Note: Descriptions are shown in the official language in which they were submitted.
~15~Z285
This invention relates to a process and an apparatus for regenerating
contaminated sulphuric acid in several stages using indirect heating in apparatus
of, or at least coated with, enamel. In the context of the invention, the term
"enamel" applies to glass-like coatings which are acid-resistant and are capableof withstanding fluctuating temperatures.
More particularly, the present invention provides a process for regen-
erating contaminated sulphuric acid using indirect heating in apparatus of, or at
least coated with, enamel which is characterised in that the acid is first con-
centrated in an indirectly heated, single-stage or multiple-stage preconcentra-
tion unit to an output concentration of from 60 to âO % by weight of sulphuric
acid, subsequently introduced into a high concentration unit where it is concen-trated at temperatures of from about 160C to 250C and under a pressure of fromabout 30 Torr to 100 Torr to a concentration of from 90% by weight to 98.3% by
weight of sulphuric acid, and then delivered to a purification stage in which a
temperature of from about 220C to 330C is maintained, and the acid is purifiedunder atmospheric pressure or reduced pressure.
The present invention also provides an apparatus for regenerating sul-
phuric acid by preconcentrating the acid to 60% to 80% by weight and then concen-
trating the acid to 90 to 98.3% by use of temperatures in the range of 160C to
250C and a pressure of 30 to 100 Torr, whereby the acid is purified, said appar-
atus comprising heat exchangers of enamelled double-jacketed tubes heated by heat
carriers and provided for heating th0 acid in a preconcentration unit; said pre-concentration unit including a first preconcentration stage comprising a first
heat exchanger followed by a first evaporation vessel; a following, second pre-
concentration stage comprising a second heat exchanger formed by double-jacketedtubes of enamelled steel and followed by a second evaporation vessel; and a final
concentration stage following said preconcentration unit comprising a heat exchan-
~ ~5'~Z8S
ger formed by double-jacketed tubes of enamelled steel and followed by an evapor-
ation vessel; and a purification stage comprising a radiation-heated quartz glass
tube, a heating zone for heating the acid, a following reaction zone and a follow-
ing after-reaction zone. The first heat exchanger advantageously comprises a nest
of tantalum tubes.
It has been found that, using enamel-coated installaticns, it is possi-
ble to maintain the high temperatures mentioned above and, hence, to concentrate
the acid to the azeotropic point. The acid is completely purified in an addi-
tional step carried out in a separate apparatus, for example made of quartz or
coated with enamel. This is carried out by heating the concentrated acid for a
certain period (residence time) to 290 - 350C under reduced pressure or at s-
pheric pressure, if necessary in the presence of an oxidising agent, preferably
nitric acid.
Enamel coatings are applied to steel at elevated temperature. On acc-
ount of the different thermal expansion coefficients of the steel support and the
enamel coating, a compressive stress is developed in the enamel layer on cooling.
This stress is desirable and necessary for preventing the formation of cracks in
the enamel layer. At an ambient temperature of 25C, the compression of the ena-
mel layer corresponding to this compressive stress amounts to approximately
0.0015 m/lm. The compressive stress increases with decreasing temperature and
decreases with increasing
-- 3 --
~5;~2135
- 4 -
temperature~ so that it will normally be zero for an
average temperature of the order of 400C. Accordingly,
if it were to be assumed that a compressive stress must
always be present in the enamel layer, it would be
possîble under ideal conditions to use enamelled
components up to a temperature of around 400C. Under
normal service conditions, however, the effect of
various factors in conventional evaporators is that,
even at low ~ean temperatures, the compressive stre3s
becomes zero or even negative, which almost inevitably
results in unwanted hairline cracks. The~e factors
include inter alia the transfer of heat from the steel
side through the enamel layer to a medium to be treated.
