DESC:Technical Filed:
This invention relates to an improved process for production of nitric acid using non-thermal plasma technology. More particularly, the invention relates to green process for production of nitric acid using a non-thermal plasma reactor to achieve good yields.

Background and Prior Art:
Nitrogen is the key limiting nutrient for crop production. Usually, most of the plant available form of nitrogen is as nitrates. The conventional processes of producing nitrogen fertilizer either through natural gas steam methane reforming or by the electrolysis of water are both well understood.

Nitric acid (HNO3) is used in the manufacture of paints, fertilizers, plastics, fabrics, dyes, detergents, and explosives. Nitric acid is commonly manufactured by Ostwald process, wherein, ammonia is converted into nitric acid. Many modified Ostwald processes for the production of nitric acid is disclosed, for example, US1362418, US1989267, US2142646, US3868443, US6165435, US6737034, US7258849, US8263036, US858904, US9199849 etc., the contents of which are incorporated for reference.

The major drawbacks of these existing thermal plasma processes are that they need high temperature and pressure, and also high capital and operational costs. High temperature and pressure conditions require special equipment / reactors and other safety measures in place. This makes the whole process expensive thereby increasing the capital and operational costs. In case of non-thermal plasma (NTP) methods the reactants are only partly ionized where the energy is stored mostly in the free electrons and the overall temperature remains low. The NTP methods are widely used for many years in various applications such as low-temperature plasma chemistry, waste management and also further expanded into new biological areas like health and food industry.
CN 102583278 A entitled “process for producing nitric acid by dielectric barrier discharge nitrogen fixation” comprising a tooth-shaped stainless steel cylinder, wherein an insulation tube is sheathed outside the tooth-shaped stainless steel cylinder. The gap between the SS cylinder and the insulation tube is 3-30 mm. They reported maximum concentration of HNO3 (0.072 m mol/L) by applying 15.64kV peak voltage, 5-20 kHz frequency and the amount of nitric acid produced for 1kWh plasma power consumption is 2.36 grams.

WO 2016063302 A2 entitled “Process for combustion of nitrogen for fertilizer production” discloses a materials, methods and system useful for conversion of nitrogen gas into nitrogen compounds that can be assimilated by plants. GB 915,771 generates a dielectric barrier discharge via radio frequency power which is ignited at low pressure. Concentrations of up to 5% NO in the process exhaust gas are detected, which is about 20 kWh/kg HNO3 in the best case.

Kaoru Harada et al., carried out the reductive fixation of molecular nitrogen with water using glow discharge. It was found that ammonia and nitrate ions were formed in the aqueous solution. The amount of ammonia and nitrate ion found in the reaction mixture increased with the reaction time, and the concentrations of ammonia and nitrate ion were 0.45 mmol/20 ml and 0.38 mmol/20 ml, respectively after 24 hours.

Wenjuan Bian et al., used pulsed high voltage discharge for Nitrogen fixation into HNO3. By applying pulsed high voltage discharge to a needle-mesh reactor that using seven acupuncture needles as discharge electrode and stainless steel wire mesh as ground electrode. At the end of the 36 min discharge, the HNO3 concentration with bubbling air was 2.215 m mol L-1 at an applied voltage of 25 kV, pulse repetition frequency of 140 Hz and ground electrode mesh of 20 x 20. The energy yield was about 1.22 g (HNO3) / kWh.

From the above literature it is evident that the cost of producing nitric acid with the existing thermal plasma processes is very expensive compared to cold plasma technology. Therefore, there remains a need in the art to provide an efficient process for production of nitric acid using atmospheric air through cold plasma process to overcome the above problems. Therefore, it is an objective of the present invention to provide process for production of HNO3 under non-thermal plasma conditions, using both pulsed corona discharges and dielectric barrier discharges.

