GB2435883A

GB2435883A - Autothermal cracking process for ethylene production

Info

Publication number: GB2435883A
Application number: GB0604874A
Authority: GB
Inventors: David Charles Griffiths; Stephen Hardman
Original assignee: Ineos Europe Ltd; Innovene Europe Ltd
Current assignee: PetroIneos Europe Ltd
Priority date: 2006-03-10
Filing date: 2006-03-10
Publication date: 2007-09-12
Also published as: GB0604874D0

Abstract

The present invention relates to a process for the production of ethylene by autothermal cracking of ethane, said process comprising: <SL> <LI>(i) separating an ethane-containing stream from LNG at an LNG regasification plant, <LI>(ii) autothermally cracking the ethane-containing stream from step (i) by contacting with a catalyst capable of supporting combustion beyond the fuel-rich limit of flammability in the presence of oxygen, to produce a product stream comprising ethylene, and <LI>(iii) passing the product stream comprising ethylene to a separations train, to separate ethylene from other components in the product stream, wherein the separations train comprises one or more steps in which refrigeration is provided from the LNG regasification plant. </SL>

Description

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PROCESS

The present invention relates to a process for the production of ethylene by autothermal cracking of ethane separated from liquefied natural gas (LNG).

Natural gas comprises a mixture of paraffinic hydrocarbon gases, and may typically be found in either isolated gas fields ("stranded gas") or associated with crude oil. The principal component of natural gas is methane, with smaller amounts of ethane, propane and butane. The natural gas usually needs to be shipped to where it is desired. In order reduce the volume of natural gas so that it can be economically shipped the natural gas is liquefied, typically by cooling to approximately -16 1 C at atmospheric pressure in an LNG process, to form liquified natural gas (LNG).

Once shipped the LNG is regasified (regas LNG) for use. The natural gas is typically used directly as a fuel for a purpose built power station or passed to a suitable natural gas distribution grid. Because the regas LNG can have a varying composition, ethane (and higher hydrocarbons) in the LNG may need to be separated to meet lean gas specifications for use. The volumes of recovered ethane are too small to provide enough feed for a conventional steam cracking process for the production of ethylene, such processes generally requiring over lmtpa (million tonnes per annum) of ethane to be economically attractive.

However, it has now been found that ethane from an LNG regasification plant can be advantageously utilized to produce ethylene in a co-located autothermal cracking process for production of ethylene.

Hence, in a first aspect, the process of the present invention provides a process for the production of ethylene by autothermal cracking of ethane, said process comprising: (i) separating an ethane-containing stream from LNG at an LNG regasification plant, (ii) autothermally cracking the ethane-containing stream from step (i) by contacting with a catalyst capable of supporting combustion beyond the fuel-rich limit of flammability in the presence of oxygen, to produce a product stream comprising ethylene, and (iii) passing the product stream comprising ethylene to a separations train, to separate ethylene from other components in the product stream, wherein the separations train comprises one or more steps in which refrigeration is provided from the LNG regasification plant.

The present invention takes advantage of the fact that autothermal cracking processes can be economically operated at lower scale than other ethylene production processes, such as steam cracking processes.

In particular, the autothermal cracking process of step (ii) could operate at a scale below I 000ktpa (kilotonnes per annum) of ethane. Preferably the autothermal cracking process of step (ii) is at a scale of 500ktpa or above of ethane, for example in the range 500ktpa to 700ktpa.

Thus, the ethane for the autothermal cracking step (ii) can be provided solely from ethane separated from LNG during regasification. This enables the ATC process (including the separations train) to be located close to the LNG regasification plant such that refrigeration for the ATC separations train can be provided from the LNG regasification plant. The heat which must be removed from the downstream separation section of the ATC plant is thus used to provide energy for regasification of the LNG, thereby maximising the heat integration of the two processes.

The ethane-containing stream separated in step (i) generally comprises predominantly ethane, by which is meant at least 50 vol% ethane, with smaller quantities of higher hydrocarbons, especially propane and butane, and a further advantage of the process of the present invention is that autothermal cracking processes are more tolerant of variations in the feed composition than steam cracking processes. This can be important since the LNG supply may come from sources with different compositions. In particular, varying quantities of propane and butane may be fed with ethane to the autothermal cracker in the process of the present invention whilst still maintaining high ethylene selectivities.

Variations of propane and butane in the ethane-containing stream may be due to natural variations in the composition of the LNG from which the ethane-containing stream has been separated, but the process of the present invention also allows different amounts of propane and butane to be deliberately fed with the ethane.

For example, in one embodiment, propane and butane in the LNG may be separated with the ethane from the LNG as a single, mixed, stream, and the mixed stream is passed to the autothermal cracking step (ii). The autothermal cracking process is tolerant to changes in the relative amounts of ethane, propane and butane due to variations in the LNG source, and the overall process has the added advantage that no separation of ethane from propane and butane is required during the LNG regasification.

