PROCESS FOR SEPARATING SO.SUB.2 FROM A CURRENT OF GAS, WITH PRODUCTION OF SULPHURIC ACID BY THE NITRIC OXIDE PROCESS

PROCEDE DE SEPARATION DE SO.SUB.2 ET D'UN FLUX GAZEUX, AVEC PRODUCTION D'ACIDE SULFURIQUE PAR LE PROCEDE A L'OXYDE NITRIQUE

English Abstract

Case 72-9818+
Canada
Process for separating SO2 from a current of gas, with
production of sulphuric acid by the nitric oxide process
Abstract of the Disclosure
A process for separating SO2 from a current of gas, with pro-
duction of sulphuric acid by the nitric oxide process in a tower
system is described, in which the SO2-containing gas is introduc-
ed into a denitration zone and, after passing through the latter
at a nitric oxide concentration of at least 1 % by volume in
the gas phase,onward into an SO2-processing zone comprising
at least one packed tower, in which it is brought into intimate
contact with sulphuric acid containing less than 70 % by weight
of H2SO4 (dilute acid, the acid being used to absorb the
nitrogen oxides in the towers downstream from the SO2-processing
zone having a concentration of between 70 and 85 % by weight of
H2SO4 (absorption acid): this process is improved by
a) heating the nitrose-containing acid intended for the denit-
ration zone which, relative to the direction of gas flow, is
located upstream from the first tower, trickle-fed with dilute

acid, of the SO2-processing zone, by indirect heat exchange to
above 60°C before it comes into contact with the gas stream
which leaves the denitration zone, and
b) holding the sojourn time of the gas between leaving the
denitration zone and entering the first tower of the nitric
oxide absorption zone, at concentrations of at least S % by
volume o SO2 and at least 10 V/o by volume of °2 in the gas
entering the denitration zone, at less than 30 seconds, and,
at lower SO2 contents or lower O2 contents in the said entry
gas, limiting the residence time to a maximum length calculated
from the equation
<IMG>
in which formula Zmax is the sojourn time in seconds,
[SO2] denotes the SO2 content in the gas entering the denit-
ration zone, in % by volume, and [O2] denotes the oxygen con-
tent in the same entry gas, in % by volume.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for separating SO2 from a cold, moist current of gas
containing up to about 8% by volume of the same, with attendant production of
sulfuric acid by the nitrogen oxide process in a system of reaction zones,
which system is adapted for the production of a strong sulfuric acid having
a H2SO4-content of at least 70%, by weight, in which system the current of
SO2-containing gas having a sufficiently high oxygen content for carrying out
this process in practice is caused to flow, at substantially atmospheric pres-
sure, successively
(a) through an acid denitration zone;
(b) through an SO2-processing zone in which said current is brought into
intimate contact with dilute sulfuric acid having a concentration of less
than 70% H2SO4 and being substantially free from nitrose, flowing in a cycle
through the latter zone; the pressure on said gas current being sufficiently
low to avoid an increase of the nitrose content in said acid cycle; and said
SO2-processing zone being of sufficient length to have SO2 present therein
substantially to the end thereof, thereby substantially avoiding dissolution
of nitrogen oxides in said cycle of dilute acid;
(c) through a nitrogen oxides-absorption zone; all of the nitrose-
containing acid exiting from said absorption zone being indirectly heated to
a temperature above 60°C? and then introduced into said denitration zone by-
passing said SO2- processing zone and having nitrogen oxides withdrawn in said
denitration zone from said nitrose-containing acid into the gas current; there-
by obtaining a denitrated sulfuric acid exiting from the denitration zone which
acid has an H2SO4-concentration of between 70 and 85% by weight, is substanti-
ally free from nitrose and is withdrawable from said system as product acid;
a portion of said denitrated sulfuric acid being conveyed through said nitro-
gen oxides absorption zone and absorbing nitrogen oxides from the gas current
in said absorption zone; said process further comprising the steps of
(?) removing a portion of the said dilute sulfuric acid, substantially
free from nitrose, from said SO2-processing zone and bringing the last-mentioned
portion into contact with a cold moist current of said SO2-containing gas in a

pretreatment zone upstream of said denitration zone in said system;
(B) reintroducing a portion of the acid formed in (?) more diluted
by moisture from said gas and free from nitrose into the sulfuric acid from
the SO2-processing zone, this portion of the moisture content of the SO2-
containing gas thus bypassing the gas flowpath leading to the denitration zone;
and
(?) at least when the water-content of the more diluted acid exiting
from the pretreatment zone increases to the extent that the H2SO4-content of
the denitrated sulfuric acid leaving the denitration zone drops substantially
below 70% by weight of H2SO4, removing a portion of the dilute sulfuric acid
from said cycle and conveying the last-mentioned portion through an acid-
dehydration zone, wherein the last-mentioned portion of dilute sulfuric acid
is heated to evaporate water there-from, and then returning the resulting more
concentrated acid to said cycle.
2. The process of claim 1, wherein the temperature imparted to the
nitrose-containing acid prior to entering the denitration zone is above 80°C.
3, The process of claim 1, wherein the current of gas passing through said
denitration zone is brought counter currently into contact with nitrose-con-
taining sulfuric acid having an H2SO4 concentration of above 70 and up to 80%
by weight, from said absorption zone.
4. The process of claim 1, wherein said gas current passing from
said denitration zone to said SO2-processing zone leaves said denitration zone
with a nitrogen oxides-concentration of at least 1% by volume.
5. The process of claim 1, wherein, the said small portion of di-
lute acid is indirectly heated before entering said acid-dehydrating zone.
6. The process of claim 1, wherein the nitrose containing acid fed
to the denitration zone is heated upstream of the latter by indirect heat
exchange with the denitrated acid discharged from the latter zone and is addi-
tionally indirectly heated to at least 60°C.
31

