Sieve Trays: Harder to install than bubble caps, needs two people to install.
Valve Trays: Basically the same as a sieve tray, however installation time can be very lengthy if the valves do not come pre-fitted.
Shower Trays: Easy installation, however time and cost come into consideration as they require replacing quite regularly.
d liquid/vapour handling capacity
Bubble Cap: Have an average handling capacity due to quite large surface area of each plate. The problem that it does have is that it can only have small quantities of liquid surrounding each bubble or the liquid will move through the vapour downcomer preventing most efficient contact places.
Sieve Trays: Have a greater liquid/vapour handling capacity than shower trays, but not as great as valve trays.
Valve Trays: Has greatest liquid/vapour handling capacity of all the trays.
Shower Trays: Least effective at handling liquid and vapour.
e plate efficiency
Bubble Cap: Bubble cap trays are not very efficient, about 70% as efficient as sieve trays and valve trays. In recent years, most columns that have used bubble caps before are now filled with the sieve or valve trays due to the increased efficiency.
Sieve Trays: More efficient than bubble cap trays as they can be installed closer together in the column. Individually not as efficient but over more.
Valve Trays: Most efficient type of tray due to a high surface area making a longer contact time between down coming liquids and rising gases. Now being installed as upgrades to old bubble caps.
Shower Trays: Least efficient, only about 50% efficient overall and not really a feasible project if used on a large scale.
f pressure drop across plate
Bubble Cap: Bubble caps offer the largest pressure drop compared to other types of trays and packing. However, this on its own does not overcome all of the other disadvantages.
Sieve Trays: Quite a high pressure drop, but not as great as bubble caps or valve trays. A good pressure drop compared to the price.
Valve Trays: Nearly as much of a pressure drop as bubble caps, but other advantages make it a more profitable investment.
Shower Trays: Very little pressure drop compared to other trays, even with the closest packing possible.
g entrainment
Bubble Cap: Has a high rate of entrainment compared to other trays
Sieve Trays: Lower rate than bubble caps, more trays can be fitted in smaller space though giving about even performance over a certain height
Valve Trays: Less efficient than sieve trays, lower rate due to less surface area.
Shower Trays: More efficient than no trays, but not worth the money.
h dumping.
Bubble Cap: Slight chance of dumping due to raised sides to the holes in the trays. The cap on top also reduces dumping.
Sieve Trays: Higher chance of dumping than bubble caps and valve trays, but much more effective than shower trays.
Valve Trays: The smallest chance of dumping due to the valves on the plates restricting the flow of liquid down the column.
Shower Trays: Not effective due to the lack of methods to prevent or slow down the dumping process.
4. describe the construction, operation and application of packed columns.
Packed columns are basically the same as tray columns however instead of being filled with the trays listed about, they are filled with packing.
Structured Packing.
Packing Fills the column from top to bottom. It provides a very large surface area for gas-liquid interaction to occur. There are still points at the side of the column for taking off product, however this can be more difficult than taking off product in a tray column. Packed columns are generally used where two products are being separated, one can be taken off the top and the other at the bottom.
5. for the main types of column packing listed. Give typical examples of their use.
a Raschig rings
Ceramic Raschig Rings.
Raschig rings are a means of reducing pressure in a column. They are a form of random packing used in distillation columns. They maintain good efficiency.
b Lessing rings
Lessing rings are used in applications of mass transfer and direct heat transfer. They are made in two parts, metal strips bent into ring shapes and then the ends connected with a projection and indentation.
A Lessing Ring
c Ben saddles
Saddles are used for denitrifying aqueous solutions. As long as the solution contains a nitrate concentration in excess of one kilogram nitrate per cubic meter, it will remove some of it. They are installed as part of the general packing of the column.
d interlox saddles
Manufactured to about 1” in size, they are used in the purification of hydrochloric acid.
e Pall rings.
Pall rings are used in biological filtration systems. They will float above bottom inlet and outlet ports and deter the build up of solids around the ports.
