Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-12-04
2003-06-17
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S630000, C062S935000
Reexamination Certificate
active
06578378
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns in general and in a first of its aspects, the chemical industry and, in particular, a method for high-yield recovery and purification of ethylene as well as other products originating from a gas produced by pyrolysis of hydrocarbons. This invention also concerns an installation and equipment for exploiting this method on an industrial scale.
BACKGROUND
A large number of papers and patents addressing the production, recovery, and purification of olefins show their industrial importance and the problems encountered in the exploitation of the various processes.
Recently, the production capacity of ethylene units has attained and even exceeded the level of 1 million tons per year for a single line; which requires a new approach in the design of the process, equipment, and the controllability of the unit.
In systems of recovery and purification, particularly for ethylene, the elimination of acetylene is a key element in purification. Because of its relative volatility with respect to ethylene and ethane, it cannot be separated by distillation. In industrial practice, only two processes are applied: absorption of acetylene by a solvent and hydrogenation to ethylene and ethane.
The first method involves the use of a solvent which is usually N,N-dimethylformamide (DMF) or N-methylpyrrolidone (NMP), which allows for preferential recovery of dissolved acetylene.
The second method, catalytic hydrogenation, is generally carried out by treatment of all the gas from cracking before separation of the hydrogen contained in it, or a separate treatment of the cuts containing C
2
hydrocarbons after addition of sufficiently pure hydrogen to transform all the acetylene into ethylene and ethane. These two types of hydrogenation use palladium-based catalysts with different formulations.
The stage of hydrogenation of acetylene has also been the subject of a number of papers and inventions dealing with the catalyst system and the formulations of the catalyst, and exposing the specific disadvantages connected with each of the hydrogenation technologies.
Thus, in the case of treatment of all the cracking gas originating from the pyrolysis of hydrocarbons in a hydrogenation reactor, a racing reaction may occur, corresponding to an acceleration of the kinetics of the reaction transforming the acetylene into ethylene (and also undesirable secondary reactions) because of a significant increase in the temperature of the catalyst together with the presence of a large excess of hydrogen (50 to 100 times the quantity required by stoichiometry). The ethylene can then be transformed into ethane and may thereby cause a significant rise in temperature, which requires immediate depressurizing of the reactor to prevent an explosion.
In the case of treatment of the C
2
cut alone, polymerization of the acetylene and progressive deactivation of the catalyst may occur, because of the large concentration of unsaturated hydrocarbons in the cut to be treated, which necessitates regeneration or periodic replacement of the catalyst charge. Generally, a reserve reactor is installed to avoid interrupting production. In addition, it is necessary to use a purified hydrogen current for the reaction, and these two aspects tend to increase the investments for reserve equipment or the equipment used only for the purpose described.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the known previous techniques by purification of the ethylene-rich fraction at an intermediate stage of the process.
Thus the invention concerns, according to one of its aspects, a process for fractionation of a large anhydrous gas resulting from pyrolysis of hydrocarbons containing hydrogen and hydrocarbons, particularly C
1
to C
3
hydrocarbons, including ethylene, propylene, and acetylene, and at least one current enriched with hydrogen and/or methane, at least one current enriched with ethylene and poor in acetylene, and at least one propylene-rich current, including stages wherein:
a) the gas resulting from the pyrolysis of hydrocarbons under pressure is cooled and liquefied progressively by passage into a series of increasingly colder heat-exchange zones. At least one condensate is separated from the pyrolysis gas after passage into each heat-exchange zone, at least one of the condensates being propylene-enriched and at least one other condensate being ethylene and ethane enriched, and containing in solution a smaller proportion of hydrogen, methane, and acetylene, and the residual hydrogen-rich gas is collected;
b) at least part of the ethylene- and ethane-enriched condensate and the propylene-enriched condensate is evaporated by a decrease in pressure. They are reheated, independently or not, in at least one of the heat-exchange zones by thermal exchange with fluids to be cooled, including at least the gas resulting from the pyrolysis, to provide, respectively, a fraction that is at least partly evaporated due to the reduction in pressure and reheating of the ethylene- and ethane-enriched fraction, and a fraction that is at least partly evaporated due to the reduction in pressure and the reheating of the propylene-enriched fraction, to provide at least part of the cold needed for cooling and for progressive liquefaction of at least said gas resulting from the pyrolysis of hydrocarbons upon passage into said successive heat-exchange zones;
c) the fractions which are at least partly evaporated, resulting from stage (b), are introduced into part of a distillation column called a de-ethanizer, the ethane- and ethylene-rich partly evaporated condensate being admitted into a point of the part of the distillation column called the de-ethanizer, higher than the propylene-enriched partly evaporated condensate, the part of the distillation column called the de-ethanizer operating under conditions of temperature and pressure allowing the separation, in an upper part, of a first current of ethylene- and ethane-enriched head gas containing, in a smaller proportion, acetylene, hydrogen, and methane, and in a lower part, a first bottom current of propylene-enriched fluid, which is collected;
d) the first current of ethylene- and ethane-enriched head gas from stage (c) in a zone of acetylene elimination by extraction with solvent and/or by selective hydrogenation of the acetylene by means of hydrogen containing in the first gaseous head current, to provide a current essentially devoid of acetylene, and
e) in the part of the distillation column called the de-methanizer, the gas current which is essentially devoid of acetylene from stage (d) is cooled and fractionated in a second hydrogen- and/or methane-enriched head gas fraction, which is collected, and a second bottom liquid fraction which is enriched with ethylene and ethane, and is essentially devoid of acetylene, and which is also collected.
The charge gas is generally essentially free of water to prevent deposits of ice in the low-temperature circuits. Thus, a water content lower than 10 ppm by volume, preferably less than 1 ppm, is desirable.
According to one of its aspects, the process according to the invention may use the gas current from the pyrolysis of hydrocarbons at a pressure of 15-50 bar, preferably 28-38 bar, and the distillation zone called the de-ethanizer may be at a pressure of 10-30 bar, preferably 14-24 bar, lower than the pressure of the pyrolysis gas.
According to one of its aspects, the process according to the invention may use evaporated condensates introduced into the part of the distillation column called the de-ethanizer; these condensates contain dissolved hydrogen in a proportion such that the first head gas current contains 2 to 10%, preferably 4 to 5%, in moles, of hydrogen, and stage (d) may be implemented by essentially ethylene-selective hydrogenation of the acetylene contained in the first head gas current by means of the hydrogen contained in the first head gas current of stage (c), the temperature of the hydrogenation zone being between 0 and 160° C., inclusive.
According to one of its aspects, the
Kaiser Victor
Laugier Jean-Paul
Simon Yvon
Capossela Ronald
Technip-Coflexip
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