PROCESS FOR THE MANUFACTURE OF SULFUR DIOXIDE

Abstract: A process is described for manufacture of sulfur dioxide in which molten sulfur and liquid sulfur trioxide are reacted in a reaction medium of oleum. The crude gaseous SO₂ produced is removed from the reaction zone and purified.

Description: PROCESS FOR THE MANUFACTURE OF SULFUR DIOXIDE The invention relates to an improved process for the manufacture of sulfur dioxide (SO2). Background Art The reaction between sulfur and sulfur trioxide (S03) to make S02 is known in the art. For example, U.S. Patent 2,813,007 describes a process for making S02 by reacting molten sulfur with an excess of sulfur trioxide. In the process described in this U.S Patent the reaction is conducted under such conditions that a portion of the SO3 is maintained in the liquid phase and SO2 is released as a gas. The crude SO2 is then rectified and fractionated under reflux conditions in a two column system to produce the SO2 product. Summary of the Invention According to the present invention there is provided a process for the manufacture of sulfur dioxide by the reaction of sulfur with sulfur trioxide characterised in that the reaction is conducted in a reaction medium of oleum and at a temperature and pressure at which the reactants are liquid. One practical form of the invention comprises continuously feeding molten sulfur and about a stoich iometric amount of liquid S03 into a heel of oleum in an agitated reaction zone, maintaining said zone 0 at temperatures from 50-200 C and pressures from 70-500 psig, withdrawing a crude gaseous S02 stream overhead from the reaction zone and feeding it to a single column purification zone to remove S03 and recover S02 as a product. The use of oleum as the reaction medium gives several advantages. One is that higher reaction temperatures can be employed, i.e., up to 2000C and this gives a higher rate of reaction and'permits a reduction in reactor size. The process also can be conducted under lower pressures, i.e., the process can be operated at the autogenous pressure of the SO2 in the condenser in the recovery section of the process. These lower pressures make it easier to recover and purify the S02 product. The process of the invention also avoids the problem of having a recycle of S203 and the need for equipment associated with such a recycle. A recycle is a feature of many of the prior art processes and ultimately it necessitates an undesirable release of SO2 to the atmosphere. Description of the Drawing The Figure is a flow chart of the process of the invention. Description of the Invention The process will now be described in reference to the Figure. The raw materials for the process are sulfur trioxide and sulfur. The S03 will be fed to the process as a liquid and will normally be at ambient temperature. In some cases the S03 may contain a stabilizer to prevent it from polymerizing into solid form. The sulfur will be fed to the reaction In the molten state, e.g., at a temperature of 125-1450C, and can contain the usual impurities associated with commercially available sulfur. The SO3 and sulfur are fed into a reactor which is heated by suitable means. In the Figure the reactor illustrated has a steam jacket into which steam is fed to control the temperature of the reaction. The reactor has suitable means for agitation to insure mixing of the reactants. The reaction takes place in a heel of oleum. By oleum is meant sulfuric acid containing 10-65, preferably 15-40%, even more preferably 20-30%, by weight of sulfur trioxide. This heel acts as a medium for the reaction and produces the advantages previously set forth. Initially the reactor should be filled with sufficient oleum to initiate the reaction. In operation, the S03 and sulfur are fed into the heel at a rate such that the volume of the reaction mass remains about constant. The feed is also regulated such that the sulfur and S03 are fed into the oleum in about stoichiometric amounts. The oleum is maintained at a temperature of 50-2000C, preferably 100-1500C, and most preferably, 120-1500C. The reactor and a condenser are maintained at the autogenous pressure of the SO2 gas produced by the reaction, i.e., within the range of 483-3447 kPa gauge (70-500 psig), preferably 552-1724 kPa (80-250 psig). The pressure can be maintained at the desired level by adjusting the flow of cooling water to the condenser in the purification portion of the process. Under these conditions the desired reaction to produce S02 takes place. This reaction is illustrated by the following equations: The overall reaction is: EMI3.1 This reaction occurs in two steps as illustrated by the following EMI4.1 The heat added to the oleum enables the reaction of step (ii) to take place. The reaction creates a crude S02 gas stream which contains some SO3, and this stream is withdrawn from the top of the reactor. A purge of sulfonated hydrocarbons produced from the organic impurities in the sulfur can be taken from the bottom of the reactor to prevent their buildup. The crude SO2 is fed into a rectification column, which can be part of the reactor or separate, as illustrated. The purpose of the column is to remove the S03 present in the crude 502. Normally, a column packed with ceramic saddles will be used to provide the desired intimate mixing necessary. Within the column the crude gaseous S02 is fed countercurrent to a liquid reflux stream. The contact between the two streams will take place at 35-1100C, preferably 45-800C, and under the autogenous pressure of the SO2, i.e., 483-3447 kPa (gauge). This contact can reduce the SO3'content of the S02 gas stream to a very small amount, e.g., less than 1 ppm. The liquid stream from the bottom of the column, containing SO2-SO3, will be fed back to the reactor. If desired, liquid SO3 can be fed into the bottom of the rectification column. This SO3 prevents carryover of sulfur from the reactor into the packing of the column. The purified SO2 gas is taken off at the top of the column and fed to a condenser. In this condenser, the SO2 is condensed to a liquid by indirect contact with cooling water. The liquid SO2 is then drawn off as a product, with a portion being fed to the column as reflux. The reflux ratio will normally range from 0.5-2. One of the advantages of the process of the invention is that the rectification procedure produces a commercially acceptable grade of SO2 without the need for further purification steps, i.e., the S02 product from the rectification need not be sent to a fractionation column. The process of the invention will now be illustrated by the following example: Into a reactor having a holdup of 95 liters is fed 181.4 kg of 20% oleum to form a heel. The reactor is heated to 1500C by a steam jacket and is pressurized to 1380 kPa (200 psig). Into the reactor is fed 113.4 kg/hour of liquid SO3 and 22.7 kg/hour of molten sulfur at 1400C. The reaction mass is agitated at the rate of 6.5 HP/3785 liters. The gaseous S02 produced is fed to a column packed with 305 cm of ceramic "Intalox" saddles. The column is operated at 720C and 1380 kPa (200 psig) and has a reflux ratio of 0.7. The gaseous SO2 taken from the column is condensed, part is refluxed and the rest removed as product.