6. compare packed and plate columns with respect to efficiency, cost and operational
problems in terms of the following:-
a corrosion
Packed columns: will corrode at un-even rates which can lead to a drop in efficiency, increased operational costs and more problems.
Plate columns: plates will corrode un-evenly which can lead to them failing. As plates corrode the efficiency will decrease while operational costs and problems increase.
b foaming
Packed Columns: due to the reduction in room throughout the column, and more importantly at the top, the packing helps reduce foaming. This in turn will keep a higher efficiency, while lowering both the operational costs and problems.
Plate columns: can suffer badly from foaming. The foam produced is very thick and although it doesn’t last very long it can lead to major operational problems. Its production is dependant on the vapour rate of the column. Foam will decrease the efficiency of a column and increase the costs and problems associated with them.
c fouling
Packed columns: suffer if it contains closed packing such as Raschig rings and Pall rings. If open sided packing is used the much lower chance. If fouling does occur, there are major operational problems, costs dramatically rise and efficiency falls greatly.
Plate columns: more susceptible to fouling due to the larger open areas and smaller holes on the trays. Can lead to the column filling, increased cost of operation due to less area for the liquid/vapour interaction and lower efficiency.
d heat evolution
Heat evolution leading to “run-away”.
Packed columns: not as susceptible to heat evolution as plate columns, however when it does occur it can have a bigger impact. The efficiency of the column will decrease dramatically while the costs of operating and the problems that can be encountered would become extremely high.
Plate column: high chance of heat evolution occurring however it can be more controllable that in a packed column if it does start to occur due to the extra room to add cold feed and reduce temperature. If it does occur, again the costs and problem count would increase while the efficiency would drop.
e liquid hold up
Packed columns: does not occur that often due to the construction. As there are many routes for the gas and liquid to take, there is less of a chance of liquid hold up occurring. Also, there is also only going to be a small surface area on which the rising gas can act, again reducing the chance. This will increase the operating costs, but lower the efficiency and problems that are encountered.
Plate columns: liquid hold up is a common thing. Rising gases stop all the liquid from falling down the column, ensuring there is a constant level on each plate. This can improve efficiency, lower costs and operating problems.
f pressure drop.
The pressure drop across a packed column is almost constant, where as in a plate column it varies as you move upwards from the bottom. This gives a difference in application situations, the packed columns are more suited to operating when only two or maybe three products need separating such as Isobutane from butane, where as plate columns are able to fraction multiple components out. If either is used in the wrong situation, the operating problems can be tremendous as plate columns will not separate IC4 from C4 and crude oil will not be split in a packed column properly.
7. explain the following operating problems associated with plate and packed columns.
a variations in temperature
Packed columns have a small temperature difference between the top and bottom of the column. This means that if the temperature were to rise slightly in the column, then the product that should come out of the bottom of the column may leave at the top. The second product will either also leave with it, or increase the pressure of the column, in turn raising the temperature again.
Plate columns have a wide variation in the column between the top and the bottom. If the temperature increases by a few degrees, this can lead to a chain reaction of the pressure increasing, which in turn will further raise the temperature. The increase can lead to major operational problems as the feed rate and burner rate will have to be lowered to try and decrease the temperature, possibly leading to a full shut down.
b variations in pressure
Variations in pressure can lead to major problems. This is because as the pressure increases, the temperature will also rise. This is the case for both column types. As both of these rise, the product output will change on the trays or at each end of the column, which may lead to problems further down the line. Too much of an increase could possibly lead to over-pressuring, a column shutdown or an explosion.
c variations in composition
As the composition of the incoming feed varies, the pressure and temperature of the column may need to change. If this is not done correctly, then there may be problems in other areas where the column outputs are going. This can lead to the production of off-spec products and major operational and financial discrepancies.
d variations in reflux ratio
If the reflux ratio alters, this can affect the temperature and/or the pressure depending how the size of the alteration. If it is large, it could lead to over-pressuring or too greater temperature being present in the vessel, possibly resulting in column shutdown or an explosion.
e boil up
Boil up (the point at which reflux begins) can be a dangerous time. If the rate of boil up is too great, it can lead to exponential temperature and pressure increases within the column, whether it is packed or plated. This can lead to explosions or unit/area shutdowns if it not controlled closely.