Summary of the Invention:
The main object of the present invention is achieved by providing a simple process, wherein oxygen and nitrogen in the air may be fixed in the plasma device for production of nitric acid in water, in order to facilitate the use of nitrogen in agricultural activities. In the present invention, air is exposed into plasma conditions in the reactor, where oxygen and nitrogen are ionized in a discharge process to form the oxygen and nitrogen plasma along with the water mist and a liquid phase plasma chemical reaction is induced simultaneously to generate the nitric acid from the atmospheric air.

In accordance with the above objective, the present invention provides a process for preparation of nitric acid by using a non-thermal plasma reactor that is operated between 3 to 5 kV and 10 to14 kHz and the other parameters such as air flow rate, nitrogen to oxygen ratio, height of the water bucket, along with other variables that are to be added to the water are optimized to increase the production of nitric acid.

This process requires a start gas having nitrogen and oxygen, preferably of environmental air. The process of the present invention encompasses the reaction of energetic electrons present in the plasma zone with nitrogen (N2) and oxygen molecules (O2) to generate the respective radicals, wherein the energetic electrons are bombarded with water molecules to form hydroxyl radicals (•OH). Then the nitrogen oxides (NOx) generated in the plasma zone react with these hydroxyl radicals (•OH) and water molecules present in the aqueous medium to form nitric acid, HNO3.

Reactions Involved in Plasma Technology Process:
1. High Voltage Ionisation:
N2 + 2O2 -------------> 2NO2 (gas)
2. Wet Reactor:
3NO2 + H2O -------------> 2 HNO3 + NO (Untreated) - recirculation with high voltage ionisation & cold plasma
3) 2NO + O2 -------------> 2NO2 (gas)
4) 2NO2 + 2H2O -------------> 2HNO3
Overall Reaction:
2N2 + 5O2 + 2H2O ---------------> 4 HNO3

Brief description of drawings:
Fig.1 depicts Non thermal Plasma reactor for producing nitric acid
Fig. 2 shows Effect of air flow rate on HNO3 production
Fig. 3 shows the effect of % H2O2 addition on HNO3 production

Detailed Description of the Invention:
The invention will be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be fully understood and appreciated.
Accordingly, in a preferred embodiment, the invention provides a process for preparation of nitric acid under non-thermal plasma conditions, by using pulsed corona discharges in cold plasma reactor comprising:
(a) providing two electrodes in a cold plasma generator, one stationary and other rotating electrode arranged one surrounding the other to operate between 3 to 5 kV and / or 10 to14 kHz for generating sufficient difference in electric potential between stationary and rotating electrodes plasma;
(b) providing small holes / perforation on rotating electrode for sucking the mist (air + water) into the corona plasma zone situated between the two electrodes;
(c) optimizing the atmospheric air and water flow rates in the reactor to get very fine mist generation and optimum residence time in the plasma region;
(d) maintaining nitrogen and oxygen ratio in the plasma reactor to provide sufficient oxygen and conversion of un-reacted NO to NO2; and
(e) adding H2O2 and other radical initiators to plasma medium containing water to provide additional reactive species to increase the nitric acid production.

In accordance with the above objective, the present invention provides process for preparation of HNO3 using a non-thermal plasma reactor that is operated at 3-5 kV and 10-14 KHz. The other parameters such as air flow rate, nitrogen to oxygen ratio, height of the water bucket along with other variables which are added in the process such as H2O2 to the water are optimized to increase the production quantity of nitric acid.

The process of the present invention requires a start gas, i.e., atmospheric gas having nitrogen and oxygen, preferably the environmental air. The energetic electrons present in the plasma zone react with nitrogen (N2) and oxygen molecules (O2) to generate the respective radicals (Eq. 1 to 4), whereas, the energetic electrons bombarded with water molecules results in the formation of hydroxyl radicals (•OH - Eq. 7 & *). Then the nitrogen oxides (NOx) react with hydroxyl radicals (•OH) and water present in the aqueous medium to form HNO3 (Eq. 5).

