WO2014102286A1

WO2014102286A1 - Process for the preparation of propylene and ethylene from fischer-tropsch derived gas oil

Info

Publication number: WO2014102286A1
Application number: PCT/EP2013/077998
Authority: WO
Inventors: Pieter HUIZENGA; Peter LOECKER; Muhammad ZAFAR
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2012-12-28
Filing date: 2013-12-24
Publication date: 2014-07-03
Also published as: TW201443219A

Abstract

The present invention provides a process for the preparation of propylene and ethylene, the process comprising at least the following steps: (a) providing a Fischer-Tropsch derived gas oil, which Fischer-Tropsch derived gas oil comprises paraffins having from 9 to 25 carbon atoms; (b) mixing the Fischer-Tropsch derived gas oil provided in step (a) with a dilution gas thereby obtaining a mixture; (c) heating the mixture obtained in step (b) thereby obtaining a mixture of diluted gas and evaporated Fischer-Tropsch derived gas oil; and (d) subjecting the mixture obtained in step (c) to a thermal conversion step thereby obtaining a product stream which comprises propylene and ethylene.

Description

PROCESS FOR THE PREPARATION OF PROPYLENE AND ETHYLENE FROM FISCHER-TROPSCH DERIVED GAS OIL

The present invention relates to a process for the preparation of propylene and ethylene from a Fischer- Tropsch derived gas oil.

It is known to use Fischer-Tropsch derived products as obtained in a Fischer-Tropsch process as steam cracker feedstock. For example in "The Markets for Shell Middle Distillate Synthesis Products", Presentation of Peter J. A. Tijm, Shell International Gas Ltd., Alternative Energy '95, Vancouver, Canada, May 2-4, 1995 on page 5, it is mentioned that SMDS naphtha, the Fischer-Tropsch derived naphtha fraction of the Shell MDS process, is used as steam cracker feedstock in for example Singapore. Fischer-Tropsch derived naphtha comprises paraffins having from 5 to 8 carbon atoms and a boiling range of from 40 to 160°C.

A problem of using heavier steam cracker feedstock, viz. steam cracker feedstock comprising paraffins having more than 8 carbon atoms and a higher boiling range than 160°C, is that the feedstock may comprise carbon

residues. Due to the presence of carbon residues in the heavy steam cracker feedstock coke is formed. Coke formation due to the presence of carbon residue is known in the art and is for example described in Chapter 5 of "Petroleum Technology", John Wiley & Sons, Inc., and WILEY-CBH verlag GmbH & Co. KGaA, Weinheim, 2007, ISBN

978-0-470-13402-3, pages 759 - 761.

It is an object of the present invention to solve or at least minimize the above problem. It is a further object of the present invention to provide a process for preparing a high yield of propylene and ethylene from a heavy steam cracker feedstock.

One of the above or other objects may be achieved according to the present invention by providing a process for the preparation of propylene and ethylene, the process comprising at least the following steps:

(a) providing a Fischer-Tropsch derived gas oil, which Fischer-Tropsch derived gas oil comprises paraffins having from 9 to 25 carbon atoms;

(b) mixing the Fischer-Tropsch derived gas oil provided in step (a) with a dilution gas thereby obtaining a mixture ;

(c) heating the mixture obtained in step (b) thereby

obtaining a mixture of diluted gas and evaporated Fischer-Tropsch derived gas oil; and

(d) subjecting the mixture obtained in step (c) to a

thermal conversion step thereby obtaining a product stream which comprises propylene and ethylene.

It has now surprisingly been found according to the present invention that a heavy steam cracker feedstock comprising Fischer-Tropsch derived product comprising paraffins having from 9 to 25 carbon atoms contain a low amount of carbon residues.

Carbon residues are sometimes referred to as compounds with a higher final boiling point than the final boiling point of the steam cracker feedstock. These carbon residues are therefore not evaporated when the stream cracker feedstock is heated to be evaporated prior to being steam cracked.

The Fischer-Tropsch derived gas oil according to the present invention can thus be evaporated at a high temperature in an efficient manner with minimal risk of coke formation.

A further advantage of the present invention is that due to preheating of the Fischer-Tropsch derived gas oil at a high temperature prior to the thermal conversion of the Fischer-Tropsch derived gas oil, the high temperature at which the thermal conversion takes place, is faster reached. In this way, conversion to propylene and

ethylene initiates earlier than when the preheating temperature is much lower than the temperature at which thermal conversion takes place, and results therefore in a high yield of propylene and ethylene.

