INTRODUCTION TO COLUMN INTERNALS The pressure vessels are designed to various national and international codes since the design aspect are well established. In the separation process, column internals are installed on adjusted or on existing column attachments. The requirements are carefully verified and specified for each situation. `have been provided for many columns. Types of Column internals – 1. Trays - Trays of various designs are used to hold up the liquid to provide better contact between vapour and liquid, hence better separation. A tray essentially acts as a mini-column, each accomplishing a fraction of the separation task. From this we can conclude that the more trays there are, the better the degree of separation and that overall separation efficiency will depend significantly on the design of the tray. Trays are designed to maximize vapour-liquid contact by considering the Liquid distribution and Vapour distribution on the tray. This is because better vapour-liquid contact means better separation at each tray, translating to better column performance. Lesser number of trays will be required to achieve the same degree of separation leads to less energy usage and lower construction costs. 2. Packings - Instead of trays, 'packings' are used to enhance contact between vapour and liquid. There is a clear trend to improve separations by additions of packings. Packings are passive devices that are designed to increase the interfacial area for vapour-liquid contact. The strangely shaped packing pieces are supposed to impart good vapour-liquid contact when a particular type is placed together in numbers, without causing excessive pressure-drop across a packed section. This is important because a high pressure drop would mean that more energy is required to drive the vapour up the distillation column. 3. Packings versus Trays – A tray column that is facing throughput problems may be de-bottlenecked by replacing a section of trays with packings. This is because: a. Packings provide extra inter-facial area for liquid-vapour contact b. Efficiency of separation is increased for the same column height c. Packed columns are shorter than tray columns d. Low liquid hold up compared to trays 4. Packed columns are called continuous-contact columns while tray columns are called staged-contact columns because of the manner in which vapour and liquid are contacted. 5. A mass transfer tower also requires other internal equipments besides the packing or trays. 6. Feed plates, dispenser plates, distributors and redistributors to spread the liquid and vapour evenly within the tower. 7. Packing support plates and bed limiters are used to physically support and retain inside the tower. Collector plates capture liquid withdrawal from the tower. 8. Tray support rings, tray decks, manhole in tray decks, bolting bars, down comers, seal pans, inlet and exit weir, support grids and multi beam support are used in tray columns. 9. Different types of trays are – Sieve trays, Bubble cap trays, Valve trays, Chimney trays specialty trays. 10. Tray design parameters, which impact on column operation are – active area, downcomer area, tray spacing, downcomer clearance, outlet weir height, flow path length, tray open area, and number of flow path etc. 11. The contents of packed column vary based on application and performance requirements. Most packed columns are custom designed and column is characteristically operated with counter flow of the phases. The packed column design parameters are selection of packing material, surface availability, interface regeneration, pressure drop, weight, corrosion resistance and cost. Packings are essentially of 2 types namely Random packing – Pall rings, Raschig rings, Intalox saddles, and cascade mini rings Berl saddles etc. Structured packing – sheet based / wire mesh structured, mist eliminators, Coalescers
Typical columns with various internals
Structured Packed column
Typical Tray column
Typical column tray arrangement
Random Packed column
Typical column tray Support
Various types of Trays
Sieve trays - Simply metal plates with holes in them. - A perforated tray with typically 8% to 15% open area - Vapour passes straight upward through the liquid on the plate. - The arrangement, no. & holes size are design parameters - Good anti fouling and pressure drop characteristics
Bubble Cap trays - Riser or chimney fitted over each hole - The cap is mounted so that there is a space between riser & cap to allow the passage of vapour. - Vapour rises through the chimney and is directed downward by the cap. - Finally discharging through slots in the cap, & finally bubbling through the liquid on the tray.
Valve trays - Perforations are covered by lift able caps. - Vapour flows lifts the caps, thus self creating a flow area for the passage of vapour. - The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays
Chimney trays - Function as a collector device either for feeding to a liquid distributor (particularly flashing feeds) or for a liquid draw off. - Used to mix liquids before distribution to packed bed. - This is especially important where high vapour rates are encountered due to the low pressure drop across most packed beds. - Hats are used to prevent direct impingement of vapour on to the device above Specialty trays - Tray hydraulics plays important role in efficient functioning and process performance. - Excellent fouling resistance. - Prototype design - Smaller valve provide more capacity due to reduced pressure drop. - Lower front height, less entrainment due to lateral vapour release. - Standard sizes are – SVG, MVG, MMVG
Typical Random Packings
Riser cap
Pall rings Characteristics - Design provides higher capacity and lower pressure drop than trays and other random Packings - The interior and exterior contacting surfaces of the ring provide an effective distribution of liquids & gasses and resist plugging, fouling & nesting. Applications - Absorption, Aeration, Degassing, Desorption, Distillation, Stripping, Heat Recovery and Extraction
Raschig Ring Characteristics - Raschig Ring structure provides one of the largest surface areas among tower packing options - Handles heavy loading, process upsets and temperature shocks. Applications – - Petrochemical distillation and extraction applications - Absorption in gas processing and combustion plants - Desorption in water treatment Intalox Saddles Characteristics - Good Capacity and Low Pressure Drop - Higher Liquid Hold-up and Residence Time - Versatile Standard Packing Applications – Absorption, scrubbing, and stripping services Pulp and paper service, such as bleach plant absorbers Versatile alternative to ceramic saddles Berl saddles Characteristics - Improved liquid distribution compared to Raschig rings and Intalox saddle - Easier to manufacture - Low pressure drop with higher surface area Applications – Acid Absorption, Gas drying, Scrubbing and Cleaning
Typical Structured Packings - A range of specially designed materials for use in absorption and distillation columns and chemical reactors. - Typically consist of thin corrugated metal plates or gauzes arranged in a way that they force fluids to take complicated paths through the column - Thereby creating a large surface area for contact between different phases - vapour passes straight upward through the liquid on the plate. - Structured Packings provide high capacity, higher efficiency & lower pressure drop
Miscellaneous auxiliaries
Hold down grids / Bed limiter – - Placed directly above random packed bed to limit the packing bed from moving or lifting and getting packing pieces entrained away from the bed. - Either screwed to the column wall or loosely placed over the packing. - Made from rods and bars in combination with screens Support grid – - Required to support packing. - Can be easily installed with a minimum of beams and welding attachments.