In an embodiment, the invention provides process for preparation of HNO3 in non-thermal plasma using PCD reactor that is operated at 3-5 kV and 10-14 KHz. In another embodiment, the invention provides optimized flow rate of the desired gas so as to increase the production efficiency of nitric acid. Accordingly, plasma treatment was carried out with various flow rates between 600 LPH (liters per hour) to 1200 LPH for one hour for the production of HNO3. It was observed that when the flow rate was fixed at 900 LPH, the amount of HNO3 produced is around 1.3 g and it is further decreased to 0.8 and 1.0 g for 600 LPH and 1200 LPH, respectively. Accordingly, it can be concluded that the air flow rate at 900 LPH showed maximum HNO3 production out of all the tested flow rates.

In a further embodiment, height of the water bucket in the plasma reactor is optimized, as it also plays an important role due to the gravity of water. According to this embodiment, when the height of the bucket increased from 21-41cm, the amount of water driving inside the reactor was also increased which further increases HNO3 yield and followed the order: 21 cm (0.33 g) < 31 cm (1.20 g) < 41 cm (1.3 g) and the inside water volume is 0 ml < 130 ml < 390 ml. This experiment concludes that the height of the bucket in the reactor can be optimized to obtain the maximum production of nitric acid.

In one embodiment, it is observed that at the end of 60 min of plasma treatment, the highest amount of HNO3 was obtained at flow rate of 900 LPH and the height of the bucket is 41 cm. In an alternate embodiment, the present invention provides another process variant for the production of nitric acid. According to this embodiment, Fenton process is used for the generation of hydroxyl radicals wherein in-situ decomposition of H2O2 results into •OH in the presence of a Fe2+ salt (Eq. 9). This process may further increase the HNO3 production.
Fe2+ + H2O2 •OH + OH- + Fe3+ (9)

In this embodiment, the plasma treatment is given by varying two different solutions viz., H2O2 and H2O2 + Fe2+ salt. H2O2 present in the water may be converted in to hydroxyl radicals (•OH) during the discharge process which may increase the HNO3 production (Eq. 5). The present invention is currently working on the optimized concentration of H2O2 so as to increase the production of HNO3.
The HNO3 thus produced using non thermal process (NTP), is used to manufacture the fertilizers such as Ca(NO3)2, NH4NO3, KNO3 for at least two end-applications. In the first application, small units can be placed in the field connected to micro-irrigation systems to generate the required amount of nitrogen as HNO3 or the corresponding nitrate salts of metal ions present under-ground or surface water (is used) for the crops or the big units may be constructed to commercial production of HNO3.

The process of the present invention can be depicted as flow chart shown below.

The use of non-thermal plasma (NTP) / cold plasma for the production of nitric acid in accordance with the invention comes with various advantages such the utilization of cheaper and abundantly available raw materials such as air and water, solar energy, green/clean technology and moderate operational conditions such as low temperature and pressure and thus devoid of special material of construction to sustain high pressure or high temperature. No tail gas emissions and harmful effluents generated in the process. There is no risk of explosion as the process is safe to handle. In view of the decentralized production capability at demand site, the process negates huge investments, distribution / transportation costs and related carbon emissions and hence enables lower capital and operational expenses thereby reduces overall manufacturing cost. The present invention is devoid of using hydrocarbons as feedstock and thus brings associated benefits of having zero carbon foot-print.

In another embodiment, the invention provides a cold plasma reactor for production of nitric acid comprising:
(a) two electrodes are provided in the reactor, one stationary and other rotating electrode arranged one surrounding the other to operate between 3 to 5 kV and / or 10 to14 kHz for generating sufficient difference in electric potential between stationary and rotating electrodes plasma;
(b) an electrical motor is provided to drive the rotating electrode;
(c) small holes / perforations are provided on rotating electrode for sucking the mist (air + water) into the corona plasma zone situated between the two electrodes;
(d) air atomizing nozzle is provided in the reactor for ionization of atmospheric air and water in the plasma reactor to obtain very fine mist and optimum residence time in the plasma region;
(e) the reactor is fed to a discharge voltage AC power of 3 to 5kV, frequency of 10-14 kHz and AC input power of around 200W.

