6 replies to this topic

[veronika.hs]

Hi,

I'm designing a distillation column in Hysys and I need to optimize it. In order to achieve this I need to work out the outlet temperature of the cooling water that it's used in the condenser that I have in the top of my column. I tried to do some energy balances but I don't have the cooling water mass flow. I only know the condenser duty, the inlet and outlet temperature of the stream that comes from my column and the inlet temperature of the cooling water.

Does anyone want to help me?

Please, I would appreciate if someone could help me.

Thank you.

[djack77494]

veronika,
In the design phase of a project, things like this are typically defined. In Imperial Units, I have often seen cooling water supply (CWS) temperatures somewhere near 90F. That allows for a small margin above the worst case wet bulb temperature at your locale. You save on the expense of pumping around large quantities of cooling water if you set the cooling water return (CWR) temperature at as high a value as practical. However, there are opposing considerations. High CWR temperature = higher tendency to form scale and less cooling available for your process. So, like everything in engineering, you must strike the optimum balance to solve this problem. (CWR temperatures are often set in the vicinity of 115F.)

Once you have set your CWR temperature, then you can estimate a reasonable approach temperature (say 10 to 12F) to determine the process side temperature of you condenser. If that's not close to the temperature you seem to have predetermined, then you may have a problem. (Alternately, you may adjust the column pressure to get a better fit, if that is practical.) There's a lot to this type of problem - Happy Designing.

[katmar]

One addition to the good advice from djack - consider the functioning of the cooling tower as well. In my experience, the cooling water return (CWR) temperature is usually 8 to 10 degrees C above the cooling water supply (CWS) temperature, but this can vary. If the CWR temperature goes higher you get the disadvantages like scaling, corrosion and bacterial growth mentioned already but the heat rejection capacity of the cooling tower improves because it now has a higher temperature driving force. On the other hand I have installed a cooling tower where the CWR temperature was only 3 deg C above the supply temperature. This severely degrades the heat rejection capacity of the cooling tower and it is best to keep such a circuit totally separated from the other cooling water and have a dedicated cooling tower.

In an industrial situation you would most probably find the CWS and CWR temperatures specified in the design document, but your problem seems to be a classroom exercise. If the professor has not specified the CWR temperature then allow a rise of 8 to 10 deg C if that is possible. If you get to the situation mentioned by djack where the CWR temperature gets too close to the condensing temperature you either have to do as I described above where the CW temperature rise was restricted to a few degrees, or raise the distillation column pressure (and therefore the condensing temperature) as described by djack. A low temperature rise means a larger (more expensive) cooling tower and a pressurised distillation column means a thicker (more expensive) shell.

Edited by katmar, 25 February 2010 - 06:33 AM.

[ankur2061]

Hi,

I'm designing a distillation column in Hysys and I need to optimize it. In order to achieve this I need to work out the outlet temperature of the cooling water that it's used in the condenser that I have in the top of my column. I tried to do some energy balances but I don't have the cooling water mass flow. I only know the condenser duty, the inlet and outlet temperature of the stream that comes from my column and the inlet temperature of the cooling water.

Does anyone want to help me?

Please, I would appreciate if someone could help me.

Thank you.

veronika,

You can approach the problem in following manner:

Calculate a CW flow rate for a given CW inlet line size with a velocity of say 1.5 m/s using the equation:

V = A.v

where:

V = volumetric flow rate m3/h
A = c/s area of pipe, m2
v = velocity in m/s (1.5 m/s should be good for starters)

Convert the volume flow rate to mass flow rate by multiplying with density (1000 kg/m3 is a good figure to start with)

The mass rate as obtained above may then be used in the heat balance equation:

Q = m*Cp*(T2-T1)

where:
Q = condenser duty, kcal/h
m = mass flow rate as calculated above, kg/h
Cp = Specific heat capacity of water, kcal/kg-°C (=1)
T2 = CW return temperature, °C
T1 = CW supply temperature, °C

Solve for T2 to arrive at your outlet temperature

You may also consider a CW delta T of 8 to 10°C (very commonly considered for CW circuits) across the condenser and arrive at the CW inlet flow rate as suggested by Doug & Harvey.

Hope this helps.

Regards,
Ankur.

[fallah]

Calculate a CW flow rate for a given CW inlet line size with a velocity of say 1.5 m/s

Which inlet line size?

This data isn't among the information available for Post Originator.

[Padmakar Katre]

Hi,

I'm designing a distillation column in Hysys and I need to optimize it. In order to achieve this I need to work out the outlet temperature of the cooling water that it's used in the condenser that I have in the top of my column. I tried to do some energy balances but I don't have the cooling water mass flow. I only know the condenser duty, the inlet and outlet temperature of the stream that comes from my column and the inlet temperature of the cooling water.

Does anyone want to help me?

Please, I would appreciate if someone could help me.

Thank you.

Dear,
The cooling water flow is calculated based on the,
1 Range of the cooling water and
2.The condenser approach.
In you case you have the condenser duty and cooling water supply temperature(Tc1) and the process side outlet temperature(Th2). Just find the approach(Th2 - Tc2) of 5 Deg C and set the outlet temperature of the cooling water and calculate the cooling water flow based on (Q = m*Cp*DT) now this scenario is applicatble when you have a single shell or two/mulptiple shells in parallel. In another scenario you can take the range of the cooling water which is noramaly varies from 8 to 10 deg c and calculate the cooling water flow. To optimize the overhead condenser configuration takes many points into consideration which I could list as below,
1.The minimum cooling water velcity.
2.The single shell size (min and max economical)
3.Process side allowable pressure drop (which is quite crucial)
I hope this helps a little.

[Padmakar Katre]

Dear,
You can use HTFS+ inorder to make the different cases for simulation and designs/ratings and find the optimum one.