FIELD OF THE INVENTION
[0001] The present invention relates to an improved process and system for recovery of hazardous
tail gases using ammonia vapour. Particularly, the invention relates to an improved process and system
for the simultaneous recovery of sulphur oxide gases emitted from sulphuric acid production plants
and production of ammonium sulphate in a closed loop system, using ammonia vapour. The produced
ammonium sulphate is further used in diammonium phosphate production.
BACKGROUND OF THE INVENTION
[0002] Sulphur oxides especially sulphur dioxide emission control from various sources such as
sulphuric acid plants, fossil fuel fired boilers, smelters, pulp and paper mill operations and the like is
required by law in many countries to mitigate the serious environmental and health damage that is
associated with sulphur dioxide which is a colourless gas with a characteristic of choking odour. The
most widely practiced method for sulphur dioxide control is based upon limestone or lime contact
with flue gases in the form of aqueous slurry. In most instances, the by-product is either discarded as
a land fill or converted into gypsum for use in wall board and cement manufacture. In a few instances,
other alkaline reagents, such as sodium compounds, magnesium compounds and ammonia have been
used with recovery of useful by-product.
[0003] During the sulphuric acid production process at sulphuric acid plants, sulphur dioxide is
invariably produced as tail gas which is thereafter scrubbed in alkali scrubbing media such as but not
limited to dilute caustic soda solution (NaOH solution) or 16% sodium carbonate solution (Na2CO3
solution) before passing through flue stack to ensure emission norms. The SOx-alkali scrubbing
process produces sodium sulphite (Na2SO3) which is an unstable end-product and a hazardous effluent
– utilization and disposal of which is also a big challenge for a company.

[0004] Ammonia and ammoniacal scrubbing solutions are also well known for flue gas desulfurization.
SOx scrubbing with an ammonium hydroxide solution is a proven and an established technology
which generates a stable end product i.e. ammonium sulphate ((NH4)2SO4). In U.S. Patent No.
4,690,807, which is incorporated herein by reference in its entirety, discloses that gases containing at
least sulfur oxide are treated with aqueous ammonia, and ammonium sulfate is produced in a single
vessel which includes an absorption tower and liquor reservoir. The sulfur oxide gas or gases are
removed from gases containing sulfur oxides by contact of the gas with an aqueous solution of
ammonium sulfate. As the sulfur oxide gas, such as sulfur dioxide, is absorbed by the ammonium
sulfate solution, it becomes acidic. The acidic solution in U.S. Pat. No. 4,690,807 is neutralized by
injection of ammonia into the ammonium sulfate solution to maintain a desired pH level sufficient to
prevent excessive ammonia loss. An oxidizing medium, such as air, is injected into the neutralized
ammonium sulphate solution containing the absorbed sulfur oxide gas, leading to the formation of
ammonium sulfate.
[0005] Similarly, in US Patent No. 5,362, 458, a process for the removal of sulfur oxides from sulphur
oxide-containing gas with simultaneous production of ammonium sulphate is carried out by first
passing hot sulfur oxide-containing gas through a prescrubber wherein the gas contacts saturated
aqueous ammonium sulfate liquor which is recycled in the prescrubber, followed by passing the
prescrubbed gas through an absorber wherein the prescrubbed gas contacts dilute aqueous ammonium
sulfate liquor is disclosed. The sulfur oxide in the sulfur oxide-containing gas is absorbed by the dilute
aqueous ammonium sulfate liquor in the absorber, and scrubbed gas is removed from the absorber.
The dilute aqueous ammonium sulfate liquor is treated with ammonia and air and the absorbed sulphur
dioxide is converted to ammonium sulfate in the liquor. The dilute ammonium sulfate liquor is
recycled into contact with the prescrubbed gas in the absorber. Dilute aqueous ammonium sulfate
liquor is removed from the absorber and added to the saturated aqueous ammonium sulfate liquor in
the prescrubber where it becomes saturated due to evaporation caused by the hot gas. Ammonium
sulfate crystals form in the saturated aqueous ammonium sulfate liquor in the prescrubber are
recovered as product from saturated aqueous ammonium sulphate withdrawn from the prescrubber.
[0006] Although the above mentioned solutions and many other solutions are used for the absorption
of sulphur dioxide and for the production of ammonium sulphate, very few companies are able to use