Under the effect of the known temperature profile
characterising the transfer of heat, the enamel layer
has a lower mean temperature than the steel layer, with
the result that the compressive stresses in the enamel
layer are reduced in comparison with the stationary
state. This phenomenon is further adver3ely affected
by uneven heating or cooling of the enamel-coated wall,
for e~ample due to the fact that the supply of heat
fro~ the steel side is irregular or to the fact that
the cooling effect on the enamel layer side is uneven
on account of encrustation of this layer or on account
Of irregular flow o~ the medium to be heated or its
partial evaporation. A¢oording to the invention,
therefore, the enamel layers are used in such a way
that a residual compressive stress is still present in
them. This residual stress according to the invention
3o prevents the formation of hairline cracks so that the
enamel layer affords satisfactory protection against
corrosion by the hot, highly concentrated acid. The
residual compressive stress or rasidual compression
~S2285
-- 5 --
according to the invention which should be maintained
under all working conditions should be as high a
possible under the particular workin~ conditions and
preferably no less than 0.0003 m/lm. In very special
cases, even lower residual compressive stresses are
permissible, but only if there is no danger of
deformation or other adverse effects on the enamel
layer. In cases such as these, however, a residual
compressive stress of approximately 10~, i.e. a
compression of 0.00015 m/lm, should still be present
in the enamel layer.
Another aspect of the invention concerns the
overall temperature pro~ile, the acid automatically
flowing past the enamel heat exchanger surfaces at such
a predetermined speed that no solids can be deposited
on those surfaces and, at the same time, in such a way
that the acid i9 merely heated and not evaporated on
the heat e~changer surfaces. For this reason, it is
possible in accordance with the invention to circulate
a multiple of the quantity of acid to be concentrated
in the apparatus and, at the same time, to ensure that
the static pressure prevailing at the outlet end o~ the
last heat e~changer tube is high enough, taking the
temperature of the acid into account, to avoid
25 evaporation. ~owever, this temperature profilé has to
be established in con~unction with the reqidual
compressive stress accordin~ to the invention. With
~low rates of the acid and the heat carrier of more
than about 0.8 m/sec. and pre~erably in the range of
Yrom 0.9 to 1.5 m/second, the residual compression
according to the in~rention in the enamel layer does not
fall below the minimum o~ 20~o or ~.0003 m/lm. In
general, the rate o~ ~low of the acid in the heat
~L5Z2~35
-- 6 --
exchanger tubes may be varied over a range o~ from
about 0.4 to '~ m/second. However, it is particularly
preferred to adjust the flow rate to a value of from
0.8 to 1.2 m/second. At thi~ level, any residues
present remain in the suspension.
Although the high concentration stage may be
operated with virtually any acid concentration Or more
than 30~ by weight and preferably more than 40~0 by
weight of E2~04 (input concentratio~), it is
advantageous, particularly where highly dilute acids
are used, for the high concentration stage to be
preceded by a preliminary concentration stage which,
for a high evaporation rate ? preferably operates in
several stages at di~ferent pressures on the principle
of vapour utilisation. The last evaporation ~tage of
the preconcentration unit is also heated by heat
carrier~. It is possible in this way to obtain high
temperature gradients and, hence, small heating surfaces
and economic operation By suitably selecting the
2~ prooess steps, the type of heating and the construction
of the apparatus according to the invention under the
special conditions according to the invention, it is
possible for the first time to use enamel materials in
s~ch a way that it is possible in the la~t stage of
the preconcentration unit and in the high concentration
unit to dispense altogether with metallic material~
which have only a limited resistance to corrosion. At
the same time, the speoial conditions according to the
invention also ensure that s~fficiently high temperature
differences are e~tablished between the individual
evaporator stages to be able to utilise the heat of
evaporation economically.
A suitable evaporator has proved to be the
circulation-type flash evaporator in which the acid is
~52285
-- 7 --
merely heated in the liquid phase in the heat
e~changer itself, evaporation taking place in the
evaporation vessel. This is particularly important
when the acids to be concentrated contain impurities
which have a tendency to crystallise out on the
evaporation suri~aces This i9 the case, for example,
with iron-containing acids. The solubility of iron
sulphate in sulphuric acid rapidly decreases in the
range beyond 90~0 by weight o~ H2S04 and there is a
danger of iron sulphate settling on the evaporation
surfaces. This danger does not e~ist in the
circulation-type flash evaporator because no
evaporation ta~es place on the heat exchanger surfaces
and the iron precipitating can thus remain in
suspension. Instead o~ circulation-type flash
evaporators, however, it is also possible to use other
types of evaporators, depending on the quality of the
acid, for example falling-~ilm evaporators, heated
evaporation ves~els, etc.
