Find Temperature Difference if Input Temperatures Change

[scs027]

I don't remember how to do these calculations but I know they arn't very difficult. I am working on heating ClO2 for the bleach plant at a paper mill and we are looking at steam savings.

I have attached a diagram with the data that I know. The before temps are the way the system is now. The after temps are after the ClO2 has been heated.

If you need any more infor please let me know.
Thank you.

 

[srfish]

One literature source is "Process Heat Transfer" by Donald Q. Kern. There is a chapter called "Batch and Unsteady-State Processes".

 

[zdas04]

I don't see in your block diagram where the condensate is removed, so I'm going to go out on a limb and assume that it stays with the pulp.

How is the "temp after = ?" different from the bleach tower temperature? This confuses me. It looks like the bleach tower is required to be at 105°F but I'm not sure of that.

You have to work in constant terms. Everything should be in mass flow rate terms. Use the density (at 55°F) of the ClO₂ and convert the flow rate to mass flow rate. Calculate the mass flow rate of the steam and add it to the pulp mass flow rate.

Then Q=mcΔT of the pulp-plus-steam stream (I'd use a mass-weighted average to get the system c_p) to find out how many BTU's you can give up to go from 155°F to 105°F. Then use the same equation on the ClO₂ to see how many BTU's are required to go from 55°F to 105°F.

This system is so constrained that I don't see how you hit your constant targets--something either the bleach flow rate or the outlet temperature need to be variable

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[scs027]

Thank you for your help. Here is some more information to clarify.

Currently the pulp is coming from the repulper and being heated with steam to 155F (condensate is not removed). It is then mixed with 55F chlorine dioxide and put in the bleaching tower. We do not have a measurement of the tower temperature so I think the information given is enough to find it.

Once we have found the tower temperature it is going to stay constant and we are going to heat the chlorine dioxide to 105F. If these two temperatures are known then I should be able to find the temperature that the pulp needs to be before it mixes with the chlorine dioxide.

That is what I am trying to do so that I can calculate how much steam will need to be added if the ClO2 is heated.