Alf Flashcards
Ideal distillation stage
Operates at steady-state, has a liquid and a vapour product. All vapour and liquid entering are immediately contacted and perfectly mixed. Total vapour leaving the stage is in equilib. with total liquid leaving
What can high temp lead to (crude oil) and possible solution ?
Can lead to thermal cracking, this produces lots of coke at the bottom -> vacuum distillation, so lower temp used and relative volatilities increase in magnitude.
What are weirs and active tray areas?
Weir ensures there is always some liquid (holdup) on the tray and designed so that holdup is at a suitable height, i.e so that bubble caps completely covered by the liquid. Vapour flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapour on each tray is the active tray area.
Plate design, what happens and where?
Vapour rises through the liquid pool on the tray deck and disengages from the liquid in the space above the deck. Liquid enters the tray from a downcomer above and leaves via a downcomer below. Three functional zones of a tray:
- Active area:
For mixing liquid and vapour; the zone where mass transfer occurs
- Vapour space above active area:
Zone in which liquid is separated from vapour
- Downcomer between trays:
Moves liquid from one contacting tray to another, and disengages vapour from liquid
More trays =? Trays designed to do what/what is overall purpose?
Essentially act as mini-columns that achieve a fraction of the separation task. More trays -> better degree of separation thus overall separation efficiency will depend significantly on the design of the tray. Designed to maximise vapour-liquid contact.
Bubble cap trays
Have a riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is space between the riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downwards by the cap, finally discharging through slots in the cap and bubbling through the liquid on the tray. Effectively valves on top of the tray.
Sieve trays
Metal plates with holes in them. Vapour passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters. Often used in stripping sections.
Valves trays
Perforated sheet metal decks on which round, liftable valves are mounted. The vapour flows through the valves which are installed parallel to the outlet weir. Valve trays combine high capacity and excellent efficiency with a wide operating range. Perforations are covered by liftable caps, vapour flow 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.
LOOK AT COMPARISONS OF TRAY DESIGNS
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What happens middle, bottom and top of tray: fractionation
- feed stage in the middle, make sure feed enters at point where it has same temp and comp of the material it is meeting inside
- bottom of column is stripping section where a portion of bottom product is vaporised and re-introduced as a vapour
- top of column is rectifying section where vapour is taken off the top, condensed, separated and a fraction returned to the column.
What is nitrogen used for in crude oil distillation
Used as an inert can greatly reduce partial pressure of light component in gas phase -> higher stripping efficiency
What happens during rectification?
Feed comes in at bottom already as vapour, contacted by a proportion at the top which is condensed. Feed is vapour product from stripping section.
Where does stripping of heavy components happen?
Bottom
What is structured packing?
A range of specifically designed materials for use in absorption and distillation columns. Typically consist of thin corrugated metal plates or gauzes arranged in a way that forces fluids to take complicated paths through the column, thereby creating a large surface area for contact between diff phases. Inclined flow channels -> high surface area but low resistance to gas flow. Surface enhancements chosen to maximise liquid spreading, significant performance benefits in low pressure and low irrigation rate applications.
What is packing used for ?
A tray column that is facing throughput problems may be de-bottlenecked by replacing a section of trays with packings. This is because:
-packings provide extra inter-facial area for liquid-vapour contact
-efficiency of separation is increased for the same column height
-packed columns are shorter than plate columns
Things to consider: need to support packing below by having a tray that can hold it up, need to also hold it down from above. Desired: even flow, no velocity dist.
Liquid and gas distributors
Typical for packed column. Sits at top, reflux come in through pipe. Small holes act as ‘shower head’ to distribute liquid evenly across top of packing, at bottom there is a chimney tray that allows gas to by-pass liquid without mixing - used in side drains as want no entrained vapour. Gauze mesh used to prevent liquid entrainment at top - acts as coalescer.
Why are there limits on vapour and liquid velocities?
To stop either weeping (liquid falling through the holes in the plate due to low vapour velocity), entrainment (blowing the liquid off the plate and up to the one above - also known as jet flooding), or flooding (liquid backing up on the plate, as it cannot pass through the downcomer, can be due to too high liquid loading or mechanical faults).
What can flooding be due to and what are the flow regimes?
Changes in feed and reflux conditions. Flow regime refers to the way that vapour and liquid are dispersed on the tray
- Froth: liquid phase is continuous, vapour is dispersed as bubbles in the liquid. Occurs at low to moderate vapour velocities and moderate to high liquid loads.
- Spray: vapour phase is continuous, liquid is dispersed as droplets in the vapour. Occurs at high vapour velocities and low liquid loads.
Spray entrainment flooding
Occurs at low liquid flowrates. As vapour velocity is increased, liquid on the tray becomes dispersed by the rising vapour into droplets and entrained into the tray above, rather than flowing onto tray below.
Froth entrainment flooding
Occurs at highrt liquid rates. When vapour velocity is increased, the froth height increases. If the tray spacing is small the froth may reach the tray above causing liquid accumulation on the tray above.
Downcomer back-up flooding
Occurs when aerated liquid backs up into the downcomer due to tray pressure drop, liquid height on the tray and fractional losses in the downcomer apron. All of these increase when liquid flow rate increased, while tray pressure drop also increases when vapour rate is raised. When the back-up liquid in the downcomer exceeds the tray spacing, liquid accumulates on the tray above.
Downcomer choke flooding
Occurs when the liquid flowrate increases until a point that the velocity of the aerated liquid in the downcomer exceeds a certain time limit. At this point, the fraction losses in the downcomer become excessive and the frothy mixture cannot be transported to the tray below. This causes liquid accumulation on the tray above.
Effects of total pressure on flooding
For a given mass flowrate of vapour going up column, increasing pressure will reduce gas velocity (and volume) so less likely to flood. However, this will also decrease the relative volatilities, reducing mass transfer.
What is azeotropic mixture?
‘Constant boiling’ mixtures, have inflexion point where vapour and liquid have the same composition, cannot go past it, limits separation.
- Distillation Column Operation: Continuous fractionation
Column in cylinder divided into sections by a series of perforated trays: the vapour passes through the tray, as liquid flows across it over a weir and to the tray below. Continual movement of vapour up, and liquid down through the column. At each stage, an ideal column will attain equilib between liquid and vapour, meaning that there is a transfer of MVCs from liquid to vapour and vice versa, resulting in both reduction of temp and pressure when moving up the column.
Top: Vapour condensed, portion returned as reflux as liquid to top plate
Bottom: portion reboiled and vapour sent up the column. Condenser and reboiler are top and bottom stages as contacting occurs (only if liquid product drawn off reboiler)
Feed Plate location
Ideally, should be located on the stage closest to the intersection of the operating lines -> ensures max enrichment per plate. If the feed is specified elsewhere, more plates will be required for a given separation.
Total Reflux
No feed and no product.
