Sieve-plate columns valve trays

The chemical engineering undergraduate spends most of his time sizing equipment. Usually in the problems assigned the type of equipment to be used is specified. For a distillation column the student would be told whether it is a bubble cap, a sieve plate, a valve tray, a packed column, or something else, and then asked to size it for a desired separation. In other cases he would be given the size of the specific equipment and asked to determine what the output would be for a given input. 

The basic components of a plate distillation column include a feed line, feed tray, rectifying or enriching section, stripping section, downcomer, reflux line, energy-balance system, overhead cooling system, condenser, preheater, reboiler, accumulator, feed tank, product tanks, bottom line, top line, side stream, and an advanced instrument control system. Plate columns hold trays that may be bubble-cap, valve, or sieve. Figure 6-19 shows the basic components of a plate distillation column. 

In plate columns the two phases are intensively mixed on each plate and separated between each plate (Fig. 6.7-5). For the distribution of the light phase through the liquid a lot of devices were developed. The simplest one is a perforated sieve tray, where the supercritical phase can pass through. To avoid weeping of the liquid through the holes different devices like bubble caps or valves (Fig. 6.7-6) were developed. 

The symbol used on a diagram for a plate column should indicate the type of tray used in the system bubble-cap, valve, or sieve. The first distillation column was invented in 1917. Today, a number of modifications allow modern process technicians to operate much more efficiently. The design, however, still includes the original still-on-top-of-a-still approach. The basic components of a plate distillation column are a feed line feed tray stripping section below the feed line enriching or rectifying section above the feed line overhead vapor outlet, side-stream outlet, and bottom outlet reboiler instrumentation for level, temperature, flow, pressure, and composition control outer shell and a top reflux line. 

Industrial absorption (and also distillation) columns are equipped either with trays, located about every 0.3-1 m up the column, or packed columns are used. The most widely used trays are sieve or valve trays. Sieve trays are simple flat plates with a large number of holes. The gas phase flows up through the holes, preventing the liquid from falling through. The liquid phase flows across each tray, passes over a weir, and drops into a downcomer that provides liquid for the tray below. Valve trays are built with a cap that fits over the hole in the tray and that can move up and down, providing more or less space as the gas flow rate changes. 

The vast majority of industrial distillation columns are equipped with trays or plates (sometimes called decks in the petroleum industry) located every 1-3 feet up the column. These trays promote mass transfer of light components into the vapor flowing up the column and of heavy components into the liquid flowing down the column. Vapor-liquid contacting is achieved by a variety of devices. The most widely used trays in recent years have been sieve trays and valve trays because of their simplicity and low cost. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

A number of different constructions have been developed for tray columns the essential ones are still bubble-cap, valve, sieve and grating plates. Bubble-cap plates are the oldest development and, due to the high production costs, they are rarely used today. 

The single-pass sieve tray with crossflow appears to be the most widely used type of tray today. In a tray of this type, the vapor passes upward through the perforations (or valves) and then through the liquid on the tray. The liquid flows down through the column and enters each plate by flowing under the downcomer weir. Then the liquid flows across the plate and over the outlet weir and into the downcomer to the plate below as shown in Fig. 12-3. In order for the liquid to flow across each plate in succession as it passes down through the column, a liquid head in each downcomer is required as depicted in Fig. 12-3. 

Perforated plates such as sieve trays used in absorption, distillation or extraction columns. The holes can be covered by caps or valves to avoid weeping in the range of low superficial gas or vapor velocities. The two phases are moving in a crossflow on a tray. 

Sieve, valve, or plate trays, the tray efficiency decreases as the viscosity of the liquid (at column conditions) increases at 0.2 mPa s, tray efficiency = 60% at 1 mPa s, tray efficiency =16% (F). 

Perforated plates that fit into bubble columns like sieve trays are still used in cocurrent bubble columns (11). Usually the resultant free area of the openings is very small, i.e. 1 - 5 % of the total plate area. These plates may totaly suppress backmixing by forming a gas buffer beyond each plate which prevents liquid backflow. Such plates cannot be used in countercurrent flow in as much as the required pressure drop through the holes prevents the liquid downflow. Several possibilities have been suggested for the utilization of perforated plates in countercurrent bubble columns, i.e. special kinds of downcomers with or without control valves (12) (13) or additional pulsing... 

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