US3872179A

US3872179A - Process for catalytic steam cracking

Info

Publication number: US3872179A
Application number: US382429A
Authority: US
Inventors: Kjeld Jorn Andersen; Finn Fischer; Jens Rostrup-Nielsen; Johannes Wrisberg
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 1972-08-14
Filing date: 1973-07-25
Publication date: 1975-03-18
Anticipated expiration: 1992-03-18
Also published as: DE2340904A1; GB1397315A

Abstract

The process for the manufacture of unsaturated hydrocarbons, especially ethylene and propylene, by cracking in the gas phase predominantly saturated hydrocarbons in the presence of steam over a zirconia catalyst containing an alkali metal compound is improved, especially with respect to uninterrupted operation, catalyst life and avoidance of carbon deposition, by adding an alkali metal compound to the preheated vaporized hydrocarbon feedstock prior to contacting it with the catalyst, the addition being continuous, periodical or at irregular intervals.

Description

United States Patent 1191 Andersen et al.

[ PROCESS FOR CATALYTIC STEAM CRACKING [75] Inventors: Kjeld Jorn Andersen, Tulstrup; Finn Fischer, Holte; Jens Rostrup-Nielsen, Virum; Johannes Wrisberg, Holte, all of Denmark [73] Assignees: Haldor Topsoe A/S, Soberg,

Denmark; Fluor Corporation, Los Angeles, Calif.

22 Filed: July 25,1973

211 Appl. No.: 382,429

[30] Foreign Application Priority Data Aug. 14, I972 United Kingdom 37858/72 [52] US. Cl 260/683 R, 208/121, 260/683.3 [51] Int. Cl. C07c 3/34 [58] Field of Search 260/683 R, 683.3; 208/121 [56] References Cited UNITED STATES PATENTS 2,518,354 8/1950 Meinert et al. 260/683 1451 Mar. 18, 1975 Steinhofer et al. 260/683 Bajars 260/6833 [57] ABSTRACT The process for the manufacture of unsaturated hydrocarbons, especially ethylene and propylene. by cracking in the gas phase predominantly saturated hydrocarbons in the presence of steam over a zirconia catalyst containing an alkali metal compound is improved, especially with respect to uninterrupted operation, catalyst life and avoidance of carbon deposition, by adding an alkali metal compound to the preheated vaporized hydrocarbon feedstock prior to contacting it with the catalyst, the addition being continuous, periodical or at irregular intervals.

6 Claims, No Drawings PROCESS FOR CATALYTIC STEAM CRACKING This invention relates to improvements in a process for the production of unsaturated hydrocarbons, particularly ethylene and propylene, by catalytic steam cracking of predominantly saturated hydrocarbons.

Ethylene and propylene are commonly manufactured by thermal or catalytic steam cracking of liquid hydrocarbon fractions such as naphtha, kerosene, and gas oils. A cracking apparatus for such processes, comprising in series a cracking zone, a quenching apparatus, and a heat exchanger is described in British Pat. Specification No. 1,293,260.

According to that specification the formation of coke deposits, heavy oil, and hydrocarbon polymer at critical parts of the apparatus downstream of the cracking zone can be prevented or decreased by a continuous injection of an aqueous solution of an alkali metal salt into the hydrocarbon stream at a point downstream of the cracking zone, for example to the quench water. It is stressed in the specification that in order to be effective the injection has to be made at a point following the reaction zone, since a method of injecting the alkali salt upstream from the thermal cracking zone, previously disclosed in US. Pat. No. 2,893,941, was found to be ineffective.

As disclosed in our British Patent Specification No. 1,305,568, we have found that the problem of carbon deposition in the catalytic steam cracking process can be solved by proper selection of the catalyst. A zirconium oxide catalyst promoted by 0.1-l wt.% of an alkali metal compound calculated as the oxide was found to be particularly useful. However, during prolonged use and during the necessary regenerations, the alkali metal compound will gradually be released from the catalyst.

Consequently, the alkali metal content of the zirconium oxide catalyst will gradually decrease. When it has dropped below a certain limit the rate of carbon deposition will increase which involves that regeneration operations have to be carried out more often. This tends to decrease the on-stream-factor, which is the relative time for operating the steam cracking process at its rated capacity. Furthermore, there may be an accelerated release of alkali metal during the regenerations so that the useful life of the catalyst may become reduced.

