Process and apparatus for preventing NO.sub.x emissions after emergency shutdowns of plants for the manufacture of nitric acid

The process and apparatus described seeks to prevent NO.sub.x emissions after an emergency shutdown in a process for manufacturing nitric acid by catalytic combustion of ammonia, compression of the nitrous combustion gases and subsequent absorption. Immediately on shutdown the intake nitrous gas supply to the compressor is interrupted, the residual intake side nitrous gases are conveyed to the delivery side, the delivery side gas volume is shut off, the gas on the delivery side is expanded to the intake side and then exhausted into a vacuum system. Subsequently the acid charged with NO.sub.x in the absorption stages is drawn off, degassed acid is circulated through the absorption stages until equilibrium is established, the pressure of the shutoff section is released and cooled acid is admitted to the absorption stages.

The invention relates to a process for preventing NO.sub.x emissions after 
emergency shutdowns of plants for the manufacture of nitric acid by 
catalytic ammonia combustion with air, compression of the nitrous 
combustion gases and chemical and/or physical absorption of the nitrous 
gases from the gas stream. 
The invention is also concerned with an apparatus by which this process is 
carried into effect. 
By way of example, with the process disclosed in U.S. Pat. No. 3,676,065, 
the compressed nitrous gases are cooled and are conducted through a 
chemical absorption stage, a physical absorption stage and also a 
post-absorption stage for holding back acid vapors. 
In the chemical absorption stage, a part of the nitrous gases is converted 
into over-azeotropic acid, e.g., 70 to 80% HNO.sub.3. In the physical 
absorption, the nitrous gas, which is still in the gas stream, is washed 
out under pressure down to acceptable traces, e.g., 220 ppm (V). The 
charged washing or scrubbing acid is freed from the physically dissolved 
NO.sub.x in a desorption stage by means of air being blown therethrough; 
the air with a high NO.sub.x content is sucked in by the nitrous gas 
compressor. The over-azeotropic nitric acid formed in the chemical 
absorption, after desorption of the physically dissolved NO.sub.x, is 
separated by rectification into a highly concentrated, e.g. 99% nitric 
acid, forming the final product, and an azeotropic nitric acid (68 to 69% 
HNO.sub.3) returning into the process. The rectification stage is 
operated, depending on the choice of material, under vacuum or at 
atmospheric pressure. 
In the continuous operation of nitric acid plants, the emission of NO.sub.x 
may be further controlled by other suitable absorption stages and/or 
catalytic tail gas purification stages. In the event of sudden emergency 
shutdowns of the plant, which may, for example, be caused by failure of 
the electric current and thus of the electric driving motor of the nitrous 
gas compressor, nitrous gases frequently do escape into the atmosphere. 
Such escape is generally caused by (a) opening of the blowoff valve on the 
pressure or delivery side of the nitrous gas compressor, resulting in 
release of compressor gases to prevent the "pumping" of the compressor, 
and (b) subsequent or simultaneous expansion of gases in the pressurized 
system via the chimney into the atmosphere. Attempts have been made to 
avoid the emission caused by step (a) by venting gas which is to be blown 
off through scrubbing towers at approximately atmospheric pressure. 
However, the washing or scrubbing out of the NO.sub.x at atmospheric 
pressure is not satisfactory. In connection with the release of gases 
resulting from step (b), it has not been possible to achieve a sufficient 
scrubbing of the nitrous gases trapped in the plant without the assistance 
of an emergency power unit. 
On the other hand, it is highly undesirable that the nitrous gases should 
remain in the plant, since they are capable of causing considerable 
corrosion and make difficult the normal re-starting of the plant. As a 
consequence, emergency shutdowns have heretofore always resulted in 
environmental pollution effects caused by escaping nitrous gases. 
An object of the present invention is to avoid environmental pollution 
caused by emissions of NO.sub.x, which occur after emergency shutdowns of 
plants for the manufacture of nitric acid as a consequence of the then 
necessary removal of the nitrous gas which is present in the plant. With 
this removal the nitrous gases which are trapped in the plant should not 
be lost. Furthermore, it is desirable that no additional utilities, more 
especially no fuel, and only limited additional equipment, and more 
especially no emergency power unit and no large gas vessels, should be 
needed for eliminating the nitrous gases from the plant. 