solvent extraction process efficiently to recover stack loss as the process is very expensive and
complicated. SOx scrubbing with ammonia, therefore, has a big opportunity for improvement in
respect of hazardous material handling and spent disposal. Accordingly, it is always desirable and
advantageous to improve efficiency in the scrubbing of tail gases. Owing to this long felt need, the
present invention has been conceived to reduce stack SOx emissions and produce a stable end product
i.e. ammonium sulphate ((NH4)2SO4) in a closed loop system using ammonia vapour, which is then
used as fertilizer feed stock such as diammonium phosphate.
SUMMARY OF THE INVENTION
[0007] The present invention relates to a process for recovery of sulphur oxides and production of
ammonium sulphate using ammonia vapour in a closed loop system, said process comprising the steps
of (a) vaporising liquid ammonia at a phosphatic fertilizer plant using a steam vaporiser; (b)
transferring ammonia vapour generated at the phosphatic fertilizer plant to a sulphuric acid plant; (c)
injecting the ammonia vapour into an SOx scrubber system at the sulphuric acid plant wherein the
ammonia vapour reacts with SOx in the SOx scrubber system to produce ammonium sulphate,
sulphur oxides and other tail gases; and (d) transferring the ammonium sulphate to the phosphatic
fertilizer plant for nitrogen balancing and for use as a fertilizer in the phosphatic fertilizer plant.
[0008] In an embodiment, the ammonia vapour pressure at the phosphatic fertilizer plant is
maintained at 4 to 5 kg/cm2. In another embodiment, the vapour ammonia can be directly transferred,
without the use of any pump, to the SOx scrubber system through pressure gradient developed in the
steam vaporiser. In yet another embodiment, the pH of the SOx scrubber system is controlled
through a flow control valve and wherein the pH of the SOx scrubber system is maintained between
5 pH – 7pH, prefereably at 5.70 pH for obtaining best scrubbing efficiency and reduced SOx stack
emissions. In a method embodiment, the pH of the SOx scrubber system is reduced by the injection
of ammonia in the scrubber and the SO2 emissions are maintained in the stack. In another
embodiment, the pH of the SOx scrubber system is increased by increasing the injection of ammonia
if the SO2 emissions are not maintained in the stack.
[0009] The invention also relates to a closed loop system (200) for recovery of sulphur oxides and
production of ammonium sulphate using ammonia vapour, the system comprising: (a) a steam

vaporiser for vaporising liquid ammonia at a phosphatic fertilizer plant; (b) a SOx scrubber system
(201) of a sulphuric acid plant for scrubbing SOx with ammonia vapour to produce ammonium
sulphate, sulphur oxides and other tail gases; (c) a tank (204) for collecting and transferring the
ammonium sulphate solution produced by the SOx – ammonia vapour scrubbing process to the
phosphatic fertilizer plant for nitrogen balancing and for use as a fertilizer; and (d) an exhaust slurry
transfer pump (205) for disposing off the remaining slurry produced by the SOx-ammonia vapour
scrubbing process. In an embodiment, the flow of the ammonia vapour into the SOx scrubber system
is controlled by a flow control valve (202). In another embodiment, the tail gases formed by the SOx-
ammonia vapour scrubbing process are released as stack at the release point (203) of the scrubber
system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Figure 1 illustrates a flow chart of the method steps as disclosed in the present invention.
[0011] Figure 2 illustrates a circuit diagram of the system and apparatus as disclosed in the present
invention
[0012] Figure 3 illustrates a graphical representation of the SO2 stack and scrubbing efficiency
sampling data for SO2 removal in the sulphuric acid plant using the method as disclosed in the present
invention
[0013] Figure 4 illustrates another graphical representation of the SO2 stack and scrubbing efficiency
sampling data for SO2 removal in the sulphuric acid plant using the method as disclosed in the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] It should be noted that the description and examples merely illustrate the principles of the
present subject matter. It should be appreciated by those skilled in the art that conception and specific
embodiments disclosed may be readily utilized as a basis for modifying or designing other methods
for carrying out the same purposes of the present subject matter. Furthermore, all examples recited