One way of increasing the on-stream-factor and the useful life of the catalyst is to increase its initial concentration of alkali metal. However, the rate of release of alkali metal from the catalyst tends to increase with increasing alkali metal concentration. Therefore, catalyst life will increase less than proportional to initial alkali metal concentration. Furthermore, the use of a high initial alkali metal concentration may often result in a higher initial formation of carbon dioxide. This will in some cases be a disadvantage.

A method has now been found to compensate for the loss of alkali metal from the zirconium oxide catalyst during its use in the process of steam cracking hydrocarbons.

Accordingly, the present invention provides an improved process for the manufacture of unsaturated hydrocarbons comprising a gaseous catalytic reaction step of contacting a preheated vaporized hydrocarbon feedstock consisting of predominantly saturated hydrocarbons at a temperature below 1,100C and at a pressure in the range from 1 to 30 atm. abs. in the presence of steam added in a proportion of from 0.1 to 10 parts per part of hydrocarbon feedstock with a catalyst consisting of zirconia together with 0. 1-10 wt.% of an alkali metal compound calculated as the oxide and less than 2 wt.% of other metal compounds calculated as the metal, wherein the improvement comprises a step of adding (prior to introducing hydrocarbon feedstock and steam into the reactor), at least during periods of operation of the process, to the preheated vaporized hydrocarbon feedstock an alkali metal compound in an average proportion of from 0.1 to 500 mg of alkali metal per kg of hydrocarbon feedstock.

Especially, the invention provides an improved process for the manufacture of ethylene and propylene comprising a gaseous catalytic reaction step of contacting a preheated vaporized hydrocarbon feedstock selected from the class of liquid hydrocarbon fractions ranging from naphthas to heavy gas oils at a temperature below 1,100 C and at a pressure in the range from 1 to 30 atm. abs. in the presence of steam added in a proportion of from 0.2 to 5 parts per part of hydrocarbon feedstock with a catalyst consisting of zirconia together with 0.1-10 wt.% of an alkali metal compound calculated as the oxide and less than 2 wt.% of other metal compounds calculated as the metal wherein the improvement comprises a step of adding, at least during some periods of operation of the process (and prior to contacting the hydrocarbon feedstock and steam with the catalyst) to the preheated vaporized hydrocarbon feedstock an alkali metal compound in an average proportion of from 0.1 to 500 mg of alkali metal per kg of hydrocarbon feedstock.

The reactions occurring in the catalytic steam cracking process are highly endothermic. Accordingly, the

process of the present invention is conducted in a furnace comprising catalyst-filled tubes vertically displaced in a combusting furnace. Gas or oil fired burners placed on the walls, roof and/or bottom of the combusting chamber supply the necessary heat to the catalyst tubes.

The catalyst in the catalyst tubes is in the form of small bodies such as cylinders, rings, spheres, cubes, or irregular lumps. A very suitable catalyst form is a ring having dimensions within the following ranges: height 5-20 mm, outer diameter 5-20 mm, and inner diameter 310 mm.

Any catalyst composition suitable for the catalytic steam cracking reactions may be used in accordance with the present invention. A preferred catalyst composition comprises zirconium oxide with 0.1 to 10 wt.% potassium oxide calculated as the metal. However, since an alkali metal compound periodically or continuously is added to the steam-hydrocarbon feed the cat alyst needs not contain an alkali metal compound from the start of the process, although this is usually preferred. Other metal compounds may be present in the catalyst. However, in the preferred catalyst, the total concentration of such other metal compounds is less than 2 wt.% calculated as the metal.

The mixture of steam and hydrocarbon feedstock is normally introduced at the top of the catalyst tubes and passes downwards through the catalyst beds in the tubes. The effluent from the catalyst tubes is most often subjected to further treatment. It is particularly preferred to subject it to a thermal'cracking process in which further reactions occur in the absense of a catalyst. This thermal cracking process is normally conducted in empty tubes following the catalyst tubes and placed either in the same or another furnace chamber.

The hydrocarbon feed may be a single saturated hydrocarbon or it may be a mixture of hydrocarbons, comprising saturated and unsaturated hydrocarbons as well as aromatics and naphthenes. Suitable hydrocarbons or hydrocarbon mixtures include ethane, propane, and butane fractions as well as liquid fractions of low and high boiling naphthas and gas oils.