~o In the case of acids contaminated with organic
constituents, the organic constituents, depending on
their boiling temperature, actually distil o~f at
least partly with the vapours in the various evaporator
stages. In many cases, however, there is at least a
residue o~ organic constituents which does not
evaporate and which therefo.e has to be oxidised at
elevated temperatures. Using the process and apparatus
according to the in~ention, trinitrotoluene, for
e~ample, may be completely degraded at 320C in the
presence o~ nitric acid as oxidising agent. Even lower
reaction temperatures may be maintained. In the case
of acids which may be re-used i~or the same purpose, it
is possible to accept, ~or e~ample, even lower ~egrees
. . ..
~Z285
- 8 -
of reaction, and, hence~ working up because the
impurities do not interfere with the preceding process.
It is known, for exa~ple, that the oxidation
reaction may be carried out with nitric acid in a
residence vessel of cast iron. ~owever, it has proved
to be particularly ad~antageous to use a tube reactor
o~ quartz or, optîonally, enamel in which the acid is
initially brought to the reaction and oxidation
temperature by indirect heating, ~or e~a~ple by
radiation heating, in the heating zone and subsequently
enters the reaction zone where the acid is contacted in
countercurrent with an oxidising agent, preferably
nitric acid, with the result that the organic
constituents are o~idised. The nitrosyl sulphuric acid
~(H~OS04) partly formed reacts with the remaining
organic constituents present in the ~ollowing a~ter-
reactio~ zone, so that a highly pure acid may be run
- o~ at the end of the purification stage.
The after-r~action zone i9 directly connected to
2~ the reaction zone. In the purification stage D, i.e.
in the reaction zone, the concentration of the acid
leaving the high concentration unit may be reduced or
increased to a certain extent or kept constant.
Where nitric acid is added to oxidise the organic
constitue~ts, water i9 introduced into the already
highly concentrated acid. Similarly, water is formed
in small quantities during the o~ida~ion of the organic
constituents. This minimal input of water is
sufficient to reduce the concentration of the already
3o highly concentrated acid. In the case of acids
contaminated with up to 2 _ 3do by weight or organic
constituents, the reduction in concentration will
amount to between about 2 and ~do by weight and, in
~2285
_ 9
exceptional cases, may even be higher, for eæample
from 5 to 6~o by weight.
The preferred e~bodiment of the invention is
c~aracterised in that the amount of water thus
introducel is removed again by further heating in the
reaction zone. In other words, the acid with the
concentration adjusted in the high concentration unit
is run off after the after-reaction zone. Another
possibility i9 to carry out another high concentration
in the reaction zone. I~here thiQ procedure is adopted,
a highly pure and, at the same time, highly
concentrated acid is obtained. ~owever, this procedure
would appear to be of minor significance in most cases
on economic grounds.
In the drawing, the heating zone and the following
reaction zone and the after-reaction zone are
accommodated in the same apparatus. This represents
the preferred embodiment of the invention
~owever, this arrangement is by no Deans
imperative. In one variant of the process and
apparatus ~or axample, the reaction zone and heating
zone consist of separate apparatus.
As already mentioned, t~e praconcentration unit
consists of a single stage, but usually of at least two
individual stages arranged one behind the other and
operated ~t different pressures. The preconcentration
unit will comprise fro~ one to three stages, depending
on the concentration and composition of the acids to be
worked up. It is only in special cases that it may if
necessary be constructed with an even greater number of
stages. If, in its preferred e~bodiment, the
preconcentration unit is of multistage construction,
the vapours are guided througho~t the entire pre-
~152~85
-- 10 --
concentration unit in such a way that the vapourscoming from the particular evaporator stage operating
at higher pressure are used to heat the preceding
stage (as seen fro~ the acid side) operating at lower
pressure.
According to the invention, it has proved to be
advantageous where the preconcentration unit has three
stages to adjust the conditions in the various stages
to values within the following ranges:
1st Stage
Concentration of the acids to be worked up: 15 to 35
by weight of H2S04;
Concentration of the issuing acid: 20 to 42~ by weight
f ~2S04;
~emperature range: 30 to 65C;
Pressure range: 30 to 100 Torr.
