In the pharmaceutical, petrochemical, fine chemical and other fields, there are often many liquid-phase mixtures that need to be separated or purified, and distillation technology has become the most widely used and largest mass transfer separation process.
1. How to adjust the pressure of the tower in distillation operation? What are the factors affecting the variation of tower pressure?
The tower pressure is one of the main control indicators of the
Distillation Tower. For any distillation tower operation, the tower pressure should be controlled within the specified index to adjust other parameters accordingly. Tower pressure fluctuations are too large, it will destroy the material balance and gas-liquid balance of the tower, so that the product does not meet the required quality. Therefore, many distillation towers have their specific measures to ensure that the tower pressure is stable within the appropriate range.
For the tower pressure of the pressurised tower, there are two main methods of adjustment.
① tower
Condenser for the top of the condenser, tower pressure is generally dependent on the amount of gas phase extraction to regulate. In the case of other conditions remain unchanged, the gas phase extraction volume increases, the tower pressure decreases; the gas phase extraction volume decreases, the tower pressure increases.
② tower top condenser for the full condenser, tower pressure is mostly dependent on the size of the cold dose to adjust, that is, equivalent to adjust the temperature of the reflux liquid. Under the premise of other conditions remain unchanged, increase the cold dose, the temperature of the reflux liquid is reduced, the tower pressure is reduced; if the cold dose is reduced, the temperature of the reflux liquid rises, the tower pressure rises.
For the pressure control of the reduced pressure distillation tower, there are two main methods.
①When the vacuum of the tower is obtained with the aid of the ejector pump, the vacuum of the tower can be adjusted by adjusting the cold dose of the condenser or the temperature of the refrigerant at the top of the tower so as to change the tail gas volume.
For the pressure control of the atmospheric pressure tower, there are three main methods as follows.
①For the tower top pressure stability requirements are not high, no need to install pressure control system, should be set in the distillation equipment (condenser or reflux tank) on an atmospheric pipeline to ensure that the tower pressure is close to atmospheric pressure.
② for the tower top pressure stability requirements are high or be separated when the material can not be in contact with air, the tower top pressure control can be used to pressurise the tower pressure control method.
The above method of adjusting the tower pressure does not take into account the actual need to analyse the non-condensable gas to adjust the gas phase pressure of the tower.
2. How to adjust the kettle temperature in distillation operation? What are the factors affecting the fluctuation of kettle temperature?
The kettle temperature is determined by the kettle pressure and the composition of the material. During distillation, the product quality can only be ensured if the required kettle temperature is maintained. This is why kettle temperature is one of the most important control indicators in distillation operations.
When the kettle temperature changes, the kettle temperature is usually adjusted to normal by changing the amount of heating steam in the reboiler. When the kettle temperature is lower than the specified value, the amount of steam should be increased to improve the amount of gasification of the kettle liquid, so that the content of the recombination in the kettle liquid increases relatively, the bubble point increases and the kettle temperature increases. When the kettle temperature is higher than the specified value, the amount of steam should be reduced to reduce the amount of gasification of the kettle liquid, so that the content of light components in the kettle liquid increases relatively, the bubble point decreases and the kettle temperature decreases.
There are more reasons for fluctuations in kettle temperature. When the tower pressure rises suddenly, the kettle temperature will rise and then fall. This increase in kettle temperature is due to a rise in the bubble point of the kettle liquid caused by the rise in pressure. As a result, the amount of rising vapour in the tower does not increase, but decreases due to the increase in pressure; in this way, the evaporation of the lighter components of the kettle mixture is incomplete, which will lead to a decrease in the bubble point of the kettle liquid and therefore to a decrease in kettle temperature. Conversely, when the tower pressure suddenly drops, the amount of rising vapour in the tower will increase due to the reduction in tower pressure, resulting in a rapid reduction in the liquid level of the kettle, so that the heavy components may be carried to the top of the tower. As the components in the kettle liquid become heavier, the bubble point of the kettle liquid rises and so does the kettle temperature. It would appear that the tower pressure is an important factor in the variation of the kettle temperature. Therefore, the only way to know exactly whether the kettle temperature meets the process requirements is to first control the tower pressure at the required index, otherwise this can lead to incorrect operation.
The kettle temperature also decreases as the concentration of light components in the feed increases and increases as the concentration of heavy components increases. In addition, fluctuations in kettle temperature can be caused by water in the kettle, material polymerisation in the reboiler blocking some of the column pipes, fluctuations in heating steam pressure, failure of the regulating valve and damage to the balance of the material extraction. When the kettle temperature fluctuates, the reasons for the fluctuation should be analysed and eliminated. For example, the tower top extraction volume is too small, so that the light components pressed into the tower kettle and cause the kettle temperature to drop. At this time, if you do not increase the tower top extraction, simply increase the amount of tower kettle heating steam, not only on the kettle temperature does not work, in serious cases will also cause liquid flooding. Another example is that the reboiler tube is blocked due to material polymerization, resulting in a drop in kettle temperature, at this time, the equipment should be stopped for maintenance.
3. How to adjust the reflux ratio in the distillation operation? The reflux ratio is determined according to the separation requirements of the raw material. The reflux ratio is too large or too small, will affect the economy of the distillation operation and the quality of the product. Increasing the reflux ratio increases the concentration of light components in the top product, but reduces the capacity of the tower and increases the consumption of top cooling and kettle heat. In normal operation should be maintained in the appropriate reflux ratio, to ensure product quality, under the premise of the best economic results. Only in the tower of the normal production conditions are damaged or product quality is not qualified, to adjust the reflux ratio.
