Question

Why do we need to maintain liquid level in the bottom of distillation column? (also liquid level in accumulator)? Is there relation between maintaining liquid level in the bottom of distillation column with operability of kettle reboiler?

Answer 1

It is important to maintain the proper liquid level in the bottom of distillation column as well as in the accumulator and that's why a control system is installed for maintaining the level of liquidBefore explaining why it is important I would like to explain the operation of distillation column so that you can relate the things.So what happens intially the feed is continously charged into the distillation column and part of the feed vapourised through flashing reaches to the top of the column and part of the feed which is liquid is accumulated at the bottom.The feed is continously supplied till the level of liquid in the bttom and accumulator is not fulfilled for the continous operation of distillation column.So now what we do,we start sending the entire condensed vapor as reflux to distillation column and similarly the liquid level at bottom starts being vapourised so that the system reaches steady state.At this point distillation column operates at total reflux condition.After the steady state is reached,we start drawing out the product from the top with desired composition of product and earlier we recycled the entire amount because we didn,t get the desired composition at that time and recycling increases the efficiency of the process.Now since we started drawing the product we need an additional supply of feed stream such that the pressure of the column can be maintained while maintaining the liquid level at tha bottom and top.

So as we observed we start drwaing our product from the top or other trays we first maintained the level of liquid.This was done in order to make the process continuity which is related with economy of the process.It requires a huge cost to operate a distillation column and so in order to make the process continous for large production to get economic benefit,accumulation of liquid is required.There is another reason also.Imagine the level of liquid at the bottom decreases due to increase in vapour flow rate from the reboiler so a time will come when the vapour contribution from the bottom will decreased and as such the pressure of the column may be decreased.If this pressure goes below the atmospheric pressure,the suction of air occurs from outside to the column which may form an explosive mixture or may result in combustion depending on the product and as such large heat will be generated in the column which will cause safety issue.Another reason for maintaining liquid level is that you may not get desired purity of your product.Let's imagine the liquid which you have at the bottom has already contacted with vapour and some mass transfer has taken place.So when the liquid level goes down,it will affect vapour flow rate from the reboiler i.e. it decreases and the feed which is coming from outside will now dillute our mixture because that has not been separated yet.So we require higher flow rate of vapour to increase the mass transfer and hence to improve the composition of product by maintaining liquid level.So i conclude that maintainin of liquid level in reboiler and accumulator is important to continously provide vapour flow rate from the bottom and continous reflux from the top to enhance separation and make the process more economical and safer.