GB2051766A - Recovery of liquid sulphur trioxide - Google Patents

Abstract

Sulphur trioxide is recovered in liquid form by condensation from a gas containing sulphur trioxide. The gas is cooled to a temperature from 26.7 DEG C. to 32 DEG C. (80 DEG F. to 90 DEG F.) and continually introduced into a condenser whereby sulphur trioxide is condensed and continuously removed from the condenser surfaces as it condenses. The condensed sulphur trioxide is continuously removed from the condenser and melted to produce liquid sulphur trioxide. The process is particularly suitable for condensing sulphur trioxide from a gas containing relatively low concentrations thereof. It obviates the conventional step of absorbing the sulphur trioxide in sulphuric acid and distilling the resulting oleum and condensing the vapours of such distillation.

Description

SPECIFICATION Production of liquid sulphur trioxide This invention relates to a process for the production ofsulphurtrioxide in liquid form and more particu . larly to its recovery in liquid form by condensation from gases. It is particularly useful for the recovery of sulphurtrioxide in liquid form from gases relative ly dilute in sulphur trioxide, particularly such gases containing less than 15% sulphur trioxide by volume. The process of the invention provides a method by which relatively pure sulphur trioxide can be recovered in liquid form from gases containing sulphur trioxide and other gaseous components.

Liquid sulphurtrioxide is a valuable product, being useful, for instance, as a reagent for production of lalkylbenzene sulphonates and in tertiary oil re covery.

In general, at present liquid sulphur trioxide is obtained by absorption of sulphur trioxide from a gas containing the same in sulphuric acid to form oleum, followed by boiling the oleum in a separate operation and condensing the sulphur trioxide evolved. Such a condensing operation can betrou- blesome because of freezing of sulphur trioxide on the condenser tubes.The freezing point of sulphur trioxide is about 29.5"C (85"F). In general, in conde nsing sulphur trioxide from a gas containing the same, difficulties are caused by the fact that the temperatures required for appreciable recovery of sulphur trioxide by condensation are below this freezing point, resulting in the formation of solid sulphur trioxide, which can result in plugging of the equipment, requiring steps to clear the equipment and remove the sulphur trioxide. Solid sulphur trioxide may exist in three separate forms, generally referred to as the alpha-, beta- and gamma- forms, see for instance, Brasted, "Comprehensive Inorganic Chemistry", Vol. 8, pp. 134-138 (1961). The gamma form exists in ice-like crystals and is the most volatile and easiest to melt.The beta- and alpha-forms have asbestos-like crystals and are more difficult to melt, with the alpha-form being the most difficult. In general, when sulphurtrioxide is condensed, a liquid is first formed. This then becomes converted to the gamma-solid form, which can become further con verted to the beta- and alpha-forms.

In U.S. patent 2,510,684, there is provided a substantial amount of information concerning the conditions under which sulphur trioxide can be condensed from gases containing it. However, this patent requires that, to effect the condensation of the sulphurtrioxide, the gaseous mixture contain much more than 15% sulphur dioxide. Condensation under these conditions is affected by the use of pressure and relatively high concentrations of sulphur dioxide.

Sulphur trioxide gas is conventionally produced by combustion of sulphur or sulphur-containing wastes, such as sludges containing sulphuric acid, to produce sulphur dioxide, followed by catalytic ox idation of the sulphur dioxide to sulphur trioxide in the presence of a metallic catalyst. When the combustion is conducted with air, the gases leaving the sulphur dioxide converter generally contain from 5 to 13% sulphur trioxide by volume. If oxygen is utilized for combustion, the gas leaving the sulphur dioxide converter will contain a much greater concentration of sulphur trioxide, generally from 50 to 80% by volume, and a correspondingly smaller amount of inert gases (nitrogen, carbon dioxide, etc.).However, in order to condense this sulphur trioxide according to the process of U.S. patent 2,510,684, it would be necessary to conduct an incomplete conversion of the sulphur dioxide, in order that the gases leaving the converter contain at least 15% SO2. This would require recycle of the unconverted sulphur dioxide to the converter, which provides a disadvantage as opposed to oncethrough processes such as absorption of sulphur trioxide in sulphuric acid. It is particularly difficult to recover sulphur trioxide by condensation from gases containing it in relatively dilute concentrations, for example 15 vol.% or less.

This invention provides a process for recovering sulphur trioxide in liquid form from a gas containing sulphurtrioxide comprising: (a) Adjusting the temperature of the gas to a temperature of from 27.6"C to 32"C (80"F. to 90"F.); (b) continuously introducing the gas into a condensing apparatus maintained at a temperature of from 1 1"Cto -18 C (53"F. to 0 F.) whereby sulphur trioxide is caused to condense from the gas; (c) continuously removing condensed sulphur trioxide from the condenser surfaces; (d) continuously conveying the condensed sulphur trioxide out of the condensing apparatus; and (e) melting the condensed sulphur trioxide to produce liquid sulphurtrioxide.

The process according to the invention permits the recovery of sulphur trioxide in liquid form, from gases containing it, without requiring absorption into sulphuric acid and boiling and distillation of the oleum thus produced. The process also provides a satisfactory method of recovering sulphur trioxide in liquid form from gases containing relatively low concentrations of sulphur trioxide, by direct condensation.

