Industrial manufacturing of Nitric acid

INDUSTRIAL MANUFACTURING OF

NITRIC ACID S.M. Taha Rehman Siddiqui

INSTITUTE OF CHEMICAL ENGINEERING AND TECHNOLOGY University of the Punjab, Lahore, Pakistan.

Taha Siddiqui E11-PG08 Page 1

Production of Nitric Acid

Table of Contents:

Uses Of Nitric Acid ……………………………………………………….. 1

Principle involve in the production of HNO3……………………………… 1

Raw materials use in the production HNO3…………………….………......2

Process Description of the Production of HNO3………………….………...2

Constructional and Working Profile of the Ostwald’s Converter……….…..3

Processing of Gaseous Product Mixture………………………….…………5

Construction and Working of Absorption Unit….………………………….6

The Above given sign is frequently used in the

given below notes and it represents reversible reaction.

14)


Q.1 Enlist the Uses of HNO3 OR Give Uses of HNO3.

I. In the Production of Fertilizer:

a) It is used in the production of NH4NO3.

b) It is used in the production of the Calcium Ammonium Nitrate (CAN).

c) It is used in the production of Orthophosphoric acid which is used in the production of

of triple super Phosphate.

II. In the production of Explosives:

It is used to produce organic Nitro compounds. Some of them are given below

a) Tri- Nitro Toluene (T.N.T).

b) Cellulose Trinitrate Commonly Known as Nitro Cellulose.

c) Glyceride Trinitrate Commonly Known as Nitro Glyceride.

d) Picric Acid (Tri Nitro Phenol).

e) RDX (Hexa Methyl Tetra Amine).

III. It is used in the production of Metal Nitrate such as AgNO3, Cu (NO3) 2.

IV. It is use to part Gold from impurity of Silver and Copper

V. It is use in the production of Aqua Regia (HCl:HNO3 , 3:1). Aqua Regia is use to

Dissolve Insoluble Material.

Q.2 What is the principle which involve in the Production of Nitric Acid ?

Principle Involve in the Production of Nitric Acid:

a. Oxidation of Ammonia:

The process involves the Catalytic Oxidation of Ammonia Using Oxygen Contained

in the air as an oxidizing agent.

Air use in the process must be Dust free and the Oxidative reaction is carried out

at the Temperature of 920˚-940˚C and at the pressure of 100Psig. The catalyst

which is used is the alloy of Platinum and Rhodium in the ratio of :

Pt:Rh , 90:10

Platinum is the specific catalyst and Rhodium acts as promoter.

The reaction which takes place in this step is

4NH3 + 5O2 4NO + 6H2O

This process is carried out in a reactor made up of Stainless Steel -316.

b. Oxidation of Nitric Oxide.


Nitric oxide formed in step (a) is cooled to 20˚-30˚C and then it is further oxidized

using more air to Obtain Nitrogen Dioxide (NO2) as follow:

2NO + O2 2NO2 .

This process is carried out in the absorption unit made up of Stainless Steel-316.

c. Absorption of Nitrogen Dioxide in Water:

NO2 formed in the previous step is absorbed in water as follows:

2NO2 + H2O HNO2 + HNO3 .

The Absorption of NO2 in water is carried out in an absorption Column

made up of Stainless Steel-316

Q.3 Write the Raw material use in the Production of the Nitric Acid ?

Raw Material use in the production of HNO3 is:

Liquid Ammonia (Non-Aqueous Ammonia).

Dust Free Air

Water (De-Ionized water)

Catalyst: Pt-Rh Alloy.

Q.4 Give A Detail Process Description of the Production of Nitric Acid.

Process Description of the Production of Nitric Acid:

1. Vaporization of Ammonia:

Liquid Ammonia which has -33.3˚C boiling point is vaporized continuously and

uniformly in the unit called Vaporizer Unit. Heat needed for Vaporization is get from

Steam. After giving its heat to Liquid Ammonia steam gets converted into liquid

called Steam Condensate.

Steam Condensate is connected to Absorption Column Feed Tank. Steam Condensate

is divided into TWO parts:

Part (a) is sent to Waste Heat Boiler (W.H.B) where it is converted into steam

at 150 psi.

Part (b) is fed at the top of absorption Column via Cooler Condenser.

Now, Gaseous Ammonia emerging from the vaporizer should be kept at the temperature

less than 200˚C.

Reason: The temperature should not be allowed to escalate because the construction

material of gas transmission line which is Iron has the nature to catalyze the

decomposition of NH3 into N2 and H2. Hence Loss of Raw material will take Place.


Gaseous Ammonia is Enrouted to Gaseous mixture via Ammonia Solenoid Valve and

the Ammonia Feeder.

