Industrial processes that involve separation such as distillation, stripping and absorption are often performed in packed columns — also referred to as packed towers. These packed towers require liquid distributors that are used to evenly disperse the liquid in the tower over specific sections of the packing. This process makes for a more efficient and intensive mass transfer between the various phases — plus, it helps prevent maldistribution.
Benefits of a Liquid Distributor
For industrial organizations, liquid distributors offer several distinct benefits, including:
- Uniform liquid distribution over the packed bed and structured packing.
- Resistance to plugging or fouling.
- Low vapor phase pressure drop.
- Proper operation through their turndown range.
- Mixing capability for redistribution to the next bed.
- Minimum liquid resistance time.
The Different Types of Liquid Distributors
There are two main categories of liquid distributors:
- Gravity distributors: These are capable of handling large liquid flow rates and have a high turndown ratio. Although efficient, they are sensitive to internal liquid hydraulics and contain a limited amount of drip points. They can be further divided into orifice plate distributors and trough type distributors.
- Pressure distributors: Less expensive than gravity distributors, pressure distributors feature more room for vapor flow and require smaller lead-up piping. However, given that the quality of distribution is less and there is a greater potential for corrosion, their operating costs prove ultimately higher than gravity distributors. Pressure distributors can be further divided into spray nozzle and ladder-type distributors.
Gravity Liquid Distributors
An orifice type/pan liquid distributor — the most conventional type — allows gas to pass the plate through risers while liquid flows through openings in the floor. The standard design has a turndown ratio of 2:1 and the pressure drop in water is typically 0.2 – 0.6. If higher turndown ratios are required, taller risers can be used. This type of liquid distributor isn’t recommended for fouling applications.
Drip-Tube liquid distributors are similar to orifice-pan liquid distributors, except drip-tubes are used in place of some or all of the plate orifices. This modification allows drip-tube distributors to accommodate fouling applications. When used for fouling applications, the solids settle on the deck and the liquid flows through the tubes. Turndown ratios of 10:1 are realistic.
In a trough distributor, liquid is introduced into the parting box to properly distribute the liquid onto the laterals. For 3-8 feet diameter towers, one parting box is required. For high liquid rates or larger diameters, multiple parting boxes can be used. Turndown characteristics are a minimum of 8:1. A trough distributor is used with fouling service or high liquid rates, making it a highly versatile product.
Pressure Liquid Distributors
Commonly used in stripper and scrubber services with moderate to high liquid rates, a ladder-type distributor or lateral pipe distributor can be used for a variety of applications, including conditions with limited space availability and high vapor velocities. It’s a pressure-driven liquid distributor in which the branch pipes serve to deliver the liquid through metering orifices. Generally, the pressure drop is 1-5 psi. Due to the potential for plugging of the orifices, a ladder-type distributor isn’t recommended for fouling systems — nor for systems with suspended solids or low liquid rates.
A spray nozzle distributor, or spray-type, can be designed for low liquid rates — sometimes as low as ½ GPM/ft2 of tower area. Generally, the turndown is 2:1 at a pressure drop across nozzles of 10-15 PSI. A spray nozzle distributor is predominantly used for refinery and heat transfer applications, and it’s important when short bed heights demand a uniform distribution pattern. They are not recommended for most distillation applications.
While you only ever need one liquid distributor per tower, the type of liquid distributor will depend on the application.
Benefits of Working With MACH Engineering for Your Liquid Distributor Needs
If you’re looking for custom liquid distributors and tower internals that meet your specific needs, we’re here to help. With more than 25 years of experience and a reputation of innovation and excellence, we work with you to determine which type of liquid distributor is appropriate for your application.
Once we have the exact specifications, we design your liquid distributor and run process simulations both before and after selecting the tower internals. Please contact us for more information.