Heat Transfer in Bioreactor! 2

[Jigneshpadia2002]

Hi Everyone!

I am interested in calculating the heat transfer efficiency of bioreactors. I have few basic questions and I am hoping them someone can help me understand and clarify my doubts.

1. How to calculate the heat generation due to Impeller RPM.

I have done some homework and found that the heat input is related to the powerinput of motor. So theoretical calucation can be done by assuming that 100 % of the motor input is converted into work without any loss at maximum RPM. Based on this if run at the bioreactor at 50% of RPM then we can assume 50% of the motor input would be converted into heat.

All these is fine but if I have to get a practical value by experiment how do I do that????

I do have an experiment in mind.... The idea is to run the bioreactor without any external cooling and assuming that heat loss to surrounding is negligible. If this happens to be true than I can monitor the temperatur rise with time due to impeller RPM. The problem or confusion is how do I convert this temperature time profile into heat generation Q. If I am correct Q should have unit of Watt.


2. I have some data on the heat loss from bioreactor while colling was being performed with the aid of bioreactor jacket using water as a coolant. Unfortunately I don't have the temperatur or mass flowrate of the cooling water going in/out.

If I plot the temperature loss in the reactor vs Time than the slope would give me dT/dt (temp/time). Can I use this to calculate effective cooling Q

Assumption is I could use Q = m Cp dT/dt
here
m = mass in the bioreactor (not the mass flowrate)
Cp = It is assumed to be similar to water
dT/dt = value calculated from the slope.

Is this the right approcah?


Thanks for your help in advance!

 

[BigInch]

Good. What do you define as heat transfer efficiency? The amount of heat lost by the reactor contents?

heat capacity of contents = Btu/lb-F

heat_retained_by_contents/hr = dT_of_contents/hr x heat capactiy x mass = Btu /hr

heat_lost/hr = heat_in/hr - heat_retained/hr

heat_lost/hr = heat_in/hr - dT_of_contents/hr x heat_capacity x content_mass_lbs

heat in = mix hp x (2544 Btu/hr)

eff = heat_lost/hr / heat_in/hr

your efficiency will vary until the temperatures stabilizes

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Jigneshpadia2002]

Thanks for reply

Some clarification.....

What do you define as heat transfer efficiency?
I am after the total heat transfer effcicency of the chiller in maintaing bioreactor at a specific temperature. So the objective is to understand how much heat is produced without growing the bugs and how much we are able to remove. I believe that the major one for us is heat generated by agitation (Atleast when we are not growing anything!). By doing this calculation I want to findout that how much cooling is/was possible to achieve and how fast we can cool the bioreactor. This would help to determine the timing and cooling loads during after-sterilization cool down operation.

The amount of heat lost by the reactor contents?
Yes, I would like to know the amount of heat lost by the reactor contents. Typically for bacterial processes we use combination of defined and non-defined media and for non-defined media such as yeast extract / peptone etc, we don't know the heat capacity. For some of the yeast fermentation, we use glycerol and we do know (can find out) these values for glycerol.

heat capacity of contents = Btu/lb-F
Not known - assumption would be equivalent to water. Atleast that would give us some idea about the cooling and heating loads. we will aim to have higher heat transfer than the calculated answer required for cooling by increasing flow or changing the coolant and conducting some more detailed level of experiments.

heat_retained_by_contents/hr = dT_of_contents/hr x heat capactiy x mass = Btu /hr

This is good, but unfortunately we don't have the dT_of_contents/hr. We only have dT_of_bioreactor. No one observed the change in dT_of_cooling_water.

SO than can I still use the above equation using dT/dt for the content of bioreactor???? There is enough data about the change in temperature with time. And if I can, than I guess I can use the same rule for finding the heat input by impellers with change in temperature. Is this correct?????

