GB2231057A

GB2231057A - Process and apparatus for steam cracking hydrocarbons

Info

Publication number: GB2231057A
Application number: GB9000639A
Authority: GB
Inventors: Alain Feugier; Gerard Martin; Gilbert Froment; Koenraad Herrebout
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1989-01-12
Filing date: 1990-01-11
Publication date: 1990-11-07
Anticipated expiration: 2010-01-11
Also published as: FR2641543B1; GB9000639D0; FR2641543A1; DE4000675C2; DE4000675A1; GB2231057B

Description

:2:2;3 -1 (z)!i -:,- PROCESS AND APPARATUS FOR STEAM CRACKING HYDROCARBONS

The invention relates to an improved process for the steam cracking of hydrocarbons for producing olefins and in particular ethylene and propylene.

Steam cracking has been used since 1920 for producing ethylene from ethane and has rapidly become a basic petrochemical process using ever heavier charges and extending to the vacuum treatment of gas-oils.

Its principle is based on the high temperature instability of paraffins and naphthenes compared with that of olefins and arom- atics. The main reactions are the breaking of a C-C bond by a homolytic breaking mechanism in order to bring about an olefin and a paraffin, as well as dehydrogenation. These two reactions are endothermic and consequently aided by a temperature rise. They also lead to an increase in the number of molecules, so that they are aided by low partial pressures of the hydrocarbons to be treated. Therefore said pressure is reduced to the greatest possible extent by adding water vapour to the reaction medium. The prior art is illustrated by Patents such as EP-A- 0074435, FR-A-2249942 and DE-B-1197187.

However, it was soon noted that the maintaining of a hydrocarbon charge in diameter 60 to 120 mm and length 40 to 100 m tubes heated radiatively by conventional furnaces to a temperature exceeding 8000C for approximately 0.4 to 1 second rapidly led to the formation of coke deposits, which are prejudicial in 2 - a number of different respects. There is a reduction in the heat transfer between the reactor and the charge to be cracked, there is a significant rise to the reactor skin temperature, a reduction to the useful diameter of the reactor causing an increase in the pressure drop within the reactor, which leads to the stoppage of the unit in order to carry out a decoking operation.

Coke formation is due to secondary reactions such as the forma tion of condensed polycyclic aromatic hydrocarbons, as well as to the polymerization of the olefins formed.

The latter reaction results from the tendency of olefins to polymerize when the temperature exceeds 5000C. Thus, in order to reduce the significance of this secondary reaction, it is necessary to carry out a fast cooling (often called quenching) of the reaction effluents, so as to rapidly bring them from the temperature at which pyrolysis is carried out to a temperature below 500 0 C, generally by means of an indirect heat exchanger.

The presence of this deposit makes it necessary to periodically carry out a decoking operation consisting of oxidizing the depo- sited coke by a mixture of vapour and air. This operation leads to tube fatigue and to a usage time loss with respect to the apparatus.

The second limitation is linked with the geometry and nature of the tubes used in conventional furnaces. Kinetic studies of steam cracking show that the ethylene yields increase on increasing the temperature at which the reaction is performed and on reducing the residence times. However, with conventional means, it is not possible to satisfy the kinetic requirements, because the heat transfers are limited. In order to overcome this problem, a number of solutions have.been proposed. US Patent 4 160 701 proposes using upwardly branched tubes or split coils, in order to locally increase the charge heating rate. However, the charge residence times remain long.

The third limitation to existing furnaces results from the difficulties encountered in ensuring a good distribution of the energy fluxes on the tubes. Transfer takes place radiatively is between the furnace walls and the vertically suspended tubes. Heating of the walls takes place either by a plurality of burners distributed over the furnace side walls, or by burners placed on the furnace hearth on either side of the tubes. The flux distribution quality is an important element in the operation of an apparatus, which still functions close to the metallurgical limits of the tubes. Any local temperature rise to the tube can lead to a rapid deterioration thereof. These local temperature rises also lead to increased, punctiform coke deposits, which are prejudicial to the satisfactory operation of the 23 apparatus.

A first object of the invention is to deal with the problems and disadvantages referred to hereinbefore.

The present invention therefore relates to a process for the steam cracking of a hydrocarbon or a mixture of hydrocarbons, incorporating at least two carbon atoms, leading to improved ethylene yields compared with existing processes. In more detailed manner, a mixture of said hydrocarbon with steam is made to flow in a tubular reaction zone heated in a pyrolysis zone having a first section in which a fuel is burnt with a gas cont- aining oxygen, combustion supplying smoke gases which are at least partly passed into a second section of the pyrolysis or convection zone, the effluents of the reaction then being passed into a quenching zone and the steam cracking effluents are recovered.

is More specifically, the reaction zone is heated in most or all of the convection zone in which the smoke gases are accelerated under conditions that the interstitial velocity of the gases along said convection zone is between 20 and 300 m/s.

