Distillation is one of the most widely used liquid separation techniques in the chemical industry. It typically separates two or more liquids with varying boiling points into different compositions. Distillation columns employ multiple stages or physical trays to assist in the separation of liquids. These trays distribute the falling liquid and rising vapour, creating large surface areas for contact with heat.
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Bubble Cap Trays
A bubble cap distillation tray is the most expensive and complex tray design. There is a gas-liquid contact element called a movable bubble cap. A liquid level is maintained on the top of the tray and as the hot vapour from underneath the tray pushed through the bubble cap, it contacts the liquid in a two-phase manner. The produces a high pressure drop but increases the liquid entrainment in the vapour. The bubble cap tray has stable operation and high flexibility.
Sieve trays are the most commonly used distillation tray design in the industry. It is simple, versatile, inexpensive and has high capacities. The tray features a perforated metal plate for the vapour to pass through. A weir holds a liquid level on the tray and downcomers allow any liquid overflow to pass to the tray below. Typically, there is a dual flow arrangement from tray to tray. The sieve tray is effective over a large range of flows and does not foul easily.
Valve trays are a type of distillation column tray that has design characteristics of the bubble cap and sieve trays. There are fixed holes in a valve tray similar to sieve trays but there can be a fixed vent on top of the holes. This vent allows vapour to pass through and contact the liquid level with high surface area. An variation to this design is a movable valve that allows the vapour to push the valve flow area open and contact the liquid similar to the bubble cap design. Valve trays do not foul easily and have a low cost.
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