GB1602098A - Cracking of hydrocarbons - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE CRACKING OF HYDROCARBONS (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method for controlling carbon deposits during the thermal cracking of hydrocarbons.

In the cracking of hydrocarbons, such as naphtha or ethane, the hydrocarbon to be cracked is fed into one end of a heated cracking furnace, such as a heated steel tube, and the products of the cracking collected from the other end. A problem is created by the formation of a carbonaceous layer on the inside of the heated furnace which acts as an insulation. Thus, more heat must be supplied to maintain the same temperature within the furnace. This increases fuel costs and increases the chances of carburising the steel constituting the furnace. A known solution is to conduct periodic 'de-cokes', but this involves interruption of the cracking process which is clearly undesirable on economic grounds. Another known practice is to feed steam into the heated furnace at the same time as the hydrocarbon since this is known empirically to minimise carbonaceous deposition.This does not, however, offer a complete solution and various proposals have been made for using additives for the prevention, inhibition and/or removal of carbon deposits (see, for example, U.S. Patent Specification Nos. 2,893,941; 3,364,130 and 3,437,583).

We have now found that certain other substances may be useful in this respect. Thus, the present invention provides a method for controlling carbon deposits during the thermal cracking of hydrocarbons in the presence of steam, which comprises carrying out said cracking in the presence of an additive selected from elemental Ni, Co, Fe, Pb, Sn and Cu and oxides of Ba, Sr, Ca, Mn, Zr, La and Ce by adding to the hydrocarbon, prior to cracking, a solution of salt, which salt is decomoosable under the conditions of the cracking either to a precursor which is re ducible to the additive under the conditions of the cracking, when a reducing agent for reducing the precursor to the additive is present, or to the additive.

We have carried out experiments, which are described in the examples herein, and which indicate that the additives of the invention are very effective in controlling carbon deposits. We believe that the additives may work by promoting the oxidation by the steam either of the gaseous precursor to deposition or of the deposits after they have been formed.

When a precursor is used, a reducing agent such as hydrogen must be provided in order to generate the additive in situ. For example, the solution may be fed into a cracking furnace in which the cracking takes place; together with the hydrocarbon and steam, so that the salt decomposes to the additive and is deposited on the inner surface of the cracking furnace and thereby controls carbon deposits during cracking. A reducing agent must be present when it is desired to generate elemental Fe, Co or Pb as an additive from a decomposable salt thereof. In the case of Ni, however, sufficient hydrogen is generated by the uncatalysed oxidation reaction C + H2O - > CO + H2 to ensure reduction of the oxide to elemental Ni.We prefer, where possible, that the salts used are salts of organic acids such as acetates and citrates since these generally decompose at temperatures well below generally used cracking temperatures and without causing contamination with anions which could conceivably affect oxidation kinetics. Other salts are not, however, ruled out and examples of other suitable salts in this respect are nitrates.

An advantage of the present invention is thus that it may be carried out simply and continuously and without interrupting the cracking process.

The invention will now be particularly described, by way of example only, as follows, where the general procedure is first described.

A carbonaceous deposit was formed in an ethvlene-steam cracker from a naphtha hydrocarbon feedstock. Samples of the deposit were boiled for 10 minutes in an aqueous solution of a salt decomposable to a desired addictive.

The solution was either 30% (by weight) or saturated, whichever had the lower concentration. The samples were removed after cooling in the solution and then dried under an infra-red lamp at Z 150"C. The samples were then heat treated at 900"C to constant weight in order to decompose the salt to the desired additive.

The oxidation of the samples was then followed using a C1 Electronics Mk2 controlled atmosphere microbalance. A sample was suspended within an externally heated silica reaction vessel and dry argon passed through until the desired oxidation temperature (900"C) was attained. A mixture of the water vapour oxidant at 362 mm Hg partial pressure, hydrogen at 19.4 mm Hg partial pressure for cobalt and lead to ensure reduction to the metallic state, together with dry argon as a carrier for the oxidant, was then passed over the sample. With iron, again to ensure complete reduction the water vapour partial pressure was 23.4 mm Hg and the hydrogen partial pressure was 35.1 mm Hg.