The reactor in the convection heating zone is advantageously constituted by a plurality of tubes substantially parallel to the convection zone, having a length generally between 2 and 15 m and preferably between 5 and 10 m, as well as an internal diameter generally between 5 and 30 mm, preferably between 10 and 20 mm.

1 - r. - The use of smoke or furnace gases at a temperature between 1500 and 25000C flowing at the speed mentioned hereinbefore between tubes grouped into a bundle and placed in a refractory material sleeve constituting the convective zone, makes it possible to carry out 70% at least of the heat exchange between the hot smoke gases and the reaction tubes and advantageously at least 85% of the exchange, the remainder of the exchange taking place radiatively mainly between the wall of the sleeve and the tubes. This particular configuration makes it possible to homogeneously heat a very large number of tubes and even on the tube scale ensures a uniform heating of the entire tube periphery. It also avoids the appearance of hot points resulting from the non-uniformity of heating by radiation and which generate large local coke deposits sometimes requiring a premature stoppage is of the installation. The bringing about of an almost perfect distribution of the heat supply makes it possible to move even closer to the metallurgical limits of the tubes and to increase the heat fluxes, whereas in the case of conventional installations it has always been necessary to work a little below these limits, due to the possible existence of hot points and because it was necessary to take account of a relatively significant coke thickness at the end of the cycle.

This grouping of the pyrolysis tubes into bundles leads to a reduction in the dimensions of the pyrolysis chamber, which reduces costs. It also leads to a reduction in the pressure drop, between the inlet and outlet of the pyrolysis tubes, which can be reduced by a -factor of 2 to 10 compared with conventional installations.

The use of small diameter tubes makes it possible to take advantage of a high surface to volume ratio of the unitary tube and thus to limit the charge residence times to between 20 and 150 ms and preferably to between 50 and 100 ms. Under these conditions, there is a limitation to the secondary reactions responsible for the yield reductions observed on conventional units. It is also possible to accept higher outlet temperatures, because the secondary reactions no longer have time to develop in a significant manner.

The tubes can easily operate at temperatures above 10000C by simply heating by hot combustion gases without any risk of local overheating. The decoking operation can be carried out solely by steam. Under these conditions, unlike in the prior art, there is no need to reduce the temperature of the furnace to around 6000C and carry out decoking using air-steam mixtures with the resulting risks of hot points appearing. Moreover, no matter whether in cracking or decoking, the tubes remain substantially at the same temperature, which reduces mechanical stresses and ensures that the tubes will have a longer life and will be more reliable.

According to anoLiter feature of the process of the invention, it is possible to maintain the velocity of the smoke gases substantially constant along the convection zone, by producing short, cylindrical or polygonal convection chambers containing reduced length tubes and e.g. less than 10 m.

As has been stated hereinbefore, the smoke gases are accelerated on passing through a restriction or constriction constituted by the gases passing between the inner wall of the convection chamber and the outer wall of the tubular reactor or the plurality of tubes constituting the latter. This passage can be such that the ratio of the surface of the cross-section of the convec- tive pyrolysis zone defined with respect to its axis to the surface of the cross-section of the tubular reaction zui,c is generally between 1 and 15. When this ratio is advantageously between 2.5 and 5, the smoke gases can reach preferred interstitial velocities between 100 and 25Orn/S ' which makes it poss- ible to achieve an excellent steam cracking reaction yield.

According to another feature of the inventive process, it is possible to increase the cracking reaction temperature for a short time either by carrying out at least one injection of hot smoke gases at a temperature between 1500 and 2500 0 C at at least one point of the convection chamber located at a distance from the inlet to said chamber representing at least 50% and e.g. 50 to 75% of its length, or by redistributing at the aforementioned point the gaseous mixture circulating in the tubular reactor in a larger number of small diameter tubes, 8 - or by combining hot smoke gas injection and gaseous mixture distribution as described hereinbefore, it being obvious that the total energy quantity supplied to the reactor for a given charge is substantially the same. Under these conditions, the heat exchange intensity is increased. - The tubular reactor is generally constituted by metal tubes, but that part of the bundle where the charge is at the highest temperature the tubes can be non-metallic and are instead made from refractory materials. This leads to an elimination of the constraint on the tube skin temperature and to an elimination of catalytic surfaces which favour coke deposition, as is the case with nickel contained in metal alloy tubes.

