The present invention relates to catalytic cracking of hydrocarbon feedstock.

Catalytic cracking processes are well known in the art. A more recent development is to further crack part of the product obtained thereby to obtain lower olefins besides the conventional cracking products. We now have found an improved process for separating the products obtained thereby into the different fractions.

Therefore, the present invention relates to a process for converting a hydrocarbon feed which process comprises

(a) catalytically cracking the feed,

(b) separating the mixture obtained in step (a) into spent catalyst and hydrocarbon product,

(c) separating the hydrocarbon product of step (b) into two or more fractions,

(d) further catalytically cracking at least one of the fractions obtained in step (c) ,

(e) separating the mixture obtained in step (d) into used catalyst and hydrocarbon product,

(f) subjecting one or more fractions obtained in step

(c) and hydrocarbon product of step (e) to separation such as to obtain a gas and one or more liquid fractions, (g) increasing the pressure of the gas obtained in step

(f) ,

(h) separating hydrocarbons comprising 3 carbon atoms from the product of step (g) to obtain propane-lean product, and

(i) separating the propane-lean product obtained in step (h) into a fraction containing hydrocarbons

comprising 4 carbon atoms, a fraction containing

hydrocarbons comprising 5 or 6 carbon atoms, and a heavy fraction, in which process the fraction containing hydrocarbons comprising 5 or 6 carbon atoms obtained in step (i) is sent as feed to step (d) .

The expression "butane-lean" means that the liquid in question contains a limited amount only of butane and butene, and "propane-lean" means that the liquid in question contains a limited amounts only of propane and propene .

The feed to be used in the present invention can be any feed conventionally used in fluid catalytic cracking.

The feed preferably boils in the gasoil boiling range or higher, more specifically in the range of from 210 to 750 °C, more preferably above the gas oil boiling range, more specifically of from 350 to 650 °C.

In step (a) , the feed is preferably contacted with cracking catalyst at a temperature of from 450 to 650 °C to yield a hydrocarbon product and spent catalyst. This cracking step is preferably carried out in a riser reactor .

The mixture obtained is typically separated in a separator/stripper to separate the spent catalyst from the hydrocarbon product. Preferably, steam is added to the mixture during separation.

The spent catalyst separated in step (b) is

preferably regenerated with an oxygen-containing gas to obtain regenerated catalyst, with at least part of the regenerated catalyst used again as cracking catalyst in step (a) .

The hydrocarbon product obtained in step (b) is separated into two or more fractions. A preferred

separation method is fractional distillation.

At least one of the fractions obtained in step (c) is cracked further in order to obtain lower olefins which are desirable starting compounds for many different products. The fraction to be cracked further preferably comprises at least 70 %wt of hydrocarbons comprising of from 5 to 9 carbon atoms, more preferably at least

80 %wt, most preferably at least 90 %wt . The lower olefins typically are hydrocarbons comprising of from 2 to 4 carbon atoms. The fraction to be subjected to further cracking in step (d) preferably contains

compounds boiling in the range of from 500 to 800 °C, more preferably of from 565 to 750 °C.

Used catalyst is separated from the mixture obtained in step (d) to obtain hydrocarbon product. The used catalyst contains a limited amount of coke and still has sufficiently high temperature which makes that it can be used as cracking catalyst in step (a) .

The hydrocarbon product separated off in step (e) is separated in combination with one or more fractions obtained in step (c) . As mentioned above, the separation is preferably carried out by fractional distillation. The temperature at the bottom of the fractionator preferably is of from 210 to 280 °C, more specifically of from 230 to 270 °C, most specifically of from 240 to 260 °C. The pressure applied generally will be of from 1 to 10 bar, more specifically of from 1 to 5 bar.

It is preferred to separate in step (f) a gas as top fraction, a gas oil as middle fraction and a residue as bottom fraction. Generally, only a limited amount of product is separated as gas oil specifically less than 8 %wt, based on amount of product separated, more

specifically less than 5 %wt .

The efficient work-up section according to the present invention requires to increase the pressure of the gas separated off in step (f) . This is suitably done in a number of steps whereby the gas is partly converted into a liquid which can be introduced separately into the separation column of step (h) . The pressure of the gas generally will be increased to of from 10 to 40 bar, more specifically to of from 15 to 30 bar.