In a second, alternative, embodiment, it may generally be preferred to separate at least some of the propane and butane in the LNG as a separate stream for sale as LPG (liquified petroleum gas). However, if the LPG market becomes oversupplied, this propane and/or butane may instead be fed to the autothermal cracker with the ethane-containing stream.

Thus, the process of the present invention has significant versatility to cope with variations in the LNG composition and in local market conditions.

Step (I) of the process of the present invention typically comprises separation of the ethane-containing stream during regasification. Regasification is generally achieved through the transfer of heat into the LNG, usually through at least one heat exchanger. One common technique is to burn a small amount of the LNG to produce the heat needed to gasi the stream of LNG. Another common technique for regasification of LNG uses heat from ambient water, such as seawater or river water.

In the process of the present invention, at least part of the heat required for regasification is provided from the product stream comprising ethylene of the autothermal cracking reaction.

In step (ii) the ethane-containing stream separated in step (I) is autothermally cracked by contacting with a catalyst capable of supporting combustion beyond the fuel-rich limit of flammability in the presence of oxygen, to produce a product stream comprising ethylene. In the autothermal cracking (ATC) process combustion of the feed is initiated on the catalyst surface and the heat required to raise the reactants to process temperature and to carry out the endothermic cracking process is generated in situ.

Autothermal cracking (ATC) is described generally for example in EP 0332289B; EP 0529793B; EP 0709446A and WO 00/14035.

The oxygen may be provided as any suitable molecular oxygen containing gas, such as molecular oxygen itself or air. Generally, pure or essentially pure molecular oxygen is utilised after being separated from air on-site, and in this case, the cost and energy demand for molecular oxygen separation can be reduced by also using some of the "cold" from the LNG regasification to pre-chill the air.

The hydrocarbon to be cracked (ethane and any higher hydrocarbons) and molecular oxygen-containing gas may be contacted with the catalyst capable of supporting combustion in any suitable molar ratio, provided ethylene is produced. The preferred stoichiometric ratio of hydrocarbon to oxygen-containing gas is 5 to 16, preferably, 5 to 13.5 times, preferably, 6 to 10 times the stoichiometric ratio of hydrocarbon to oxygen-containing gas required for complete combustion of the hydrocarbon to carbon dioxide and water.

The hydrocarbon is passed over the catalyst at a gas hourly space velocity of greater than 10,000 h, preferably above 20,000 h' and most preferably, greater than 100,000 h'.

It will be understood, however, that the optimum gas hourly space velocity will depend upon the pressure and nature of the feed composition.

Preferably, hydrogen is co-fed with the ethane-containing stream, oxygen and, where fed, any additional propane and/or butane feeds. The molar ratio of hydrogen to oxygen can vary over any operable range provided that the product stream comprising ethylene is produced. Suitably, the molar ratio of hydrogen to oxygen is in the range 0.2 to 4, preferably, in the range 1 to 3.

Hydrogen co-feeds are advantageous because, in the presence of the catalyst, the hydrogen combusts preferentially relative to hydrocarbon, thereby increasing the olefin selectivity of the overall process.

The autothermal cracking step may suitably be carried out at a catalyst exit temperature in the range 600 C to 1200 C, preferably, in the range 85 0 C to 1050 C and, most preferably, in the range 900 C to 1000 C.

The autothermal cracking step is usually operated at a pressure of greater than 0.5barg. Preferably the autothermal cracking process is operated at a pressure of between 0.5-4obarg.

The catalyst capable of supporting combustion beyond the fuel rich limit of flammability usually comprises a Group VIII metal as its catalytic component. Suitable Group VIII metals include platinum, palladium, ruthenium, rhodium, osmium and iridium.

Rhodium, and more particularly, platinum and palladium are preferred. Typical Group VIII metal loadings range from 0.01 to lOOwt %, preferably, between 0.01 to 20 wt %, and more preferably, from 0.01 to 10 wt % based on the total dry weight of the catalyst.

Where a Group VIII catalyst is employed, it is preferably employed in combination with a catalyst promoter. The promoter may be a Group lIlA, IVA, and/or VA metal. Alternatively, the promoter may be a transition metal; the transition metal promoter being a different metal to that which may be employed as the Group VIII transition metal catalytic component. Preferred promoters are selected from the group consisting of Ga, In, Sn, Ge, Ag, Au or Cu. The atomic ratio of Group VIII B metal to the catalyst promoter may be 1: 0.1 -50.0, preferably, 1: 0.1 -12.0.

Preferred examples of promoted catalysts include Pt/Ga, Pt/In, Pt/Sn, Pt/Ge, Pt/Cu, Pd/Sn, Pd/Ge, Pd/Cu, Rh/Sn, Pt/Pd/Cu and Pt/Pd/Sn catalysts.