The invention will be described with reference to a preferred embodiment thereof shown in the accompanying single figure of drawing which is a simplifiend flow sheet of the process of the process of the invention applied to the recovery of sulphur trioxide in liquid form from gases containing relatively low concentrations of sulphur trioxide, generally from 5% to less than 15% by volume. The principle of the invention is also applicable to the recovery of sulphurtrioxidefrom gases containing substantially greater concentrations of sulphur trioxide, for example, from gases produced by combustion of sulphur or sulphur-containing materials with oxygen or oxygen-enriched air (as opposed to air).The principle of the invention is equally applicable to the recovery of sulphur trioxide from gases containing the same resulting from any other source or method or production. In general, it is not now economically practical to recover su I phur trioxide from gases containing concentrations thereof of less than approximately 5% by volume, although the process described herein is capable of recovering sulphur trioxide from such gases containing it in low concentrations.

Referring now to the single figure of the drawings a gas containing up to 15% sulphur trioxide emanating from a source 1 and generally at an elevated temperature, is passed through line 2 into a heat exchanger 3, which may be cooled by introduction of water into the shell side thereof in line 26 and removal of heated water or steam in line 27. The gas thus cooled is then passed through line 4 into mist eliminator 5 in which acid mist is removed. The gas is then passed through line 6 into a heat exchanger 7 in which it is cooled to a temperature of from 26.7 C to 32"C (80"F. to 90 F.). The cooling is preferabiy accomplished by indirect heat exchanger with gases from which sulphur trioxide has been removed, introduced into heat exchanger 7 through line 14 and removed through line 15.

The thus cooled gas containing sulphur trioxide is passed through line 8 into a condensing apparatus indicated generally as 9, and maintained at a temperature of from 1 C. to -18"C. (53"F. to OOF.) preferably at 11"C. to -12"C. (53"F. to 10 F.) and most preferably at 5"C. to -4"C. (40"F. to 25"F.). This apparatus contains an interiorly located condensing section 10 surrounded by a cooling jacket 11. Within the condensing section 10 are contained helical scraper blades 12, driven by a motor 13, the blades being arranged so as to scrape the inner surface of the condensing chamber 10.The drawing depicts the condenser as being horizontally disposed. However, vertical or inclined condensers may be utilized.

The use of such a scraped surface condenser with helically arranged scraper blades enables the continuous removal of product sulphur trioxide from the interior surface of the consenser as it is condensed to be effected, and continuous conveying of the condensed sulphur trioxide through the condenser, preventing freezing and plugging of equipment.

Preferably, the helical blades are mounted on a drum 12a closed at both ends, the axis or rotation of which is coincidental with the long axis of the condenser chamber. This continuous removal and conveying are enhanced by warming the interior of the drum 1 2a, for example by electrical heating or by passing a warmer fluid (liquid or gas) through it, as is conventional in such equipment, to prevent sticking of the solid sulphur trioxide to the surfaces. Split flights and vanes may also be provided in the condenser, to break up lumps of solid. Preferably, the gas flow which enters is directed through a relatively narrow annulus existing between the cold condenser surface and the rotating drum 1 2a which bears the scraper blades 12.The exact clearance between the condenser and the rotating drum is a function of the desired gas velocity and the amount of sulphur trioxide expected to be condensed and removed, and is readily determinable by those skilled in the art. The amount of sulphur trioxide expected to be condensed under these circumstances depends on the concentration of the sulphur trioxide in the incoming gas and the temperature and the pressure of the environment, and is readily determinable by experiment.

The uncondensed gases are removed from the condenser 9 through line 14 and may be advantageously utilized to cool the incoming gas in heat exchanger 7. They are removed via line 15 and are either passed to treatment to remove pollutants and other undesirable material before being vented to the atmosphere, or passed to further processing, as discussed beiow.

The sulphur trioxide which condenses on the interior surfaces of the condensing chamber 10 is removed by the blades 12 and which carries it along the length of the chamber 10. The condensed sulphur trioxide may be in the form of predominantly gamma-crystals, or may be a slurry or "slush" of such crystals with liquid sulphur trioxide. The sulphur trioxide is removed from the condensing chamber via outlet 16 in which is located a star valve 17 which forms a rotating seal and also serves to extract condensed sulphur trioxide at a regular rate from the condenser. Outlet 16 and star valve 17 may be jacketed and heated in a similar manner to drum 12a, to prevent sticking of sulphur trioxide.The sulphur trioxide is conveyed by the star seal 17 into a melting tank 18 equipped with melting coils 19 through which may be introduced steam or hot water, and an agitator 26. Sulphurtrioxide is melted in melting tank 18 and liquid sulphurtrioxide is removed via line 20. Preferably the melting and removal are performed continuously.