The separation of step (h) preferably is carried out by divided wall fractional distillation, more

specifically divided wall fractional distillation which is carried out in a column containing a wall dividing the column in the longitudinal direction to form a first upper region, a second upper region and lower common region. For the separation of step (h) , it is preferred that there is no upper common region. Preferably, the dividing wall extends to the top of the column thereby preventing fluid communication between the first and second upper region in the upper part of the column.

Further, it is preferred that each of the upper regions has their separate reflux, i.e. withdraw gas at the top and recycle part thereof as reflux to the upper region from which it was withdrawn. The pressure applied in step

(f) generally is of from 10 to 50 bar, more specifically of from 15 to 30 bar.

The propane-lean product obtained in step (h) is separated further in step (i) preferably by divided wall fractional distillation, more specifically divided wall fractional distillation which is carried out in a column containing a wall dividing the column in the longitudinal direction to form a first middle region and a second middle region. The column for use in step (i) preferably comprises an upper common region and a lower common region. The pressure in the separation column of step (i) generally is of from 1 to 20 bar, more specifically of from 5 to 20 bar. Process according to any one of the preceding claims, in which the heavy fraction obtained in step (i) is

subsequently hydrotreated .

The fractions separated in step (i) mainly contain hydrocarbons comprising the number of carbon atoms mentioned but further hydrocarbons can be present as well. Each of the fractions preferably contains at least 80 %wt of hydrocarbons comprising the number of carbon atoms mentioned, more preferably at least 90 %wt .

The fraction containing hydrocarbons comprising 5 or

6 carbon atoms can be mixed with fresh feed or with a fraction obtained in step (c) before being cracked further in step (d) .

The invention shall be further elucidated by means of the following Figure 1 which shows a process flow scheme according to the present invention.

In the process of Figure 1, hydrocarbon feed is sent to the cracking process via line 1 and regenerated catalyst is added via line 2. Additionally, used catalyst can be added via line 32 if required. The mixture of feed and catalyst flows upwardly through riser reactor 3. Via line 4, the mixture of spent catalyst and hydrocarbon product is sent to stripper/seperator 5. Spent catalyst will generally be separated from hydrocarbon product with the help of a first cyclone 7, after which the gas separated off in the first cyclone 7 can be sent to a second cyclone 8. If desired, the gas separated off in the second cyclone 8 can be sent to a third means for separating solid partices. Catalyst particles which have been separated off, flow via a so-called dipleg to a regeneration zone. The separated catalyst particles flow via line 10 to regenerator 11 where oxygen-containing gas is added via line 13 and waste gas is removed via line 12. Solids can be removed from the gas flowing upwards with the help of cyclone 14 which feeds into cyclone 15. Gas which is substantially free from solids can further flow upwards via line 16. Regenerated catalyst can be used again in riser reactor 3.

The hydrocarbon product separated in

stripper/separator 5 is sent via line 6 to the main fractionator 17 where gas is removed via line 20, gas oil is removed via line 19 and the further compounds are removed via line 18. The gas removed via line 20 has a pressure of from 1 to 4 bar, which pressure is increased in several steps to of from 20 to 25 bar. This

pressurised gas is sent to divided wall column 21 which is operated at 20 bar. Gas containing propane and/or propene is removed via line 22 and a propane-lean liquid is sent via line 23 to divided wall column 24 which is operated at a pressure of from 2 to 10 bar. Gas

containing butane and/or butene is removed via line 25, a fraction containing hydrocarbons comprising 5 or 6 carbon atoms is removed via line 27 and a heavy fraction is removed via line 26. This heavy fraction is preferably hydrotreated and the hydrotreated product is separated further at atmospheric pressure in a divided wall column (not shown) .

Part or all of the fraction containing hydrocarbons comprising 5 or 6 carbon atoms is sent via line 27 to a further cracking reactor 30 together with regenerated catalyst supplied via line 27 and a fraction separated in the main fractionator 17 via line 28. The product

obtained is sent via line 31 to catalyst separator 32 to obtain used catalyst which is removed via line 33 and gaseous product removed via line 34. At least part of the gaseous product is sent to the main fractionator 17 preferably together with hydrocarbon product of line 6. At least part of the used catalyst is sent to riser reactor 3.