In step (a) of the process according to the present invention a Fischer-Tropsch derived gas oil comprising paraffins having from 9 to 25 carbon atoms, preferably from 11 to 25 carbon atoms is provided. The Fischer- Tropsch derived gas oil as provided in step (a) is derived from a Fischer-Tropsch process. Fischer-Tropsch derived gas oil is known in the art. By the term

"Fischer-Tropsch derived" is meant that a gas oil, is, or is derived from, a synthesis product of a Fischer-Tropsch process. In a Fischer-Tropsch process synthesis gas is converted to a synthesis product. Synthesis gas or syngas is a mixture of hydrogen and carbon monoxide that is obtained by conversion of a hydrocarbonaceous feedstock.

Suitable feedstock include natural gas, crude oil, heavy oil fractions, coal, biomass and lignite. A Fischer- Tropsch derived gas oil may also be referred to as a GTL (Gas-to-Liquids ) gas oil.

The preparation of the Fischer-Tropsch derived gas oil comprising paraffins having from 9 to 25 carbon atoms as provided in step (a) has been described in e.g. WO 02/070627, WO 2004/009739 and in WO 03/070857. Typically, Fischer-Tropsch derived paraffins are primarily iso-paraffins . Preferably, the Fischer-Tropsch derived gas oil according to the present invention comprises more than 90 wt . % of iso-paraffins , more preferably more than 95 wt . % of iso-paraffins .

According to the present invention, the Fischer- Tropsch derived gas oil comprises a major amount (i.e. > 50 wt.%) of Fischer-Tropsch derived paraffins having from 9 to 25 carbon atoms; preferably the amount of Fischer- Tropsch paraffins having from 9 to 25 carbon atoms is at least 80 wt.%, more preferably at least 85 wt.%, more preferably at least 90 wt.%, and most preferably at least 95 wt.% based on the total amount of Fischer-Tropsch derived gas oil.

Preferably, the Fischer-Tropsch derived gas oil as provided in step (a) has an initial boiling point of at least 140°C, more preferably at least 143°C, most

preferably at least 144°C and a final boiling point of at most 420°C, preferably at most 400°C, more preferably at most 390°C and most preferably at most 385°C at

atmospheric conditions.

By boiling points at atmospheric conditions is meant atmospheric boiling points, which boiling points are determined by ASTM D2887.

Preferably, the Fischer-Tropsch derived gas oil as provided in step (a) has a T10wt.% boiling point from 190 to 200°C, more preferably from 195 to 200°C and a T90wt.% boiling point from 330 to 360°C, more preferably from 340 to 350°C, and most preferably from 345°C to 350°C.

T10wt.% is the temperature corresponding to the

atmospheric boiling point at which a cumulative amount of 10% of the product is recovered. Similarly, T90wt.% is the temperature corresponding to the atmospheric boiling point at which a cumulative amount of 90wt.% of the product is recovered. A gas chromatographic method such as ASTM D2887 can be used to determine the level of recovery .

Further, the Fischer-Tropsch derived gas oil as provided in step (a) preferably has a density at 20°C (according to ASTM D4052) of at least 760 kg/m³, more preferably of at least 770 kg/m³, most preferably at least 780 kg/m³ and of most 850 kg/m³, preferably at most 800 kg/m³.

Suitably, the kinematic viscosity at 100°C

(according to ASTM D445) of the Fischer-Tropsch derived gas oil as provided in step (a) is above 0.8 cSt,

preferably above 0.9 cSt, more preferably above 1.0 cSt. Typically, the kinematic viscosity at 100°C (according to

ASTM D445) of the Fischer-Tropsch derived gas oil as provided in step (a) is below 20 cSt, preferably below 15 cSt, more preferably below 10 cSt, more preferably below 5 cSt and most preferably below 2 cSt.

Preferably, the cetane index of the Fischer-Tropsch derived gas oil (according to ASTM D9760) is in the range of from 75 to 100, preferably in the range from 70 to 90.

Further, the pour point of the Fischer-Tropsch derived gas oil (according to ASTM D97) is preferably below -10°C, more preferably below -20°C, more preferably below -30°C, more preferably below -35°C, more preferably below -35°C, more preferably below -40°C, and most preferably below -45°C and preferably for at most above - 50°C.

