[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.
[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 H
2S0
4, 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 H
2S0
4 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 SO
2, H
20 and O
2.
[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 263
0K 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
SO
2.
[0023] In Table II, column 4, the number in brackets means that instead of 100 g. S0
2 the indicated amount has been used.
[0024] The H
2S0
4 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.
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.