Process for the production and separation of hydrogen iodide and sulphuric acid and their respective uses in the production of hydrogen and oxygen

Abstract

 A process for the separation of hydrogen iodide and sulphuric acid produced in the reaction between sulphus dioxide, water and iodine (the Bunsen Reaction) when using liquid sulphur dioxide as a solvent includes the addition of a trialkyl phosphate or dialkyl-alkyl phosphonate to the reaction mixture to inhibit migration of the hydrogen iodide into the sulphuric acid phase.
The process may form part of processes respectively for the production of hydrogen and oxygen in which hydrogen iodide and sulphuric acid produced in the Bunsen Reaction are decomposed respectively to hydrogen and oxygen, thus providing a cycle of chemical reactions in which the net result is the production of hydrogen and oxygen from water.

Description

[0001] This invention relates to a process for the separation of hydrogen iodide and sulphuric acid produced in the Bunsen Reaction when using liquid sulphur dioxide as a solvent, and to the production of hydrogen and oxygen, respectively, by decomposition of.the hydrogen iodide and sulphuric acid so separated.

[0002] Hydrogen is an important intermediate product in the chemical industry where it is used in particular in the synthesis of ammonia and for hydrogenation processes, although it has many other uses. Consequently much effort has been directed over the years towards finding methods of producing hydrogen more cheaply.

[0003] One particularly promising possibility which has received much attention in recent years is the production of hydrogen, and also of oxygen, by means of cycles of chemical reactions in which the net result is the decomposition of water into hydrogen and oxygen. One such cycle of reactions which has been much discussed has the following steps:

[0004] Reaction (1) is well-known as the Bunsen Reaction. The Bunsen Reaction runs at ambient temperatures. Reaction (2) runs at a temperature of 400°C. and above, and reaction (3) runs at a temperature of 650°C. and above.

[0005] However, if unmodified, the Bunsen Reaction does not reach completion because at moderate sulphuric acid concentrations the reaction reaches an equilibrium. An additional problem is that the products of the reaction, hydrogen iodide and sulphuric acid, are very difficult to separate from each other.

[0006] Belgian Patent Specification No. 852674 proposes a separation process in which the Bunsen Reaction is carried out in liquid sulphur dioxide as a solvent. The sulphuric acid which is formed in the reaction is insoluble in liquid sulphur dioxide and consequently it forms a separate phase. However, when excess water is added in order to progress the Bunsen Reaction the sulphuric acid layer becomes more dilute and the hydrogen iodide produced in the reaction then tends to dissolve in the sulphuric acid phase rather than in the sulphur dioxide phase. Consequently the difficulty of separating the hydrogen iodide from the sulphuric acid again arises.

[0007] The Applicants have now discovered that if at least one water-insoluble organic compound having a high boiling point and selected from trialkyl phosphates and dialkyl-alkyl phosphonates and mixtures thereof, is added to the reaction mixture the tendency for the hydrogen iodide to migrate into the sulphuric acid phase is reduced and consequently the problem of separating the hydrogen iodide from the sulphuric acid phase is alleviated. The Applicants believe that this effect which they have discovered is due to the hydrogen iodide being solvated into the sulphur dioxide phase by the phosphate or phosphonate, together with some water, so as to form hydrate-solvate compounds in which the hydronium ion serves as a coordination centre. However, the Applicants do not wish to be bound by this theory of the mechanism by which the useful effect which they have discovered is obtained.

[0008] The phosphates and phosphonates used in the process of this invention have high boilding points by which is' meant that they boil at a temperature sufficiently high that their volatility at the temperature at which the process is carried out does not cause them,along with their useful effect, to be lost from the reaction mixture.

[0009] The alkyl groups of the phosphates and phosphonates may be the same or different. They will usually be lower alkyl groups, by which is meant generally containing no more than eight carbon atoms, and preferably no more than four carbon atoms. Either straight-chain or branched chain alkyl groups may be used.

[0010] It is a preferred feature of the process of this invention that the variable parameters of the process, including the quantity of the organic compound employed, are selected such as to produce a sulphuric acid phase having a concentration of at least 40% by weight of H₂S0₄, since it has been found that above this concentration of _H2S04 little if any hydrogen iodide tends to migrate into the sulphuric acid phase.

[0011] The phosphate or phosphonate may be diluted with an organic solvent, such as benzene, white spirit, dodecane or mesitylene, in order to enhance the effect of the phosphate or phosphonate,as is shown hereafter in the specific examples.

[0012] A particularly preferred organic compound for use in the process of this invention is tributyl phosphate (herein referred to as T.B.P.). An advantage of the use of tributyl phosphate is that at room temperature sulphur dioxide is quite soluble in it and consequently it is not necessary to use a pressure higher than normal atmospheric pressure in order to keep the sulphur dioxide in liquid form.

[0013] It will be understood that the scope of the present invention extends not only to a process for the separation of hydrogen iodide and sulphuric acid as described herein but also to processes respectively for producing hydrogen and oxygen comprising performing the separation process herein described and thereafter respectively decomposing the hydrogen iodide into hydrogen and iodine or the sulphuric acid into sulphur dioxide, water and oxygen. No invention is claimed in these decomposition procedures themselves, which may be performed by procedures well-known in the art and for which therefore the skilled man requires no detailed instruction. Consequently no detailed exemplification of these procedures are given.

