Column Internals Explained In the recent years, efforts have been made to design structured & random packings for mass transfer columns more & more effectively. These developments are done to achieve greater throughputs; lower pressure drops & better mass transfer efficiency. Column internals are also playing important role for achieving the above targets & hence needs to be designed carefully from both process & mechanical point of view. After the packed column hydraulics, column internals needs to be designed. Packing internals include support plates, distributors, redistributors, and bed-limiters, feed pipes, collector trays & mist eliminators. a) Support Plate: The primary function of the packing support plate is to serve as a physical support for the tower packing plus the weight of the liquid holdup. The packing support plate must pass both the downwardly flowing liquid phase as well as the upwardly flowing gas phase.
Support Plate For random packings, gas injection type of support plate is used. Gas injection type of support plate provides the separate passage for liquid & gas. Support grid (made up of bars) is used to support the structured packing. Support plates have a very high open area for gas & liquid passage & do not add any significant pressure drop.
b) Bed Limiter: Bed limiters are commonly used with metal or plastic packings. The primary function of these devices is to prevent expansion of the packed bed as well as to maintain the bed top surface level. In towers, the packed bed will not fluidize over the entire surface, vapour surges fluidize random spots on the top of the bed, so that after return to normal operation the bed top surface is quite irregular. Thus the liquid distribution can be affected adversely by such an occurrence.
Bed Limiter Hold-down plates are weighted plates used with ceramic or carbon tower packings. With these packings, it is especially important to prevent fluidisation of the packed bed top surface. The
hold down plate must rest freely on the top of the packed bed because beds of ceramic & carbon packings tend to settle during operation. These plates usually act by their own weight to prevent bed expansion. Weight (kg) of the hold down plates can be calculated by the pressure drop across the packed bed (kg/m2) multiplied by the column cross sectional area (m2). They weigh ~ 95-120 kg per sq meter (20 lb to 30 lb per sq ft).
c) Collector trays: Whenever there is partial or total draw off in between two packed beds, collector trays are used. However it has been noted that some packing vendors are always giving collector trays in between two structured packing beds considering the high turbulence velocity
of liquid in structured packing.
Collector Tray
Since this type of tray frequently maintains a fairly high level of liquid (and consequently a tremendous weight), special consideration should be given to its design. One consideration is the placement of the draw nozzle. A flat chimney tray can be used with the nozzle located at tray floor level, or a portion of the tray floor can be lowered to form a sump and the nozzle located at the sump floor. Both designs require the same liquid head to force the design flow rate through the nozzles. Therefore, locating the nozzle in a sump lowers the liquid level on the chimney tray by an amount equal to the head requirements. This reduces the weight, which the tray must support, but has little effect on residence time since that portion of the liquid depth corresponding the head requirements should not be considered as residence time in most cases.
A chimney height of 6 – 12″ is normally adequate for low liquid flow. 12 – 18″ is usual for high liquid flow if the draw nozzle is located in an inlet sump. All chimneys should have hats located a sufficient space above the chimney to give a peripheral area of 1.25 times the chimney area. If a leak-free design is required, the inlet sump should be seal-welded and gasketing used on the chimney tray floor. A centre sump is preferred over a side sump, as some flow restriction can occur at very high liquid rates due to the shorter weir lengths of side sumps.
d) Liquid distributor: From a process point of view liquid distributors are the most important column internal devices. A distributor is required to uniformly distribute liquid at the top of a
packed tower. Liquid distributor must provide sufficient gas passage area to avoid a high pressure drop or liquid entrainment.
An ideal distributor possesses the following attributes. Uniform liquid distribution Resistance to fouling Proper turndown capability Low gas pressure drop Minimum height Higher cross-mixing capability
Distribution quality: Quantifying the uniformity of liquid distribution is accomplished by calculating the distribution quality (DQ) of a distributor based on the Moore & Rukovena model. It relates the liquid load across the column area at the top of packed beds marking circles proportional to the liquid flow through a particular orifice & then considering the irrigated, overlapping, & un-irrigated areas of the circles.
An ideal liquid distributor should have a DQ of 100%, but practical considerations restrict the DQ to about 95%
maximum. As per Moore & Rukovena model, a low DQ indicates a high degree of liquid misdistribution & some portions of the column cross sectional area may be receiving substantially different volumes of liquid when compared to other portions of the cross sectional area. As the DQ decreases the number of stages that can be realized from a packed bed decreases, thus decreasing the separation efficiency. Following guidelines are used for deciding the no. of points per m^2 for mentioned latest type of packings.
Classification of liquid distributors: Liquid distributors are usually classified into pressure & gravity distributors. Pressure Distributors: This type of distributors provides more open area for vapour flow & tend to be less expensive, lighter, less robust & require smaller lead up piping than gravity type. Disadvantages are high operating cost, susceptibility to plugging & corrosion & a relatively inferior quality of distribution. Pipe arm & Spray type liquid distributors are the examples of pressure distributors. Gravity Distributors: These are further classified as weir type & the orifice type. Both of these types can handle large liquid flow rates. The weir type is generally one of the least troublesome distributors, handle large liquid load & has excellent turndown capability. However it can usually provide only a limited no. of drip points (minimum 30 no. of weirs per m^2), extremely sensitive to levelness & liquid surface agitation.
