(4d) Can 2-Pass Moving Valve Trays Experience Multiple Hydraulic Steady States?
- Conference: AIChE Spring Meeting and Global Congress on Process Safety
- Year: 2017
- Proceeding: 2017 Spring Meeting and 13th Global Congress on Process Safety
- Group: Kister Distillation Symposium
- Time: Monday, March 27, 2017 - 11:20am-11:50am
The maldistribution described in this paper is not due to poor distribution of vapor or liquid to the trayed section from an inlet, and it is also not caused by differences in tray geometry between one side and the other, though we have the capability to evaluate these situations.
The cause of the maldistribution is that at turndown, the valves close, and while they are closing, the dry pressure drop remains constant regardless of the vapor rate. This makes it easy for the vapor to swing from one pass to another, forming a number of alternative steady states.
Using our model, we investigated the effect of uniform weeping on the steady state multiplicity. While shifting the steady state some, uniform weeping does not largely impact the steady states. We also looked at the effect of weir loads on the steady state multiplicity. The multiplicity appears to exist both at the lower and higher weir loads. Our findings are that this is a vapor-driven phenomenon with little effect on or influence by liquid.
One strategy that appears to alleviate the effects of the multiplicity issue is using valves of different weight. This is a common practice used by valve tray manufacturers. Our study shows that this practice can significantly lessen the severity of maldistribution at turndown, but can also cause a wider range of loadings to experience some maldistribution.
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