7. The process of claim 6, wherein the nitrose containing acid is
heated, prior to entering the denitration zone, to above 80°C.
8. The process of claim 1, wherein dilute acid from said cycle
thereof is introduced intermittently into said denitration zone in order to
regulate the concentration of the acid in the latter zone.
9. The process of claim 1, wherein the sojourn time of the SO2-
and nitric oxides-containing gas between leaving the denitration zone and
entering the first tower of the nitric oxide absorption zone, while passing
through the SO2-processing zone there-between and being irrigated with dilute
acid therein, is held to less than 30 seconds when the gas entering the deni-
tration zone contains at least 5% by volume of SO2 and at least 10% by volume
of O2, and limiting the aforesaid sojourn time, when the SO2 content at the
last-mentioned entry is less than 5% by volume, to a maximum period determined
by the equation.
<IMG>
in which formula Zmax is the sojourn time in seconds, [SO2] denotes the SO2
content in the gas entering the denitration zone, in % volume, and [O2] de-
notes the oxygen content in the same entry gas, in % by volume.
10. The process of claim 1, wherein, at a nitrogen oxides-concentra-
tion of at least 2% by volume and attendant an SO2-concentration of at least
1% by volume, in the gas current entering the main sector of the SO2-process-
ing zone,
i) the latter zone comprises at least one tower filled with packing
having a surface area of more than 90 m2/m3, and
(ii) the upper limit of the sojourn time of the gas between leaving
the denitration zone and entering the first nitric oxide absorption tower, at
less than 10% by volume oxygen content in the gas, is calculated from the
equation
<IMG>

in which Zmax is the sojourn time in seconds and [O2]? denotes the oxygen
content, in % by volume, in the gas on leaving the denitration zone.
11. The process of claim 1, wherein at a nitrogen oxides-concentra-
tion of at least 2% by volume and an attendant SO2-content of at least 1% by
volume and a free oxygen content of at least 10% by volume in the gas current
entering the main sector of the SO2 processing zone,
( i) the latter zone comprises at least one tower filled with packing
having a surface area of more than 90 m2/m3, and
(ii) the upper limit of the sojourn time of the gas between leaving
the denitration zone and entering the nitric oxide absorption zone is below
30 seconds.
12. The process of claim 6, wherein the temperature of the nitrose-
containing acid being introduced into said denitration zone is kept constant
by correspondingly regulating said additional indirect heating,
13. The process of claim 1, wherein the H2SO4-concentration of the
acid leaving said denitration zone is kept constant by introduction of dilute
acid or water into said zone.
14. The process of claim 12, wherein the ratio of NO2 present in the
gas current before entering said nitrogen oxides-absorption zone is regulated
by varying the amount of nitrose-containing acid present at a given time in
the denitration zone.
15. A process for separating SO2 from a current of cold moist gas
which contains up to about 8% by volume of this substance, with attendant pro-
duction of sulfuric acid by the nitrogen oxide process, in a system of reaction
zones, which system is adapted for the production of a strong sulfuric acid
having a H2SO04-content of above 70% by weight, in which system the current of
SO2-containing gas having a sufficiently high oxygen content for carrying out
this process in practice is caused to flow at substantially atmospheric pres-
sure. successively
13

(a) through a pretreatment zone;
(b) through an acid denitration zone;
(c) through an SO2-processing zone in which said current is brought into
intimate contact with dilute sulfuric acid having a concentration of less than
70% H2SO4 flowing in a cycle through the latter zone; gauge pressure on
said gas current being sufficiently low to avoid an increase of the nitrose
content in said acid cycle; and said SO2-processing zone being of sufficient
length to have SO2 present therein substantially to the end thereof, thereby
substantially avoiding dissolution of nitrogen oxides from said gas current in
dilute acid of said cycle; and
(d) through a nitrogen oxides-absorption zone; all of the nitrose-con-
taining acid exiting from said absorption zone being indirectly heated to a
temperature above 60°C and then introduced into said denitration zone by-
passing said SO2-processing zone and having nitrogen oxides withdrawn in said
denitration zone from said nitrose-containing acid into the gas current; thereby
obtaining a denitrated sulfuric acid exiting from the denitration zone which
acid has an H2SO4-concentration of between 70 and 85% by weight, is substan-
tially free from nitrose and is withdrawable from said system as product acid;
a portion of said denitrated sulfuric acid being recycled through said nitrogen
oxides absorption zone and absorbing nitrogen oxides from the gas current in
said absorption zone; said process further comprising the steps of
(?) removing a portion of the said dilute sulfuric acid, substantially
free from nitrose, from said SO2-processing zone and bringing the last-mentioned
portion into contact with a cold moist current of said SO2-containing gas in
said pretreatment zone upstream of said denitration zone;
(?) reintroducing into the sulfuric acid from the SO2-processing zone
acid formed in (?) which acid is more diluted by moisture from said gas sub-
stantially free from nitrogen oxides, thereby diluting said sulfuric acid,
?4

this portion of the moisture content of the SO2-containing gas thus bypassing
the gas flowpath leading to the denitration zone; and
(?) removing another portion of the dilute sulfuric acid exiting from
said pretreatment zone and conveying the last-mentioned portion through an
acid-dehydration zone, wherein the last-mentioned portion of dilute sulfuric
acid is treated to evaporate water therefrom, and then returning the result-
ing more concentrated acid to said pretreatment zone.
16. A process as described in claim 15 wherein the dilute acid from the
acid dehydration zone flows through a gas-flow-preventing liquid lock directly
into the pretreatment zone.
17. A process as described in claim 15 wherein the H2SO4-concentration
of the acid contacting in the pretreatment zone the current of SO2-containing
gas is diluted below the H2SO4-concentration of the acid in the SO2-processing
zone.
18. The process of claim 1, wherein the exit gas of said system is
passed through said acid dehydration zone and conveys from said zone the water
evaporated from said dilute sulfuric acid.
19. The process of claim 15, wherein the exit gas of said system is
passed through said acid dehydration zone and conveys from said zone the water
evaporated from said dilute sulfuric acid.