Before being introduced into the catalyst tubes the hydrocarbon feed is vaporized, if it is not a gas, and mixed with steam. The weight ratio steam to hydrocarbon feed, e.g. naphtha or gas oil, may range from 0.2:1 to 6:1, especially for naphtha from 0.211 to 3:1 and for gas oil from 0.4:1 to 6:1. The mixture of steam and hydrocarbons is preheated at 400550C at the inlet of the catalyst tubes. While passing through the catalyst bed the reactants are being further heated and usually the temperature at the outlet of the bed will be 700850C. The outlet temperature may be higher, the maximum limit depending on the tube material. Construction materials available today do not permit temperature above l,lC.

The operating pressure is between atmospheric and 30 atmospheres absolute measured at the outlet of the catalyst bed.

The addition of an alkali metal compound to the steam hydrocarbon feed, whether this be done at irregular intervals, periodically or continuously during the catalytic steam cracking process, can take place in various ways. One way is to use an aqueous solution of an alkali metal compound which can be sprayed into the steam-hydrocarbon mixture at or immediately before the inlet of the catalyst tubes. A suitable alkali metal compound is potassium sulphate.

The concentration of the solution of the alkali metal compound is not very critical. The amount of water introduced with the solution should not exceed about 10 of the amount of steam in the steam hydrocarbon feed, since otherwise it will cause a considerable cooling of the feed. On the other hand, the solution should not be too concentrated, since this may result in a deposition of salt at the injection points. A suitable and preferred concentration range is from 50 to 10,000 mg of alkali metal calculated as the metal per liter of sulution. However, concentrations below or above this range may also be used although they are less suitable.

As will be understood, the solution of the alkali metal compound is preferably an aqueous solution, and preferably such aqueous solution is injected into the preheated vaporized hydrocarbon feedstock after the steam has been added thereto.

The amount of alkali metal compound added to the steam hydrocarbon feed in this way in the long run should equalize the amount of alkali metal released from the catalyst. However, the addition need not necessarily be constant and it can even be discontinued at certain intervals. It is only important that there is a balance between input and output of alkali metal when relatively long periods are considered.

The amount of alkali metal released from the catalyst during operation will depend on a number of circumstances, among which are temperature, steam and hydrocarbon feed rates, steam to hydrocarbon ratio, pressure, catalyst composition, etc. It may therefore be necessary to regularly measure the amount of alkali metal in the effluent from the steam cracking process and adjust the amount introduced into steam hydrocarbon feed in accordance with the findings.

Although the amount of alkali metal compound required for the addition may vary considerably from case to case, some general recommendations can be given. The average proportion of added alkali metal compound should be from 0.1 to 500 mg calculated as the metal per kg of hydrocarbon feed. In most cases the preferred proportion will be from 4 to 200 mg per kg.

EXAMPLE 1 Two steam cracking experiments designated as runs Nos. 1 and 2 below were conducted in a pilot plant comprising a single vertical reaction tube in a furnace. The reaction tube had two zones, an upper catalytic reaction zone followed by a lower thermal reaction zone. The catalytic zone had an inner diameter of mm and a heated length of4 m. The thermal zone had an inner diameter of 25 mm and a heated length of 5 m. The two zones were axially aligned in the same rectangular combusting chamber. Six oil-fired burners were placed on each of two opposite walls of the chamber thus enabling particular parts of the two zones to be independently heated at the desired temperatures.

Metered quantities of steam and vaporized naphtha were preheated at about 500C and fed into the catalytic reaction zone. After the reactants had passed through the catalyst bed in the catalytic reaction zone and subsequently through the thermal reaction zone the products were measured and analyzed at the reactor outlet. Further details of operating conditions and results of runs 1 and 2 are given in table 1.

' In accordance with normal practice the products in table I are calculated as weight of product per weights of naphtha feed. Further according to normal practice, ultimate yields, of C H and C H are calculated as follows:

Ultimate yield of C H Ultimate yield of C l-l The catalyst used in the catalytic reaction zone was prepared as follows: 217 kg of an aqueous paste of basic zirconium carbonate containing 46 wt.% zirconium calculated as the oxide was dried and calcined at a temperature of at least 300C to give 100 kg of zirconium oxide powder. This powder was mixed with potassium stearate and tabletted into rings having an outer diameter of 13 mm, an inner diameter of 6 mm, and a height of 13 mm. These rings were finally calcined for two hours at 850C. The potassium stearate served as a lubricant for the tabletting and at the same time it gave the final catalyst a certain content of potassium.

