PROCESS FOR PRODUCING C2 AND C3 HYDROCARBONS

Abstract

The invention relates to a process for producing C2 and C3 hydrocarbons, comprising a) subjecting a mixed hydrocarbon stream to first hydrocracking in the presence of a first hydrocracking catalyst to produce a first hydrocracking product stream; and b) subjecting the first hydrocarbon product stream to C4 hydrocracking optimized for converting C4 hydrocarbons into C3 hydrocarbons in the presence of a C4 hydrocracking catalyst to obtain a C4 hydrocracking product stream comprising C2 and C3 hydrocarbons.

Claims

1. A process for producing C2 and C3 hydrocarbons, comprising a) subjecting a mixed hydrocarbon stream to first hydrocracking in the presence of a first hydrocracking catalyst to produce a first hydrocracking product stream; and b) subjecting the first hydrocarbon product stream to C4 hydrocracking optimized for converting C4 hydrocarbons into C3 hydrocarbons in the presence of a C4 hydrocracking catalyst to obtain a C4 hydrocracking product stream comprising C2 and C3 hydrocarbons.

2. The process according to claim 1, wherein the first hydrocracking is a hydrocracking process suitable for converting a hydrocarbon feed that is relatively rich in naphthenic and paraffinic hydrocarbon compounds to a stream rich in LPG and aromatic hydrocarbons.

3. The process according to claim 1, wherein the first hydrocracking catalyst is a catalyst containing one metal or two or more metals of group VIII, VI B or VII B of the periodic classification of elements deposited on a carrier.

4. The process according to claim 1, wherein the C4 hydrocracking catalyst comprises a mordenite or an erionite.

5. The process according to claim 1, wherein the C4 hydrocracking catalyst consists of a mordenite and an optional binder or comprises sulfided-nickel/H-Erionite1 and the C4 hydrocracking is performed under conditions comprising a temperature between 325 and 450° C., a partial hydrogen pressure between 2 and 4 MPa, a molar ratio hydrogen to hydrocarbon feed of 2:1 to 8:1, wherein the number of moles of the hydrocarbon feed is based on the average molecular weight of the hydrocarbon feed and a VVH of 0.5 to 5 h.sup.−1.

6. The process according to claim 1, wherein the C4 hydrocracking is performed without compressing or expanding the first hydrocracking product stream.

7. The process according to claim 1, wherein steps (a) and (b) are performed in a single reactor having a first catalyst layer comprising the first hydrocracking catalyst and a last catalyst layer comprising the C4 hydrocracking catalyst.

8. The process according to claim 1, wherein steps (a) and (b) are performed in a single reactor comprising a layer comprising both the first hydrocracking catalyst and the C4 hydrocracking catalyst, wherein the proportion of the C4 hydrocracking catalyst at the end of the bed is higher than the proportion of the C4 hydrocracking catalyst at the beginning of the bed.

9. The process according to claim 1, wherein the temperature for the C4 hydrocracking is higher than the temperature for the first hydrocracking.

10. The process according to claim 1, wherein the mixed hydrocarbon feedstream comprises a naphtha or a naphtha-like product.

11. The process according to claim 1, further comprising the step of c) separating the C4 hydrocracking product stream to provide at least a light hydrocarbon stream comprising at least C2 and C3 hydrocarbons, a middle hydrocarbon stream comprising C4 and/or C5 hydrocarbons and a heavy hydrocarbon stream comprising at least C6+ hydrocarbons.

12. The process according to claim 11, further comprising the step of d) subjecting the heavy hydrocarbon stream to second hydrocracking in the presence of a second hydrocracking catalyst to produce a second hydrocracking product stream comprising BTX, wherein the second hydrocracking is more severe than the first hydrocracking.

13. The process according to claim 11, wherein at least part of C5 hydrocarbon in the middle hydrocarbon stream is recycled back to the first hydrocracking of step a).

14. The process according to claim 1, wherein the amount of methane in the first hydrocracking product stream is at most 5 wt %.

15. The process according to claim 1, wherein the amount of the C2-C3 hydrocarbons in the C4 hydrocracking product stream is at least 60 wt %.

16. The process according to claim 10, wherein the mixed hydrocarbon feedstream has a boiling point range of 20-200° C.

Description

EXAMPLES

Example 1

[0166] A feed consisting of n-pentane was subjected to hydrocracking in order to determine the influence of hydrocracking conditions to the product compositions. The experiments were carried out in a 12 mm reactor, wherein the catalyst bed was located in the isothermal zone of the reactor heater. The catalyst used was a mixture of 2 grams of Pt on alumina (Pt-loading of 0.75 wt %) and H-ZSM-5 (SiO.sub.2/Al.sub.2O.sub.3=80).

