Gas: heating and illuminating – Apparatus for converting or treating hydrocarbon gas
Reexamination Certificate
1999-05-03
2001-04-03
Tran, Hien (Department: 1764)
Gas: heating and illuminating
Apparatus for converting or treating hydrocarbon gas
C048S128000, C422S170000, C422S186220, C422S234000, C095S235000, C423S226000, C423S242200, C423S575000, C423S715000
Reexamination Certificate
active
06210454
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns an apparatus for treating a gaseous effluent from a Claus plant or a gas containing hydrogen sulphide and sulphur dioxide.
In particular, it concerns the treatment of effluents from Claus plants particularly from hydrodesulphurization and catalytic cracking units. It also concerns the purification treatment of natural gas.
The prior art is illustrated by French patent applications FR-A-2 411 802 and FR-A-2 336 163.
The Claus process is widely used to recover elemental sulphur from gaseous feeds containing hydrogen sulphide (H
2
S). However, the fumes emitted from these Claus type plants, even after several catalytic stages, contain non negligible amounts of acid gases. Those effluents (tail gases) from Claus plants must, therefore, be treated to eliminate the majority of toxic compounds so as to satisfy anti-pollution regulations. These regulations are becoming more and more strict and existing technology must be constantly improved.
As an example, about 95% by weight of the sulphur present can be recovered from a Claus plant; treatment of this Claus plant tail gas (using a Clauspol unit, for example) can recover 99.8% by weight of the sulphur, for example, using the reaction:
2H
2
S+SO
2
3S+2H
2
O
which uses a reaction medium constituted by an organic solvent and a catalyst comprising an alkaline or alkaline-earth salt of an organic acid. The reaction is generally carried out in counter-current mode in a reactor-contactor and its temperature is controlled by passing the solvent which is extracted from the lower end of the reactor by a circulating pump through a heat exchanger to encourage the highest possible degree of conversion to sulphur while avoiding the formation of solid sulphur. Sulphur is thus recovered in liquid form. While it is very effective, the process is limited by various constraints:
The thermodynamic equilibrium of the reaction is such that the reaction is never complete. Some hydrogen sulphide and sulphur dioxide remains, in equilibrium with the sulphur and water which are formed. The quantity of sulphur present in unreacted H
2
S and SO
2
which is found in the reaction effluent (from the Clauspol) corresponds to about 0.1% of the total sulphur in the initial feed to the Claus plant. Better conversion can be envisaged at a lower operating temperature but this temperature must be kept above the freezing point of sulphur (about 120° C.) otherwise the reactor will be blocked with solid sulphur;
The presence of unseparated liquid sulphur in the reactor-contactor, which is entrained in the solvent and catalyst which circulate, and which is recycled to the reactor-contactor. Not all of the droplets of liquid sulphur are separated from the solvent and the presence of liquid sulphur irremediably causes the presence of gaseous sulphur in the effluent due to the vapour pressure of sulphur. As an example, the quantity of unrecovered sulphur which can be attributed to vapour pressure is about 0.1% by weight of the sulphur in the initial feed.
OBJECTIVES OF THE INVENTION
The aim of the invention is to overcome the disadvantages of the prior art.
A further aim of the invention is to satisfy the strictest regulations designed to counter atmospheric pollution by sulphur-containing compounds.
A yet still further aim of the invention is to be able to modify existing installations with a Claus plant and a unit for treating the effluents from that unit (a Clauspol unit), at very low cost.
It has been shown that by eliminating all of the sulphur vapour from the effluents from gas treatment units, for example Claus plant tail gas, up to 99.9% of the total sulphur can be recovered and thus the quantity of sulphur discharged into the atmosphere when the gas is incinerated can be minimised.
PROCESS ASPECT OF PARENT APPLICATION
More precisely, the invention concerns a process for the treatment of a gas containing hydrogen sulphide and sulphur dioxide, in which the gas is brought into contact with an organic solvent containing a catalyst in at least one gas-liquid reactor-contactor at a suitable temperature, and a gaseous effluent which substantially no longer contains hydrogen sulphide and sulphur dioxide but which contains sulphur in vapour form is recovered, the process being characterized in that the gaseous effluent from the reactor-contactor is brought into contact with the same organic solvent or with another organic solvent at a temperature which is lower than the solidification temperature of sulphur (for example 95° C.) in a contactor-cooler.
In more detail, the gaseous effluent is cooled in the presence of at least one organic solvent in at least one cooling zone (
7
) so as to obtain a suspension of sulphur crystals in said solvent, the sulphur is separated from the solvent in at least one separation zone (
3
) and a gas containing water and substantially no longer containing sulphur vapour is recovered, also a solvent is recovered which is depleted in sulphur which is recycled to the cooling zone (
7
), and sulphur (
15
) is recovered from said separation zone.
The organic solvent can be cooled in different manners:
In a first variation, if the organic solvent is miscible with water, it can be cooled by heat exchange in a heat exchanger before being mixed with the gaseous effluent to be purified, by adding water at a temperature which is lower than that of the organic solvent, wherein the heat of vaporisation on contact with the gaseous effluent can reduce the temperature of the mixture, or by a combination of the above two steps. Cooling is preferably by injection of water.
In a second variation, if the organic solvent is not miscible with water, it can be cooled in the same manner as in the first variation. Cooling is preferably by heat exchange.
The following different types of solvents can be used:
In the category of solvents which are insoluble in water are hydrocarbons with boiling points at atmospheric pressure of more than 200° C., preferably dodecane, tridecane, and naphtha with boiling points in the range 225° C. to 335° C.
In the category of solvents which are soluble in water, with boiling points at atmospheric pressure of more than 200° C. are polyols containing 3 to 15 carbon atoms, preferably glycerol, thioglycol and cyclohexanedimethylethanol, acid esters containing 5 to 15 carbon atoms, more particularly trimethylpentanediol mono-isobutyrate and dimethyl adipate, glycol ethers containing 5 to 15 carbon atoms, advantageously butoxytriglycol, ethoxytriglycol, diethylene glycol butylether, ethylene glycol phenylether, terpinyl ethylene glycol monobenzyl ether, ethylene glycol butylphenylether, diethylene glycol, diethylene glycol dimethylether, diethylene glycol dibutylether, triethylene glycol, tetraethylene glycol dimethylether, propylene n-butylether, dipropylene n-butylether, tripropylene n-butylether, and polyethylene glycol with a molar mass of 200, 300, 400 or 600.
The gaseous effluent in contact with the organic solvent containing water can be cooled to a temperature which is lower than the melting point of sulphur, preferably between 50° C. and 100° C. The water contributes to this cooling as it can partially evaporate from the solution in contact with the hot gas.
During cooling, the sulphur vapour solidifies and in accordance with a further feature of the invention, the three-phase mixture of gas, solid sulphur and liquid organic solvent is sent to a contactor where the liquid captures all of the solid particles which are in suspension.
In a further feature of the invention, sulphur can be separated from the solvent for recycling to the principal reactor-contactor in liquid form after melting, or it can be recovered in a minimum quantity of solvent.
The solvent, depleted in sulphur, leaving the separation zone can be introduced into at least one filter or hydrocyclone to free it of at least a portion of the sulphur crystals before being introduced into the contact and cooling zone.
Advantageously, the solvent phase recovered from the separation zone is recycled to the cooling
Dezael Claude
Lecomte Fabrice
Viltard Jean-Charles
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Ridley Basia
Tran Hien
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