Residence time downcomer

A time of at least 3 seconds is recommended. The downcomer residence time is given by ... 

The downcomer area required for trays not only increases with the liquid-flow-rate, but also with the difficulty in achieving separation between the vapour and the liquid phases. The volume required for the downcomer (downcomer residence time) increases at a lower surface tension and a smaller density difference between vapour and liquid. Because of the large downcomer area required to handle the high liquid flow rates typical of high-pressure distillations, a trayed column cross-sectional area may be 40% to 80% greater than the active tray area calculated from the vapour flow rates for such distillations. Thus, the downcomer area becomes a significant factor in the determination of the diameter of a tray column. 

Another criterion sometimes used is to provide sufficient residence time in the downcomer to allow adequate disengagement of gas from the descending liquid, so that the liquid is relatively gas-free by the time it enters the tray below. Inadequate removal of gas from the liquid may choke the downcomer. Kister (loc. cit.) reviewed various published criteria for downcomer residence times and recommended those by Bolles (private communication, 1977) and Erbar and Maddox (Maddox, Process Engineer s Absorption Pocket Handbook, Gulf Publishing, Houston, 1985). Both sets of guidelines are similar and are summarized in Table 14-8. The residence times in Table 14-8 are apparent residence times, defined as the ratio of the total downcomer volume to the clear liquid flow in the downcomer. 

Figure .i3 Values of downcomer residence time (a to be used in the Koch correlation and in the Nutter correlation for maximum downcomer velocity [Eq. (6.26)]. Data based on Koch Engineering Co. Inc. Design Manual-Flexitray, Bulletin 960-1, Wichita, Kansas, 1962 and on Nutter Engineering Float Valve Design Manual, Tulsa, Oklahoma, 1976. 

Two different definitions are used for downcomer residence time (3]. 

The apparent residence time is the ratio of the downcomer volume to the clear liquid flow in the downcomer, The downcomer volume is the tray spacing times the average downcomer cross-section area. The true residence time is the ratio of froth volume in the downcomer to the frothy liquid flow in the downcomer. The true residence time can alternatively be expressed as the ratio of the clear liquid volume in the downcomer to the clear liquid flow. The definition adopted here is that of the apparent downcomer residence time. The author found this definition to be better and to be consistent with riublished criteria for downcomer residence times (1]. 

Kister (1) reviewed various published criteria available for downcomer residence times, and recommended those by Bolles (47) and Erbar and Maddox (49). Both sets of guidelines are similar, and express downcomer residence time as a function of the system s tendency to foam (Table 6.6). 

Downcomer residence time. This check calculates the apparent residence time in the downcomer and ensures it is within the recommended guidelines in Sec. 6.2.9. 

Apparent downcomer residence time, s 3. Operating regime 8 Emulsion 6 6... 

Using Table 7-4 to obtain both the foam factor (Foam fac) and the downcomer residence time, x, the downcomer area, is determined by... 

D3. The calculations in Example 10-3 were done for conditions at the top of the column. Physical properties will vary throughout the column, but columns are normally constructed with identical trays, downcomers, weirs, etc., on every stage (this is sinpler and cheaper). For a 12-foot diameter column, calculate entrainment, pressure drops, downcomer residence time, and weeping at the bottom of the column. The results of Problem 10.D2 are required. If the column will not operate, will it work if the gap between the tray and downcomer apron is increased to 1.5 inches ... 

Minimum Downcomer Residence Time Equation (12.47) can be converted to minimum residence time as... 

Pm valve metal density Ib/ft a surface tension dyne/in. fji liquid viscosity cP Tr downcomer residence time s (second) downcomer froth density dimensionless (p[ froth density dimensionless (pt relative froth density dimensionless valve open percentage % rjM Murphree tray efficiency dimensionless r]o column overall efficiency dimensionless... 

If a trayed column in high pressure service does not have sufficient downcomer residence time to clarify the liquid phase, the vapor/liquid mixture will be carried down to the next lower tray. This recycling of vapor lowers the capacity, as well as the separation efficiency, of the trays. Conditions preventing rapid disengagement of vapor and liquid in a tray downcomer are high liquid viscosity, low surface tension, and a small density difference between liquid and vapor. Also, any tendency for the liquid to foam will increase the downcomer residence time required. 

A depropanizer normally contains 35 to 55 actual trays and has a uniform diameter. In these systems, the surface tension of the liquid phase is below 6 dyne/cm, the liquid density is near 30 Ib/ft, and the vapor density is about 8% of the liquid density. Under such conditions, the downcomer residence time required can be a significant factor in the specification of the tower diameter for a trayed column. The downcomer normally is designed so that the froth height is no more than 70% of the tray spacing therefore, downcomer area must be large to avoid the need for excessively tall columns. 

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