The design of combining the corona plasma device with a perforated rotating electrode for sucking the centrifugal mist created, is provided to give maximum exposure of the mist (air + water) to corona plasma in the reactor, so as to obtain maximum yields of nitric acid.

Accordingly, the Cold plasma Reactor Design for producing nitric acid in accordance with the invention is described in Figure.1. This cold plasma generator device contains a stationary electrode and a rotation electrode arranged one surrounding the other one, wherein said stationary and rotating electrodes are connected to a high voltage and frequency power supply (3-5kv & 10-14 kHz) so that difference in electric potential between stationary and rotating electrodes plasma is generated. An electrical motor is used to drive the rotating electrode. Small holes / perforations are provided on rotating electrode for sucking the centrifugal mist (air + water) created into the corona plasma zone situated between the two electrodes so as to maximize the production of nitric acid. The reactor shell contains water, air inlets and air outlet ports. Air atomizing nozzles are provided in the reactor so that both air and water can enter into plasma region and dissociates into radicals. The reactor is fed to a discharge voltage AC power of 3 to 5kV, frequency of 10-14 kHz and AC input power of around 200W.

Total Nitrogen Determination:
The total nitrogen present in the solution is determined by using Devarda's Alloy Method. The solution contains nitrate ions is mixed with aqueous sodium hydroxide, adding Devarda's alloy and heating the mixture gently, liberates ammonia gas. After conversion under the form of ammonia, the total nitrogen is then determined by Kjeldahl method. In Kjeldahl method the amount of ammonia present, and thus the amount of nitrogen present in the sample, is determined by back titration. The end of the condenser is dipped into a solution of boric acid. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate solution by way of a methyl orange pH indicator.

The reduction of nitrate by the Devarda's alloy is given by the following equation:
3NO3- + 8Al + 5OH- + 18H2O 3NH3 + 8 [Al(OH)4]-
Having described the basic concepts of the instant invention reference is made to the following examples which are provided to illustrate but not limit the preferred method of the invention and other similar methods of producing metal sulfates. Other features and embodiments of the invention will become apparent by the following examples which are given for illustration of the invention rather than limiting its intended scope. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art.
Example 1 - Flow Rate of the Desired Gas:
In order to know the optimum air flow rate for the production of HNO3, plasma treatment was carried out with various flow rate of the atmospheric gas between 600 LPH to 1200 LPH for one hour. When the flow rate was fixed at 900 LPH the amount of HNO3 produced was around 1.3 g and it was decreased to 0.8 and 1.0 g for 600 LPH and 1200 LPH, respectively. When the flow rate is increased up to 900 LPH the production of HNO3 was increased, further increase in the flow rate (1200 LPH) the production of HNO3 was decreased. High flow rate (up to 900 LPH) provides more number of oxygen/nitrogen molecules in the plasma discharge zone and this may increase the HNO3 production. Hence, this can be explained as at higher flow rate (1200 LPH) the residence time of the molecules available for the reaction in the discharge zone will be less. One more possible reason for this is with increasing the flow rate the amount of water driving inside the plasma reactor also increased which may increase more contact of NOx with water and the order was followed as for 600 LPH (20 ml) < 900 LPH (130 ml) < 1200 LPH (370 ml). All these findings as depicted in table 1 conclude that the air flow rate at 900 LPH showed maximum HNO3 production.

Table. 1. Effect of air flow rate on production of HNO3 - Conditions: * Treatment time-1hr, Height of the water reservoir-41cm
S.No Volume of water taken in bucket (Lit) Air flow rate (LPH) Volume of water going inside the reactor (L) Final pH of total water & Conc. Total Power consumption (kWh/hr) HNO3 (moles) HNO3
(gms)
1 2 600 0.2 2.21 (0.00616M) 0.4 0.0123 0.763
2 2 900 0.13 1.99 (0.0125M) 0.4 0.025 1.289
3 2 1200 0.37 2.1 (0.0079M) 0.4 0.0158 0.995

Example 2 - Height of the Water Bucket:
The height of the water bucket in the reactor also plays an important role due to the gravity of water. As the height of the bucket increased from 21-41cm, the amount of water driving inside the reactor was also increased which further increases HNO3 yield and followed the order: 21 cm (0.33 g) < 31 cm (1.203 g) < 41 cm (1.289 g) and the inside water volume is 0 ml < 130 ml < 390 ml. The results are depicted in table 2.

