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Liquid Distributors

THE BEST PARTNER HANBIT SOLTECH

Introduction HPD-106/107 HDD-116/117 HCD-126/127 HCD-136/137 HTD-156 HTD-186 TPD-798 TTD-806 PAD-844 SND-1044

Liquid Distributors
Liquid distributors are used in packed towers above each bed of packing. The liquid distributor is located above the bed, typically 6 to 8 in. (150 - 200 mm) above the packing. This provides sufficient space for the gas to disengage from the bed before it passes through the distributor.
High performance distributors are used primarily for distillation services, typically where a high efficiency packing is used. Deep beds of any type of packing should require high performance distributors to maximize packed tower performance.
Redistributors share the same mechanical design characteristics as distributors, but they include gas riser covers. There are four reasons to have more than one bed of packing in a tower: feed introduction, liquid cross-mixing, distribution correction and product side-draws.
Traditional distributors are used in applications that are less demanding on packing performance, such as absorption, stripping and heat transfer services. They have the advantage of standardized design and optimal use of raw materials. These distributors generally have fewer orifices, less balanced liquid heads and less uniform distribution coverage than high performance distributors.

Liquid Distribution Quality
Perfect liquid distribution could be defined as providing equal liquid rate per unit area of the packed bed surface. Hanbit Soltech Corporation has developed a distribution quality rating system for quantifying distribution uniformity that the rating correlates to tower performance. We rate distribution quality as a percentage, where 100% quality indicates ideal uniform distribution and 0% indicates a significant portion of packing is receiving no liquid at all.

Hanbit Soltech Corporation defines two categories of liquid distributors based on our proprietary quantitative distributor rating system.
  • • High Performance Liquid Distributor
  • more than 90% quality
  • • Traditional Performance Liquid Distributor
  • generally 40% - 85% quality
A tower with many more stages of high-efficiency packing (HMTP random packing, wire gauze or structured packing) is sensitive to liquid distribution quality. The high performance distributors are always recommended to maximize packed tower performance.

Conversely, a tower with fewer stages of traditional packing (Pall rings, Saddles, Raschig rings, etc...) is less sensitive to distribution quality. This tower permits the use of a less expensive traditional distributor.

Distribution Point Density
The number of distribution points per unit area is defined as distribution point density, which has an influence on the efficiency of the packed bed. When using larger packings with relatively low efficiency, the effect of the number of distribution points is insignificant, but this effect can be considerable when using smaller, high efficiency packings.

Hanbit Soltech Corporation has established the minimum recommended guidelines for the drip point density on the liquid distributor as shown below.
  • • Distribution Point Density, pts/㎡
  • 60-80
  • 80-100
  • 100-120
  • • Structured Packing, ㎡/㎥
  • <250
  • 250-350
  • >350 & Wire Gauze Packing
  • • Random Packing, ㎡/㎥
  • <210
  • 210-300
  • >300
The number of distribution points per unit area can be designed as required, however, mechanical considerations usually limit the number to about 200 points/m2.

Distribution Orifice Sizing

As for the gravity-fed distributor, the orifice size is dependent upon the hole count, the specific liquid rate, the liquid head and the operating range, which varies from min. 3 to max. 25 mm.

The approximate orifice size is indicated in figure provided the distributor with an operating range of 60 to 120% of the design flow and this applies to gravity-fed distributors with fixed minimum liquid head.

Operating Range
Hanbit Soltech Corporation provides distributors which usually give a flow variation at any point of the distributor of better than ± 5% and at minimum rates better than ± 8%. This will generally give an operating range of 2:1 for a single-level orifice distributor. If a wider flow variation can be acceptable at the minimum flow rate, a wider operating range is possible.

We generally design to have a maximum flow variation, defined as the coefficient of variation(Cv), equal to no more than 5% at design rate and no more than 10% at turndown rates. Special designs of distributor can be made for processes requiring lower flow variations. Designs of better than ± 3% at 100% design rate and better than ± 5% at a 2:1 turndown are feasible.

Fouling

There are several sources to foul a distributor such as dirts, scales, debris entering to the column from outside of the column or inherent process character like polymerization, coking, sourced within the column itself, which can lead to maldistribution and reduce the packing performance. Using external strainers are far easier way to eliminate fouling outside of the column than distributor cleaning regardless of the degree of fouling resistance for distributor.

V-notch weir or side wall orifices are generally less prone to plugging than orifices in the bottom of a distributor, but distributors designed with V-notch weirs may have flow variation outside the high performance range. Distributors with bottom orifices are not normally recommended for orifice sizes of 6 mm or less when the fouling tendency is expected.

