EP0127519B1 - Hydrocarbon steam-cracking process - Google Patents

Abstract

The steam and thermal cracking of hydrocarbons is facilely carried out by in situ generating a stream of hot combustion gases including steam, advantageously in the configuration of a downstream axially extending, axially symmetrical helical flowstream, by combustion of steam-producing reactants in a combustion first reaction zone, and serially directly contacting and intimately admixing a liquid hydrocarbon feedstock with said gas of combustion in a downstream isodistribution, non-multi-tubular second reaction zone, advantageously first at a zone of reduced pressure thereof, the momentum of said gas of combustion at the point of direct contact being such as to provide all of the thermal and mechanical energy and heat transfer required to autogenously vaporize, entrain and effect cracking therein of said liquid hydrocarbon feedstock.

Description

The present invention relates to a process for steam cracking of hydrocarbons, optionally in the presence of hydrogen. We know that steam cracking of hydrocarbons, or cracking in the presence of water vapor, was used in the UNITED STATES between 1930 and 1940 for the production of ethylene and aromatic hydrocarbons.

It was then a process according to which the load to be cracked, diluted, circulated in tubes heated from the outside. The evolution of technology has made it possible to increase production capacities and to use increasingly heavy hydrocarbon loads.

• - «elle abaisse la pression partielle de l'hydrocarbure et favorise ainsi un craquage sélectif en oléfine
• - elle réduit la pression partielle des hydrocarbures aromatiques à haut poids moléculaire et diminue leur tendance à former du coke avec la charge de craquage et à déposer des résidus lourds sur la surface des échangeurs et des canalisations
• - elle a un effet oxydant suffisant sur les tubes métalliques afin de diminuer, d'une façon significative, les effets catalytiques du fer ou du nickel pour la formation du carbone».

It is known that, in such a process, water vapor fulfills several functions. Thus, according to B. BLOURI (State of knowledge on the heat treatment of heavy products - Review of the French Petroleum Institute, Vol. 36 N ° 1 - JANUARY-FEBRUARY 1981), the introduction of water vapor to three main duties:

• - "it lowers the partial pressure of the hydrocarbon and thus promotes a selective cracking into olefin
• - it reduces the partial pressure of high molecular weight aromatic hydrocarbons and decreases their tendency to form coke with the cracking charge and to deposit heavy residues on the surface of the exchangers and pipes
• - it has a sufficient oxidizing effect on the metal tubes in order to significantly reduce the catalytic effects of iron or nickel for the formation of carbon ”.

There is also a process by which it is sought to replace water vapor with pressurized hydrogen. The objective is to obtain a large proportion of alkenes by cracking a charge without catalyst, in a very short residence time and at a temperature which, if the charge is naphtha, is of the order of 930 ° C.

The use of hydrogen has two advantages: the conversion of heavy products into light products is improved and the formation of deposits is reduced.

Unfortunately, the presence of a large amount of hydrogen causes hydrogenation reactions which compete with the cracking reactions themselves. There is then inhibition of the formation of alkenes.

Finally, whatever the case, the parasitic effects of the walls appear as a constraint in the steam cracking processes.

In general, attempts have been made to avoid using multi-tubular reactors. It has long been proposed to carry out a cracking by direct contact with combustion gases, for example by means of a thermal shock as in US Pat. No. 2,790,838. More sophisticated forms of contact have also been claimed. Thus, in US-A-4136015, the charge is introduced in atomized form into the gas stream obtained by combustion of the heavy product by an oxidant, in a burning and mixing zone, followed by a cracking and d 'a quenching zone.

The reactor has the shape of a convergent-divergent nozzle.

However, this reactor supposes that one knows how to control a complex flow of fluids, as noted by Barendregt - Information Chimie, N ° 231, Nov. 82, p. 217-221.

“The fundamental condition for any cracking process with olefin production is the appropriate setting of the residence time / temperature ratio, which is possible in a tubular reactor. As the recent trend is towards very short residence times, it will be more difficult to create this fundamental condition and the development of new technologies will prove to be essential ”.

However, this implies that the mixture between a viscous liquid at the start and a gaseous phase is produced homogeneously in a time significantly less than the residence time, that is to say that the spraying, vaporization and mixing time is the most short possible.

To these problems, we must add those posed by the need for load changes, insofar as we use loads ranging from the lightest to the heaviest and where we want to control the process, depending of the final product that we want to favor: ethylene or other.

Thus, GB-A-1 174870 has proposed a device for cracking hydrocarbons, leading to the manufacture of ethylene and acetylene, by treatment by means of a vortex gas flow, but the quantities of ethylene and acetylene obtained remain low.

