Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
977~
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Purification and Reconcentration
of Waste Sulphuric Acid
This invention relates to apparatus and processes for
the purification and production of concentrated sulphuric
acid and, more particularly, to the production of concentrated
sulphuric acid from waste sulphuric acid using microwave
radiation.
Many processes in the chemical industry which use
sulphuric acid either as a reagent or solvent produce a waste
acid s~ream which must be dealt with. In some instances, it
is possible to re-~se the waste acid stream directly, but, in
the majority-of cases some upgrading treatment of the waste
acid must be carried out. However, because virgin sulphuric
acid is relatively cheap almost any regeneration scheme is
economically unattractive. While the disposal of waste acid
has long been a problem, a solution has become increasingly
urgent since these wastes, if not treated, present a threat
to the environment. Thus, due to these disposal concerns
processes have been developed to regenerate specific waste
acids.
Spent sulphuric acids can be broadly categorized into
two groups - that group whose principal contaminant is water
(e.g. sulphuric acid from drying operations), and that group
having principally an organic contaminant (e.g. sulphuric
acid from petroleum alkylation). To-date, no generally
accepted universal process has been developed suitable
for the recovery of sulphuric acid from both groups of
waste acids.
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The main problems to be addressed in any process are
the high energy costs associated with the removal o~ water
from the spent acid, the accelerated corrosion characteristics
associated with upgrading waste acids, and the conditions
required for the removal of inorganic and/or organic
contaminants.
Thus, processes which are capable of dealing with all
of the vast array of waste sulphuric acids produced are
extremely rare. In the case of slightly contaminated, but
dilute, waste acids, these have been treated by, i) the
Simonson-Mantius concentrator (brick-lined vessel under
vacuum), ii) Chemico drum concentrator (hot furnace gas
in direct contact with the waste acid), and iii) the
Pauling concentrator (acid is heated in cast iron vessels).
All of these processes are found to have certain limitations,
including the materials of construction limiting the
processing temperatures and formation of deposits on the
heating surfaces which impair the heating efficiency.
In the case o the treatment of waste acids which
contain high levels of non-volatile organic materials, this
usually involves the attainment of temperatures which effect
the the~mal decomposition of the acid to sulphur dioxide and
water, via sulphur t~ioxide, with subsequent re-oxidation of
the sulphur dioxide to sulphur trioxide for use in sulphuric
acid manufacture. This method is high in capital cost and
energy requirement. Decomposition of the waste acid to
sulphur dioxide for it to be re-oxidlzed to sulphur trioxide
is an expensive redundancy. Further, certain gaseous
impurities which remain in the sulphur dioxide gas after the cleaning
process may have a deleterious effect on the oxidation step
in a sulphuric acid plant.
The thermal decomposition of sulphuric acid is an
endothermic process and requires external heat energy. The
heat requirement amounts to 275 kJ/mol, in accordance with
the reaction equation:
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~2S04(1)_~ H20(g) ~ 1/2 02(g) + S02(g)
The composition is formed in two steps, namely:
H2S04 (1) ~ H2(g) + S03(g) ..... (1)
SO3(g) ~_ SO2(g) ~ 1/2 O2(g) ..... (2)
One method of recovering waste acid is to subject it to
thermal degradation to produce sulphur trioxide and water
vapour, and to recover said ga~ and water vapour. Some water
may be retained in the form of sulphuric:acid vapour, ~ut as
10 the degradation temperature increases, the equilibrium between
SO3, H2O and H2SO4 is overwhelmingly d~splaced towards SO3
and H2O. At 500C, there is almost complete degradat$on of
acid into H2O and SO3. At higher temperatures the unwanted
thermal decomposition of sulphur trioxide to sulphur dloxide
15 (equation 2) occurs. Thus, it can be see~ that ~n both of
equations 1 and 2 appreciable amounts of energy are requi~ed,
with the first st~p consuming more heat than the second.
One source of energy avallable for the heating of certain
materials ls that produced by microwave irradiatlon. Mlcrowave
20 energy in the 800-3000 M8z range has ~een widely used for
cooklng and reheating of foods in mlcrowave ovens. Frequencies
of approximately 915 and approximately 245~ MHz are the ones
primarily used in North America for thls purpose, although
other frequencies, notably 5850 MHz and 18000 MHZ9 are also
25 available. In Western Europe, 896 MHz is generally used, and
in Japan 100-450 kHz or 40-50 MHz is generally used.
