9 replies to this topic

[Dmitry]

Hello Dear members,

 

On our Oil Refinery we have old fired heater which needs to be replaced  by new. The purpose of this heater is to preaheat condensate in the bottom of Distillation column.

 

The same time we have available steam - (T=250 C, P=16 bar.) and we consider replace this heater by steam reboiler.

 

My question is how to compare using steam rebioler and fired heated on fuelgas. Is we burn gas to preheat condensate using Heater or we generate steam to preheat condensate.

 

Regards,

Dmytro

 

[S J]

Dmitry

To check each option, we need to know process fluid temperature, operating cost and intallation cost.

For installation, fired heater will be more expensive.

If ambient is very cold, steam condensate could be freezed. Does your plant use steam ?

And you should check MTD for steam heat exchanger.

[Dmitry]

Dmitry

To check each option, we need to know process fluid temperature, operating cost and intallation cost.

For installation, fired heater will be more expensive.

If ambient is very cold, steam condensate could be freezed. Does your plant use steam ?

And you should check MTD for steam heat exchanger.

Hello and thanks,

 

Now we need to select ot steam reboiler or new heater. The cost oh new heater is 1 800 000 USD. Amdient is middle and we use now steam for process.

 

Available steam flow is 20 t/hour

Temperature is 250 deg. C

Pressure is 16 barg.

 

The fluid is condensate C5+, SG is 0.72

Temperature upstream heateris +130 deg C

Flow is 50-100 t/hour

Required temperaure downstram is  +150

Edited by Dmitry, 22 June 2016 - 05:15 AM.

[fallah]

Dmitry,

 

The main points based on which you may decide about using reboiler or fired heater in distillation columns are as follows:

 

-Available source of heat at required temperature; if steam is available at required temperature level using reboiler is preferred...

 

-Flammability of the handling fluid; if it's flammable and explosive using reboiler is preferred...

 

-If the fluid isn't flammable and explosive and also the required temperature level is so high; using fired heater is preferred

 

In the case you described appears using reboiler would be preferable...

[Dmitry]

Dmitry,

 

The main points based on which you may decide about using reboiler or fired heater in distillation columns are as follows:

 

-Available source of heat at required temperature; if steam is available at required temperature level using reboiler is preferred...

 

-Flammability of the handling fluid; if it's flammable and explosive using reboiler is preferred...

 

-If the fluid isn't flammable and explosive and also the required temperature level is so high; using fired heater is preferred

 

In the case you described appears using reboiler would be preferable...

 

Dear Fallah,

 

We have available saturated steam at 250 deg. C/16 bar and overheated steam at 450 deg. C/39 bar.

We use now saturated steam for another reboilersm however can we use overheated steam for reboilers? It can give more heat transfer to condensate I suppose

 

Dmytro

[srfish]

For the saturated steam condition and 16 bar pressure, I get 201.4C, not 250C.

That is a long temperature range for the desuperheated steam. The heat transfer of the steam at the upper end of the temperature range will be relatively low. That will be detrimental to the higher heat transfer at the saturated condition.

[Dmitry]

For the saturated steam condition and 16 bar pressure, I get 201.4C, not 250C.

That is a long temperature range for the desuperheated steam. The heat transfer of the steam at the upper end of the temperature range will be relatively low. That will be detrimental to the higher heat transfer at the saturated condition.

Yes, you are right 250 C is more than saturation temperature. 

Could you please explaine with more details regarding differents of heat transfer between 201,4 and 250 deg C? Or what do you mean?

 

Dmytro

[fallah]

 

however can we use overheated steam for reboilers? It can give more heat transfer to condensate I suppose

 

 

Dmitry,

 

Yes, you can, provided that you will use desuperheater before entering the steam to the reboiler...in fact the best form of heat transfer in the reboilers is losing the latent heat via condensation of saturated steam while using superheated steam will reduce the overall heat transfer due to transfer of sensible heat before condensation...

 

[srfish]

This is confusing. In the statement to Fallah it said "available saturated steam at 250C. If the steam is saturated, the pressure would have to be higher than 16 bar. Do you mean there is a desuperheat zone from 250c to 201.4C and the entering steam is not saturated?

[S J]

Dmitry

In my experience,

if steam header is operated as superheated , superheated steam is injected into reboiler.

Installing desuperheater is not easy.

If steam header is operated as saturated, saturated steam is injected into reboiler.

Superheated steam is normally good for transferring and less condensate is generated in the piping.

If saturated steam is moving in the piping, piping design may be used up to the limitation of piping pressure and temperature.

In a view of thermal rating, I checked both cases of saturated and superheated. It was not much different.

Anyway, for economics of chemical process,

I recommend to use cheaper type including operating cost such as fuel or steam price and maintenance cost (crane...)

Operating cost will be much higher than 1800000 USD.

And you could also find some heat integration system. For example, other column overhead could be used for reboiler because column overhead requires cooling and reboiler requires heating.

Edited by S J, 22 June 2016 - 09:42 PM.