herein are principally intended expressly to be for pedagogical purposes to aid the reader in
understanding the principles of the present subject matter and the concepts contributed by the
inventor(s) to furthering the art and are to be construed as being without limitation to such specifically
recited examples and conditions. The novel features which are believed to be characteristic of the
present subject matter, both as to its organization and method of operation, together with further
objects and advantages will be better understood from the following description when considered in
connection with the accompanying examples.
[0015] These and other advantages of the present subject matter would be described in greater detail
with reference to the following examples. It should be noted that the description merely illustrates the
principles of the present subject matter. It will thus be appreciated that those skilled in the art will be
able to devise various arrangements that, although not explicitly described herein, embody the
principles of the present subject matter and are included within its scope.
[0016] Without limitation, in a preferred embodiment, the present invention in Figure 1 discloses a
novel method of simultaneous recovery of sulphur oxides and production of ammonium sulphate
using ammonia vapour in a closed loop system. In an embodiment, the method commences with the
step of vaporising liquid ammonia at a phosphatic fertilizer plant using a steam vaporiser wherein the
ammonia vapour pressure at the phosphatic fertilizer plant is preferably maintained at 4-5 kg/cm2.
The use of liquid ammonia as a scrubbing media for SOx scrubbing at sulphuric acid plants has been
known in the art, however, the inventors after conducting various experiments and through rigorous
analysis interestingly found that the use of ammonia vapour as a scrubbing media for SOx scrubbing
at sulphuric acid plants instead of the known liquid ammonia produces significantly surprising and
beneficial results. Specifically, the addition of vapour ammonia helps to control the volume of liquid
effluent generation at the sulphuric acid plant and the scrubbing efficiency is also maintained at more
than 92%. Additionally, unlike liquid ammonia, vapour ammonia can be directly transferred, without
the use of any pump, to the SOx scrubber through pressure gradient developed in the steam vaporiser
wherein the pH in the SOx scrubber is controlled through a flow control valve.
[0017] In a method step of the preferred embodiment, after the liquid ammonia is vaporised in the
steam vaporiser at the phosphatic fertilizer plant, the ammonia vapour is transferred to the sulphuric
acid plants through pipelines. The ammonia vapour is thereafter injected into the SOx scrubber system

