MANUFACTURE OF AMMONIA (Haber Process)

THE AMMONIA MANUFACTURING PROCESS


Ammonia is produced in a process known as the Haber process, in which nitrogen and 
hydrogen react in the presence of an iron catalyst to form ammonia. The hydrogen is formed 
by reacting natural gas and steam at high temperatures and the nitrogen is supplied from the air.
. Other gases (such as water and carbon dioxide) are removed from the gas stream and 
the nitrogen and hydrogen passed over an iron catalyst at high temperature and pressure to form the ammonia.



The raw materials for this process are hydrogen and nitrogen. Hydrogen is obtained by reacting natural gas - methane - with steam, or through the cracking of oil. Nitrogen is obtained by burning hydrogen in air. Air is 80 per cent nitrogen; nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen, leaving nitrogen behind.

Nitrogen and hydrogen will react together under these conditions:

a high temperature - about 450ºC
a high pressure - about 200 atmospheres (200 times normal pressure)
an iron catalyst
The reaction is reversible.

nitrogen + hydrogen --> ammonia

N2(g)  + 3H2(g) --> 2NH3(g)

The (g) indicates that the substance is a gas.

The flow chart shows the main stages in the Haber process. The reaction is reversible, and some nitrogen and hydrogen remain mixed with the ammonia. The reaction mixture is cooled so that the ammonia liquefies and can be removed. The remaining nitrogen and hydrogen are recycled.

The Haber process for making ammonia

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MATERIAL BALANCE (with chemical reactions)

MATERIAL BALANCE WITH CHEMICAL REACTIONS:

Introduction:

Chemical reactions play a vital role in manufacturing process. For design of chemical
process equipment, the operating conditions such as pressure, temperature,
composition and flow of the streams should be known. The material balance and energy
balance calculations come to the rescue of the designer and allows him to calculate the
various flow rates and temperature of the streams. Assuming that the kinetic data of the
reaction is available, the overall material balance of the steady state condition will be
discussed here.



Material balances:

The general mathematical statement can be written as

Total mass entering the unit = Total mass of products leaving the unit

It should be noted that in chemical reactions, the total mass of the input remains
constant, but the total moles may or may not remain constant.

Example: Consider the shift reaction

                                                            CO +H2O  - ->    CO2 +H2

 In this, it can be observed that two moles of reactants react with each other and
produce also two moles, thus the number of moles of the reactants entering the reaction
equals the number of the products leaving the reaction.


                                               1 mole CO= 1 mole H2O
                                                                    1 mole H2
                                                                    1 mole CO2

Limiting reactant / component:

It is the reactant which is present in such proportion that it’s compete consumption by
the reaction will limit the extent to which the reaction can proceed


Excess Reactant:

Percentage excess reactant is defined as % excess quantity taken based on theoretical
requirement.
It is the amount in excess of Stoichiometric (theoretical) requirements expressed as the
percentage of Stoichiometric / theoretical requirement.

Consider a reaction
                                                                 A + B --> C
Where B is the excess reactant, then
Percent excess of B
 = mole of B supplied or fed – moles of B theoretically requiredx100
                                                    Moles of B theoretically required

Consider for example, the reaction
                                                      SO2 + 1/2O2--> SO3
 
And suppose that 100 moles SO2 /hr and 75 moles O2 /h are fed to the reactor. SO2 is
clearly the limiting reactant and to be in stoichiometric proportions, moles of O2 would
have to be 50 kmol / hr. 

ANS.                 The percent excess of O2 is therefore
                               = [(75 – 50) / 50] x 100 = 50


CONVERSION:

                                                          A + B--> C
Where A = a limiting reactant
            B = the excess reactant.

Then the conversion or fractional conversion of A is the ratio of amount of A reacted to
the amount of A charged or fed to a reactor. The percentage conversion of A is the
amount of A reacted expressed as the percentage of amount of A charged or fed to a
reactor.
The amount of A can be expressed in moles or weight of the amount of A is expressed
in moles then
                          % conversion of A = Moles of A reacted x 100
                                                               Moles of A charged or fed


YIELD and SELECTIVITY:

Consider the multiple reactions, namely a series parallel reaction, a series reaction and
a parallel reactionSeries parallel reaction 
                                                     A + B--> C
                                                     C + B--> D

Series reaction                           A --> C -->D

Parallel reaction                         A--> C
                                                    A--> D
Where C is a desired product, D is an undesired product, A is a limiting reactant.
 
