What is a Conversion Ratio?

The conversion rate refers to the percentage or fraction of the conversion of a certain reactant, and the conversion rate is for the reactants. If there are more than one reactant, the conversion values calculated according to different reactants may be different, but they reflect the same objective fact. Therefore, it is possible to calculate the conversion rate by any of the reactants.

Polyvinyl chloride (PVC) is a thermoplastic synthetic resin commonly used at home and abroad, which has excellent performance and low price, and occupies a large share in plastic applications. Polyvinyl chloride has excellent chemical resistance and electrical insulation, and is widely used in industrial, agricultural, construction, electronic and electrical, transportation, power and packaging fields.

The most common production process of polyvinyl chloride is mainly acetylene. The quality of vinyl chloride monomer determines the quality of polyvinyl chloride. Common production processes of vinyl chloride mainly include conversion, water-alkali washing, and rectification. Conversion, as the first step of vinyl chloride monomer formation, occupies a very important position in the entire vinyl chloride production. Increasing the conversion rate of vinyl chloride helps to improve the purity of vinyl chloride monomer, and can reduce the production load of subsequent equipment, which is of great significance for saving energy consumption and reducing danger.

Conversion rate of vinyl chloride production process

The principle of vinyl chloride synthesis is to preheat the acetylene and hydrogen chloride gas after sufficient dehydration and mixing. After the gas reaches a certain temperature, the vinyl chloride gas is synthesized through the first stage of the converter and the second stage of the converter. The main reactions and side reactions are as shown in formulas (1) to (3).

Main reaction:

side effects:

The process of the conversion process of vinyl chloride is that the acetylene gas sent from the acetylene plant is cooled and dehydrated, and the cooled and dehydrated hydrogen chloride gas sent from the hydrochloric acid plant is thoroughly mixed in a mixer according to a certain molecular ratio. The gas undergoes freezing and dehydration again, and then heats up through the mixed gas preheater to heat up. Finally, one or two sets of fixed-bed converters are reacted to be converted into crude vinyl chloride gas.

Process control for improving conversion rate of vinyl chloride

The main process flow of the vinyl chloride conversion section is shown in Figure 1.

figure 1

To improve the conversion rate of vinyl chloride, it should be controlled from the purity and dehydration effect of the feed gas, the mixture ratio of the mixed gas, the conversion reaction temperature, and the catalyst activity.

Effect of Purity of Raw Gas and Dehydration Effect on Conversion

It can be known from the preparation process of acetylene and hydrogen chloride that the preparation of these two raw material gases will necessarily contain certain moisture and impurities. For example, acetylene contains moisture, sulfur, and phosphorus impurities, and hydrogen chloride gas contains free chlorine, hydrogen, and water. These impurities have a serious effect on the conversion of vinyl chloride.

First, the sulfur and phosphorus contained in acetylene will block the activated carbon catalyst micropores attached to the mercury in the converter, resulting in a reduction in the contact area between the mixed gas and the catalyst, which is not conducive to the reaction. At the same time, due to the characteristic that hydrogen chloride gas is very soluble in water, if the water contained in acetylene and hydrogen chloride gas is not effectively removed at the front end of the mixer, then these two gases will dissolve the hydrogen chloride gas in water in the mixer. The formation of hydrochloric acid leads to a reduction in hydrogen chloride gas and ultimately to errors in the mixing ratio. In addition, due to the operation error of the hydrogen chloride preparation section, free chlorine is generated in the hydrogen chloride gas, and a large amount of free chlorine will react violently after mixing with the acetylene gas, and a large amount of heat will cause an explosion accident. Therefore, controlling the purity of the feed gas and the content of impurities has an important effect on improving the conversion rate.

To improve the purity of the raw material gas, the first step is to strictly control the operation and process parameters of the raw material gas production device. Second, set up sampling points at the raw material gas inlet section to periodically check the purity of the raw material gas. If there is any abnormality in the acetylene gas, notify it in time. The acetylene preparation workshop takes measures. If the free chlorine of hydrogen chloride exceeds the standard and the temperature of the mixer rises, it should decisively start the emergency shutdown plan to prevent accidents.

