Regarding the effect of the reboiler circulation rate on steam consumption
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The tower operates under negative pressure, with an absolute pressure of 0.5 Kpa at the top of the tower and 1.5 Kpa at the bottom. The same quality of steam is used as the heat source. The material at the bottom of the tower contains 10% light components; to maintain this concentration, the material at its bubble point is fed into the reboiler. The flow rate of the pump at the bottom of the tower is increased, but the amount of vaporization must remain constant. Will the amount of steam consumed differ between low flow rates and high flow rates? 2. If an increase in flow rate is considered, resulting in a greater pressure difference, will the amount of steam consumed change under low-flow and high-flow conditions?
If this relates to design, then it’s a different matter; but when discussing operational issues, it’s best to follow the control parameters specified by the equipment. For reference.
Here, the heat provided by steam is primarily reflected in the heat of phase change of the material; other losses such as heat loss due to insulation are much smaller compared to the heat of phase change. At this point, your flow rate increases while the amount of vaporization remains unchanged, indicating that there is little change in the phase transition compared to before; therefore, the impact of the steam flow rate does not change significantly.
If we take into account that an increased circulation rate leads to a higher pressure difference inside the heat exchanger, but since the outlet is connected to the tower and the pressure remains constant, will the amount of steam used change?
In actual tower operation, it is usual to maintain a constant heating load on the reboiler, with the steam consumption remaining unchanged, and it is basically independent of the circulation rate of the reboiler.
It’s not that simple in reality. It is simply assumed that if the total amount of vaporization remains unchanged, the amount of steam will also remain unchanged. But in reality, when you adjust the flow rate of the pump, the vaporization rate changes. This change in vaporization rate affects the heat exchange efficiency of the heat exchanger. If the design margin of the reboiler is not large enough, there will not be sufficient heat exchange area, which leads to a decrease in the tower temperature. If the actual system adjusts the steam supply based on the temperature at the bottom of the tower, then more steam must be supplied, but the effect will not be significant.
None of the assumptions made by the original poster hold true. When the circulation rate increases, the heat transfer efficiency should theoretically improve, the amount of vaporization will increase, and the steam consumption will change as well； If the steam flow rate is restricted, increasing the circulation volume will not result in significant changes; this is a systematic issue that requires comprehensive consideration.
When the flow rate is high, the tower pressure increases and the pressure difference rises; as a result, the amount of steam increases before decreasing again. The bubble point drops, and the light components cannot be separated, which in turn leads to a decrease in the amount of steam. The opposite occurs when the flow rate is low