Then yield of C is given as

                   Yield of C = moles of A reacted to produce C x 100
                                           Total moles of A reacted


Consider parallel reaction
                                          A --> C and A--> D
 
Where C is a desired component, D is an undesired component
In such cases, the selectivity is given as
 
Selectivity of C relative to D = moles of C (desired product) formed
                                                        Moles of D (undesired product) formed



EXAMPLE:


1. Ethylene oxide is produced by oxidation of ethylene. 100 kmol of ethylene are fed to
a reactor and the product is found to contain 80 kmol ethylene oxide and 10 kmol CO2.
Calculate
a) the % conversion of ethylene and b) the percent yield to ethylene oxide.
 

ANS:  Basis: 100 kmol ethylene fed to the reactor.

Reactions:
C2H4+1/2 O2 --> C2H4O........ ( 1 )
C2H4+3O2 --> 2CO2+2H2O....... (2)
 
80 kmol ethylene oxide is produced and this is possible only by reaction 1. As per
Stoichiometry 1kmol ethylene oxide will be produced per kmol of ehylene. Therefore
ethylene reacted for reaction 1 is 80 kmoles.
 
10 kmol of CO2 is produced and this possible through reaction 2. As per Stoichiometry 2
moles CO2 will be produced per mol ethylene.

 Therefore kmol ethylene reacted by
reaction 2 is 10/2=5 kmol.
 
Total ethylene reacted towards equation1and 2 = 80 + 5 = 85 kmol.
Total kmol of ethylene taken = 100 kmol
Therefore
 % conversion = (ethylene reacted / ethylene taken) x100
                          = (85/100) x100= 85%
 
% yield of ethylene oxide= (moles ethylene reacted to ethylene oxide /Total moles of
ethylene reacted) x100
                                           = (80/85)100=94.12%

 

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Preperation Of Dyes

METHYL ORANGE INDICATOR:

Methyl orange is a pH indicator and due to its clear color change it is very often used in
titrations. Methyl orange changes color at the pH of a mid-strength acid and is usually
used in titrations for acids. Unlike a so called universal indicator, methyl orange does not
have a full spectrum of color change, but has a sharper end point.

Structure Of Methyl Orange

PREPERATION:

1. Although sulfanilic acid is insoluble in acid solutions, it is nevertheless necessary to carry out the diazotization reaction in an acid (HNO2, nitrous acid) solution. This problem can be circumvented by precipitating sulfanilic acid from a solution in which it is initially soluble. The precipitate which is formed is a fine suspension and reacts instantly with nitrous acid. The first step is to dissolve sulfanilic acid in basic solution.
2. In order to obtain the nitrosonium ion (NO+), sodium nitrite has to be treated with hydrochlorid acid. During the addition of the acid, the sulfanilic acid is precipitated out of solution as a finely divided solid, which is immediately diazotized
3. The finely divided diazonium salt is allowed to react immediately with dimethylaniline in the solution in which it was precipitated.

                                 

MALACHITE GREEN:      

                     

Malachite green is an organic compound that is used as a dyestuff and has emerged as a controversial agent in aquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper.

PREPARATION:

Grignard reagents are highly susceptible to water and much care will go into ‘drying’ the reaction before 

you begin. In order to insure water is kept out of this reaction you will flame dry your glassware, use 

anhydrous ether as the solvent and your reaction vessel will contain a drying tube to dry the 

atmosphere with in the reaction apparatus. 

Before you begin this lab you may want to recrystallize 4-bromo-N,N-dimethylaniline using methanol as 

solvent. Once recrystallized make sure that all methanol has evaporated from the crystals.

