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ESTIMATION OF PLATINUM GAUZES CATALYST FOR AMMONIA OXIDATION IN

NITRIC ACID PRODUCTION

Article · December 2005

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061-064•estimation of platinum 2/9/06 3:38 PM Page 61

ESTIMATION OF PLATINUM GAUZES CATALYST FOR

AMMONIA OXIDATION IN NITRIC ACID PRODUCTION
A. W. NurSulihatimarsyila1, T.G. Chuah1, Thomas S. Y. Choong1 and B. Thayananthan2
1
Department of Chemical and Environmental Engineering, Faculty of Engineering,
Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor
2
CCM Chemicals Sdn Bhd, Lot PT, 200 Persiaran Selangor, 40000 Shah Alam, Selangor

ABSTRACT
This paper illustrates design aspects of a shallow bed reactor involving a very rapid reaction, which is a diffusion controlled
reaction. The design of the bed composed of a small number of catalyst screens, is normally based on past experience, scale-
up on the basis of equal velocity, and careful planning for good distribution. Methods needed to estimate the number of
meshes, diameter, height and volume of catalyst bed are discussed. This design method can be useful for economic or
operating studies.

Keywords : Ammonia Oxidation, Gauze, Kinetic, Nitric Acid, Shallow Bed Reactor

1.0 INTRODUCTION from absorber will be mixed with the dilute nitric acid from
There were approximately 65 nitric acid (HNO) condenser to produce the final product of nitric acid (Figure 2).
manufacturing plants in the U. S. with a total capacity of 11
million tons of HNO per year. The plants range in size from Generally, there are three steps involved in nitric acid
6,000 to 700,000 tons per year. About 70 percent of the nitric production:
acid produced is consumed as an intermediate in the (i) Catalytic oxidation of ammonia with atmospheric oxygen
manufacture of ammonium nitrate (NH4 NO3), which in turn is to yield nitrogen monoxide:
used in fertilisers. Another 5 to 10 percent of the nitric acid
produced is used for organic oxidation in adipic acid 4NH3(g) + 502(g) 4NO(g) + 6H2O(1)
manufacturing. Nitric acid is also used in organic oxidation to
manufacture terephthalic acid and other organic compounds. (ii) Oxidation of the nitrogen monoxide product to nitrogen
Explosive manufacturing utilises nitric acid for organic dioxide or dinitrogen tetroxide:
nitrations. Nitric acid nitrations are used in producing
nitrobenzene, dinitrotoluenes, and other chemical 2NO(g) + O2(g) 2NO2(g) or
intermediates. Other end uses of nitric acid are gold and silver
separation, military munitions, steel and brass pickling, 2NO(g) + O2(g) N2O4(g)
photoengraving, and acidulation of phosphate rock.
Much of the nitric acid produced in the world is manufactured
via a high-temperature catalytic oxidation of ammonia. This
process consists of three main steps: ammonia oxidation, nitric
oxide oxidation, and absorption. This process can be performed
at one or multiple pressures. This paper focuses on the single
pressure process. Newer processes typically operate at a low and
a high pressure to favour the reactions.
In nitric acid production, first of all, ammonia is vaporised
in a vaporiser before it is mixed with compressed air and send
to a shallow bed reactor as shown in Figure 1. In the reactor,
ammonia oxidation will occur, where nitrogen monoxide will
be produced. The reaction in the reactor will take place at a
temperature of 840-880°C. Due to the high temperature
involved in the reactor, energy can be recovered by cooling the
gaseous mixture in a waste heat boiler and cooler-condenser.
Along waste heat boiler and cooler-condenser, the nitrogen
monoxide will be oxidised to nitrogen dioxide, and dilute nitric
acid will be condensed out at the condenser. The nitrogen
dioxide will enter the absorber where absorption with water
will take place and finally, nitric acid will be produced. Tail gas Figure 1: Shallow bed reactor for catalytic ammonia oxidation
will be produced at the absorber, and the nitric acid produced with integrated waste-heat recovery system

Journal - The Institution of Engineers, Malaysia (Vol. 66, No. 4, December 2005) 61
061-064•estimation of platinum 2/9/06 3:38 PM Page 62

A. W. NURSULIHATIMARSYILA et al.

where PA is the partial pressure of ammonia in bulk fluid and

aWR is the surface area per unit volume of screen. For shallow
beds reactors, axial diffusion must surely be important,
however, at very high flow rates encountered in commercial
equipment the effect of axial dispersion is less important than
the problem of ensuring uniform flow distribution across the
gauzes. The ammonia oxidation rate determining step in the
reaction of NO is the transport of ammonia to the catalyst
surface, and the reaction kinetic, ksgA is given as [2]:
0.865NRe-0.648 G (2)
ksgA =
PNsc2/3Mmεw

where NRe is the Reynolds number, G is the superficial flow rate

(g/cm2 .sec), NSc is Schmidt number, Mm is the molecular weight
of mixture of gases and εw is the porosity of gauzes.