The present invention is accordingly concerned with a process for 
preventing emissions of NO.sub.x after emergency shutdowns in plants for 
the manufacture of nitric acid by catalytic combustion of ammonia with 
air, compression of the nitrous combustion gases in a nitrous gas 
compressor having an intake side and a delivery side, and chemical and/or 
physical absorption of the nitrous gases from the gas stream in one or 
more chemical or physical absorption stages, which process comprises the 
steps of immediately upon such emergency shutdown-- 
(a) interrupting the supply of nitrous gas to the intake side of the 
nitrous gas compressor; 
(b) conveying the nitrous gases which are on the intake side through the 
nitrous gas compressor to the delivery side; 
(c) shutting off the gas volume which is on the delivery side of the 
nitrous gas compressor thereby establishing shutoff pressure sections; 
expanding the gas volume which is in the nitrous gas compressor to the 
intake side of the compressor; and drawing off by suction the gas volume 
which is then on the intake side into a vacuum system; 
and thereafter-- 
(d) drawing off acid which is charged with NO.sub.x and which is in the 
chemical and/or physical absorption stages; 
(e) circulating degassed acid through the chemical and/or physical 
absorption stages, and 
(f) after establishing equilibrium in the chemical and/or physical 
absorption stages, expanding the gases in the shutoff pressure section 
downstream of the absorption stages while simultaneously charging said 
absorption stages with cooled acid. 
The process according to the invention thus serves to reduce emissions of 
NO.sub.x from nitric acid plants, which consist of at least the following 
apparatus or plant components: 
nitrous gas compressor 
absorption system for nitrous gases (on the pressure or delivery side of 
the nitrous gas compressor). 
The process and apparatus of this invention are not limited to specific 
absorption means. For example, the absorption system may consist of only a 
chemical absorption stage or only of a physical absorption stage or of a 
combination of both absorption stages. In addition to the absorption 
system, the following processing stages may also be employed for 
increasing the NO.sub.x content: physical absorption combined with a 
desorption or chemical absorption combined with a decomposition stage. 
In the manufacture of highly concentrated nitric acid, plant components may 
be connected downstream of or in parallel with the absorption system, as, 
for example: 
unit for rectification of over-azeotropic nitric acid into highly 
concentrated acid and azeotropic acid, 
bleaching column with N.sub.2 O.sub.4 liquefaction and HNO.sub.3 formation 
with oxygen in autoclaves (e.g., the HOKO process, as described in 
Hydrocarbon Processing, 45, 183-188 (November 1966). 
The presence or absence of the latter stages does not have any deleterious 
effect on the process according to the invention. 
The process of the present invention accordingly covers the approximate 
time interval from the emergency shutdown due to power failure or 
compressor failure up to substantially complete depressurization of the 
gas in the plant. 
After such process is carried out, the plant is in a state such that it can 
be re-started under conditions similar to those occurring after a normal 
shutdown. The process of the invention comprises two phases. The first 
phase, with the process steps (a) through (c), has to start in practice 
immediately upon emergency shutdown, for example, caused by power failure, 
because it is by these stages that the nitrous gas compressor is mainly 
protected against "pumping" and this must take place immediately. The 
second phase of the process (process steps (d) through (f)), can be 
carried out subsequently or later, when all conditions are favorable for 
the purpose, i.e. electrical energy and other auxiliary means are once 
again available. By the automatic shutting-off of the nitrous gas volume 
in the plant on the delivery side or downstream side of the nitrous gas 
compressor, any escape of such nitrous gas in the meantime is thereby 
avoided. 
