DISTILLATION COLUMNS (or TOWERS)
Distillation columns or towers are constructed to behave in the same way as a series of separate stills as discussed earlier. Each 'still' section consists of a number of 'TRAYS' or contacting devices arranged vertically above one another in the column. These trays or contactors bring liquid and vapour into intimate contact in order to obtain the required separation of the mixture. The height of the tower and the number of trays or contacting devices it contains, depends upon the purity of the 'Fractions' required.
Columns for the distillation process can be of the following types:
- The 'PACKED' Tower
- The 'TRAY' Tower
1. THE PACKED TOWER
As its name implies, the packed tower is a vertical, steel column which contains 'Beds' of packing material which are used to bring the rising vapours into intimate contact with falling liquid within the tower. The heat added to the mixture before entering the tower partially vaporises the mixture and the vapours rise up the tower and begin to cool.
The liquid falls towards the bottom of the tower. At the tower bottom, in general, more heat is added to the liquid by a 'Reboiler' which may be steam heated or a fuel fired furnace type.
The addition of heat here causes more vapours to rise up the column. As the two phases of the mixture - falling liquid and rising vapour - come together, light components are stripped out of the liquid and enter the gas phase while heavy components in the vapour are condensed into the liquid phase.
In this way, as the vapour rises and gradually cools, it becomes lighter and, as the liquid falls, it becomes hotter and heavier.
With this type of distillation column there is generally only a top and bottom product. The quality of the products depends upon the height of the tower, the number of contacting devices, the tower temperature and pressure and their control, and the velocity of the rising vapours.
The type of packing materials used, also plays a part in the separation process. The packing can be of such types as:
Ceramic Raschig Rings, Stainless Steel Pall Rings or Ceramic Saddles .. etc. See Figure: 10.
2. THE TRAY TYPE TOWER
This is also a tall, cylindrical column. Inside, a series of trays are placed, one above the other. The trays are used to bring the rising vapour and falling liquid into intimate contact. Tray towers do the same job as packed towers but they are very much more efficient in the separation process than packed towers and, they are also more costly. There are various types of tray in use and the type selected depends upon the degree of product purity required, the type of fluids, fluid velocity and other process parameters of the system.
The types of tray used in distillation columns are as follows:
- THE SIEVE TRAY is simply a metal plate containing drilled holes through which the rising vapour can pass into the liquid flowing across the tray. Figure: 11
- THE VALVE TRAY is similar to the sieve type but, each hole is fitted with a flapper valve which opens as vapour passes through the hole. This type is used where vapour velocity is not constant and the valves prevent liquid from dumping through the holes at times of low gas velocity. Figure: 12
- THE BUBBLE-CAP TRAY is the most efficient separation device but, is also the most costly. It consists of a number of 'Chimneys' or 'Risers' (small, short pipes set into the tray), through which the vapour can pass. Fitted over the riser is a 'Cap' which causes the rising vapour to turn through 180 °. This forces the gas to 'Bubble' through the liquid flowing across the tray. The liquid level on the tray is maintained below the top of the riser to prevent dumping of liquid down the tower. Figure: 13
Each of the above trays also has a 'WEIR' that maintains the liquid level on the tray. As the liquid flows over the weir, it enters a 'DOWNCOMER' - (a short pipe), that carries the liquid down to the tray below. The downcomer outlet is below the surface of the liquid on the tray below, acting as a seal to prevent gas from bypassing the tray above.
The liquid is prevented from dumping through the perforations by the velocity of the up-flowing gas passing through them. The 'WEIR' maintains the liquid level on the tray and the gas is forced to bubble through the liquid. This gives intimate contact between the gas and liquid.
With the 'VALVE' tray, a non-return valve is fitted over each hole. This will close due to the weight of liquid at times of low gas velocity.
