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.
Effects of increasing reflux ratio
Makes more expensive as column diameter increases as D is a fixed value, condenser duty also increases.
Heat and cooling duties
Heat loss from a large insulated column is relatively small, and the heat effects of the entire unit can be considered in terms of the condenser and reboiler. Assuming no heat losses, if the feed is at its boiling point then the heat supplied to the boiler is equal to the heat removed by the condenser.
Differential distillation
With a single stage pot is the simplest form of distillation, is severely limited and only useful when the volatilities of the components involved are very different.
Flash vaporisation
Single stage operation (usually continuous). Liquid mixture is partially vaporised and allowed to contact the residual liquid. The system is at equilib. The resulting vapour and liquid phases are separated and removed from the equip.
Consists of vaporising a definitive fraction of the liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid. The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced. The still is essentially a separator in which the liquid and vapour produced by the reduction in pressure have sufficient time to reach equilibrium. Vapour is removed from the top of the separator and is then usually condensed, whilst the liquid leaves from the bottom.
Constant molar overflow assumption
Molar flowrate of vapour = molar flowrate of distillation and reflux
How many columns needed to separate a mixture with ‘n’ components
n-1 columns
What are the heavy and light keys
Heavy: heaviest component in the top product
Light: lightest component in the bottom product
As well as being the distillate, the light key is present in the bottoms in important amounts, whilst components lighter than the light key are not. As well as being in the bottoms product, the heavy key is present in the distillate in important amounts whilst the components heavier than the heavy key are not.
Difficulty of multicomponent separation
Measured by the number of trays required at a given reflux ratio, and is fixed by the concentrations of the key components in the top and bottom product streams. Depends on the ‘K’ value closeness
What is a sharp separation
Where the components are adjacent in the rank order of volatility. The keys are the only components that appear in both product streams in any appreciable concentrations.
Distributed and undistributed components
A distributed component is found in both the distillate and the bottoms product, whereas an undistributed component is only found in one product.
How to improve multicomponent separation
- Low pressure operation: increase relative volatility
- Relax the quality specification
- Experiment with pre-purification
Primary objective of distillation column control
To maintain the specified comp of the top and bottom product, correcting for disturbances in:
- feed flow-rate, composition and temperature
- steam supply pressure
- cooling water pressure and header temp
- ambient conditions which cause changes in the internal reflux
Rectification control
Want to keep tight control of top product composition so maintain pressure as constant as possible, as temp increases heavy key becomes more volatile -> more is driven up, reverse happens at the bottom.
Increasing reflux ratio will reduce temp. Separation efficiency in rectification section improves if increasing reflux ratio, more liquid falling down column, to maintain previous equilib you want more vapour rising up so increase reboil duty.
How is feed flow to column set?
Set externally or could be a level controller
Control requirements summary
- Maintain constant overpressure
- Operate over a range of feed flowrates
- Optimise for desired top product composition
- Optimise for desired bottom product composition
Semi-batch column control
Bottom level is controlled via feed flowrate
Bottoms product is a function of bottoms quality
What variables does pumped reboiler circuit give to control?
reboil flowrate
heat input
Aircooled overhead condenser duty can be controlled by
variable speed fans
lourves
turning fans off/on
How are non-condensibles managed?
By setting overhead condenser pressure controlled by pressure control valve to off gas line
Common and unusual overpessure scenarios
Common:
- High reboiler heat duty
- Loss of cooling to the condenser
- External heat source (e.g pool fire)
Unusual:
- Extreme high bottom liquid level (texas city accident)
- High level in overhead condenser
Batch distillation
Processes carried out in batch, very convenient to distill each batch separately. In these cases, the whole of a batch is run into the reboiler of the still, and on heating, the vapour is passed into a fractionation column. As usual, the composition of the top product depends on the still composition, the no of plates on the reflux ratio used. When the still is operating, since the top product will be relatively rich in the MVC, the liquid remaining in the still will become steadily weaker in this component. As a result, the purity of the top product will steadily fall. Thus, the still may be charged with S1 moles of a mixture containing a mole fraction xS1 of the MVC. Initially, with a reflux ratio R1, the top product has a composition xd1. If after a certain interval of time, the composition of the top product starts to fall, if the reflux ratio is increased to R2, it will be possible to obtain the same composition at the top as before, although the composition in the still will be weakened.
Advantages of batch distillation
More than one product may be obtained, thus a binary mixture of alcohol and water may be distilled to obtain initially a high quality alcohol. As the comp in the still weakens w.r.t alcohol, a second product may be removed from the top with a reduced comp of alcohol.
When a large number of plates is used and the reflux ratio approaches the minimum value, then the continuous distillation has the lowest reflux requirement and hence operating costs. However, if a smaller number of plates is used and high product purity is required, then batch distillation is probably more attractive.
A continuous distillation is the thermodynamically snd economically efficient method for producing large amounts of material of constant composition. However, when small amounts of products of varying compositions are required, a batch distillation provides several advantages over the continuous distillation. Versatile and commonly employed for producing biochemical, biomedical and/or pharma products, in which the production amounts are small but very high purity.
Semi-batch distillation
Fill up base of batch distillation column to start with, with initial material, concentrate it in the bottom. Sometimes want bottom product where it is particularly useful or of importance as a continuous product stream but only when it reaches a certain concentration. So at beginning, may be too dilute but once the batch column has reached a particular place, you want to start drawing out and introduce fresh feed as you do so. No stripping section, just rectification. Analysis will tell you concentrations and analyser will then open outlet valve, bottoms product flowrate determined by conc in bottom batch section, liquid level will controller in bottom controls the addition of fresh material -> at beginning will look exactly like batch.