2.1.1 NUMBER OF GAUZES

From literature, at temperature, T = 900°C and pressure, P=
100 psig (7 bar), a quantity of 80 mesh gauze (nw), with wire of
0.003 in. in diameter (dw), equivalent to 2 Troy oz/ton of acid
produced and a cross sectional area of 2.7547 sq ft/100 daily
tons of HNO3 produced is required [3]. The equations below
are used in calculating the number of gauzes, after further
deriving and simplifying from mass transfer concept as shown
in equations (1) and (2):
Figure 2: Schematic diagram of nitric acid production
In(1-XA) = -KgAs Pns fw MF ) (3)
G
(iii)Absorption of the nitrogen oxides to yield nitric acid:
- In(1-XA)εw0.352dw0.648 G0.648µf0.019
ns = (4)
3NO2(g) + H2O(l) 2HNO3(1) + NO(g) (5.81761 x 10-5)fwTi0.333(28.85+11.82yAO)0.667

where fw is the wire area per gauze cross-sectional area

2.0 AMMONIA OXIDATION for one gauze, εw is porosity of mesh, dw is wire diameter, µf
Ammonia oxidation that takes place in the reactor can be is viscosity, yAo is initial gaseous mole fraction of A. Perfect
categorised as the reaction between gas phase and a solid plug flow model is assumed in calculating the number of
catalyst. The most suitable type of reactor is the thin beds and gauzes, which is approached in the unit with a special gas
wire gauzes. It is also known as shallow bed reactor. This kind distributor.
of reactor is usually used in fast catalytic reactions that must be
quenched rapidly. The materials to be reacted are in contact 2.1.2 HEIGHT OF CATALYST BED
with wire screens or thin layers of fine granules. For example, The height of catalyst bed needed in this detailed design
ammonia in a 10% concentration in air is oxidised by flow project is calculated by:
through a fine gauze catalyst made of 2 to 10% Rh in Pt, 10 to
30 layers, and 0.075 mm diameter wire. Contact time is hc = 2dw ns (5)
0.0003s at 750ºC and 7 atm followed by rapid quenching [1].
2.1.3 DIAMETER OF CATALYST BED
2.1 REACTOR DESIGN Gillespie and Kenson [3] proposed a method in determining
In the reactor, ammonia oxidation will occur, and for the the cross sectional area of catalyst bed. It is found that the area
reaction to happen at faster conversion rate, platinum catalyst is 2.7547 sq ft/100 daily tons HNO3.
is used. High temperatures and high velocities will produce
essentially total conversion of ammonia. The catalyst is placed The diameter can be determined using:
on wire gauzes and the parameters of the catalyst used and
calculated will be discussed in detail in following sections. (6)
On the basis of mass transfer control the rate of ammonia
oxidation may be written in terms of a mass transfer coefficient
with the ammonia partial pressure at the catalyst surface 2.1.4 WEIGHT OF CATALYST
assumed to be zero for this rapid reaction. From literature, weight of 80-mesh gauze is 1.71 troy oz/ft2
and the catalyst needed is 2 troy oz/daily ton [3]. The following
-rA = kgAsaWRPA (1) equation is used in calculating total weight of catalyst bed.

62 Journal - The Institution of Engineers, Malaysia (Vol. 66, No. 4, December 2005)
061-064•estimation of platinum 2/9/06 3:38 PM Page 63

ESTIMATION OF PLATINUM GAUZES CATALYST FOR AMMONIA OXIDATION IN NITRIC ACID PRODUCTION

awrdw
wc = [1.71 x Area(ft2) x ns] + [2 x Daily production rate] Porosity, εw = 1-
(7) 4

2.1.5 VOLUME OF CATALYST BED = 1- 258.5(0.003)

4
After calculating the height and diameter of the catalyst
bed, the volume of the catalyst bed can be determined by: = 0.806

Volume of catalyst = 1 πD2(hc ) (8) i) Calculating mass velocity, G:

4
Flowrate of gases
Details of the example calculation are shown in Appendix and G=
Table 1 is the summary of the calculated results. Cross sectional area of catalyst needed

237286.50
3.0 CONCLUSION =
It is always not easy to estimate the amount of catalyst used 3600 x 3.10
in ammonia oxidation. Conventional packed bed reactor design
method is not suitable in estimating the catalyst needed. A = 21.26kg/m2.s
simple estimation method of catalyst gauze thickness has been
proposed in this work. These design method can be useful in = 2.126g/cm2.s
economic or operating studies. ■
ii) Calculating superficial velocity based on outlet conditions:
Table 1: Result for chemical detailed design of reactor
us = g/ρmixture
Parameter Value Unit
Temperature 840 °C Table A-1: Mass flowrate, molar flowrate and mole fraction of
components in reactor feed
Pressure 8 bar
Mesh size 80 in-1 Components Mass flowrate Molar flowrate Mole raction
(kg/hr) (kmol/hr)
Wire diameter 0.003 in
NH3 14,450.00 850.00 0.10
Porosity 0.806 -
O2 51,952.02 1,623.50 0.19
Mass velocity of gasses 2.126 g/cm2.s N2 170,884.05 6,103.00 0.71
Superficial velocity of gases 29.14 g/cm.s Total 237,286.07 8,576.50 1.00
Number of gauzes needed 21 -
Height of catalyst bed 0.32 cm Average molecular weight,
Diameter of catalyst bed 1.987 m M = 0.10(17) + 0.19(32) + 0.71(28)
Weight of catalyst and gauzes 112.70 kg = 27.67 kg/kmol
Volume of catalyst 0.010 m3
Operating pressure, P = 8 bar