2nd Stage
2i~ Concentration o~ the acids to be worked up: 20 to 42~o
by weight of H2S04;
Concentration of the issuing acids: 28 to 54~ by weight
of H2S4;
Temperature range: 65 to 100C;
Pressure range: 150 to 500 Torr,
3rd Stage
Concentration o~ the acids to be worked up: 28 to 54do
by weight of H2S04;
Concentration of the issuing acids: 60 to 80~o by wsight
of H2S04;
Temperature range: 120 to 125C;
Pressure range: 500 to 1500 ~orr.
~52285
l~ere the preconcentration unit comprises two
stages, the following conditions are established in
accordance with the invention:
5 ~
Concentration of the acids to be worked up: 25 to 50~p
by weight of fI2S04;
Concentration of the issuing acids: 34 to 617ot by weight
of H2S04;
lO Temperature range: 40 to 100C;
Pressure range: 30 to 200 Torr.
2nd Stage
Concentration of the acids to be worked up: 34 to 61a~o
15 by weight of ~[2S04;
Concentration of the issuing acids: 60 to 80~o by weight
~ ~[2S4;
Temperature range: 125 to 225C;
Pressure range: 500 to 1500 Torr.
In special circumstances, for example when the
acid actually accumulates with a sufficiently high
concentratio:l, there may be no need for the pre-
concentration unit to have several stages, i.e. it may
be operated as a single-stage unit. In cases such as
25 the~e, the following condit-ions have proved to be
effective in accordance with the invention:
Single Stage
Concentratio~ of the acid to be worked up: 30 to 60do
3o by weight of H~S04;
Concentration of the issuing acid: 60 to ~Op by weight
~ ~2S4;
'remperature range: 60 to 160C;
13 5~2Z~35
- 12 -
Pressure range: 30 to 200 Torr.
A particularly advantageous embodiment of the
process and of the apparatus required for carrying it
out is described by way of exalnple in the following
with reference to the accompanying drawings, in which:
Figure 1 is a general flow chart of the process using
a two-stage preconcentration unit and
diagrammatically illustrates the apparatus:
and
Figure 2 diagrammatically illustrates one e~ample of an
e~bodiment.
A 30~0 by weight crude sulphuric acid contaminated
by organic constituents i9 to be regenerated to a
98.3~o ~2S04 in a two-stage preconcsntration unit, a high
concentration unit and a following purification stage.
In Figure 1, the letters A and B denote two stages
of the preconcentration unit, the letter C denotes the
high concentration unit and the letter D denotes the
purification stage with the reaction zone and after-
reaction zone.
Crude acid (3333 kg/h, 4000 ~OC, 30~ ~2S04) is
delivered at 1 by means of a rotary pu~p 2 to a heat
exchanger 3 in which the crude acid is preheated by
heat transfer from the product acid. The preheated
~47C) thin acid is then introduced at 4 into the
circuit of the first stage A. The flow of crude acid
is regulated by keeping the level constant in the
3~) evaporation vessel 6 of the first stage. The
circulating sulphuric acid is heated in the heat
exchanger 5 (for example, a nest of tantalum tllbes).
It is only in the evaporation vessel 6 that water is
~S2Z85
-- 13 --
evaporated commensurately with the amount of heat
absorbed, so that crusts are prevented from forming on
the he~ting surfaces.
The first stage ~ is heated by the heat of
condensation of the vapours (pipe 18) fro~ the second
stage B. Stage A is operated in vacuo (50 ~orr,
41.5% of ~2~4~ 5~) The virtually acid-free
vapours from the fir~t stage A (924 kg/h, 50C, 50
Torr) pass via a drop ~eparator 7, in which entrained
drops of sulphuric acid are retained, through a pipe 8
into a mixing condenser 9 where the vapours are
condensed by spraying in cooling water. The light
organic constituents distilled off with the vapours
are separated off as iar as possible from the remaining
condensate in the following separation vessel 10.
Generation of the vacuum and extraction of the
non-conde~sed organic vapours and the inert gasss is
carried out, for exa~ple, by mea~s of a water ring
vacuum pump 11.
Suitable construction materials for the circuit
and the svaporation vessel are, for e~ample, glass-
fibre-reinforced plastic materials, PVC, graphite,
glas~ or enamel.
The first stage ~ may eve~ be formed by a
falling-film evaporator, in which case the sulphuric
acid is concentrated in a single passage. In both
cases, the heat exchanger 5 i9 made of a metal, in the
present case tantalum. Instead of using a metal,
however, it is also possible to use graphite, glass or
enamel.