See Figure: 12
OPERATION OF VALVE TRAYS
OPERATION OF BUBBLE-CAP TRAYS
SIMPLE CONTINUOUS DISTILLATION PROCESS
Refer to Figure: 14, as you read on. This represents a basic Crude Oil distillation column where the feed to, and the products from, the unit is a continuous operation.
In the distillation process, the crude oil feed is first heated by exchanging heat with some of the hot products leaving the column. This cools the products and, at the same time reduces the fuel requirements in the main heater - the fuel fired furnace.
The hot feed now enters the tower into the 'Flash Zone'. At this point, due to the greatly increased volume of the column, the lighter components of the crude oil ' Flash Off ' (vaporise), and rise up the column. The hot liquid will fall towards the column bottom.
The bottom section of the column, below the Flash Zone, called the 'Stripping Section', contains trays - generally Bubble-cap or Sieve type. The tower bottom liquid is re-circulated & re-heated in a steam or fired 'Reboiler' which drives off vapours of light ends and some of the heavy ends contained in the liquid. These vapours rise upwards through the trays and contact the down-flowing liquid. This action further removes (strips out), light ends from the liquid.
The top section of the tower, above the flash zone, is called the 'Rectifying Section'. Here again, the rising vapour passing through the trays, contacts the liquid flowing across them.
Action of the Trays Each tray in the tower is acting like a single still as discussed in 'Batch Distillation'. As we rise above the flash zone, each succeeding tray is slightly cooler than the tray below.
The down-flowing liquid, as it passes across the trays is becoming hotter and heavier as light ends boil off into the vapour phase. Conversely, the rising vapour is becoming cooler and lighter as heavier ends condense into the liquid on the tray.
The down-flowing liquid is called 'Internal Reflux' and works in the same way as in Figure: 7 (Page 14), where the liquid is returned to the preceding still.
At pre-determined points in the column, the process conditions (mainly temperature and pressure), are such that, the liquid components are at the required purity to meet the specification desired as a product - like 'Kerosene' for example. At these points, the tower will contain 'Collecting Pans' from which the desired product can be drawn from the tower.
The lightest components of the crude oil mixture leave the top of the tower as vapour. This is fed through condensers - generally water-cooled -and the condensate, usually Naphtha and water, passes into the 'Overhead Receiver or Accumulator'.
In the receiver, light gases also build up. The control of these gases, (to a fuel system or flare), also controls the pressure on the distillation process at the required level.
The Naphtha liquid forms an interface above the water. The water is drained away under control, to disposal. The Naphtha, also under level control, is divided into two - some is returned to the tower top tray as 'External Reflux' which is used to control the tower top temperature and thereby help to control the naphtha quality.
The remaining naphtha from the receiver is piped to storage and / or to other processes. The products leaving the side of the column -called 'Side-streams', are usually passed through 'Stripping Towers' where an injection of superheated steam removes final traces of light ends to meet the specification required for the product. The light ends and steam are passed back into the tower.
The control of the quality of the side-stream products is generally helped by a controlled flow of 'Intermediate Reflux' of some of the product into the column just above the section producing the product.
The side-stream products pass from the stripping towers through feed / product exchangers and water coolers to storage.
The tower bottom product as already mentioned, is reheated in a reboiler to remove light ends and to provide stripping gases in the tower. The final bottom product, such as heavy fuel oil is pumped away via feed/product heat exchangers and water coolers to storage. In some distillation systems, superheated steam may be injected into the tower bottom to assist the stripping process.
Crude oil distillation is often carried out under vacuum conditions. The vacuum is produced by pulling the overhead vapour from the tower by steam ejectors via surface condensers.
The explanation of crude oil separation given above, is that of a basic system. Crude oils also produce chemicals, waxes, gasolines, lubricants and many other products in everyday use.
DISTILLATION COLUMN CONTROL
- REFLUX RATIO
- TEMPERATURE GRADIENT
1. REFLUX RATIO
The reflux to a tower top is used to control the top temperature thereby controls the purity of the overhead product.
