Process For Producing Ethylene

Abstract

Hydrocarbons having a boiling point above 300.degree. C (fuel oils) are vaporized by means of direct superheated steam; after separation of any liquid components a further amount of superheated steam is added to obtain a feed at 450.degree. -550.degree. C for a tubular pyrolysis reactor operating at 700.degree.-800.degree.C. The total steam content in the feed is 0.5 to 5.0 parts by weight steam per part by weight of the hydrocarbon.

Parent Case Text

CROSS REFERENCE

This application is a continuation-in-part of applicant's copending application Ser. No. 704,578, filed Feb. 12, 1958, now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns the production of ethylene by pyrolysis of fuel oils.

More particularly, this invention concerns the production of gaseous mixtures having an ethylene content exceeding 40 percent by weight, by pyrolysis, in the presence of steam, of heavy hydrocarbon mixtures having an initial boiling point in excess of 300.degree.C, particularly heavy fuel oils.

2. Description of the Prior Art

In latter years, ethylene has attained very great importance in a variety of fields, such as the preparation of plastic materials or in modern processes for obtaining products formerly prepared from acetylene. For these reasons, the demand for this compound has been greatly increased and production techniques have been correspondingly improved.

At the present time, the pyrolysis of hydrocarbons is the process which is by far the most used for the production of ethylene.

A number of industrial methods are known for the pyrolysis of normally gaseous hydrocarbons such as the natural gases, ethane, propane, butane or relatively light hydrocarbon liquids, for example, the mixture of hydrocarbons found in commerce and known as "Naphtha."

Certain pyrolysis processes utilize hydrocarbon mixtures (e.g., crude petroleum) containing low boiling liquid fractions. In said processes, the low boiling liquid fractions are first separated from their mother-mixture (for example by means of steam) and the pyrolysis is carried out on only these separated fractions. In pyrolysis processes carried out in the presence of steam, the hydrocarbons are subjected to elevated temperatures for a very short period, the products of the pyrolysis then being cooled rapidly and finally fractionated.

These procedures are inconvenient in that they utilize primary feed materials which are relatively expensive in the production of pyrolysis products having a concentration of ethylene not greater than about 30 percent by weight.

It has now been discovered that gaseous mixtures having an ethylene content exceeding 40 percent by weight can be obtained by high-temperature pyrolysis, in the presence of steam, of hydrocarbon mixtures having an initial boiling point exceeding 300.degree.C, in particular by pyrolysis of heavy fuel oils.

SUMMARY OF THE INVENTION

The process of this invention comprises:

A. VAPORIZING AN AMOUNT UP TO 50 WT. PERCENT OF A HYDROCARBON MIXTURE HAVING AN INITIAL BOILING POINT ABOVE 300.degree.C by admixture with 0.1 to 0.5 parts by weight of superheated steam (primary steam) per part by weight of the hydrocarbon mixture charge;

b. separating liquid components from gaseous components at a temperature of 300.degree. to 450.degree.C; point

c. admixing the gaseous components with a further amount of superheated steam (secondary steam) thereby to obtain, as the feed for a pyrolysis reaction, a mixture of steam and hydrocarbons at a temperature of 450.degree. to 550.degree.C, which mixture contains 0.5 to 5.0 parts by weight of steam per each part by weight of hydrocarbons;

d. pyrolysing said feed, for example, in a tubular reaction zone at a temperature of 700.degree. to 800.degree.C over a period of from 1 to 3 seconds, thus producing from said feed an amount of normally gaseous hydrocarbons up to 50 wt. percent with respect to the hydrocarbons contained in the feed;

e. recovering said normally gaseous hydrocarbons from the pyrolysis product obtained.

DETAILED DESCRIPTION OF THE INVENTION

As in previous processes of this type, the process of the present invention is advantageously carried out by use of a cyclically operated tube-furnace in which the periods of pyrolysis are alternated with periods of regeneration to eliminate the deposits of coke within the tubes, although the quantity of deposited coke does not become excessive (less than 1 percent in proportion to the oil introduced) in the tube furnace during operation of the present process. This regeneration is conventional and is preferably effected at temperatures of 750.degree. to 900.degree.C, by passing a mixture of air and steam through the pyrolysis tube furnace.

In the process of the present invention, the hydrocarbon is first preheated and then mixed with primary steam in a proportion of 0.1 to 0.5 parts by weight to each part by weight of hydrocarbon.

As a result, a mixture comprising a gaseous phase and a liquid phase at 300.degree.-450.degree.C is obtained, from which the liquid phase is subsequently separated in a gas/liquid separator maintained within this temperature range. By operating under these conditions, an amount of up to 50 wt. percent of hydrocarbon components is vaporized. It should be pointed out that, considering the nature of the hydrocarbon charge employed, a vaporization of an amount exceeding 50 percent hydrocarbons would give rise to undesirable deposits in the gas/liquid separator comprising carbonaceous and/or pitchy residues. Moreover, the hydrocarbons which would then be present in the feed to the pyrolysis furnace would contain undesirably high amounts of fractions capable of immediately coking in the furnace (with consequent clogging of the furnace tubes). Thus, it is important to partially vaporize the hydrocarbon charge using only a relatively small amount of steam and to dilute the vaporized fraction with a further amount of steam only subsequently to the initial vaporization and separation steps.

To the gaseous mixture thus obtained is added the secondary steam in a proportion such that the weight ratio of steam/hydrocarbon at the inlet to the pyrolysis furnace is between 0.5 to 5.0 and preferably between 1.5 and 3.0 The temperature of the mixture may vary between 450.degree. and 550.degree.C.