at the sulphuric acid plant through a purger wherein the flow of the ammonia vapour into the SOx
scrubber is controlled by a flow control valve. Before the ammonia vapour is injected into the SOx
scrubber, it is ensured that the SOx scrubber is thoroughly washed so that there is no caustic/soda
solution present in the SOx scrubber. The pH of the SOx scrubber system is maintained between 5
pH - 7 pH, preferably at 5.70 pH throughout for obtaining best scrubbing efficiency, and reduced
SOx stack emissions.
[0018] In a method embodiment, the vapour ammonia thereafter reacts with SOx in the scrubber
system to produce ammonium sulphate along with stack emissions such as but not limited to SO2,
SO3, and acid mist etc. While the process is carried out, the stack emissions are monitored at every 30
minute interval. In an embodiment, the pH of the SOx scrubber system is reduced by the injection of
ammonia in the scrubber and the SO2 emissions are maintained in the stack. In another embodiment,
the pH of the SOx scrubber system is increased by increasing the injection of ammonia if the SO2
emissions are not maintained in the stack. In case, visible and dense stack is produced while increasing
the pH of the SOx scrubber, addition of ammonia vapour is stopped and the SOx scrubber is
continuously washed with water till the stack emissions are stabilized. During the scrubbing process,
it is carefully ensured that pH is not increased to more than 7.5 at any point of time. In a method
step, the specific gravity of the ammonium sulphate solution produced by the vapour ammonia – SOx
scrubbing process is checked and the ammonium sulphate solution is thereafter transferred to the
phosphatic fertilizer plant for nitrogen balancing and for use as a fertilizer in the phosphatic fertilizer
plant.
[0019] Figure 2 illustrates the circuit diagram of the ammonia vapour – SOx scrubbing system 200 of
the present invention. The ammonia vapour generated from the steam vaporizer of the phosphatic
fertilizer plant is injected into the SOx scrubber system 201 of the sulphuric acid plant wherein the
flow of the ammonia vapour into the SOx scrubber is controlled by a flow control valve 202. The pH
of the SOx scrubber system is maintained between 5pH - 7 pH, preferably at 5.70 pH throughout for
obtaining best scrubbing efficiency and reduced SOx stack emissions. Once the ammonia vapour
reacts with SOx in the SOx scrubber 201 of the sulphuric acid plant, a neutralization process takes
place forming acidic tail gas comprising sulphur di-oxide, sulphur tri-oxide and acid mist - which gases
are then subsequently released as stack at release point 203 of the scrubber system. The ammonium
sulphate solution produced by the vapour ammonia – SOx scrubbing process at the sulphuric acid

plant is collected at tank 204 and is continuously recycled back to the phosphatic fertilizer plant for
nitrogen balancing and for subsequent use as a fertilizer in the phosphatic fertilizer plant such as
diammonium phosphate. The remaining slurry produced from the SOx-ammonia vapour scrubbing
process is transferred to the exhaust slurry transfer pump 205 and disposed off separately. The entire
process is conducted in a closed loop system which ensures maximum scrubbing efficiency, reduced
SOx stack emissions, as well as continuous recycling of the ammonium sulphate solution produced
in the sulphuric acid plant to the phosphatic fertilizer plant for use as a fertilizer in the phosphatic
fertilizer plant.
[0020] The following table provides illustrative data on stack emissions produced after the SOx was
treated with ammonia vapour in the SOx scrubber in the sulphuric acid plant at a pH range of 5.90-
6.60 using the method as disclosed in the present invention:
TABLE 1
Sulphuric Acid Plant stack sampling data analysis at pH range 5.90 – 6.60:

S.
No. SO2 at
Inlet of
Scrubber
(mg/Nm3) SO3 at
Inlet of
Scr.
(mg/Nm3) Acid Mist
at Inlet of
Scr.
(mg/Nm3) SO2 at the
Outlet
(mg/Nm3) SO3 at the
Outlet
(mg/Nm3) NH3 at the
Outlet
(mg/Nm3) Acid Mist
at Inlet of
Scr.
(mg/Nm3) Scrubbing
Eff. %
w.r.t. SO2
1 1809 252 86.07
2 1809 164 < 30 90.93
3 1800 160 91.11
4 1800 160 91.11
5 2022 186 90.80
6 2000 191 90.45
7 2000 194 90.30
8 2022 1100 126 180 107 13 91.10
9 1740 162 90.69
10 1656 1286 121 156 81 10 90.58
11 1719 1105 151 152 69 31 12 91.16
12 1719 1105 151 149 47 38 12 91.33