The followings are the fouling resistance of the various metering arrangements.

• “V” notch weir
• Slotted weir
• Side wall orifice with orifice strainer
• Side wall orifice
• Bottom orifice with orifice strainer
• Bottom orifice

Model HPD-106/107

Pan Distributor/Redistributor

Orifices in the pan bottom are arranged to provide optimum distribution quality together with gas risers set between the orifices. When used for higher flow rates, this distributor will have large orifices that will not tend to foul. For lower flow rates, orifice strainer caps or the Model HCD-136/137 should be considered if the system is fouling. The standard turndown range is 2:1.

• Column diameters : 250 to 900 mm
• Liquid rates : above 5 m3/hr/m2
• Orifices in pan base

Construction Features :
Standard connection to the tower wall is by bolting to clips. Optionally, the distributor can be supported on lugs or a ring trapped between body flanges. All joints in multi-piece pans are gasketed. Height for gas risers varies by turndown range. The standard design for Model HPD-107 redistributor requires gas riser covers.

Model HDD-116/117

Deck Distributor/Redistributor

The deck-type construction allows good liquid cross-flow between gas risers to be set between orifices in deck specifically spaced for a high distribution quality. As this deck distributor normally handles higher liquid flow rates, the orifices are usually large and allow moderate fouling resistance. The standard turndown range is 2:1.

• Column diameters : greater than 900 mm
• Liquid rates : above 30 m3/hr/m2
• Orifices in deck

Construction Features :
The distributor is clamped onto a support ring with clamps provided. Therefore, distributor levelness is determined by the ring levelness. All joints are gasketed. Height for gas risers varies by turndown range, but standard design passes through 18 in. (450 mm) nominal manway. The standard design for a Model HDD-117 redistributor includes gas riser covers to collect and redistribute liquid raining from above.

Model HCD-126/127

Channel-Type Trough Distributor/Redistributor

Orifices in the channel base are placed to optimize distribution quality. The main channel is connected to the distribution channels directly. The liquid flow in all the channels of the distributor is hydraulically connected to each other. The vapor passage is provided by the space between the channels. The channel-type construction allows minimal leakage due to few joints and easy distributor leveling, which is critical in large diameter towers. The standard turndown range is 2:1.

• Column diameters : greater than 900 mm
• Liquid rates : 5 - 40 m3/hr/m2
• Orifices in channel base

Construction Features :
The distributor rests on a support ring or is suspended from a ring or beams. All joints are gasketed. Height for gas risers varies by turndown range, but standard design passes through 18 in. (450 mm) nominal manway. The standard design for Model HCD-127 redistributor includes gas riser covers to collect and redistribute liquid raining from above.

Model HCD-136/137

Channel-Type Trough Distributor/Redistributor

Orifices in the side-wall of the channels are placed to optimize distribution quality. The liquid from each discharge point is conducted into the lower vapor velocity region below the distributor troughs. This results in low entrainment levels. The resistance to fouling of this distributor is higher than that of deck-type distributors because it features a debris collection zone below the side-wall orifices. The main channel provides structural support as well as equalization of liquid between channels. The standard turndown for a single side-wall orifice is 2:1. However, the turndown range can be extended by using two or three orifices at different levels of the same discharge point. Turndown ratios up to 10:1 are achievable.

• Column diameters : greater than 250 mm
• Liquid rates : 0.75 - 30 m3/hr/m2
• Side-wall orifices

Construction Features :
The distributor rests on a support ring or is suspended from beams or clips. All joints are gasketed. All distributors and redistributors include overflow orifices which help prevent distributor flooding during liquid surges. Height for gas risers varies by turndown range, but standard design passes through 18 in. (450 mm) nominal manway. The standard design for Model HCD-137 redistributor includes gas riser covers to collect and redistribute liquid raining from above.

Model HTD-156

Trough Distributor

Orifices in the side-wall of the troughs are placed to issue liquid against an splash baffle that greatly multiply into many more drip points before the liquid is released onto the packing. Since this multiplying effect gives more drip points even at the low point density, the side-wall orifices can be designed as large as possible for fouling resistance. This trough distributor with baffle has good distribution characteristics particularly at low liquid rates.

• Column diameters : greater than 760 mm
• Liquid rates : 0.5 - 20 m3/hr/m2
• Side-wall orifices with splash baffle

Construction Features :
The distributor is suspended from beams for use with structured packing. However, it can also be constructed as a self-supporting unit for use with random packings. When used between packed beds for redistribution, a liquid collector is required to catch the liquid from the bed above. More column height is required for this liquid collector.

