FR2683542A1 - Process for thermal pyrolysis of hydrocarbons using a gas generator and using an external combustion of a fuel with compressed air - Google Patents

Abstract

Improvement to a process and to a device for carrying out endothermic chemical reactions such as a steam-cracking reaction for producing ethylene. The heating gases under pressure travelling in the calandria (14) of a heat exchanger-reactor (11) are conveyed into at least one power-recovery turbine (22) which puts into action at least one air compressor (51) into which an appropriate volume of air is conveyed. The air is compressed to a determined pressure and heated in a combustion zone (55) distinct from that (1) feeding the heat exchanger-reactor. The compressed heating effluents are also conveyed into the calandria of the heat exchanger-reactor (1). The energy recovered in the power-recovery turbine thus makes it possible to reduce the number of gas generators (1) supplying heating gas to the heat exchangers-reactors.

Description

The invention relates to an improvement to a method and a device for carrying out endothermic chemical reactions by passing the charge through a heat exchanger under pressure and at high temperature.

It applies in particular to the implementation of a non-catalytic reaction such as the steam cracking of a hydrocarbon feed such as naphtha or ethane leading to the production of ethylene and to the implementation of a reaction catalytic like the production of aromatic hydrocarbons from a feed comprising mainly of saturated aliphatic hydrocarbons, from 2 to 20 carbon atoms or the production of olefinic hydrocarbons from this same feed.

The applicant has already described in two French patent applications filed under No. 91 / 04.688 and 91 / 04.689 a process of steam cracking from a gaseous mixture of a hydrocarbon feed containing from 2 to 20 carbon atoms and of vapor of water, comprising a heating phase in a heating zone under appropriate conditions, which delivers a hot effluent of steam cracking rich in ethylene and a rapid cooling phase of said effluent in a cooling zone under appropriate conditions and one recovers said cooled steam cracking effluent.

Said mixture is preferably introduced beforehand, in an adequate hydrocarbon-to-water vapor ratio at one end of at least one reactor and shell and tube pressurized heat exchanger, and the mixture is circulated inside a plurality of tubes contained in said exchanger reactor.

The heating phase of the gas mixture is carried out in the reactor-heat exchanger according to the following steps: a) an amount of air and a
quantity of fuel in at least one gas generator, suitable for delivering
heating gases at adequate pressure and temperature and
containing 10 to 20% oxygen by volume; b) Said gases are sent to an inlet of the shell of the reactor-heat exchanger
heat at one end, preferably on the introduction side of the mixture, and
said gases are circulated in the shell of the reactor-heat exchanger of
so as to exchange heat indirectly with the gas mixture; c) An in situ combustion is optionally carried out in at least part of
the shell of the reactor-exchanger, preferably in the middle of the reactor
exchanger, an appropriate quantity of gaseous fuel and possibly
inert gas by at least part of said gas, the fuel being injected
under conditions such that the temperature is maintained substantially
constant all along said tubes, and said heating gases and
combustion in situ through an outlet from the calender at the other end, and
collects a hot ethylene-rich steam cracking effluent that is sent
immediately in the cooling zone.

The pressure of the heating gases is generally from 4 to 20 bar (1 bar = 105 Pa) and their temperature from 600 to 1400 "C. This temperature is at least partly obtained by the combustion of an appropriate quantity of fuel thanks to the minus part of the oxygen-containing gases from the gas generator, in a post-combustion zone downstream of the latter. The heating gases as well as the combustion gases resulting from the in-situ combustion in the shell which have circulated through it are generally evacuated at a temperature of 580 to 12500C and under a pressure of 1 to 19 bar and are sent to a second heat exchanger in which the mixture of hydrocarbons and steam is preheated, preferably at counterflow of the gas flow direction. These heating gases which have passed through the second heat exchanger are usually recovered at a
temperature from 550 to 4500C and under a pressure from 3 to 19 bar and are sent to
at least one power recovery turbine.

The power recovery is, for example, from 50 to 60 MW for a unit of
ethylene production of 400,000 tonnes per year, involving for example two
gas generators of the type GE LM 5000.

Generally, for a unit of this size, the energy requirement does not exceed 20 to 30 MW and it can be provided at least in part by recovering the reaction heat (process heat) which has been transformed into high pressure steam . We are then faced with a problem of excess energy which is not always recoverable on the site of the unit and which harms the economy of the process.

