What are the ten areas that affect the process of distillation operations?
The distillation operation process is influenced by the following factors: feed position, tower temperature, tower pressure, feed status, feed volume, feed composition, feed temperature, rising steam velocity in the tower, reboiler heating, return flow rate, top cold dose, top of the tower extracted, bottom of the tower extracted, and so on. The operation of the tower is to regulate these influencing factors according to the requirements of the composition of the product at the top and bottom of the tower.
1、The influence of feed plate position on distillation operation
The most suitable feed plate position is the same number of theoretical plates and the same operating conditions, with the maximum separation capacity of the feed plate position or in the same operating conditions require the least number of theoretical plates feed plate position. In the chemical industry, most distillation towers are equipped with more than two feed plates, adjust the position of the feed plate is based on changes in the feed component. When the light component of the feed component is lower than normal operation, the position of the feed plate should be moved downward to increase the number of plates in the distillation section, thereby increasing the separation capacity of the distillation section. Conversely, the position of the feed plate is shifted upward in order to increase the number of plates in the distillation section so as to increase the separation capacity of the distillation section. In short, the content of light components in the feed component on the feed plate should be less than the content of light components on the lowest plate of the distillation section and greater than the content of light components on the uppermost plate of the distillation section. This will make the feed does not destroy the composition of the material in the tower on the various layers of the plate, so as to maintain smooth operation.
2, changes in the composition of the feed on the impact of distillation operations
Changes in the composition of the feed, directly affecting the distillation operation, when the concentration of recombinant components in the feed increases, the distillation section of the load increases, for the fixed distillation section of the number of plates of the tower, will cause the recombination of components brought to the top of the tower, so that the product quality at the top of the tower is unqualified. If the concentration of light components in the feed increases, the distillation section of the load increases, for the fixed distillation section of the tower plate number, will cause the distillation section of the light components of the incomplete evaporation of the light components of the kettle liquid in the light components of the loss increases. Changes in the composition of the feed will also cause changes in the tower material balance and process conditions. Components become lighter, the top fraction increases, the kettle liquid discharge is reduced. At the same time, the whole tower temperature decreases, the tower pressure increases. Components become heavy, the opposite is true. Feed composition changes, the following measures can be taken. 1) adjust the feed port. When the component becomes heavy, the feed port is moved down; when the component becomes light, the feed port is moved up. 2) Change the reflux ratio. When the component becomes heavy, increase the reflux ratio; when the component becomes light, reduce the reflux ratio. 3) Adjust the temperature of the tower top and kettle. According to the composition of changes in the situation, adjust the tower top and kettle temperature accordingly, through the reflux liquid, gas reflux temperature or reflux flow rate adjustment to maintain the top, kettle product quality unchanged. Increase the heating dose of the kettle, you can increase the kettle temperature, the kettle product light component content is reduced, the content of heavy components, but may lead to the top of the tower product heavy components, then, need to reduce the temperature of the tower reflux liquid, or improve the return flow rate to maintain the quality of the product at the top of the tower.
3, the impact of changes in feed temperature on distillation operations
Changes in feed temperature on the distillation operation is a great influence. In general, the feed temperature decreases, will increase the bottom of the evaporation kettle heat load, reduce the top of the tower condenser cold load; feed temperature increases, increase the top of the tower condenser cold load, reduce the bottom of the tower evaporation kettle heat load. When the magnitude of the feed temperature change is too large, it usually affects the temperature of the whole tower, thus changing the vapor-liquid equilibrium composition. In the case where the feed temperature is too low and the tower kettle is not rich in heating vapor, it will increase the light component content in the bottom fraction of the tower. For example, a tower designed for bubble point feed, when changed to cold liquid feed, the distillation section of the number of plates is too much, the distillation section of the number of plates is not enough, the result is that the product quality at the top of the tower may be improved, while the kettle liquid in the light components of the evaporation is incomplete. If it is changed to gas-liquid mixed feed or saturated steam, superheated steam feed, the distillation section of the number of plates is not enough, the distillation section of the number of plates is too much, the result is that the top of the tower product content of recombinant components exceeds the provisions of the kettle liquid light component content is lower than the prescribed value, and at the same time increase the top of the tower condenser refrigerant consumption, reduce the consumption of thermal agent of the tower kettle. The change of feed temperature means the change of feed state, and the change of feed state will affect the distillation section, distillation section load change, and then product quality, material balance will be changed, therefore, the feed temperature is one of the important factors affecting the operation of the distillation tower. The more ideal feed is bubble point feed, which is the most commonly used feed state.