2. Air Compression:

Air Contain 79% by Volume of Nitrogen and 21% volume of Oxygen. It is drawn

from Air through the Air Intake Filter. The Filtered air is compressed in a power

recovery compressor to a pressure of 100 Psig.

The power Recovery Compressor is coupled with an expander turbine operated by

reheated Tail gas. Compressed air further filters through another air filter to ensure

complete removal of dust from it. The dust free compressed air then heated in an Air

Heater. It gets heated against the draft of hot gaseous product coming out of the

reactor.

The heated air is divided into TWO parts:

The major part of the air also called primary air at the temperature of 205˚-

230˚C is directed to the gaseous Mixture.

The second minor part is cooled to 20˚-25˚C is Called Bleaching or

Secondary air is passed through absorption Column via Solenoid Valve and

introduce at the bottom of absorption Column.

3. The Salient Features of Construction and Working of Ostwald’s

Converter: OR

Operational and Constructional Profile of Ostwald’s Converter:

Ammonia Gas from the Gas Feeder is mixed with air which is compressed at 100Psig

and at the temperature of 205˚-230˚C. The composition of Air-NH3 is extremely

critical as it can lead into an explosion.

Composition: At 18˚C

NH3 27-16%

Air 73-84% Explosive.

At 950˚C

NH3 11.5-12%

Air 88.5-88% Explosive

The converter is therefore operated at 10.6% Ammonia and 89.4% Air. This gaseous

mixture is at a temperature of 205˚-230˚C.


Reaction Occurring in Ostwald’s Converter:

a. 4NH3 + 5O2 4NO + 6H2O

b. 2NH3 + 2O2 N2O + 6H2O

c. 4NH3 + 3O2 2N2 + 6H2O

d. 4NH3 + 6NO 5N2 + 6H2O

The Reaction occurring in step (a) is a most desirable reaction, where NH3 is converted

into NO. The reaction (b) and (c) lower the yield of NO as Ammonia gets converted into

N2O and N2 respectively. In the reaction (d) NH3 as well as NO , both are lost forming

Nitrogen.

It is therefore necessary to suppress the reaction (b), (c) and (d).

Catalyst:

It is observed that Platinum metal has high tendency and specificity for

reaction (a) and therefore use as a catalyst. In the presence of Pt , 90-95% NH3 gets

converted into NO.

Pt used as Catalyst is alloyed with Rhodium in a weight ratio of 90:10 to achieve the

following objectives:

i. By Alloying , the tensile strength of catalyst Gauze increase.

ii. The alloy has Higher Ductility.

iii. Rhodium also acts as promoter to some extent.

The Pt-Rh alloy is drawn into silk-fine Wire of 0.003 Inch diameter. The wire is

converted into gauze of 80 meshes.

About 15 to 20 pieces of Pt-Rh gauzes are stacked and placed in Catalyst cylinder. The

catalyst cylinder is made up of steel and has the dimension 12 X 8 inches ; 12 inches

length and 8 inch diameter. In some cases 30 pieces of gauzes are packed in the catalyst

cylinder.

The Ostwald’s converter used for oxidation of NH3 to Nitric Oxide is made up of

Stainless Steel-316. The gaseous mixture which enters into the converter is at a

temperature of 205˚-230˚C and pressure of 100psig.

Temperature:

The First Gauze of the stack of catalyst is heated to a temperature of 700˚-800˚C to

initiate the NH3 oxidation Process.

4NH3 + 5O2 700˚-800˚C 5NO +6H20

The NH3 oxidation process is exothermic (ΔH= -416Kcal/mol) and heat evolved in this

process is capable of maintaining a temperature of 920˚-950˚C in the Converter.

The reaction of NH3 proceeds un-interrupted due to evolution of heat. The overheating

of the converter is avoided by providing converter with a cooling Casket and most

appropriate temperature is maintained.

Material of Construction of Converter is Stainless Steel-316.


Pressure:

4NH3+5O2 4NO +6H2O

The reaction involved suggests that process should be carried out at a pressure less than

atmospheric pressure. However the actual pressure is 7-5 atm to achieve the following

advantages:

a) At higher pressure more NH3 comes in contact with the surface of the catalyst and

hence the conversion of Ammonia to NO per unit volume is increased with the

increase in Pressure.

The space velocity of gaseous mixture is increase and contact time is decreased. The time

of contact has its significance as it is apparent from the following table:

Contact time (sec) of NH3-Air mixture with catalyst

Percentage Formation of NO

0.28 seconds 82%

0.23 seconds 92 % At 920˚-950˚C and 100psig

3x10-4 seconds 95-97%

b) At 100psig the concentration of HNO3 obtain is between 60-70% whereas it only

50-60% at 1 atm.