Description

~7~7~
The invention relates to a process for separating S02
:Erom a current of gas, with production o:E sulpl uric acid by the
nitric oxide process in a tower sys-tem, in which the S~2-con-
taining gàs is in-troduced into a denitration zone~and, a~ter
passing -through the latter ~t a nitric oxide con-
cen-tration of at least 1% by volume in the gas phase,
onward into an S02-processing zone comprising a-t least one
packed tower, in which it is brought into intimate contact
with sulphuric acid containlng less than 70% by weight of
H2S04 (dilute acid), the acid which is then used to ab~orb
the nitrogen oxides in the towers downstream from the
S02-proce~sing zone having a concentration o~ between 70
and 85 % by weight, preerably 74 to ~0 % by we~ght, of
H2S04 (absorption acid) and containing nitrose.
Such a process is alr~ady known, as emerges from -the
following desc.ription o:~ the development and current sta-te of
the ar-t in the field of separating S02 from wa~te gases and
converting the separated-off S02 into sulphuric acid~
As in the known nitric oxide/sulphuric acid process,
three process steps can be distinguished in the process of the
invention, namely: (l) denitration, (2) S02~proc~ and
~3) ~itr~ 'd~ , For the processing of S02, an
acid of less than 70% streng-th is used, in the process accord-
ing to the invention7 in at least one tower; it is referred to
as di.lute acid In the old li.tera~lre, the term
"chamber acid" corresponds -to this ter~l. The old term
"Glover to~er" cor.responds to -the term "denitra-tion tower"
and the old term"Gay-Lussac tower" is replaced ~y
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7477
"absorption tower". During deni,ration, there occurs necessarily
some working up of S02 with formation of sulphuric acid.
In the description of -the presen-t invention~ all towers of a
system which discharge gas con-taining more gaseous nitric
oxide than is contained in the feed gas of the tower are taken
to belong to the denitration zone. So-called "production"
-towers of a Petersen tower plant, wl.~ich co~ply with
this condition, are thus regarded, in accordance with the
definition (also in the patent claims) as belonging to the
denitration zone. Whether the acid discharged from a
~.ower contains nitrose or is free frorn nitrose is not ernployed
herein as a criterion for applying the t~rm "denitration tower"
thereto.
As part o~ the endeavours to keep the atmosphere clean,
the separation of S02 ~rom curremts of waste gas is a pressing
problem, Diverse endeavours are being made to recover the S02,
separated from the waste gases, in a utilisable form, ~or
example as sulphur or sulphuric acid. However, at lo~ S02 con
centrations, cost of recovering sulphuric acid is very high.'
The sulphuric acid contact process requires extensive gas puri-
fication in order to avoid excessively rapid poisoning of the
catalyst, and -this purification makes i-t necessary to cool
the gases. Warming the gases to -the start temperature of the
catalyst, and drying the gas be~ore it reaches the catalyst,
entails high costs~ For this reason attempts have been made
to bring about the formation of sulphuric acid at a lower
temperature by using active charcoal or by means o~ ca-talyti.
-- 3 -- '
, , ,, : , ., ,:

7~7t~
cally active dissolved metal salts (for exam~le manganese).
However, the processes which resul-t are relatively expensive
and complicated if pure acid containing more than 40% of
H2S04 is to be produced.
The nitric oxide process for the manufacture of
sulphuric acid is over 100 years old. The process is
described, for example, in the follo~/ing books: Winnacker-
K~chler, Chemische Technologie (Chemical Technology?, volume
II~ Carl Hanser-Verlag, Munich, 19701 page 38 et seq.;
Ullmann~s Enzyklopaedie der Technischen Chemie,1964, volume 15,
~age k32 et seq. The second literature reference employs
the term "nitrose process".
The limi-ts of -the known nitr~c oxlde process result
from the water balance o~ the plan-t, because, in the prod~lction of
sulphuric acid, water is also necessarily absorbed from the
gas. The reason for the import~mce o~ the wa-ter balance is
tha-t in order to be able -to store the sulphuric acid obtained
in iron tanks, the acid should,if possible,contain more than
65% by weight of H2S04, tha-t is to say it must not be diluted
too much.
If a gas contains more water vapour than is consumed
in producing an acid of a concentration sufficien~ for ni~ric
oxide absorption, this fact interferes with the operation o~
a nitric oxide/sulphuric acid process A further disadvantage
of known processesisthat -the reaction chambers must have a
large vo]ume.
.. ~ . . ..
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7~77
Thus, when processsing moist gases with an S02 content
of less than 6 % by volume at temperatures below 60C, the
known nitric oxide/sulphuric acid processes only afford a
daily produc~ion of limited amounts of sufficiently concentrated
sulphuric acid per m3 of reaction space. When processin~ gases
containing ~l ~o 6 % by volume of S02, modern Petersen tower
installations produce less than 150 kg of a sulphuric acid
containing 7~,~ by weight of H2S04 (Glover acid) per m3 of
packed space per day. Only with more concentrated gases are
higher space-time yields achieved
Was~ ga~s h~ngthe above low concentration of S02 are
produced,for example, in plants in the metallurgical industry.
E`lue gases of power stations which burn oil or coal contain
a few grams, for example 2 to ~ g, of S02/Nm3 (l Nm3 = 1 m3
at l atmosphere and 0C). When ~lpplying known enrichment
processes, for example the process according to
U S, Patent 3,721,066, to gases of such low ~ ;
S02 content, the regeneration of the solid or liquid sorption
agents ~ields gases which contain varying amounts of S02,
at concentrations which are frequently only a few percent
by volume, and which have a relatively high water content
A "relatively high" water content is to be regarded here
as a water content which exceeds 50/ of the weight of S02
which the gas contains per m3, that is to say, for example,
a water content of more than 50 g of H20/m3 if lOO g of S02/m3
are present~
The decomposition of waste sulphuric acid also
- 5 -
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~A7~7'~ .
produces waste ~es with rela-tively low S02-conten-t (for
example from 50 to 150 g/m3) because a subs-tantial proportion
o~ the oxygen is consumed by the fuel in the decompositi.on
oven.
It is furthermore kno~n, in Petersen -to~er
installations, to place, ups-tream from the towers of -the
nitric ox.ide absorption zone which follows the S02 processing
zone, a tower which is trickle-fed with dilut.e acid. In
accordance with the S02-processing taking place in this tower,
formation of N0 is expected, and it was therefore regarded as
nec~sary -to ensure a su~ficient sojourn time of the gases there~n
before they enter the absorption zone, so -tha-t N0 is converted
into N02 For this reason, a special regenera-tion chamber
was interposed in Petersen -tower ins-tallations i~ in the
said dilute acid production tower in the S02-processing zone,
it is necessary -to worl~ up not only gases containi.ng small
remnants of S0~ but also gases containing a few percènt by
volume o~ S02 (for example 2 to 5/0 by volume)0
It is also known that in tower systems a high
proportion of the reaction space requlred is a-ttributable to
the absorption system for the nitric oxides (Gay-Lussac towers).
It can be seen from the initially men-tioned literature
reference in Ullmann, page 435, 18th line from the bottom,
et seq., tha-t it is considered advantageous that srnall arnounts
f S2 should pass in-to the absorption system In con-trast
thereto it was however already recognised ~that the absorpti.on
system can be made subs-tantially sma31er if prac-tica31y
~ 6

