GB2213828A

GB2213828A - Process for the preparation of ethylene

Info

Publication number: GB2213828A
Application number: GB8729554A
Authority: GB
Inventors: Santen Rutger Anthony Van; Petrus Anthonius Joha Hendriks; Der Zwet Gerardus Petrus Van
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1987-12-18
Filing date: 1987-12-18
Publication date: 1989-08-23
Also published as: GB8729554D0

Abstract

Process for the preparation of ethylene by pyrolysis of low molecular weight hydrocarbons, by contacting a flow of gas, consisting of low molecular weight hydrocarbons with an inert particulate carrier material having a surface area of > 0.01 and <2 m<2>/g, at a temperature in the range of from 1000 to 1400 DEG C and at a GHSV of from 100 to 6000 hr<-1>, during a period in the range of from 1 minute to 60 minutes, followed by recovering ethylene gas produced in said reaction from the product stream, and calcination of said inert particulate carrier material in a flow of an oxidizing gas to remove the carbonaceous deposits on the carrier material surface, at a temperature in the range of from 600 DEG C to 1000 DEG C and a GHSV of from 100 to 6000 hr<-1>, for at least one minute.

Description

PROCESS FOR THE PREPARATION OF ETHYLENE The invention relates to a process for the preparation of ethylene by pyrolysis of low moleculair weight hydrocarbons and more particularly methane.

Such a process is known from e.g. " Catalytic Pyrolysis of methane", Treliant Fang and Chuin-Tih Yeh, J. Chinese Chem. Soc., 29,265-273 (1981), relating to the investigation of the product distribution of the methane decomposition over different metal oxides/SiO2 compositions at 1400 K, teaching that ThO2/SiO2 was found to be the most effective catalyst for the methane conversion in terms of activity and ethylene selectivity.

Moreover, it was found for this ThO2/SiO2 composition, that at low reaction conversions, ethane and ethylene were the major products, while yields of ethylene and other unsaturated products are sensitively inhibited by NO impurities in the methane.

From UK-patent application No. 2,148,935A a catalytic process is known for the production of higher molecular weight hydrocarbons e.g. ethylene and benzene from lower molecular weight hydrocarbons such as methane, utilizing a metal compound-containing catalyst, reaction temperatures greater than 1000 "C and gas hourly space velocity greater than 3200 hr . The catalyst to be used contains a metal compound of the Group IA, IIA, IIIA, IVB or actinide series metals, such as lithium, calcium, aluminium, zirconium or thorium.

From US-4,507,517 a catalytic process for the production of e.g. ethylene and benzene from lower molecular weight hydrocarbons such as methanes is known, using a boron compound containing catalyst, a reaction temperature of greater than 1000 "C and a gas hourly space velocity of greater than 3200 hr . Preferably as catalyst a boron carbide or boron nitride is used.

From US-4,450,310 a process is known for the conversion of methane into olefins and hydrogen comprising contacting methane in the absence of oxygen and in the absence of water at a temperature of at least 500 C, with a catalyst comprising the mixed oxides of a first metal selected from lithium, sodium, potassium, rubidium, cesium and mixtures thereof and a second metal selected from beryllium, magnesium, calcium, strontium, barium and mixtures thereof. These catalysts additionally comprise an oxide of a promotor metal selected from copper, rhenium, tungsten, zirconium, rhodium and mixtures thereof, while the preferred reaction temperature is said to be between about 700 C to about 1000 "C.

From US-4,563,197 a process is known for the production of ethylene and other hydrocarbon compounds, such as benzene, from coal, comprising the steps of (a) reacting coal particulates entrained in a flow of gas consisting essentially of methane with said methane gas at a temperature in the approximate range of 850 "C to 1000 "C and at a partial pressure of approximately 50 psi for a period of time of approximately 1.5 seconds, and recovering ethylene gas produced in said reaction from the product streamcomprising in addition to said ethylene, said methane and other reaction products.

It will be appreciated that when using these before-mentioned prior art processes, in all cases several by-products are formed in addition to the desired ethylene, on account of which the ethylene selectivity will invite for further improvement in order to come to a real economical, industrial process.

On the other hand ethylene is a very important raw material in the plastic and polymer industries and it is anticipated that the demand for ethylene will continue to increase in the future.