4, the distillation tower operating pressure changes on the distillation operation
Distillation tower design and operation are based on a certain tower pressure, so the general distillation tower is always the first to maintain a constant pressure. Tower pressure fluctuations will have the following effects on the operation of the tower.1) Affect product quality and material balance. Changing the operating pressure will change the composition of the gas-liquid balance on each plate. The kettle pressure increases, then the gas phase in the recombination is reduced, the corresponding increase in the concentration of light components in the gas phase, the liquid phase in the light component content increases compared to the previous, but also changes the weight ratio of the gas-liquid phase, so that the amount of liquid phase increases, the amount of gas phase decreases. The total result is: the concentration of light components in the fraction at the top of the tower increases, but the quantity is relatively reduced; the concentration of light components in the kettle liquid increases, and the amount of kettle liquid also increases at the same time. Similarly, the kettle pressure decreases, the amount of the top fraction increases and the concentration of light components decreases; the amount of kettle liquid decreases and the concentration of light components decreases. In normal operation, a constant pressure should be maintained. However, if the operation is not normal, caused by the increase in the concentration of heavy components in the product at the top of the tower, it can be used to appropriately increase the pressure, so that the product quality is qualified, but at this time the kettle liquid loss of light components increased. 2) Change the relative volatility of the components of the components of the relative volatility of the components of the components of the relative volatility of the components. Kettle pressure increases, the relative volatility between the components decreases, the separation efficiency decreases, on the contrary, the relative volatility between the components increases, the separation efficiency increases. 3) Change the capacity of the tower. The kettle pressure increases, the heaviness of the components increases, and the capacity of the tower increases.4) Fluctuation of the tower pressure. Fluctuations in tower pressure will cause confusion between kettle temperature and the correspondence between components. In the operation of the temperature is often used as an indirect measure of product quality standards, but this is only under the premise of a constant tower pressure is correct. When the tower pressure changes, the bubble point of the mixture, the dew point changes, causing the temperature of the whole tower changes, the temperature and product quality correspondence will also change. From the above analysis, it can be seen that changing the operating pressure, will change the whole tower operation, so in normal operation should be maintained at a constant pressure (process indicators), only in the tower of the normal operation of the tower is damaged, can be based on the above analysis, in the process indicators allow the range of the tower pressure to make appropriate adjustments. It should be noted that, in the distillation operation, feed volume, feed composition and feed temperature changes, tower kettle heating steam volume changes, back to the flow rate, reflux temperature and refrigerant pressure (in terms of the reflux tower) changes, as well as the tower’s clogging, etc., may cause fluctuations in the tower pressure, at this time should be the first to analyze the reasons for fluctuations in the tower pressure, and timely treatment, so that the operation to return to normal.
5, the tower rising steam speed and evaporation kettle heating fluctuations on distillation operations
Tower rising steam speed size, directly affecting the effect of mass transfer. Generally speaking, the maximum speed of rising steam in the tower should be smaller than the liquid flood speed. Process often choose the maximum permissible hole velocity of 80% of the velocity of the liquid flood generated. Too low a speed will make the tower plate efficiency drops significantly. The main factor affecting the rate of rising steam in the tower is the reboiler heat. In the case of the kettle temperature remains stable, the heating amount increases, the rate of rising steam in the tower increases; the heating amount decreases, the rate of rising steam in the tower decreases. It should be noted that the heating capacity adjustment range is too large, too fierce, may cause liquid flooding or leakage.
6, the size of the reflux ratio on the impact of distillation operations
Operation to change the size of the reflux ratio to ensure product quality. When the top fraction of the tower when the content of recombinant fractions increased, often used to increase the reflux ratio of the method of recombinant fractions will be pressed down to make the product quality qualified. When the light fraction of the distillation section down to the distillation section caused by the lower temperature of the tower, you can use the appropriate reduction of the reflux ratio in order to raise the temperature of the lower tower. Increase the reflux ratio, from the top of the tower to get the product distillation tower, can improve product quality, but to reduce the tower’s production capacity, increase the consumption of water, electricity, steam. The reflux ratio is too large, will cause excessive circulation of materials in the tower, and can even lead to liquid flooding, destroying the normal operation of the tower.
