Inlet tray column

Reflux and Intermediate Feed Inlets for Tray Columns... 

This chapter examines common practices of introducing reflux and intermediate feed into tray columns, outlines the preferred practices, highlights the consequences of poor practices, and supplies guidelines for troubleshoot-ii and for reviewing designs of reflux and intermediate feed inlets. 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

A tray column s vapour feed will need an additional space between trays to accommodate the vapour main and so unrestricted flexibility cannot easily be provided. Nevertheless, a choice of two or three vapour feed inlets could be made available as long as the tray spacing is designed appropriately. 

Fig. 5.4-1 Perspective view of a tray column (a) downcomer, (b) tray support, (c) sieve trays, (d) man way, (e) outlet weir, (f) inlet weir, (g) side wall of downcomer, and (h) liquid seal...

Internal pipe distributors are frequently used for feed or reflux inlets for trayed columns or towers (fractionation or distillation columns). They are used to distribute the inlet stream to a particular point on a tray or uniformly across a tray. The distributor consists of one or more pipes, with or without branches, containing a series of holes, slots or spray nozzles. This procedure addresses the design of these distribution devices. 

In an olefins plant, the gas stream at a pressure of 140 to 180 psia is washed with a caustic soda solution to remove all acidic components present. Typically, the inlet gas contains 300 to 1,000 ppm of H2S plus CO2. The exit gas specifications will be 1 to 5 ppm by volume total acid gases in order to prevent freeze-up in the cold section fractionators. This caustic scrubber usually has been constructed with 30 to 50 total trays. The top three to five trays serve as a water-wash section to prevent caustic carry-over in the exit gas stream. The remaining trays constitute two or three pumparound sections. Fresh caustic solution is fed to the upper section along with the pumparound return liquid. Excess partially spent solution overflows a trap tray at the bottom of the section and goes to the next lower section where it is mixed with pumparound return liquid from the bottom of that section. The liquid effluent from the bottom of the column is a salt solution from which 65% to 75% of the sodium hydroxide has been consumed. The pressure drop through such a trayed column normally is 5 to 8 psi. 

A distillation column is operating at 27.5 inches mercury vacuum, referenced to a 30-inch barometer. This is the pressure at the inlet to the ejector. Due to pressure drop through a vapor condenser and trays of a distillation column, the column bottoms pressure is 23 inches vacu-... 

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. 

Holes for drainage must be adequate to drain the column in a reasonable time, yet not too large to interfere with tray action. Draining of the column through the trays is necessary before any internal maintenance can be started or before fluid services can be changed, when mixing is not desirable. The majority of holes are placed adjacent to the outlet or downcomer weir of the tray. However, some holes are placed in the downcomer inlet area or any suspected low point in the mechanical layout of the column. 

Solution Ideal gas behavior is a reasonable approximation for the feed stream. The inlet concentrations are 287mol/m of methane and 15mol/m of carbon dioxide. The column pressure drop is mainly due to the liquid head on the trays and will be negligible compared with 8 atm unless there are an enormous number of trays. Thus, the gas flow rate F will be approximately constant for the column as a whole. With fast reaction and a controlling gas-side resistance, c = 0. The gas-phase balance gives everything that is necessary to solve the problem ... 

Liquid inlets. Liquid enters the top tray via a hole in the column shell, often discharging against a vertical baffle or weir, or via a short, down-bending pipe (Fig. 17), or via a distributor. Restriction, excessive liquid velocities, and interference with tray action must be avoided, as these may lead to excessive entrainment, premature flooding, and even structural damage. Disperser units (e.g., perforations, values) must be absent in the liquid entrance area (Fig. 17) or excessive weeping may result. 

Liquid flow patterns. Liquid entering a single-pass tray flows in a diverging channel until reaching the tray centerline, then in a converging channel as the outlet weir is approached. The liquid has little incentive to move sideways and follow the curved walls of the column. Instead, it seeks the shortest path from inlet to outlet, and channels through the tray center (Fig. 7.75). This leaves stagnant zones near the curved walls on the side. 

The number of valve caps that can be fitted on a tray is at best an estimate unless a detailed tray layout is prepared. However, a standard has evolved for low- and moderate-pressure operations a 3 x 2 y2 in pattern that is the tightest arrangement available, accommodating about 14caps/ft2 (150 caps/m2). The active area does not take into account liquid-distribution areas at the inlet and outlet, nor edge losses due to support rings, nor unavailable space over tray-support beams. In smaller columns, it is possible that as much as 25 percent of the active tray area may not be available for functioning valves. For this column, which operates at low pressure, select the standard 3 x 2 /2 in pitch. 

Figure 10.4 gives results for 20 percent changes in the toluene recycle flowrate. The change in benzene production is the same as with the 18CF decrease in reactor inlet temperature. Now, however, the control tray temperature in the product column is disturbed about 53F, compared to 2 when reactor inlet temperature was changed. 

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