Model HTD-186

Trough Distributor

The Model HTD-186 trough distributor is similar to the Model HCD-136 channel distributor, however, the Model HTD-186 uses a parting box over the distribution troughs instead of a center channel at the same level as the lateral troughs for the crossflow. Therefore, this distributor requires more column height, but it is easier to seal and requires no gasket. Because of no center channel for the crossflow, the parting box design is crucial to this distributor performance.

• Column diameters : greater than 900 mm
• Liquid rates : 0.75 - 20 m3/hr/m2
• Side-wall orifices

Construction Features :
The distributor rests on a support ring or is suspended from beams or clips. All distributors include overflow orifices which help prevent distributor flooding during liquid surges. Height for gas risers varies by turndown range, but standard design passes through 18 in. (450 mm) nominal manway. When used between packed beds for redistribution, a liquid collector is required to catch the liquid from the bed above. More column height is required for this liquid collector.

Model TPD-798

Pan Distributor

The Model TPD-798 “Weir-Riser” distributor is used for highly fouling services in towers less than 1200 mm ID. Cylindrical risers with “V” notched weirs are used as liquid downcomers. This distributor effectively handles a wide turndown range due to the weirs, which permit greater flow as the liquid head increases. However, maximum liquid flow is limited by the gas flow since they are flowing counter-currently in the same passages.

• Column diameters : 300 to 1200 mm
• Liquid rates : 2.5 - 20 m3/hr/m2
• Weir in risers

Construction Features :
The pan of the distributor is supported by equally spaced lugs or a full ledge. The lugs raise the pan above the distributor support ledge to provide an annular space between the distributor and the tower wall. Gas rises partially through the annular space and partially through the risers. Gasketing is required on the plate joints for the flow rates less than 10 m3/hr/m2.

Model TTD-806

Trough Distributor

The Model TTD-806 “Weir-Trough” distributor is the most versatile distributor and it incorporates a compound weir design that enables it to handle effectively a wide range of liquid flow requirements. It is particularly effective in large towers when large volumes of gas and liquid are handled. It is also quite resistant to fouling and can handle large volumes of suspended solids in the liquid. Liquid is metered to the closed-end troughs by one or more parting boxes and overflows by gravity through the weirs of the troughs. The number of parting boxes depends on the tower diameter and liquid flow rate. The standard turndown ratio is 2.5:1, but higher turndown ratios can be achieved with special parting box design.

• Column diameters : greater than 900 mm
• Liquid rates : 5 - 100 m3/hr/m2
• Weirs in troughs

Construction Features :
The distributor rests on a full annular support ring. Towers larger than 3700 mm ID may require mid-span support beams. Standard design passes through 18 in. (450 mm) nominal manway.

Model PAD-844

Pipe-Arm Distributor

The Model PAD-844 ladder pipe distributor is made of a series of pipes with orifices in the bottom of the pipes for distribution. This ladder-type distributor requires little column elevation to accomplish its distribution task, and it provides high open area for high vapor flow. It is designed primarily for use where feed liquid under pressure is available. It is sometimes used in towers where low residence time is required, such as where polymerization can occur. The Model PAD-844 should be used only with clean liquids. The normal turndown ratio is 2.5:1.

• Column diameters : greater than 430 mm
• Liquid rates : 4 - 25 m3/hr/m2
• Orifices in the bottom of the pipes

Construction Features :
The distributor is designed with the flanged header section for easy passage through manways and assembly. Flanged horizontal feed from the side of the tower is standard. The laterals are flanged and removable to permit passage through manways. Header seat and/or wall clips are welded to vessel wall.

Model SND-1044

Spray Nozzle Distributor

The Model SND-1044 spray-type distributor is constructed of a header pipe and laterals that supply liquid to a pattern of spray nozzles. Full cone spray nozzles with an angle of 90° or 120° are the standard design for most applications. This spray distributor can be designed for very low liquid rates because each spray nozzle covers a large area of the tower. Fouling or plugging can be a concern with spray nozzles. The number of sprays per square meter of the cross sectional area is much smaller than the number of drip points in a gravity distributor. This permits larger spray nozzle openings compared to gravity distributors, which reduces their plugging tendency. The normal turndown ratio is limited to 2:1 by the range of effective operation of the spray nozzles.

• Column diameters : no restrictions
• Liquid rates : 0.5 - 120 m3/hr/m2
• Spray nozzles

Construction Features :
The distributor is designed with the flanged header section for easy passage through manways and assembly. Flanged horizontal feed from the side of the tower is standard. The laterals are flanged and removable to permit passage through manways. Header seat and/or wall clips are welded to vessel wall.