Reboiler calcs 1

[ccaissie]

Thanks all, for previous help in this methanol distillation design, discussed under methanol volatilities:

What would you recommend for simple calculations to design a reboiler. I know there are canned programs out there. This design is simple. All we need is more heat until it works, but...

Would you consider the vaporization of the entire feed to be a good first shot at heat input at the reboiler?

I'm thinking that although the 70% methanol/30% water feed means that about 98% water wind up as bottoms, doesn't this water, during the process, need to be vaporized and then condensed and be part of the reboiler duty?

I'm not a math whiz, can eventually do the math, but need the instinctive feel for what's going on here.

Thanks again,
CSC

 

[sshep]

hey ccaissie,

For a bubble point feed, the reboiler duty is going to be the heat required to generate the reflux (internal) plus distillate. The required reflux will depend on the number of stages in the tower, so to simply assume the the heat required to 100% vaporize the feed is far too simple of a statement.

Once you get the duty and the tower bottom properties, such as temp and HVap, you can start the reboiler design. Choose an outlet vapor fraction based on the type of reboiler you envision. The process design will involve calculating the utility side (steam) condensing pressure, steam flowrate, and process circulation rate.

1) You can choose a condensing temp (pressure) at some reasonable approach (10-20 degC) to process side bottoms temp. The closer approach to the bottoms temp, the larger and more costly the exchanger; while a high margin in condensing will mean high chest pressure and flash losses in the condensate system. This condensing pressure decision affects the relief rate for uncontrolled heat input, control valve size, condensate return, exchanger cost, etc, so it is not a casual decision.
2) Calculate the steam flow based on the duty and the chosen condensing pressure using steam tables.
3) Calculate the process side circulation rate and boilup. For a vertical thermosyphon, reasonable outlet vapor fraction numbers are 15%-25%.
4) Estimate the size of the exchanger by zone analysis (steam desuperheating and then condensing) using heat transfer coefficients appropriate to those services via A=Q/(U*MTD), and Atotal=Adesup + Acondensing. The superheat will depend on the steam supply pressure vs the condensing pressure. This estimated size if for costing and reasonability check of the HX vendors design.

I would not do any serious reboiler design work without getting your duty from a simulation of a distillation tower for your system, using your relative volatility data, and nstage vs reflux decision.

good luck and best wishes,
sshep

 

[ccaissie]

Thanks, SShep:
I am studying Kister's "Distillation Design", and have worked through the column design via McCabe-Thiele graphical methods. Packing suppliers have confirmed the basic design.

Kister does not describe "internal" reflux as such. I assume we are talking of the descending liquid flow L as seen in the mass balance at each theoretical stage. It would seem that at some stages, this is in a sense, a condensate. Since we are choosing a q=1, the split has liquid descending, and vapor rising.

This design will operate with a reflux of about 1.5 and 8 stages in the column. Total condenser,"non-ideal stage" reboiler design. Since it is a small column we are looking at 20% overbuilding it in terms of stage height, expect to adjust feed quality and will have 2 alternate feed points.

Kister is a bit light regarding energy inputs in this book, and does not go into explicit detail on deriving thermal duty values. I have not "got it".

I feel like I'm on the edge of the "AHA!". What is the missing piece that will get me to heat duty? Just a better understanding of thermodynamics? Or is it just more crunching?

CSC

 

[sshep]

Hey ccaissie,

I understand. McCabe Theile graphical methods do not reference enthalpy, and therefore this detail is hidden to you, but your material and energy balance will give you the reboiler duty. There is no subcooling accounted for in the basic method of McCabe Theile, so the reflux flow is internal reflux. I assume the 1.5 you gave for reflux above is actually a reflux ratio.

First two cautions:
1) The basic McCabe Theile method that uses straight operating lines assumes the heat of vaporization of methanol and water are the same. This is not reality. Kister probably discusses this, but it may not be emphasized- I have his text but haven't looked at his design book in forever. Let us know if you need some elaberation on this point.
2) Kisters rule of thumb (when I saw him many years ago) on packing was to never use an HETP of less than 12" no matter how good the vendor claimed it was.

Now to the reboiler duty. By energy balance: 0=QR+QC+F*Hf+D*Hd+B*Fb
where QR= reboiler duty, QC= condenser duty, Hf= enthalpy of feed, etc, and get the signs right for streams and energy entering or leaving the process.

The condenser duty can be calculated from the reflux and distillate flows, and though it should be calculated from the vapor and liquid enthalpy differences. Because the top product is almost pure, it will condense as a narrow boiling mixture and we usually just say QC=Hvap*(R+D). Hvap is the heat of vaporization of the distillate product (methanol). QC is negative because heat is removed.

Really though, who does this stuff by hand anymore except to gain understanding. This is a few minutes light work with a simulation. If you give the feed flow, feed composition, tower pressure, and the desired product purity, I am sure there are many on this forum who would run a quick check on your design just for fun.

best wishes always,
sshep

 

[ccaissie]

Sshep:

Thanks for reply. Yes, reflux RATIO is 1.5

It does make sense that the operating lines would NOT be straight, as composition changes. Yeah, HVap of MeOH is very different than water.

Vendor (Mr. K.Patel) is comfortable with 24" HETP, and so am I. Flood calcs work out well, now at 43%.

I'm willing to relay basic data for a professional comment, and I list it below. If you know of someone in the forum who would like to help by looking it over, would you give me direction, or advise if I should start a new thread?

Much appreciated help, sshep. The light is coming on re: the thermal stuff, but I've got a seemingly big number and want to check it. Maybe time to go back to school?

Thanks again,
CSC

Feed composition 70%+ MeOH, bal. water.
Bottoms 2-5% MeOH
Product 97-98% MeOH
0.75 gpm
feed at stage 4
q=1.0

Continuous operation of one feed composition over 12-18 hrs. Feed comp. may occasionally rise to near 90% for some runs. Feed at stage 3

Column 6.35 i.d. x 16'-0 packed height.
5/8" Metal Pall rings
Reflux ratio minimal (1.25-1.5)
HETP 24", 8 packed stages
Total condenser
Kettle reboiler

Heat duty of reboiler initial estimate 59kW
Heat duty of condenser initial estimate -54kW

Simulation run desired to confirm heat duty range.

 

[sshep]

Hey ccaissie,

I get about 80kW for reboiler duty. As a hand check assuming a distillate + reflux flow of 250kg/hr, and HVap of 1100kJ/kg, then

250kg/h * 1100kJ/kg * 1Kw-s/kJ * 1hr/3600s = 76.4kW

I attached a summary of results which you can use as a check.

best wishes always,
sshep