is the gas that the liquefaction unit is not able to condense. It consists of the Cl2 gas and the gaseous impurities that were not removed upstream. The Cl2 measurement at this point provides an indicator of how effectively the liquefaction unit is working as well as how well the upstream reactor and purification steps are functioning.
Nitrogen Trichloride (NCl3) is one of the impurities found in Cl2. It is formed by the electrolytic cell if ammonium ions or organic nitrogen is present in the original brine. Only 1 ppm of NH3 in the brine is enough to create concentrations of 50 ppm NCl3 in liquid Cl2. This is dangerous. NCl3 is very unstable and has been shown to be capable of accelerated decomposition at concentrations greater than 3%. This reaction is strongly exothermic and has caused several explosions in various chlor-alkali processes. Since NCl3 has a higher boiling point than Cl2, any NCl3 present in the chlorine gas will concentrate in the liquid phase during liquefaction. Therefore, the NCl3 should be monitored in the dry Cl2 gas prior to liquefaction as well for safety. The OMA-300 Chlorine Process Analyzer can measure the NCl3 and the Cl2 concentrations simultaneously.
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If a Cl2 scrubber is used on the tail gas, then the vent gas after scrubbing that is released to the atmosphere needs to be measured. Typically, the Cl2 measurement should be very low, since it is being removed from the gas by the scrubber. However, the Cl2 content needs to be measured for EPA reporting, and to confirm proper operation of the scrubber. The OMA-300 Chlorine Analyzer can measure the Cl2 content in this vent gas.
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The final stage in the chlor-alkali process is liquefaction. While a vast majority of the gas coming into the liquefaction unit is liquefied, some lighter byproducts of the electrolysis reaction not already removed through processing are vented as the tail gas. The tail gas coming out of the top of the liquefaction unit also contains residual chlorine. Depending on the efficiency of the liquefaction unit, the Cl2 concentration can be anywhere from 1 to 50%. It is important that this chlorine is recovered before the gas is vented, for environmental and safety reasons. One of the main methods of removing the residual Cl2 is with an NaOH scrubber. NaOH is used up in the process, reacting with the Cl2 to form NaCl, NaClO and water. Monitoring the level of NaOH in the reactor and adding NaOH to make up what is lost ensures that no Cl2 gas escapes to vent.
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Liquefied Chlorine is one of the final products of the chlor-alkali process. At this point, most of the impurities have been removed and the Cl2 is ready to be transported to the next process as a feed stock. Residual moisture in the stream will create corrosive compounds if the moisture exceeds the solubility of water in chlorine. Monitoring the moisture here allows the plant to ensure that it is kept under these levels. This keeps the plant and further downstream operations, storage, and transportation units from being subjected to unanticipated corrosion. Measuring the Cl2 in this location assures that the product meets the standard to be distributed to other processes and users.
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The direct chlorination reactor vent gas contains the side products from the reaction as well as the excess reactants fed into the reactor. Ethylene measurement in the vent gas provides feedback control for the flow rate of the ethylene feedstock into the direct chlorination reactor. While the tendency to use excess ethylene is justified by the effect of reducing Cl2 and HCl in the EDC and preventing corrosion, overshooting the optimal flow rate leads to waste of an expensive raw material. Measuring ethylene at AT-8 allows for real-time optimization of the ethylene feedstock flow rate into the initial stage of VCM production. O2 can be fed into the reactor to minimize side reactions. The O2 concentration is monitored here to make sure the vent gas remains safely below explosive limits.
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The direct chlorination catalyst is known to contaminate the effluent EDC with FeCl3 — a highly corrosive compound which fouls machinery. Reasons for controlling FeCl3 and Cl2 levels using analysis at this point include: coke formation, loss of VCM product quality, degradation of oxychlorination catalyst, corrosion of stainless steel piping, unwanted reactions, reaction inefficiency, and downstream FeCl3 formation due to free chlorine.
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The vent gas of the oxychlorination reactor contains the side products and excess reactants from the reactor. Ethylene measurement in the vent gas provides feedback control for the flow rate of the ethylene feedstock into the oxychlorination reactor. While the tendency to use excess ethylene is justified by the effect of reducing Cl2 and HCl in the EDC and preventing corrosion, overshooting the optimal flow rate leads to waste of an expensive raw material.
The CO/CO2 ratio measurement in the oxychlorination reactor vent gas is also an excellent indicator of reaction efficiency (an increasing ratio signifies decreasing efficiency). Additionally, the concentration readings provide a control parameter for the reactor temperature, as higher temperature promotes CO and CO2 formation instead of the desired products. The O2 concentration is monitored here as well to make sure the vent gas remains safely below explosive limits.
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Moisture is known to form a highly corrosive mixture with HCl and Cl2. Above 100 ppm, H2O will aggravate the corrosive effects of the produced EDC and VCM streams. The moisture also carries other undesirable impurities which reduce the quality of these products. Since removal of moisture is more economically viable than removing other components of the corrosive mixture, this is the main target of corrosion control.
VCM process operators typically measure 0-200 ppm moisture at these analysis points for process control.
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Moisture is known to form a highly corrosive mixture with HCl and Cl2. Above 100 ppm, H2O will aggravate the corrosive effects of the produced EDC and VCM streams. The moisture also carries other undesirable impurities which reduce the quality of these products. Since removal of moisture is more economically viable than removing other components of the corrosive mixture, this is the main target of corrosion control.
VCM process operators typically measure 0-200 ppm moisture at these analysis points for process control.