Preparation of acetylene and synthesis gas

Details Preparation of acetylene and synthesis gas Preparation of acetylene and synthesis gas

2000-03-20

2002-04-02

Griffin, Walter D. (Department: 1764)

Chemistry of hydrocarbon compounds

Unsaturated compound synthesis

Triple-bond product

C585S534000, C585S536000, C585S538000, C585S540000

Reexamination Certificate

active

06365792

ABSTRACT:

The present invention relates to a process for the preparation of acetylene and synthesis gas.
Numerous processes for the uncatalyzed preparation of acetylene are based on pyrolysis or partial oxidation of hydrocarbons. Suitable feedstocks here are short-chain species, in the C region of methane, up to long-chain compounds of crude oil. A process development, the submerged-flame process, enables high-boiling fractions, such as residual oils, to be employed in addition. In principle, thermodynamic and kinetic parameters have a crucial effect on the choice of the reaction conditions in pyrolytic or oxidative processes for the preparation of acetylene. Important prerequisites of corresponding processes are rapid supply of energy at a high temperature level—the maximum reaction temperature must be above 1400° C.—extremely short residence times of the starting materials or reaction products of from 10
−2
to 10
−3
seconds, a low partial pressure of the acetylene and rapid quenching of the gases formed. In pyrolysis and partial oxidation, acetylene is produced in a gas mixture, known as cracking gas. The cracking gas normally contains from about 5 to 20% by volume of acetylene. The latter is extracted from the cracking gas by selective solvents, such as N-methylpyrrolidone, dimethylformamide, kerosene, methanol or acetone, and purified in further steps.
The individual processes in which acetylene is prepared differ, in particular, with respect to the generation of the high reaction temperatures. The provision and transfer of heat energy plays the crucial role here. A distinction can be made here between two types of process which differ with respect to the principle:
A allothermal pyrolysis processes, usually with electric heating
B autothermal processes, in which the heat from the partial combustion of the starting material is utilized for oxidative pyrolysis
REGARDING A
This includes the electric arc process. In this process, hydrocarbons having a boiling point of up to 200° C. are pyrolyzed with the aid of a stabilized electric arc with a length of about 1 m which has a temperature of up to 20,000° C. in its center. At the end of the “burner”, the gas mixture has, at an operating pressure of about 1.2 bar, a temperature of about 1800° C., which is rapidly lowered to about 100° C. by spraying in water. The residence time in the burner zone is a few milliseconds, and the yields of acetylene or ethylene (in the case of ethylene generally only with pre-quenching with hydrocarbons) reach 1.0 or 0.42 tonnes respectively per 1.8 tonnes of hydrocarbon employed. Another electric arc process has been trialed in two industrial pilot plants. H
2
as heat-transfer medium is firstly heated to from 3000 to 4000° C. in an electric arc and dissociated to the atoms to the extent of from 30 to 65% in the process. In the subsequent reactor, all types of hydrocarbons from methane to crude oil can then be sprayed into the plasma and cracked. The cracking gas is quenched rapidly and separated. On use of light gasoline, acetylene/ethylene yields of about 80% by weight are obtained if byproducts are recycled into the cracking process. The acetylene concentration in the cracking gas reaches almost 14% by volume.
REGARDING B
An autothermal cracking process developed for this purpose is suitable for feedstocks such as methane, liquid gas or light gasoline. The majority of the plants built worldwide are based on natural gas as feedstock; only a few use naphtha as raw material. In the industrial process, methane and oxygen, for example, are separately pre-heated to from 500 to 600° C., mixed and brought to reaction in a special burner with formation of a flame. The O
2
/CH
4
ratio is set, at about 1:2, in such a way that only incomplete combustion can take place. Both the exothermic oxidation of some of the CH
4
and the endothermic dehydrodimerization of the CH
4
into acetylene and hydrogen take place in the flame. After a residence time of a few milliseconds, the reaction gas is quenched by spraying in water or quenching oil, since otherwise acetylene would decompose to soot and hydrogen. However, the formation of soot cannot be prevented completely—about 5 kg of soot are produced per 100 kg of acetylene. The acetylene is usually separated off using an extractant, such as N-methylpyrrolidone or dimethylformamide. Fractional desorption and suitable rectification steps then serve to separate off accompanying components which are also dissolved. The proportion by volume of acetylene in the cracking gas is about 8%. The principal components are hydrogen, with 57% by volume, and carbon monoxide, with 26% by volume. In this ratio, the principal components represent a highly suitable synthesis gas. The autothermal preparation of acetylene is always associated with the preparation of synthesis gas.
Some terms used will be defined below:
“Heating” is taken to mean all measures and processes which result in a temperature increase. A medium, for example a starting mixture for the preparation of acetylene and synthesis gas, can be heated, for example, by ignition (thus initiating an exothermic reaction), by the supply of energy (for example from the outside) or by exothermic reactions with simultaneous or prior supply of energy (for example by pre-heating).
“Starting mixture” is taken to mean the mixture employed for the process of the preparation of acetylene/synthesis gas. This can basically vary, and it contains different starting materials depending on the desired synthesis gas. The starting mixture always contains molecular oxygen and/or one or more compounds containing the element oxygen. Molecular oxygen can be provided to the starting mixture in the form of air, air/oxygen mixtures or pure oxygen. The compounds containing the element oxygen can be provided in the form of steam and/or carbon dioxide. In addition, the starting mixture contains one or more hydrocarbons. The starting mixture frequently, in particular if methanol synthesis gas is to be prepared, comprises a large proportion of natural gas, but also, for example, liquid gas, such as propane or butane, light gasoline, such as pentane or hexane, benzene or other aromatics, pyrolysis gasoline or distillation residues from oil refining. Conversion of the starting mixture into a mixture containing acetylene/synthesis gas is referred to as thermal treatment. The underlying reaction types are predominantly combustion (total oxidation), partial combustion (partial oxidation or oxidative pyrolysis) and pyrolysis reactions (reactions without participation of oxygen). “Indirect cooling” is taken to mean cooling of the reaction mixture where the coolant employed does not come into direct contact with the reaction mixture. In “direct quenching”, conversely, the coolant comes directly into contact with the reaction mixture.
Common features of the known methods for the preparation of acetylene are that the reaction temperature is above 1400° C. and that the residence times are in the region of milliseconds. In order to avoid subsequent reactions (for example the formation of soot), the reaction gas must then be quenched rapidly by direct quenching, in which a quenching agent is sprayed in directly. During this operation, the corresponding mixture is cooled to different extents depending on the quenching agent employed—to about 300° C. in the case of oil as quenching agent, and to about 100° C. in the case of water as quenching agent. The acetylene is washed out of the resultant mixture by selective solvents.
The above principle for the preparation of acetylene/synthesis gas is disclosed in DE-A-44 22 815. In this process, the starting mixture is produced in a mixing chamber after separate pre-warming. The reaction subsequently takes place at a burner block in a combustion chamber. The reaction is terminated in a quench tank. Since acetylene is thermodynamically unstable at high temperatures and tends toward rapid decomposition, the acetylene-containing product mixture must be cooled suddenly (by direct quenching)—indirect cooling would be too slow.

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