The steam proportion mixed with the charge is a function of the latter. The mass ratio of the steam flow rate to the charge flow rate is generally between 0.1 and 2 and advantageously between 0.2 and 1. The charges which can be cracked in the presence of steam can be ethane, propane, naphtha, atmospheric gas-oil, vacuum gas-oil, taken singly or in mixed form.

The invention also relates to the steam cracking apparatus for mainly performing the process. It comprises a pyrolysis chamber 1 thermally insulated.. fuel supply means 4 and comb- ustion gas supply means 5 connected to the chamber, at least one tubular reactor 6 maintained within the chamber 1 and having an inlet 13 and an outlet 14, hydrocarbon supply means 9 and steam supply means 11 connected to the tubular reactor inlet 13 and means 18 suitable for carrying out a quenching of the pyrolysis effluents connected to the tubular reactor outlet 14.

More specifically, the pyrolysis chamber comprises a first part or combustion chamber 2 containing at least one pressurized burner able to carry out a combustion of.a fuel and fuel gas and generate smoke gases and a second part comprising at least one convection chamber 8 linked with the first part and which is elongated and incorporating most or all of the tubular reactor 6, said convection chamber 8 having accelerating means suitable for accelerating the smoke gases along the tubular reactor to a velocity between 20 and 300 m/s.

The distance between the axes of the different tubes is generally between 1.2 and 4 times the external diameter of the tubes and is advantageously between 1.4 and 1.8 times said diameter and the distance between the inner wall of the convection chamber and the outer wall of the tubular reactor or envelope representing the tube group is generally between 0 and 2 times the external diameter and is preferably 0.3 to 1 time said diameter.

The invention is described in greater detail hereinafter relative to a non-limitative embodiment and with reference to the attached drawings, wherein show:

Fig. 1 an axial section through the reactor.

Fig. 2 a variant of the apparatus showing the presence of a second burner and a second chamber for the injection of smoke gases into the convection chamber, so as increase the temperature of the reaction zone.

According to fig. 1 the steam cracking chamber 1 surrounded by a refractory material envelope 16 comprises a combustion chamber 2 in which are arranged two pressurized burners 3 on the periphery of said chamber 2 and suitable for burning a fuel supplied by a line 4 in the presence of air supplied by a line 5. The flame of said burners releases thermal energies stored in the smoke gases, which heat to a temperature between 1500 and 2500 0 C a reduced length portion of a tubular reactor 6 constituted by a plurality of metal tubes 7 substantially parallel to the axis of the furnace. These tubes e.g. have an internal diameter of 20 mm, a length of 8 m and a distance between the tube axes of approximately 1.45 times the external diameter of the tube.

The tubes are supplied at their inlet by hydrocarbon charge supply means 9 and by steam supply means 11. The charge can be constituted by a petroleum fraction, e.g. a naphtha fraction. The steam weight proportion mixed with the hydrocarbon fraction represents approximately 29% of the total mixture.

1 At chamber 2, the tubes can be covered with a refractory material or can be surrounded by a sleeve 15 also made from a refractory material, so that the peripheral tubes of the bundle exposed to the radiation of the surrounding chamber do not receive more heat than the central tubes.

The smoke gases resulting from the combustion leave the combus tion chamber 2 and are accelerated in a second part of the steam cracking chamber 1 continuing onto the combustion chamber and subsequently referred to as the convection chamber 8. The sub stantially cylindrical, vertical chamber 8, which is substant ially parallel to the axis of symmetry of chamber 2 has a narrow passage section for the tubes all along its length in which the smoke gases f low. The distance between the envelope const ituted by the contour of the tube bundle and the inner wall is of the convection chamber approximately corresponds to e.g.

a value between 0.3 and 1 time the external diameter of the unitary tube 7. In other words, the ratio of the surface of the cross-section of the convection chamber to the surface of the cross-section of the tubes is advantageously between 2.5 and 5.

Under these conditions, the smoke gases circulate in the convection chamber at a velocity advantageously between 100 and 250 m/s and preferably between 150 and 200 m/s.

The convection chamber 8 receives most of the tubes in which the reaction mixture flows. The smoke gases are accelerated at the inlet 8a of said convection chamber and this starts off the heat transfer necessary for the steam cracking reaction, which takes place in these tubes under very short residence time and optimum temperature conditions.