An example of the industrial use o microwave radiation
has been described in U. K. Patent No. 1,567,129, in the name
of Foster Wheeler Energy Corporation. In Patent No. 1~567,129,
30 microwave radiation is used to desorb any adsorbed sulphur
dioxide from coal, coke or char by subjecting the coal and
the like to radiation at a frequency sufficient to cause
arcing, thereby oxidizing a portion of the carbon and thus
increasing the temperature of the coal~
However, despite the successful applicat1on of microwave
, ... ..
37~5
~ 4 - C-I-L 683
radiation to foodstuffs, the stigma of microwave equipment
poorly designed to meet the needs of the chemical process
industry has severely hampered its utility in this field.
5 In addition, microwave technology generally suffers from
the reputation of being prohibitively expensive.
Surprisingly, we have now discovered a process for the
purification and production of concentrated sulphuric acid
from waste sulphuric acid which is applicable to both dilute
10 and heavily organic-contaminated waste sulphuric acids. The
process further pro~ides reduced overall energy conswlption,
lower capital cost with improved simplicity of operation, and
reduced corrosion of acid recovery plant components.
A specific object of this invention is to provide a
15 method of utilizing microwave energy for the purification and
production of concentrated sulphuric acid.
~e have found that the application of microwave radiation
as an energy source to a body of waste sulphuric acid may
produce a two-fold result~
When dilute, slightly contaminated, waste sulphuric acid
is irradiated the acid is concentrated by the loss of water
vapour. In additio~ we have found that non-volatile organic
materials may be removed by steam distillation along with
excess water vapour to produce a concentrated and purified
25 sulphuric acid.
When waste sulphuric acid highly contaminated with
org~nic material is irradiated to a sufficient degree, sulphur
trioxide and water vapour, with relatively minor amounts of
sulphur dioxide, are liberated. The sulphur trioxide is
30 generally free of organic material and can be efficiently used
to manufacture fresh sulphuric acid.
We have found that the application of microwave energy
to spent sulphuric acid can be used for the purification
and/or concentration, as the case may be, of sulphuric ac~d
35 and thus provide a general universal process ~hich yields
-5- C-I-L 683
economic savings in plant cost, maintenance of operation
through reduced corrosion, and energy costs.
Accordingly, the invention provides a process for the
production of concentrated sulphuric acid from waste sulphuric
acid comprising subjecting said waste acid to microwave energy
for a sufficient period of time to effect production of water
vapour, whereby said waste acid is concentrated to a desired
degree; removing said water vapour; and collecting said
concentrated acid.
The removal of excess water from the waste acid can be
effected by microwave irradiation either in a batch process
or, preferably, in a continuous process. ~ batch system
generally involves the use of a reaction pot or vessel
inside a microwave oven and conta~ng the waste acid during
the irradiation step and means, such as a vacuum system,
for removing the excess water produced as vapour. After the
waste acid has been concentrated to the desired degree the
concentrated acid is removed from the pot as product. The
time required to achieve the re~uired concentration depends
upon expected parameters such as the nature and concentration
of the feed waste acid, acid strength desired and wavelength
of the microwave radiation used. Such parameters can be
easily determined by those skilled in the art.
In a preferred process, the waste acid is fed continuously
through a micro~ave field applicator to interact with the
microwave energy. In such a process, the degree of interaction
between the waste acid and the microwave field is controlled hy
the attenuation constant and penetration depth. These are, of
course, dependent on the dielectric properties of the waste
sulphuric acid. If it turns out that the penetra~ion depth
is either considerably less or considerably greater than the
dimensions of the volume of sulphuric acid subjected to
irradiation in the microwave field applicator, the efficiency
of the interaction between the microwave energy and the
waste acid will decrease. In this situation, special
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applicator techniques may be required to realize acceptable
utilization of microwave power. Penetration depth can be
controlled by wavelength, ~ . In simple systems, 915 MHz
energy has a penetration depth roughly three times that of
2450 MHz energy.
The concentrated acid product emerges from the base of
the applicator while the excess water, volatile, and possibly
non-volatile organic materials as steam distillate exit from
the top of the applicator.
Thus, in a preferred form the invention provides a
process as hereinbefore defined further comprising continuously
feeding said waste acid to a microwave applicator; subjecting
said waste acid to microwave energy in said applicator for a
sufficient period of time to effect production of water
vapour whereby said waste acid is concentrated to a desired
degree; removing said water vapour from said applicator; and
collecting said concentrated acid from said applicator.