The object of the present invention is to remedy this drawback and therefore to make the process more economical.

More generally, it relates to a process for thermal pyrolysis from a gaseous mixture of a hydrocarbon feed containing from 2 to 20 carbon atoms, comprising a heating phase in a heating zone under appropriate conditions which, delivers an effluent hot hydrocarbon, according to which heating gases containing air supplied by at least one gas generator are passed at an adequate temperature and pressure in at least one heat exchanger-reactor comprising tubes through which the gas mixture circulates and a calender where the heating gases circulate, an in situ combustion is optionally carried out, in at least part of said calender, of an appropriate quantity of gaseous fuel by at least part of said gases, said gases are sent together with any combustion gases in a second heat exchanger and then they are recovered at a temperature and at a pressure sufficient for the e send to at least one power recovery turbine, said method comprising recovering said effluent
More precisely, according to the present invention, a suitable volume of air is generally introduced into at least one compressor activated by at least one power recovery turbine, the air volume is compressed to an adequate pressure, the combustion of a determined quantity of preferably gaseous fuel is carried out under appropriate conditions by at least a portion of said volume of air in at least one external combustion zone separate from the gas generator supplying the reactor-heat exchanger and compressed heating effluents are obtained which are sent to the inlet of the shell of the heat-exchanger reactor.

Thus at least part of the energy available in the power recovery turbine is recovered to set in motion at least one axial air compressor. The compressed air, then reheated to high temperature, provides the additional heat necessary for the endothermic reaction, which makes it possible to minimize the power of the gas generator or to reduce the number of gas generators.

The volume of compressed air per unit of time in the power recovery turbine usually corresponds to 30 to 60% of the amount of air per unit of time circulating in the exchanger reactor and preferably from 45 to 55%.

According to a characteristic of the process, the volume of air is generally compressed to a pressure of 4 to 20 bar and preferably from 6 to 10 bar. The temperature of the compressed air before the external combustion of the gaseous or liquid fuel introduced may be from 200 to 500 "C and preferably from 350" C to 400 "C.

According to another characteristic of the process, the temperature of the heating gases and effluents is usually from 600 to 14000C and preferably from 1000 to 12000C.

 It may be advantageous to carry out, by suitable means, controlled combustion in situ in at least part of the calender of an appropriate quantity of gaseous fuel and possibly inert gas with at least part of the heating gases and effluents, the fuel being injected under conditions such that the temperature is kept substantially constant all along the tubes.

The present invention also relates to a device for carrying out an endothermic reaction such as a steam cracking reaction comprising a heating reactor, said heating reactor comprising at least one gas generator (1), at least one reactor- elongated pressure heat exchanger (11), with tubes (12) and calender (14) comprising at one of its ends a heating gas inlet connected to the gas generator and connected to said reactor calender- exchanger in which said gases circulate and heat said tubes where a hydrocarbon charge circulates and where the reaction takes place, said reactor-exchanger comprising at the other end an outlet for said gases, connected to at least one power recovery turbine (22 ), said device being characterized in that it comprises at least one air compressor (51) actuated by said power recovery turbine (22), means supplying air (53) to said compressor, first means (54) for transferring compressed air to combustion means (55) defined below, means (55) for combustion of a fuel by l compressed air, external and separate from the gas generator comprising a fuel supply (5c) and adapted to deliver compressed heating effluents, and second means (56) for transferring the compressed heating effluents to said inlet of the calender ( 14) of the reactor-heat exchanger.

According to a preferred embodiment, a second heat exchanger can be interposed between the outlet of the heating gases from the shell of the reactor-heat exchanger and the inlet of the power recovery turbine. This heat exchanger is generally suitable for preheating the charge which must circulate in the reaction tubes of the reactor-exchanger.

 It is understood that the reaction tubes could be filled with at least one catalyst, zeolitic for example, in which a hydrocarbon feedstock possibly comprising hydrogen, for example, would circulate.

The invention will be better understood in view of the attached figure illustrating an embodiment applicable to the production of ethylene and propylene by steam pyrolysis of naphtha water, mainly highlighting the implementation of heating and mechanical energy recovery. A gas generator 1, which can be a jet engine (Jet Engine) comprises an axial air compressor supplied by a line 3 which compresses it to about 10 to 20 bar. The air temperature increases by approximately 300 to 450 Oc by compression. The compressed air passes through a combustion chamber which is an integral part of the jet engine and which is supplied with fuel by lines 5 and 5a. This fuel can be from methane to fuel oil. An exothermic combustion is initiated and takes place at constant pressure to increase the air temperature to around 1000 to 12000C. The very hot air and combustion products are then extended to a high pressure power recovery turbine (not shown) which drives the air compressor.

