Tank Fluid Heating with Recirculation and Heat Exchanger. 1

[Pavan Kumar]

Hi All,

I am working on a Anaerobic Digestion Project. The process involves heating the contents of the Hydrolysis and Digester tanks to 37 Deg C using a external Heat exchanger( with Glycol on the shell side) with a re-circlation pump. To develop a calculation spreadsheet I want to start with simple case assuming the tank contents to be just water. I want to calculate the heat input to this Heat exchanger to heat the tank contents from 10 Deg C to 37 Deg C. Pasted below is quick sketch. I want to know how to proceed with this calculation. I know the water temperature is not constant but would like get a methodology to start the calculations. Once I understand the methodology I will use the actual fluid properties. Any guidance would be very helpful.

Thanks and Regards,

Pavan Kumar

 

[LittleInch]

Your key variable is how long you want this heat up to take followed by the mass of water to be heated. Take the heat energy required to raise your mass 27 C (J) then divide by the time in seconds to get Power (W). Multiply by about 1.5 to allow for various losses and then figure out pump rate for water and the glycol rate.

Is the digestor insulated?

What's the ambient temperature?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Assuming there is no feed to the tank (ie batch heating operation), the appropriate expression for this is equation 11-35f on page 11-18 of Perry Chem Engg Handbook (external heat exchanger with non isothermal heating medium). Note that θ in this expression is the time in seconds. This expression is derived in DQ Kern's Process Heat Transfer book in the chapter on batch and unsteady state operations on page 629.

Since there are solids in the feed to this HX from the tank, a spiral HX would be preferable.

 

[Compositepro]

Hot liquids are less dense than cold and have a strong tendency to rise (natural convection). If you want a uniform temperature in your tank, you should return the heated fluid to the bottom of the tank. But be careful to avoid short circulating the flow in and out of the tank to the heater.

 

[MJCronin]

My two cents ...

1) - Strongly agree with LittleInches comment about your most important consideration being time. With solids in the stream and high potential for fouling, tank heating could take a long, long time

2) - How fast must your tank be heated ?.. Your MBA/Plant manager will insist on two hours heatup, right up until you show him the equipment costs . This is a big deal .. Find reasonable people in your organization

3) - I suggest that you perform a calculation using water first, then modify it if necessary

4) - Sometimes in-tank heating coils are used as back-up, if your tank is new aand if it is suitable for your fluid (and your tank is new) -- Have you explored these ?

5) - I disagree with the suggestion of a spiral HX ... These are rare and very expensive, in my experience .. "Plate Frame" Heat exchangers are a better choice here (easy to clean, too !!)

6) - I disagree with your proposed scheme of dumping heated fluid into the top of tank ...This will cause probable foaming/fouling and other problems -- Consider dip tubes or, as suggested above, discharge into lower nozzles on the new tank

7) This may be a good time to consider tank overpressurization ... What happens if your HX cannot be controlled or springs a leak ?

Please keep us posted about other design details ... How big is your tank ?... Will all componets be new ?

Regards

MJCronin
Sr. Process Engineer

 

[georgeverghese]

Though plate type heat exchangers can handle solids in the feed, gasketed plate heat HXs' are prone to leaks, and all welded PHEs' may not be a prudent choice in this application. Spiral units, on the other hand, are much less prone to leaks, can handle slurries and can be cleaned. Agreed, initial cost would be higher for SHE, but in the long run, its much less trouble for Operations. Both types enable high overall htc with close temperature approach for all countercurrent flow configuration. See Perry Chem Engg Handbook, 7th edn in the chapter on heat transfer equipment for narratives on both types.

Presume this tank has a mechanical agitator, else you could get flow maldistribution and the pump may get bunged up with solids. Antiscale chemical dosing may be necessary if you have high dissolved Ca / Mg in the HX feed. If the antiscale chemical reduces bacterial activity in the tank, then offline chemical cleaning would be the approach. Ca / Mg salt deposition rate on heat transfer surfaces in the HX would be lower if feed glycol temperature is lower, so avoid high glycol feed temp.