At present ethylene is produced mainly through thermal and catalytic hydrocracking of hydrocarbons.

Therefore a rapidly growing interest in an economical process for the preparation of ethylene from low molecular weight hydrocarbons occurring in LNG and more particularly methane has led to an intensive research and development, directed to such a process.

Thus it is an object of the present invention to provide an economical process for the preparation of ethylene from low molecular weight hydrocarbons and more particularly methane, i.e. a process showing high activity and selectivity for ethylene.

As result of extensive research and experimentation there has now been surprisingly found such an improved economical process for the preparation of ethylene by pyrolysis of low molecular weight hydrocarbons, characterized in that a flow of gas consisting of low molecular weight hydrocarbons is contacted with an inert particu 2 late carrier material having a surface area of > 0.01 and < 2 m as measured by the BET method using krypton, at a temperature in the range of from 1000 to 1400 "C and at a GHSV of from 100 to 6000 hr -l during a period in the range of from 1 minute to 60 minutes, followed by recovering ethylene gas produced during said contacting from the product stream, and calcination of said inert particulate carrier material in a flow of an oxidizing gas to remove carbonaceous deposits on the carrier material surface, at a temperature in the range of from 600 "C to 1000 "C and at a GHSV of from 100 to 6000 hr 1, for at least one minute.

With the term "inert, particulate carrier material" is meant any material which under the reaction conditions and/or calcination conditions does chemically not interfere with the reaction.

As suitable examples of carrier materials may be mentioned silicium oxide, aluminium oxide, magnesium oxide, calcium oxide, silicium carbide, talc and the like.

More preferably alpha -A1203 or SiO2 in the form of quartz or any of its dense polymorphs are applied.

It will be appreciated that contrary to the generally adopted prior art conceptions, it has been found that the process according to the present invention can provide attractive conversion and selectivity results independent on the type of inert carrier material, provided that the specific surface area of the carrier as well as the other relevant parameters have values in the indicated ranges. Therefore the process can actually be carried out with relatively simple and cheap carriers.

Preferably an inert carrier material is used, having a surface 2 area in the range of from 0.05 to 1.5 m /g as measured according to the BET-method (Journal Am. Chem. Soc. vol. 60, pp 309 (1938) using 2 krypton and more preferably in the range of from 0.1-1 m /g.

With the term "low molecular weight hydrocarbons" as used throughout the specification, are meant hydrocarbons containing 1-3 and more preferably 1-2 carbon atoms.

Moreover, it was found that the most attractive results are obtained when using inert carrier materials having a water pore volume of < 0.6 ml/g and more preferably in the range of from 0.25 to 0.50 ml/g.

The main hydrocarbon conversion reaction is preferably carried out at an operational temperature of from 1100 to 1400 C.

Preferably the applied GHSV is in the range of from 700 to 5000 hr and more preferably 3000-4000 hr -1 while a preferred period for leading a hydrocarbons containing gas through the reactor will be in the range of from 3 to 20 minutes.

The calcination period during which carbonaceous deposits (deposited coal and/or tar) is substantially removed from the inert carrier material, is preferably in the range of from 5 to 20 minutes. The calcination step is preferably carried out with air as oxidizing gas, using a GHSV in the range of from 700 to 5000 her 1 and/or a temperature in the range of from 700 to 800 "C.

It will be appreciated that contrary to the teachings of e.g.

US patent 4,450,310, column 5, it has been found now that an attractive ethylene formation can be reached when using a non-basic inert carrier material.

It will be appreciated that the low molecular hydrocarbons feedstock preferably will consist of a predominantly methane and minor amounts of ethane containing gas, derived from natural gas, optionally mixed with inert diluents such as nitrogen, or helium and the like.

The process of the present invention may be carried out in a great variety of tube reactors, wherein the inert carrier material is incorporated by methods, generally known in the art. The inert carrier material may comprise one continuous column of a sufficient height to reach an attractive conversion after one pass through, but may also be divided over several reactors in series or over several, separate, interconnected compartments, incorporated in one column reactor, or the inert material may be recirculated through one or more reactors.