The oxidation was followed by the continuous measurement of the loss in sample weight.

The results are summarised in the Table below.

TABLE

No. of atoms of Factor by which additive element per Average gasification Possible chemical initial active reaction rate rate of deposit Example Salt state of additive carbon site mg/g.h. was enhanced 1 Stronitium nitrate oxide 67 41 2 2 Barium nitrate oxide 145 83 4 3 Lead acetate metal 22 125 (A) 6 4 Iron citrate metal 36 230 (B) 11 5 Cobalt acetate metal 25 160 (A) 8 6 Nickel acetate metal 47 100 5 7 Calcium nitrate oxide 90 38 1.8 8 Manganese acetate oxide 39 46 2.2 9 Copper acetate metal 15 33 1.6 10 Zirconium nitrate oxide 86 35 1.7 11 Tin nitrate metal 39 35 1.7 12 Lanthanum acetate oxide 61 36 1.7 13 Cerium nitrate oxide 86 44 2.1 (A) Maximum rate in PH2O = 353 mm Hg, after PH2 = 19.4 mm Hg added.

(B) Rate in PH2O = 23.4 mm Hg, after PH2 = 35.1 mm Hg added.

The enhancement columns are in comparison with tests on carbonaceous deposits lacking an additive and clearly show the success of the particular additives in oxidising the deposits.

WHAT WE CLAIM IS:- 1. A method for controlling carbon deposits during the thermal cracking of hydrocarbons in the presence of steam, which comprises carrying out said cracking in the presence of an additive selected from elemental Ni, Co, Fe, Pb, Sn and Cu and oxides of Ba, Sr, Ca, Mn, Zr, La and Ce by adding to the hydrocarbon, prior to cracking, a solution of a salt, which salt is decomposable under the conditions of the cracking either to a precursor which is reducible to the additive under the conditions of the cracking, when a reducing agent for reducing the precursor to the additive is present, or to the additive.

2. A method according to claim 1 wherein the solution is fed into a cracking furnace in which the cracking takes place, together with the hydrocarbon and the steam.

3. A method according to any of the preceding claims wherein the salt is a salt of an organic acid.

4. A method according to claim 3 wherein the salt is an acetate or a citrate.

5. A method according to claim 1 or claim 2 wherein the salt is a nitrate.

6. A method for controlling carbon deposits during the thermal cracking of hydrocarbons in the presence of steam substantially as described herein with reference to any of the examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. The enhancement columns are in comparison with tests on carbonaceous deposits lacking an additive and clearly show the success of the particular additives in oxidising the deposits. WHAT WE CLAIM IS:-

1. A method for controlling carbon deposits during the thermal cracking of hydrocarbons in the presence of steam, which comprises carrying out said cracking in the presence of an additive selected from elemental Ni, Co, Fe, Pb, Sn and Cu and oxides of Ba, Sr, Ca, Mn, Zr, La and Ce by adding to the hydrocarbon, prior to cracking, a solution of a salt, which salt is decomposable under the conditions of the cracking either to a precursor which is reducible to the additive under the conditions of the cracking, when a reducing agent for reducing the precursor to the additive is present, or to the additive.

2. A method according to claim 1 wherein the solution is fed into a cracking furnace in which the cracking takes place, together with the hydrocarbon and the steam.

3. A method according to any of the preceding claims wherein the salt is a salt of an organic acid.

4. A method according to claim 3 wherein the salt is an acetate or a citrate.

5. A method according to claim 1 or claim 2 wherein the salt is a nitrate.

6. A method for controlling carbon deposits during the thermal cracking of hydrocarbons in the presence of steam substantially as described herein with reference to any of the examples.