~7477
complete working up of S02 is ensured before the gases enter
the firs-t tower of the absorption zone. A Iurther known
prerequisite for reducing -the space required for nitric oxide
absorption is accurate re~ulating of the -co-tal system, so
that the NO N02 ratio in the gases entering the absorptiQn
is an optimum.
Furthermore, German Pa-tent 1,031,292
(Pe-tersen) poin-ts out that it is advisable to keep the feed
acid of a production tower, which) as men-tioned above, is to
be considered part of the denitration zone because, in it,
nitric oxides are released into the stream of gas, a-t a
temperature of 50 to 60, As is further stated in -this
German Petersen Patent, this produc~ion tower is-
-trickle-fed in a circula-tion system shared with the firs-t
Gay-Lussac tower (nitric oxide absorption zone), The exit
temperature of the acid from the Gay-Lussac -tower accordingly
corresponds to the feed tempera-ture of the acid to the
denitration tower.
However, as already men-tioned, a space-time yield of
at most 150 kg of 78% strength sulphuric acid per cubic metre
per day is achieved with such kno~n measuresO This space-
time yield is also no-t improved if, in order to be able
bet-ter to expel -the nitrose fumes from -the acid, the trickle
acid charged with nitrose is kept a-t a temperature of 50 -to 60,
or if, as is also already known, attemp-ts are made to raise
the acid entry -temperature of a denitration tower by mixing
a part of the hot acid discharged from the same tower, or
~ 7 ~
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from another denitration -totrer, with the feed acid~ This
process en-tails a reduc-tion in the nitrose content o:E
the feed acid ~ which has a mos-t unfavourab:Le ef.feot since
the reaction space needed. remains undesirably large.
The invention is therefore in-tended,above all,-to
solve the problem of improving -the ~rocess ~escribed at the
ou-tse-t so -that when processing gases contain1.ng S02,
especially also gases of ]ow S02 conoentra-tion, of less th.an
6% by volume of S02, the space--time yield, calculated as 78%
strength sulphllric aoid, is increased substantially, preferably
to more than -twice ~hat has hitherto been attainable, and
at -the sc~me time, preferably, to reduce the large volwne,
hitherto necessary in most ca.qes, of reac-tion chambers, and
to achieve these impro~ements substantiall~ independently o:E
the water content of the feed gases.
These objects are attc~ ed in a process o~ the type
initially described, which .is characterised in that
a) the nitrose-containing acid to be introduced into the
denitration zone which, relative to the direction of gas
fLow, is located upstream from the first tower, .rickle-fed
with dilute acid, of the S02-processing æone, is heated by
indirect heat exchange to above SOC before it comes into
contact with the gas stream w~ch leaves the d.enitration zone,
and that
b) the sojourn time of the gas between leaving the
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7~77
deni~ration zone and e-ntering -the first -tower of the nitric
oxide absorption zone is J at concentrations o~ a-t least ~% b~J
volume of S02 and at least 10% ~y volume of 0? .in the gas enterin~g
the denltration zone, less than 30 seconds,:~,n~e,at lower S02
contents or lower 2 contents in the said entry gas -the upper
li mit of the sojourn -time is calculated from the equation
Z max = ~ 00
[SO2~0[o2]
in which formula Z max is the s~jo-urn time in seconds, [S0
denotes the S02 content in the gas entering the denitration
zone~ in % by volume, and [0~ denotes the oxygen content in
the sarne entry gas, again in % by volume.
The tower system in whicll the process according to
the invention is carried out can be fed wlth S02~ containing
gas which consists of waste gases of known type~ especially
flue gases or roasting~ off-gas~s, such as arise on combustion
of fuels of low sulphur content or ~n roasting of ores contain--
ing sulphur, for example when producing copper, or as arise
~rom other industrlal processes, especially also from processes
of pre~concentration of waste g~s -to increase the sulphur
content therein, and which usually contain between 0.~ and L~S'
of sulphur, preferably from 1 to 3% of sulphur, in the form of
S02, that is to say insufficient S02 for usein the contact
process.
In the waste gas to be treated in accordance with the
process of the invention, i e. in the gas as it- enters the de-
nitrati.on zone, the S02 content should prefe~ably not be less
than 0.2 % by volume. 9
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~79L~7
I-n -the process according -to the invention9 the -towers
of the S02-processlng zone worl~ with a concentration of ~aseous
nitrc~loxides above 1% by volume if' the gas en-tering this
zone con-tains less than 2% by volurne of S02~ whilst if the
entry gases have a higher S02 conte-nt the nitric oxide
concentration in -the gas phase should be more than 2% by
.~
volume.
It is no~ possible to work up gases having an S02
content of the order of less than 2% by volume, preferably of
1 to 2% by vol.ume, and having a water vapour content correspond~
ing to a saturati,on temperature hig~ier than 35G (36 g/m3 of H20),
using a nitric oxide concentratio:n of less than 0.2yo by volume~to
ob~n a sulphuric acid of more than 75~6 strength by weight, by
bringing the moist S02 gases into contact wi,th dilute ac,id in
a drying tower upstream o~ the denitration zone, whereby a
part of their water vapour content is removecl from -the gases,
and water from the entry gases is u-tilised ~`or sulphuric acid
forma-tion by exchange between acid from the drying tower and
the dilute acid production -tower, without -this por-tion o~ the
water passing into -the denitration zone by a gas path~
~ o achieve sufficient denitration i-t is necessary to
heat the acid which is to be charged into the denitration
-tower to above 60C.
According to the invention this is achi.eved, in contrast
to the method of warming described ~urther above, by heating the
nltrose-containing acid which is intended Eor the denitration
tower upstream -~rom the first tower~ tricl;le~fed ~.lt~ dilu-te
- 10 -`
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77
acid, of the S02 processing zone, to above 60C by .indii~ect
heat exchan~e before the acid comes in-to contac-t with the
gas stre~nn
In or~er -to enable the amoun-t of water fed to the
system to be kep-t as low as possible, the concentration of
the acid in the denitration zone is regula-t.ed by adding dilu-te
acid, i.n place of ~la-ter, to the acid charged into the
deni-tration tower.
In -the case of gases containing more than 2% by
vo]ume of S02 an upstream drying tbwer also ofiers ..
advantages as regards the operation of the system, but is
not absolutely essential. I-Ieating to above 60C the nitrose-
containing acid, whlch is passed into the denitr~tion tower,
which with regard to the direction of gas flow is upstream
o~ the first to~er, which is trickle-fed with dilute aci.d,
; permits a drastic reduction in the reaction space for S0~-
processing if
a) bodies of paclcing having a total surface area of more
than 90 m /m are used in the diLute acid production tower or
towers~ and
b) the sojourn time of the gas between leaving the
denitration zone and entering -the first nitric oxide absorption
tower, at 10 or more % by vol~e oxyge-n content in the gas,
is less than 30 seconds.
A-t lower oxygen contents, the upper limit of the
sojourn time, Z ma~for -these gases con-taining more than 2%
by volume of S02 is calculated from the e~uation
~, - 11 -