Thus, despite repeated efforts and numerous solutions proposed for the past fifty years, the basic problems of steam cracking remain without satisfactory solution.

However, and this is what is the subject of the present invention, the Applicant has found that these disadvantages could be overcome in a direct, non-tubular contact process.

According to the invention, the combustion gas is formed in situ and contains water vapor. In addition, it serves to provide the mechanical energy necessary to produce, in situ, the transfer surface as well as the thermal energy necessary for cracking; heat transfer is therefore carried out without the inconvenience of a tubular exchange surface and the combustion gas and the charge are mixed intensively.

According to the method of the invention, in a first zone 1, gaseous reagents are introduced, so that the resulting gaseous phase has the form of a symmetrical helical flow, said reagents reacting in this zone, so as to produce a gas containing superheated water vapor which is evacuated in the form of a well-vortex flow in an isorepartition zone 2 into which the charge of hydrocarbon to be cracked is introduced, at the level of the zone in relative depression generated by the well-vortex flow, the momentum of the combustion gas containing the superheated steam being sufficient relative to that of the liquid charge to cause said liquid charge to rupture, to be sprayed and to take charge of each drop formed by an associated gas volume element, the amount of heat provided the combustion gas being sufficient to allow the vaporization of this charge, overheating and cracking at the desired temperature. In practice, the momentum of the volume elements of the gas phase from zone 1 is at least 100 times and preferably between 1000 and 10000 times that of the associated volume elements of the liquid phase.

Contrary to what happens with conventional atomization processes with random distribution of the volume elements of the liquid phase, heat transfers can be carried out in a very short time in an isorepartition zone, due to the quality of contact between the two phases (so-called isoflash effect).

It suffices to choose, in this case, pressures on the gases, low compared to the pressure in the contact zone.

The speed of introduction of the liquid, which can be optionally predispersed, is generally low, less than 10 m / s and preferably less than 5 m / s.

According to the method of the invention, the gas-well-vortex phase is obtained, in situ, by means of the gases serving to generate the combustion gas containing the steam of the steam cracking process.

The temperature of this combustion gas will depend in particular on the nature of the charge to be cracked and on the final products which it is desired to favor, but it must in any case be sufficient to cause cracking of the charge, in any case higher than the cracking temperatures in tubular reactors which are of the order of 800 ° -900 ° C.

The temperature of this gaseous well-vortex phase will therefore advantageously be as high as possible, taking into account the possibilities of the apparatus, that is to say practically between 1000 and 25θ 0 ° C.

This combustion gas containing superheated steam can be obtained by combustion of a hydrocarbon.

The superheated steam produced in zone 1 can also be obtained by direct combustion of hydrogen and oxygen, at least one of these reactants being introduced in a symmetrical helical flow into said zone.

It is also possible to introduce excess hydrogen. This hydrogen can fulfill several functions as a thermal diluent in zone 1 but also as a chemical participant in cracking in zone 2.

A non-pure oxygen source can also be used.

Optionally, it is possible to introduce a very small proportion of oxygen into the contact and isorepartition zone 2.

Surprisingly, it has been found that this superheated steam, produced immediately before the isorepartition zone in which cracking occurs (at least partially), overcomes the drawbacks of the prior art.

Although we are not in a position to explain with certainty the action of water vapor under the conditions of the invention, it is believed that the physicochemical state in which water vapor s is not indifferent.

As said previously, the method according to the invention has great flexibility with respect to the load introduced.

• - une chambre amont (1 à à l'intérieur de laquelle a lieu la combustion nécessaire à la production de vapeur d'eau surchauffée
• - une chambre aval (2) dans laquelle a lieu la réaction de craquage
• - des moyens d'injection tangentiels et de répartition symétrique d'au moins une source gazeuse du groupe de l'hydrogène, des hydrocarbures et de l'oxygène
• - un passage restreint aval, de manière à conférer au flux gazeux l'allure d'un écoulement puits-tourbillon symétrique
• - au moins un moyen d'introduction axiale de la charge d'hydrocarbure à l'entrée de la chambre de craquage. Eventuellement, la charge peut être pré- dispersée afin de favoriser sa pulvérisation par l'écoulement puits-tourbillon issu de la chambre de combustion (1)
• - des moyens pour effectuer une trempe sur les produits issus de la réaction.

It can be implemented in a device which comprises:

• - an upstream chamber (1 inside which takes place the combustion necessary for the production of superheated steam
• - a downstream chamber (2) in which the cracking reaction takes place
• means of tangential injection and symmetrical distribution of at least one gaseous source from the group of hydrogen, hydrocarbons and oxygen
• - a restricted downstream passage, so as to give the gas flow the appearance of a symmetrical well-vortex flow
• - At least one means of axial introduction of the hydrocarbon charge at the entrance to the cracking chamber. Optionally, the charge can be pre-dispersed in order to favor its spraying by the well-vortex flow coming from the combustion chamber (1)
• - Means for carrying out a quenching on the products resulting from the reaction.