Run No. 1 was not conducted in accordance with the process of the invention, since no alkali metal compound was added to the steam hydrocarbon feed. The catalyst used in this run had an initial content of 0.5 wt.% potassium calculated as the metal. Other details of the operating conditions are given in Table l.

The run was operated during a total period of 258 hours. Carbon deposition had to be removed from the catalyst by regeneration is steam and air after 100, 189, 236, and 258 total hours of operation. From the naphtha feed rate, 75.2 kg/h, the amount of naphtha converted before a regeneration becomes necessary can be calculated to be as follows: 7520, 6693, 3534, and 1654 kg. In other words, the rate of carbon deposition increased during the run so that still less naphtha could be converted between the regenerations.

The amount of potassium released from the catalyst during run I and collected with the condensate was determined from time to time. At the start of the run the rate of release of potassium corresponded to 0.75 g per hour, while at the end of the run the rate had decreased to about 0.07 g per hour.

After run 1 the catalyst was withdrawn from the catalytic reaction zone and samples from various positions were analysed for potassium. The results of these analyses are given in table II. It is seen that the alkali concentrations in the major part of the catalyst bed has decreased to about a tenth of the original concentration.

Run No. 2 was conducted in accordance with the process of the invention. The catalyst was similar to the one used in run No. 1, except that its initial content of potassium was 0.3 wt.% calculated as the metal. A solution of potassium sulphate was continuously added to the steam hydrocarbon feed at a point before the reactor tube. The concentration of this solution was 28 g potassium metal per liter. The rate of potassium addition was varied during the run from 4 to 19 mg potassium metal per kg of naphtha feed. Other details of the operating conditions are given in Table I.

The run was operated for a total period of 202 hours. Carbon deposition was removed by regeneration in steam and air after 70 and 202 total hours of operation. Because of the high naphtha feed rate, 215 kg/h, this corresponds to a much higher total amount of naphtha converted between regenerations than in run No. 1 without addition of alkali metal. Before the first regeneration 15,000 kg and between the regenerations 28,400 kg of naphtha were converted in Run No. 2. In other words, the amount of naphtha that could be converted before a regeneration became necessary had been more than doubled.

The run was not conducted for a period long enough to give a steady state in which there was a balance between addition and release of potassium. Therefore, there was during the run an accumulation of potassium in the catalyst as well as between the catalyst particles. Analysis of catalyst samples from various positions are given in Table II and confirms that there was an accumulation of potassium in part of the catalyst bed, while in another part there had been some decrease, but not below half of the original concentration.

Temperatures, C:

TABLE I-Continued Inlet Catalyst 529 470 Outlet Catalyst 780 780 Outlet thermal zone 850 850 wutlet) atm.abs. 1.5 1.5 Ultimate Yields of Gaseous H 1.22 0.88 C0 C0 0.27 0.99 1 15.2 13.0 CH, 32.6 33.2 Gil- 15.5 111.7 C H 4.4 4.9 (1H,, 3.8 5.7

Weight of product per weights of naphtha l'ced.

TABLE II Run 1 Run 2 Unused: 0.5 wt.'7( K 0.3 wt.'7r K Used:

(distance from inlet) 0.00 m 4.9 wt.7( K 0.33 m 0.29 wt.7t K 0.50 m 1.74 wtfil K 1.00 m 0.06 wt.7t K 0.52 wtf/t K 1.70 m 0.06 wL /t K 2.00 m 0.48 wtf/l K 2.30 m 0.05 wtf/r K 3.00 m 0.03 wt.7t K 0.35 WLV! K 3.50 m 0.21 WL(7( K 3.70 m 0.04 wt.% K 3.90 m 0.18 wt.% K

EXAMPLE II Two steam cracking experiments designated as Runs Nos. 3 and 4 below were conducted in a laboratory bench scale reactor and in accordance with the process of the present invention. The reactor had one reaction zone, a catalytic reaction zone, in a reactor tube having an inner diameter of 20.5 mm and a heated length of about 1.5 m. The tube was placed in an electrically heated furnace having separate heating elements. This arrangement made it possible to adjust the temperature along the length of the reactor tube so that the desired temperature profile through the catalyst bed could be obtained.

