PROCESS AND PLANT FOR THE CONDENSATION OF SULFUR TRIOXIDE FROM HOT STARTING GASES

The present invention relates to a process and a plant for the condensation of sulfur trioxide from hot starting gases containing sulfur trioxide. Such processes are usually employed for separating and recovering sulfur trioxide from gases containing sulfur trioxide.

On a technical scale, sulfur trioxide generally is obtained from oleum, i.e. con- centrated sulfuric acid with sulfur trioxide dissolved therein. For this purpose, the sulfur trioxide dissolved in the oleum is first expelled from the oleum in an evaporator, before the sulfur trioxide is separated from the produced gas mixture rich in sulfur trioxide by condensation in a condensation device. For producing the starting gases rich in sulfur trioxide, oleum with a concentration of free sulfur trioxide between 28 and 64 wt-% is conventionally leaned to oleum with a concentration of free SO ₃ between 5 and 30 wt-%. As heat transfer medium for the evaporator, there is usually employed process gas from a sulfuric acid plant or heating air from a heating-air generator. Due to their lower oleum inventory, the evaporators frequently are designed as falling-film apparatuses, in which because of their continuous operation as compared to other types of evaporator fewer liquid particles are entrained by the gas stream, so that the gas rich in sulfur trioxide contains fewer impurities.

For the condensation of the sulfur trioxide from the hot starting gases rich in sulfur trioxide, there is usually employed a condensation device in the form of a shell-and-tube or falling-film heat exchanger operated with water as cooling medium, in which the starting gas is first cooled to saturated-steam temperature, and subsequently the SO ₃ is condensed out of the cooled starting gas. However, the use of water as cooling medium in the condensation device involves some safety risks, as in the case of damages to the coolant circuit water can be

mixed with the sulfur trioxide and hence a strongly exothermal reaction can occur. Apart from this, the dual function of the condensation device, i.e. cooling of the hot starting gas rich in SO ₃ and subsequent condensation of sulfur trioxide from the cooled starting gas, allows no optimum design of the condensation device.

Therefore, it is the object underlying the present invention to provide a process for the condensation of gaseous sulfur trioxide from a hot starting gas containing sulfur trioxide, which is sufficiently safe even in the case of damages in the coolant circuit, supplies liquid sulfur trioxide of high purity, which is in particular largely free of water and nitrosyl sulfuric acid, and can be operated optimally in terms of energy efficiency.

In accordance with the invention, this object is solved by a process as men- tioned above, which comprises the following steps:

a) cooling the starting gas to a temperature of 40 to 45°C in at least one heat exchanger, in which a gas, preferably air, is used as cooling medium, b) introducing the cooled starting gas into a condensation device, c) condensing the sulfur trioxide in the condensation device, in which a liquid chemically inert to sulfur trioxide is used as cooling medium, and d) withdrawing the condensed sulfur trioxide from the condensation de- vice.

Due to the fact that at least one heat exchanger for cooling the hot starting gas is provided upstream of the condensation device, in which both the cooling of the hot gases containing SO ₃ and the condensation of the sulfur trioxide is ef- fected in accordance with the known processes, the energy demand of the total

process can be decreased significantly, as the at least two parts of the plant can each be designed optimally for their respective purpose. In the heat exchanger, all components with a dew point between 50 and 120 ⁰ C are removed, so that by means of the process of the invention liquid sulfur trioxide of high purity is ob- tained, which is at least substantially free of water and other impurities, such as nitrogen oxides. In addition, by omitting water as cooling medium, a safe procedure is achieved, as even in the case of damage to the coolant circuit no significantly exothermal reaction can occur between the cooling medium and the sulfur trioxide. Furthermore, the use of gas as cooling medium in the heat exchanger provided upstream of the condensation device provides for utilizing the cooling gas withdrawn from the heat exchanger for heating purposes in other parts of the plant used in the process or in other plants, such as in a conventional sulfuric acid plant. On the whole, the process in accordance with the invention thus provides for the safe and inexpensive production of liquid sulfur trioxide of high quality.

In principle, the process of the invention can be used for the condensation of sulfur trioxide from all hot gases containing sulfur trioxide, independent of their SO ₃ concentration and of the kind and quantity of possible impurities. Particu- larly good results are obtained, however, when a gas mixture produced by heating oleum with a content of dissolved sulfur trioxide of 28 to 64 wt-% in an evaporator is used as starting gas. In this case it turned out to be expedient to dispose the condensation plant downstream of a sulfuric acid plant.

