Choosing Trays and Packings For Distillation

Distillation towers are one of the most imperative equipment’s to the chemical process industry; therefore, they need to be properly designed and operated. Choice of the right internal equipment is the most important part since it is difficult to change the equipment once it is in operation. If the internal equipment fails, then the entire process is likely to suffer.

Selecting distillation tower internals requires an in-depth understanding of the purpose of separation, required range of flow rates, and physical properties of the vapor and liquid throughout the tower. Towers can have trays, random packing, or structured packing.

Tray Operations

Distillation Trays provide for a hold up for a portion of the liquid through which vapour flows to accomplish the required vapour-liquid mass transfer. In a conventional tray, the liquid travels horizontally across the tray into a downcomer and the gas flows up through holes or passages in the tray and across the liquid flow. In a counter current tray (such as dual flow trays), there is no downcomer and liquid & vapour compete for passage through holes.

Nowadays, Trays are available in various types depending on the operating conditions and limits. The most widely used are Bubble cap trays, valve trays, and sieve trays.

Manufacturers of bubble cap trays, valve trays, and sieve tray manufacturers are also able to recommend the right type of tray for your application

Generally, trays are used in applications with liquid rates of 30 m3/m2 per hour and above, and / or those where solids are present or fouling is a concern

Random and Structured packings

Packed towers provide a continuous contact of the liquid and vapor phases. Due to Because of their large open area, packed towers and operate at with lower overall pressure drop and are often specified for vacuum service where reboiler pressures and temperatures are low. Other advantages are lower tower height, ease of manufacturing, and installation and cost effectiveness. However, they require vigilant maintenance for reliable service over long periods of time. Structured packings are more efficient than random packings but are more expensive to manufacture and time consuming.

Random Packings come in a wide range of geometries: cylindrical rings, saddles, spheres etc. Traditional shapes such as raschig rings and intalox saddles have been replaced by modern designs such as pall rings, nutter ring, IMTP packings, and other proprietary designs. They are manufactured in various thicknesses and materials depending on the service conditions.

Structured packings are corrugated, perforated sheets fabricated into a structured packing element made from thin sheets of metal. Wire gauze packings fabricated from woven metal cloth are used for high efficiency applications at low liquid loadings. The main process advantages of structured packings are seen at low pressures and liquid rates due to its effective vapour handling ability.

Important Criteria when selecting packing:

System Pressure

Column Diameter

Number of stages required

Thermal degradation of the product

Manufacturers of Random Packings and Structured packing can recommend the right product and materials based on the service conditions.

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