The difference between simulated and actual flow rates?
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When performing distillation simulations, since volume flowmeters are used in practice, I chose to use volume flow rates in my simulations as well; however, the results obtained were quite strange. When using Proii for simulation, the mass balance is maintained when the inlet and outlet material flows of the distillation tower are calculated based on volume flow rates. However, this balance is not maintained in actual operations (although the liquid level in the distillation tower and the reflux tank remains constant). Why does this happen? ?
Simulations are generally carried out based on the conservation of mass; if this principle is not respected, the calculations cannot converge. Of course, if the conditions remain largely unchanged and mass is conserved, volume may also be conserved. But it’s hard to clarify on the spot; whether the measurement itself is accurate is an issue, and likewise, whether the operation is stable. By how much do you say the conservation is not satisfied?
It’s 0.2–0.3 m³/h short; according to the mass conservation principle, there’s density involved in the volume flow rate at the site, so making the conversion is quite troublesome
As for the percentage, it depends on the specific situation – was less output generated, or more? The key is whether the measurements in all aspects are accurate. And have you calculated a long enough time? If the operation is relatively stable, extending the measurement time may reduce errors.
What do you mean by measurement time? Of course, it refers to the traffic after the system becomes stable, not at a specific point in time. For example, I supply 1.7 m³; 1.45 m³ is recovered at the top of the tower, and theoretically 0.25 m³ should be recovered at the bottom. In reality, only 0.08 m³ is recovered. It’s strange because the liquid level in the tower doesn’t increase either – where did the rest go? ?
How can volume be conserved? If this is conserved, it’s basically just by coincidence; the temperature, pressure, and composition of the feed differ from those at the top and bottom of the tower, so volume conservation cannot be used for judgment – it’s mass conservation that matters here. When calculating based on on-site data, errors are bound to occur; in such cases, calculations can be done using the composition and conditions of the feed, with the simulated output compared to the on-site data
The calculation is steady-state, but the actual tower in operation is necessarily dynamic; at any given moment, the mass of the materials present on site may not even be conserved. One can look at the average flow rate over a period of time – it should be the mass flow rate, as volume flow rate is not conserved.
The main component of the feed material entering my tower is at a concentration of over 99.99%, so the remaining trace impurities are present in ppm levels. Moreover, their density is not very different from that of the main component. Therefore, can’t we apply the principle of volume flow conservation to this tower? But in reality, it is not conserved
Even if the feed is very pure, the temperature of the output material is not necessarily the same as that of the feed; the density changes, and therefore volume is not conserved!
We also need to consider the accuracy of the flow meter; a slight deviation is quite normal.
I firmly support this post – let’s boost it again!