13 1740 957 148 146 45 43 12 91.61
14 1740 957 148 142 35 33 13 91.84
15 1638 868 128 138 32 43 13 91.58
16 1715 1009 76 136 34 58 10 92.07
17 865 88 48 BDL 89.83
18 1637 1362 252 140 29 12 91.45
MAX 2022 1362 252 252 107 58 13 92.07
MIN 865 868 76 88 29 31 10 86.07
[0021] Figure 3 illustrates a graphical representation of the above SO2 stack data and scrubbing
efficiency data for SO2 removal in the sulphuric acid plant using the method as disclosed in the present
invention at a pH range 5.90-6.60.
[0022] The following table provides illustrative data on stack emissions produced after the SOx was
treated with ammonia vapour in the SOx scrubber in the sulphuric acid plant at a pH range of 6.20-
6.30 using the method as disclosed in the present invention:
TABLE 2
Sulphuric Acid Plant stack sampling data analysis at pH range 6.20 – 6.30:

S.
No. Scr. I/L
SO2 Conc.
(mg/Nm3) Scr. I/L
SO3 Conc.
(mg/Nm3) Scr. I/L acid
mist Conc.
(mg/Nm3) Scr. O/L
SO2 Conc.
(mg/Nm3) Scr. O/L
SO3 Conc.
(mg/Nm3) Scr. O/L acid
mist Conc.
(mg/Nm3) Scr. O/L
NH3 Conc.
(mg/Nm3) SO2 scr.
Eff. %
1 1680 1211 155 142 53 12 33 91.55
2 1668 886 177 148 61 13 22 91.13
3 1715 1034 150 146 44 10 0 91.49
4 1678 1110 151 138 42 11 0 91.78
5 1808 962 202 136 57 13 0 92.48
6 1722 1067 251 119 43 12 0 93.09
7 1637 890 197 125 50 12 0 92.36
8 1657 731 151 127 31 10 33 92.34

9 1636 1012 224 122 44 11 15 92.54
10 1562 908 227 117 48 13 35 92.51
MAX 1808 1211 251 148 61 13 35 93.09
MIN 1562 731 150 117 31 10 0 91.13
[0023] Figure 4 illustrates a graphical representation of the above SO2 stack data and scrubbing
efficiency data for SO2 removal in the sulphuric acid plant using the method as disclosed in the present
invention at a pH range 5.90-6.60.
[0024] The key findings of the above stack emissions sampling data indicate that at a pH range of
5.90 – 6.60, the SO2 removal efficiency was observed in the range of 86-93% and when the pH is
steady between 6.20 to 6.30, maximum efficiency of SO2 removal was observed at 93.09%. It was also
noticed that even at a pH of 5.60 (not shown in the above table) with specific gravity of 1.102, the
SO2 removal efficiency was found to be approximately at 90.6%. As indicated in the above data,
maximum efficiency of 93.09% was observed with scrubber liquid pH at 6.23 and with a specific
gravity of 1.154. For the data obtained in Table 2, the SO2 removal efficiency was found to be in the
range of 91.13 to 93.09% with specific gravity range from 1.133 to 1.157.
[0025] The following table shows the effect of stack emissions visibility with lowering of the SOx
scrubber liquor pH. It is pertinent to note that by lowering the pH of the SOx scrubber liquor pH
from 6.4 to 5.6, the stack visibility is considerably improved without any change in scrubbing
efficiency.

TABLE 3

Effect of lowering scr. Liq. pH with stack visibility and scrubbing efficiency
DATE Scr. Liq. pH Scrubber inlet gas analysis Scrubber outlet gas analysis Scr. Eff. %
w.r.t. SO2 Remarks

SO2 at Inlet
of Scrubber
(mg/NM3) SO3 at Inlet
of Scrubber
(mg/NM3) Acid Mist at
Inlet of Scr
(mg/NM3) SO2 at the
Outlet
(mg/NM3) SO3 at the
Outlet
(mg/NM3) NH3 at the
Outlet
(mg/NM3) Acid Mist at
Inlet of Scr
( mg/NM3)