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AMMONOLYSIS

MANUFACTURE OF ANILINE FROM CHLOROBENZENE BY AMMONOLYSIS

Raw materials

Basis:1 ton aniline

Chlorobenzene = 2,500 lb

Ammonia solution(28%) = 7,450 lb

Cuprous oxide = 350 lb

  Manufacture

Chlorobenzene is charged into a series of horizontal, rotating high - pressure rolled steel autoclave. Approximately 0.1 mole of cuprous oxide and 4 to 5moles of 28 to 30% aqueous ammonia per mole of chlorobenzene are added. The reaction is initiated at a temperature of 180oC and is later maintained at 210 to 220°C under constant agitation. The pressure rises to 750 to 850 psi. The active catalyst is cuprous chlorine produced from cuprous oxide by the product ammonium chlorine as follows:

Cu2O + 2NH4CI  →  Cu2CI2 + 2NH3 + H2O

A large excess of ammonia solution is used to suppress the phenol producing side reaction (C6H5CI + NH3 + H2O → C6H5OH + NH4CI). If the indicated ratio of reactants is used, the rate of aniline formation is about 20 times greater than the rate of phenol formation.

The reaction products are cooled below 100°C and run to a separator. The free ammonia continues to absorption and condensing system for recovery. The settled reaction mass separates into two layers: aniline rich lower and an aqueous upper layer. The approximate distribution of reaction products not including unreacted chlorobenzene in the two layers is as follows: aniline layer 82% aniline, 5% phenol, and 1% diphenylamine; water layer 5 % aniline, 0.5% phenol, 9% chlorine ion (NH4CI), 3% cuprous oxide and 14% ammonia.

The aqueous layer is drawn from the top of the separator and is run to a neutralizer, where it is treated with sodium hydroxide or lime. A sufficient amount of alkali is used to react with the ammonium chloride and phenol. The solution is fractionally distilled, and the liberated ammonia expelled first is recovered in an absorption system. The second fraction consists of aniline and water, which are separated by decantation. The residual solution of sodium phenate and sodium chloride is filtered to remove the precipitated copper oxides, which are reused in subsequent runs.

The aniline layer is withdrawn from the bottom of the separator and treated with 50% sodium hydroxide solution. Approximately 0.2 percent of the volume of the aniline layer is used. The solution is fractionally distilled, yielding first aniline -water mixture, which is further treated as described previously. The second fraction is technically pure (97 to 90%) aniline, the residue is steam distilled, yielding diphenylamine. The phenol is recovered by acidifying the residue mostly sodium phenate and distilling. The yield of aniline is 96% based on chlorobenzene.

 m-NITROANILINE 

100 parts of dinitrobenzene is added to 1000 parts of water at 900C contained in a reducer fitted with reflux condenser and a propeller type stirrer. Upon emulsification, 245 parts of sodium sulfide (9H2O), dissolved in a minimum of water, is gradually run in. The dinitro compound is gradually reduced to m-nitro aniline, the end point being determined by the formation of a definite black streak when ferrous sulfate solution is added to filter paper spotted with some of the reducer liquor.

A modification in the preceding process involves the use of an organic solvent, which is immiscible with water, for the m-dinitrobenzene. Accordingly, 100 parts of technical dinitrobenzene, 90% purity and 160 parts of either solvent naphtha or toluene are put into reducer, and the mixture is warmed to 600C to effect solution. Then, 4000 parts of hot water is added, and the m-dinitrobenzene solution is stirred and heated to 950C. A hot polysulfide (Na2S3) solution – made by heating 720 parts of 7% Na2S with 40 parts of flowers of sulfur – is then added rather rapidly. The reaction of polysulfide is distinctly exothermic, and the charge boils vigorously, but overheating is avoided because of vaporization of solvent. Reduction of the dinitrobenzene to m-nitro aniline is found to take place quickly under such conditions.

To hot reduction mass is first filtered to remove any free sulfur, and the solvent naphtha in the filtrate is distilled with steam. The dissolved m-nitro aniline crystallizes out in the form of bright yellow crystals when the residual liquor is cooled. After washing, the product has a melting point of about 1130C and can be used directly in the manufacture of azo dyes. A yield of approximately 90–92% of theory is attainable, and the process is applicable to other m-dinitro compounds, e.g., m- dinitro derivatives of toluene and xylene.

     

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BECHAMP REDUCTION

MANUFACTURE OF ANILINE FROM NITROBENZENE BY BECHAMP REDUCTION

1 Raw materials 

Basis: 1 metric ton aniline

Nitrobenzene: 1390 kg

Iron borings: 1600 kg

Hydrochloric acid: 125 kg

2 Manufacture 

Crude nitrobenzene is charge into a reducer (reactor) fitted with an efficient reflux condenser. The reducer is a steam jacketed cast iron enclosed cylindrical vessel containing an agitator. Cast iron borings (turnings) or powder (free from oil and non - ferrous metals), water and catalyst are added gradually, in small quantities, to the nitrobenzene. Generally 10 to 20% of the total iron is added at the beginning and the mixture is heated by steam to reflux temperature (200°C). The remaining iron is added over a period of time at a rate determined by the proper pressure temperature balance. The addition rate is fast enough to maintain lively reflux by the heat generated from the exothermic reaction, yet slow enough to prevent excessive hydrogen, pressure build up.