Step (a) of the process of this invention serves to substantially prevent 
formation of any new nitrous gas. Consequently, the volume of nitrous gas 
to be eliminated from the plant is that volume which is present in the 
plant after step (a) has taken place. During step (b) the nitrous gas 
compressor advantageously continues to run for a period of at least about 
10 seconds, preferably from about 10 to about 30 seconds. During this 
period the gases are passed into a separate vessel kept under atmospheric 
or a lower pressure during normal operation. Depending on the 
characteristic of the compressor, the mean final pressure will be in the 
range of approx. 50% of the normal operating pressure. By continuing to 
run in this manner, the nitrous gas compressor draws off by suction the 
nitrous gases which are in the ammonia combustion section and in the 
desorption section, respectively, or which are in the decomposition stage 
of the plant, and conveys these gases into the pressure section of the 
plant. As a consequence, these nitrous gases are replaced by air drawn in 
from the atmosphere. This air which is drawn in is thereafter generally 
not completely free from nitric oxide but, as a consequence of 
after-desorption, it still contains relatively small quantities of nitric 
oxides. The energy for this time limited run-on of the nitrous gas 
compressor may be partly supplied by the expansion of tail gas in the tail 
gas expansion turbine, which supplies energy to the compressor. Such 
energy may also be supplied from the rotational or inertial energy of the 
compressor and/or of another part coupled to the compressor. The 
compressor thus has sufficient mass so that rotational energy stored 
therein, combined with any other means employed, provides the desired run 
on time. The further running time of the compressor is, in any case, such 
that at least substantially all the volume of nitrous gas which is on its 
suction or intake side of the compressor is conveyed into the described 
system on its pressure or delivery side. This time interval and the size 
of the vessel required can be easily determined from the gas volume to be 
delivered and the characteristics of the compressor. At the same time, in 
step (c), the gas volume which is on the pressure side of the nitrous gas 
compressor in the absorption section of the plant is shut off 
hermetically, so that it is unable to escape from this section of the 
plant during subsequent operational steps. After the time interval 
described above, the blowoff valve to the suction side is opened, so that 
the gas volume still present in the nitrous gas compressor and the 
adjoining pipe section is able to flow to the suction side of the 
compressor and the undesired "pumping" of the compressor is avoided. 
Finally, about simultaneously with the opening of the blowoff valve, or 
shortly thereafter, the volume of nitrous gas present in the plant on the 
intake side of the nitrous gas compressor, in the nitrous gas compressor 
and the connecting pipe section on the pressure or delivery side and, if 
present, in the desorption section or the decomposition stage is sucked 
out into a vacuum system which is ready for that purpose. Whereas 
substantially all of the nitrous gas on the suction side of the nitrous 
gas compressor and from the compressor itself is removed by step (b) and 
is replaced by air, an additional safeguard is provided by the suction of 
the indrawn air into the vacuum system, since this system is 
advantageously so designed that it is in turn able to take all the gas 
volume of those apparatuses connected on the suction side of the 
compressor. This suction step thus also removes from the intake section of 
the plant those small quantities of nitrous gases which pass by subsequent 
desorption into the gas phase. Removal of gas by suction into a vacuum 
system also prevents emissions of NO.sub.x in the more unusual event of an 
emergency shutdown of the compressor. Such a shutdown could be caused, for 
example, by interruption of the main drive, e.g. by electric motor, and 
the tail gas turbine. 
The second phase of the process of this invention may start immediately 
following the first stage or may take place at a later time. The second 
phase serves to eliminate from the delivery or pressure side of the plant 
the trapped volume of nitrous gas which is under pressure, that is, the 
shutoff pressure section. 