APPENDIX: Operating temperature, T = 840ºC = 1113K

Catalyst calculations
ρmixture = MP/RT
From literature [3]: 27.63(8 x 105)
=
8.314(1113)
Mesh size, nw = 80 in-1
= 2,391.92 g/m3
Wire diameter, dw = 0.003 in.
= 2,391 x 10-6 g/cm3
Surface area per unit volume, awr = πlwnw2, 2.126
Therefore, us =
2,391 x 10-6
where lw = [(1/nw)2 + dw2]0.5
= 888.33 cm/s
Therefore, awr = πnw [(1/nw) + d ]
2 2
w
2 1/2 = 29.14 ft/s
= π(80)2[(1/80)2 + 0.0032]1/2
= 258.5 in-1 Thus us = 29.14 ft/s constitutes a more general criterion based
on these reported data.
Wire area per gauze cross sectional area,
fw = awr 2dw iii) Calculating number of gauzes needed:
= (258.5)(2)(0.003) - In(1-xA)ε0.352dw0.648 G0.648µf0.019
= 1.55 ns =
(5.81761 x 10-5) fwTi0.333(28.85 + 11.82yAO)0.667

Journal - The Institution of Engineers, Malaysia (Vol. 66, No. 4, December 2005) 63
061-064•estimation of platinum 2/9/06 3:38 PM Page 64

A. W. NURSULIHATIMARSYILA et al.

Where, XA = conversion of ammonia feed = 1 π(1.987)2 (0.0032)

yAo = mole fraction of ammonia in feed 4
µf = viscosity of mixture = 0.010 m3

µf = (12.5 + 29.20 x 10-3T) x 10-5 g/cm.s = 3.97 m

= [12.5 + 29.20 x 10-3 (1113)] x 10-5
= 4.5x10-4 g/cm.s
Therefore, ns
[-In(0.04)](0.806)0.352(0.0076)0.648(2.126)0.648(4.5x10-4)0.019 REFERENCES
=
(5.81761x10-5)(1.55)(1113)0.333(28.85+1.17)0.667 [1] Perry R.H. and Green D.W. Perry’s Chemical
= 20.17 Engineer’s Handbook. 7th ed. McGraw-Hill. (1997).
= 21 gauzes
[2] Satterfield C. N. and Cortez, D. H. Ind. Eng. Chem.
iv) Calculating height of catalyst bed: Fundam, 9, p.613 (1970).

Height of catalyst bed = 2dw [3] Gillespie G. R. and Kenson R. E. Chem. Tech., Oct.
(1971).
For 21 gauzes, height of catalyst bed, hc = 21 x 2 x 0.003
= 0.126 in
= 0.32 cm
PROFILE
v) Calculating diameter of catalyst bed:
A. W. NurSulihatimarsyila
From literature, cross sectional area Department of Chemical and
= 2.7547 sq ft/100 daily tons HNO3 Environmental Engineering, Faculty of
Engineering, Universiti Putra Malaysia,
43400 UPM, Serdang, Selangor.
Since the daily production rate is 1,212.12 tons/day,

Therefore, cross sectional area = 2.7547(12.12) Dr T.G. Chuah Abdullah

= 33.39 sq ft (= 3.10 m2) Department of Chemical and
Environmental Engineering, Faculty of
Diameter of catalyst bed, D = Engineering, Universiti Putra Malaysia,
43400 UPM, Serdang, Selangor.

= 1.987m Thomas S. Y. Choong

Department of Chemical and
vi) Calculating weight of catalyst: Environmental Engineering, Faculty of
Engineering, Universiti Putra Malaysia,
2
From literature, weight of 80-mesh gauze = 1.71 troy oz/ft , 43400 UPM, Serdang, Selangor.
catalyst needed = 2 troy oz/daily ton.
B. Thayananthan
Weight of one gauze = 1.71(33.39) CCM Chemicals Sdn Bhd,
= 57.10 troy oz Lot PT 200, Persiaran Selangor,
4000 Shah Alam,
Selangor.
Weight of 21 gauze = 21(57.10)
= 1,199.05 troy oz

Weight of catalyst needed = 2(1212.12)

= 2,424.24 troy oz

Total weight of catalyst and gauze = 1,199.05 + 2,424.24

= 3,623.29 troy oz
= 112,696.87
= 112.70kg

vii) Calculating volume of catalyst:

Volume of catalyst = 1 πD2 (hc)

4

64 Journal - The Institution of Engineers, Malaysia (Vol. 66, No. 4, December 2005)

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