Liquid Distributors – An Important Tower Internal (Part 1)

Industrial processes that use separation methods like distillation, absorption and stripping often occur in packed columns or towers. These towers require liquid distributors to disperse the liquid evenly across specific sections of the packing throughout the structure, creating a more efficient transfer between the various phases.

Liquid distribution tower internals are important because they provide adequate liquid distribution onto the packed bed and structured packing. The liquid should be distributed equally across the packed column area to maximize the liquid interface surface area. Since a larger bed height can only compensate for poor initial distribution to a limited degree, liquid distributors are essential for overcoming maldistribution.

Liquid Distributor Performance

Liquid Distributor

An ideal liquid distributor will perform with the following characteristics:

  • Uniform liquid distribution
  • Proper operation through its turndown range
  • Low vapor phase pressure drops
  • Resistance to plugging or fouling
  • Minimum liquid Residence Time
  • Mixing capabilities for redistribution to the next bed

Classifications of Liquid Distributors

The liquid distributors in packed columns can be further classified as pressure or gravity-fed. Each type offers a unique solution for maximum performance in specific operating conditions. Choosing the optimal distributor type can depend on several factors, including the liquid’s properties, flow rate, pressure drop limitations and the desired distribution uniformity.

Liquid Distributor

Pressure Distributors

Pressure distributors provide more passage for vapor flow and are comparatively less expensive than gravity distributors. Also, they require smaller lead-up piping than the latter. These pressure-type distributors are used where space in the tower is limited and packed bed heights are small.

However, their operating cost is high and they are more likely to experience corrosion and plugging. Therefore, the quality of distribution is less compared to the gravity type. The spray nozzle and ladder-type distributors are examples of pressure-type distributors.

  • Spray nozzle: Ideal for heat transfer or refinery applications, spray nozzle liquid distributors work well for low liquid rates or when short bed heights require a uniform distribution pattern. Typical turndown ratios are 2:1. These designs are not suitable for distillation.
  • Ladder: Ladder type or lateral pipe distributors work well for various applications, including conditions with high vapor velocities or limited space. Because of the possibility of plugging in the orifices, ladder distributors are incompatible with fouling systems or systems with low liquid rates or suspended solids.

Liquid Distributor

Gravity Distributors

Gravity distributors can handle larger liquid flow rates. Weir-type or trough-type gravity distributors are typically the least troublesome distributors with a high turndown ratio. However, they contain a limited number of drip points and are sensitive to internal liquid hydraulics. Gravity distributors include specific designs like orifice plates, drip tubes and troughs:

  • Orifice plate: Orifice or plate liquid distributors feature the most conventional designs, allowing the gas to pass through the plate with risers while the liquid flows through the floor’s openings. Standard orifice design types typically offer 2:1 turndown ratios.
  • Drip tube: Highly similar to orifice-type distributors, these designs use tubes instead of several or all the plate orifices. This modification accommodates fouling applications with turndown ratios of up to 10:1.
  • Trough: Trough distributors use one or more parting boxes to distribute the liquid properly onto the laterals. With turnover ratios exceeding 8:1, these designs are ideal for fouling applications or towers with high liquid rates.

Load Ranges

The following table shows the range of liquid loads that specific distributors can handle:

Distributor-Type Liquid Load (m3/m2h)
Spray type 0.5 to 120
Ladder type 4 to 25
Gravity type 5 to 100

Liquid Distributor Design Parameters

It is highly challenging to distribute

small amounts rather than large amounts of liquid. The main design criteria for any liquid distributor is determining the number of drip points in proportion to the column area or irrigation density.

A crucial parameter in the design of a distributor is the liquid load uL required for the separation process. Together with the head ho, it allows the flow velocity at an outlet uo and thus the number of outlets required, to be determined in distributors with liquid discharge or overflow. Assuming the coefficient is approximated as 1, the usual equation for flow through orifices is:

blog-liquid-Part1-2-eq-1

Since the cross-sectional area of the outlet is known for the basic shape distributors or can be determined with the help of equations and correlation reported in other literature, the volumetric flow rate l can be obtained from:

blog-liquid-Part1-2-eq-2

Once l is known, the number Z of distributor outlets per square meter of column cross-section that is required to cope with a liquid load uL in m3/(m2hr) can be obtained from:

blog-liquid-Part1-2-eq-3

discover-more-with-mach-engineering

Discover More With MACH Engineering

Without a proper liquid distributor installed in the tower, an efficient mass transfer fails to occur on the packing. MACH Engineering offers different types of liquid distributors to suit the needs of chemical processes in place. With our expertise in AutoCAD, we can design and fabricate liquid distributors according to customers’ requirements. For more information regarding liquid distributors or column internals, contact us today.

 

 

 

 

 

 

 

 

Liquid distributors are an important component of column internals. They provide adequate distribution of the liquid onto the packed bed and structured packing. To achieve an intensive mass transfer between the phases, the liquid should be distributed equally across the packed column area. It serves to overcome maldistribution and, poor initial distribution can only be compensated to a limited degree by a larger bed height

An ideal liquid distributor will perform with the following characteristics:

  • Uniform liquid distribution
  • Proper operation through its turndown range
  • Low vapor phase pressure drop
  • Resistance to plugging or fouling
  • Minimum liquid resistance time
  • Mixing capability for redistribution to the next bed

The liquid distributors in packed columns can be further classified as pressure fed distributors or gravity fed distributors.

Pressure Distributors: These liquid distributor types provide more passage for vapor flow and are comparatively less expensive than gravity distributors. Also, they require smaller lead-up piping than the latter. However, its operating cost is high and more likely to experience corrosion and plugging. Therefore, the quality of distribution is less compared to the gravity type. The spray nozzle and ladder type distributors are examples of pressure type distributors

Gravity Distributors: They can handle large liquid flow rates. The weir type (trough-type) distributors prove to be the least troublesome distributors with a high turndown ratio. However, it contains a limited number of drip points and is sensitive to internal liquid hydraulics.

The following table shows the range of liquid load that each type of distributor can handle.

It is obviously difficult to distribute small than large amounts of liquid. The main design criteria for any liquid distributor is to determine the number of drip points in relation to the column area (irrigation density).

A crucial parameter in the design of a distributor is the liquid load uL required for the separation process. Together with the head ho, it allows the flow velocity at an outlet uo and thus the number of outlets required, to be determined in distributors with liquid discharge or overflow. The usual equation for flow through orifices is

 

Since the cross-sectional area of the outlet is known for the basic shape distributors or can be determined with the help of equations and correlation reported in other literature, the volumetric flow rate l can be obtained from

 

Once, l is known, the number Z of distributor outlets per square meter of column cross-section that is required to cope with a liquid load uL in m3/m2h can be obtained from

 

Without a proper liquid distributor installed in the tower, an efficient mass transfer fails to occur on the packing. MACH Engineering offers different types of liquid distributors to suit the need of chemical process in place. With our expertise in AutoCAD drawing, we can design and fabricate liquid distributors according to customers’ requirements. For more information regarding liquid distributors or column internals, contact us.

Archives