Types Of Packing Used In Absorption 363xa

Types of Packing used in Absorption

A common apparatus used in gas absorption and certain other options is the packed tower . The device consists of a cylindrical column , or tower, equipped with a gas inlet and distributing space at the bottom ; a liquid inlet and distributor at the top; gas and liquid outlets at the top and bottom, respectively ; and a solid ed mass of inert solid shapes , called tower packing. The packing is typically a screen , corrugated to give it strength , with large open area so that flooding does not occur at the , The inlet liquid , which may be pure solvent or a dilute solution of solute in the solvent is usually a weak liquor distributed over the top of the packing by the distributor , uniformly wets the surfaces of packing. The packing provides a large surface area of between the liquid and the gas and encourages intimte between the phases. Various types of packings made of different types of materials of construction are available, and both random and structured packings are commonly used. Examples of random packings as shown in the Figures below – left and right - are Raschig rings, Pall rings, Berl saddles, etc.

Random vs. Stacked Random packings, as the name implied, are dumped into a column during installation and allowed to fall in random. Small packings poured randomly into a vessel is certainly the more popular and commonly employed form of packed-tower design. However, in certain instances where exceptionally low pressure drop and very high flow rates are involved, stacked or oriented packings have also been used. Dry packing avoids high hydrostatic liquid head, and prevents the introduction of water into a dry process. It is also quicker and less expensive than wet packing, and it minimizes rusting of metal packings. In any case, it is not suitable for plastic packings, as plastic typically floats on water. Wet packing applications are preferred when the packings are constructed of breakage-prone materials, such as ceramic or carbon. The column is first filled with water and the packings are gently poured down the column. The water cushions the fall and promotes randomness of settling. This tends to increase column capacity and improve the column pressure drop characteristics. Wet packing also minimizes compression and mechanical damage to packing materials. The main disadvantage is the need to remove the water after loading and dry the packings.

Structured Packing Structured packings are considerably more expensive per unit volume than random packings. They come with different sizes and are neatly stacked in the column. Structure packings usually offer less pressure drop and have higher efficiency and capacity than random packings.

Issues and Problems in Distillation Column Distillation tower turndown The loss of tray efficiency due to low vapor velocity – is commonly called turndown. It is the opposite of flooding, which is indicated by loss of tray efficiency, at high vapor velocity. To discriminate between flooding and weeping Ideally there is a smooth flow of liquid across a tray in a distillation tower and down the downcomer to the tray below. At the same time there is a smooth flow of vapors from the tray below through the valve caps or bubble caps.

Flooding

In a flooding condition there is too much liquid coming down the column and it swamps the downcomers and causes the liquid on the tray to build up. This puts exerts extra pressure on the vapor and it can not flow up through the valve caps. This cause the pressure from the vapors to build up until the pressure is high enough to overcome the extra liquid on the tray Weeping Weeping is when liquid flows downward through the holes in a distillation tray. Normally vapor rises up through the holes and s the liquid on the tray. If the vapor rate is too low the liquid may be able to drop to the next stage through the holes, resulting in less than optimal vapor/liquid (and therefore less than optimal separation). Condensed liquid from above trays is generally distributed onto a distillation tray via a weir.