High-pressure steam generator

[tgmcg]

I'm looking for some suggestions as to the best type of heat exchanger to use for the following application.

We wish to generate saturated steam at pressures between 250-1000 psig through heat transfer with hot, high-pressure effluent from a chemical process. The effluent from the chemical process is a hot vapor at approx 2000 F at pressures between 250-1000 psig. The pressure differential between the steam and process effluent is always less than 50 psig.

A once-through type design on the steam side with 80% yield or more would be preferred, but not mandatory.

Total heat duty for the exchanger is about 9.0 MMBTU/hr. Water flowrate to the boiler is 20 USGPM.

Thank you for your help.

Best regards,

Tom

 

[rmw]

You don't give any indication of the volume of the gas stream. That would be necessary to analyze your question.

rmw

 

[tgmcg]

rmw,

The gas stream MW=23. The flowrate is 18,000 lbm/hr.

Per Kern, it appears that a forced-circulation reboiler arrangement might work, with hot gas stream on the tube side. But perhaps there's a better approach.

Best regards,

Tom

 

[mbeychok]

tgmcg:

I would suggest a kettle type exchanger (just like the ones used for distillation column reboilers) with the hot gas in the tubes. Such a reboiler automatically provides a de-entrainment space between the surface of the boiling water and the steam outlet.

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

 

[rmw]

I'm going with the process gas on the tube side because you don't want boiling on the tubeside in a tubular Hx. It's fine for boilers and evaporators, since they have special zones for steam/water separation, and for such issues as recirculation and blowdown of the BFW, but you don't want to invent that wheel.

Now, within the delta T's that you have to work with, do you expect any condensation of the process gas on the tubeside?

rmw

 

[tgmcg]

rmw/Milton,

A kettle-type reboiler arrangement (forced or natural-circulation) sounds feasible. That's great news...always better to use a well-proven exchanger design.

rmw...the process gas contains perhaps 60% (mol) water vapor with balance non-condensables, and the bulk gas temperature is far above it's water dewpoint. Since the shell-side fluid is at it's saturation temperature (ie. it's a boiler), the inner tube wall temperature should at all times be above the water dewpoint.

Does the 2,000 F process gas temperature cause you any concerns? I suppose that in a fire-tube boiler, the gas temperatures can reach these levels and that the tube is protected from overtemperature by water convective/evaporative cooling at the outer wall....so long as there is not excessive scaling and/or max heat flux < 30,000 BTU/hr-ft2.

Best regards,

Tom

 

[dan21703]

At 250 psig you may be more economical with a firetube type waste heat boiler (tubes in a shell, firetube optional). The higher pressures are most economically handled by watertube boilers.

I know of two types of tubular boilers that would work for your situation as stated. One is a traditional drum type boiler. The other is a once through steam generator. Both have their good and bad points. There could be details regarding your process that would tip the balance one way or the other between the three options discussed.

The drum type boiler will be more stable if your process is not stable. I suggest hiring an experienced boiler engineer to evaluate your process.

If I understand your flow rates it would seem that if you anticipate evaporating 20 gpm of flow, the heat recovery application should be economical.

 

[tgmcg]

Dan,

It is my understanding that watertube boilers are preferred for high system pressures when the hot process gas is at low pressure. However, in this case, both the water and process gas are at nearly the same pressure..the water pressure is perhaps 30-50 psig above process gas pressure. The process will also be stable. Any changes in process gas flow are matched by proportional changes in water flow, but the flows are expected to be constant during normal operation. The kettle-type reboiler seems a good choice.

Regards,

Tom

 

[dan21703]

The critical factor for cost is the heat transfer efficiency. If the heat transfer rates are high the kettle boiler will be more cost effective in the case you cite.

However most gas flows and especially with waste heat where radiative or condensing transfer is not available. Watertube configurations can result in overall lower cost as it is economical to apply fins to the tubes.

Kettle reboilers are a traditional solution that many plant operators have lots of experience with. It may be that if you selected a watertube configuration while overall lower in cost may be a more difficult sell. Kettle reboilers will tolerate higher scale and are percieved to be lower maintenance than watertubes. However if you get your chemistry right both can last a lifetime.