The steam flow rate to the reboiler never reaches the normal value; please help identify the cause.
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In the distillation workshop where I work, there is a reboiler that uses 4 kilograms of steam to heat silicon tetrachloride; it has a fixed tube sheet design, a double tube sheet structure, with steam flowing in the shell side and silicon tetrachloride flowing in the tube side. It operated normally at the beginning of its operation, but within a few months it was found that the steam flow rate never reached the desired level, as a result of which the liquid level in the bottom of the distillation column often rose to full capacity. As a solution, it was necessary to reduce the reflux rate, but this meant that the quality of the product could no longer be guaranteed. Initially, the workshop supervisor thought that the steam trap for the return water from saturated steam condensation was clogged, but replacing it with several new ones did not solve the problem. A bypass was also added to the steam trap, yet nothing worked even with the valve fully open. The steam pressure, as indicated by the gauge on the steam pressure regulator downstream of the reboiler, is fine at 4 kilograms. However, when the drain at the very beginning of the saturated steam and condensate return line is opened, what comes out is mostly steam, with very little water. I’ve been unable to figure out what the problem is, so I’m turning to everyone for help – could it be that the reboiler is blocked?
Possible cause analysis for reference: Scaling on the material flow side wall inside the heat exchanger tubes, resulting in a **reduction in the heat transfer coefficient**
What is the temperature of the reactor liquid? Is 4 kg of steam sufficient if heated to over 100 degrees? Additionally, silicon tetrachloride tends to form scabs inside the tube side, reducing the heat exchange area; it can be removed for cleaning.
Thank you both! The temperature at the bottom of the tower is usually around 104 degrees, and it often rises to 108 degrees as well. My thought is that if it’s scaling in the tube bank, the steam flow won’t be able to increase; I’m not sure if that’s correct.
Could you help check what the inlet and outlet temperatures for the shell and tube sides are in the design drawings? Being in the workshop, it’s easier to determine what the actual inlet and outlet temperatures of the tube side and shell side are
Q=K·A·△t. As Q→ and △t→, if K↓, then A must ↑. Of course, this does not solve the fundamental problem; it is necessary to find ways to reduce the frequency of scaling in the tube side, such as by using a forced-circulation reboiler system
I’m sorry, the drawings do not specify the inlet and outlet temperatures for the shell side and the tube side; only the design temperatures are indicated, which are 180°C for the shell side and 130°C for the tube side.
Main reasons: 1. Scaling on the inner wall of the pipe, resulting in a **reduction in the heat transfer coefficient**； 2. Severe scaling may lead to blockages, further reducing the heat exchange area ； 3. The design margin of the reboiler is not large. If the tube bank gets scaled, the evaporation rate is insufficient, and the steam flow cannot be increased. We often encounter this kind of fouling in reboilers; we start by gradually increasing the steam pressure and then stop the operation for cleaning.
Carefully test various parameters and conduct heat transfer analysis. Since it works fine when driving, it’s probably a problem with dirt in the pipeline.
Do the materials in the tube side tend to form scale when exposed to high temperatures? If so, you need to address the issue of scaling in the tube side.
The previous speakers mentioned issues such as scaling, blockages, and the designed heat exchange area. If everything functions properly every time the system is started up, there is still one detail to consider during operation: it is necessary to remove the non-condensable gases from the reboiler from time to time, as these gases can also affect the efficiency of heat exchange.