But the present invention will be more readily understood with the aid of the following examples, given by way of illustration, made using the device illustrated in the figure attached, with the couple H _{2/0 2} for forming vapor superheated water.

This comprises a combustion chamber (1) which has an envelope (3), closed at its downstream part by a restricted passage (4), a pipe (5) for tangential supply of hydrogen, a supply pipe (6) oxygen. The envelope (3) ends downstream by a convergent (7) in which ends, along the axis of rotation symmetry of the chamber (1) an injection device (8) of the hydrocarbon charge, substantially at the restricted passage (4), a contact chamber (2) extending downstream the chamber (1) along the same axis of symmetry. Furthermore, the chamber (1) has an ignition means (9), an annular cooling space (10) around the injection device (8) and another cooling circuit (11) around said chamber (1 ). Finally, a quenching device (12) is provided at the outlet from the contact chamber (2).

Examples

The hydrocarbon charge consists of a liquid hydrocarbon fuel (domestic fuel oil according to French standards, type 2 according to ASTM standard).

In the examples, the temperature / residence time couple is varied with a device of 20 cm overall size.

Examples 1 and 2 are produced with a chamber 2 in the form of a bicone as in the figure. In Example 3, the bicone is replaced by a tube.

• - que le taux de transformation de la charge en moins de C₅ est supérieur à 80%, c'est-à-dire qu'il est excellent, aussi bon que pour un naphta plus léger, par un procédé conventionnel (réacteur tubulaire), ce qui correspond à une meilleure performance,
• - que le rendement en C₂H₂ + C₂H₄ + C₃H₆ est bon, de l'ordre de 50%,
• - que l'on peut faire varier simplement la sélectivité notamment en acétylène,
• - que ces résultats sont obtenus avec un dispositif de faible dimension eu égard à l'encombrement de la chambre de réaction.

On these examples we observe:

• - that the rate of conversion of the charge into less than C ₅ is greater than 80%, that is to say that it is excellent, as good as for a lighter naphtha, by a conventional process (tubular reactor) , which corresponds to better performance,
• - that the yield of C ₂ H ₂ + C ₂ H ₄ + C ₃ H ₆ is good, of the order of 50%,
• - that the selectivity can be varied simply, especially in acetylene,
• - that these results are obtained with a device of small size having regard to the size of the reaction chamber.

It has been verified on a heavy hydrocarbon (heavy fuel oil according to French type 6 standards according to the ASTM standard) that the mixture consisting of the following light hydrocarbons (CH ₄ - C ₂ H ₂ - C ₂ H ₄ - C ₃ H ₆ ), under the same conditions as in test 3, has a composition similar to that obtained from the preceding light hydrocarbon, as indicated in Table III.

Claims (6)

1. Process for the steam cracking of hydrocarbons by direct contact with in situ formation of the combustion gas containing steam, characterised in that in a first zone 1, gaseous reactants are introduced in such a way that the gaseous phase resulting therefrom is in the form of a symmetrical helical flow, said reactants reacting in said zone, so as to produce a combustion gas containing superheated steam which is discharged in the form of a symmetrical axial flow-vortex flow configuration into an isodistribution zone 2 into which the hydrocarbon charge to be cracked is introduced, at the location of the zone in which there is a relative depression generated by the axial flow-vortex flow configuration, the momentum of the combustion gas containing superheated steam being sufficient with respect to that of the liquid charge to cause said liquid charge to be broken up and atomised and to cause each drop formed to be taken in charge by an associated element of gaseous volume, the amount of heat provided by said combustion gas being sufficient to permit vaporisation of said charge, superheating thereof and cracking thereof at the desired temperature.

2. A process according to claim 1, characterised in that the momentum of the elements of the gaseous phase is at least 100 times and preferably 1000 to 10000 times that of the elements of volume of the liquid phase.

3. A process according to one of claims 1 and 2, characterised in that the temperature of the combustion gas is higher than 800°C and is advantageously between 1000 and 2500°C.

4. A process according to one of claims 1 to 3, characterised in that the combustion gas containing the superheated steam is produced by the combustion of a hydrocarbon.

5. A process according to one of claims 1 to 4, characterised in that the superheated steam is produced by the direct combustion of hydrogen and oxygen, at least one of said reactants being introduced as a symmetrical helical flow.

6. A process according to one of claims 1 to 5, characterised in that an excess of hydrogen is introduced.

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