Table. 2. Effect of height of the water bucket on production of HNO3
S.No Height of the water bucket (cm) Volume of water taken in bucket (Lit) Air flow rate (LPH) Volume of water going inside the reactor (L) Final pH of total water & Conc. Total Power consumption (kWh/hr) HNO3 (moles) HNO3
(gms)
1 21 2 900 0.0 2.57 (0.00269M) 0.3 0.00538 0.333
2 31 2 900 0.13 2.02 (0.00954M) 0.4 0.019 1.203
3 41 2 900 0.39 1.99 (0.0102M) 0.4 0.020 1.289

Example 3: H2O2 addition
H2O2 present in the water may converted in to hydroxyl radicals (•OH) during the discharge process which can increase the HNO3 production (Eq. 5). The present invention shows the optimized concentration of H2O2 to increase the production of HNO3. When 0.01% H2O2 added to the water, the production of HNO3 was increased up to 35%, however, a 10 fold r increase in the H2O2 concentration, the production of HNO3 was increased marginally. Therefore, addition of 0.01% H2O2 appears to be enough to generate sufficient •OH radicals at present experimental conditions and increases the HNO3 production. The effect of % H2O2 addition on HNO3 production is shown in figure 3.

Table. 3. Effect of addition of H2O2 on production of HNO3: *Conditions - Treatment time-1hr, Height of the water reservoir-41cm
S.No Volume of water taken in bucket (L) % of H2O2 added to the water Air flow rate (LPH) Volume of water went Inside the reactor (L) Final pH of total volume & Concentration Total Power consumption (kWh/hr) HNO3 moles HNO3 (gms)
1 2 0 900 0.13 1.99 (0.0125M) 0.4 0.025 1.289
2 2 0.01 900 0.79 1.85 (0.01412) 0.5 0.0282 1.75
3 2 0.1 900 0.60 1.83 (0.0147) 0.5 0.0294 1.82

Example 4: Nitrogen to Oxygen ratio
The production of HNO3 is investigated with increasing O2% in the feed. The percentage of oxygen in the feed gas has a strong impact on the amount of HNO3 produced. Presence of additional O2 provides higher amounts of activated oxygen species (O and O3), which accelerates the NO oxidation to NO2. The effect of addition of O2 on production of HNO3 is shown in table 4.

Table 4. Effect of addition of O2 on production of HNO3:
Air Flow HNO3(grams)
900 LPH 1.289
750 Air+150 O2 1.526

Example 5 - The Mechanism Involved in the HNO3 Formation:
The energetic electrons present in the plasma zone react with nitrogen (N2) and oxygen molecules (O2) to generate the respective radicals, whereas, the energetic electrons bombarded with water molecules to form •OH. Nitrogen oxides (NOx) react with hydroxyl radicals (•OH) and water present in the aqueous medium to form HNO3 (Eq. 1-8). The temperature during the operation of the reactor increases by about 10 to 15 degrees centigrade. Room temperature and pressure are the optimum conditions that the reactor is operated in.
*e- + N2 *N + *N + e- (1)
*e- + O2 *O + *O + e- (2)
*N + *O NO (3)
NO + *O NO2 (4)
NO / NO2 + •OH HNO2 / HNO3 (5)
NO2 + H2O NO3- + 2H+ (6)
H2O + e- H2O+ + 2e- (7)
H2O+ + H2O •OH + H3O+ (8)