~ he preconcentrated sulphuric acid flowing off
from the first stage ~ (2409 kg/h, 50C, 41.5% H2S04)
i9 introduced through a pipe 13 of a rotary pump 14
~5ZZ8S
_ 14 -
into the circuit of the second stage B. The flow
volume is regulated by keeping the level constant in
the evaporator 16 of the second stage B.
The sulphuric acid circulated by the rotary pump
19 is heated in the heat exchanger 15, consisting of
enamelled double-jacketed tubes arranged onebehind the
other, and then evaporated solely in the evaporation
vessel 16 so that crusts are pravented ~rom forming on
the enamelled heating ~ur~ace.
The heat exchanger 15 o~ the second stage B is
heated by a heat carrier oil, the heat carrier being
passed through the inter-jacket space o~ the enamelled
double-jacketed tubes in countercurrent to the
sulphuric acid. The concentration and pressure
prevailing in the evaporation vessel 16 are selected in
such a way that the concentration of acid in the
vapours can be kept at substantially zero witho~t any
additional column and the heat of condensation o~ the
vapours can be utilised in the first stage A (75
~ S04, 1 bar, 185C).
The vapours ~ormed in the evaporation vessel 16
(1076 kg/h, 1 bar, 100C3 pass via a drop separator 17,
in which entrained drops of sulphuric acid are retained,
through a pipe 18 into the heat exchanger 5 of the first
stage A where they are condensed. Finally, the
condensate formed is introduced into a separation
vessel 20 at 21 (1076 kg/h, 1 bar, ~5C)
In this separation ves~el, the organic constituents
distilled o~f wlth the vapours (for example, all having
a boiling point lower than 185C) and condensed in the
heat e~changer 5 or crystallised may be separated ~rom
the~conde~sate. The inert gases are removed through the
exhaust system.
z~s
- 15 -
The interme~iate produ^t acid lowing off from
the second stage B is delivered by a rotary pump 23
through a pipe 22 to a rectification column 26 of the
high concentration stage C. Providing the second and
third stages A and B are suitably arranged, there is
no need for the pu~p 23. The input (1333 ~g/h, 185C,
75~ H2S04) is regulated by keeping the level constant
in the evaporation vessel 25 of the high concentration
stage C
In the separation column 27, the liquid undergoes
a material e~change with the vapours ascending from
the rotary evaporator 25, the liquid becoming enriched
with relativel~ high boiling constituents and the
vapour with relatively low boiling constituents.
Providing the intermediate product concentration is
correctly selected (75% ~2S04), the ~aterial and heat
exchange results in complete absorption of the ~2S04
component in the vapours. If the final concentration
of stage B is too high (for example higher than 75~
H2S04), the rectification column of stage C has to be
supplemented b~v a concentration column which has to be
operated with water.
The sulphuric acid circulated by the rotar-~ pump
29 is heated in the heat exchanger 24, consisting of
enamelled double-jacketed tubes arranged one behind the
other, and is then evaporated solely in the evaporation
vessel 25, thereb~v preventing crusts from forming on
the enamelled heating surface. The heat e~changer 24
is heated b~ a heat carrier oil, the heat carrier being
3o passed through the inter-jacket space of the enamelled
double-jacketed tubes in countercurrent to the
sulphuric acid.
It has surprisingly been found that enamelled
~52;~85
-- 16 --
steel shows ~ery high che;nical resistance to sulphuric
acid, particularlvv to very highly concentrated
sulphuric acid (98.3%), under the conditions according
to the invention. This is all the more surprising
insofar as it is known that enamel develops hairline
cracks at elevated temperatures, with the result that
there is no alternative but to work under the special
conditions a~cording to the invention. On the other
hsnd, it is imperative for the temperatures or
temperature differences not to fall below or exceed
certain levels. For this reason, it has proved to be
best to operate the high concentration sta~e C und~r
reduced pressure. The following conditions have prove~
to be safe in operation: vacuum in the evaporator (25)
30 to 100 Torr, preferably 50 to 70 Torr, temperature
160 to 250C, boiling te3~perature of the 98.3~ acid
240C, heat carrier temperature at the end of the heat
exchanger (24) 310 C, acid temperature at this point
2~0C, heat carrier temperature at the entrance to the
heat exchanger (24) 290C and acid ~emperature at this
point 2~0C. The vapours formed in the evaporation
vessel 25 differ in their acid content according to the
final concentration. This acid is exchanged in the
rectification column 26 by material exchange with the
inflowing sulphuric acid.