But the increase in operating pressure is accompanied with the following

DISADVANTAGES:

I. Loss of catalyst is more at 100psig than the lost at 1 atm.

II. 7% platinum is lost through sprout.

III. The working life of catalyst is shortened to 6-10 weeks.

IV. The wall thickness of converter as well as an absorption unit is needed to be increased

and hence the installation cost of the plant is increased.

Product Mixture:

The product mixture emerging out of converter contains 10-12% of NO, small quantity

of Unreacted Oxygen, Nitrogen and Water Vapors.

4. Processing of Gaseous Product Mixture:

Gaseous Mixture contain 10-12% NO at temperature of 920˚-950˚C enters into the Re

heater/ air heater and heat up tail gas and compressed dust free air to temperature of

205˚-230˚C.

The product mixture leaves the air heater at 800˚c and is directed to Waste Heat Boiler

where the heat of the stream of Gaseous Mixture is used for generation of steam of

100psig.

Steam produced in W.H.B is use for the vaporization of NH3 in the vaporizer.


The Gaseous mixture cooled to 200˚-300˚C is passed through Platinum filter and

further cool to 20˚-30˚C in cooler condenser.

The cooled gaseous mixture is divided into TWO parts as follows:

The first part is introduced in the Absorption Unit at its bottom, where the unit

contains packing of Rasching Rings. Rings are placed above the acid trap.

The second part is introduced in the upper half of absorption Column.

Secondary Air from the air heater/re heater is introduced at the bottom of absorption column

to provide oxygen needed for oxidation. The secondary/Bleaching air temperature is 25˚C.

5. Construction and Working of Absorption Unit: OR

Working Profile of Absorption Unit:

1) Absorption unit is tall upright thick walled gas-Liquid absorption unit of the type of

Bubble and Cap.

2) The wall thickness of absorption unit is co-related to pressure.

3) The Absorption of NO2 in water is carried out in an absorption Column made up of

Stainless Steel-316.

4) The absorption column is provided with the cooling chest to keep the temperature of

the unit between 18˚C-20˚C.

5) The height of a column is such that nitric oxide is introduced into it gets converted

into nitric acid via NO2 and tail gas leaving at the top contains only traces of oxides of

Nitrogen.

6) Cold water for absorption of the oxides of nitrogen is introduced at the top whereas

gaseous stream containing NO is introduced at two different points in the absorption

unit.

7) At the bottom of absorption unit, an acid trap is provided which separates liquid from

gases and secondary air also called as bleaching air is introduced at this point at 25˚C.

8) The Bleaching air removes any NO or NO2 contain in HNO3. These gases impart

yellow color to HNO3. HNO3 is basically colorless compound.

9) The area above the acid trap is packed with Raschig rings.

10) The secondary air oxidizes nitric oxide into NO2 by providing O2.

Reaction in the Absorption Unit:-

Principle reaction:

2NO + O2 2NO2

Side reaction:

NO2 + NO N2O3

NO2 + NO2 (NO2)2 N2O4

11) Formation of NO2 (Mechanism) :


The mechanism of the formation of NO2 is thought to be as follows:

NO + NO [NO2]2

[NO2]2 + O2 2NO2

Both reactions are reversible and exothermic.

The 2nd step the dimeric NO is converted into NO2 is slower than the rate of preceding

reaction and hence it is the rate determining step.

The rate of oxidation of NO to NO2 can be increased 100 times using coke as catalyst.

However this catalyst demands that oxidation should be carried out in non-aqueous

medium since production of Nitric Acid involves absorption of NO2 into water hence

the reaction is carried out in the absence of any catalyst.

12) 90% of NO gets converted into NO2 is less than 100 second provided the gaseous

mixture containing NO has 10% of NO and 8% of oxygen from secondary air is kept at

18˚C.

13) Formation of Nitric Acid takes place through series of following reaction:

Reaction in Absorption Unit:

i. 2NO2 + H2O HNO2 + HNO3

ii. N2O3 + H2O 2HNO2

iii. N2O4 + H2O HNO2 + HNO3

iv. HNO2 + NO2 HNO3+ H2O + NO

14) The cycle of conversion of NO in NO2 and subsequently into HNO3 and continues as

the gaseous mixture moves upward in the unit.

15) The contents of Nitric oxide progressively will reduce to almost negligible as gaseous

mixtures reaches at the top of the unit.

16) Ostwald’s process is carried out at a pressure of 100psig the concentration of HNO3

obtained at the bottom Absorption Column is 60-70%.

17) Tail Gas Leaving at the Top of Absorption Column contain chiefly traces of Oxides of

N2, NO, entrained Water and small quantity of Oxygen and traces of Oxides of Nitrogen.

Good Luck

Industrial manufacturing of Nitric acid

Engineering

Transcript of Industrial manufacturing of Nitric acid