The Fischer-Tropsch derived gas oil has preferably a cloud point according to ASTM D-2500 in the range of -8°C to -33°C, more preferably in the range of -13°C to - 27°C. The cold filter plugging point of the Fischer-Tropsch derived gas oil (according to ASTM D6371) is preferably below 0°C, more preferably below -10°C, more preferably below -15°C, more preferably below -20°C, more preferably below -30°C,and most preferably below -40°C and

preferably for at most above -45°C.

In a preferred embodiment, the Fischer-Tropsch derived gas oil as provided in step (a) is heated to obtain a partly evaporated Fischer-Tropsch derived gas oil. The Fischer-Tropsch derived gas oil is preferably evaporated for at least 80 wt.%, more preferably for at least 90 wt.%.

Preferably, the Fischer-Tropsch derived gas oil as provided in step (a) is heated to at least 150°,

preferably at least 350°C and at most 430°C, preferably at most 400°C, thereby obtaining a heated Fischer-Tropsch derived gas oil.

Suitably, the heated Fischer-Tropsch derived gas oil has a temperature of at least 150°C, preferably of at least 195°C. The upper limit of the temperature of the heated Fischer-Tropsch derived gas oil is 430°C.

In a preferred embodiment, in step (a) the heated Fischer-Tropsch derived gas oil is mixed with a dilution gas thereby obtaining a mixture. The Fischer-Tropsch derived gas oil is preferably evaporated for at least 80 wt.% and preferably for at most 90 wt.%.

In step (b) the Fischer-Tropsch derived gas oil provided in step (a) is mixed with a dilution gas thereby obtaining a mixture.

Examples of a dilution gas are methane, ethane, nitrogen, hydrogen, natural gas, dry gas, refinery off gases, vaporized naphtha and steam. Preferably, the dilution gas mixed with the Fischer-Tropsch derived gas oil in step (b) comprises steam or hydrogen, more

preferably the dilution gas comprises steam.

Preferably, the weight ratio of dilution gas to Fischer-Tropsch derived gas oil in step b) is from 0.3 to 0.8, preferably from 0.3 to 0.5, more preferably from 0.3 to 0.45.

Typically, the temperature of the dilution gas is in the range of 140 to 800°C, preferably in the range of 150 to 600°C and more preferably in the range of 200 to 550°C.

The pressure of dilution gas is not particularly limited. Typically, the pressure of the dilution gas is in the range of 6 to 15 bar.

In step (c) the mixture as obtained in step (b) is heated thereby obtaining a mixture of diluted gas and evaporated Fischer-Tropsch derived gas oil.

The mixture as obtained in step (c) of the present invention, comprises Fischer-Tropsch derived gas oil which is preferably evaporated for at least 95 wt~6 , more preferably for at least 99 wt . % and most preferably for at least 100 wt . % .

Suitably, the temperature in step (c) is in the range of from 420 to 620°C, preferably in the range of from 450 to 610°C, more preferably in the range of from 500 to 610°C and most preferably in the range from 595 to

610.

Typically the presence of carbon residue in the Fischer-Tropsch derived gas oil according to the present invention leads to coke formation in step (c) .

Suitably, the amount of carbon residue is measured by Micro Carbon Residue Technique (MCRT) according to ASTM D4530. MCRT according to ASTM D4530 is known in the art and for example described in "Handbook of Petroleum Product Analysis", John Wiley & Sons, Inc., Hoboken, New Jersey, 2002, ISBN 0-471-20346-7, pages 222-223.

In order to determine the carbon residue of the Fischer-Tropsch derived gasoil samples, they need to be pre-thickened . Pre-thickening was accomplished by vacuum distillation in a pot still at a temperature below 250°C. Initial and final amount of sample are weighed.

After performing MCRT on the pre-thickened sample, the carbon residue is calculated as final weight divided by initial weight times the measured MCRT. A detection limit of 0.02 wt . % has been demonstrated by first

exposing an empty vial to the ASTM D-4530 test procedure, subsequently weighing it and only then adding a weighed amount of test sample. This procedure was applied in the ASTM D-4530 test described in the present application.

Preferably, a sample of the Fischer-Tropsch derived gas oil as provided in step (a) , this sample comprises less than 10 ppm of carbon residue, more preferably less than 5 ppm of carbon residue, and most preferably less than 2 ppm of carbon residue and at most 20 ppm carbon residue as determined by the method as described above.

