Anaerobic Digester heat input

[JohnWeal]

Good morning,

I am looking at sizing a heat exchanger for the heating requirements for an anaerobic Digester.
The digester is a concrete tank with steel roof and is thermally insulated so that heat loss is limited to 1 degree C per day. The set point temperature is 36 deg C.
The inside wall diameter is 22.86m with a sidewalk height of approx 11m. There is also a cone base. The roof is being replaced by a steel cover which is approx 20 deg angle.
The volume of sludge is 4897m3 and is 4-6% dry solids content.

The raw sludge enters the digester by joining a recirculation loop. The entry point is downstream of the heat exchanger thereby introducing the raw sludge into the digester sludge at a tenth of the recirculation flow rate which doesn’t cause any disruption to the digestion process.

To work out the amount of kW the heat exchanger must have, what are the calculations I must consider?

I believe these are the two I must calculate:

1) to heat the raw sludge feed from 5 deg C to 36 deg C
2) to calculate the heat loss from the digester

I would be grateful if anyone can provide any typical calculation examples / website links that I could base the design on.

Regards
John

 

[willard3]

First find the specific heat of the sludge.

 

[chicopee]

Isn't sludge mostly water for a specific heat value?

 

[georgeverghese]

Presume this is some kind of inground insulated concrete tank; post us a sketch of this arrangement so the picture is clearer.

Assuming this is a continous operation, we have the following heat balance

Heat input at HX(A) + heat stream from incoming fresh feed(B) = exit heat stream from volumetric displacement(C) + tank heat loss(D)

A= 10.x.(T-35).Cp
B= x.5.Cp
C= x.36.Cp, since tank level is kept constant
D= Qloss

Where x = fresh feed rate in kg/hr, T = HX exit stream temp

Here, x can be varied to suit your feed rate, while T is the unknown, and Qloss can be estimated. Presume there is an upper permissible value for T, to avoid killing the bacteria.

Is there some agitation in this tank with some impeller - how do you keep the solids in suspension, since this is anaerobic? The greater the agitation, tank inside skin Nre will increase, but net effect on external heat loss may be negligible since insulation resistance will predominate. Heat loss through the vapor space roof should also be taken into account.

Qloss will be highest at min ambient temp and some reasonable coincident cross wind velocity.

Choice of HX and materials selection will be critical, since fouling and corrosion due to acidic conditions is otherwise very likely - 2x100 % units may be required, to allow for offline cleaning. Though this is not to be decided here and now, you'll have to cross this bridge soon. High skin temps should also be avoided to prevent killing bacteria here.

So you have 2 max permissible temperature values at the HX to decide on also; also related to detailed HX thermal design (a) max bulk fluid exit temp and (b)max heat transfer surface skin temp. From this point of view, the lower the heating utility temp, the better for the bacteria, but HX cost will increase, and so may recirculation rate.

Tell us if this has already thrown you off; it is going to get much more complicated with my next response(which will be after you reply). Else, get a senior process engineering consultant with heat transfer skills to do this for you. I cannot lay it all out for you anyway; I'll give you methods and procedures to use to get Qloss, but you'll have to run the calcs yourself. More iterations and revisits on these calcs may be required when you find some limitations with HX detailed thermal design. Would suggest you take what you've got here and get some help from a local consultant.

 

[georgeverghese]

Qloss in this case would be whats left over after subtracting permissible heat loss from the actual total heat loss.

Qloss = Qtloss - Qploss, where Qploss would be computed with MCpdt, where dt = 1/24 degC to represent hourly temp drop, while M would be the mass of the sludge - water mix in the 4900m3 tank.