The above processes may be simply modified to be applicable
to the production of concentrated and purified acid by
irradiating the waste acid in such manner as to not only boil
off the excess water but also to liberate sulphuric acid as
vapour in equilibrium with water and sulphur trioxide. This
modified process is highly desirable when the waste acid
contains relatively large amounts of organic or inorganic
contaminant. We have found that although the sulphur trioxide
liberated may have organic material present, it can be collected
and used to manufacture fresh sulphuric acid. In addition,
under the relatively mild conditions for the production of the
sulphur trioxide as described hereinabove there is a minimal
amount of degradation to sulphur dioxide caused by the reduction
of the sulphur trioxide by organic species in the vapour.
Accordingly, the invention further provides a process as
hereinbefore defined wherein said waste sulphuric acid contains
organic or inorganic contaminant and wherein said waste acid is
1~5~77~
~7- C-I-L 683
subjected to microwave energy for a sufficient period of
time to effect production of a gaseous mixture comprising
sulphuric acid, sulphur trioxide and water vapour, collecting
all or part of said gaseous mixture and producing sulphuric
acid therefrom.
It will be appreciated that when the process of the
invention is used for the purpose of concentrating sulphuric
acid by the removal of water vapour, only acid of a desired
strength up ~o the azeotropic mixture strength can be
obtained. Further irradiation will convert the liquid
azeotrope to the gaseous mixture. Thus, if the waste
sulphuric acid is clean and merely dilute, concentration
up to the azeotropic strength is possible, with the
concentrated product acid remaining behind. Further
irradiation of the clean concentrated acid is unnecessary
and wasteful of energy.
If the waste sulphuric acid is dilute and contaminated,
concentration of the acid up to the azeotropic strength
followed by continued irradiation forms the azeotropic
gaseous mixture contaminated with organic matter.
Condensation of this gaseous mixture will produce clean
sulphuric acid of azeotropic strength. If concentrated
contaminated acid of ~ azeotropic strength is
irradiated, condensation of the resultant gaseous mixture
will produce clean concentrated acid of similar strength
as that of originating waste acid.
It is well known that thereis extremely poor interaction
between microwave radiation and material in the vapour state.
This is because the molecules in the vapour state already
have appreciable amounts of rotational, vibrational and
translational energy, and also because the vapour cannot
provide a sufficient depth of penetration for the microwave
radiation to permit interaction with the molecules. Thus,
it is found that the temperature of the vapour does not
~597~5
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increase beyond that at which it is produced from the bulk
liquid. This is to be contrasted with the conventional art
heating methods wherein the coolest entity in the system is
the bulk liquid to be vaporised. Heating o~ the liquid
container wall in prior art processes causes the resultant
vapour to interact with said w~lls to effect incre~ses in
vapour temperature. It can thus be seen that there is a
reduced risk of causing sulphur trioxide vapour breakdown
to sulphur dioxide in the processes according to the
invention.
We have found that some organic materials are
unexpectantly removed under the influence of the microwave
energy. These materials may have been destroyed through
lS destruction in the strong acid, vaporised directly or, for
example, by steam distillation, or converted to a more
volatile or steam-distillable material. Such destruction
or removal is thus more possible with microwave energy than
with conventional energy processes since hotter, localised
temperatures are attainable in the bulk liquid phase since
no heat transfer surfaces are required as microwave heating
is independent of the container wall surface.
In the case of non-volatile inorganic contaminants, these
may be removed by subsequent processing, e.g. filtration of
cooled waste acid following irradiation. Such filtered
waste acid could be recycled back through the microwave
system.
The processes according to the invention can be carried
out utilizing equipment similar to that used in commercial
microwave heating in the food industry. The apparatus would,
of course, have to be modified because of the peculiarities
of sulphuric acid processing and the volumes of vapours evo]-ved.
Accordingly, in a further feature the invention provides
apparatus for the production of concentrated sulphuric acid
from waste sulphuric acid comprising container means for
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- 8a - C-I-L 683
containing said waste acid, microwave means for subjecting
said waste acid in said container means to microwave energy
for a sufficient period of time to effect production of
S water vapour whereby said waste acid is concentrated to a
desired degree, and means for removing said water vapour.