[0167] The feed stream was fed to the reactor. The feed stream enters a vaporizer section prior to the reactor where it is vaporized at 280° C. and mixed with hydrogen gas. The conditions used throughout these experiments were: WHSV=1/hr, pressure was 1379 kPa (200 psig) and the molar ratio H2/hydrocarbons was 3. The temperature of the isothermal zone of the reactor was varied between 375 and 450° C. The effluent of the reactor was sampled in the gas phase to an online gas chromatograph. Product analyses were carried out once per hour.

TABLE-US-00001 TABLE 1 Compositions of hydrocracking product effluent Component 375° C. 400° C. 425° C. 450° C. Methane (wt %) 0.5 1.1 2.2 3.9 Ethane (wt %) 3.3 7.2 12.7 19.4 Propane (wt %) 16.3 24.4 32.8 39.7 Butanes (wt %) 16.9 19.8 20.8 19.0 i-Pentane (wt %) 11.9 13.8 13.4 9.6 n-Pentane (wt %) 49.0 32.3 17.3 7.2 C6+ (wt %) 2.1 1.4 0.8 1.2 Selectivity (—) 98.7 98 96.8 95.3

[0168] The compositions of the product effluent at different reactor temperatures are provided in Table 1. The selectivity was defined as (100%−(amount of methane formed/amount of C5 converted)). The amount of C5 converted is defined as (total amount−(i-pentane and n-pentane)). By comparing the results in Table 1, it was observed that when the reactor temperature is decreased, the overall selectivity is increased during hydrocracking. It is anticipated that a similar trend will be observed when a feed consisting of butanes is subjected to hydrocracking (based on experiments using different carbon number paraffin feeds and conversions and production rates obtained using naphtha type feeds).

[0169] It can therefore be concluded that a higher selectivity can be achieved by operating at a lower temperature.

Example 2

[0170] A feed consisting of a normal paraffin was subjected to hydrocracking in order to determine the influence of hydrocarbon chain length to the extent of conversion. The experiments were carried out in a 12 mm reactor, wherein the catalyst bed was located in the isothermal zone of the reactor heater. The catalyst used was a mixture of 2 grams of Pt on alumina (Pt-loading of 0.75 wt %) and H-ZSM-5 (SiO.sub.2/Al.sub.2O.sub.3=80).

[0171] The feed stream was fed to the reactor. The feed stream enters a vaporizer section prior to the reactor where it is vaporized at 280° C. and mixed with hydrogen gas. The conditions used throughout these experiments were: WHSV=1/hr, pressure was 1379 kPa (200 psig) and the molar ratio H.sub.2/hydrocarbons was 3. The temperature of the isothermal zone of the reactor was varied between 300 and 500° C. The effluent of the reactor was sampled in the gas phase to an online gas chromatograph. Product analyses were carried out once per hour.

TABLE-US-00002 TABLE 2 Single-pass conversion of normal paraffins 300° 350° 375° 400° 425° 450° 500° Feed component C. C. C. C. C. C. C. n-Pentane 51.03 67.74 82.70 92.82 n-Hexane 92.76 96.35 98.20 98.96 99.67 n-Heptane 92.76 99.10 99.51 99.73 99.90 99.98 100 n-Octane 99.89 100

[0172] The conversion level at different reactor temperatures is provided in Table 2. The conversion level was defined as ((n-paraffin effluent concentration in wt %−100)/100). By comparing the results in Table 2, it was observed that when the chain length of the normal paraffin is reduced, the extent of conversion is reduced at a similar temperature. Alternatively, increased reaction temperatures are required to achieve sufficient conversion levels for normal paraffins with shorter chain length. By interpolation of the data presented in Table 2, the temperature required to achieve 80% conversion could be estimated for n-pentane, n-hexane and n-octane. The estimated reaction temperatures are depicted in FIG. 3. Extrapolation of the data confirms that significantly higher reaction temperatures are required to achieve sufficient conversion of n-butane.

[0173] As illustrated by Example 1, the feed components that are to be exposed to these higher temperatures should be minimized to achieve high selectivities. This could be achieved by sending the butanes and pentanes to a dedicated hydrocracker optimized for converting C4 to C3 instead of subjecting them to second hydrocracking having severe conditions.