At the outlet 8b of the convection chamber, an expansion chamber 17 recovers the smoke gases at a temperature between 800 and 1500 0 C and preferably between 950 and 11000C. The velocity of the smoke gases is brought to a value between 5 and 30 m/s in said expansion chamber. The smoke gases are discharged through an opening 10 with a view to preheating the charge and e.g. generating vapour.

is The outlet 14 of the tubes 7 of the bundle is extended into the indirect quenching enclosure 18 insulated from the convection chamber 8. Enclosure 18 is supplied with water by inlet and outlet pipes 12a, 12b and cools the cracking effluents. The energy withdrawn from the effluents is used for generating steam.

In the presence of a charge containing a gas-oil, it is possible to advantageously carry out a direct quenching in enclosure 18. Under these conditions,' the effluents are recovered in said enclosure, or a fuel oil recycling is introduced for cooling them. The resulting mixture is then fractionated and the effluents collected.

A According to another embodiment of the apparatus according to the invention illustrated in fig. 2 and which has the same references as in fig. 1 for the same means in order to raise the reaction temperature for a short time just prior to the cooling in the quenching enclosure 18, hotter smoke gases can be supp lied via a second combustion chamber 19 equipped with pressurized burners 3a located at a distance from the inlet 8a of convection chamber 8 equal to approximately 0.7 times the length of the reaction tubes in said chamber 8. The smoke gases released can be accelerated in the same way as in the first case illustrated by fig. 1 by a forced passage into the upper reduced crosssection part of the convection chamber 8.

The reaction temperature can also be increased by a distribution of effluents or reaction mixture into a larger number of small diameter tubes 20 and optionally made from refractory material. To this end, a distributor 21 is located in the immediate vicinity and preferabiy upstream of the second combustion chamber 19 and redistributes the cracking effluents and the reaction mixture which has not reacted.

Fig. 1 only shows a single convection chamber 8. However, it is obvious that the apparatus can have several convection chambers containing reaction tube bundles and in which the smoke gases are accelerated and circulate at velocities defined according to the process.

Fig. 1 also shows the reaction zone 6 heated to a minor extent in the combustion chamber 2. However, the reaction zone can be entirely in the convection chamber, where the smoke gases will be deflected and then accelerated by passage into the const- ricted sections of said chamber in accordance with the inventive process.

The following example illustrates the scope of the invention in a nonlimitative manner.

Example 1 A according to the invention.

A naphtha charge with a boiling point of 35 to 1750C, a density at 200C of 0.68 and the weight composition given in Table I is cracked in the presence of steam in accordance with a steam to charge mass ratio of 0.5 in a tubular reactor having 37 8.5m long and 20mm internal diameter tubes. The distance between the tube axes is 1.48 times the external diameter of the tube.

Table I

Paraffins Isoparaffins Aromatics Naphthenes Olefins 45% 34% 5% 15.5% 0.5% - is - The smoke gases resulting from the combustion of a mixture mainly consisting of methane, which is the by-product of the steam cracking reaction are at a temperature of 2050 0 C and are accelerated and kept at a velocity of 190 m/s in a cylindrical convec- tion chamber. The convection ratio of the cross-sectional surface of the convection chamber to the cross-sectional surface of the tubular reactor is equal to 2.91.

The discharge temperature from the convection chamber is approximately 945 0 C. The mixture spends 75 ms in the reaction zone.

The smoke gases are heated in the convection chamber at a point 5 m from the chamber inlet by a complementary supply of smoke gases at 1900 0 C resulting from the combustion of the same fuel.

The energy supply at this level represents 25% of the total supply resulting from the combustion of the same fuel as that used initially 90 % of the total heat exchange is carried out between the hot smoke gases and the reaction tubes.

The steam cracking effluents are cooled by indirect water quenching.

At the outlet, effluents have the following weight composition (Table Il):

Table II

H 2 CH c 2H4 c 3 H 6 C4 H 6 other C 4 petrol Fuel 0.8% 14.1% 34.2% 14.8% 8.0% 4.5% 21.3% 2.3% Example IB according to the prior art. The charge of example 1A is steam cracked in a tubular reactor 45m long and with an internal diameter of 65 mm, radiatively heated in a steam cracking furnace. The steam to charge ratio is 0.6. The charge temperature on leaving the furnace is 855 0 c is and the charge spends 350 ms in the furnace.

The effluents are cooled by indirect water quenching, as in example 1A and recovered. They have the following weight composition (Table III):

Table III

H CH 4 c 2 H4 c 3 H 6 c 4 H 6 other C 4 petrol Fuel 0.9% 15.2% 28.5% 17.5% 4.0% 7.2% 22.6% 4.1% These comparative examples show that the inventive process makes it possible to obtain more ethylene, less methane and less fuel.