In a highly preferred embodiment, after step (c) , the mixture is further heated to a temperature just below the temperature at which thermal conversion starts to occur. This temperature is preferably in the range of from 580 to 620, more preferably in the range of from 590 to 610, and most preferably in the range of from 595 to 610.

In step (d) the mixture as obtained in step (c) is subjected to a thermal conversion step thereby obtaining a product stream which comprises propylene and ethylene.

A thermal conversion step may generally be referred to as a conversion step wherein a "cracking" reaction is performed. In such a thermal conversion step, larger molecules are broken into smaller ones. This can

generally be done via a catalytic cracking method, or preferably via a thermal cracking process.

Preferably, the thermal conversion step is executed as a steam cracking step. Steam cracking is known in the art and therefore not discussed here in detail. Steam cracking is for example described in "Petroleum

Technology", John Wiley & Sons, Inc., and WILEY-CBH verlag GmbH & Co. KGaA, Weinheim, 2007, ISBN 978-0-470-

13402-3, page 805.

Suitably, the temperature in step (d) is in the range of from 700 to 900°C, preferably in the range of from 750 to 850°C, more preferably in the range of from 780 to 830°C.

The pressure in step (d) is generally in the range of from 1 to 3 bar absolute, more preferably from 1.2 to 1.98 bar absolute.

In step d) the evaporated Fischer-Tropsch derived gas oil of the mixture obtained in step c) is thermally converted to a product stream which comprises propylene and ethylene. Preferably, in step d) the evaporated

Fischer-Tropsch derived gas oil of the mixture obtained in step c) is steam cracked to a product stream which comprises propylene and ethylene. Further products of the thermal conversion reaction include, but are not limited to, butadiene, benzene, hydrogen and methane and other associated olefinic, paraffinic, and aromatic products.

Preferably, the product stream comprises from 20 to 35 wt . % ethylene, more preferably, from 25 to 35 wt . % ethylene, and most preferably, from 30 to 35 wt . % ethylene based on the total amount of Fischer-Tropsch derived gas oil as provided in step (a) . The amount of ethylene is determined by GCxGC- internal test methodology.

Preferably, the product stream comprises from 15 to 25 wt . % propylene, more preferably, from 17 to 25 wt . % propylene, and most preferably, from 18 to 25 wt . % propylene based on the total amount of Fischer-Tropsch derived gas oil as provided in step (a) .

The amount of propylene is determined by GCxGC- internal test methodology.

The temperature of the product stream in step d) is preferably in the range of from 750 to 850°C, more preferably in the range from 780 to 830°C.

Typically, the temperature of the product stream of step d) is quickly reduced to terminate any unwanted reactions to a temperature of below 400°C. The product stream is generally cooled by indirect quenching in transfer-line exchangers and or by direct quenching by injection of oil. Transfer-line exchangers and quench oil fitting are known techniques in the art and therefore not discussed here in detail. Transfer-line exchangers and quench oil fittings are for example described in

"Petroleum Technology", John Wiley & Sons, Inc., and WILEY-CBH verlag GmbH & Co. KGaA, Weinheim, 2007, ISBN 978-0-470-13402-3, page 761 to 769.

Preferably, the temperature is reduced to below

400°C by means of a transfer line exchanger and further reduced below 240°C by means of quench oil fitting.

Further processing of the cooled product stream of step d) to recover propylene and ethylene is known in the art and therefore not discussed here in detail. Further processing of the product stream including the recovery of propylene and ethylene from the product stream is for example described in "Petroleum Technology", John Wiley & Sons, Inc., and WILEY-CBH verlag GmbH & Co. KGaA,

Weinheim, 2007, ISBN 978-0-470-13402-3, page 769 to 787.

Recovery of propylene and ethylene of the product stream as obtained in step d) has been described in e.g. WO 03/062352.

This preferred embodiment will be described in more detail with reference, where appropriate, to Figure 1, which is not intended to limit the scope of the present invention in any way.

For the purpose of this description a single

reference number will be assigned to a line as well as a stream carried in that line. The cracking furnace setup is generally referred to with reference numeral 1.

