Method for reboiler and condenser sizing using HYSYS or UniSim
In its distillation calculations, HYSYS/UniSim calculates the duty of reboilers & condensers, but not the heat exchange area A or the utility requirements; for capital cost estimation A is needed, and the utility usage may be needed for operating cost estimation. Below we describe a method to simulate condensers and reboilers using the heat exchanger unit in HYSYS/UniSim. However, the problem with the heat exchanger unit in HYSYS/UniSim is that it can handle only one product stream, so you can't have a partial condenser or partial reboiler. In such a case, you must first combine the product streams for the heat exchanger calculations. First read the notes at the end.
Heuristics (rules-of-thumb) are cited in several places below. See those listed at Heat Exchangers.
1. Open your converged case-study pfd. Click on Tools, PFDs and select a distillation column to view.
2. Click on Column Environment.
3. Double click on the condenser. (The size and dimensions of the condenser given at the Rating tab are meaningless defaults. Do not use them.) Through the Worksheet tab, write down or print the T, P, molar flow, and composition mole fractions for the To Condenser Stream. Write down the T and P for any of the streams exiting the condenser (they are all the same). The Heat Flow shown for the coolant is the duty of the condenser. From the Worksheet/Properties page write down the Mass Density and the Actual Volumetric Flow of the reflux stream, as you will need these to estimate the power required by the reflux pump.
4. In HYSYS/UniSim, preferably in a new case, create a new stream with the T, P, molar flow and mole fractions for the To Condenser Stream. It should now converge. Close the stream Worksheet, right-click on the stream, Cut/Paste Objects, Clone Selected Object. This will create a new identical stream. Double-click on the new stream, name it From Condenser and change its T and P to that found in 3. This should converge, with its vapour fraction being the fraction of the overhead that is a gas.
5. Create a heat exchanger and connect its tube side to To Condenser and From Condenser.
6. Select an appropriate cooling fluid (see the heuristics cited above and the important notes below). Open the Heat Exchanger Design/Connections page and enter the information for the coolant on the shell side.
7. On the Design/Parameters page enter the appropriate pressure drop for the coolant from the heuristics cited above. Select Exchanger Design (Weighted) for the Heat Exchanger Model. Increase the number of intervals for the Individual Heat Curve Details calculations.
8. On the Worksheet/Compositions page enter the composition of the coolant.
9. On the Worksheet/Conditions page enter reasonable inlet and outlet conditions for the coolant stream (per heuristics). If everything's okay, the heat exchanger calculations will converge.
10. On Performance/Details make certain the minimum approach does not violate heuristics. Write down UA (which is actually q/DTaverage). Click on Performance Plots. Change the X-axis to Heat Flow and the Y-axis to T. If the curves do not cross, print out this plot. If they do cross, make appropriate changes to the coolant conditions.
11. Print out the Performance Details and the Worksheet Conditions for the heat exchanger.
12. Calculate A by dividing the UA in step 11 by suitable values of U and F. Submit these calculations with the above material that you printed out. Indicate the source(s) of your U and F values. (See e below.)
13. Repeat for the reboiler. If the temperature of the bottoms stream exceeds that which can be heated by steam (heuristics), then assume a fired heater will be used. To cost this, you will need will need only the reboiler duty. Alternately, you could use a high-temperature heat-transfer fluid (see http://people.clarkson.edu/~wilcox/Design/HeatExch.htm). In that case, you’d have to add a circulating loop to heat this fluid.
a. When steam is used for heating the reboiler, the entering steam must be specified as all vapor (vapor fraction 1), and either P or T but not both. The exiting water should be specified as all liquid (vapor fraction 0) at a P slightly lower than the entering P (per heuristics). The flow rate should not be specified, as HYSYS/UniSim will calculate it from the heat required by the process stream and the change in specific enthalpy of the steam/water. Note that such heat exchangers normally allow only water to exit, so they are self regulating (just like a steam radiator for heating a room). See utility heuristics to select the steam pressure or temperature. You are not free to choose any old value you want. Since the maximum T for high pressure steam is 254oC, you cannot use steam to heat above 244oC. (CAPCOST accepts steam temperatures of 160oC, 184oC and 254oC. For higher T, CAPCOST accepts "Thermal Systems" with Ts of 330oC, 400oC and 60oC.)
b. When a condenser is cooled by water going to steam, the entering boiler feed water should be specified as all liquid at a specified P or T but not both. The exiting steam should be specified as all vapor at a slightly lower P. See heuristics to select the pressure or temperature. You are not free to choose any old value you want.
c. When a condenser is cooled by a refrigerant, such as ethane or ammonia, the entering refrigerant should be specified as all liquid at a high pressure and the exiting refrigerant as all vapor at a much lower pressure. (See, for example, Perry's 7th page 11-77.)
d. When cooling water is used for a condenser, specify T and P, but not vapor fraction. If you specify a vapor fraction of 0, you are telling HYSYS/UniSim that the water is saturated (would be in equilibrium with steam) and HYSYS/UniSim will give you a ridiculously low P. Note that the heuristics for CW indicate it should enter the condenser at 30oC and exit no higher than 45oC. With a minimum approach temperature of 10oC, this means that you cannot cool the condenser below 40oC and you’ll have to use another coolant. For lower temperatures, CAPCOST gives you the choices of refrigerated water and refrigeration to 5oC, -20oC and -50oC.
e. Heating and cooling fluids included in the database are given at http://people.clarkson.edu/~wilcox/Design/therfld.xls. If a Fluid Package Basis has not already been created for a desired fluid, go to the Basis Environment and create it now. Alternately, you could calculate log-mean-delta T as an estimate of the more exact value that HYSYS/UniSim would calculate by the weighted method.
f. We have assumed that the flow is truly countercurrent (which is not true for shell-and-tube exchangers). To be more accurate, to get A you need to divide q/UDTaverage by the correction factor F for non-countercurrent flow, which can be obtained from graphs given in Perry's, Walas and Seider et al. Since F<1 and sometimes even <<1, this will give you a larger value for A.
g. For a condenser operating under a vacuum (below 1 atm), read http://people.clarkson.edu/~wilcox/Design/condvac.pdf.
h. UniSim also offers the possibility of doing a detailed design of a heat exchanger, assuming that the UniSim Heat Exchangers modelers have also been installed along with UniSim Design. Select this method on the Design/Parameters page, and then use the corresponding tab. Alternately, you can open the particular heat exchanger modeler separately.
Last revised June 1, 2009. Please email questions, comments and suggestions to W.R. Wilcox.