 

[pierreick]

Hi,
If you want to improve the homogenization and stability of your heating process you should consider adding an Eductor to your system.
Pierre

 

[Pavan Kumar]

Hi LittleInch,

Your key variable is how long you want this heat up to take followed by the mass of water to be heated. Take the heat energy required to raise your mass 27 C (J) then divide by the time in seconds to get Power (W). Multiply by about 1.5 to allow for various losses and then figure out pump rate for water and the glycol rate.

Is the digestor insulated? [highlight #3465A4]The plant is located in Canada and so I am sure it will have to be insulated to prevent heat loss[/highlight].
What's the ambient temperature? [highlight #3465A4]In the winter time it will go to -15 Deg C[/highlight].

Thank You Littleinch.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi georgeverghese,

Thanks for guidance. I will work through the Perry Section you mentioned get back to you once I have my spreadsheet ready.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi Compositepro,

Thanks for your correction. I will connect the re-circulation to the bottom of the tank.

Thanks and Regards,
Pavan Kumar.

 

[Pavan Kumar]

Hi MJCronin,

1) - Strongly agree with LittleInches comment about your most important consideration being time. With solids in the stream and high potential for fouling, tank heating could take a long, long time. [highlight #3465A4]There are going to be solids as this is one of the tanks where Anaerobic digestion is going to take place. I will check this against the residence time. The fluid is going a slurry which less than 15% solids.[/highlight]

2) - How fast must your tank be heated ?.. Your MBA/Plant manager will insist on two hours heatup, right up until you show him the equipment costs . This is a big deal .. Find reasonable people in your organization. [highlight #3465A4]Thanks I will.[/highlight] [highlight #BABDB6][/highlight]

3) - I suggest that you perform a calculation using water first, then modify it if necessary. [[highlight #3465A4]b]Yes that is my plan to get the spreadsheet and then make improvement for the actual service.[/b][/highlight]

4) - Sometimes in-tank heating coils are used as back-up, if your tank is new aand if it is suitable for your fluid (and your tank is new) -- Have you explored these ? [highlight ]I have not explored these yet. The process needs good mixing to activate the acidogenesis and acetogensis bacteria, so having an internal heating tubes won't help unless there is an agitator. I am sure if there is a Biogas generated if they will go with a Agitator and a seal. I am not sure, I will find out more soon.[/highlight]

5) - I disagree with the suggestion of a spiral HX ... These are rare and very expensive, in my experience .. "Plate Frame" Heat exchangers are a better choice here (easy to clean, too !!) [highlight #3465A4]Ok. How about Shell and Tube or Double Pipe HXs?.[/highlight]

6) - I disagree with your proposed scheme of dumping heated fluid into the top of tank ...This will cause probable foaming/fouling and other problems -- Consider dip tubes or, as suggested above, discharge into lower nozzles on the new tank. [highlight #3465A4]I agree. I will put the re-circ line to the tank bottom nozzles.[/highlight]

7) This may be a good time to consider tank overpressurization ... What happens if your HX cannot be controlled or springs a leak ? [highlight #3465A4]The vent line ( or the Biogas line) is open to the Gas Holder which a line that can go to an emergency Flare. So do you still Overpressurization or Vacuum generation will be a concern?.[/highlight]

Please keep us posted about other design details ... How big is your tank ?... Will all componets be new ? The project is at Basic Engineering. [highlight #3465A4]I donot yet have the details. I am working to see what the energy requirements for the Hydrolysis and Digester tanks are at this point. My intention is to have a spreadsheet ready to be used when the actual details come in.[/highlight]

 

[Pavan Kumar]

Hi Pierre,

The Plant's PFD shows a Biogas Bubbler installed in the tank. This serves the same purpose as an eductor?.

Thanks and Regards,
Pavan Kumar

 

[LittleInch]

As said your key input is time and how many times this operation takes place.

Once a day and it makes a relatively short time cost effective though more expensive, ( say 1 or 2 hours)

Once a month and you could afford probably to make that 10 to 12 hours. Operations will curse you and tell you you underpowered the heater, but if you chose some random time and then don't look at different time options you could find yourself with a heat demand 4, 5 or 10 times more than what you need to keep it warm. Good insulation will also speed things up.