The container means can be a reaction pot or vessel or
preferably, a glass or quartz tubular reactor disposed within
a microwave applicator.
Thus, in a preferred feature the invention provides
apparatus as hereinabove defined wherein said container means
comprises a glass or quartz tubular reactor disposed within
a microwave applicator, and further comprising means for
feeding said waste acid to said tubular reactor and means
for collecting said concentrated acid product from said
tubular reactor.
In yet a further feature the invention provides
7~5i
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apparatus as hereinbefore defined modified for the production
of concentrated sulphuric acid from waste acid containing
relatively high amounts of organic or inorganic materials,
which apparatus comprises said microwave means capable of
subjecting said waste acid in said container means to micro-
wave energy for a sufficient period of time to effect
production of a gaseous mixture comprising sulphuric acid,
sulphur trioxide and water vapour; means for isolating all
or part of said gaseous mixture, and means for producin~
sulphuric acid from said isolated gaseous mixture.
An advantage of using microwave radiation accordtng to
the processes of the invention is that sulphur;c acid absorbs
the electxomagnetic energy uniformly, inside the container
means and not just at the surface of the container. The haat
buildup takes place extremely rapidly and is controlled,
almost instantaneously, by the power applied. Since the
energy transfer occurs by radiation rather than surface-to-
surface contact, the attainable temperature is limited
primarily by the thermal decomposition temperature of the
material being processed.
Not all materials absorb microwave radiation. Some
materials, such as metals, reflect microwaves, while others,
such as paper, glass and many plastics, transmit the waves
without interaction. Thus, in the process and apparatus
according to the inven~ion, any material which transmits
microwave radiation without interaction and is not attacked
by sulphuric acid represents a desirable material of
construction - glass or quartz are preferred examples. The
desirability of glass or glass-lined container means of use
in instant invention may be contrasted to prior art
sulphuric acid recovery systems wherein glass is generally
not employed because of the thermal shock that can be produced
due to the surface-to-surface heating and because of its
poor heat transfer coefficient. ~owever, in instant process,
77~
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these disadvantages are not present. Indeed, since glass is
more corrosion resistant than iron or, if sodium ions are
present in the spent acid, acid brick - especially at
elevated temperatures, glass Gffers an additional advantage.
Another advantage of using microwave radiation according
to the process of the invention is that relating to energy
costs. In the majority of the prior art techniques, steam
is consumed, in addition to the direct energy requirements
for the waste acid treatment, in order to maintain a vacuum
in the reactor and permit the reconcentration to occur at a
relatively lower temperature. The cost of this additional
steam requirement can be substantial and may be as high as
$10/tonne of waste acid and easily outweigh the fact that
the conversion of electric line power to microwaves is
approximately 75% e~ficient. The importance of this feature
can be clearly highlighted by the fact that the total power
C05t to generate sufficient microwave radiation to
` reconcentrate 70% ~2SO4to 98% is $10.60/tonne (electric
power = 2.6¢/kw-hr).
In the processes of the invention acid recovery
efficiencies as high as 95~ have been obtained. During
irradiation, exit gas temperatures of 100 - 320C have been
observed after five minutes of start up, with the temperature
obtained being dependent on the nature of the waste acid. In
all instances, the exit vapours (water and/or sulphuric acid
and sulphur trioxide) were colourless, even though it was
noted that some decomposition of organic material accompanied
the waste acid treatment. Waste acid as low as 20~ acid has
been concentrated to 96 - 97% strength. However, the
processes according to the invention may be used to
concentrate spent acid obtained from processes involving
pickling of metals and having an acid strength of, say, 8%,
to provide acid at the useful concentration of 18% strength.
~977s
~ C-I-L 683
Preferably, the microwave energy is applied at a
frequency of 915, 2450, 5850, or 18000 MHz.
In order that the invention may be better understood,
several embodiments will now be described by way of example
only and with reference to the drawings wherein:
Figure 1 is a schematic diagram of a preferred
apparatus and process according to the invention; and
Figure 2 is a schematic diagram of an alternate
embodiment o~ the apparatus and process ac~ording to the
invention.
Figure 1 shows a tubular quartz reactor (2.5 cm O.D.)
1 fitted inside a microwave applicator 2 linked to a
commercial microwave generator 3.