For the avoidance of doubt, the Group VIII metal and promoter in the catalyst may be present in any form, for example, as a metal, or in the form of a metal compound, such as an oxide.

The catalyst may be unsupported, such as in the form of a metal gauze, but is preferably supported. Any suitable support may be used such as ceramic or metal supports, but ceramic supports are generally preferred. Where ceramic supports are used, the composition of the ceramic support may be any oxide or combination of oxides that is stable at high temperatures of, for example, between 600 C and 1200 C. The support material preferably has a low thermal expansion co-efficient, and is resistant to phase separation at high temperatures.

Suitable ceramic supports include corderite, lithium aluminium silicate (LAS), alumina (a-A1203), yttria stabilised zirconia, alumina titanate, niascon, and calcium zirconyl phosphate. The ceramic supports may be wash-coated, for example, with y-Al203.

The catalyst capable of supporting combustion beyond the fuel rich limit of flammability may be prepared by any method known in the art. For example, gel methods and wet-impregnation techniques may be employed. Typically, the support is impregnated with one or more solutions comprising the metals, dried and then calcined in air. The support may be impregnated in one or more steps. Preferably, multiple impregnation steps are employed. The support is preferably dried and calcined between each impregnation, and then subjected to a final calcination, preferably, in air. The calcined support may then be reduced, for example, by heat treatment in a hydrogen atmosphere.

The product stream comprises ethylene, and may also comprise propylene and butylene. The product stream is quenched as it emerges from the reaction chamber to avoid further reactions taking place. Usually the product stream is cooled to between 750-600 C within less than 1 OOmilliseconds of formation, preferably within 5Omilliseconds of formation and most preferably within 20milliseconds of formation e.g. within lOmilliseconds of formation. The heat from the quenching is used to generate high-pressure steam, which is used to provide power for those parts of the overall process requiring it.

In addition to olefins, the autothermal cracking reaction produces hydrogen, carbon monoxide, methane, and small amounts of acetylenes, aromatics and carbon dioxide.

In step (iii) of the process of the present invention this product stream is passed to a separations train to separate ethylene from the other components in said stream. The carbon dioxide is usually removed first, typically using an amine-based absorption system such as MEA or TEA (or mixtures of both), or any other commercially available CO2 removal process. The reaction products are then treated in a refrigeration facility (cryogenic separation unit) to separate methane, hydrogen and carbon monoxide from ethylene and higher hydrocarbons.

In the process of the present invention, the refrigeration for this refrigeration facility is provided from the LNG regasification plant.

This has significant synergistic advantages since a separate refrigeration facility for the autothermal cracking process is not required, saving cost and energy requirements. In addition, the heat which must be removed from the downstream separation section of the ATC plant provides energy for regasification of the LNG.

The ethylene may be separated from ethane and higher hydrocarbons by any suitable technique, for example by passing to a de-ethaniser to separate ethane and ethylene from propane, propylene and any higher hydrocarbons, followed by subsequent separation of ethane from ethylene.

In a preferred embodiment of the process of the present invention, the LNG regasification and ATC processes also share one or more other process facilities (services) such as power, control room and flare facilities.

The autothermal cracking process also produces, as one or more by-products streams, methane, hydrogen and carbon monoxide. In a typical ATC process, a process stream comprising methane, hydrogen and carbon monoxide is used as fuel gas in a gas turbine to provide energy for the overall autothermal cracking process. Where the LNG is --7 to be utilised directly as a fuel in a power plant, this process stream may instead be supplied as a fuel gas stream to the power plant to generate energy this way. Thus, a separate turbine can be avoided.

The ethylene produced, along with any other olefins, may be used as feedstocks for olefin derivative processes, such as polymerization processes to produce polyethylene, polypropylene and other polymers. Such processes may be located with the ATC process and LNG regasification plant, or may be remote from said processes.

Production of ethylene according to the process of the present invention has the added advantage that the scale of the ethylene production can be matched to the feed requirements of a single downstream ethylene derivative plant. In contrast, for a "conventional" steam cracking process of I mtpa scale several derivative plants are required to utilise all the ethylene produced.

Claims

CLAIMS

1. A process for the production of ethylene by autothermal cracking of ethane, said process comprising: (i) separating an ethane-containing stream from LNG at an LNG regasification plant, (ii) autothermally cracking the ethane-containing stream from step (i) by contacting with a catalyst capable of supporting combustion beyond the fuel-rich limit of flammability in the presence of oxygen, to produce a product stream comprising ethylene, and (iii) passing the product stream comprising ethylene to a separations train, to separate ethylene from other components in the product stream, wherein the separations train comprises one or more steps in which refrigeration is provided from the LNG regasification plant.