~@ ~ 7 ~7 7
max = ~3~ Q
[ 2]
wherein [0~]' denotes the oxygen conlen-t in % by volume .in the
gas on leaving the denitration zone.
~ hilst hitherto~ as already mentioned above, the sojourn
time of the gases between the SO~processing æone and the ni~ric
oxide absorption. zone was extcnd2d, for example by i.nterposing
a regeneraticn tower between the two zones, a di.fferent approach
was recognised to be successful in the process according to the
invention, namely to shorten the sojourn t.~me as much-as possible
between the denitration and the nitric oxide absorption, but,
on the other hand, to use packings of sufic:Len~ surface area
so that very intensive material exchange between gas and liquid
is achieved.
Thus, according to the invention, at a lowered oxygen
content, the increase in N02 concentration is not brought about
by using an empty chamber, as in l:he case o~ the Petersen process,
but by increasing the reaction chamber which is charged with
packings and trickle-fed with dilute acid Therefore, in contrast
to known systems, the p~ cess according to the invention aYoids3
as far as is at all possible,leaving any emp~y space, not filled
with packings, intermediate the denitration and the nitric oxide
absorption. It is only the combination of the several features
of the invention which makes it possible to increase the effi-
ciency of a tower system to the extent described, that
is to say?to augment its yield, even when processing
- 12 -
.

7'7
gases containing less than 60~J by volume of S02~ to such a
degree that a daily production of 300 kg of 7~' strength
sulphuric acid per m3 of packed space 9 or even more, can be
achieved,
Pre:ferably9 the plant for carrying ou-t the process acco~dirlg
-to -the invention is so designed, or the process is so con-
trolled, that the reaction space for the nitric oxide absorp-
tion is at least equa] to, or preferably greater -than, -the
sum of the reaction spacesfor denitration and S02~processlng.
If this is not ~he c~se, ni~rogen oxide~ required for carrying
out the process are lost because the nitric oxide absorp~ion
depends prim~rily on the gas volume and not on ~he concentration
oE the nitrogen oxides in the gas. By devising tlle reaction
spaces in the above-mentioned proportions, the total ins~allation
ca~ be kept s~lbstantially smaller than the known installations
for processes of the type initiaLly dcscribed.
It is particularly advantageou~ to carry out the he~t
ing of -the acid fed to the denitration to~er by heat exchange
~i-th the acid discharged ~rom this tower. The temperature
can be regula-ted by par-tially by-passing the heat exchanger.
In the case OI S02 gases containing less than 2% by vol-ume of
S2 it is necessary to employ heat no-t origina-t:ing from the
tower system. The addi-tional amount of heat required is
astonishingly low,and steam or other heating media of a rela-
tively low temperature level can be utilised for the indirect
heating of the acid. In industrial plan-ts~ such heat energy
is frequently available in abundance îrom cooling processes.
:
: . . .; :: ,
., . . :

7~7
Tlle process according to -the invention makes it pos-
sible -to process gases enterlng the denitration tower whlch are
a-t a tem~erature below 60C En-try gases at a temperature of
less than 45C can also still be processed to produce sulphuric
acid even though they have an S02 content of only 1 to 1.5 %
Raisin~ -the -temperature of the acid, before i-t enters
-the denitra~ion tower, to above 80C permits a further reduc- -~
-tion o~ the reac-tion space required for -the S02~processin~.
Sui-table packings for the dilute acid production towers have
proved to be packings according to U.S. Pa~enl 2,867,4~5 or
U S. Patent 3,752,453 and especially those of the type described
further beLow, the manufacture of whlch is simpler, because they
have a large surface area coupled with ~ very low resistance ~o
gas flow.
According to a ~urther eature of the invention, the
water balance of the system can be relieved by bringing the
dilute acid into contact with the current of waste gas in a
tower do~mstream from the last nitric oxide absorption tower,
whereb~ this acid loses water and can be fed at a somewhat higher
concentra~ion to the dilute acid production tower or the deni-
tration tower. The concentrating action o this additional
downstream tower can be boosted considerably by heating the acid
fed thereinto.
In order to effect this heating,hot acid from the tower
system can be employed, as is in itself known for gases of
higher S02 concentration. _ 14 -
..
..
, ~
- :
~ ` ; -

Lt is also possible according to the inventio~, as an
improvemen~ o~ the s-tate of the art described above, to
regulate the NO:N02 ra-~io rellably by keeping the temperature
of the acid entering the last denitration tower constant through
re~ation of the hea-t supply in the acid heater. The concen-tra-
tion of -the acid discharged from the last denitration tower
is a~ -the s~ne -time kep-t constant by introducing dilute acid
or wa-ter into the tower.
In order to regulate the NO:NO~ ratio in the gas before
it enters the nitric oxide absorption towers, the amount of
nitrose~con~aining acid being fed to the denitration tower,
is controlled. According to the invention, it is the variation
in the amotmt of acid for the denitration which serves as the
means of control in the tower system and not the addition of
water, as is mostly customary in known tower systems. This
control system em~loyed in accordance with the invention makes
it possi~le to ~utoma~e th~ tower system.
~ preferred mode o~ carrying out the process according
to the invention in practice is now described in more detail
in relation to the installation sho~l in the-drawing:
In the latter, Fig. 1 shows the assembLy of a plant for
the treatment of S02-containirlg waste gas, for example in
~mounts of 500 Nm3/hour. In the drawing, the reference numbers
1 to 7 denote the towers of the nitric oxide/sulphuric acid
plant; the reference numbers ~ to 12 denote heat exchangers
for liquids, the reference numbers 13 to 16 denote vesséls for
- 15 -
, .