Typically, a cracking furnace 1 comprises a

convection zone 2, which comprises a feed preheating zone

3, first preheating zone 4, a second preheating zone 5 and a cracking zone 8 (also known as radiant section) . Between the feed preheating zone 3 and the first

preheating zone 4 an inlet 6 for dilution gas is located. To the inlet 31 of the feed preheating zone 3 a stream 10 comprising Fischer-Tropsch derived gas oil comprising paraffins having from 10 to 35 carbon atoms is fed. The pressure and temperature at which the Fischer-Tropsch derived gas oil 10 is fed to the inlet 31 of the feed preheating zone 3 is not critical; typically the

temperature is in the range of from 0 to 400°C,

preferably lower than 430°C.

The pressure within the feed preheating zone 3 is not particularly limited. The pressure is generally in the range of 4 to 21 bar.

In a preferred embodiment, in the feed preheating zone 3 the Fischer-Tropsch derived gas oil 10 is heated to obtain a partly evaporated Fischer-Tropsch derived gas oil 11.

Suitably, the heated Fischer-Tropsch derived gas oil 11 as obtained in the feed preheating zone 3 has a temperature of at least 150°C, preferably at least 195°C.

The upper limit of the temperature of the heated Fischer- Tropsch derived gas oil 11 as obtained in the first preheating zone 3 is below 400 °C.

Suitably, a dilution gas 12 is added to the inlet 5 of the convection zone. Typically, the temperature of the dilution gas 12 at the inlet 6 of the convection zone 2 is in the range of 140 to 800°C, preferably in the range of 150 to 600°C and more preferably in the range of 200 to 550°C.

The pressure of dilution gas 12 is not particularly limited, but is preferably sufficient to allow injection at the inlet 6 of the convection zone 2. Typically, the pressure of the dilution gas 12 is in the range of 6 to 15 bar.

The heated Fischer-Tropsch derived gas oil 11 as obtained in the feed preheating zone 3 is preferably mixed with the dilution gas 12 at the inlet 6. Typically, the mixture 13 obtained is led to the first preheating zone 4.

Optionally, the heated Fischer-Tropsch derived gas oil 11 as obtained in the feed preheating zone 3 is fed directly to the first preheating zone 4 and mixed with dilution gas 12 in the first preheating zone 4 to obtain a mixture 13.

Suitably, the conditions of the heated Fischer-

Tropsch derived gas oil 13 at the inlet 41 of the first preheating zone 4 is similar as the conditions as

described above for the feed preheating zone 3. Suitably, a dilution gas 14 is added to the inlet 15 of the convection zone. Typically, the temperature of the dilution gas 14 at the inlet 15 of the convection zone 2 is in the range of 140 to 800°C, preferably in the range of 150 to 600°C and more preferably in the range of 200 to 550°C.

The pressure of dilution gas 14 is not particularly limited, but is preferably sufficient to allow injection at the inlet 15 of the convection zone 2. Typically, the pressure of the dilution gas 14 is in the range of 6 to 15 bar.

The heated Fischer-Tropsch derived gas oil 13 as obtained in the feed preheating zone 4 is preferably mixed with the dilution gas 14 at the inlet 15.

Typically, the mixture 16 obtained is led to the second preheating zone 5.

Optionally, the heated Fischer-Tropsch derived gas oil 13 as obtained in the feed preheating zone 3 is fed directly to the first preheating zone 4 and mixed with dilution gas 14 in the second preheating zone 5 to obtain a mixture 16.

Suitably, the temperature of the heated Fischer- Tropsch derived gas oil 16 at the inlet 51 of the second preheating zone 5 is at least 150°C, preferably of at least 195°C.

In the second preheating zone 5 the mixture 16 is preferably heated further to a temperature just below the temperature at which thermal conversion starts to occur. Suitably, the temperature in the second preheating zone 5 is in the range of from 450 to 650°C, preferably in the range of from 500 to 645°C, more preferably in the range of from 610 to 645°C and most preferably of from 610 to 630°C. Typically, the presence of carbon residue in the Fischer-Tropsch derived gas oil 10 leads to coke

formation in the convection zone 2 of the furnace 1.

The mixture 17 as obtained in second preheating zone 5 of the convection zone 2 is led to the cracking zone 8 of the cracking furnace 1. The mixture 17 is preferably thermally converted in the cracking zone 8 of the

cracking furnace 1, more preferably, the mixture 17 is steam cracked in the cracking zone 8 of the cracking furnace 1.

Suitably, the temperature in the cracking zone 8 is in the range of from 700 to 900°C, preferably in the range of from 750 to 850°C, more preferably in the range of from 780 to 830°C.