For example, the top of the tower product in the recombination content increases, the quality of the decline, to increase the appropriate reflux ratio. The tower load (feed volume) is too low, in order to ensure a certain rising steam rate in the tower, also to increase the reflux ratio appropriately. For large production units, when the different types of tower plate structure and rising steam speed requirements and instrumentation design work range and the actual production of the size of the contradiction, you can correspondingly appropriate to change the reflux ratio. Another example is the floating valve tower plate, if the processing capacity of only 50-60% of the design capacity, in order to make the floating valve in the appropriate range of work, in order to let the instrumentation measurement range in the appropriate range of work, should be taken to increase the reflux ratio to ensure the stability of production, which is extremely important for large production units to maintain the necessary stability of production.
There are several ways to adjust the reflux ratio:
① Reduce the amount of tower top extraction to increase the reflux ratio.
② tower condenser for the condenser, can increase the amount of tower coolant to improve the amount of condensate, increase the reflux ratio.
③ have reflux liquid intermediate storage tank forced reflux, can temporarily increase the reflux flow to improve the reflux ratio, but shall not reflux storage tank empty.
4. In the distillation operation how to adjust the differential pressure of the tower? Tower pressure is the main factor to measure the size of the gas load in the tower, but also to judge the distillation operation of the feed, out of the material is balanced one of the important signs. In the feed and discharge material to maintain a balance, the reflux ratio is constant, the tower pressure difference is basically constant. When the normal material balance is disrupted, or the tower temperature, pressure change, will cause the tower to rise in the vapour flow rate change, as well as the tower plate liquid seal height change, thus causing the tower pressure change.
Distillation operation, to cause the tower pressure change for the reasons corresponding to the adjustment, commonly used methods have three.
① in the case of unchanged feed, with the top of the tower of liquid phase extraction to adjust the tower pressure difference. Product extraction more, then the tower of rising vapour flow rate is reduced, the tower pressure drop; extraction volume is reduced, the tower of rising vapour flow rate increases, the tower pressure rise.
②In the case of constant extraction amount, the feed amount is used to adjust the tower pressure difference. Increase the amount of feed, the tower pressure increases; decrease the amount of feed, the tower pressure decreases.
③In the scope of the process index, the tower pressure difference is adjusted by the change of kettle temperature. Raise the kettle temperature, the tower pressure difference rises; lower the kettle temperature, the tower pressure difference falls.
For the equipment problems caused by the differential pressure change, should be specific problems specific treatment, serious should stop the maintenance.
5. In the distillation operation how to adjust the temperature of the top of the tower? Tower temperature is an important factor in determining the quality of the product at the top of the tower. In the premise of the tower pressure remains unchanged, the top temperature increases, the top of the tower products in the recombination content increases, the quality decreases.
The tower top temperature adjustment method, mainly for two kinds: one is fixed return flow, adjust the return temperature; a fixed return temperature, adjust the return flow. Due to the increasing size of the production unit, the method of regulating the return flow rate has been widely adopted in consideration of the stability of production. The specific regulation methods are as follows:
① Use the return flow rate to control the top temperature. The return flow rate increases, the top temperature decreases, this regulation method is mostly used when the tower top is a full condenser.
② When the tower top of the refrigerant used in the heat transfer process has a phase change, the evaporative pressure of the refrigerant and the top temperature series adjustment to control the top temperature. Evaporation pressure is reduced, the corresponding evaporation temperature is also reduced, causing the top temperature to decrease. This method can be used to change the return flow rate when the top condenser is a sub- condenser; when the top condenser has a sub-cooling effect, it can be used to change the return temperature.
③ when the tower top of the refrigerant in the heat transfer process without phase change, available refrigerant flow and top temperature series adjustment to control the top temperature. If the flow rate increases, the top temperature decreases. This method can change both the return flow rate and the return temperature.
④ Use the top of the tower condenser heat transfer area to adjust the top temperature. Raise the refrigerant level, the heat transfer area increases, the top temperature decreases. This method can change both the return flow rate and the return temperature.
⑤ When the material concentration in the distillation section is relatively high, the temperature difference between a certain two plates can be used to adjust the top temperature. The temperature difference increases, the reflux volume increases and the top temperature decreases.
6. In the distillation operation, sometimes the kettle temperature does not rise for what reason? Distillation towers may not rise in temperature during the start-up process and normal operation.
The reasons why the kettle temperature does not rise during the start-up process may be:
① the water evacuator (or drainage air blocking valve) of the heating system is not working;
② the return water valve of the water raising station is not open;
③ The steam condensate in the reboiler is not emptied and steam cannot be added to it;
④ there is a large amount of water in the tower kettle material (water and material are not soluble);
⑤ the equipment structure is unreasonable, so that the kettle liquid circulation is obstructed;
⑥ due to improper operation (reboiler heating too late or feed volume is too large, too violent), resulting in reflux to the tower kettle light component volume is too large, a kettle temperature is difficult to mention normal, especially the low temperature liquid phase feed tower, very easy to appear this phenomenon. At this time should change the amount of feed, feed composition or increase the amount of tower top extraction, in order to adjust the operation.
Normal operation, caused by the kettle temperature can not be brought up for the following reasons:
① blockage of the liquid phase circulation pipe of the reboiler at the bottom of the tower, so that the kettle liquid does not circulate;
②The material in the reboiler column tube is coking or the column tube is blocked;
③Drainage and gas blocking valve failure;
④The tower kettle component is too heavy and the existing thermite cannot heat the kettle liquid to the bubble point, resulting in poor circulation of the kettle liquid;
⑤ a drop in the pressure of the heating kettle thermite;
⑥The kettle liquid level is too low or too high.
Source: Reproduced
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