Claims: CLAIMS 1. A process for the manufacture of sulfur dioxide by the reaction of sulfur with sulfur trioxide characterised in that the reaction is conducted in a reaction medium of oleum and at a temperature and pressure at which the reactants are liquid. 2. A process according to claim 1 wherein the oleum contains 10 to 65% by weight of sulfur trioxide. 3. A process according to claim 1 wherein the oleum contains 15 to 40% by weight of sulfur trioxide. 4. A process according to claim 1 wherein the oleum contains 20 to 30% by weight of sulfur trioxide. 5. A process according to any one of claims 1 to 4 wherein the sulfur and sulfur trioxide are employed in substantially stoichiometric proportions. 6. A continuous process for the manufacture of sulfur trioxide which comprises feeding molten sulfur and liquid sulfur trioxide to a reaction zone containing oleum, maintaining the reaction zone at a temperature 0 from 50 to 200 C and a pressure from 70 to 500 psig (480 to 3450 kPa) and withdrawing a crude gaseous sulfur dioxide stream overhead from the reaction zone. 7. A process according to claim 6 wherein the oleum contains 10 to 45% sulfur trioxide. 8. A process according to claim 6 or claim 7 wherein the sulfur and sulfur trioxide are fed to the reaction zone in approximately stoichiometric amounts. 9. A process according to any one of claims 6 to 8 wherein the crude gaseous sulfur dioxide is purified by feeding it to a single column purification zone. 10. A process according to claim 9 wherein the purification zone is a rectification column having a reflux ratio of 0.5 to 2.

Inventors: BURRUS, HARRY OTTO
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1980
Main classification: C01B17
IPC codes: C01B17