The air and the products of combustion leave the gas generator 1 at a pressure of approximately 5 to 8 bar and at a temperature of approximately 700 to 800 "C and are sent to the outlet of the turbine, to a post-combustion not shown in the figure in which fuel is introduced via line Sa. The temperature of the combustion fumes and of the air (heating gas) which may still contain from 15 to 19% by volume of oxygen can These gases are used as a heat source in a reactor-heat exchanger below and their temperature can be controlled by appropriate means controlling a fuel supply valve connected to the post-combustion.

This heat exchanger-reactor has been described in the French patent application of the Applicant under the number 91 / 04.687.

 I1 are sent by a line 10 to the lower end of a plurality of reactors-heat exchanger 1 1 vertical and elongated. Each tube and shell type reactor-exchanger comprises an enclosure coated with refractory material containing a shell 14 made of non-alloy steel for example, adapted to withstand a pressure of approximately 20 bar. This grille includes internal elements, not shown, adapted to achieve a gas baffle circulation and promoting turbulence and the mixing of these gases, for example internal elements alternately comprising a central solid disc and a pierced disc (disc and donut; GA Skrotzki, Power, June 1954).

The reactor-exchanger also contains a plurality of reaction tubes 12, about a thousand of 20 mm in diameter, made of Incoloy type steel, with high nickel content, substantially parallel to each other and substantially parallel to the axis of the reactor-exchanger of heat. These tubes are adapted to receive, thanks to a parallel supply, a mixture preheated to 580-6500C under 1.5 to 3 bar, of steam and hydrocarbons, a naphtha cut for example, by a line leading to the lower end of the reactor 11, so that the gaseous hydrocarbon mixture flows from bottom to top in the reactor-exchanger under conditions such that its residence time is limited to approximately 100 to 300 ms.

The tubes 12 are heated essentially by convection substantially at the temperature of the heating gases which circulate in the calender 14 connected to the line 10 via, for example a nozzle not shown in the figure at the base of the calender. Thus, the oxygen-rich heating gases (10 to 19%) circulate in the same flow direction as that of the gaseous mixture in the tubes, which promotes a greater heat supply at the very beginning of the reaction.

The reactor-exchanger comprises at one of its ends, on the discharge side of the gaseous effluent for example, a chamber for mixing the gaseous fuel and dilution water vapor, substantially cylindrical, impervious to the heating gases and to the mixture gaseous constituting the charge. It is delimited, on the end side of the reactor-exchanger, by a floating circular wall which supports the reaction tubes passing through this chamber and on the shell side by a floating circular wall supporting injection tubes (15) which open into said chamber.

This is connected to introduction means 5b, gaseous fuel (methane for example) and water vapor. It is suitable for preheating the fuel and water vapor and cooling the walls of the chamber. It also contributes to the mixture of fuel and water vapor which is all the better achieved when the fuel and water vapor arrivals in the cylindrical chamber are tangential and diametrically opposite.

The mixing chamber is for example located at the end on the discharge side of the gaseous effluent. According to this particularly advantageous embodiment, it allows a pre-tempering of the effluent from 5 to 500C for example, in the reaction tubes cooled by the mixture, over a length representing from 1/50 to 1/10 of their length. and advantageously from 1/25 to 1/15 of their length.

In order to keep the temperature substantially constant around 1000 "C throughout the reaction, a plurality of fuel injection tubes 15 are installed, in incoloy (about 5% of the number of reaction tubes), the upper end of which opens out. in the mixing chamber and the other lower end of which is closed, these tubes being adapted to achieve controlled combustion in the calender. These tubes are substantially parallel to the reaction tubes and are arranged so as to be substantially in the center of a circle. formed by the neighboring reaction tubes. These gaseous fuel injection tubes connected to a gaseous fuel supply are pierced at their periphery with calibrated orifices arranged in circles judiciously distributed along these tubes, for example every meter, so that the local temperature increase resulting from the combustion of the fuel and part of the heating is generally about 20 to 1000C and advantageously 40 to 600C. These holes are usually located between the discs constituting the internal elements. Taking into account the pressure drop existing in the grille, the preferably circular orifices have smaller and smaller opening surfaces, in the direction of the flow of gases in the grille so as to be able to introduce substantially the same flow rate at each injection level.