The vapours of stage C, whioh have an acid content
corresponding to the equilibrium of the acLd introduced,
flow via the drop separator 28, in which entrained drops
of sulphuric acid are retained, through a pipe 30 into
a miæing condenser 31 whera the vapours (313 kg!h, 50
Torr, 185C) are condensed by spraying in water. The
organic constituents distilled off with the vapours
(for example all those having a boilin,g point below
~!L15Z285
-- 17 --
240C) and condensed in the mixing condenser 31 or
crystallised are separated as far as possible from the
remaining condensate in the following separation vessel
32. Generation of the vacuum and extraction o~ the
non-condensed organic vapour~, the waste gases of the
~xidised organic constituents and the other inert
gases, is carried out by means of the water ring
vacuum pump 33.
Where the acid is contaminated by organic
constituents, a reaction normally takes place between
the organic constituents and the So3-gas present in
the evaporation vessel. In order to avoid reduction
of the S03 to S02 and the presence of undesirable
sulphurous acid ~2S03 in the condensate, an oxidising
agent, pre~erably HN03, i~ added.
In the case of iron-containing acids, the
saturation state (approximately 20 ppm of Fe at 98.3
S04 and 240C) is generally e~ceeded in the high
concentratio~ stagé C, resulting in the precipitation
~20 of iron (II) sulphate. By virtue oi the continuous
circulation of the sulphuric acid in the circuit, the
iron sulphate remains in suspension and does not have
any opportunity to settle in the evaporator.
In order to protect the enamel against the
abrasive effect o$ precipitated iron sulphates, the
rate of circulation in the double-jacketed tubes and
in the circuit pipes mny be limited to lm/s.
The high concentration stage i9 preierably made
o~ enamelled steel. An alternative material for the
3o rectification column is glass. ~he circulation pump
may also be made of a material other than enamelled
steel, preferably cast silicon.
~he highly concentrated, but only partly purified,
.
- .
~L5ZZ85
- 18 -
sulphuric acid issuing from the high concentration
stage C (1020 kg/h, 98% H2S04, 240C, 20~0 ppm TOC) i~
delivered by a metering pump 35 through a pipe 3'~ to
a vertical q~artz glass tube 36 which is filled with
acid or through which acid trickle~. In thi~ quartz
tube, the concentrated sulphuric acid is heated under
normal pressure to a temperature close to its boiling
point (for example ~rom 240C to 320C).
In o~e variant, reduced pressure is applied at the
tube entrance (top of the quartz glass tube 36) and in
a possible rectification column 41. Providing the
reduced pressure is correctly selected, normal pressure
prevails in the purification zone under the effect of
the column of liquid in the quartz tube 36. Anot~er
possibility is to introduce the sulphuric acid in the
~orm of a falling ~ilm in the upper part of the heating
zone 38 in order to increase the transfer o~ heat.
The heat is transferred by radiation, i.e. the
quartz tube 36 is surrounded by a concentric radiation
jacket which is in turn surro~nded by an electrical
resistance heating system ~7 (or possibly by another
heat source, ~or e3ample smoke gases). The heat is
transferre~ by radiation irom the heating system to
the radiation jacket and1 ~rom there, the ra~iation
emitted inwards is absorbed by the ~uartz and the acid.
In the heating zone 38~ a sulphuric acid l~hich has
not been conc~ntrated to the azeotropio point (for
example 90 to 97~0 by weight ~ ~23~) may i~ desired be
concentrated to 98% by weight or to 98.3~o by weight.
In ca~es where strong vapours are given of~, it is
advisable to provide the heating zone and~ in this case,
the concentration zone standing beneath a column of
liquid with a filling, most desirably in the form of a
~L5~Z85
-- 19 --
packing.
The ~apours given off during concentration in the
heating zone 38 difPer in their acid content according
to the input concentration, pressure and temperature
of the acid. This acid has to be eYchanged in a
rectification column 41 by material exchange with a
liquid to be intro-~uced. Water may be used as the
liquid, in which case the outflowing washing water
enriched with sulphuric acid may be returned to the
p~rification stage or may even be introduced together
with the thin acid into the high concentration stage.
