DISTILLATION COLUMN BASICS
The liquid mixture that is to be processed is known as the feed feed and and this is introduced usually somewhere near the middle of the column to a tray tray known known as the feed tray. tray. The feed tray divides the column into a top (enriching (enriching or or rectification rectification)) section and a bottom (stripping ( stripping)) section. The feed flows down the column where it is collected at the bottom in the re-boiler re-boiler..
Heat is supplied to the re-boiler to generate vapour. The source of heat input can be any suitable fluid although in most chemical plants this is normally steam. !n refineries the heating source may be the output streams of other columns. The vapour raised in the re-boiler is re-introduced into the unit at the bottom of the column. The liquid removed from the re-boiler is known as the bottoms product or product or simply bottoms.. bottoms The vapour moves up the column and as it exits the top of the unit it is cooled by a condenser condenser.. The condensed liquid is stored in a holding vessel known as the reflux drum. drum. "ome of this liquid is recycled back to the top of the column and this is called the reflux reflux.. The condensed liquid that is removed from the system is known as the distillate distillate or or top product. product. Thus there are internal flows of flows of vapour and liquid within the column as well as external flows of feeds and product streams into and out of the column.
CRUDE DISTILLATION SYSTEM The crude needs to be heated up before entering the fractionation column and this is done at first in a series of heat exchangers where heat is taken from other process streams which require cooling before being sent to rundown. Heat is also exchanged against condensing streams from the main column. Typically, the crude will be heated up in this way upto a temperature of 200 - 20 0!, before entering a furnace.
"s the raw crude oil arri#ing contains quite a bit of water and salt, it is normally sent for salt remo#ing first, in a piece of equipment called a desalter. $pstream the desalter, the crude is mixed with a water stream, typically about % - &' on feed. (ntense mixing takes place o#er a mixing #al#e and )optionally* as static mixer. The desalter, a large liquid full #essel, uses an electric field to separate the crude from the water droplets. (t operates best at +20 - +0 0!, hence it is con#eniently placed somewhere in the middle of the preheat train. The role of a platformer is to pa#e the way for this by a process which reforms the molecules in low octane naphtha to produce a high octane gasoline component. This is achie#ed by employing a catalyst with platinum as its acti#e compound hence the name latformer. /or many refinery catalyst applications, a promoter is used, and in the platforming process, it is a chloride promoter which stimulates the acidity of the catalyst and thereby the isomerisation reactions. 1ften, a bimetallic catalyst is used, i.e. in addition to the platinum, a second metal, for instance henium is present on the catalyst. The main ad#antage is a higher stability under reforming conditions. The disad#antage is that the catalyst becomes more sensiti#e towards poisons, process upsets and more susceptible to nonoptimum regenerations
PLATFORMING PROCESS The main reactions of platforming process are as follows3 4ehydrogenation of naphthenes, yielding aromatics and hydrogen 4ehydro-isomerisation of alkyl cyclopentanes to aromatic and hydrogen (somerisation of paraffins and aromatics 4ehydrocyclisation of paraffins to aromatics and hydrogen Hydrocracking of paraffins and naphthenes to ligher, saturated paraffins at the expense of hydrogen • • • • •
FCC Units (Catalytic cracking Hot feed, together with some steam, is introduced at the bottom of the riser #ia special distribution no55les. Here it meets a stream of hot regenerated catalyst from the regenerator flowing down the inclined regenerator standpipe. The oil is heated and #aporised b y the hot catalyst and the cracking reactions commence. The #apour, initially formed by #aporisation and successi#ely by cracking, carries the catalyst up the riser at +0-20 m6s in a dilute phase. "t the outlet of the riser the catalyst and hydrocarbons are quickly separated in a special de#ice. The catalyst )now partly deacti#ated by deposited coke* and the #apour then enter the reactor. The #apour passes o#erhead #ia cyclone separator for remo#al of entrained catalyst before it enters the fractionator and further downstream equipment for product separation. The catalyst then descends into the stripper where entrained hydrocarbons are remo#ed by in7ection of steam, before it flows #ia the inclined stripper standpipe into the fluidised catalyst bed in the regenerator. "ir is supplied to the regenerator by an air blower and distributed throughout the catalyst bed. The coke deposited is burnt off and the regenerated catalyst passes down the regenerator standpipe to the bottom of the riser, where it 7oins the fresh feed and the cycle recommences.
The flue gas )the combustion products* lea#ing the regenerator catalyst bed entrains catalyst particles. (n particular, it entrains 8fines8, a fine dust formed by mechanical rubbing of catalyst particles taking place in the catalyst bed. 9efore lea#ing the regenerator, the flue gas therefore passes through cyclone separators where the bulk of this entrained catalyst is collected and returned to the catalyst bed. The /!! $nit can be a real margin impro#er for many refineries. (t is able to con#ert the residues into high #alue products like :;, 9utylene, ropylene and
ia product blending, expensi#e treating steps can be a#oided and the units prepare excellent feedstock for eachother3 desulfurised residue or hydrowax is excellent /!! feed, while the /!! cycle oils are excellent Hydrocracker feed.