To control the thermal level of the reaction, to possibly avoid hot spots and to minimize the risk of cracking of the fuel, in particular hydrocarbons other than methane, water vapor can be added to the fuel supply. makes it possible to dilute the mixture, generally in a low pressure vapor to fuel ratio of for example between 0.2 and 20 and advantageously between 0.8 and 5 by weight. Finally, the very high temperature difference and therefore the possibility of carrying out a very large rapid thermal transfer combined with local additions of thermal energy contribute to inducing a substantially square temperature profile throughout the reaction, which promotes a better ethylene yield.

The above heating gases and combustion gases still containing 10 to 12% by volume of oxygen are collected and evacuated at the top of the reactor heat exchanger at approximately 900-10,500C and at approximately 5 to 7 bar via a line 17 and directed towards the upper end of a conventional gas-gas heat exchanger 18. This is suitable for preheating against the current by convection in appropriate tubes the gaseous reaction mixture of hydrocarbons and steam at 580-650 "C. The mixture, once preheated, is sent to the lower end of the reactor-heat exchanger.

The heating gases circulating in the shell of this preheater exchanger 18 leave the latter at a temperature of 700-9500C and approximately 4 to 6 bar. They have, under these conditions, as much potential mechanical energy as they would have had in a gas turbine configuration, equivalent, that is to say at the output of the power recovery turbine coupled to the generator. gas 1. The heating gases are therefore directed through a discharge line 20 to a guard container 21 (known as KO
Drum) then are expanded in a power recovery turbine 22 to extract the mechanical energy which will be used for example to compress either the gaseous effluent or, as will be seen below, air. expanded gases, substantially at atmospheric pressure and at a temperature of 400-5500C are brought by a line 23 to the inlet of a heat recovery chamber 30 allowing to carry out by convection a first preheating of the hydrocarbon feedstock and water vapor around 350 to 4500 C.

The hydrocarbon gaseous effluent at the outlet of the reaction tubes 12 of the exchanger reactor It is collected at a temperature of 800 to 900 "C by a suitable internal collector and provided with an expansion joint. A very short transfer line is connected to the upper end of the reactor-exchanger and leads the effluent from the collector to a vertical member, not shown, of conventional type, adapted to achieve rapid cooling (quench) by a counter-current circulation of demineralized water and which generates steam at very high pressure, around 100 bar.

The cooled effluent is then collected and led to downstream separation units, not shown in the figure.

In the figure illustrating the invention there is shown a gas generator 1 delivering pressurized heating gases to four reactor-exchangers 1 1 arranged in parallel.

Four preheating exchangers 18 of the load receive the heating gases having passed through the reactor-exchangers and a power recovery turbine 22 recovers the energy of these gases released by each pair of reactor-exchangers, ie two turbines. A first turbine 22a is connected to a compressor 51a of the gas charge for example while the second turbine 22 is connected to an axial air compressor 51 by a transmission shaft 52 rotated by said turbine. This cold air is pumped through a line 53 connected to the compressor and comes out of it by a line 54 under a pressure of 4 to 20 bar, typically 7 to 8 bar and a temperature of 350 to 4000C. n then enters a combustion zone 55, external and distinct from the gas generator 1, where it is brought into contact with fuel, gaseous for example, brought by a line 5c. The fuel flow rate is adjusted by regulating means 60 controlling a control valve in connection with the temperature of the heating effluents downstream of the combustion zone 55. The resulting combustion can reach a temperature of the compressed combustion effluents from 1000 to 12000C under a substantially identical pressure. These combustion effluents are sent by a power supply line 56 in parallel to the inlet of the calender 14 of the heat exchanger-reactors thus providing the additional energy necessary for their implementation.

Under these conditions, compared with those of the applicant's patent applications requiring two gas generators, a single gas generator of the GE LM 5000 type is necessary for the operation of a unit of 400,000 tonnes per year.

One of the two power recovery turbines of around 40 MW is used to pump and compress the air 450 T / h at 7 to 8 bar, which is added to that (450 T / h) which is delivered by said gas generator.

The other power turbine of around 10 MW is used to compress the feed gases.