A zirconium oxide catalyst similar to the one used in runs Nos. 1 to 2 was used in runs Nos. 3 to 4. However, because of the smaller diameter of the reactor tube, the catalyst for the bench scale reactor was crushed and a sieve fraction from 3 to 5 mm was collected for use in the reactor tube. The height of the catalyst bed was about 1.5 m.

Liquid water and liquid gas oil feed were separately metered through calibrated pumps and evaporated before being introduced into the reactor as a mixture of steam and hydrocarbon vapour. After having passed through the catalyst bed the products were collected at the outlet for analysis. An aqueous solution of potassium sulphate was sprayed into the steam hydrocarbon mixture immediately before the inlet to the catalyst bed. The concentration of this solution was from 290 to 580 mg potassium per liter calculated as the metal.

Operating conditions and results from runs Nos. 3 and 4 are given in Table III. In run No. 3 the catalyst had an initial alkali metal oxide content of 5 wt.% potassium calculated as the metal. The major part of this had been added by impregnation. The experiment was conducted for a short period only, and failed eventually due to experimental difficulties. Therefore, no details were obtained about the condition of the catalyst after the experiment. However, while it was operating, the experiment demonstrated the feasibility of the process of the invention to operate on a gas oil feed.

After certain modifications of the experimental equipment run No. 4 was conducted during a period of 290 hours. The catalyst used in this run had an initial content of alkali metal oxide of 0.2O.3 wt.% of potassium calculated as the metal. Furthermore the catalyst contained a nickel oxide added by impregnation in a concentration of 1.1 wt.% calculated as the metal. The operating data for this run are given in Table 111. They relate to the conditions after 284 hours operation which correspond to the conditions during the major portion of the total period of operation. There were no regenerations during this run or at the end of the run. After the run the catalyst was found to be practically free of carbon deposition, since it contained less than 1 wt.% carbon. The alkali concentration of the catalyst after the run is given in Table W at various positions of the catalyst bed. There has been a slight increase in alkali metal near the top of the catalyst bed and a slight decrease at the bottom of the catalyst bed. However, on the whole the continuous alkali addition has made it possible to operate the process for a prolonged period of time without carbon deposition and without any regeneration being necessary. During the run the alkali addition was varied from 40 to 170 mg potassium metal per kg gas oil.

TABLE 111 Run No. 3 4 Catalyst: ZrO- 5% K ZrO (ll-0.3% K

1.1% Ni Volume. cc. 500 500 Gas Oil Feed:

Sp. gravity. g/cc 0.86 0.86 l.B.P.. 195 1'95 F.B.P.. C 405 405 Flash point. 103 103 Pour point. "C 3 3 Viscosity (C) C.stokc 12.5 12.5 Sul hur. wtf/t 0.66 0.66 Feed ates:

Gas Oil. g/h 2420 627 Water. g/h 3365 2890 Space velocity. vol/vol/h 5.6 1.5 Alkali addition. mg/kg oil 29 54 Temperatures. C:

lnlet 475 530 Outlet 790 790 Pressure (outlet) atm.:tbs. 3.5 4 Ultimate Yields of Gaseous Products. \vt.7(

H; 1.2 2.5 C0 CO 4.5 14.5 CH, 9.9 18.7 C H 26.8 34.0 a a 17.2 16.6 4 1, 5.8 5.5 C H 7.1 3.5

* weight of product per 100 weights ofgas oil feed.

TABLE IV Run No. 4 Unused catalyst 0.20.3 \vtf/r K Used catalyst (distance from inlet) 0 cm 1.07 30 cm 0.28

8 TABLE lV-Continued 60 cm 0.18 cm 0.28 cm 0.14 cm 0.16

We claim:

1. An improved cracking process for the manufacture of unsaturated hydrocarbons comprising a gaseous catalytic reaction step of contacting a preheated vaporized hydrocarbon feedstock consisting of predominantly saturated hydrocarbons at a temperature below 1,100C and at a pressure in the range from 1 to 30 atm. abs. in the presence of steam added in a proportion of from 0.1 to 10 parts per part of hydrocarbon feedstock with a catalyst consisting of zirconia together with 0.1-l0 wt.% of an alkali metal compound calculated as the oxide and less than 2 wt.% of other metal compounds calculated as the metal, wherein the improvement comprises a step of adding, at least during periods of operation of the process, to the preheated vaporized hydrocarbon feedstock an alkali metal compound in an average proportion of from 0.1 to 500 mg of alkali metal per kg of hydrocarbon feedstock.