Orifice distributors are usually of the pan/deck type or of the tunnel (trough) type. An orifice pan distributor consists of a pan equipped with the circular or rectangular risers for vapour flow & perforations in the pan for liquid flow. Orifice trough type distributors consist of parallel troughs with perforations in the trough floors for liquid. Vapour rises in the space between the troughs.
The troughs are often interconnected by cross channels (called as parting box) that equalize liquid levels in different troughs. These trough type liquid distributors provide more open area for vapour flow & are easier to support. Orifice type may suffer from corrosion & plugging, but it can be designed with a large no. of drip points to provide superior liquid distribution. Orifices are sized to maintain a minimum liquid head at desired turndown conditions & to avoid distributor overflow during turn up condition. Very small orifice diameters (<4mm) are avoided to prevent fouling. Selection of a liquid distributor: Specific liquid distributor for particular service is chosen by considering the tower diameter, flow rate, turndown ratio, available height (in case of revamp) & fouling resistance. Please refer the packing vendor catalogue for more details on liquid distributor models/types.
Following types of liquid distributors to be used in applications that are less demanding on packing performance, such as heat transfer applications, absorption & stripping services.
Following are the some important design points of liquid distributor to be checked before finalising its design.
Maximum & Minimum head: Maximum liquid head is the actual liquid head on the deck of the distributor at turn up condition. The overall height (riser/trough) of the liquid distributor shall be higher than this head by following formula. Riser (Trough) height = {(Maximum head) 1.10 + 25} mm
It is always required to estimate the minimum head on the distributor at turndown condition. At low liquid head there are chances of liquid vortexing & entrainment due to insufficient resistance to orifices. Also if minimum heads are fallen short of, considerable misdistribution can occur. Hence minimum liquid head on the distributor shall be around 35mm. It is always recommended to use punched orifices in the direction of liquid flow instead of drilled orifices. Pressure Drop & open area requirement for vapour: Liquid distributor must provide sufficient gas passage area as per gas load to avoid a high-pressure drop or liquid entrainment. In case of very low open area required for gas, a minimum of 12% open area is used. Pressure drop of liquid distributor shall not exceed a value of 20mm WC. However it will depend upon gas load, liquid load & a particular service.
% Flow variation: For low liquid loads & small column diameters, flow variation will increase due to low liquid head. Flow variation is a strong function of mechanical points such as plate levelness, distributor deck flatness as etc. Excessive flow variation will cause liquid misdistribution on below packed bed, entrainment & hence needs to be minimised. Maximum of 12% flow variation is allowed for liquid distributor or redistributor.
Liquid redistributors: One of the main functions of a liquid redistributor is to remix the liquid phase so as to bring the entire liquid flow onto the next lower bed at a more uniform composition.
Deck type Liquid Redistributor If liquid tight pans or boxes are used for a redistributor, gas riser covers & wall wipers must be used. Whenever the liquid falls directly onto a deck type redistributor, the gas risers must be provided with covers to prevent liquid from raining into this area of high vapour velocity. All covers should have located a sufficient space above the risers to give a peripheral area of 1.25 times the riser area.
Feed pipes: Process demand various feed to be introduced into the column at various locations. The feed being introduced could be liquid only, liquid & vapour above a packed & vapour only below a packed bed.
Liquid Feed Pipe For liquid only feed devices, liquid is fed into the distributor via feed pipe & feed pipe design depends upon distributor type, liquid flow rate, its operating range as etc. Each feed pipe meters flow to one or more appropriate feed areas, matching the hydraulic requirements of the distributor. Excessive turbulence, hydraulic jump & horizontal flow velocity in the liquid distributor are eliminated in a feed pipe design. A submerged feed is recommended in all cases in large towers with low or high flow rates. Submerging the feed maintains uniform liquid temperature & reduces hydraulic jump of liquid in the distributor. Liquid feed pipes are designed with a velocity of 1m/s to max 3.5m/s.
For liquid & vapour feed devices above a distributor, separating the two phases is of primary response. The primary design factors are the feed flow rate, desired turndown, column height needed for flashing vapour distribution & mixing of the inlet liquid with overhead liquid. Flashing feed devices such as flash feed gallery, flash feed chamber or flashing feed pipe are used based
on the composition of vapour in liquid.
Flashing feed pipe Vapour only feed devices are required for reboiler returns or to introduce vapour feed or to introduce vapour or gaseous feed. If the column offers adequate pressure drop (> 4-5 mm of WC per m of packed bed height), the packings themselves tend to mix the vapours. In the event of very low pressure drop across the column (i.e. < 4-5); vapour channelling can become a serious problem. The kinetic energy of the vapour (F factor) & its composition at the point of introduction are the two main factors considered in designing the vapour entry device. Vapour or gas feed pipes are designed with a velocity of max 20m/s.
ii) Vapour composition: When the vapour is introduced between packed beds, consider the degree of mixing of the inlet vapour with rising vapour. If a gross mismatch of composition &/or temperature exists, mixing of the two vapour optimizes packing performance above. g) Mist Eliminator: In every process involving contact between liquid and flowing gas, tiny mist droplets are carried away with the gas. This phenomenon is called entrainment. Mist eliminator provides a large surface area in a small volume to collect liquid without substantially impeding gas flow. It work on the principles of inertia whereby small droplets of liquid entrained in vapours are coalesced into large drops & fall downward due to gravity.
Mist eliminators can be either demister pads (knitted or coknitted wire mesh for fine mist) or vane type separators.