Liquid collector tray - Used to collect all the liquid, which drips down, from the packed section. - Basically it is used below packed bed for the purpose of redistributing the fluids on the lower bed - Performs the task of redistributing the liquid uniformly
Liquid distributors - Used to distribute the liquid feed depending upon the service requirements. - Pan type, spray nozzle are typical types used - Crucial for a proper running packed bed.
Vapour distributors - Used to distribute the vapour only where flow energy is excessive - Used to reduce inlet stream energy by dividing the inlet stream and then directing it tangentially to each sidewall. - Pressure drop is relatively low
Vapour distributor - Schoepentoeter - Device is used for 2-phase feed inlet. - Best suited for flashing feeds and reboiler returns. - It gives excellent distribution, phase separation and operates at negligible pressure drop.
Conceptual process design guidelines for tower internals The process design guidelines is intended to provide guidelines for sizing and specifying trayed and packed towers and their internals for absorption, stripping and distillation applications and they do not address the calculation required to establish the no. of theoretical stages and vapour/liquid traffic data. The conceptual design variables are – Types of internals used which forms the basis for tower pressure drop allowance A pressure profile across the tower system including condenser and vent condenser if applicable The no. of theoretical stages in the tower The reflux ratio The feed location Determination of optimum selection for these variables is an iterative process where tower system installed cost and operating cost are estimated until the economics of the operation are optimal. The scope of contractual liability for internals suppliers varies with the internals being used and generally following types of guarantees requested from suppliers – Mechanical guarantee - Normally for sieve and dual spatiality dual flow trays Mechanical and hydraulic performance guarantee - typical for valve trays Full mechanical, hydraulic, and separation performance guarantee - typical for packed towers. The calculation method for simulating tower operation depends on types of fluid being processed, the level of purity to be achieved, the operating pressure and temperature range, etc. Tray efficiency – The no. of trays is determined by No. of theoretical stages / tray efficiency. Tray efficiencies vary from 100% at high vapour density to as low as 15%.Efficiency increases with significantly with increasing vapour density and with decreased % of flood. The efficiency drops significantly when approaching flood and when weeping becomes significant. HETP – To determine pressure profile for packed column, the height equivalent per Theoretical plate (HETP) and estimated pressure drop per height of packing need to be determined. Downcomers – Downcomer can limit the maximum capacity of the tray if the downcomer area results in a high downward velocity vapour that will cause entrainment. Straight downcomer is the usual choice because the design is simpler and cost for installing weld-in parts is lower. Slope-sided and step-sided downcomers are used to increase bubbling area. Generally, downcomer weir length shall not be less than 60% of tower dia. for single pass tray and 50% of tower dia. for multipass trays. In general downcomer area is used in computer programs = 10000 GPH/ft 2.
Outlet weirs – Maintain a minimum head on the tray. Weir height may vary from 1 ½” to 3”. If pressure drop criteria in the system is critical, lowering the weir height reduces tray pressure drop and weepage, and lower weepage allows one to increase for sieve trays hole area thereby reducing pressure drop further. Lowering the weir heights causes increase in maldistribution on trays. Downcomer clearance – In general practice, the downcomer clearance (i.e. distance between bottom of downcomer and the tray below) of ½” less than the overflow weir height to provide static liquid seal. Ex. With 2” outlet weir height the downcomer clearance is 1 ½”. Sometimes the velocity under the downcomer is used to limit to 1.3 ft/sec. Inlet weirs – An inlet weir can be provided when a positive seal can’t be provided by downcomer. This is most likely to occur when low pressure drop and low outlet weir heights are used. When inlet weirs are used to provide a positive liquid seal, the inlet weir height shall be ½” higher than the downcomer clearance in order to maintain appositive liquid seal at the bottom of the downcomer. Turndown ratio – It is the ratio of Maximum allowable vapor rate at or near flooding condition to the minimum vapor rate when weeping or liquid leakage becomes significant. Turndown can be applied to all types of trays, however it is more relevant to sieve and valve trays. 1. For sieve trays, turndown ratio is ~ 2-3 : 1 2. For valve trays, turndown ratio is ~ 4-5 : 1 3. For bubble cap trays, turndown ratio is ~ 10 : 1 Tray spacing - Typically use 18” tray spacing. Greater tray spacing may be used based on economic considerations or based on providing access under beams in larger diameter towers. Increase tray spacing in 6” increments. In general, have maximum 20 trays between manways. 12. Design of Internal support Beds Internal beds must be supported by vessel shell. Sand filters, packed columns, and reactors with catalyst beds are often supported by combination of beam(s), grating and circumferential ring which supports the periphery of the grating. The beams are attached to the shell wall by either clips or beam seats.