It is not necessary at this point for us to obtain the value per hour. But at some point in the calculation we would come across this and it would be nice to have an idea but overall cooling requirement is more important. Wich would be KW of cooling

Please let me know your view.... Thanks again

 

[BigInch]

I'd suggest you rough it in with the above logic. Maybe you'll get some good ideas and can refine it as you go. As far as I know, it should get you close, but I'm not certain exactly how much heat you would generate via mixing with a mixer with a certain HP input. Maybe someone else can tell us. For me it sounds reasonable to assume most is eventually disipated as heat into the fluid as the fluid ultimately converts its motion to heat via viscos friction forces. When a fluid is stirring at steady state, you should only be adding enough energy to overcome friction alone.

If its mostly water, use water, or glycol use glycol. Bugs (and humans) are mostly water anyway, so their heat capacity will be close to water.

It wouldn't have to be per hour, I just used /hr as a representative time value. It could be calculated over the total time, from start of mixing, heating up and eventually stabilizing at a certain temperature. I think that actually, that would be the preferred method, but you could plot it for awhile and see where it looks like it will level off. The Temp vs time curve should initially rise and eventually start to level off.


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Jigneshpadia2002]

Question still remains that can I use the equation where

Q = M Cp (T2 - T1)

M = Total Mass
T2 = Temperature of broth and time t = 0
T1 = Temperatur of the broth at time t = t

 

[BigInch]

Why not? Besides, there's no alternatives?

But, that equation is accounting for the heat retained when heating up the mixture's mass only.

The system control volume equation is,
Q M Cp dT = Q_input - Q_lost_vessel_wall

You will have to consider the heat lost through the walls of the reactor. That you can do that by calculating the OTHC, overall heat transfer coefficient at the vessel wall. It will give the heat lost through the wall, equivalent to units of Btu/ft2-Hr-ºF. When integrated over the vessel wall area and through the change of temperature and over time, will give heat lost.

More about OHTC here,

http://www.engineersedge.com/heat_transfer/overall_heat_transfer_coef.htm

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[quark]

The heat addition due to agitation should be based on power drawn by motor. You can get this by taking current reading.

The chilled water flowrates should be, rather, based upon the highest load from bioreactor during a production cycle. This is generally when you cool down the reactor after sterilization. If you are reviving the cell and bacterial seeds from cold conditions, you should be able to drop the temperature from 121C to 4C. You can calculate this by considering the mass of the bioreactor and a time duration of half an hour time.

The process of bacterial growth is generally exothermic and if you are maintaining 37C, you should have good temperature control mechanism for both heating as well as cooling.

 

[Jigneshpadia2002]

Thanks Quark,

I was waiting and hoping that you would eventually give some of your expert input. I am glad your did.


For some strange reason we have to maintain our fermentation at 25 C and we are able to do it at 5L scale because of the Height to dua ratio and availability of the bigger chiller but at large scale it seems to be impossible to maintain this....

I hope that you had a look at the method I am intending to use to calculate the heating load. I wasn't sure but BigInch clarified that the idea is ok and can be used. Any other suggestions.

Thanks to all of you for great help.

 

[Yuman]

Hi,
I don't know what kind of liquid you use, but a should a reaction takes place and thus a heat generation will be produced , you have also a heat coming with the hot temperature due to motor driving ( may be you use a compressor) and the two factors can effect on the bioreactor effeciency (what is the operating temperature of the bioreactor ?).
For the calculation of the heat transfer you have to take account many factors such as:
1- the weather condition( summer with a max . of humidity and winter with a minimum temperature)
2- you have to calculate Reynolds number, Nu.. ect.. to calculate the heat coefficient h
3- To calculate the heat transfert by convection and by conduction.
4- then the heat will be Q= U*A*(DELTA T)
Knowing the amount of the heat you have you can know what kind of cooler you will use to maintain decrease the bioreactor with a high efficiency.
good luck

 

[25362]

The achievable heat transfer is a common limitation in batch reactors. Sometimes the heat transfer (heating or cooling) takes as long as the reaction itself. In this cases loop reactors are generally considered, and the heat transfer is carried out in external heat exchangers.

 

[Jigneshpadia2002]

the optimum set temerature is 25 C for our process. At present for small scale, 5L we are using 1 tones of cooling...

we have to scale-up the process to 750L and it the reactors have only 8 tones of chilling so now the question is is the caluclations correct and if correct how do we overcome this heating issue without affecting fermentation parameters