The present invention relates to catalytic cracking of paraffinic hydrocarbon feed.

Catalytic cracking is well known in the art. Fischer- Tropsch hydrocarbon feed was found to give a

catalytxcally cracked product which can be worked up more efficiently than product of conventional hydrocarbon feedstock.

Therefore, the present invention relates to a process for cracking a Fischer-Tropsch hydrocarbon feed which process comprises:

(a) contacting the feed with cracking catalyst at

elevated emperature,

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

(c) removing heat from the hydrocarbon product obtained in step (b) with the help of a heat exchanger and a liquid to obtain cooled product and heated liquid, and (d) separating the cooled product obtained in step (c) by fractional distallation into gas and a heavy fraction.

Removing heat in step (c) makes it possible to remove heat more efficiently as this stream still has a

relatively high temperature. Further, the fractional distillation column of step (d) can be smaller due to the reduced volume of the cooled product and the slurry recycle in the bottom of the column can be reduced in size as the stream entering the main fractionator has already been reduced in temperature.

A heat exchanger is a device that transfers heat from one liquid to another without allowing them to mix.

The hydrocarbon product obtained in step (b)

generally will have a temperature of from 400 to 700 °C, more specifically of from 450 to 600 °C. The cooled product generally will have a temperature of from 150 to 550 °C, more specifically of from 200 to 500 °C, more specifically of from 250 to 500 °C, most specifically of from 350 to 450 °C.

The hydrocarbon product obtained in step (d) is relatively hot which makes that the liquid which is to be heated can become relatively hot as well. Such high quality streams for heating other liquids are in high demand in refineries. Exactly which liquid is to be heated depends on the lay-out of a refinery and depends on the local circumstances. The liquid to be heated will have a temperature which is at least 50 °C lower than the temperature of the hydrocarbon product from which heat is to be removed, more specifically at least 100 °C, most specifically at least 150 °C.

The feed is prepared by so-called Fischer-Tropsch hydrocarbon synthesis wherein synthesis gas is converted into alkanes with the help of a catalyst. Generally, the product obtained in Fischer-Tropsch synthesis is

subsequently cracked and separated by fractional

distillation in fractions of various boiling ranges. The feed for use in the present invention preferably

comprises at most 5 %wt of contaminants other than hydrogen and carbon, more preferably at most 2 %wt, more preferably at most 1 %wt, more preferably at most 0.1 %wt, most preferably at most 0.01 %wt. Most preferably, the feed consists of hydrogen and carbon. The feed preferably comprises at least 70 %wt of hydrocarbons obtained in a Fischer-Tropsch hydrocarbon synthesis process, preferably at least 90 %wt. The feed preferably boils in the gasoil boiling range or higher, i.e. in the range of from 210 to 750 °C, more preferably above the gas oil boiling range, i.e. 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 mixture of hydrocarbon product and spent catalyst. This process is preferably carried out in a riser reactor.

The mixture obtained is typically separated in a separator/stripper to obtain spent catalyst and

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) .

Part of the hydrocarbon product obtained in step (b) can be cracked further in order to obtain lower olefins which are desirable starting compounds for many different products. Therefore, the present invention further relates to a process according to the present invention which process further comprises

(g) further cracking part of the hydrocarbon product obtained in step (b) with cracking catalyst to obtain product comprising olefins of from 2 to 4 carbon atoms,

(h) separating the mixture obtained in step (g) to obtain hydrocarbon product and used catalyst, and

(i) using at least part of the used catalyst obtained in step (h) as cracking catalyst in step (a) .

The hydrocarbon product subjected to step (g) preferably contains compounds boiling in the range of from 500 to 800 °C, more preferably of from 565 to 750 °C. The main fractionator used in step (d) separates the cooled product into gas and a heavy fraction. The amount of heavy fraction generally will be less than 10 %wt, based on total amount of hydrocarbon product, more specifically at most 5 %wt, most specifically at most

3 %wt.

The heavy fraction obtained in step (d) can be separated further in various ways. A preferred method comprises

(e) separating the gas obtained in step <d) into a gas containing propane and/or propene and propane-lean liquid, and

(f) separating the remaining liquid obtained in step (e) to obtain a gas containing butane and/or butene and butane-lean liquid.

The separation of step (e) is generally carried out converting the gas into a liquid before separation. The gas can be converted into a liquid by lowering the temperature and/or increasing the pressure. Preferably, the pressure is increased. The pressure in the separation column of step (e) generally is of from 10 to 50 bar, more specifically of from 15 to 30 bar.

The separation of step (f) is generally carried out by decreasing the pressure of the liquid obtained in step (e) . The pressure in the separation column of step (f) generally is of from 1 to 20 bar, more specifically of from 5 to 20 bar.

Preferably, at least part of the hydrocarbon product obtained in step (h) is sent to step (d) together with product of step (c) .

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 .

Further, it is preferred to carry out step (e) and/or (f) by divided wall fractional distillation. Such

distillation 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. Above the first and second upper regions there can be a common top region if desired.

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

In the process of Figure 1, Fischer-Tropsch

hydrocarbon feed is sent to the cracking process via line 1 and regenerated catalyst is added via line 2. 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 the 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. The catalyst thus regenerated 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 with in-line heat removal with the help of heat exchanger 19. The heat exchanger can be any heat exchanger known in the art to be suitable for the

specific circumstances. Gas is removed from the main fractionator 17 via line 20 and slurry oil is removed via line 18. The gas removed via line 20 is increased in pressure to obtain a mixture of gas and liquid which is separated further in column 21 to obtain a gas containing propane and/or propene which is removed via line 22 and a propane-lean liquid which is sent via line 23 to column 24 where the liquid is separated further by reducing the pressure. Gas containing butane and/or butene is removed via line 25 and butane-lean liquid is removed via line