Initially, in step (d) acid which is charged with dissolved NO.sub.x which 
is obtained from the sumps of the chemical and/or physical absorption 
stages (coming from the plates and from the packing layers, respectively) 
is drawn off into a vessel provided for this purpose. Then in the 
following step (e) degassed, cold nitric acid is supplied from another 
vessel into the chemical and/or physical absorption stages and is 
circulated through said stages until a state of equilibrium has been 
established. The time necessary for this purpose may be from a few minutes 
to several hours, and is generally in the range of from about 0.5 to about 
2 hours. With this circulation of acid and depending on the operating 
conditions, a part of the nitrous gas present in these stages can be 
chemically converted into nitric acid. Another part of such nitrous gas is 
physically dissolved in the nitric acid. As a result, a gaseous NO.sub.x 
in the absorption stages can be lowered substantially. For example, a 
diminution of the amount of such gaseous NO.sub.x by about 50% or more can 
be achieved. In a plant which produces 200 t/d of concentrated HNO.sub.3, 
the volume of circulated acid can be as low as about 4 cubic meters, and 
preferably is at least from about 6 to 8 cubic meters. After the state of 
equilibrium has been established in step (e), the circulation of acid is 
stopped and the pressure of the gas in the plant is released. During the 
release cooled nitric acid is delivered to the absorption stage or stages, 
and the absorption is run with substantially the same operating conditions 
as with the normal production, i.e. with substantially the same acid 
concentration, substantially the same acid temperature and substantially 
the same specific flow rate of acid per cross sectional area. The 
after-absorption also remains in operation. The nitric oxide content is 
further lowered by the contact of the gas escaping into the atmosphere 
with the acid supplied to the absorption. The supply of acid is 
immediately stopped when atmospheric pressure is reached in the absorption 
system. During this release of pressure, with a plant producing 200 t/d of 
concentrated HNO.sub.3, from about 10 to about 30, preferably abut 20 
cubic meters of acid are supplied to the absorption stage. The time of 
pressure release in this case amounts to about 20 minutes. The quantity of 
acid delivered and the quantity of acid remaining from step (e) in the 
plant remain in the system for restart-up of the plant. During the period 
of pressure release, e.g. about 20 minutes, the average NO.sub.x content 
of the blow-off gas amounts to about 600 ppm (volume basis) with the mixed 
gas blower being in operation. 
In accordance with a preferred embodiment of the process of this invention, 
the gas volume which is in the compressor is expanded in step (c) into the 
inlet of the gas-cooling means after the combustion of ammonia. Since the 
gas volume is sucked off from a point downstream of the outlet of the 
gas-cooling means into the vacuum system, the warm gas coming from the 
compressor flows through the gas cooler before it is sucked off, as a 
result of which, the effective gas volume is slightly reduced. 
In accordance with a preferred embodiment for the manufacture of 
concentrated nitric acid, the gas volume existing on the suction or input 
side of the compressor is sucked off into a rectifying column operating 
under vacuum. It is unnecessary in this case to provide a separate vacuum 
vessel. If necessary, the rectifying section operating under vacuum can be 
designed to be of such a size that substantially all the gas volume 
existing on the suction or intake side of the compressor can flow off into 
the rectifying system. 
In a particular embodiment of the process of the invention, the acid 
circulation through the absorption stages according to step (e) is carried 
out after adding oxygen or hydrogen peroxide. As a result of the 
circulation of acid in the presence of additional oxygen or hydrogen 
peroxide, the quantity of nitric oxide in the gas phase is further reduced 
by additional formation of nitric acid. In a plant, as mentioned above, 
with a capacity of 200 t/d of concentrated HNO.sub.3, the acid 
circulation, in the presence of additionally about 50 cubic meters of 
O.sub.2 causes a further reduction of the amounts of nitric oxide of the 
gas phase by about 25%, i.e. in step (e), altogether by about 75%. Since 
emergency shutdowns do not occur too frequently and oxygen is readily 
available in pressure vessels, e.g. for welding purposes, the additional 
use of oxygen can be economically justified if very strict regulations as 
regards the emission of NO.sub.x exist. 
The supply of nitrous gas in step (a) may be interrupted by shutting off 
the supply of ammonia for the combustion and possibly interrupting the 
return of acid charged with NO.sub.x to the desorption or decomposition 
stage. Care is to be taken in every case that, from the instant of the 
emergency shutdown, no new nitrous gas is any longer formed and/or passes 
into the gas phase on the suction or intake side of the nitrous gas 
compressor. 
The apparatus by which the process of this invention is carried into effect 
is characterized in that the running time of the nitrous gas compressor, 
in the event of emergency shutdown, is such that at least substantially 
all of the volume of nitrous gas which is on the suction or intake side of 
the compressor is conveyed into the delivery side system. Such running 
time is generally at least about 10 seconds, and is preferably in the 
range from about 10 to about 30 seconds. This running time can be 
accomplished by drive by the tail gas turbine or an increased mass of its 
rotating parts as compared with the normal construction or both. This is 
valid under the provision that the pressure on the delivery side of the 
compressor is reduced as compared to the normal operating pressure on that 
side. 