The amount of reflux compared to the product is known as the 'Reflux Ratio'.
An example of this is as follows:
The overhead liquid from a distillation column is divided into 4 m3 per hour reflux and 2 m3 per hour product. Therefore:
When reflux ratio is increased, the amount of reflux increases. Reflux represents cooled, condensed top product returned to the tower top and, as such it is being reprocessed. The top product will therefore be purer. In general, the higher the reflux ratio, the fewer the number of trays required for a given separation.
However, too high a ratio may cause flooding in the tower resulting in poor separation and causing 'off-spec' products throughout the system. The reflux rate is normally controlled by a temperature controller in the vapour outlet which operates a control valve in the reflux pump discharge. An increase in tower top temperature will cause the valve to open, increasing the reflux rate, and vice versa.
2. TEMPERATURE GRADIENT
Temperature (and pressure) control of a distillation tower will govern the purity of the products. The control of top temperature is as discussed above in 'reflux'. Control of the feed inlet temperature and that of the reboiler are also very important.
Again, if feed and bottom temperatures are too high, too much heavy vapour will rise up the tower and put side-stream products off-spec. This condition, combined with high reflux rate will again lead to flooding and poor separation. Opposite conditions can lead to liquid starvation across the trays and again, a very upset process will result.
The careful control of top temperature, feed and reboiler temperatures, together with pressure control, will give the desired temperature profile across the tower.
Remember, changes in pressure will affect the boiling points of the components in the crude oil. The vapour pressures therefore, will also be affected and again, if the control parameters are incorrect, the system will be inefficient.
- High top temperature will result in heavy components in the overhead product.
- Low top temperature will result in a lighter top product.
- High feed temperature will give heavier side-streams and vice-versa.
- High reboiler temperature will produce heavier bottoms product and pass heavier vapours up the tower to affect the side-streams.
- Increased pressure in the system will give lighter components in the overhead LIQUID product and decrease its Initial Boiling Point, whereas the FBP is governed by the tower top temperature.
It can be seen that careful, accurate control of the variables is very important in order to achieve the required quality control of the products.
Also, with regard to the purity of the side-streams, control of the stripping towers' steam supply is very important.
A further point is, that high water content in the crude feed will cause pressure surges as the water vaporises in the tower. The crude oil should be as water free as possible.
Many modern distillation units are operated under high vacuum. This method, due to the vacuum decreasing the BP's of the components of the mixture to be separated, also reduces the amount of heat energy needed to vaporise the components.
VACUUM DISTILLATION TOWER OVERHEADS SYSTEM
In the above diagram, the surface condenser is a 'Total Condensing' unit. This means that all fluids that can be condensed are changed to liquid. Due to this, a vacuum is formed in the tower the level of which depends upon the degree of condensation allowed to take place. This is governed by the level of distillate and how much of the condensing surface is covered. The liquid level and therefore the amount of condensing surface available will decide the level of vacuum (Absolute Pressure) of the system.
The PRC is therefore controlling the available condensing area on the cooling tubes. Increasing absolute pressure (decreasing vacuum), will open the control valve, the liquid level in the condenser will fall thus presenting more condensing area to the vapour. More vapour will condense and therfore the pressure will drop back again - and vice-versa.
A small quantity of uncondensible gases will tend to build up in the tower and the surface condenser which, if allowed to build up, will slowly destroy the vacuum.
The ejector is used to remove the uncondensibles which are passed into the separator after passing through the ejector condenser which condenses the ejector steam – thus helping to maintain the vacuum, while the uncondensed gases are fed via a check-valve or control valve to atmosphere or flare system. The barometric seal loop holds a head of liquid which will prevent the vacuum pulling gases back out of the separator.
About the Author
Norrie is a retired professional who has been working in Oil and Gas and LNG production in Marsa-el-Brega, Libya for 30 years.
Norrie used to be in the Training Dept. and prepared Programmes for Libyan Trainees