Outcome 3: List and explain the function of ancillary equipment required by distillation plant
1. describe the construction, operation and application of ancillary equipment
a re-boilers
Re-boilers are essentially a shell and tube heat exchanger located at the base of a fractional distillation column. They are made up by passing a cool steam of liquid from the column through tubes which are surrounded in a hot liquid or gas, generally steam. This partially vaporises the cool liquid before it re-enters the column, with the vapours driving the separation of products.
The operation is essential. They require constant monitoring to ensure that all the temperatures and flow rates are correct to ensure maximum efficiency.
b condensers and coolers
These are used to lower the temperature and possibly change the state of a product once it has been through the column. It is possible to have them similar to a shell and tube exchanger or more like a fridge. Here, air is possible to use as the coolant for the hot liquid. Fin fans are an example of using air as a coolant. Powerful fans blow cold are over tubes that are cover in fins to increase the surface area.
c reflux ratio dividers
Pieces of electronic equipment which are set up in line with the column. They measure the feed rate and output rate total, temperature and pressure and work this out into a ratio. This value is then compared to a pre-set ratio. The divider will either open to allow more reflux feed into the column or close to reduce it depending what the outcome of the comparison is.
It is constructed in a small, earthed casing containing an intrinsically safe circuit for performing the calculations.
d pipe furnaces and pre-heaters
Both pipe furnaces and pre-heaters are large bits of equipment. The main purpose of both is to provide a hot feed for the column. This allows optimum efficiency for operation and also reduces the time the products need to be in the column for.
Rotating Air Preheater.
e vacuum and pressure equipment.
The vacuum distillation column is designed to operate at a pressure lower than the atmosphere. This means that the fractions that are remaining in it will boil at a lower pressure, as for something to vaporise the vapour pressure must exceed the ambient pressure.
The vacuum is created in one of two methods. Either a tower is installed and the Venturi effect applies, lowering the internal pressure of the tower and as such drawing air from the column to replace it, or a compressor is installed with the suction connected to the top of the tower. This then will draw the air in the vacuum column away, lowering the internal pressure.
2. explain how the following process variables are controlled using ancillary equipment
a column temperature
The column temperature can be controlled in many ways. The main two ways are to alter the reflux ratio and increase a colder feed of reflux back into the column. The other main way is to lower the fuel and air supply to the burner, thus making them fire at a lower rate producing less heat to keep a high temperature. The opposite can be used to raise the temperature. By adding a hotter reflux and increasing the fuel and air to the burner, the temperature can easily be raised. A third method is to control the initial feed temperature.
b feed rate
The feed rate is controlled using valves and pumps. If the inlet valves are opened more from where they were originally, the rate will increase. The opposite effect will occur if the valves are closed. If a pump is added to the line of the feed, the rate will dramatically increase compared to natural circulation.
c reflux variations
The reflux is controlled using a reflux ratio divider (see Outcome 3, Question 1.c.).
d feed temperature
Feed temperatures are controlled by using various forms of heaters and exchangers. Heating a feed can be done either by it passing through a pre-heater (Outcome 3, Question 1.d.) or through a heat exchanger. In the heat exchanger, the feed would be passed counter flow to a hot product though a set of tubes.
The heat transfers through the walls of the tubes and into the oncoming feed product.
e column pressure
Column pressure can be controlled in a number of different ways. A PSV (Pressure Safety Valve), a PVSV (Pressure Vacuum Safety Valve) or a TSV (Thermal Safety Valve) can be fitted. If the temperature, pressure or vacuum levels exceed a set limit, these will lift until the problem has been resolved. If the pressure internally is required at less than atmospheric pressure, then a compressor with the suction attached to the top of the column can be fitted. This will lower the pressure, thus lowering boiling points.
f product analysis.