In this respect, the increased mass which results in a higher angular 
momentum may be provided not only in the nitrous gas compressor, but also 
in other drive units coupled with such compressor, e.g. the tail gas 
turbine. As a result of the longer slowing time, it is ensured that the 
gas volume located on the suction or intake side of the nitrous gas 
compressor in the plant is conveyed at least once completely to the 
pressure or delivery side. 
Furthermore, valved blowoff pipes advantageously connect the delivery side 
of the nitrous compressor with the intake side of the compressor, at a 
point between the ammonia combustion unit and the nitrous gas cooler. The 
point of connection of the blowoff pipe on the delivery side is upstream 
from the delivery side shutoff valve. By means of this blowoff pipe, 
process step (c) allows the gas in the nitrous gas compressor or on its 
delivery side before the shutoff valve to be expanded into the suction or 
intake side. Furthermore, provision may be made for a valved pipe 
connecting the delivery side of the compressor, upstream from the shutoff 
valve, with a vacuum system. By means of the said pipe, the gas volume 
located before the shutoff member can be sucked off into the vacuum 
system. The valves and shutoff members which are provided are for the 
major part operated automatically.

Air and ammonia gas are respectively fed by way of the pipes 1 and 2 to a 
mixed gas blower 3. The gas mixture is burned to form nitrous gas in the 
ammonia-combustion unit 4 which is equipped with a waste-heat boiler. The 
nitrous gas flows at about atmospheric pressure, e.g. from about 0.8 to 
about 5 bar, preferably from about 1 to 2 bar, more preferably about 1 
bar, through the pipe 6 and gas cooler 5 to the nitrous gas compressor 7, 
in which the stream of nitrous gas is compressed to a pressure of from 
about 5 to about 15 bar, preferably from about 8 to 12 bar, most 
preferably to about 9 bar. The compressor 7 is driven by an electric motor 
8 and a tail gas turbine 12. The compressed nitrous gas flows from the 
compressor 7 via pipes 6 and 9 to a gas cooler 15, where it is cooled from 
its temperature of about 150.degree. C. to a temperature from about 
30.degree. to about 100.degree. C., preferably from about 50.degree. C. 
to about 70.degree. C., and most preferably about 60.degree. C. The 
nitrous gas then flows into the chemical absorption column 16, in which it 
is brought into contact with azeotropic acid supplied through pipe 24. 
Some of the nitric oxide is converted into nitric acid. The over-azeotropic 
acid leaves the chemical absorption stage 16 through pipe 30. The effluent 
gas of the chemical absorption state 16, which gas still contains 
NO.sub.x, flows via pipe 9 to the physical absorption stage 17, in which 
the residual nitrogen oxide is physically scrubbed almost completely from 
the gas stream. The gas then flows through an after-absorption stage 18 
for removing acid vapors and acid droplets from the gas stream. The 
effluent tail gas is heated by heat exchange in the stages 4 and/or 7 
and/or 15 and, with the valve 34 being closed and the valve 11 being open, 
is then expanded in the tail gas turbine 12 and is conducted via pipe 19 
to the stack 20. The acids discharging from the absorption stages 16 and 
17 by way of pipes 30 and 28, respectively, are freed from gas in the 
desorption stage 27 by means of air supplied from the atmosphere via pipe 
26. The degassed, over-azeotropic acid coming from the chemical absorption 
stage 16 passes by way of pipe 30 into the rectifying stage 23, in which 
it is split up into product acid, having a concentration of 99% which is 
discharged via pipe 25, and azeotropic acid, which is recycled via pipe 24 
into the stage 16. The absorber acid from the absorption stage 17 which is 
degassed in the desorption stage 27 is recycled via pipe 29 into the said 
stage 17. The air charged with the desorbed nitric oxides and coming from 
stage 27 is once again drawn in by the nitrous gas compressor 7 via pipe 
31 having the shutoff valve 32. 