The pressure in the cracking zone 8 is generally in the range of from 1 to 3 bar absolute, more preferably from 1.2 to 1.98 bar absolute.

In cracking zone 8 the mixture 17 obtained in the second preheating zone 5 is thermally converted to a product stream 18 which comprises propylene and ethylene.

Preferably, in cracking zone 8 the evaporated Fischer- Tropsch derived gas oil of the mixture 17 is steam- cracked to a product stream which comprises propylene and ethylene. Further products of the thermal conversion reaction include, but are not limited to, butadiene, benzene, hydrogen and methane and other associated olefinic, paraffinic, and aromatic products.

The temperature of the product stream 18 is

preferably in the range of from 750 to 850°C, more preferably in the range from 780 to 830°C.

The temperature of the product stream 18 is quickly reduced by a transfer-line exchanger 9 to terminate any unwanted reactions to a temperature of below 400°C. Further processing of the cooled product stream 19 to recover propylene and ethylene is known in the art and therefore not discussed here in detail. Further

processing of the product stream and thus also the recovery of propylene and ethylene from the product stream is for example described in "Petroleum

Technology", John Wiley & Sons, Inc., and WILEY-CBH verlag GmbH & Co. KGaA, Weinheim, 2007, ISBN 978-0-470- 13402-3, page 769 to 787.

The present invention is described below with reference to the following Examples, which are not intended to limit the scope of the present invention in any way.

Examples

Example 1

Preparation of a Fischer-Tropsch derived gas oil

comprising paraffins having 9 to 25 carbon atoms

The Fischer-Tropsch derived gas oil was obtained by the process as described in Example 3-4 of WO 02/070627. The properties of the Fischer-Tropsch derived gas oil are listed in Tables 1 and 2.

Table 1

Fischer-Tropsch derived gas oil comprising paraffins having 9 to 25 carbon atoms

Kinematic viscosity at 100°C 1.25

According to ASTM D445

[mm²/s ]

Kinematic viscosity at 40°C 2.27

According to ASTM D445

[mm²/s ]

Content of aromatics 0.6

according to IP 368 [weight

% molecules] Content of nitrogen 0.0001 according to ASTM D-5762-98

[%w]

Content of sulphur according 0.0002 to ASTM D-2622-98

[%w]

Pour point according to ASTM -27

D97

[°C]

Cloud point according to <-22 ASTM D2500

[°C]

Cold Filter Plugging Point <-21 (CFPP) according to ASTM

D6371

[°C]

Cetane index according to 74.6 ASTM D976

[°C]

Density at 20°C according to 785 ASTM D4052 [kg/m³]

Table 2

Mixing Fischer-Tropsch derived gas oil with dilution gas

In a Pyrolysis Milli Scale Unit similar to the one described in ind. Eng. chem. Res. 2001, 40, 470-472, the Fischer- Tropsch derived gas oil with the properties as listed in Tables 1 and 2 was pumped at a flow rate of between 61.4- 65.0 ml/hr to an evaporator.

To this evaporator helium at a flow rate of between 562-597 Nml/min and nitrogen at a flow rate of between 41.30 Nml/min were pumped to obtain a mixture of Fischer- Tropsch derived gas oil, helium and nitrogen. Helium was used as a dilution gas (in lieu of steam, which is used commercially) , and nitrogen was used as internal standard for the GC . The simulated steam to Fischer-Tropsch derived gas oil ratio was 0.6 on a weight basis.

Heating the mixture comprising Fischer-Tropsch derived gas oil and dilution gas The temperature of the evaporator was increased to 550°C to fully evaporate the Fischer-Tropsch derived gas oil mixture to obtain a mixture comprising evaporated Fischer-Tropsch derived gas oil, helium and nitrogen. Thermal conversion of Fischer-tropsch derived gas oil to a product stream comprising propylene and ethylene

This mixture was then transferred to a glass reactor tube (diameter of glass tube was 2mm) . The reactor tube was heated for 0.240 (s) to several high temperatures (see Table 3: Experiments A, B, C, and D) to thermally convert the evaporated Fischer-Tropsch derived gas oil to a product stream comprising propylene and ethylene. The pressure in the tube was 2.25 bar absolute.

The product stream was cooled in a series of quench vessels to approximately 96°C. The composition of the product streams obtained in Experiments A, B, C and D was analysed with GCxGC internal testing methodology and shown in Table 3.

