2 replies to this topic

[GSaikrishn]

Dear Sirs,


We have a Naphtha Splitter column at our process unit which splits the naphtha from Crude unit into light naphtha and heavy naphtha.

The column gets its reboiler duty from 4 MP steam reboilers.

Operating conditions:

a) Column feed temp: 110 deg C, 655 TPH capacity, Column operating pressure: 1.2 Kg/cm2(g), Bottom temperature: 165 Deg C, column overhead temp: 95 deg C. Overhead/bottom split: 0.5/0.5

Reboiler duty: Inlet MP steam: 210 Deg C, 16.5 Kg/cm2(g)., 100 TPH Steam consumption.

Reboiler condensate outlet: 175 Deg C.

This condensate is further flashed in a vessel to form Low pressure steam (150 Deg C, 4.5 Kg/cm2(g).

On account of reduced LP steam usage, dumping of LP steam is being carried out.

My query is: Can this Reboiler steam condensate be utilized to heat up other low temperature streams rather than flashing (Process fluid to be heated up: ~ temp: 50 Deg C, Qty: 325 TPH, Sp gravity: 0.8 (Low temp fluid is hydrocarbon).

Assuming the condensate temp to reduce to 90 Deg C on heating this low temp fluid, my net energy savings work out to be 8 Gcal/hr. Based on simple heat equation: Q: mcp delT, my steam side condensate duty works out to be 100*1000*1*(170-90): 8 gcal/hr. My process fluid temperature after the exchanger is ~ 75 Deg C. Cold side temperature approach is assumed to be 15 Deg C.

I know i have to carry out a detailed HTRI study, but for starters i am not aware if steam condensate can be utilized to heat up hydrocarbon fluid.

Again i have heard that the surface area requirements will be very high for such a heat transfer to take place, which will increase my net capex. Please correct me if im wrong.

Any suggestions as to how to proceed with this study.

Attached drawing for reference.

Regards
Saikrishna

Attached Files

•  Book1.xlsx   33.34KB   274 downloads

[Art Montemayor]

Saikrishna:

From what you describe, I can advise you that it is very sensible and common place to carry out a heat exchange between a steam condensate fluid and a process fluid.

I certainly encourage you to carry this proposal further by generating an estimated heat exchanger sizing using the reboiler condensate as your heating medium. Attached you find my workbook where I come up with the same 34 million kJ/hr that you do. This is a sizable amount of energy that can potentially justify an energy recovery project. The estimated sizing should be a good, quick exercise because you are dealing with a liquid-to-liquid, sensible heat transfer operation that is straight-forward. I would first generate a PFD and a preliminary P&ID.

I would not worry about the heat transfer area required for now. First, make the basic heat and mass balance and then make the preliminary exchanger sizing. Use a conservative overall “U” – perhaps a 100 – 120 Btu/hr/ft2 value. Select an exchanger configuration that avoids a temperature cross and yields a good, counter-current LMTD. You are dealing with a lot of energy exchanged here and the result will be a corresponding large amount of heat transfer area. Your condensate will be a clean fluid, so you don’t have to worry about fouling. If your selected process fluid is also clean, you could easily have a very attractive proposal.

The key to this type of application is the existence of a “need” for heat transfer in a process fluid that is relatively cold and results in a decent LMTD. If this is your case, then the local cost of generating steam calories should give you a quick idea of what order of magnitude you are dealing with in regards to the attractiveness of the project.

I wish you good luck on this promising challenge. The rewards could be great for you in experience and personal compensation. Please keep us informed as to the outcome.

Attached Files

•  Heating with Steam Condensate.xlsx   59.06KB   362 downloads

[Technical Bard]

Art has (as usual) covered this very well.

One option could also be to connect the LP flash steam to a heat exchanger and condense it. If you flash the MP condensate to just above atmospheric pressure, you can get a condensing steam at a temperature above 100C. The LMTD of your exchanger would still be good and the hot side heat transfer coefficient would be better. Of course, you can't get as much heat, because your net condensate would still be about 100C or more (and flashing to vacuum risks air ingress to the condensate).