Distillation column level control 1

[nrxbra001]

Hi all,

I have a question related to the control of distillation column bottoms level. I see often in practice that the level of a distillation column is of importance and is almost always controlled at a set point. Usually I see the level is cascaded to the bottoms flow.

My question is, what is the importance of controlling the bottoms level? Consider a distillation column where the bottoms product is on flow control and there are no control loops keeping the bottoms level at its set point. Hence any disturbance will cause the level to fluctuate. What is the detriment of having a fluctuating bottoms level?

Can we restrict the argument to forced reboilers.

Thanks in advance,

 

[EmmanuelTop]

Changing the level set point will affect product flow only during transient times. Once when level reaches the new set point, flow of bottoms product will restore at its initial value. For steady state operation and constant heat input, Flow (1) = Flow (2), where (1) and (2) refer to conditions with initial and final level set point.

For thermosyphon reboilers, changing level inside the column obviously affects the circulation rate. Not having control over circulation rate will reflect on column operation (column will likely enter into some sort of pulsation, due to changes in reboiler outlet liquid flow/regime). SO, controlling liquid level in the column bottoms for thermosyphon systems is quite essential thing.

For kettle reboilers, column bottom sump is just a surge volume where liquid product from the reboiler is accumulated, and hence there is no level control on the column but on the reboiler.

For pumped systems, and assuming reboiler outlet on temperature control, keeping constant flow through reboiler (not constant level in the column) means keeping heat input to the tower constant at all times. Now, if feed to the column decreases (increases) by any amount, the actual heat input will be higher (lower) than what the system needs. So you inject more (less) heat into the column, and as the result get warmer (colder) overhead product, if reflux is on flow control. Warmer (colder) tower top temperature means overhead product off-spec (reduced yield), as well as bottom product reduced yield (off spec).

The answers may vary depending on how the real system looks like and what kind of changes we are talking about, so posting a sketch can help in developing true sequence of events.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Presume this is about product control and not the reboiler controls. Dont see how you can have product exit control based purely on flow without running into high or low level situations with varying feed flows. LC cascade on FC is possible.

Else you have to maintain constant column feed flow in order to maintain constant bottoms exit flow. This will in turn mean the upstream feed vessel will need have adequate surge capacity to handle fluctations in feed flow. This may work only if feed composition in terms of the volume fraction appearing in the bottoms is constant.

Why would you want to do this anyway ?

 

[mbt22]

For pretty much every control problem, I start with material balance and then look at quality control.

Bottoms on flow control, so I need at least one of controlled level on a liquid product, pressure (i.e. inventory) on a vapour product, or feed flow off a level.

Quality control can now act on any remaining variables to set product purity within component balance constraints.

This can give something straightforward, such as reboiler and overheads on level control, reflux ratio and reboil on internal temperatures. It also works with something wierd, such as fixed bottoms flow, bottoms level onto reboiler power, reflux flow on an internal temperature and overheads on level.

*Something* needs to control bottoms level, whether it be directly on product flow or vaporisation or indirectly, or just a really steady process than an operator manually tweaks every so often.

Matt

 

[MatthewL]

Another reason is to keep a liquid level in the bottom of the column, to prevent running the heater "dry". On a side note, especially important if you have a bayonet style electric heater, as they tend to burn out/coke up if allowed to run dry.

Regards,

Matt

Matt

Quality, quantity, cost. Pick two.