In the pyrolysis furnace, formed of tubes heated from the outside, the hydrocarbons are pyrolysed, the temperature of the interior of the tubes preferably being maintained between 700.degree. and 800.degree.C and the residence time of the mixture in the reactor proper being maintained between 1 and 3 seconds, preferably 1.5 and 2 seconds.

By operating under these conditions, the hydrocarbons in the pyrolysis furnace are converted to normally gaseous products in an amount up to 50 wt. percent of the hydrocarbon feed. The composition of the feed (steam: hydrocarbon) is to be maintained within the limits stated hereinbefore, in order to avoid deposits of coke in the furnace tubes. Moreover, the pyrolysis conditions (relatively high temperatures and short residence times) prevent coke formation in furnace tubes and contribute towards formation of ethylene-rich pyrolysis gases from the specific hydrocarbon stock employed.

The pyrolysis product leaving the pyrolysis reactor is quenched to about 300.degree. to 400.degree.C, preferably by admixture of the product with water to make use of the heat of vaporization of the latter. The products are thus separated into liquids and gases which are then fractionated.

The values set forth above with regard to the amount of hydrocarbon stock to be vaporized by admixture with primary steam (50 wt. percent) and the amount of conversion of hydrocarbon pyrolysis feed into normally gaseous hydrocarbons (50 wt. percent) are to be considered as maximums for operation in accordance with the present invention. Any positive value less than these is operable in the process of the present invention, the minimum acceptable values being dictated by purely economical considerations.

Employing the process of the present invention, a pryolysis gas is obtained in which the content of ethylene exceeds 40 percent by weight, thus allowing subsequent separation to be effected in a very economical manner.

Finally, the present process has the advantage of utilizing inexpensive starting materials, such as heavy fuel oils or other heavy petroleum fractions.

The following example illustrates the present invention without limiting the scope thereof.

EXAMPLE

A pilot plant is fed with 100 kg/h of a fuel oil having the following characteristics:

API: 20.4.degree.

Flow point:36.degree.C

Lower calorific value 9,800 calories/kg

Boiling point: 380.degree.C

The oil is preheated and mixed with about 23 kg/h superheated steam at a temperature of 500.degree.C. The mixture, at a temperature of 360.degree.C and pressure of 1.1 atm. abs. is passed through a gas-liquid separator.

The gaseous products obtained are admixed with about 93 kg/h of superheated steam at a temperature of 780.degree.C. The new mixture, containing about 27.degree. by weight hydrocarbons, is introduced at a temperature of 530.degree.C and under 1.1 atm. abs. into a pyrolysis reactor formed by a tube furnace heated from the outside.

The pyrolysis conditions correspond to a residence time of the mixture of 1.7 seconds, the products reaching the furnace outlet at a temperature of about 770.degree.C. The products leaving the reactor are quenched to about 400.degree.C in a mixing tower fed with about 43 kg/h water which is instantly vaporized.

The heavier hydrocarbons are separated in a separating tower at a temperature of about 110.degree. C, while the lighter products constituting the pyrolysis gas, the water and the hydrocarbons boiling at about the boiling point of petrol are successively cooled in a heat exchanger.

In this manner, normally gaseous products are recovered which can be separated by normal techniques at a rate of 24 to 24.5 kg/h, calculated as dry products.

The pyrolysis product has the following composition determined by gas chromatography :

methane 24.9% by weight ethylene 46.3% by weight ethane 4.5% by weight C.sub.3 hydrocarbons 8.6% by weight C.sub.4 hydrocarbons 2.9% by weight benzene 3.8% by weight water 1.8% by weight CO 2.8% by weight CO.sub.2 2.2% by weight hydrogen 2.2% by weight

In a series of tests with the pilot plant described, pyrolysis periods of 10 to 12 hours can be alternated with regeneration periods of 70 to 100 minutes. This regeneration operation is effected at temperatures exceeding preferably by 100.degree. C the pyrolysis temperature, that is to say between 750.degree. and 950.degree. C by passing air and steam at a rate of 20 to 50 m.sup.3 /h.

Claims

What is claimed is:

1. A process for the production of a gaseous mixture containing above 40 wt. percent ethylene comprising:

a. vaporizing up to 50 wt. % of a heavy fuel oil hydrocarbon mixture having an initial boiling point above 300.degree.C by admixing said mixture with 0.1 to 0.5 parts by weight of superheated steam per part by weight of hydrocarbon

b. separating liquid components from gaseous components by passing said mixture through a gas-liquid separator at a temperature of 300.degree. to 45.degree.C

c. admixing said gaseous components with a further amount of superheated steam to obtain a feed mixture of steam and hydrocarbons having a temperature of from 450.degree. to 550.degree.C, the total proportion of steam present in said feed mixture being from 0.5 to 5.0 parts by weight per part by weight of hydrocarbon,

d. pyrolyzing said feed mixture at a temperature of 700.degree. to 800.degree.C for from 1 to 3 seconds to produce normally gaseous hydrocarbons in an amount of up to 50 wt. percent of the weight of hydrocarbons subjected to pyrolysis, and

e. recovering said normally gaseous hydrocarbons from the pyrolysis product obtained.

2. The process of claim 1 wherein the hydrocarbon mixture is a heavy fuel oil.

3. The process according to claim 1 wherein the proportion of steam present in said feed mixture is from 1.5 to 3.0 parts by weight per part by weight of the hydrocarbon.

4. The process according to claim 1 wherein said feed mixture is pyrolyzed for from 1.5 to 2 seconds.

5. The process according to claim 1 wherein said normally gaseous hydrocarbons are quenched to about 300.degree. to 400.degree.C.

6. The process according to claim 5 wherein said normally gaseous hydrocarbons are quenched with water.

7. The process according to claim 1 whereby deposits of coke are reduced.