In chemical plant production, the
Distillation Tower is the most common and typical separation equipment, any person engaged in chemical production will not be unfamiliar with the distillation tower, but the common outstanding problems in the operation of the distillation tower, such as: liquid flooding, flooding tower, flushing tower phenomenon, the reasons are not very clear, the problem occurs when the response to the tower parameters changes are not sensitive, so often delay the solution to the problem, affecting the production of the device.
A detailed analysis of the causes of the above mentioned problems, plus examples to show the parameter changes and incorrect operation when problems occur in production!
Firstly, let's look at the most familiar phenomenon of liquid flooding
► What is liquid flooding?
In a distillation column, the accumulation of liquid phase beyond the space in which it is located for various reasons is called liquid flooding. Liquid flooding can be divided into liquid flooding of the drop tube, liquid flooding of the mist entrainment, etc.
Liquid flooding refers to the accumulation of liquid phase in the descending tube to the last layer of the tower plate. Mist entrainment liquid flooding refers to the open space on the tower plate of the gas phase flow rate reaches a certain speed, so that the liquid phase on the tower plate along with the rising gas phase into the upper layer of the tower plate.
The operating condition when liquid flooding occurs is called the liquid flooding point. When designing a distillation tower, the liquid flooding rate must be maintained within a certain range to ensure the stable operation of the distillation tower. When liquid flooding begins, the pressure drop of the column rises sharply and the efficiency drops dramatically. Subsequently, the operation of the column is disrupted.
► What causes the liquid flooding phenomenon?
1. Liquid in the descending tube flows backwards back to the upper plate
As the tower plate has resistance to the rising airflow, the pressure above the lower plate is higher than the pressure above the upper plate, and the height of the foam in the descending tube is equivalent to the static pressure head to overcome this pressure difference, the liquid can flow downwards.
When the liquid flow rate remains the same and the gas flow rate increases, the pressure difference between the lower plate and the upper plate increases and the liquid level in the descending tube rises.
If the gas flow rate increases to make the liquid in the descending tube rise to the top of the weir, the liquid in the tube will not only not flow down, but begin to backflow back to the upper plate, the plate will begin to accumulate liquid; to operate when the liquid is constantly sent from the tower, and finally will make the whole tower full of liquid. On the formation of the liquid flood.
If the gas flow rate is certain and the liquid flow rate is increased, the resistance of the liquid through the descending tube increases, as well as the plate on the liquid layer thickening, so that the pressure difference between the plate up and down increases, will make the liquid level in the descending tube rise, thus leading to liquid flooding.
2.Excess liquid foam entrained to the upper plate
Air entrainment to the upper plate of liquid foam, can make the plate on the liquid layer thickening, normal increase to a certain extent, the thickening of the liquid layer will be significant up (the amount of liquid on the plate increased, bubbles add more, increase). Air flow through the thickened liquid layer of the liquid foam brought out and further increase. This excess liquid foam entrainment so that the top of the foam layer and the distance between the bottom of the upper plate is reduced, liquid foam entrainment continues to increase, large droplets are easy to spray directly to the upper plate, foam can also be bubbled to the upper plate, and finally the whole tower is filled with liquid.
► Liquid flooding phenomenon is divided into several kinds?
1, the bottom of the tower and the top of the tower pressure difference increases;
2, the temperature difference between the bottom of the tower and the top of the tower is reduced;
3.The level of the reflux tank at the top of the tower decreases;
4, the product yield at the bottom of the tower is reduced;
5.The product quality at both the top and bottom of the tower is not satisfactory.
► What methods are used to deal with it?
1.Refurbishing the bottom gap of the lowering plate;
2.Reducing the amount of rising steam;
3.Reduce the amount of feed;
4.Reduce the amount of steam, return flow.
Note: Of the above two causes of liquid flooding, the more common one is excessive liquid foam entrainment.
The second common outstanding problem is flooding the tower
In the distillation process, from a certain tower plate up the liquid gradually accumulated, to fill part of the tower section, so that the rising gas is blocked, gas, liquid two-phase heat transfer process can not be carried out properly, this is called flooding tower.