The following example makes it possible to determine the thermal balance of a steam cracking unit suitable for treating 400,000 tonnes per year in 4 reactors-heat exchangers.

Amount of energy consumed: GJ / h
- Primary and post combustion in the generator
GE LM 5000 gas heating to 1 1500C and including 450 T / h
of air in the presence of 13 T / h of fuel 577
- In-situ combustion in each reactor-exchanger of
3.5 T / h of fuel i.e. 14 T / h 515
- External combustion of 450 T / h of compressed air
and 8.3 T / h of fuel 367
TOTAL 1,460 o Energy recovered
- Heat of reaction 686
- Preheating of the load 118
- waste heat recovery 334
Sub Total 1,138
- Power recovery in turbine 50
TOTAL 1,188, representing a thermal efficiency of 81.5%.

Claims (10)

1. Method of thermal pyrolysis from a gaseous mixture of a feed  hydrocarbon containing from 2 to 20 carbon atoms, comprising a phase  heating in a heating zone under suitable conditions, which  delivers a hot hydrocarbon effluent by which gases are passed to  an adequate temperature and pressure in at least one reactor  heat exchanger comprising tubes through which the gas mixture circulates and a  calender where the heating gases circulate, an optional  in-situ combustion, in at least part of said calender, of a quantity  suitable gas fuel by at least a portion of said gases,  sends said gases as well as any combustion gases in a second  heat exchanger then they are recovered at a temperature and at a pressure  sufficient to send them to at least one recovery turbine  power, said process comprising the recovery of the hydrocarbon effluent,  and being characterized in that:  - an appropriate volume of air is introduced into at least one compressor  in action by at least said power recovery turbine,  - said air volume is compressed to a determined pressure,  - one carries out, under appropriate conditions, the combustion of a quantity  adequate fuel from at least part of said volume of air  compressed, in at least one external combustion zone separate from  that of the gas generator and we obtain heating effluents  tablets which are sent to the shell of said reactor heat exchanger  heat. 2. The method of claim 1 wherein the volume of air is compressed to  a pressure of 4 to 20 bar and preferably 6 to 10 bar. 3. Method according to one of claims 1 to 2 wherein the air temperature  compressed before said external combustion is 200 to 500 "C and preferably  from 350 to 400 "C. 4. Method according to one of claims 1 to 3 wherein the temperature of  compressed heating effluents is 600 to 14000C and preferably 1000  at 12000C. 5. Method according to one of claims 1 to 4 wherein one performs a  controlled combustion in situ in at least part of the radiator grille  reactor-heat exchanger, of an appropriate quantity of fuel  gaseous and possibly inert gas by at least part of said gases, the  fuel being injected under conditions such that the temperature is  maintained substantially constant along said tubes. 6. Method according to one of claims 1 to 5 wherein the filler contains  water vapor. 7. Device for carrying out an endothermic reaction such as a  steam cracking reaction comprising a heating reactor, said reactor  heating comprising at least one gas generator (1), at least one reactor  elongated pressure heat exchanger (11), with tubes (12) and  grille (14) comprising at one of its ends a gas inlet for  heating connected to the gas generator and connected to said reactor shell  exchanger in which said gases circulate and heat said tubes where  a hydrocarbon charge circulates and where the reaction takes place, said reactor  exchanger comprising at the other end an outlet for said gases, connected to the  at least one power recovery turbine (22), said device being  characterized in that it comprises at least one air compressor (51) placed in  action by said power recovery turbine (22), a means  supply air (53) to said compressor, first transfer means  (54) from compressed air to combustion means (55) defined above  below, means of combustion (55) of a fuel by compressed air  external and separate from the gas generator comprising a supply (5c) of  combustible and suitable for delivering compressed heating effluents, and  second means of transfer (56) of the compressed heating effluents to  said inlet of the shell (14) of the reactor-heat exchanger. 8. Device according to claim 7 wherein the recovery turbine  power (22) comprises a drive shaft (52) setting in motion  said compressor (51). 9. Device according to one of claims 7 to 8 wherein the reactor  heat exchanger comprises at least one combustion means (15)  checked in at least part of the grille, a fuel and a  part of said gas, adapted to maintain the temperature of the tubes (12)  substantially constant, said controlled combustion means being connected to a  fuel supply means. 10. Device according to one of claims 7 to 9 wherein a second  heat exchanger (18) is interposed between the shell and the turbine  power recovery, said heat exchanger being adapted to preheat  load.