The generator 3 further comprises a slide screw tuner
4 to minimize reflected power, directional couplers 5 and 6
to sample forward and reflected power, and circulator 7 to
prevent reflected power from reaching microwave power source
8. Each of above form part of a waveguide 9. The waveguide
in the applicator terminates with a shorting plate 10.
The microwave applicatox is designed to efficiently
deliver the microwave field to the reactor 1. The dimensions
of the waveguide are designed to support a standing wave and
can be easily determined by those skilled in the art. In
the embodiment shown the WR340 waveguide dimensions of lS cm
long by 8.5 cm high by 4.2 cm deep support a standing
wave of 2450 MHz and provides an interaction zone of
approximately 30 cm.
Reactor 1 has a waste acid inlet 11 and outlet 12,
to which outlet 12 is attached lower and upper condenser
and receiving vessel systems 13 and 14,respectively, which
constitute a liquid-vapour separator system. A control
thermometer 15 is provided to signal a temperature
controller (not shown).
7~S
-12- C-I-L 6~3
In operation, chlorine drying waste dilute sulphuric
acid (70% H2SO4) was fed through inlet 11 to reactor 1 at a
rate of 30 g/min. Microwave power of 740 watts provided a
standing wave of 2450 MHz to the waveguide in applicator
chamber 2 sufficient to irradiate and concentrate the waste
acid through the loss of water vapour and inert gases to
~3.8% at a temperature of 292C. The concentrated acid
falls to the lower condenser and receiving system 14 for
recovery while the water vapour and gaseous impurities exit
through the upper condenser and receiving system 13.
An energy balance demonstrated that greater than 90%
of the microwave energy was transferred to the waste
sulphuric acid. A summary of the analytic work is as follows:
Before After
Acid strength, ~ 70 93O8
Chloride, ppm 6 3
Total Organic Carbon~ ppm 37 9
Free chlorine, ppm 2.8 2.5
Colour, Hazen units 30 30
When energy transfer to the waste acid and acid
temperature is sufficiently high sulphuric acid vapour is
liberated in equilibrium with sulphur trioxide and
water vapour. The acid and sulphur trioxide are recovered
in the upper condenser and receiving system 14 to generate
fresh sulphuric acid. By this latter approach the process
may be used to regenerate waste acid containing appreciable
amounts of contaminates. In this case the sulphuric acid
vapour, sulphur trioxide and water ~apour are collected in
upper system 13 whereas enhanced contaminated~coDcentrated
waste acid is collected in lower system 14. In one run
of such a process a waste acid containing 75% sulphuric
acid, 20% organic contaminant and 5~ water was irradiated
and resulted in the recovery of a mixture containing 94%
sulphur ~rioxide (g) and 6% sulphur dioxide (g)~ Other
spent acids such as alkylation waste acid and sulphonation
5~7~i
-13- C-I-L683
waste acids may produce varying but still favourable sulphur
trioxide/sulphur dioxide ratios. It can thus be seen that
this technique circumvents the necessity of the re-oxidation
of sulphur dioxide and is, thus, economically attractive.
Figure 2 shows an alternative embodiment of a process
and apparatus according to the invention involving a non-
continuous or batch process. It shows a Litton commercial
microwave oven chamber 1 modified to accept a glass
distillation pot 2 connected to a heat exchanger 3. In this
process, the glass vessel 2 contains waste sulphuric acid 4
irradiated with microwave energy (2450 MHz~ sufficient to
produce a stream of water vapour, sulphuric acid vapour,
sulphur dioxide, sulphur trioxide and a small amount of other
gases. These gases may be cooled by means of heat exchanger
3 and recovered. The relative concentrations of these
vapours in the stream is dependent upon the nature of the
waste acid.
In one specific run, chlorine drying waste acid
(164.3 g, 70% H2SO4) was heated for 50 minutes using 2450
MHz radiation. Within 2 minutes, vapours began to exit from
the oven at 100C, and after 20 minutes the temperature of
the vapour had risen to 230C. A summary of the analytic
work is shown, thus:
Before After
Acid strength, % 70 94.2
Chloride, ppm 4 C 0.31
Total organic carbon, ppm 30.5 3.89
Colour Yellow Colourless
The above embodiments describe continuous and batch
processes for the purification of sulphuric acid. The exact
conditions necessary to carry out the processes will be
dependent on factors such as the nature and concentration
of the feed waste acid, acid strength desired, acid
35 flow rate and wavelength of microwave radiation used,
and can be easily determined by those s~illed in the art.
