Design related to the downcomer structure of plate towers

This post was last edited by springflower on 2015-8-10 21:59. I don’t know what you’re talking about. It was the first time I saw someone understand the outlet flow velocity of this downcomer in this way. . . m3/h/m2 = m/h. . . It can’t be said to be your fault, but generally, design isn’t considered in this way. . The liquid level in the downcomer indeed determines the outlet flow rate. . But it’s both a fixed value and something that needs to be implemented – I was completely stunned when I saw that. The outlet flow velocity of this downcomer generally needs to be given more attention only in foamy systems or when a dynamic seal cannot be maintained at extremely low flow rates. Then the range of values for this parameter is quite wide, and the method of adjustment is extremely simple; therefore, it is generally not set to a fixed value – as long as the upper and lower limits are not exceeded, it is acceptable. 1. This is called the bottom gap of the downcomer; it is generally set at 1/10 of the plate spacing. Determine the downcomer residence time, the outlet flow velocity of the downcomer, and the assist at the downcomer outlet. In the following situations, reducing the height is applicable: when there is a lot of foam, to increase the residence time and allow the liquid to settle. There is little liquid; keep it sealed. By increasing the height, with a larger volume of liquid and less foam, the height of the liquid layer in the liquid pipe can be reduced. The outlet flow rate is high. If that still doesn’t work, add an inlet weir; if you find it troublesome, use a concave liquid receiving tank. 2. In terms of dynamics, the liquid layer cannot be too thin so as to lack a dynamic seal, nor can the calculated foam layer accumulate up to the upper layer. In fact, the dwell time also needs to be considered – dwell time, dwell time. Say important things three times. Considerations of power and resistance are truly secondary. Generally, what is 20%~50% of the board spacing height? How is it calculated? There are many formulas; please refer to the *Chemical Engineering Principles course design. 3. It’s difficult to calculate, but the downcomer is there to ensure that the foam layer turns into a clear liquid that flows downward; I don’t care about how the layers are structured or what the height ratio is. All that matters to me is that the foam layer doesn’t cover the upper trays, and that the liquid flowing out is clear.

First of all, thank you for typing so much; you’ve worked hard. What I meant in the first paragraph is that I thought the width of the downcomer, or the length of the overflow weir, was determined by the flow velocity of the liquid within the downcomer. Since, according to the law of conservation of mass, the flow rate of liquid entering the downcomer remains constant, it is sufficient to determine the flow velocity of the liquid in order to find the cross-sectional area of the downcomer. But then I suddenly realized that the length of the overflow weir is usually determined by the intensity of the fluid flow; in other words, the flow velocity of the liquid in the downcomer is merely a reference for calculations and not a particularly important design parameter Furthermore, the residence time of the fluid in the downcomer is very important; therefore, I would like to know what heights are required to achieve gas-liquid separation within the downcomer for systems with no foaming, mild foaming, and severe foaming, respectively You also mentioned that the gap at the bottom of the downcomer is usually an empirical value, and this value is increased only when the flow rate is too low and liquid sealing may not be possible. In other words, the residence time can be determined primarily from the perspective of gas-liquid separation in the downcomer. But how can we avoid the situation where a too small gap at the bottom of the downcomer leads to an excessive liquid layer, thereby causing flooding in the downcomer (i.e., excessive resistance in the downcomer)? Thank you again for your code words^_^

Think of it this way. Do you still remember the problem in middle and elementary school involving a tank that was both releasing water and collecting it? This is a variation of this problem: once all the dimensions are determined, it is assumed that there is little resistance at the liquid inlet. But since the outlet of the liquid discharge tube is relatively small, isn’t there some resistance there? How can this obstacle be overcome? That can only be overcome by the liquid level height; of course, the liquid level height is also determined in part by the tray pressure drop. No one reduces the clearance right from the start, causing the liquid level to be too high. Overflowing caused by an excessively small bottom clearance generally only occurs due to design errors or excessive deviations in the operating liquid level. Even if you increase the bottom clearance when there is more liquid, overflow will still occur because your downcomer can no longer hold it. There is no strong correlation between height and liquid-gas separation; with the same amount of liquid, if I make the downcomer larger, will the liquid level be lower? If I make my downcomer smaller, will the liquid level be higher? Design liquid layer height for A is 300 mm, and for B it is 400 mm. But whichever it is, a design that has sufficient dwell time is a qualified design. Assuming the bottom gap height is 20 mm, the initial design is based on a minimum liquid seal. Then I’ll tell you that the liquid level in this layer of trays can vary by a very large amount. Just how large? The maximum liquid volume could be 20 times the minimum liquid volume. Well, then I’ll adjust the bottom clearance height; 60mm should work, to accommodate the maximum amount of liquid. After all, the lack of a liquid seal only leads to lower efficiency; preventing flooding is still the top priority. If the clearance isn’t sufficient, just adjust it. You still remember the several lines of the tower design, right? Within these lines, the problem you mentioned generally does not occur; if it does, there must be a significant deviation, and it would be better to consult a professional. The design should not go to extremes; it’s best to keep it in the middle ground. None of the parameters within it have fixed values – only ranges are specified. A good design is one in which all parameters fall within these specified ranges. If it really can’t be guaranteed, then decide what’s the most important. The bottom gap of the downcomer results in a relatively low level of pressure drop at the outlet. For domestic exchanges, it is sufficient to ensure three parameters: the valve orifice kinetic energy factor, the downcomer residence time, and the opening ratio. Other tweaks are just icing on the cake; try to include them if possible, but it’s fine if not.

Thank you. By the way, I have a side question: are jet trays really that excellent as people say? What are the limitations in usage? It talks about high flux, high operational flexibility, low pressure drop, and high efficiency – it has all the good features

All products are carefully selected. The advantages of vertical spray towers are obvious, but they also have drawbacks; for example, they are expensive, can only be used in conditions with a high gas-liquid ratio, and there is a higher likelihood of leakage in towers with insufficient gas flow. No single product can suit all situations, and they are also not suitable for liquids with high viscosity or containing particles. In the case of highly foamy systems, problems can arise if the design is not proper.

Thank you. According to his principle, if the liquid volume is very small while the gas volume is large, could that also cause the gas phase to pass through the tower?

That’s not the case; when designing it, it’s necessary to keep the gas velocity within a reasonable range.