While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.
,CLAIMS:A process for production of nitric acid under non-thermal plasma conditions using cold plasma reactor comprising:
(a) providing two electrodes in a cold plasma generator, one stationary and other rotating electrode arranged one surrounding the other to operate between 3 to 5 kV and / or 10 to14 kHz for generating sufficient difference in electric potential between stationary and rotating electrodes plasma;
(b) providing small holes / perforation on rotating electrode for sucking the mist (air + water) into the corona plasma zone situated between the two electrodes;
(c) optimizing the atmospheric air and water flow rates into the plasma reactor to get very fine mist and optimum residence time in the plasma region;
(c) maintaining optimum nitrogen and oxygen ratio in the plasma reactor to provide sufficient oxygen for conversion of un-reacted NO to NO2; and
(d) adding H2O2 and other radical initiators to plasma medium containing water to provide additional reactive species to increase the nitric acid production.
2. The process as claimed in claim 1, wherein the optimized flow rate of the air and water for plasma treatment is in the range of 600 LPH to 1200 LPH for one hour for the production of nitric acid.
3. The process as claimed in claims 1 and 2, wherein the optimized flow rate is 900 LPH which provides effective mist generation at the height of plasma region, with more number of oxygen / nitrogen molecules in the plasma discharge zone leading to increase in nitric acid production.
4. The process as claimed in claims 1 and 2, when the flow rate is fixed at 900 LPH with the maximum production of nitric acid (1.3 g) with a decrease in nitric acid production to 0.8 and 1.0 g for 600 LPH and 1200 LPH, respectively.
5. The process as claimed in claims 1 and 2 or 3, wherein the height of the water reservoir is kept in the range of 21 - 41cm, to drive maximum amount of water inside the reactor to increase the yields of nitric acid yield.
6. The process as claimed in claim 1, wherein the increasing O2 percentage in the feed gas provides higher amounts of activated oxygen species (O and O3), which accelerates the NO oxidation to NO2 facilitating higher yields of nitric acid.
7. The process as claimed in claim 1, wherein the addition of H2O2 and other radical initiators to the water facilitates the conversion of water into hydroxyl radicals (•OH) during the discharge process for increasing nitric acid production.
8. The process as claimed in claims 1 and 2, wherein the mist or the air and water mixture enters into non thermal plasma reactor through air atomizing nozzle and ionization takes place in the reactor leading to production of nitric acid.
9. The process as claimed in claim 8, wherein the reactor is operated with discharge voltage AC power fed to the reactor is 5kV, frequency 14 kHz and AC input power is around 200W.
10. The process as claimed in claim 1, wherein the design of combining corona plasma device with a centrifugal mist sucking device gives maximum exposure of the mist (air + water) to corona plasma in the reactor.
11. A cold plasma reactor for production of nitric acid comprising:
(a) two electrodes are provided in the reactor, one stationary and other rotating electrode arranged one surrounding the other to operate between 3 to 5 kV and / or 10 to14 kHz for generating sufficient difference in electric potential between stationary and rotating electrodes plasma;
(b) an electrical motor is provided to drive the rotating electrode;
(c) small holes / perforations are provided on rotating electrode for sucking the mist (air + water) into the corona plasma zone situated between the two electrodes;
(d) air atomizing nozzles are provided for ionization of atmospheric gas (air) and water in the plasma reactor to obtain very fine mist and optimum residence time in the plasma region;
(e) the reactor is fed to a discharge voltage AC power supply of 3-5kV, frequency of 10-14 kHz and AC input power of around 200W.
12. The cold plasma reactor as claimed in claim 11, wherein the reactor can be placed in the field connected to micro-irrigation systems in-situ farm fields to generate the required amounts of nitrogen from the nitric acid produced or the corresponding nitrate salts of metal ions present under-ground or surface water (is used), for the crops as liquid fertilizer.
13. The cold plasma reactor as claimed in claim 11, wherein, the reactor can be used for commercial production of nitric acid from renewable atmospheric air.
14. The cold plasma reactor as claimed in claim 11, wherein, the reactor can be in miniaturized versions to be fit to a mobile unit for field applications.
15. The cold plasma reactor as claimed in claim 11, wherein the corona plasma reactor can also be run with alternate energy sources, such as solar, wind and hybrid systems.