The product flow can be analysed in various methods. The normal method though is to run a small line off the main process line to a testing station. Here, it is run through equipment which detects various content, such as H20, H2S and benzene. These can be relayed to a visual method of display for further interpretation. If needs be, the feed can be changed to correct the problem.
Outcome 4: Demonstrate a knowledge of the safety hazards associated with distillation processes and the precautions necessary to minimise them
1. state the precautions which will minimise the following hazards associated with distillation plant
a corrosion
Corrosion can be prevented by ongoing maintenance, product analysis and inspection. If the plant is kept in good working order, then there will be less chance of a problem turning up with a valve or other piece of equipment. The product analysis is important as this can be a tell tale sign of anything going wrong internally. If there are high levels of a certain impurity, then it may mean that there is something not being removed in the process of that part of the internal structure is starting to fail. Inspection can keep a close monitoring of the bulk of the structure. One hazard is pin hole’s forming in the main structure and leaking. With monitoring of the situation, these can be kept to a minimum and patches attached prior to a problem arising. Painting will also reduce corrosion.
b explosion
Fitting of PSV’s, TSV’s and PVSV’s to column’s to control the limits at which it can run, this will reduce the chance of an explosion. Also, by keeping the area free of ignition sources then this risk will reduce further.
c implosion
Implosion can only occur on vessels under a vacuum. To prevent this, Vacuum Safety Valves are fitted to allow an influx of air to raise the pressure should the vacuum become too strong.
d toxicity of materials
Toxic materials must be marked up at all times, with visual signs and possible audio sirens as well. They must be kept in secure locations and have a log of amounts, uses and the data sheets available encase of emergency. Toxicity is also controlled by the requirement of increased PPE (Person Protective Equipment).
e flammability of materials
Materials that are flammable must be clearly marked and identified. A data sheet must also be provided with material stating the ignition point and other information. Flame retardant clothing must be worn while dealing with these to further reduce the possible risks, along with keeping sources of ignition of the area.
f overheating and auto-ignition
Due to the nature of the operation, overheating may occur. These temperatures can be constantly monitored. If a temperature is seen to be rising too quickly, removal of the heat source (commonly a burner) or cooling via external means (such as a fire hose spraying water) is the usual methods of reducing these.
Auto-ignition is in place for the places where a source of ignition is required, such as the base of a burner. The system is set up so that on the press of a button, both the fuel and air feeds are applied and then an ignition spark to create the pilot, all done from a safe distance. This means that there is reduced chance of injury as there are no persons stood directly below the burner when the fuel is being ignited.
g pressure surge
A pressure surge can be controlled in two ways. One is to have a reduced in the line, which will only allow a certain amount of product through, such reducing the pressure of it. The other is to have PSV’s fitted to the column’s and vessels. If the pressure increases and exceeds a set limit, the valve lifts and the pressure escapes through the valve.
h static electricity
Static electricity is a major problem. The only way to reduce the chance of a problem from it is to have all vessels, columns, equipment and people earthed. By doing this, the charges that build up are dissipated into the ground in a controlled manner, lowering the likelihood of a problem.
i spillages and leaks.
Spillages and leaks can be controlled in two ways. By having a designated spills response team, if a problem does arrive then there are already people designated to deal with it. Also, the construction of bunds around columns and vessels helps to control the problem is it does occur. To prevent them, regular inspection of everything is required to check thicknesses and strengthen any point which is vunurable.
2. state the function of the special chemicals used to reduce operational hazards
- anti-foaming agents
Anti-foaming agents are used to prevent foaming in the columns (see Outcome 2, Question 6.b.) Due to the value and efficiency of the chemical, only a small amount is needed to control the hazard. It eliminates the build up of small bubbles. Commonly used agents are insoluble oils, dimethyl polysiloxanes and other silicones, certain alcohols, stearates and glycols.
- corrosion inhibitors.
Corrosion inhibitors remove oxygen from the product. This helps to reduce the amount of internal corrosion of vessels and pipe’s, as most chemicals require the presence of oxygen to react with metal.