7'
liquid, the reEerence n~lm~ers 131, 1~], 151 and lGl denote acid
pumps, the reference numbers 101, 201~ 401, 501, 601 and 70]
deno~e droplet-ca~chi.ng devices faS described in Eelgian rate~lt
Specification 814,91$ ~nd in the corresponding Ge~m2n Offen-
legungsschri.f~ 2,32!~520 and 6~5 deno~es a ~as scL-ubber as
described in German Offenlegungsschrift 2,414,3L7.
S2 gases are introd-lced illtO a d-ryi-ng ~ow~r 1
through an inlet pipe 102 at the abovementioned r~-t~ of
500 ~m /hour, Dilute acid from the tank 13 is
charged into the drying -tower ] through a line 103, The acid
leaves the tower, which con-tains packings, -throu.gh a line 104
and flows back into the vessel 13oTlleple^dried S02 gases pass
-through a gas line 202 into the denitration zone, which i.n the
presen-t ins-talla-tion consists~the denl-tratiorl tower 2, in
which the gases flow upwards -l;hrough a packing, The -tower
2 is fed, through ~ :Line 20~, with ni.tr~se cont~ining acid
from the vessel 15,-the amoun-t of -the acid being regulted by
means of a valve 2031, The acid i.s warmed in the heat
exchangers 8 and 9, S-team serves as the heat trans~er mediurn
for the heat exchanger ~, The deni-trated acid leaves the
tower 2`through a l.ine 20~4 and is cooled ln the hea-t exchan-
gers 8 and 10 before it reaches the vessel 16, The sulphuric
acid ob~aine~ as final. product in Lhe plan~ ls t.ken ofi~
through a valve 2032 in an outle-t branch of line 204 and
is passed into a s-torage tank,
The heat exchanger lO is fed wi-th ccoling wa-ter,
The exi-t gases from -tower 2, wflich co;ltai~ i~;ric
~~ 16 ~
.
. .

7~77
oxide, pass through a line 302 into the tower 3s that is -to
say the first tower of the S02-processing zone, which,
in the plant shown, also includes -the -tower ~. The gases
flow downwards through a pack:ng, preferably- of bodies of the
type desribed further belowO The to~er 3 is ed through line
303 with dilute acid ~rom the vessel 13, the acid being cooled
in the heat exch~ng~r 11. W~er at 15C is usec' as the coolant.
Due to the reaction t~king ~lace in the tower 3, the acid warms
up and is returned to vessel 13 through ~ line 304.
The gases lea~e the -tower 3 a~t the bot-tom and pass
through a line 402 in-to the second S02~processi~g tower 4,
which is provided with the sc~e -type of packings as -tower 3.The
gas ~c~ path in -this tower ~l is u~warcls ln co~mter-current to ~he
acid which is introduced'at the ~op o~ the tower ~ through a line
403. The acid passes from -tower 4 into tower 3 -through
line 404, and from -tower 3 back to vessel 1~ through -the
line 304. The differerlce in -temperature of -the acid be-tween
entering and leaving the -tower 4 is measured con-tinuously; it
is kept below 2 degrees Centigrade. This difference is a
measure of the amoun-t of S02 which en-ters the -tower 4. If the
difference is -too great, -the trickle feed to the tower 3 is
increased so that more S02 is worked up in -this tower~
No significan-t amoun-t of nitric, oxide is taken up by
the dilute acid in -the towers 3 and 4. The concentra-tion of the
nitrogen oxides in the gas remalns ~nchangecl bu-t -the N02
content increases at the expense of -t'ne N0 content.
17 _
- ,.,.. ~

7~ r
'l'he S02-free gases which contain -the nitrogen oxides in
-the volume rati.o of NO:N02 ~- 1:1 pass -through a line 502 into
the first tower 5 of the nitric oxide absorption zone, which
comprises the towers 5 and 6.
Tower 5 is trickle-fed through line 503 with sul-
phuric acid containlng between 74% by weight and ~0% by weight
of H2S0~l, and th:i.s acid absorbs nitro~en oxides.The acid leaves
the tower 5 through a line 504 and passes in-to the vessel 15.
The exi.--, gases of the tower 5 are drawn in by a :Ean
17 and forced through a line 602 in-to the tower 6, -through
which -they flow upwards. Acid from -the vessel 16 is ~ed into
-the -towe:r 6 throllgh a line 603, Vessel 16 receives
nitrose-free aci.d from the tower 2 via -the line 204 already
referred -t.o,
The acld issuing from the tower 6 passes through a
line 60~ in-to the tank 16, The gases leave the second ni-tric
oxide absorp-tion tower 6 through a line 702 and flow upwards
through the acid dehydra-tion tower 7 ~rhich is downstream from
the nitric oæide absorption zone,
The tower 7 receives dilute acid from the tank 14,
and this acid can be warmed indirectly by means of s-team in
the heat exchanger 12. A small degassing tower, which is not
shown in -the drawing, can additionally be fi-tted into a line
703, upstream from the heat exchanger 12. In this degassing
tower, traces of nitrogen oxides can be flushed out of the acid
wi-th a small amount of air. The amount of air required is
less than 20 Nm'/hour and can be introduced. in-to the
1.8 -
'' , : '

374~
plant upstream from -the fan 17
In the tower 7J the acid loses water and its -tempera--
ture drops before the a~dis returned to the -tank 14 throu~
a line 7040
A small amount of acid can be fed con-tinuously in-to
the ve~ell~ through a valve 7041 in line 704 A liquidlevel-
equalising line 133 is provided be-tween -the tanks 1~ and ll-~.
The volume in the dilute acid circulation, which takes
place via vessel 13, increases in accordance with the forma-
tion of sulphuric acid in the S02-processing -towers 3 and 4.
In accordance with the amount produced, dilute acid is passed in
-to -the top of the denitration -tower 2 through a valve 1031 in
a branch 105 of the line 10~.
The reference n~bers 4021, 5021. 6021, 7021 -and 8021
d~note transparent lengths of l.ine in which the colour of
the stream o:E gas can be observed
Finally, the S02-free gases are led into the atmos-
phere through a line 705.
A metering device 18 serves to add water through li.ne
104 to the dilute acid circulation. The supply of nitric acid
to the system to form. nitrose is not shown. Ni-tric acid
is added at the top of -the denitration tower 2 by means of a
metering device and the level of nitrose in the systern
is kept at the desired magnitude.
The operation of -the installation shown in Figure 1 of
the drawing is further explained hereina~ter, with the aid of
a number of ill-ustrative examples:
- 1~
. . ... ...... .
. . . . :

7~77
Ex2.mE~ ,lel
Washed gases from a sulphide roasting process contain
1 to 1.5% by volume of S02 and about 0.05S~ by volume of ni~-ogen
oxides. The gases are a-t a -temperature of 35 - 40C and are
saturated wlth steam. Processing in an ins-tallation according
to Figure 1 results in the following operating conditions:
- 20 -
,.
.: , . ... .
, ~ . ~ . , .:
. .. .. ... , ., . -.. ` i. .. , :