7, the size of the cold dose of the top of the tower on the impact of distillation operations
The tower using internal reflux operation, the size of the cold dose, the impact of distillation operation is more significant; but also the main factors affecting the return flow fluctuations. For columns with external reflux, the operation of the distillation columns will also be affected to varying degrees by fluctuations in the cold dose. For example, a reduction in the amount of cold, will make the condenser role worse, the condensate volume is reduced, and in the top of the tower product of the liquid phase extraction for a fixed value of regulation, the return flow is bound to reduce. If the condenser also has the role of subcooling (i.e., commonly known as condensation cooler), the cold dose reduction, but also cause the return liquid temperature increase. These will make the top temperature of the distillation tower, the top of the tower products in the recombinant content increased, the quality decreased.
8, the size of the top of the tower extracted on the impact of distillation operations
The size of the top of the tower and the size of the tower feed volume has a reciprocal relationship, the feed volume increases, the amount of extraction should be increased. As we all know, the extraction volume only with the feed volume changes, in order to maintain a fixed reflux ratio in the tower, to maintain the normal operation of the tower, otherwise it will destroy the gas-liquid equilibrium in the tower. For example, when the feed amount is unchanged, the tower using internal reflux, if the top of the tower to increase the amount of extraction, the reflux ratio will inevitably reduce the amount of reflux caused by the plate to reduce the amount of reflux, the gas-liquid contact is not good, the mass transfer efficiency decreases; at the same time, the operating pressure will also drop, the composition of the gas-liquid phase on the plate changes. The result is that the recombined components are brought to the top of the tower, and the quality of the product at the top of the tower is unqualified. In the operation of forced reflux, if the feed volume remains unchanged and the top extraction volume suddenly increases, it is easy to cause the reflux liquid tank to be pumped out. The reflux liquid is interrupted, the top temperature rises, which will also affect the top product quality decline. If the amount of feed increased, but the top of the tower to extract the same amount, the consequences of the reflux ratio increases, the tower material increases, the rising steam speed increases, the top of the tower and the tower kettle pressure difference increases, and will cause liquid flooding in serious cases.
9, the size of the bottom of the tower extracted on the impact of distillation operations
In the operation of the distillation tower, the need to maintain the top of the tower and kettle product stability, retaining the material balance of the distillation unit is a necessary condition of the distillation tower steady state operation, usually by the kettle level to control the material balance of the distillation tower. Tower kettle to maintain a stable liquid level, is to maintain a constant kettle temperature of the first condition. Changes in kettle liquid level, and mainly determined by the size of the bottom of the tower extraction. When the bottom of the tower extracted too much, it will cause the kettle liquid level reduction or pumping, which will make the kettle liquid circulation through the reboiler is reduced, resulting in poor heat transfer, the light component is not evaporated, the top of the tower, the bottom of the tower products are unqualified. If the use of column tube reboiler, due to the circulation of liquid volume is too small, so that the kettle liquid through the upper half of the column tube when the formation of superheated gas, manifested in the volatile tube of the gas temperature is higher, while the kettle temperature is lower. If the bottom of the tower extraction is too small, will cause the tower kettle liquid level is too high (in serious cases, will exceed the volatile tube or even flooding tower), increasing the kettle liquid circulation resistance, also caused by bad heat transfer, kettle temperature drop. In particular, it should be pointed out that, for easy polymerization of materials, kettle liquid level is too high or too low, will cause longer residence time, increasing the possibility of polymerization. In addition, to maintain a certain kettle liquid level also plays a role in liquid sealing to ensure safe production.
10, the impact of auxiliary equipment on distillation operations
Distillation operation is inseparable from the bottom of the tower reboiler, tower top condenser and transfer pumps. In the normal production of these equipment should meet the operating conditions of the distillation tower operation requirements, otherwise the capacity of the distillation tower will be limited, and in serious cases, will make the whole process can not go on. For example, the reboiler in the production process, may be due to the polymerization of certain unsaturated hydrocarbons, and blocked tubes, so that the heat transfer area is reduced, resulting in a decline in the production capacity of the tower or even forced to stop. The condenser at the top of the tower may also be used due to poor water quality, tube fouling and reduce the condensation efficiency, so that the tower’s capacity to reduce the normal process conditions can not be implemented. When the normal production process is damaged, not only from the distillation tower process to find the reason, but also should be taken into account the condition of auxiliary equipment.