Reboiler process design size'ing 3

[v8landy]

Hi

I am looking for help sizing a reboiler for a new project.

70 million L per year of pertol /diesel mix.

Petrol over top, diesel as bottoms product. Not using a kettel.

I have column reseved on the used market, 52 trays bubble cap 1 m diam x 15.540 m tall.

I am thinking about introducing feed at a mid point, with pre heating via steam??? Size of pre heater???

Main reboiler will be via hot oil. We have 2 hot oil heaters 1 at 1.76 MW other at 0.76 MW

I would be looking for about 7 - 8 T per hour boil up

Thanks

 

[chusker15]

What is the temperature of the fluid entering the reboiler? What is the required exit temperature of the reboiler? What temperature is the hot oil available at? Is there a minimum hot oil outlet temperature? What is the flow of the fluid to the reboiler and what are its physical properties (viscosity, heat capacity, boiling curve, etc.)? At what pressure will the reboiler be operating? Have you considered using the hot bottoms product to preheat the feed? If you're not using a kettle reboiler, will you be using a thermosyphon? If you are, will it be vertical or horizontal? What is the required turndown? Preheating the feed will decrease the reboiler duty while increasing the condenser duty, do you have sufficient condenser area to handle this?

 

[v8landy]

Hi Chusker15

I will try and give answers to your question, but given that we are effectly starting from scratch most will be blank.

Hot oil. We can heat it to 300 deg c. Two units on site one at 1.76MW other at 0.76MW.

Good point about using bottoms for pre heating feed, but would we not still require addtional source of heat to get things going i.e before bottoms comes back at temp?

Condenser, again will be new, so size as required.

Reboiler, process will run max at 2bar i.e in tubes product.
Orientation of reboiler is as design states, we are not fixed for it horizontal or vertical.

Crude, at this moment only know it is a mix of petrol and desiel.

 

[sshep]

Hey V8,

In brief for process design:

1) Choose type of exchanger based on fouling characteristics of the bottom stream. A vertical thermosyphon is cheapest, horizontal thermosyphons cost more but are easier to clean (pullable bundle located at grade), pumped system is most expensive.

2) Calculate the duty from the boil-up (M) and your heat of vaporization, Q=M*Hvap. If boilup of 7-8 ton/hr is actually the overhead vapor rate then don't forget to account for feed energy requirement. Best if you use a simulation to get the requirement at the reboiler for a given overhead (reflux + distillate) rate.

3) Estimate the total circulation rate based vapor outlet fraction typical for the the type of exchanger choosen- ie 15-20% for vertical thermosyphon, 30-40% for horizontal thermosyphon. I note that you gave a 2bar pressure "in the tubes" which implies that a vertical exchanger will be used.

4) Specify a reasonable hot oil flow (M) based on the calculated duty and choice of a reasonable return temperature, Q=M*Cp*dT, same Q as above. As the hot oil already exists, heat capacity data should be readily available to you.

5) Estimate the area (A) based on an appropriate overall heat transfer coefficient (U) from Q=U*A*LMTD, again Q is already determined from the boilup rate. LMTD is calculated from your bottoms temp data (inlet and outlet), and hot oil temp data (supply and return). The process side bottoms temperatures will of course depend on the bottoms pressure.

6) A vendor should be contacted to figure how to distribute the required area between tube size, length, number of tubes, and exchanger type (for a vertical thermosyphon, the length is an important factor to get the circulation rate right). Fill out a spec sheet with the process data (duty, flowrates, temperatures, transport properties, etc), and let them recommend the mechanical details- they will design one to be the most economical dimensions that can do the job.

These are the general steps anyway. Others may give more guidance, but this should get you started.

best wishes,
sshep

 

[dcasto]

I get a 1 meter tower feed 150 liters/min of a 50% mixture, at a 1.8 reflux ratio with a feed preheat with bottoms product to 150C requires 1.1 MW reboiler and a .88 MW condensor. I just used 1.8 reflux ratio without knowing compositions and specifications and I but 1% of the light ends in the bottoms.

 

[v8landy]

Hi Dcasto

We are looking at a 1:1 reflux ratio.

 

[dcasto]

That will lower the reboiler duty and allow slightly more throughput. The bottoms temperature was 220 C. So your hot oil should be enough driving force. This was at 3atm absolute.

 

[mbeychok]

v8landy:

With all due respect to dcasto who has kindly been helping you, if you are going to spend about $1,000,000 as you have already estimated, then you should really spend a few thousand dollars to have a consulting chemical engineer provide you with a complete process design.

Trying to get the information by exchanging emails and spreadsheets in an online discussion forum is not the optimum way to go about it.

This is in now way meant to belittle the helpfulness or competence of dcasto. It is only meant to offer you some good advice.

Milton Beychok
(Visit me at www.air-dispersion.com)
.