The condenser 9 is maintained at the appropriate temperature bythe use of a liquid or gaseous refrigerant in a typicai refrigerant cycle. When using a gas as a refrigerant, it is introduced into the shell 11 of the condenser 9 via line 21 and removed via line 22, passed through a compressor 23, cooled in heat exchanger 24, passed through expansion valve 25 and returned to the shell 11. As the refrigerant there may be used any gas which will provide the satisfactory cooling effectforthe condenser, such as sulphur dioxide.Preferably the jacket temperature is maintained at approximately -4"C. (25"F.). A liquid refrigerant may be similarly used, without the em ploymentofan expansion valve; it will bevapou- rised in shell 11 by the condensation of the sulphur trioxide and re-liquefied in compressor 23.

The pressure of the condenser and other equipment utilized to process sulphurtrioxide gas is maintained substantially at atmospheric pressure.

However, pressures may be as high as 0.56-0.7 kg/cm2 gauge, (8-10 psig) or, in a high pressure plant, 5.25 kg/cm2 gauge (75 psig).

Sulphur trioxide thus recovered is substantially pure sulphurtrioxide, although small amounts of sulphur dioxide may also be condensed and contained in the product; thus, the process according to the invention is capable of producing a relatively pure liquid sulphurtrioxide product from gases containing relatively small amounts of this substance. In general, as known in the art, the condenser will only serve to condense a portion of the sulphur trioxide contained in the gases treated. However, the refrigerant gas utilized to cool the condenser 9 can be varied in order to recover more sulphurtrioxide.

For example, if additional recovery is desired, a refrigerant capable of producing lower gas temperatures in the condenser, such as dichlorodifluoromethane, could be utilized. Alternatively, the pressure at the inlet of compressor 22 could be lowered. These techniques for producing desired gas temperatures in a condenser by adjustment of a compression-expansion cycle are well known to those skilled in the art.

In general, recovery of additional sulphur trioxide from the uncondensed gases in line 15 is not advantageous since it would be necessary to compress such gases to a higher pressure. However, if such were desired, the gases would be re-introduced into the system via line 8. Preferably, the sulphur trioxide values in these gases would be utilized by passing the gases through line 15 into sulphuric acid drying tower or an absorption tower in an oleum plant. Alternatively, the gases could be passed through purification equipment to remove sulphur trioxide and/or other pollutants, and then vented.

The following Example further illustrates the invention.

In this Example the apparatus described in the drawing is employed.

Example A gas containing 8.16% by volume sulphur trioxide and having a flow rate of 100 Ib. mol. per hour (45.4 kg mol. per hour) and at a temperature of 177 to 260"C. (350-500"F.) is introduced into a water-cooled heat exchanger 3 in which it is cooled to a temperature of 54.4 C. (130"F). The gas is then passed into a mist eliminator 5 and then further cooled to a temperature of 26.7 to 320C. (80 to 90 F.) in heat exchanger 7 by heat exchange with exiting gases in line 14. The gas is then conveyed via line 8 into a condenser 9 equipped with helically arranged blades 12 mounted on a rotating drum 12a driven buy a motor 13. The condenser is equipped with a refrigerating jacket 11 supplied with pure sulphur dioxide refrigerant such that the jacket temperature is -4"C.

(25"F.) and the pressure is 0.31 kg./sq.cm. (4.38 psig).

In the condenser, the sulphur trioxide containing gas is cooled to - 1 'C (30"F.), whereupon approximately 30% of the sulphur trioxide in the gas is condensed, removed from the condenser through outlet, 16 melted in tank 18 and recovered as liquid product.

Claims (10)

1. A process for recovering sulphur trioxide in liquid form from a gas containing sulphurtrioxide comprising: (a) Adjusting the temperature of the gas to a temperature of from 26.7"C. to 32"C. (80"F. to 90"F.); (b) continuously introducing the gas into a condensing apparatus maintained at a temperature of from 11"C. to -18"C. (53"F. to 0 F.) whereby sulphur trioxide is caused to condense from the gas; (c) continuously removing condensed sulphur trioxide from the condenser surfaces; (d) continuously conveying the condensed sulphur trioxide out of the condensing apparatus; and (e) melting the condensed sulphur trioxide to produce liquid sulphurtrioxide.

2. A process as claimed in claim 1 in which the gas to be treated contains between 5 and 15% by volume of sulphurtrioxide.

3. A process as claimed in claim 1 in which uncondensed gas leaving the condenser is used to cool the gas in step (a) by indirect heat exchange.

4. A process as claimed in any of claims 1 to 3 in which the condenser is maintained at a temperature of from 11 C.to -180C. (53 F.to 10 F.).

5. A process as claimed in claim 4 in which the condenser is maintained at a temperature of from 5"C. to -4"C. (40"F. to 25"F.).

6. A process as claimed in any of claims 1 to 5 in which the condenser is maintained at approximately atmospheric pressure.

7. A process as claimed in any of claims 1 to 6 in which the condensed sulphur trioxide is continuously melted in step (e) and liquid sulphurtrioxide is continuously recovered from the melting step.

8. A process for the recovery of sulphurtrioxide in liquid form substantially as herein described with reference to the drawing.

9. A process for the recovery of sulphur trioxide in liquid form substantially as herein described with reference to the Example.

10. Sulphurtrioxide in liquid or other form when recovered by a process as claimed in any of claims 1 to 9.