77
___ o . ~o ~o __. _ . .
_ _ O Lr~ ___ _ __ ~ . .~ ,
~) ~t ~) ~1 ~ J O ri ~
O L~ _ O L(~ ____ ~ ___ ~O _._ __
~ _ _. _ _~. _~ 0 ___.
~ J O ~0 J ~ J O; ~, J
~' 1-- -- - - ~- - ---
O ~ _ J ~_ O J Lr~ _____ ____
t`~l ~i ~ ~., (~ O O O, __~0
~1 O ~O L 1~ ~ l J ~
~__ __ _ __ _ ~
~ -1'
.~ ~ (\1~ N~3 O O ~e o bl~ O O
. O 't~ .
0 ~
o o h o,~) o o ~ ~, ~ o
bD tH t~ CH t~ ~ t.) ~0 ~ bD
¦ ~ ~ X h ~ ~0 (> h
h o ~ E~ . ~ Z.~ ~
.
21
. . .
~ .~ : '. `. :
. :: :.: . : ::: : .: . ... :
. - : ::~

7~
The gas washing device 605 in the upper part of the
-tower 6 shown in Figure l of the drawing t iS not used in this
example, Tower 6 opera-tes as a normal packed tower. Tower
2 is insulated against hea-t losses.
The hea-t energy supplied by means of steam is about
goo Kcal/kg of lO0,'. s-trength H2S0~. Because of -the con-tent of
nLt~P~en oxides in -the gases entering the installa-tion, the manu--
facture o~ H2S04 does no-t cons~e any nitric acid. At -the
exit of -the deni-tration zone 9 the oxygen conten-t is abo~rt 8o
by volume and -thc nitric oxide conten-t abou-t 1.8% by vol~e~
The optimum entry temperature of the acid into tower 2 is 83C,
A fan pressure of a -total of 250 mm wa-ter column is required to
convey -the gases -through a11 Seven LoWerS,
The S02 con-ten-t of -the exi-t gases is less than 0.003Yo
by volume,
F,xam~ 2
S02-contailling gases rom a plan~ for splitting wast:e sul-
phuric acid are cooled in a purification installa-tion and freed
from the hydrogen halide acids which -they con-tain, The gases
are at a temperature of about 50G and are saturated ~ith
water vapour, The S02 conten-t ls 4 to 6~o by volume and the 2
conten-t is more than lO~' by volume, Processing -the gases in
an installation as shown in -the drawing, but wi-thout tower 7
and withou-t vessel 14~gives the following operating conditions:
- . ~
: - . : . . , :
. . .
. :

3Lg;~7~77
_ ___ __ __ , _ _~ - .
~D ~ ~ ~ O ~ ~
_ __ _. _ _ _ ---- --~ 1 ----- -- ------
~ O ~'~ ~D ~ ~ ~ ~
_, ~i ~i
. __ ___ _ _ _ __
~D O ~ ~ ~D O
. ~ ~ ~ . .
~ _ o _ ._,._ __ _ o __ .._.._
a) ~ ~ o co ~ O ~ ~
-~ ~ ~ 2 ~ ~ ~ o ~ ~
~1 ~ _ __ __ _ I ~ _. ... _
o r-~ C\l
. L~ O ~ ~ C~ O ~ C~l
~ ~ ~i C~l ~ ~ O' ~i ~
_ _._. __ .___ __~. ___ _ .__ . ...
~D ~D u~ O ai~ ~ ~1
r-l O C~ ~ ~ ~ l ~i L~
_ _ . . _ __ - - - 1 -------
,~
?
.~' ~ ~
.~, C\l ~ o ~ C~ V
~ r(~ ~ ~E3 oV (~) ~ ~ u~
~ __. __ _ ------- ~d ~ ~
rl, ~ ..
o S:~ ~ ~ ,~
~ ~ Q~ a) ~ ~ c)
g ~ ~ ~ ~ ~ ~, ~ a~
+~ V ~1 h c~ O ~ ~
~ ,~ O ~ ~ ~V $;' +~ 4-1
$ ~ O F ~ ~V ~ O
O ~ C~ ~ (V ~ ~ ~
c~ ~ o (V 1~ a) ~ ~v
~ ~V .~ o -~ h
~0 ~ ~ ~1 ~ C~ (V a> ,~ ~v
~ O ~ ~ ~0 ~, ~0 -1' ~0
.,1 a~ a~ ~ ~1
c~ c~ ~ ~ ~ cq ~ +~ ~ ~ c~
~i c~ ~: ~v a~ o ,~:' ,~ a
~1 ,~ ~ ,~ ~ o ~0 C) ~ C~
cV I_I ~1 ~ ~q Cq ~ Cq ~r-¦ C~ '~
O ~ r5 __E-~_ ~) _~ ~ ~ ~ -~i L ~ -~
. ... . ,. . ~ .. . ........ ...
.. ,.. ~ .. ~ ,. . ,. -; ~,
. . - ~ ,
., ~......... . .
.. . .. .

4L7'~
The nitric oxide concentratlon in -the gas between
denitration and nitric oxlde absorption is 4.5 to 5~5% by
volume. The temperature of -the aci.d when entering tower 2 is
~1C. The acid produced has a conGentra-tion oi 79O by weight
and contains less than 0. 01~o by volume of ni-trose, calcula-ted
as HN03 (IOOYo). I-t is no-t necessary to supply hea.t during
-the operation. However, when starting up -the plant~'it i's o~
advantat~e to bring -the acid temperature in ~ower 2 r~pidly to
.. . . ... . . .. ..
the optimum value, for which purpcse an indirectly steam-heated
heat exchanger is suitable. o.6 kg of HN03 (100~ s-trength)
is consumed per 100 kg of H2S04 (100io streng-th) when the gas
scrubber 605 (Figure 1) is in ox~er.cltion.In thclt case, -the gas
resistance of -the sys-tem consis-tin~ of 6 -towers is ~0 mbars.
Withou-t operating the gas scrubber in the upper par-t of -the
tower 6, the ~lN03 conswnp-tion is 1. lSo by weight and the gas
resistc~nce is 24 mbars. The sojou:rll ti.mc of the gases bet-
ween denitrati.on and nitric oxide absorpt:ion is 19 seconds.
The exi-t gases contain l.ess than 0.003~0 by volume of S02.
The space-time yield is 310 kg of sulphuric acid, con-taining
78% by weigh-t of H2S04, per day per m3 of packed space of -che
tower system.'
Com~ e~ le ~
Processing the same S02-containing gases as in
Examples 1 and 2~ but in an installation without indirect
heating of the ni.trose-containing acid for the
de-nitration tower, and wi.-thout an upstreaM dryi-ng -to~,~er and a
do~nst~eam acid deh~drati.on -tower proves -to be imposslble
because th~ concen-tra-tion of the acid for -t:he ni.-tric o,~ide
:
, . , ~ ~ . .
: ,
, ~ . , :
.