Mechanical Vapour Recompression
(any distill column) heat required at the bottom, heat removed at condenser in the top/ Heat required at bottom is higher temp than that being removed from the top -> temp profile (gradient) up through column, but:
In mechanical vapour recompression (used e.g orange juice concentration) (when advantageous to use reduced pressure). Compress the vapour coming off the top, the higher temp that you get is slightly greater than temp at the base of the column -> allows for heat integration. Would have to start up heater to get column up to a particular temp to get it going, once vapour generation is begun, using a compressor you end up elevating the temp. Heat input is the mechanical work of the compressor - inefficiency of it results in heat being given to the fluid.
Shortcut design characteristics
- Simplify assumptions to enable quick calculations
e. g constant molar overflow and 100% tray efficiency - Calculations tend to be resolved for the system as a whole
Rigorous design characteristics
Starts from the mathematical description of a fundamental unit and works out to the whole system
Calculations are very time consuming if done by hand and can require many iterations or differential equations
Calculation objectives
Rate, composition & condition of feed
Number of stages
Feed stage number (plus products and pumparounds if applicable)
Product composition specification
Column pressure profile
MESH equations
For each finite unit the following equations must be solved:
Material balance
Equilibrium (vapour-liquid mass transfer)
Stoichiometric equations or composition constraints
Heat or energy balance
Independent and dependent variables
Independent Variables:
Product flowrates and compositions
Internal vapour and liquid rates and compositions
Stage temperatures
Dependent Variables
Equilibrium constants
K values
Mixture enthalpies
Calculation protocol
Problem set-up
Introduce initial values for the MESH variables
Perform the calculations to resolve the MESH equations
Solution test (eg. vs shortcut design or simulation)
Non-equilibrium approaches
Don’t get mixing characteristics all across the tray. Could have deviations on downcomers, or could be larger bubbles in froth not achieving mixing -> bulk liquid, bulk vapour, interface.
Equal vapour dist? What is happening at the wall? edges? blank spaces? baffles? is all the liquid crossing the tray in the same time?
Rules of process modelling
All models are wrong.
Rubbish in equals rubbish out. Make sure you have high quality reliable input data.
Models can only perform calculations that can be done by hand. If you treat them as a ‘black box’ that produces answers then they are worse than useless.
Uses of a rigorous model
Obtain accurate and reliable design data.
The model can be tuned to plant data and used for trouble-shooting, optimisation, debottlenecking or other upgrade projects.
Start up considerations
Start-up consideration
Speed and efficiency
Reliability
Unsteady state operation more likely to expose equipment to beyond design basis conditions (ie. low flow, temperature spikes etc…)
Safety
Accidents are also more likely under start-up or shutdown for the same reasons, see Texas City for an example of this.
Start up sequencing basic
Initial confirmation of equipment readiness (particularly important post-outage.
This will be signed off by operations and engineering
Commission instrumentation and control schemes
Check output signals from level legs, thermocouples etc…
Ensure all controllers are functioning (eg. level control)
Purge system of oxygen if required (eg. with nitrogen or steam)
Introduce feed and establish flowrates and liquid levels
Commission heating and cooling systems
Start up sequencing - Generic for distillation
Introduce feed, establish bottom level and then recycle the bottom back to the feed inlet (short recycle) or back to the feed tank (long recycle). Feed heat exchangers will be bypassed.
Add heat to reboiler and establish column pressure and temperature at the top of the column.
Once there is a level in the overhead accumulator then reflux can be returned to the column.
Heat exchanger bypasses can be carefully closed and column temperature and pressure controllers can be established.
Start up - What can go wrong?
Loss of site of liquid levels (see Texas City disaster)
Temperature fluctuations & pressure bumps
Mis-direct off-spec products
Operate equipment outside of design parameters leading to damage such as centrifugal pump vibration
How do you increase % benzene in the distillate?
How do you increase % benzene without reducing distillate flowrate?
Increase reflux ratio, but will also reduce top temp, will increase ratio of liquid to vapour at top but will also swamp product by reducing flowrate of distillate
To maintain quality whilst increasing purity:
Add heat to the column feed or reboiler, increase amount of vapour -> increased vapour and liquid traffic with higher reflux ratio you then drive more material up to the top of the column and increase separation efficiency -> hard limits -> jet flooding, downcomer back-up, hydraulic limits.
List as many ways as you can to improve fractionation efficiency.
How can you remedy a column that is pinched in the rectification section?
- Operating at lower pressure -> greater relative volatility. Higher reboil and reflux ratios.
- Can reduce the reflux ratio or reduce the feed q.
Trouble shooting general
Conduct regular test runs of the column and maintain a tuned simulation model.
Q. What data would you need to collect in your testrun?
Reboiler data, pressures and temperatures, flow data, compositions from sample of feed, top and bottom products.
If in doubt, perform a material and energy balance with live plant data. This always helps!
Use shortcut calculations such as McCabe Thiele to further analyse the plant data and to test hypotheses.
Common faults
Tray damage / fouling / coking / corrosion.
Pump failure (rotating equipment needs frequent maintenance
Other equipment failure (eg. valve fails to fully open, SRV spuriously lifts).
Instrument failure (eg. thermocouple reads too low or controller not properly configured).
Humans