Effect of Reasonably Designed Feed Gas Ratio on Conversion

The design of the two gas mix ratios has a great influence on the conversion of vinyl chloride. It can be known from the main reaction (1) that increasing the amount of acetylene or hydrogen chloride can promote the reaction to move forward, which is conducive to improving the conversion rate. However, it can be known from the side reaction (3) that if the acetylene is excessive, the catalyst will be denatured and deactivated. At the same time, the excessive acetylene will increase the load in the rectification section and affect the pressure swing adsorption process. Therefore, hydrogen chloride gas is used in excess in actual operation. Usually in actual production,

The ratio of HCl to HCl is operated at a molecular ratio of 1.00: (1.02 to 1.10), and the effect of mixing and dehydration in the early stage is good.

Regarding the regulation of the mix ratio, it should be controlled according to the following test parameters. Reasonably control the flow of acetylene gas and hydrogen chloride gas according to the flow meter. By analyzing the conversion rate of each group of converters, the mix ratio was adjusted. At the same time, when necessary, a sampling port can be set at the main pipe of the mixed gas cooler, and the mixture ratio analysis can be performed by using the property that hydrogen chloride is easily soluble in water.

Effect of conversion reaction temperature on conversion

The commonly used converter in vinyl chloride synthesis is a tubular fixed bed reactor. A catalyst is installed in the tube to provide free radicals required for the reaction, and the circulating hot water at about 97 ° C is provided in the jacket to provide the heat required for the conversion and take away the heat generated by the reaction. In order to test the reaction temperature inside the converter, two sets of thermocouple thermometers which are symmetrical to each other are usually arranged in the tube, and each thermocouple thermometer is provided with 4 temperature measurement points to measure the temperature of the converter. Generally, it is required that the temperature of the two temperature measurement points of the converters be symmetrical to each other at 120 ° C, and the single point temperature should not exceed 160 ° C.

A suitable method should be adopted for the control of the reaction temperature of the converter, too low is not conducive to the reaction, too high side reactions will increase the catalyst fire and cause an explosion accident. There are two main ways to control the temperature of the converter. The first is to control the flow rate of the mixed gas. If the flow rate is too large, the reaction will be too violent, and some gases will not participate in the reaction. If the flow rate is too small, the output will be reduced. The second type controls the temperature and flow of circulating hot water. If the temperature of the circulating hot water of the converter is too low, the heat required for the conversion cannot be provided, and the reaction does not occur or the reaction is incomplete. If the temperature of the hot water is high, the hot water cannot take away the reaction heat. In the same way, the hot water flow is large and the reaction zone is fast. The small flow will cause the temperature of the converter to increase and it is easy to cause accidents.

Effect of catalyst activity on conversion

The catalyst is a catalyst that provides free radicals for the entire reaction and has a great effect on the conversion rate. In general polyethylene production, in order to improve the conversion rate, two sets of converters are generally used in series. The contents of the active ingredients of the catalysts placed in the two groups of converters are different. The first group of converters has a high purity of raw material gas, and the reaction is violent. Generally, the content of active ingredients in the catalyst in this group of converters is low, and is generally controlled between 6% and 8%. In the second group of converters, due to the complex composition of the mixed gas, in order to increase the conversion rate, the effective content of the catalyst is generally controlled to be high, about 12%.

Insufficient dehydration in the raw material gas results in water content in the mixed gas, which will inevitably lead to agglomeration of the catalyst and decrease in the contact area of the catalyst when it is passed into the converter. Therefore, the general converter should be replaced or reinstalled after a certain period of use. The common way is to replace the 1 group and 2 groups of converters symmetrically. The replaced two groups of catalysts can be directly used in one group of converters. The catalysts used in one group of converters should be sealed and handed over to the relevant units to prevent environmental pollution. At the same time, in order to improve the catalyst activity, the catalyst should be activated before the mixed gas is introduced. The activation medium is hydrogen chloride gas. Its role is to dry the catalyst and to increase the catalyst activity in the acid environment.

In addition to the above four factors that mainly affect the conversion rate of polyethylene, there are also factors such as system pressure, flow rate, and preheater temperature that will affect the conversion rate. In actual production, we should comprehensively consider and analyze carefully according to the specific conditions.

Conversion rate

The quality of polyvinyl chloride has a greater relationship with the purity of polyethylene. Increasing the purity of polyethylene can promote the efficient use of raw materials while reducing unnecessary economic waste. Increasing the conversion rate of polyethylene production has a great effect on reducing the load of water-alkali washing and distillation processes, saving energy, and reducing environmental pollution. Improving the conversion rate of polyethylene should not only be solved from the control of process parameters, but also start from strengthening workshop management and improving the technical level of employees. Polyvinyl chloride production is a further comprehensive utilization of China's coal, lime, and salt resources, which will greatly promote China's economic development. ^[1]