forced circulation reboiler 1

[jangochem]

We have a batch distillation unit with a forced circulation reboiler.we are using 8 bar steam as heating media.Total boilup ( reflux + top draw) of the column is approximately 5500 kg/hr.can any one suggest how to calculate the optimum circulation rate through the reboiler.I want to resize the circulation pump of the reboiler.

Thanks in advance

 

[Chance17]

For heavy fouling service the tube velocity should be somewhere
between 7-15 ft/sec depending just how much fouling is expected.
Velocity of 20 ft/sec is absolute maximum due to metal erosion.
As a counter check, the vaporization per pass should be about 10%.

 

[sshep]

Chance's %vaporization guidence would suggest 5500/0.1= 55000kg/hr. The suggested vaporization sounds like a good target. More circulation is often better for most reboilers in that the heat transfer is better and tubewall temperatures are lower, but ultimate limits will depend on your existing exchanger and piping.

I have often wondered what real problems can come from too much circulation in various systems. Norm Liebermans video on the subject suggests that foaming from too much recirculation is his main concern, to which we might add several other hydraulic problems (velocity into the tower, impingment, level stability, etc). For a pumped system there is an energy cost, but with dedicated reciculation pumps (i.e. seperate bottoms pumps) the head should be low which keeps energy costs down. Often engineers specify some sort of control to reduce the flow (even in vertical thermosyphons) in the event there is a problem, and then find it can be left well open.

Hopefully this is not a suppressed vaporization system (control valve on reboiler outlet) because such systems need alot of flow to achieve the flash vapor target (your 5500kg/h) without high temperature. A picture is always nice to allow peers to visualize the system.

best wishes always,
sshep

 

[jangochem]

Thanks friends for your valuable advice.I am attaching a flow diagram of the system.My query is that if i increase the circulation rate beyond a particular level will the steam be able to transfer heat to the process.process is not a fouling operation

 

[sshep]

Hey jangochem,

The steam will have no problem transfering to the process, regardless of the circulation rate so long as it is sufficiently high to return a mixed phase. No worry there.

Since the drawing doesn't show controls, we speculate that the steam flowrate will be used to control the kettle level, and that the kettle will be dumped as needed based on temperature- is that right?

best wishes,
sshep

 

[jangochem]

Steam flow rate is used to control kettle temperature.Mostly we operate by varying the steam flow rate so as to maintain sufficient boilup( 4500 kg/hr reflux + 1000 kg/hr product draw from top)

As you mentioned steam flow control wrt kettle level,is it a better way for controling boilup ?