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Question 1

Ideal distillation stage

Answer 1

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

Question 2

What can high temp lead to (crude oil) and possible solution ?

Answer 2

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.

Question 3

What are weirs and active tray areas?

Answer 3

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.

Question 4

Plate design, what happens and where?

Answer 4

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

Question 5

More trays =? Trays designed to do what/what is overall purpose?

Answer 5

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.

Question 6

Bubble cap trays

Answer 6

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.

Question 7

Sieve trays

Answer 7

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.

Question 8

Valves trays

Answer 8

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.

Question 9

LOOK AT COMPARISONS OF TRAY DESIGNS

Answer 9

-

Question 10

What happens middle, bottom and top of tray: fractionation

Answer 10

• 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.

Question 11

What is nitrogen used for in crude oil distillation

Answer 11

Used as an inert can greatly reduce partial pressure of light component in gas phase -> higher stripping efficiency

Question 12

What happens during rectification?

Answer 12

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.

Question 13

Where does stripping of heavy components happen?

Answer 13

Bottom

Question 14

What is structured packing?

Answer 14

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.

Question 15

What is packing used for ?

Answer 15

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.

Question 16

Liquid and gas distributors

Answer 16

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.

Question 17

Why are there limits on vapour and liquid velocities?

Answer 17

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).

Question 18

What can flooding be due to and what are the flow regimes?

Answer 18

Changes in feed and reflux conditions. Flow regime refers to the way that vapour and liquid are dispersed on the tray

1. 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.
2. Spray: vapour phase is continuous, liquid is dispersed as droplets in the vapour. Occurs at high vapour velocities and low liquid loads.

Question 19

Spray entrainment flooding

Answer 19

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.

Question 20

Froth entrainment flooding

Answer 20

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.

Question 21

Downcomer back-up flooding

Answer 21

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.

Question 22

Downcomer choke flooding

Answer 22

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.

Question 23

Effects of total pressure on flooding

Answer 23

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.

Question 24

What is azeotropic mixture?

Answer 24

‘Constant boiling’ mixtures, have inflexion point where vapour and liquid have the same composition, cannot go past it, limits separation.

Question 25

- Distillation Column Operation: Continuous fractionation

Answer 25

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)

Question 26

Feed Plate location

Answer 26

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.

Question 27

Total Reflux

Answer 27

No feed and no product.

Question 28

Effects of increasing reflux ratio

Answer 28

Makes more expensive as column diameter increases as D is a fixed value, condenser duty also increases.

Question 29

Heat and cooling duties

Answer 29

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.

Question 30

Differential distillation

Answer 30

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.

Question 31

Flash vaporisation

Answer 31

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.

Question 32

Constant molar overflow assumption

Answer 32

Molar flowrate of vapour = molar flowrate of distillation and reflux

Question 33

How many columns needed to separate a mixture with 'n' components

Answer 33

n-1 columns

Question 34

What are the heavy and light keys

Answer 34

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.

Question 35

Difficulty of multicomponent separation

Answer 35

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

Question 36

What is a sharp separation

Answer 36

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.

Question 37

Distributed and undistributed components

Answer 37

A distributed component is found in both the distillate and the bottoms product, whereas an undistributed component is only found in one product.

Question 38

How to improve multicomponent separation

Answer 38

- Low pressure operation: increase relative volatility - Relax the quality specification - Experiment with pre-purification

Question 39

Primary objective of distillation column control

Answer 39

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

Question 40

Rectification control

Answer 40

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.

Question 41

How is feed flow to column set?

Answer 41

Set externally or could be a level controller

Question 42

Control requirements summary

Answer 42

- Maintain constant overpressure - Operate over a range of feed flowrates - Optimise for desired top product composition - Optimise for desired bottom product composition

Question 43

Semi-batch column control

Answer 43

Bottom level is controlled via feed flowrate | Bottoms product is a function of bottoms quality

Question 44

What variables does pumped reboiler circuit give to control?

Answer 44

reboil flowrate | heat input

Question 45

Aircooled overhead condenser duty can be controlled by

Answer 45

variable speed fans lourves turning fans off/on

Question 46

How are non-condensibles managed?

Answer 46

By setting overhead condenser pressure controlled by pressure control valve to off gas line

Question 47

Common and unusual overpessure scenarios

Answer 47

Common: 1. High reboiler heat duty 2. Loss of cooling to the condenser 3. External heat source (e.g pool fire) Unusual: 1. Extreme high bottom liquid level (texas city accident) 2. High level in overhead condenser

Question 48

Batch distillation

Answer 48

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.

Question 49

Advantages of batch distillation

Answer 49

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.

Question 50

Semi-batch distillation

Answer 50

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.

Question 51

Mechanical Vapour Recompression

Answer 51

(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.

Question 52

Shortcut design characteristics

Answer 52

- 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

Question 53

Rigorous design characteristics

Answer 53

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

Question 54

Calculation objectives

Answer 54

Rate, composition & condition of feed Number of stages Feed stage number (plus products and pumparounds if applicable) Product composition specification Column pressure profile

Question 55

MESH equations

Answer 55

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

Question 56

Independent and dependent variables

Answer 56

Independent Variables: Product flowrates and compositions Internal vapour and liquid rates and compositions Stage temperatures Dependent Variables Equilibrium constants K values Mixture enthalpies

Question 57

Calculation protocol

Answer 57

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)

Question 58

Non-equilibrium approaches

Answer 58

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?

Question 59

Rules of process modelling

Answer 59

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.

Question 60

Uses of a rigorous model

Answer 60

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.

Question 61

Start up considerations

Answer 61

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.

Question 62

Start up sequencing basic

Answer 62

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

Question 63

Start up sequencing - Generic for distillation

Answer 63

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.

Question 64

Start up - What can go wrong?

Answer 64

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

Question 65

How do you increase % benzene in the distillate? | How do you increase % benzene without reducing distillate flowrate?

Answer 65

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.

Question 66

List as many ways as you can to improve fractionation efficiency. How can you remedy a column that is pinched in the rectification section?

Answer 66

- Operating at lower pressure -> greater relative volatility. Higher reboil and reflux ratios. - Can reduce the reflux ratio or reduce the feed q.

Question 67

Trouble shooting general

Answer 67

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.

Question 68

Common faults

Answer 68

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