Discussion on the method of vacuuming the vacuum port of the vacuum condenser
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This post was last edited by sdfczhangkai on 2018-12-25 12:38 Dear Sichuan friends, in the vacuum distillation design work, we often encounter various methods of vacuuming after the condenser. Here I list two methods of vacuuming directly at the condenser (the gas phase balance of the receiving tank and other issues are not considered for the time being). Here we only discuss the feasibility of vapor-liquid separation and the actual effect in application. The first is the shell process of cooling water, and the material steam is condensed on the tube side (the material properties are special and the process requires that the material be transported on the tube side). The condenser is installed on a slope, and the vapor and liquid are separated at the right head. The condensate is directly discharged at the lower side of the head, and the upper side is directly evacuated, and is collected by cold hydrazine before entering the vacuum pump. The second is a common cooling water route through the pipe side. The material vapor is condensed on the shell side, and the condensate is discharged on the lower side of the shell side. The tail gas port on the upper side of the shell side is directly evacuated, and is collected by cold hydrazine before entering the vacuum pump. Now I want to discuss with you, how effective are these two vacuuming methods in vapor-liquid separation in practical applications? If the vapor-liquid separation is not good, how can it be improved? ———————————————————————————————————————————————————————————————— Updated December 25: Today we will continue to discuss with you the vacuum condensation and vapor-liquid separation of vertical condensers. The figure below also compares the above four vacuum condensers. Which one should be used under which working conditions? Is it better to use tube-side condensation for high vacuum 2-3mmHg? Have you ever used horizontal tube-side condensation?
The first type of vapor-liquid separation is mainly at the pipe box, and the vapor-liquid separation space is relatively small.； The second type has a larger vapor-liquid separation space and a better effect.
Thanks for the answer. We use the first one a lot in practical applications and feel that the effect is pretty good. ； The second method has never been used. Recently, I proposed to use the second method in our company. It was opposed by many people who have never done design. These people subjectively think it is not feasible. However, through analysis and data review, I found that this vapor-liquid separation effect is very good, so I would like to open a special post here to listen to everyone's opinions.
It should be fine. The first type is that the cooling water flows through the shell. Once it is scaled, it is difficult to clean. ； Second, during the shelling process of the material, attention should be paid to the selection of the baffle spacing and form, and the pressure difference of the material passing through the condenser should be controlled within a certain range.
The first type is better. The right side is equivalent to a gas-liquid separation chamber, while the second type requires multiple baffles on the shell side to prevent the material vapor from being directly pumped away by the vacuum pump. However, the baffles will form liquid accumulation and reduce the condensation effect.
Personally, I prefer the first clean medium with high steam pressure, which is suitable for piping.
If the two solutions use the same separation device, then the key to the final recovery rate is the effect of the heat exchange equipment. In this type of condensation condition before the vacuum pump, the gas flow rate is very high. Whether the conventional heat transfer element is on the shell side or the tube side, under the premise of reasonable design, even if the mixed gas outlet temperature reaches the outlet temperature of the process design, the separation device will separate most of the droplets entrained by the gas. However, the high flow rate gas at the outlet will still take away some droplets that will not be condensed in the future. Therefore, there is a certain gap between the recovery rate calculated in theory and the actual equipment operation indicators. If your company's products require running inside the pipe, then abandon the second option, because if the heat exchange equipment is the same, placing the separation device at the rear will not bring significant changes. It is recommended to start with heat exchange equipment. At this time, the heat transfer between the gas side of the tube and the cold water side is the thermal resistance side that needs to be solved. Taking into account the distribution of air flow and improving the droplet condensation effect, it is recommended to use plug-in heat transfer elements. For example, the pre-pump solvent recovery devices I make for some pharmaceutical industries mostly use internal micro-spiral fin condensers, and the recovery rate can reach 99%. For some information, please refer to my signature file forum link for some introduction.
This post was last posted by symc on 2018-12-5 18:26 Editor "...For example, the pre-pump solvent recovery devices I make for some pharmaceutical industries mostly use internal micro-spiral fin condensers, and the recovery rate can reach 99%." This also violates the physical and chemical properties of the medium and the leakage of non-condensable gas...:L, alas... For example, let's not talk about the recovery of methylene chloride before the vacuum pump. Let's calculate it based on the recovery of C. If it reaches 90% before the vacuum pump, I will buy it in bulk!
There is no doubt that the following one is better. The good thing is in two places. First, the heat transfer effect is better because it can have smaller subcooling. Second, the separation effect will be much better than the above one, which is determined by the flow rate. Third, the condensable gas on the tube side will form a liquid film on the inner wall of the tube side, which not only reduces the overall heat transfer coefficient, but also increases the degree of subcooling. Check out the HEI surface condenser standards to take a look.
In addition, in addition to the traditional shell and tube condenser, this conceptual condenser can be said to have one ugly thing - pressure drop! The pressure drop under vacuum will have a great impact on the selection of vacuum equipment, including the process. If you haven't seen it, look through the condenser standards. Which standard uses various other heat exchange types?
Nowadays, the first type is the most widely used. The second type must consider the form and setting accuracy of the baffle to prevent the material vapor from short-circuiting.