The liquid to be introduced may even be formed by the
thin acid (where the concentration i9 below 75~o by
weight of ~2S04)-
The ~apours given ofP during concentration pass
through the rectification column 41, in which the
above-mentioned exchange of material takes place, into
a condenser 42. Generation of the vacuum and e~traction
of the non-condense~ organic vapours, the waste gases of
the oxidised organic constituents and the other inert
gases is carried out by means of the vacuum pump 43.
The preheated, concentrated, but still contaminated
sulphuric acid (1020 kg/h, 9710 by weight of ~2~4~
320C) ~lows from the heating zone 38 into the reaction
25 zone 39
The reaction zone, which consist~ oP the lower part
of the quartz tube 36, ls surrounded by a co~centrlc
radiation jacket which is in turn surrounded by an
electrical resistance heating system 37 (or possibly
3~ by another heat source, for e~ample smoke gases). The
heat is transferred by radiation from the heating system
to the radiation jacket and, from there, the radiation
emitted inwards i9 absorbed by the quartz and by the
~1~;2285
- 2~ -
acid.
In the lower part of the quartz tube 36, the
organic constituents are oxidised by means of an
oxidising agent (for example 65~o nitric acid) which is
added at 44 and which is introduced in vapour form in
the lower part of t~e purification column, ~or example
through a per~orated plate, to pro~uce small uniformly
distributed vapour bubbles. In the case of highly
contaminated su~phuric acids, it is advisable to
provide the reaction zone 39 with a ~illing, most
desirably with a packing. Ii necessary, the filling
may be e~tended over the entire quartz tube. The
sddition o~ nitric acid may even result in the partial
formation of nitrosyl hydrogen sulphate which is
unwanted in the product acid. This nitros-~l hydrogen
sulphate is partly degraded again by reaction with the
organic constituents in an after-reactor 40.
It has proved to be advantageous to arrange the
heating zone and the possible concentration zone with
the purification zone one be~ind the other. The S03-
gase3 forme~ during concentration in the lower part o~
the heating zone 38 are re-absorbed by the colder acid
in the upper part of the quartz glass tube 36. The
non-reduced N0~-gasies ascending from the reaction zone
are able ~urther to react with the organic const1tuents
in the heating zone. According to the invention, there
i9 no strict ssparatio~ between the heating zone and
the reaction zone.
The inco~pletely reduced N0x-gases issuing ~rom
3o the puri~ic~tion stage D and the re~ction gases may be
introduced through a pressure-reducing val~e in the
pipe 45 into the evaporation vessel 25 of the high
concentration stage C where the excess N0x-gas may
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react with the organic constituents and, at the same
time, at least partly prevent the possible reduction
of S03.
The hot concentrated sulphuric acid (320C, 98.3
of ~2S04) issuing ~rom the after-reactor 40 is cooled
with secondary circuit acid of the same purity and
concentration in a mixing condenser 46 arranged
adjacent the after-reactor. Be~ore being introduced
into the mixing condenser 46, the secondary circuit
acid is cooled in the heat excha~ger 3 by the crude
acid and by a ~ollowing water-cooled heat e~changer
47. The product acid is removed ~rom the secondary
circuit and, if necessary, is introduced into a plate
separator 54 where the suspended iron sulphate is
separated off.
In the puri~ication stage ~, the sulphuric acid
has an input concentration o~ from 90 to 93do by ~eight,
an output concentration of from 90 to 93.3% by weight
and a temperature of from 225 to 330C.
The heat e~changers 15 and 24 o~ the second and
third stages B and C are heated by means of a central
heating system consisting of a high temperature
heater 48, a burner 49, a circulation pump 50, an air
preheater 51, a combustion air fan 52 and a chimney 53.
This heating system has the a~vantage over smoke gas
heating that eveIl sulphur-containing he~vy oil may be
used as fuel. On the heat carrier side, the two heat
e~changers 15 and 24 may be connected in series or in
parallel.
3l~ It is also possible to reduce the ~O~-gases prasent
in the waste gas in a sub-stoichiometric atmosphere in
a spscial burner 49.
Accordingly, tbe described process carried out
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_ 22 -
with the illust:rated apparatus gives satisfactorily
purified sulphuric acid concentrated to the aze~tropic
point.
2~
3o