The water required for the reaction is generally in the form of aniline water recovered from the separator or column and is added to the reducer in bulk at the start or in small quantities along with the iron additions. Dilute (30%) HCl acid is added along with the water as catalyst. The acid reacts with the iron borings. Forming catalytically active iron salts. Subsequent runs may utilize aniline hydrochloric acid mother liquor as the source of the catalyst and some of the reaction water; the weight ratio of reactants is approximately 115 parts of iron borings, 0.27 parts of 100% hydrochloric acid and 60 parts of water per 100 parts of nitrobenzene. After the last addition, the reaction is heated with steam to maintain lively reflux.

At the end of the reduction (about 10 hrs. for 2250kg charge), the aniline is separated from the reducer charge by one or more of several methods, The liquid water aniline mixture may be separated from the solid iron oxide iron hydroxide sludge by steam distillation, vacuum distillation, filtration, centrifugation or siphoning. For example the finished reduction product may be neutralized with a small amount of sodium carbonate (about equal to the amount of hydrochloric acid in the charge) and allowed to settle. Most of aniline and some water siphoned off and the residual aniline is separated from the sludge by steam distillation. The sludge consisting of ferric oxide, water and small amount of ferric oxide is dumped and may be marketed after drying.

The water aniline mixture from the reducer is run to a separator where the lighter aniline separates and is withdrawn from the upper. The top layer, which contains 3 to 5% aniline, is partially distilled until the aniline content in the water is low. The residual aniline water is returned to the reducer for subsequent runs. The aniline in the distillate is separated by decantation and the water layer is redistilled to obtain the remaining aniline. An alternate procedure is to extract aniline from the aniline water with nitrobenzene.

The aniline streams from the separator and decanter are united and vacuum distilled to yield purified aniline. The yield based on nitrobenzene is approximately 95% by weight.

MECHANISM OF BECHAMP REDUCTION

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AMINATION

INTRODUCTION:

The process by which amine group is introduced in an organic compound thereby producing primary, secondary or tertiary amine or its derivatives is known as amination process.

SIGNIFICANCE OF AMINATION

 Amination is a very important unit process as the products Amines are of very great importance as intermediates in the chemical process industries. They are used in production of dyes, rubber chemicals, nylon, pharmaceuticals, gasoline additives, surfactants, textile auxiliaries, photographic chemicals, chelating agents, sweetening agents, agricultural chemicals, polyurethanes, inks, plastics etc.

In recent years, the production of diamines by reductive methods has become increasingly important. Hexamethylenediamine is a prime intermediates, together with adipic acid, in the production of nylon 66. m-phenylenediamine is used as a crosslinking agent in epoxy resin. Toludenediamines are intermediates in the production of toluene disocyanates, from which are made polyurethane foam, rubbers, coatings and adhesives.

TYPES OF AMINATION:

• AMINATION BY Reduction
• Ammonolysis

 Amination by reduction involve the synthesis of amines by reductive methods. Amines can be produced by reducing nitro, nitroso, hydroxylamino, azoxy, azo and hydrazo compounds as well as oximes, amides, nitriles and azides. In each case, a carbon to nitrogen bond already exists.

Amines may also be formed by ammonolysis i.e. reacting compounds containing certain labile groups (e.g. halogens, hydroxyl and sulfonic) with ammonia. It involves substitution of –X, -H,-OH or –SO3H group by –NH2 group as well hydroamonolysis of carbonyl group.

• AMINATION BY Reduction

Amines can be easily produced by a great variety of reduction methods as shown below:

.

• The reduction of nitro, nitroso, hydroxylamino, azoxy, azo and hydrazo compounds.
• The reduction of nitrites, amides, oximes and azides.
• The replacement of labile groups, such as nitro, halogen, hydroxyl and sulfonic acid by reaction with ammonia or ammonia progenitors such as urea.
• Intramolecular replacement of (a) hydrazobenzenes and hydroxylamines (b) amides and (c) secondary and tertiary amines.
• The hydrolysis of N-substituted amides.
• Direct amination by means of hydroxylamine and sulfuric acid.

