Tray efficiency improvement

Sakata [180] evaluates the degree of mixing of the liquid as it flows across a tray and its effect on the tray efficiency, Figure 8-30. For plug flow the liquid flows across the tray with no mixing, while for partial or spot mixing as it flow s over the tray, an improved tray efficiency can be expected. For a completely mixed tray liquid, the point efficiency for a small element of the tray, Eog> tray efficiency, E V, are equal. 

Removing water from the lean oil. Installation of water draws and/ or a coalescer can improve recovery. Water can become trapped in the tower and cause poor tray efficiencies, foaming, and premature flooding. 

In order to improve tray efficiency, it will be necessary to increase the vapor velocity through the trays, so as to increase the pressure drop to at least 4 or 5 in of liquid per tray. If the reboiler duty were simply increased, the concentration of the heavy component—normal... 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

Weir Height Taller weirs raise the liquid level on the tray in the froth and emulsion regimes. This increases interfacial area and vapor contact time, which should theoretically enhance efficiency. In the spray regime, weir height affects neither liquid level nor efficiency. In distillation systems, the improvement of tray efficiency due to taller weirs is small, often marginal. 

Case studies were reported (170,174) of large-diameter (> 15-ft) towers with sieve trays not reaching the expected efficiency. Maldistribution was cited as the culprit or at least one of the causes. Improving liquid flow patterns, often among other modifications, was the fix. The only other evidence that channeling adversely affects tray efficiency comes from the above-mentioned theoretical models. 

An important reference often used for design is O Connell s correlation of tray efficiencies. This work is an improvement of an earlier publication by Drickamer and Bradford. In O Connell s correlation (Fig. 10), it is assumed that mass and heat transfer depend on diffusivities and liquid viscosity, and that both effects can be correlated by using the liquid viscosity and the component relative volatility for the separation. 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

O Connell derived his correlation from binary systems in distillation service with bubble-cap trays. Calculated values are slightly conservative for sieve and valve trays. Credit for the slight improvement in valve and sieve tray efficiency should be ignored and counted as a design margin. A separate correlation was developed for absorption services. 

Swept-back weirs (Fig. 6.76) are sometimes used at high liquid loads. They extend the weir length, which in turn lowers the effective liquid load (gallons per minute per inch of weir length), without changing tray or downcomer area. Swept-back weirs reduce tray pressure drop and downcomer backup, improve liquid distribution on the tray, and improve tray efficiency by inducing liquid flow into peripheral stagnant zones. However, the above improvements are usually small. 

The equilibrium-stage model can be made more realistic by including tray efficiencies (see Sect. 5.4.1.4) in the E-equations (5.2-68). Another possibility for improving column simulation is the use of the concept of transfer units (see Sect. 5.2.1.1) instead of the concept of equihbrium stages. This concept is well suited for packed columns. Such models, called rate-based models, simultaneously solve the relevant thermodynamic and mass transfer equations describing the complex mechanisms in a column (e.g., Taylor et al. 1994 Kloecker et al. 2005). Thus, rate-based models are much more complex since, for instance, the mass transfer coefficients and the interfacial area (see Sect. 5.4.1.4) must a priori be known. 

In distillation systems, the improvement of tray efficiency due to taller weirs is sm l (6), Koch Engineering (8), Ereis and R b (28), and Kalbassl et al. (184) observed little effect of weir height on distillation tray efficiency for weirs 1.5 to 3 in, 1 to 2 in, and 0,5 to 1 in tall, respectively. Finch and Van Winkle (185) reported an efficieniy increase of the order of 5 to 10 percent as weir height is raised from 1 to 3 in a similar increeise was reported by Prado and Fair (110,144) in humidification and stripping tests. 

Figure 49.4 shows the overhead system of the Coastal Refinery crude distillation tower in Aruba. The island of Aruba is a beautiful country in the Caribbean Sea off the coast of Venezuela. 1 often take my wife, Liz, on a romantic vacation to this tropical paradise. During one such exotic trip I was assigned by Coastal to find a plan to improve fractionation between naphtha and jet fuel. The problem was that the naphtha contained 20 percent jet fuel. A computer simulation of the crude tower indicated that the apparent tray efficiency of the upper five trays was... 

In Table 8-2 Proctor [178] compares efficiencies of sieve and bubble cap trays (plates). He concludes that the sieve design provides a 15% improvement in plate efficiencies. To fully evaluate the actual efficiencies in any particular system, the physical properties, mechanical details of the trays, and flow rates must be considered. See Reference 2 also. 

Nye Tray, 10-20% increased tray (over sieve or valve) capacity and good efficiency. More capacity from existing column. Improved inlet area for sieve or valve tray with greater area for vapor-liquid disengagement. 

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