Table 3

Example 2

Determination of high temperature on coke formation

In order to determine the amount of coke formed during heating a mixture comprising evaporated Fischer- Tropsch derived gas oil (with the properties as listed in Tables 1 and 2), helium and nitrogen, to high

temperatures, which temperatures are just below the temperatures at which full thermal conversion takes place, the following experiments were performed.

The same procedure was performed as for Example 1, with the exception that the reactor tube was heated for 6.5-7 hours to several high temperatures (see Table 4:

Experiments E, F, G, and H) . The transparency of the last 20 cm of the reactor tube was used as a measure of coke formation at several temperatures (see Table 4:

Experiments E, F, G and H) . Table 4

Discussion

The results in Table 4 show that heating of a mixture comprising evaporated Fischer-Tropsch derived gas oil to high temperatures did not lead to the formation of coke, as visually observed from the transparency of the reactor tubes in Experiments E to G. The results in Table 3 show that by thermal cracking of Fischer-Tropsch derived gas oil high yields of propylene and ethylene were obtained. This indicates that heavy Fischer-Tropsch derived gas oil with a low amount of carbon residue can be converted to high yields of propylene and ethylene without the risk of coke formation.

Claims

C L A I M S

1. Process for the preparation of propylene and

ethylene, the process comprising at least the following steps :

(c) heating the mixture obtained in step (b) thereby obtaining a mixture of diluted gas and evaporated

Fischer-Tropsch derived gas oil; and

(d) subjecting the mixture obtained in step (c) to a thermal conversion step thereby obtaining a product stream which comprises propylene and ethylene.

2. Process according to claim 1, wherein the Fischer- Tropsch derived gas oil as provided in step (a) has an initial boiling point of at least 140°C, preferably at least 143°C, more preferably at least 144°C and a final boiling point of at most 420°C, preferably at most 400°C, more preferably at most 390°C, and most preferably at most 385°C.

3. Process according to claim 1 or 2, wherein the

Fischer-Tropsch derived gas oil as provided in step (a) has a T10 wt . % boiling point from 190 to 200°C,

preferably from 195 to 200°C and a T90 wt . % boiling point from 330 to 360°C, preferably from 340 to 350°C, and more preferably from 345 to 350°C.

4. Process according to any one of claims 1 to 3, wherein the Fischer-Tropsch derived gas oil as provided in step (a) has a density at 20°C according to ASTM D4052 of at least 760 kg/m³, preferably of at least 770 kg/m³, more preferably of at least 780 kg/m³ and of at most 850 kg/m³, preferably at most 800 kg/m³.

5. Process according to any one of claims 1 to 4, wherein the Fischer-Tropsch derived gas oil as provided in step (a) has a kinematic viscosity at 100°C according to ASTM D445 is above 0.8 cSt, preferably above 0.9 cSt, more preferably above 1.0 cSt.

6. Process according to any one of claims 1 to 5, wherein the dilution gas comprises steam or hydrogen, preferably the dilution gas comprises steam.

7. Process according to any one of claims 1 to 6, wherein the weight ratio of dilution gas to Fischer- Tropsch derived gas oil in step (b) is from 0.3 to 0.8, preferably 0.3 to 0.5, more preferably 0.3 to 0.45.

8. Process according to any one of claims 1 to 7, wherein the temperature in step (c) is in the range of from 420 to 620°C, preferably in the range of from 450 to 610°C, more preferably in the range of from 500 to 610°C and most preferably in the range of from 595 to 610°C.

9. Process according to any one of claims 1 to 8, wherein the Fischer-Tropsch derived gas oil as provided in step (a) comprises less than 10 ppmwt of carbon residue, preferably less than 5 ppmwt of carbon residue and more preferably less than 2 ppmwt of carbon residue and at most 20 ppmwt of carbon residue.

10. Process according to any one of claims 1 to 9, wherein the product stream comprises from 20 to 35 wt.%, preferably from 25 to 35 wt.%, more preferably from 30 to 35 wt.% ethylene based on the total amount of Fischer-

Tropsch derived gas oil as provided in step (a) .

11. Process according to any one of claims 1 to 10, wherein the product stream comprises from 15 to 25 wt . preferably from 17 to 25 wt.%, more preferably from 18 25 wt.% propylene based on the total amount of Fischer Tropsch derived gas oil as provided in step (a) .