Hello
As a process engineer working on the design of a Crude Distillation Unit (CDU), designing the preheat train can be tricky. This involves transferring heat from hot streams to cold feed streams before sending the feed to the distillation column.
On one hand, we want efficient energy integration, reusing as much heat as possible within the system. This shall lead to reducing the required duty of the fired heater before introducing the crude to the distillation column. In addition, it shall reduce the required cooling water to cool down the products before sending them to storage.
However, taking this too far can mean the temperature difference between the hot and cold streams is too small.
For example, if we have a hot stream is 180°C and we want to cool it to 150 C using a cold stream at inlet temperature of 170°C (stream A), this means a low driving force, which can lead to a significantly oversized heat exchanger.
On the other hand, if we used another stream B at 120 C, this would lead to much more efficient heat transfer, which means a much smaller heat exchanger.
If a stream has a significantly lower flow, which means that cannot fulfill the duty to cool the hot stream. We may use Stream A at 170 C inlet then use Stream B at 120 C and yield two exchangers which have a smaller size than using a very large exchanger with multiple shells using Stream A only.
Remember:
Q = U A (MTD)
If MTD is too small, this shall lead to a very large A, which is the heat transfer area, or in other words the dimensions of the exchanger, which can even lead to a multishell exchanger if we are requiring high area.
The key is optimizing the design. We aim for maximum energy reuse without impractical heat exchanger sizes. This requires a deep understanding of heat transfer, fluid flow and costs. Using simulation tools like HexTran or Aspen Energy Analyzer in addition to validating the design using HTRI is necessary to get the required temperatures and specifying the duty of each exchanger that would lead to efficient energy integration along with reasonable exchanger sizes.
In the article "Understand Thermal Design Aspects of Shell & Tube Heat Exchangers", we have gone through the important considerations that we should take care of when sizing a shell and tube exchanger. You can check it out by clicking on the below button:
What about you ? Have you carried out thermal design of a shell and tube exchanger? Wishing to know more about that. Just hit a reply to this email!
Regards
Mohamad
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