2. An improved cracking process according to claim 1, in which the alkali metal compound is added as an aqueous solution containing from 50 to 10,000 mg of alkali metal per liter, and in which said aqueous solution is injected into the preheated vaporized hydrocar bon feedstock after the steam has beed added thereto.

3. An improved cracking process according to claim 2 in which the alkali metal compound to be added to the preheated vaporized hydrocarbon feedstock is potassium sulphate.

4. An improved cracking process for the manufacture of ethylene and propylene comprising a gaseous catalytic reaction step ofcontacting a preheated vaporized hydrocarbon feedstock selected from the class of liquid hydrocarbon fractions ranging from naphthas to heavy gas oils at a temperature below 1 100C and at a pressure in the range from 1 to 30 atnrabs. in the presence of steam added in a proportion of from 0.2 to 5 parts per part of hydrocarbon feedstock with a catlyst consisting of zirconia together with 0.1-l0 wt.% of other metal compounds calculated as the oxide and less than 2 wt.% of other metal compounds calculated as the metal wherein the improvement comprises a step of adding, at least during periods of operation of the process, to the preheated vaporized hydrocarbon feedstock an alkali metal compound in an average proportion of from 0.1 to 500 mg of alkali metal per kg of hydrocarbon feedstock.

5. An improved cracking process according to claim 4 in which the alkali metal compound is added as an aqueous solution containing from 50 to 10,000 mg of alkali metal per liter, and in which said aqueous solution is injected into the preheated vaporized hydrocarbon feedstock after the steam has been added thereto.

6. An improved cracking process according to claim 5 in which the alkali metal compound is potassium sulphate added in an average proportion of from 4 to 200 mg per kg of hydrocarbon feedstock.

Claims

1. AN IMPROVED CRACKING PROCESS FOR THE MANUFACTURE OF UNSATURATED HYDROCARBONS COMPRISING A GASEOUS CATALYTIC REACTION STEP OF CONTACTING A PREHEATED VAPORIZED HYDROCARBON FEEDSTOCK CONSISTING OF PREDOMINANTLY SATURATED HYDROCARBONS AT A TEMPERATURE BELOW 1,100*C AND AT A PRESSURE IN THE RANGE FROM 1 TO 30 ATM. ABS. IN THE PRESENCE OF STEAM ADDED IN A PROPORTION OF FROM 0.1 TO 10 PARTS PER PART OF HYDROCARBON FEEDSTOCK WITH A CATALYST CONSISTING OF ZIRCONIA TOGETHER WITH 0.1-10 WT.% OF AN ALKALI METAL COMPOUND CALCULATED AS THE OXIDE AND LESS THAN 2 WT.% OF OTHER METAL COMPOUNDS CALCULATED AS THE METAL, WHEREIN THE IMPROVEMENT COMPRISES A STEP OF ADDING, AT LEAST DURING PERIODS OF OPERATION OF THE PROCESS, TO THE PREHEATED VAPORIZED HYDROCARBON FEEDSTOCK AN ALKALI METAL COMPOUND IN AN AVERAGE PROPORTION OF FROM 0.1 TO 500 MG OF ALKALI METAL PER KG OF HYDROCARBON FEEDSTOCK.

2. An improved cracking process according to claim 1, in which the alkali metal compound is added as an aqueous solution containing from 50 to 10,000 mg of alkali metal per liter, and in which said aqueous solution is injected into the preheated vaporized hydrocarbon feedstock after the steam has beed added thereto.

4. An improved cracking process for the manufacture of ethylene and propylene comprising a gaseous catalytic reaction step of contacting a preheated vaporized hydrocarbon feedstock selected from the class of liquid hydrocarbon fractions ranging from naphthas to heavy gas oils at a temperature below 1100*C and at a pressure in the range from 1 to 30 atm.abs. in the presence of steam added in a proportion of from 0.2 to 5 parts per part of hydrocarbon feedstock with a catlyst consisting of zirconia together with 0.1-10 wt.% of other metal compounds calculated as the oxide and less than 2 wt.% of other metal compounds calculated as the metal wherein the improvement comprises a step of adding, at least during periods of operation of the process, to the preheated vaporized hydrocarbon feedstock an alkali metal compound in an average proportion of from 0.1 to 500 mg of alkali metal per kg of hydrocarbon feedstock.