IMPACT OF THE INCREASED ACTIVE SURFACE OF THE PLATINUM CATALYST ON THE TOTAL AMMONIA RECOVERY COEFFICIENT

In order to increase the active surface of platinum catalysts for ammonia oxidation and on the basis of theoretic considerations and tests in industrial environment, we have finally decided on their specific design. Efficiency on the newly designed catalyst was checked in industrial circumstances. A comparative analysis of the total ammonia recovery coefficient between the mentioned new catalysts and previously applied platinum catalysts was carried out. All advantages of catalysts with increased active surfaces were confirmed and a new method of their manufacturing process was selected.


Introduction
During the process of ammonia oxidation to nitric monoxide (NO), a set of catalyst gauzes was used based on platinum and platinum alloys.Usual thickness of catalyst gauzes was 1024 appertures/cm², while the wire diameter ranged between 0.06 mm to 0.08 mm.The number of gauzes in one set was 3 to 40, depending on the quantity and pressure of the gas mixture.During ammonia oxidation, the mixture of ammonia and air reacted with the platinum catalyst at the pressure of 0.1 to 1.0 MPa and t=780-920°C in a very short period of time, 1-2x10 -4 sec.For that time, 92-98% ammonia oxdized to NO and the rest transfered to N 2 or N 2 O.In the ammonia oxidation process, platinum and rhodium evaporated, the catalyst activity was reduced and the life time was shorter.Depending on the gas mixture pressure, gauzes can be used from three months to one year.Platinum catalyst using time, platinum and rhodium loss during the ammonia oxidation and the degree of ammonia oxidation are the subject to numerous research projects.Different deactivation mechanisms for platinum catalysts have been studied by Sandjykov and Bartholomew [1,2,3].
Refining of platinum metals from used platinum catalysts to obtain platinum metals of high purity was the subject of research of numerous authors [4,5,6].
Impact of impurities on platinum catalyst activities have been studied by many authors [7,8,9,10].
A number of other research projects were directed to the phenomena occurring on the very surface of platinum catalysts during their use with a particular accent on the kinetics of the mass transported over them in the function of the time and active surface [11][12][13][14][15][16][17].
However, even with many research projects, new techniques for platinum catalyst testings are always actual and with one aim only and that is to icrease the total ammonia recovery in the process nitric sideline production.
The purpose of this paper is: 1. to determine the ammonia consumption values per tonne of produced nitric acid (N) and the total ammonia recovery coefficient by applying platinum catalysts manufactured by weaving techniques.
2. to determine the ammonia consumption value per tonne of produced nitric acid and the total ammonia recovery coefficient by applying platinum catalysts with an increased active surface.
A set of catalysts comprised of woven and knitted catalysts in the weight ratio of 50 : 50.

Materials and Research Techniques
An ammonia oxidation reactor was used for testing on line I in the Nitric Acid Plant, in Pančevo.The reactor production capacity is 257 t of nitric acid per day, the temperature of the process 920°C and the working pressure 0.75 MPa.For the first group of tests, knitted platinum catalysts were used and presented in figure 1 with the following characteristics: 1. Chemical assays -90%Pt, 5%Rh, 5%Pd 2. Round catalyst diameter -1300 mm 3. Density of the material, 1024 appertures/cm², wire diameter 0.076 mm 4. Packaging weight, app.16 kg Measurements were done and the following results were obtained for the ammonia consumption, the quantity of obtained nitric acid, the percentage of oxygen, pressure and volume of ammonia in the feed, ammonia oxidation coefficient, absorption coefficient and the total ammonia recovery coefficient.

Comparative analysis of obtained results
For the second group of tests, platinum catalysts were used.They are shown in picture 2 in the weight ratio of 50% woven and 50% knitted and the constant packaging weight of 16 kg.The wire diameter, catalyst diameter and chemical assays remained unchanged.The woven structure of the catalysts with increased active surface was selected after certain theoretical considerations and tests performed on industrial knitting machines.
For the second group of tests, same measurements and calculations were done as for the first experiment

Results and discussion
For the first group of platinum catalyst tests, woven ones were used.Obtained values and calculations for the ammonia consumption per tonne of produced nitric acid (n), ammonia oxidation coefficient, apsorption coefficient and the total ammonia recovery coefficient (k) are given in table 1.
The parameter is calculated according to the formula: N = A / C The total ammonia recovery coefficient is calculated according to the formula: k = molar ammonia mass / molar nitric acid mass x 1 / N In the time of exploitation which lasted 102 days, 23,581t of nitric acid was produced in the reactor.For this quantity, 6990t of ammonia was used.The mid parameter value was N = 0.297.The mid value of the total ammonia recovery coefficient with this parameter was 0.91.Metal loss during the use of the catalyst was 9,237.49gor 0.392g/t of produced acid.
Dependence of ammonia consumption per one tonne of produced acid in the function of time is shown in figure 3.
Dependence of the total ammonia consumption per one tonne of produced acid in the function of the catalyst explotation time is shown in figure 4.  When the mentioned platinum catalysts were used for 112 days, 24,369t of nitric acid were produced.To produce this quantity, 7046 t of ammonia was used.Mid value of the parameters for this quantity was 0.289.
Mid value of the total ammonia recovery coefficient was 0.94.Metal loss during the catalyst exploitation was 8.821g or 0.0362g platinum per tonne of produced nitric acid.
Dependence of ammonia consumption per tonne of produced nitric acid on the exploitation time is shown in figure 5.
Dependence of the total ammonia recovery coefficient on the catalyst exploitation time is shown in figure 6.
Obtained results depend on the quantity of ammonia used to produce one ton of nitric acid and the total ammonia recovery coefficient in the exploitation time are presented in figure 7 and

Conclusion
When results of application of woven and a combination of woven and knitted catalysts were comparatively analyzed, the following was concluded: -When platinum catalysts with an increased active surface were used, ammonia consumption per tone of produced nitric acid dropped while the total ammonia recovery coefficient increased.
-Ammonia consumption was 8 kg per ton of produced nitric acid lower when a combined package of catalysts was applied.
-Total ammonia recovery coefficient when combined catalysts were applied was by 2.87% higher in comparison to the same when only woven catalysts were applied.
-Packagings with combined catalysts had a life longer by 10 days.
-Platinum loss when woven and knitted catalysts were combined was by 416.49g lower than with woven catalysts.Obtained results have significantly contributed to the studies on the impact of changes of active surfaces to the total ammonia recovery coefficient in the nitric acid production process.

Fig. 5 .Fig. 6 .
Fig. 5. Dependence of ammonia consumption per tonne of produced nitric acid on the exploitation time

Fig. 7 .Fig. 8 .
Fig. 7. Dependence of parameters on the exploitation time with used woven and combined woven and knitted catalysts

Table 1 .
Measured values and calculated values for woven catalysts A R T I C L E I N P R E S S A R T I C L E I N P R E S S

Table 2 .
Measured and estimated values for the combination of woven and knitted platinum catalysts 8.
A = ammonia mass, t B = nitric acid mass, t C = ammonia pressure in MPa D = ammonia temperature in ° C E = ammonia volume in Nm³