With the emergency shutdown, for example caused by failure of the power for 
the driving motor 8 of the nitrous gas compressor 7, the emission of 
NO.sub.x is almost completely prevented in the following manner, when 
using the process according to the invention: The supply of ammonia to the 
ammonia combustion unit 4 is interrupted by closing the valve 2a, and the 
supply of acid charged with nitric oxides to the desorption stage 27 is 
interrupted by closing the valves 36 and 37. As a consequence of the 
short-time run-on of the compressor set, the compressor continues to run 
until the gas volume which is on the suction or intake side of the 
compressor, i.e. the gas volume in the apparatus 4, 5, 27 and the intake 
side pipe sections 6 and 31 is conveyed to the pressure or delivery side 
of the compressor respectively into the vessel 38. The valve 39 is 
thereafter closed and valve 14 consequently opened, so that the gas volume 
which is present in the nitrous gas compressor 7 is able to flow by way of 
pipe 13 to the nitrous gas pipe 6 on the intake side of the gas cooler 5. 
After opening of valve 39 the volume of nitrous gas on the pressure side 
of the nitrous gas compressor is shut off by closing the valves 10 and 11 
with the valve 34 being closed, so that such gas is confined to the plant. 
Finally, the valve 22 is opened, so that the gas volume which is present 
in the apparatus 4, 5, 7, 27 and the pipes 6, 13, 31 is able to flow via 
pipe 21 into the rectifying system 23 which was maintained under vacuum. 
The gas volume which is trapped under pressure in the absorption section 
between the valves 10, 11 and 34 is removed from the plant in the 
following manner at a later time, when the driving energy for the 
circulation pumps of the absorption columns 16, 17 and for the mixed gas 
blower 3 is once again available. Initially, the acid which is charged 
with NO.sub.x and which is still present in the absorption stages 16 and 
17 is drawn off into a separate vessel (not shown) and is replaced by 
degassed acid. This acid is circulated through the absorption stages 16 
and 17, until the equilibrium has been substantially established, i.e. the 
nitric oxide contained in the columns has been partially converted 
chemically into nitric acid and has been partially dissolved physically in 
the circulated acid. After this equilibrium has been achieved, the 
circulation is stopped. The gas trapped in the absorption sections 9 and 
15-18 is expanded by opening the valve 34, via the pipes 33 and 19 to the 
stack 20 with the absorption column 17 being simultaneously charged with 
cooled azeotropic acid. In this manner the major part of the NO.sub.x 
still contained in the gas passes from the gas into the acid. The tail gas 
flowing to the stack is further diluted by air conveyed by way of blower 3 
into the pipe 35, if this is necessary. 
EXAMPLE 
In a plant, such as described by reference to the drawing, 200 t/d of 
concentrated HNO.sub.3 are produced. By an appropriate design of the 
nitrous gas compressor, and the vessel, a guarantee is given that it 
continues to run for 15 seconds after emergency shutdown of the driving 
means. With the emergency shutdown, the supply of ammonia for combustion 
and the desorption are simultaneously interrupted. With the running down 
of the nitrous gas compressor, the absorption section is shut off, the 
pressure or delivery side of the compressor is released to the intake side 
and the intake side is connected to the rectifying system which is under 
vacuum, so that the gases which are on the suction or intake side are able 
to flow into the column. The acid in the absorption columns is then 
replaced by approximately 6 cubic meters of degasified azeotropic acid, 
and 50 cubic meters of oxygen are passed into this section of the plant. 
The acid is circulated for one hour through the columns. Thereafter, the 
pressure of the pressurized section is released during a period of 20 
minutes, and 20 m.sup.3 of azeotropic acid is simultaneously admitted to 
the physical absorption column. During the 20-minute pressure release 
period, the mean NO.sub.x concentration of the exhaust gas amounts of 
about 600 ppm (V) and the total discharged quantity of NO.sub.x, 
calculated as NO.sub.2, amounts to about 20 kg. 
By contrast, if the quantity of nitrous gas present at the time of an 
emergency shutdown in the same plant is blown out of the plant without 
using the process according to the invention, it has to be expected that 
there will be a discharge of NO.sub.x which is about 20 times greater.