First four are the most important methods from chemical engineering point of view.

The reduction agents used are enlisted as follows:

1. Metal and acid: Iron and acid (Bechamp method) is the major example in this category, but other metals (tin, zinc) have also been employed. Generally, hydrochloric acid is preferred, but sulphuric acid, acetic acid and formic acid have also been used.
2. Metal and alkali: This method is used mainly for the production of azoxy, azo and hydrazo compounds. The latter are important in the manufacture of the benzidine series.
3. Metal hydrides
4. Catalytic: This method involves the use of hydrogen (or hydrogen - containing gases) and a catalyst such as nickel, copper, platinum, palladium or molybdenum sulfide.
5. Sulfide: This is used mainly for the partial reduction of polynitro aromatic compounds to nitro-amines and for reduction of nitro-anthraquinones to amino- anthraquinones.
6. Sulfite (Piria method): The reaction of sodium sulfite and bisulfite on an aromatic nitro compound leads to a mixture of amine and aminoaryl sulfonic acid.
7. Sodium hydrosulfite (hyposulfite).
8. Electrolytic
9. Sodium and sodium alcoholate.
10. Strong caustic oxidation-reduction.
11. Hydrogenated quinolines and naphthalene.

During the general course of reduction various intermediate reduction products are obtained. By a proper selection of reducing agent and careful regulation of the process, reduction may often be stopped at intermediates states and valuable products other than amines obtained. Metal and acid reduction is most vigorous and usually yields amines as end products. When nitrobenzene is treated with zinc and a mineral acid, the resultant product is aniline. When an alkaline solution is employed, hydrazobenzene is generally obtained, but very vigorous conditions sometimes result in the formation of aniline. When zinc dust and water are used, reaction product is phenyl hydroxylamine as shown below.

Selective reduction is normally carried out in presence of alkaline sulphide. When the compound to be treated contains more than one nitro group, the product of reduction depends upon the agents used. Thus, m-phenylenediamine is obtained by the iron and acid reduction of m-dinitrobenzene, while the alkaline sulfide reduction yields m-nitro aniline.

In case of nitro compounds consisting acid or alkali sensitive groups (e.g. ester or amides), it is necessary to adjust the pH carefully to avoid decomposition and side reactions. In such reductions on the acid side, it is generally advisable to replace the normally used mineral acids with organic acid like acetic acid. An example of the latter is the reduction of nitroanilides, for here the presence of mineral acids would tend to hydrolyze the anilide to an amine.

The method to be used will therefore, depend upon the degree of reduction desired, the sensitivity to the process of both starting material and final product, the need for avoiding contaminants and the overall economics.

The reduction of nitro compounds involves the progressive removal and replacement by hydrogen of the oxygen in the -NO2 group. The intermediate products in the reduction of nitrobenzene, whose interrelationships are shown in the following figure are obtained by control of the reduction potential of the system.

Sulphide reduction: though this is an expensive method of reduction but find uses in case of partial and selective reduction. Like reduction of dinitro compounds to natramines, reduction of nitrophenols,  reduction of nitroantraquinons.

Since alkali sulphide can easily take up oxygen, it causes reduction of nitro compound. The reaction may involve following steps:

4 RNO2 +6 Na2S + H2O → 4 RNH2  + 3 Na2S2O3 + 6 NaOH

RNO2 + Na2S2 + H2O →  RNH2  + Na2S2O3 

4 RNO2 +6 NaHS + H2O → 4 RNH2  + 3 Na2S2O3 

In dye intermediate industry, ammonium sulphide is used extensively to reduce dinitrobenzene derivative to nitroamines.

Catalytic Hydrogenation: This is the process of reducing nitro compounds into amines by hydrogen gas in presence of any catalyst. Catalytic reduction can be carried out in different conditions by using different catalysts, promoter, poison, temperature, pressure solvent etc. A slight modification in any of these, have a marked influence on the course of reduction and rate of hydrogenation.

The general course of reduction follows following order:

Nitrobenzene → nitrosobenzene → hydrazobenzene → aniline

Generally, used in preparation of aniline and xylidenes.

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