- How does distillation work?
- Crude distillation
- Propane deasphalting
- Solvent extraction and dewaxing
- Crude oil often contains water, inorganic salts, suspended solids, and water-soluble trace metals.
- Step 0 in the refining process is to remove these contaminants so as to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning catalysts in processing units.
- The two most typical methods of crude-oil desalting are chemical and electrostatic separation, and both use hot water as the extraction agent.
- In chemical desalting, water and chemical surfactant (demulsifiers) are added to the crude, which is heated so that salts and other impurities dissolve or attach to the water, then held in a tank to settle out.
- Electrical desalting is the application of high-voltage electrostatic charges to concentrate suspended water globules in the bottom of the settling tank. Surfactants are added only when the crude has a large amount of suspended solids.
- A third (and rare) process filters hot crude using diatomaceous earth.
- The crude oil feedstock is heated to 65-180°C to reduce viscosity and surface tension for easier mixing and separation of the water. The temperature is limited by the vapor pressure of the crude-oil feedstock.
- In both methods other chemicals may be added. Ammonia is often used to reduce corrosion. Caustic or acid may be added to adjust the pH of the water wash.
// How does distillation work?
- Distillation is defined as:
- a process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat.
- Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points.
- Thus, when this vapour is cooled and condensed, the condensate will contain the more volatile components. At the same time, the original mixture will contain more of the less volatile components.
- Distillation is the most common separation technique and it consumes enormous amounts of energy, both in terms of cooling and heating requirements.
- Distillation can contribute to more than 50% of plant operating costs.
- Distillation columns are classified by the manner in which they are operated:
- Batch, in which the feed to the column is introduced batch-wise. That is, the column is charged with a ‘batch’ and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced.
- Continuous columns process a continuous feed stream. No interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the most common of the two types.
// Continuous distillation columns
Classified according to:
- Nature of the feed that they are processing:
– binary column – feed contains only two components;
– multi-component column – feed contains more than two components.
- Number of product streams they have:
– multi-product column – column has more than two product streams.
- Where extra feed exits when used to help with the separation:
– extractive distillation – where the extra feed appears in the bottom product stream;
– azeotropic distillation – where the extra feed appears at the top product stream.
- Type of column internals:
– tray column – trays of various designs used to hold up the liquid to provide better contact between vapour and liquid;
– packed column – packings are used to enhance vapour-liquid contact.
// Main Components of Distillation Columns
- A vertical shell where separation of liquid components is done.
- Column internals e.g.trays/plates and/or packings which are used to enhance component separations.
- A reboiler to provide the necessary vaporization for the distillation process.
- A condenser to cool and condense the vapour leaving the top of the column.
- A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column.
// Trays and plates
Bubble cap trays
A riser or chimney is fitted over each hole, and a cap covers the riser. The cap is mounted with a space to allow vapour to rise through the chimney and be directed downward by the cap, finally discharging through slots in the cap, and bubbling through the liquid on the tray.
Bubble cap trays
Perforations are covered by caps lifted by vapour, which creates a flow area and directs the vapour horizontally into the liquid.
Sieve trays are simply 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.
Liquid and vapour flows in a tray column
• Each tray has 2 conduits called downcomers: one on each side. Liquid falls by gravity through the downcomers from one tray to the tray below.
• A weir ensures there is always some liquid (holdup) on the tray and is designed such that the the holdup is at a suitable height, e.g. such that the bubble caps are covered by 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 called the active tray area.
• Packings are passive devices designed to increase the interfacial area for vapour-liquid contact.
• They do not cause excessive pressure-drop across a packed section, which is important because a high pressure drop would mean that more energy is required to drive the vapour up the distillation column.
• Packed columns are called continuous-contact columns while trayed columns are called staged-contact columns because of the manner in which vapour and liquid are contacted.