In accordance with a development of the invention it is proposed to connect two heat exchangers in series for cooling the starting gas before introducing the same into the condensation device. In this way, the process can easily be controlled such that in the first heat exchanger impurities in the starting gas, in particular those with a dew point between 50 and 120 ⁰ C, are removed particu- larly reliably by condensation, and in the second heat exchanger the gas rich in

- A -

sulfur trioxide is adjusted to an optimum temperature for the final condensation to obtain liquid sulfur trioxide. Thus, liquid sulfur trioxide of particularly high quality is obtained.

In accordance with another particular embodiment of the invention it is provided to use a gas chemically inert to sulfur trioxide, preferably air, as cooling medium both in the first and in the second heat exchanger. As a result, undesired reactions between the starting gas and the cooling medium, which can damage the coolant device, are reliably avoided on the one hand, and on the other hand the heated coolant gases can be used as heating gas in other parts of the plant, and the energy demand of the process can thus be decreased. A particularly effective procedure is obtained when the first heat exchanger is operated countercurrently and the cooling medium has an inlet temperature of 25 to 35°C.

Condensate obtained in the first heat exchanger when cooling the starting gas, which in particular includes compounds with a dew point between 50 and 120 ⁰ C, is preferably recirculated to the oleum evaporator, in order to expel sulfur trioxide contained in the condensate and increase the total yield of the process.

In accordance with a development of the invention it is proposed to cool the starting gas in the first heat exchanger to a temperature between 50 and 70 ⁰ C and in the second heat exchanger to a temperature between 40 and 45°C.

Preferably, the condensate possibly obtained in the second heat exchanger, in dependence on its sulfur trioxide concentration, which can be adjusted by the procedure, is either recirculated to the evaporator for again expelling the sulfur trioxide contained in the condensate or is introduced into the condensation device.

In accordance with another particular embodiment of the present invention it is proposed to use a heat transfer oil chemically inert to SO ₃ as cooling medium in the condensation device. Due to their high thermal stability and their higher thermal capacity as compared to gases, heat transfer oils are excellently suited for cooling the gas to an extent sufficient for the condensation of sulfur trioxide, even with a high volume flow rate of the gas passed through the condensation device. As these heat transfer oils are chemically inert to SO ₃ as compared to water, a high process safety is ensured even in the case of damages inside the coolant circuit.

In accordance with a development of the invention it is proposed to use a heat transfer oil with a viscosity as measured at 30°C between 0.1 and 100 mPa-s, preferably between 0.5 and 20 mPa-s, and particularly preferably between 1 and 1.2 mPa-s, as cooling medium in the condensation device.

Particularly good results are obtained when a heat transfer oil based on silicone oils is used as cooling medium in the condensation device.

In accordance with the present invention, heat transfer oils based on silicone oils, in particular those based on polydimethyl siloxanes, as they are sold for instance by the firm Dow Chemicals under the trade names SYLTHERM ^® XLT ₁ SYLTHERM ^® 800 and SYLTHERM ^® HF, likewise turned out to be useful for this purpose.

Quite particularly preferably, SYLTHERM ^® XLT is used as cooling medium in the condensation device.

When using other heat transfer oils on a commercial scale, laboratory experiments must be made to determine their reactivity with liquid SO ₃ . In accordance with the invention, the heat transfer oils should have a low reactivity with liquid

SO ₃ , so that no exothermal reaction can occur. This can be determined by means of a measurement. Silicone oils exhibit no quick exothermal reaction with liquid SO ₃ .

As condensation device for the process of the invention, all apparatuses known to those skilled in the art for this purpose can be used in principle, in particular shell-and-tube heat exchangers, plate-type heat exchangers and falling-film heat exchangers.

In accordance with a development of the invention, it is proposed to operate the condensation device cocurrently, the cooling medium preferably having a temperature between 25 and 35°C in this case.

Alternatively, the condensation device can also be operated countercurrently, the cooling medium preferably having a temperature between 30 and 35°C in this case.

In accordance with another particular embodiment of the present invention, the cooling medium of the condensation device is guided in a closed circuit and upon discharge from the condensation device is first recooled in a liquid-liquid heat exchanger, before it is again introduced into the condensation device.

Another subject-matter of the present invention is a plant for the condensation of sulfur trioxide from a hot starting gas containing sulfur trioxide, which can in particular be used for performing the process of the invention, comprising:

a) a first heat exchanger comprising supply and discharge conduits for coolant and starting gas containing sulfur trioxide, in which a gas, preferably air, is provided as cooling medium,

b) a second heat exchanger comprising supply and discharge conduits for coolant and starting gas containing sulfur trioxide, in which a gas, preferably air, is provided as cooling medium, and c) a condensation device, in which a liquid chemically inert to sulfur tri- oxide is provided as cooling medium.

