To produce a high purity product, two distillation columns are operated in series. The overhead stream from the first column is the feed to the second column. The overhead from the second column is the purified product. Both columns are conventional distillation columns fitted with reboilers and total condensers. The bottom products are passed to other processing units, which do not form part of this problem. The feed to the first column passes through a preheater. The condensate from the second column is passed through a product cooler. The duty for each stream is summarized as follows:

Find the minimum utility requirements for this process, for a minimum approach temperature of 10 °C.  

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No. 1 2 3 4 5 6 Stream Feed preheater First condenser Second condenser First reboiler Second reboiler Product cooler Type cold hot hot cold cold hot Source Temp. °C. 20 70 65 85 75 55 Target Temp. °C 50 60 55 87 77 25 Duty, kW 900 1350 1100 1400 900 30