For heat up you could always use any spare heaters or similar to speed things up, so 2 x 100%.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Pavan Kumar]

Hi georgeverghese,

Assuming there is no feed to the tank (ie batch heating operation), the appropriate expression for this is equation 11-35f on page 11-18 of Perry Chem Engg Handbook (external heat exchanger with non isothermal heating medium). Note that θ in this expression is the time in seconds. This expression is derived in DQ Kern's Process Heat Transfer book in the chapter on batch and unsteady state operations on page 629.

Since there are solids in the feed to this HX from the tank, a spiral HX would be preferable.georgeverghese

I went through the derivations for the tank/vessel heating with External Heat Exchanger with non-isothermal heating medium and got the equation to be used. However the estimation of the Overall Heat Transfer Coefficient\, U has to be done independently based on Chapter 20 of D. Q. Kern. When I checked Chapter 20, I did not see derivation for estimation of U for the External Heat Exchanger. Is it that U should be estimated by estimating the tube side and shell side film coefficients that is normally done for 1-1 Shell and Tube Heat Exchangers?. If yes then I have to make some assumptions regarding the transport properties of the tube side ( tank material, which is actually the bio-degradable material). I fear this might make the calculation inaccurate. Can you please share some guidance in this regard.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

Abstracting from Perry for plate HX (spiral HX values would be similar), average overall U is 6500w/m2/degK for water/water service, and 3100w/m2/degK for glycol/glycol service. By some manipulation, I get an overall U estimate of 4200w/m2/degK for glycol/water service. See Perry 7th edn page 11-54.

Shell and tube HX or double pipe HX are not suited for this solids laden service. Overall U will be much much lower than for plate/ spiral compact HX.

If your gas holder design pressure is very low, it may not be able to withstand the built up backpressure from the emergency flare during an overpressure scenario due to the high vent resulting from temperature control failure at this HX. This is a critical process safety concern, so ask some one to study this.

 

[Pavan Kumar]

Hi georgeverghese,

Is shell and tube HXs are not suitable for slurries then how come Plate and frame Hxs or spiral Hxs suitable? Won't they plug up as well. Won't placing the slurry in the shell side and glycol on the tube side not help. If not is the internal heating coil inside the tank the only solution.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

Slurry on shellside will present many dead zones where solids will collect. Cleaning the shellside will be almost impossible.

With slurry on tubeside, velocity is most likely not adequate to keep solids in suspension, especially in the tubeside channel.

In plate HX and spiral HX, the narrow channels enable much higher velocity where solids are far less likely to collect.

If you use an internal heating coil, cleaning the tube bundle of caked up Ca/Mg salts will be a major maintenance exercise, and involve considerable downtime. Entire tank will have to be drained out to pull out the tube bundle.

 

[pierreick]

Hi,
Are you sure that the best way is to use an external HX? I'm worried with the solid content. If the stream is not homogenous you will have solid accumulated at the bottom of the tank and risk to clog the suction line and the heat exchanger. An internal coil should be more appropriate.
My view only.
Pierre

 

[Pavan Kumar]

Hi georgeverghese,

Slurry on shellside will present many dead zones where solids will collect. Cleaning the shellside will be almost impossible.

With slurry on tubeside, velocity is most likely not adequate to keep solids in suspension, especially in the tubeside channel.

In plate HX and spiral HX, the narrow channels enable much higher velocity where solids are far less likely to collect.

If you use an internal heating coil, cleaning the tube bundle of caked up Ca/Mg salts will be a major maintenance exercise, and involve considerable downtime. Entire tank will have to be drained out to pull out the tube bundle.

Can I still use Equation 11-35f from Perry's Handbook 9th edn to calculate the heating time for Plate type HX?. If so how do I calculate the "U" for Plate Type HX?.

Thanks and Regards,
Pavan Kumar

 

[LittleInch]

You might want to consider a pipe in pipe or jacketed pipe system or even an agitated tank with external jacketed pipes.

Current design with any sort of slurry or bits looks problematic.

Heat design it makes no real difference. Time to heat up is still your critical issue which you haven't let on so far....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.