Preferably, an oleum evaporator is provided upstream of the first heat exchanger of the plant in accordance with the invention.

The invention will subsequently be explained in detail with reference to embodiments and the drawing. All features per se or in any combination form the subject-matter of the invention, independent of their inclusion in the claims or their back-reference.

The plant as shown in the only Figure for the condensation of sulfur trioxide from a hot starting gas containing sulfur trioxide, which is produced by expelling sulfur trioxide from oleum, comprises an oleum tower 1 as storage tank for oleum, an oleum evaporator 2, two heat exchangers 3, 4, and a condensation device 5.

For producing the starting gas, oleum is first withdrawn from the oleum tower 1 , in which oleum with a content of free sulfur trioxide between 28 and 64 wt-% is circulated for cooling purposes via conduit 6 and the oleum circuit cooler 7, and is then supplied via conduit 8 to an oleum preheater 9 for preheating purposes, before the preheated oleum is introduced into the oleum evaporator 2 supplied with cooling gas via conduit 10, where it is further heated, until a large part of the sulfur trioxide dissolved in the oleum has been expelled from the oleum. While the leaned oleum with a content of free sulfur trioxide between 5 and 30 wt-% is recirculated to the oleum preheater 9 via conduit 11 , the expelled start- ing gas containing sulfur trioxide with a temperature between 100 and 180 ⁰ C is

introduced into the first heat exchanger 3 via the gas conduit 12, in which heat exchanger the starting gas containing sulfur trioxide is countercurrently cooled down to 50 to 70 ⁰ C by means of air, which at the inlet of the heat exchanger 3 has a temperature between 25 and 35°C. Condensate obtained in the first heat exchanger 3 is recirculated via a conduit (not shown) to the oleum evaporator 2, whereas the heated cooling air is discharged from the first heat exchanger 3 and utilized for heating purposes. Via the gas supply conduit 13, the precooled gas rich in sulfur trioxide is introduced into the second heat exchanger 4, where it is further cooled to 40 to 45°C by using air as cooling medium, before it is intro- duced into the condensation device 5 via the gas supply conduit 13'. Condensate obtained in the second heat exchanger 4 can selectively be supplied to the oleum evaporator 2 or to the condensation device 5.

In the condensation device 5, which preferably constitutes a shell-and-tube heat exchanger or a plate-type heat exchanger, the SO ₃ is condensed out of the gas by a suitable cooling medium, preferably by a heat transfer oii chemically inert to sulfur trioxide. The cooling medium is circulated via the conduits 14, 14' and the thermal recooler 15, which for instance constitutes a spiral heat exchanger or a plate-type heat exchanger, in which the cooling medium is recooled to a tem- perature desired for operating the condensation device 5 by using cooling water normally present in the plant. If the condensation device 5 is operated cocur- rently, the cooling medium preferably has a temperature between 25 and 3O ⁰ C at the inlet of the condensation device 5, in the case of a countercurrent operation, however, preferably a temperature between 30 and 35°C.

The liquid sulfur trioxide condensed in the condensation device 5 can be discharged, for instance by gravity, to a storage tank (not shown) via corresponding conduits which constitute double-walled conduits and are connected to the high- temperature end of the cooling circuit.

Example

A condensation plant as shown in the Figure, which is provided downstream of a conventional sulfuric acid plant with a capacity of 108 tons per day, was oper- ated continuously.

In the oleum evaporator 2, oleum was heated to a temperature of about 120 ⁰ C for expelling the sulfur trioxide dissolved therein, and the produced starting gas rich in sulfur trioxide was supplied to the first heat exchanger 3 with a tempera- ture of about 135°C and cooled down therein to about 60 ⁰ C. In the second heat exchanger 4, the gas then was cooled to about 40 ⁰ C, before it was supplied to the condensation device 5, which was operated with SYLTHERM ^® XLT as cooling medium, a heat transfer oil based on silicone oil commercially available from Dow Chemicals, and was condensed.

There were obtained 18 tons of condensed sulfur trioxide per day.

List of Reference Numerals:

1 oleum tower

2 oleum evaporator 3 first heat exchanger

4 second heat exchanger

5 condensation device

6 oleum circulating conduit

7 oleum circuit cooler 8 oleum supply conduit

9 oleum preheater

10 cooling gas supply conduit

1 1 oleum return conduit

12 starting gas supply conduit 13,13' gas supply conduit

14,14' coolant circulating conduit

15 thermal recooler