Last week, we were tasked to do a homework re: heat exchange networks. The problem is stated for your reference:
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 condenser from the second column is passed through a product cooler. The duty for each stream is summarized below:
The minimum approach temperature used was 10degC.
The temperature was divided into 10 segments as shown in the table below. From here, the total heating and cooling loads were calculated and the pinch point was determined to be at 70degC with external heating load of 2300kW and external cooling load of 1600 kW. The solution is shown in the figures below. The respective CP’s were calculated using the equation Q=CP*(deltaT).
Now we need to determine which streams are going to be paired with each other, knowing the pinch point temperature and the external heating and cooling loads.
Above the pinch (T>70degC), we only have stream 4 and 5, which cannot be paired together since there is not overlap in temperatures and since both are cold streams. Thus, an external heating load is calculated using Q=CP*(deltaT) to get 2300 kW required external heating, which is what we were able to compute earlier. Below the pinch, there are 4 streams – 1, 2, 3, and 6 which correspond to C1, H1, H2, and H3, respectively. Below the pinch the number (and heat capacity) of cold streams must be less than or equal to the number (and heat capacity) of hot streams. Both of these conditions are satisfied so there is no need to split the streams. The cold stream, C1, can be paired with either stream 2 or 3 (H1 or H2). Pairing it with H2 will lower the H2 temperature to ~56.8degC and will need external cooling to lower the temperature further down to 55 degC. Adding the external cooling required for H1 and H3 will result in a total of 1600 kW external cooling load required. Which was what we calculated. A similar calculation can be done by pairing C1 with H1. The same external cooling would also be calculated. Now to select which of these we should use, we must take into consideration the costing and other trade-offs of equipment design. That is another matter which we may possibly discuss in future blog entries. 😉
Engineering Perspectives 