Heating of the reactor to provide the needed reaction heat may be carried out by e.g. an external electric heating equipment around the reactor tube wall or by heating the feedstock indirectly via preheated particles or directly before the entrance of the reactor(s).

It will be appreciated that a more preferred embodiment of the process of the present invention is formed by operating several reactors simultaneously, wherein the respective reaction and calcination stages are different, in order to reach an almost continuous output of ethylene containing product stream.

The advantage of the process of the present invention is that it enables the use of relatively simple and cheap inert carrier materials, giving attractive Space-Time-Yield and rather constant selectivity as to ethylene and therefore provides a significant economical process for ethylene preparation.

The following example further illustrates the invention however without restricting its scope to these specific embodiments.

EXAMPLE The test reactor comprised a 450 mm long vertical quartz reactor tube with an inner diameter of 3.2 mm. The central 120 mm of the reactor tube were heated by a high temperature electric furnace (Setaram, model four PL 200). The top of the reactor tube was connected to a gas flow system, which permitted methane, nitrogen (or helium) or an nitrogen (or helium)-oxygen mixture to be passed through the reactor at a measured rate using pre-calibrated flow-meters. The gas leaving the reactor tube was injected into a CLC equipped with a flame ionization detector and optionally a mass spectrometer or thermal conductivity detector was used to measure hydrogen, carbon monoxide and carbon dioxide.

Different GLC columns were used to separate the light aromatics (mainly benzene, with small amounts of toluene and naphthalene) and the aliphatic hydrocarbons.

The central part of the reactor tube was filled with the inert carrier material. The material bed was supported by a quartz frit, which was located 30 mm below the heating zone to prevent the deposition of carbonaceous deposits on the frit during the reaction. The first 40 mm of the reactor tube above the frit were filled with 14-25 mesh particles of the desired material. These particles were relatively large in order to avoid pressure build-up in the reactor. On top of the 14-25 mesh particles, the tube was filled with 30-80 mesh particles of the same material over a length of 80 mm, which corresponds to the central part of the heating zone. A total of about 1.2-1.8 ml of inert carrier material was charged to the reactor.The part of the reactor tube below the frit consists of a capillary (inner diameter of 1 mm) in order to facilitate a fast removal of the reaction products from the central heating zone.

The experiments were performed in a pulsed mode as follows: after the reactor tube had been filled with the inert carrier material and connected to the gas flow system, a nitrogen (or helium) flow of 40 ml/min was carried through the reactor and the reactor was heated to the desired operating temperature. The temperature was measured by a thermocouple mounted in the furnace wall. The actual temperature in the reaction zone was calculated from calibration experiments in which a thermocouple was mounted in the reaction zone and compared to the thermocouple in the furnace wall. Once the reaction temperature was reached, the nitrogen flow was replaced by methane with a predetermined flow rate. Space velocities were calculated on the basis of the methane flow rate and the total volume of the test material. After 5 minutes 0.5-1.0 ml of the exit gas were injected into the GLC.Subsequently, the methane flow was replaced again by nitrogen and the reactor was cooled down to a temperature in the range 600-800 "C. For a period of 1-20 minutes, oxygen at a rate of 20 ml/min was added to the nitrogen flow in order to remove the carbonaceous deposits from the inert carrier material and the reactor wall. After this regeneration period, the oxygen flow was stopped and the reactor was heated again to the operating temperature under a nitrogen flow. Then the next methane pulse could be applied, etc.

In these experiments the exit gas of the reactor was analyzed at different GHSV's of the methane and the methane conversion and selectivities of the different reaction products were calculated.

Corrections had to be made for the increase in volume due to the formation of hydrogen. The amount of coke/tar formed was calculated on the basis of a fixed C/H ratio in the coke/tar fraction determined in calibration experiments.

Several experimental runs according to the above description are listed below in table 1.