► The phenomenon of flooding tower is: tower top temperature drop; reflux tank liquid level drop; tower bottom liquid level and pressure increase.
► The causes of the flooding of the tower occur for several reasons:
1.Sinker tube blocked, reflux liquid can not flow down.
Start iron chips, welding slag and other debris, the normal production of equipment corrosion deposits, or solid precipitation in the liquid, the solution of the self-polymer, are prone to cause the descending liquid tube blockage.
2, the amount of liquid is too large, so that the descending liquid tube overload.
► The treatment methods are these two: 1, appropriate to reduce the amount of feed and return flow. 2, such as equipment failure, then shut down to deal with.
The last common problem is the flushing tower
In normal operation of a distillation tower, the gas-liquid phase load is relatively stable. When the gas-liquid phase load are too large, the gas through the tower plate pressure drop increases, will make the descending liquid tube in the liquid surface height increases; liquid phase load increases, the liquid surface height on the exit weir increases. When the liquid is filled with the whole descending tube, the upper and lower tower plate is connected into one, fractionation is completely destroyed, there will be a flushing tower.
► The reason for flushing tower is: all the factors that form the tower gas-liquid phase load is too large can cause flushing tower, such as the processing volume of crude oil, the nature of the raw material is too light, crude oil into the tower water content, tower bottom blowing steam volume, into the material temperature is too high, reflux interruption or uneven distribution, etc..
► Phenomenon: the occurrence of flushing tower, because of the tower fractionation effect becomes bad, destroy the normal mass transfer heat transfer, resulting in the tower top temperature, pressure, side line distillation outlet temperature, reflux temperature are rising, tower low liquid level suddenly drop, distillation oil colour becomes black.
► The principle of processing is to reduce the vapour-liquid load, that is, to reduce the return flow and the amount of steam heated at the bottom of the tower, if the processing volume is too large, can reduce the amount of feed. If necessary, you can interrupt the feed, turn off the bottom heating steam and wait for the temperature of each layer of the tower tray to fall back to below the normal value, then reheat and feed.
► Data analysis
As seen by the changes in the stabilisation tower parameters:
a) The separation effect of the tower has deteriorated and the purity of the product at the bottom of the tower has decreased, resulting in the sensitive plate temperature remaining below the normal production index with increased steam volume;
(b) With the same pressure at the top of the tower, the return flow rate is increased and the temperature of the tower plate under the return flow is still higher than the normal index value, indicating that the purity of the product at the top of the tower has decreased and the separation effect has become worse;
(c) sensitive plate (the third layer of the tower plate) and the 21st layer of the tower plate temperature difference is significantly smaller, indicating that the lower tower plate light components increase, the upper tower plate regrouping increased, while the bottom of the tower level can still be controlled normally, the tower can be judged to have a serious liquid flooding phenomenon. If the tower is flushed, the level at the bottom of the tower will decrease rapidly, which is the obvious difference between flushing and liquid flooding.
► What causes it?
For a distillation tower that has been designed and is in normal operation, with little change in the composition of the raw material, when a washout or liquid flooding occurs, it should be analysed primarily from an operational point of view.
As can be seen from the comparative data in the above graph when the tower is stabilised with liquid flooding, the stabilised tower return flow and the amount of steam heated at the bottom of the tower are both higher than normal, which is the most common operation leading to liquid flooding.
Operators are not experienced and do not have a deep understanding of distillation tower operation, when the sensitive plate temperature is low, increase the amount of heating steam at the bottom of the tower, when the tower top temperature is high and increase the return flow, so repeatedly, resulting in the amount of heating steam and return flow are too large, the gas-liquid phase load are significantly more than the design load of the tower, resulting in liquid flooding, the tower gas-liquid balance is damaged.
After the liquid flooding phenomenon in this stabilised tower, the return flow rate and the amount of steam at the bottom of the tower were readjusted, but after 16 hours the stabilised tower still did not reach a normal equilibrium. Finally, measures were taken to turn off the heating steam, stop feeding and reduce the temperature, and the tower was put back into operation to adjust properly.
Source: Reprint
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