29.07.2020 5.8 1910 1053 175 153 53 54 16 91.99 Stack visibility improved
30.07.2020 5.8 1964 807 202 156 55 56 17 92.06 Stack visibility improved
06.08.2020 5.6 2148 1354 150 156 50 19 92.74 Stack visibility improved further
07.08.2020 5.6 1910 1228 251 149 49 34 14 92.20 Stack visibility improved further
[0026] The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure,
are intended to cover a non-exclusive inclusion, such that a method that comprises a list of steps does
not include only those method steps but may include other steps not expressly listed or inherent to
such method. In other words, one or more steps in a method proceeded by the expression
“comprises… a” does not, without more constraints, preclude the existence of other steps or
additional steps in the method.
[0027] It will be understood by those within the art that, in general, terms used herein are generally
intended as “open” terms (e.g., the term “including” should be interpreted as “including but not
limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should
be interpreted as “includes but is not limited to,” etc.).
[0028] It will be further appreciated that functions of a plurality of steps may be combined into a
single step, or the functions of one-step may be split among plural steps. The present invention
contemplates all of these combinations. In addition, while a feature of the present invention may have
been described in the context of only one of the illustrated embodiments, such feature may be
combined with one or more other features of other embodiments, for any given application. The use
of “comprising” or “including” also contemplates embodiments that “consist essentially of” or
“consist of” the recited feature.

We claim:

(1) A process for recovery of sulphur oxides and production of ammonium sulphate using ammonia
vapour in a closed loop system, the said process comprising the steps of:
vaporising liquid ammonia at a phosphatic fertilizer plant using a steam vaporiser;
transferring ammonia vapour generated at the phosphatic fertilizer plant to a sulphuric acid
plant;
injecting the ammonia vapour into an SOx scrubber system at the sulphuric acid plant wherein
the ammonia vapour reacts with SOx in the SOx scrubber system to produce ammonium sulphate,
sulphur oxides and other tail gases; and
transferring the ammonium sulphate to the phosphatic fertilizer plant for nitrogen balancing
and for use as a fertilizer in the phosphatic fertilizer plant.
(2) The process as claimed in claim 1, wherein the ammonia vapour pressure at the phosphatic
fertilizer plant is maintained at 4 to 5 kg/cm2.
(3) The process as claimed in claim 1, wherein vapour ammonia can be directly transferred, without
the use of any pump, to the SOx scrubber system through pressure gradient developed in the steam
vaporiser.
(4) The process as claimed in claim 1, wherein the pH of the SOx scrubber system is controlled
through a flow control valve.
(5) The process as claimed in claim 1, wherein the pH of the SOx scrubber system is maintained
between 5 pH – 7pH, prefereably at 5.70 pH for obtaining best scrubbing efficiency and reduced SOx
stack emissions.

(6) The process as claimed in claim 1, wherein the pH of the SOx scrubber system is reduced by the
injection of ammonia to maintain the SO2 emissions in the stack.
(7) The process as claimed in claim 1, wherein the pH of the SOx scrubber system is increased by
increasing the injection of ammonia if the SO2 emissions are not maintained in the stack.
(8) A closed loop system (200) for recovery of sulphur oxides and production of ammonium sulphate
using ammonia vapour, the system comprising:
a steam vaporiser for vaporising liquid ammonia at a phosphatic fertilizer plant;
a SOx scrubber system (201) of a sulphuric acid plant for scrubbing SOx with ammonia vapour
to produce ammonium sulphate, sulphur oxides and other tail gases;
a tank (204) for collecting and transferring the ammonium sulphate solution produced by the
SOx – ammonia vapour scrubbing process to the phosphatic fertilizer plant for nitrogen balancing
and for use as a fertilizer; and
an exhaust slurry transfer pump (205) for disposing off the remaining slurry produced by the
SOx-ammonia vapour scrubbing process.
(9) The system as claimed in claim 8, wherein the flow of the ammonia vapour into the SOx scrubber
system is controlled by a flow control valve (202).
(10) The system as claimed in claim 8, wherein the tail gases formed by the SOx-ammonia vapour
scrubbing process are released as stack at the release point (203) of the scrubber system.