Table 1a Selectivity (vol % C) Ex. Material SA PV(H20) GHSV Conv. C2= C2-- arom. coke/tar STY (m2/g) (ml/g) (hr ) (%) (kg/l hr) 1 a-alum. 0.43 0.37 3300 20 24 11 3 58 0.17 2 a-alum. 0.03 0.19 1100 20 24 16 14 42 0.06 3 quartz 0.025 0 600 20 23 16 18 38 0.03 4 silicium 0.045 < 0.05 800 20 22 6 14 53 0.04 carbide 5 a-alum. 0.94 0.49 5200 20 22 9 3 63 0.21 6 a-alum. 2.0 0.35 5700 20 16 4 0.3 77 0.16 7c - - -- y=2.70 20 23 17 28 27 0.03 8 d -alum. 231 0.75 3700 45 1 0 0 99 0.01 9 a-alum. 0.43 0.37 50 53 2 0 0 98 0.06 a. In all cases methane pulses are applied for a 5 min. period at a reaction temperature of 1125 "C and a pressure of 1 bar.

Regeneration of the test material occurs at 750 "C during a 20 min. period.

b. STY means the Space Time Yield of ethylene.

c. In the case of an empty reactor, without inert carrier material, GHSV is replaced by a residence timer, which is defined as the time that the methane spends in the zone which is normally packed with the inert carrier material.

Experiments 1-5 illustrate specific embodiments of the invention using various test materials. Experiment 1 is particularly preferred. Experiments 2-4 show that, while different materials (c.q. a-alumina, quartz and silicium carbide) are employed, the results are similar, if and when the specific surface areas are about the same.

Experiments 8 and 9 show that a similar process fails when the specific surface area is too high (ex. 8) or the GHSV is too low (ex. 9), whereas experiment 6 shows that the selectivity begins to 2 decrease at a specific surface of 2,0 m /g.

Claims

1. Process for the preparation of ethylene by pyrolysis of low molecular weight hydrocarbons, characterized in that a flow of gas, consisting of low moleculer weight hydrocarbons is contacted with an inert particulate carrier material having a surface area of 2 > 0.01 and < 2 m /g as measured by the BET method using krypton, at a temperature in the range of from 1000 to 1400 "C and at a GHSV of from 100 to 6000 hr , during a period in the range of from 1 minute to 60 minutes, followed by recovering ethylene gas produced during said contacting from the product stream, and calcination of said inert particulate carrier material in a flow of an oxidizing gas to remove carbonaceous deposits on the carrier material surface, at a temperature in the range of from 600 "C to 1000 "C and a GHSV of from 100 to 6000 hr , for at least oe minute.

2. Process according to claim 1 characterized in that an inert carrier material is used having a surface area in the range of from 2 0.05 to 1.5 m2/g, as measured according to the BET method as specified hereinbefore.

3. Process according to claim 1 or 2 characterized in that an inert carrier material is used having a surface area in the range 2 of from 0.1 to 1 m2/g.

4. Process according to one or more of claims 1 to 3 characterized in that an inert carrier material is used having a water pore volume of < 0.6 ml/g.

5. Process according to one or more of claims 1 to 4 characterized in that an inert carrier material is used having a water pore volume in the range of from 0.25 to 0.50 ml/g.

6. Process according to one or more of claims 1 to 5 characterized in that inert carrier materials are selected from silicium oxide, aluminium oxide, magnesium oxide, silicium carbide, calcium oxide and talc.

7. Process according to one or more of claims 1 to 6 characterized in that the hydrocarbon conversion reaction is carried out at a temperature of 1100 to 1400 "C.

8. Process according to one or more of claims 1 to 7 characterized in that the main hydrocarbon conversion reaction is carried out at a GHSV in the range of from 700 to 5000 hr

9. Process according to one or more of claims 1 to 8 characterized in that the main hydrocarbon conversion reaction is carried out at a GHSV in the range 3000 to 4000 hr

10. Process according to one or more of claims 1 to 9 characterized in that the hydrocarbon conversion reaction is carried out over a period in the range of from 3 to 20 minutes.

11. Process according to one or more of claims 1 to 10 characterized in that the calcination period for the removal of carbonaceous deposits is in the range of from 5 to 20 minutes.

12. Process according to one or more of claims 1 to 11 characterized in that the calcination step is carried out with air, at a GHSV in the range of from 700 to 5000 hr

13. Process according to one or more of claims 1 to 12 characterized in that the calcination step is carried oWt a temperature in the range of from 700 to 800 "C.

14. Process according to claim 1 substantially as described hereinbefore with reference to the Example.

15. Ethylene obtained by the process according to anyone of the claims 1 to 14.