absorption falls below 74 % by weight and the system loses
its nitrogen ox~ides durlng ope~a-tion ~ithin a few hours.
5~ _
Processing the same S02-containirlg gases as in the
preceding exalilples, but without heatillg the nitrose-containing
acid for the denitrati.on tower in a sys~em according to
Figure 1 is impossible because under these circumstances
sufficiently denitrated acid cannot be produced.
If a production tower which is fed, in the manner
of a PETERSEN to~er plant, with acid from the first nitric
oxide absorp~ion tower (nitration tower), i.s inserted
upstream from the first dilute acid production tower, the
system can only be operated if the amount o~ gas being
processed is reduced. The result~ of the ~bove-mentioned
intercalation of the production tow~r in tlle plant can be seen
from the previo-usly ci~ed Iiterat~lrë reference of Winnacker- ;
Kuchler,.page 45, Figure 26a. In -this case, the ma~irnurn nitric
oxide concentration in the gas is abou-t 2.8% by vol~ne against
5.5S~ by volume in the process of the invention. In order to
achieve the low nitric acid conswnption of the process accord--
ing to t.he invention, the gas throughput in the known process
.. . . . . . . .. ..... . . . . .. . . . . .. . . .. . .. ... . . .
.lnust be throttled so much that the space-time yield. alls off to
ol~yl40 kg of sulphuric acid, containing 78q' by weight o.. H2S04,
per day per m' of packed space of the tower sys-tem.
:
. :- .

1~747~
The packings employed in -the dilute acld towers of -the
S2 processing zone are i.n-tended to produce as large a surface
area as possible within -the res-tric-ted volume of the -~ower,
Hitherto, packings in the form of wire-like st.ructures,
in partlcular, have provecl especially effec-tive for -this
purpose, since with such packings the ratio of the space
occupied bytto the space actually :~.illed with,the material
c~ the packings is extremely high. One of the best hithert~ -
known packings of this t~7pe is described in U.S. Patent
No. 2,867,425. I~ consists of synthetic plastic material
and is in. the forrn of a spiral of which the begilming and the
end are connectetl to one another, Ho~ever, l,his kno~l packing
has the disadva-ntage that because of its complicated shape i-t
can only be pro~uced with relative difficulty and is corres-
pondingly expens.ive,
This di.sadv~ntage is avo:i~ed with 1 packing which con-
sists of small bars arranged subs-tantially parallel to one
another on a carrier,
In the tex-t which follows, this packing is explained
in more ~etail in relation to -the illustra-tive embodiment shown
in the drawing. In this:
Figure 2 shows a view a-t right angles -to the small
bars~ and
Figure 3 shows -the illustrative embodiment in a view
parallel to -the small bars.
As sho~n in the drawing, th2 packing consists of a
plurality of paralle]. srnall bars 190f cirGul.ar cross-sec-tion,
- 26 -
. . ~ ,
. . .,~
.. .. . . . ..

~ ~. 7 47 i~
which are arranged o.n a join~ carrier 20. Tne carrier 20 is
axially symmetrical relative -to an c~xis parallel to the small
bars 19, The small bars 19are so arranged on the carrier
and their leng-ths are so chosen, that -the erltli~e packing 21
occupies a space of ~pproxi~ately spheric~1 sllape. Of course,
a different envelope, for exaiDple an ellipsoidal envelope, of
the packing ~ould also be possi.ble,
The carrier 2 comprises three parallel rings 2a, 2b
and 2c and a disc 2d, The three rings and the disc are held
together by means of ribs 2e and in particular in such a way
that they form an essentially conical basket, Ihe resul-t of
-this arrangemen~t is that the interspaces between any two rings
and two ri.bs do no-t become too small and thus do not o:Efer a
high resistance -to flow, The largest ring 2a is provided on
its outside wi-th radial extens iOllS 2f which each carry a
small bar 1 at their ends,
Instead of a conical b~sket, a pyramidal basket can
also serve as the carrier for the small bars, In -that case,
t.he parallel rings are rèplaced by polygons such as, for
example, triangles, squares or hexagons. It is advantageous
to provide at least two rings each wi-th more than 3 small bars,
The packing described above can be manufactured very
easily by injection moulding.since the mould required for the
purpose is itself relatively slmple, The mould need onl~J con-
sist of two parts, with the parting surface being conical. and
coinciding with the outer envelope of the ri~s 2e, The nega
tive cavities corresponding -to t'ne rings and ribs are then,
- 27 -

`7~7
respec-tivel~ sim~ly annular grooves,or ~l~y are'g~ves r~n~ngin~C
direc-tion of ~he envelope lines of -the -two n~ould pa~ in~r sur-
faces. Parallel bores in the two mould halves corresporld to
the small bars. The bores can be cylindrical or p~eEerably
conical, the la-tter for easier mould release
In or~er -that the packing described can perlec-tly
fulfil the f~c-tlon in-tended for it, -the distances between a~y
two parallel small bars should no-t be less than about twice
their diameter. Preferably, the distances are about ~hree to
-ten times the diameter of the small bar~ Furthermore, the
meshes of the basket-like carrier, that is to say -the in-ter-
spaces each defined by -two rings and -two ribs, shoulcl a]so be a~
large as possible. If -these conclitions are fulf'illed, -th~
packing~ has no pronounced preferred direc-tions in re3pec-t of
resis-tance to flo~g that is to say it is approximate]y equall~
e~fec-tive in any ~esired posi-tion. Measurclncnts in a gas
scrubber installation have shown ~'hat,especially a~ higher
flow speeds and lower concentrations of the materials to be
removed by scrubbing, this preferred packing is superior, in
respect of the degrees of absorption achievable therewith,
to the known splral packings, mentioned earlier, even when
the latter are used under normal conditions.
The terms ~Initrose acid'l or l~ni~rose--con~aining acidl'
are used herein to designate a dilu~ed sulfuric acid having a
content of tlp to 85 weight percent H2S04, in which acid nitro-
sylsulfuric acid, (NO)HS04~ and/or nitrogen 1rioxide (N203)
are dissolved. Such acid is alsn reerred ~o as llnitrclted acid'
- 2~ ~
. :
;:

7~
in some textbooks. "Nitrose" des;c~nates a mixture of nitrogen
oxides the nitrogen-to-oxygen ratio of which corresponds to
the formula N20 .
- 29 -
, - ,
. ~ , ; . ,.
.