[0014] Following however is a description by way of example of a specific separation process in accordance with the invention carried out on a laboratory scale followed by Tables in which are set out the results of different runs of the process using different process parameters. For the propose of this detailed description the organic compound used was tributyl phosphate and fixed amounts of iodine and sulphur dioxide were employed, namely respectively 10 g. iodine and 100 g. sulphur dioxide, except where otherwise indicated. The procedure was as follows:-

[0015] The iodine, sulphur dioxide, organic compound and organic solvent diluent, if employed, were mixed together and the water added dropwise with stirring until the Bunsen Reaction was complete. The completion of the reaction was indicated when the reaction mixture became clear. At this part two phases had been formed, an upper aqueous phase, the sulphuric phase and a lower sulphur dioxide phase, the sulphurous phase. The two phases were then separated from one another.

[0016] The sulphur dioxide phase contains most, if not all, of the hydrogen iodide, and a small amount of sulphuric acid. This phase is first washed with a little water to remove the sulphuric acid. The washings could then be recycled to the main process. Most of the sulphur dioxide is then removed by evaporation and the hydrogen iodide extracted with water. The extracted solution may then be fed to an electrolytic cell for decomposition of the hydrogen iodide to hydrogen and iodine. Complete electrolysis is not required because it is advantageous to have some hydrogen iodide remaining in which the iodine formed can be dissolved. Iodine is very soluble in hydrogen iodide solutions. The resulting iodine solution is then recycled for a successive hydrogen iodide extraction.

[0017] The aqueous sulphuric phase contains most of the sulphuric acid produced in the reaction and a little hydrogen iodide if the concentration of H₂S0₄ is below 40% by weight. This phase is readily concentrated from its usual concentration of around 50-55% e.,g. by a multi-effects evaporator, to 65-70% and then thermally decomposed to SO₂, H₂0 and O₂.

[0018] Procedures for recovering hydrogen and oxygen respectively from the hydrogen iodide and sulphuric acid produced in the Bunsen Reaction are described in German Offenlegungsschrift No. 2516441.

[0019] In the accompanying Tables are set out the results of runs of the afore-described specific procedure carried out by way of experiment using different amounts of tributyl phosphate without and with various organic solvent diluents. In Table I the reaction temperature was 263⁰K whereas in Table II the reaction temperature was in the range from 303°K to 313°K. In fact there was little difference in the reaction and extraction steps when carried out at the different temperatures.

[0020] The details in the Tables are largely self-explanatory. However, the following should be mentioned:

[0021] In the Tables the name of the diluent in column3 is indicated by a reference number.

ref. 1 means : no diluent

ref. 2 means : benzene

ref. 3 means : white spirit having a boiling range 40-60°C

ref. 4 means : dodecane

ref. 5 means : mesitylene

ref. 6 means : white spirit having a boiling range 60-80°C

ref. 7 means : white spirit having a boiling range 80-100°C

ref. 8 means : mesitylene

ref. 9 means : white spitit having a boiling range 80-100°C

[0022] Run 38 was performed at a pressure higher than atmospheric pressure using 60 g. of SO₂.

[0023] In Table II, column 4, the number in brackets means that instead of 100 g. S0₂ the indicated amount has been used.

[0024] The H₂S0₄ concentration in the sulphuric phase depends on the amount of T.B.P. used. When its concentration is above 40% by weight, no hydrogen iodide is present, nor T.B.P. which is water insoluble.

[0025] The amount of sulphur dioxide required by the reaction can be reduced by increasing the amount of T.B.P. or water, or otherwise by increasing the reaction time and the stirring rate. In fact in all of these experiments the water added for the reaction could be reduced by at least 20% if the reaction time had been longer. Consequently the H₂SO₄ concentration becomes higher.

Claims

1. A process for the separation of hydrogen iodide and sulphuric acid produced in the Bunsen Reaction when using liquid sulphur dioxide as a solvent characterised in that at least one water-insoluble organic compound having a high boiling point and selected from trialkyl phosphates and dialkyl-alkyl phosphonates and mixtures thereof, is added to the reaction mixture.

2. A process as claimed in claim 1 wherein the variable parameters of the process,including the quantity of the organic compound employed, are selected such as to produce a sulphuric acid phase having a concentration of at least 40% by weight H₂SO₄.

3. A process as claimed in claim 1 or claim 2 wherein the organic compound or one of the organic compounds employed is tributyl phosphate.

4. A process as claimed in any one of the preceding claims wherein the organic compound is diluted with an organic solvent.

5. A process for producing hydrogen which process comprises performing the process as claimed in any one of the preceding claims and thereafter decomposing the hydrogen iodide into hydrogen and iodine.

6. A process for producing oxygen which process comprises performing the process as claimed in any one of claims 1 to 4 and thereafter decomposing the sulphuric acid into sulphur dioxide, water and oxygen.