Claims

CLAIMS is Process for the steam cracking of hydrocarbons with at least two

carbon atoms, in which a mixture of said hydrocarbon with steam is made to circulate in a tubular reaction zone heated in a pyrolysis zone incorporating a first section in which combustion takes place of a fuel with a gas containing oxygen, the combustion supplying smoke gases which are at least partly passed into a second section of the pyrolysis or convection zone, wherein the effluents of the reaction are passed into a quenching zone, after which the sLeam cracking effluents are recovered, characterized in that said reaction zone is heated in most or all of the convection zone in which the smoke gases are accelerated under conditions such that the interstitial velocity of the said gases along said convection zone is between 20 and 300 m/s.
2. Process according to claim.-. 1, wherein the velocity of the smoke gases is maintained substantially constant along the convection zone.
3. Process according to claims 1 or 2, wherein the ratio of the cross-sectional surface of the convection zone to the cross-sectional surface of the reaction zone is between 1 and 15.

Z - 19
4. Process according to one of the claims 1 to 3, wherein the residence time of the mixture in the convection zone is between 20 and 150 ms.

is
5. Process according to one of the claims 1 to 4, wherein the smoke gases and the mixture circulate in co-current manner in the convection zone.
6. Process according to one of the claims 1 to 5, wherein the temperature of the tubular reaction zone is increased for a short time by: a) either a stage of introducing smoke gases at a temperature between 1500 and 2500 0 C at at least one point of the convection zone located at a distance from the start of said zone representing 50 to 75% of the convection zone length, b) or a stage of redistributing at this point gaseous mixture into a tubular reaction zone having a larger number of small diameter tubes.
c) or by a combination of stages a) and b).

Apparatus for performing the process according to one of the claims 1 to 6 comprising a pyrolysis chamber (1) therm ally insulated, fuel supply means (4) and comb ustion gas supply means (5) connected to the chamber, at least one tubular reactor (6) maintained within the chamber (1) having an inlet (13) and an outlet (14), hydrocarbon supply means (9) and steam supply means (11) connected to the tubular reactor inlet (13) and means (14) for quenching the pyrolysis effluents connected to the tubular reactor outlet (14), characterized in that the pyrolysis chamber (1) comprises a first part or combustion chamber (2) containing at least one pressurized burner suitable for carrying out the combustion of a fuel and combustion gas and for generating smoke gases and a second part comprisin&t least one convection chamber (8) continuing onto the first part, having an elongated shape and incorporating most or all of the tubular reactors (6), said convection chamber (8) having accelerating means able to accelerate the smoke gases along the tubular reactor to a velocity between 20 and 300 m/s.
8. Apparatus according to claim 7, wherein the convection chamber has an axis of symmetry and wherein the tubular reactor is constituted by a plurality of tubes (7) substantially parallel to the axis, having a length between 2 and 15 m and an internal diameter between 5 and 30 mm.
9. Apparatus according to one of the claims 7 and 8, wherein the convection chamber is tubular and wherein the ratio of the crosssectional surface S of the convection chamber A with respect to the axis to the cross-sectional surface T of the tubular reactor with respect to said axis is between 1 and 15.
10. Apparatus according to one of the claims 7 to 9, wherein the convection chamber is cylindrical or polygonal.
11. Apparatus according to one of the claims 7 to 10, wherein the distance between the tubes or axes is between 1.2 and 4.0 times the external diameter of the tubes.
12. Apparatus according to one of the claims 7 to 11, wherein the distance between the inner wall of the convection cham- ber and the outer wall of the tubular reactor or the envelope of said tubes is between 0 and 2 times the external diameter of the tubes.
13. Apparatus according to one of the claims 1 to 12, wherein the convection chamber (8) has at least one smoke gas injection means (19) located at a distance from the inlet of said chamber representing 50 to 75% of the length of said chamber.

22
14. Apparatus according to Claim 7, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.
15. A process according to Claim 1, substantially as hereinbefore described in Example 1A.
16. A process according to Claim 1, carried out in apparatus according to any one of Claims 7 to 14.
17. Olefins obtained by a process according to any one of Claims 1 to 6, 15 and 16.

Pliblished 1990 asThe Patent Office, State House.6671 1Lgh Holborn, London WC1R 4TP.Further copies maybe obtainedfrom The PatentMce. Sales Br. inch. St Mary Cray, Orpington, Kent BR5 3RD. Printed by MiUtiplex techmques ltd. St Mary Cray. Kent. Con, 1187