02-07-2010, 11:16 PM
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INTRODUCTION TO AMMONIA
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Ammonia is one of the most important basic chemical of the world, ranking with materials such as sulfuric acid and sodium carbonate. In the fertilizer field, anhydrous ammonia itself has become the major supplier of the fertilizer nitrogen in India and it is also important inter mediate in production of more complex chemicals.
The major use of ammonia, both directly and indirectly, is as an intermediate in the fertilizer area. There are many other uses, although relatively minor, in both organic and inorganic chemical production for e.g. manufacturing of explosives and acrylonitrile. The main function of ammonia, both as an end product and as an intermediate, is to supply nitrogen in a relative form. Ammonia is unique in that, unlike the other basic chemicals, the main constituents, nitrogen is readily available without need for transport and in unlimited quantity.
Unfortunately, element nitrogen is a very uncreative and inert material, of little use unless converted to a chemically reactive form. To accomplish such conversion, it has been necessary to adopt extremes of temperature and pressure that are not required for the other basic materials. Conversion of atmospheric nitrogen to usable form is often referred to as Nitrogen Fixation. That is converting to solid or liquid form i.e. reactive enough to be useful. Some plants have roots nodules in which fixation of nitrogen by microbiological means takes place during the growing process. Unfortunately, only a few plant types can do this more over, the process is so slow that it is not adequate in modern farming practice. The production of ammonia has been found to be more economical than either fixation by plants or production other nitrogen compounds.
Since nitrogen is so readily available, the main problem is getting hydrogen that will react with it to form ammonia, water, also readily available is an obvious raw material for making hydrogen. Therefore, the starting material of main importance is the hydrocarbon or coal use to tie up oxygen in the water molecule, thereby reading hydrogen. The major materials used, the order of importance are natural, liquid hydrocarbon. All are widely available, but bringing them to surface, purifying and transporting them involve costs. Each differs in delivered cost and in processing difficulty in hydrogen production.
A direct application of ammonia as a fertilizer involved injection of ammonia under the surface of the soil where high pressure ammonia after release of pressure is held by adsorption on soil practical until converted by soil or plant mechanisms to other forms. This unusual application procedure of ammonia (82% nitrogen) has associated with some hazards because the liquid is handled under pressure and it is toxic chemical. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers. It is either directly or indirectly building block for synthesis of many pharmaceuticals. In spite of its wide usage, ammonia is both caustic as well as hazardous.
Ammonia as used commercially is often called anhydrous ammonia which signifies the absence of water in the material. Its heat of vaporization is sufficiently high so that it can be readily handled in ordinary beakers in a fume hood.
Names of Ammonia
Sr. No. Types of Name Name
1. IUPAC Name Azane
2. Other Name Ammonia
Hydrogen nitrite
Spirit of Hartshorn
Nitro-sil
Vaporole
¢ Production Capacities:
In 2007, ammonia production all over the world was estimated at 150.7 million tones.
Industries Producing Ammonia in India
Sr. No. Industry Name Capacity (MTPD)
1. GSFC (Baroda) 1350
2. Nagarjuna Fertilizers (Kakinada) 900
3. Tata Chemicals Ltd. 1350
4. IFFCO (Kalol) 1160
5. GNFC (Bharuch) 1350
6. Shriram Fertilizers (Kota) 600
7. KRIBHCO (Hazira) 2600
8. National Fertilizers (Panipat) 900
Ammonia Production in the World
Sr. No. Country/ Continent Capacity (MMTPA)
1. North America 20.7
2. United states of America 18
3. Western Europe 17.3
4. Asia 40.6
5. Africa 3.5
2. HISTORY
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The history of ammonia production can not be separated very well from the larger subject of nitrogen supply to the fertilizer and chemical industries there was no synthetic ammonium, dependence was placed unnatural and waste product of various kinds. The principle nitrogen source for agriculture before 1800 were byproduct organic materials of various types, including animal manures, seeds meals, fish crap, leather scrap, and various water from meat animal slaughtering. The supply of fertilizer was very mall.
Nitrogen for the limited chemical production before 1800 came from natural salts as saltpeter (potassium nitrite) found in limited quantities in the industrial country of that day, salts such as ammonium chloride, recovered from animal sources, were also used in some areas.
In 1809, a very large deposit of sodium nitrite was discovered in a desert area in Chile. Mining scone began and by 1850 the material was supplying a major portion of the world nitrogen supply. Chilean nitrite supply about 70% of the world fertilizer from nitrogen from 1850-1900 and about 50% up to 1914. The remoteness plus other factors caused a rapid decline in usage of sodium nitrite in beginning of 20th century. One adverse factor was the growing product of byproduct ammonium sulfate and aqua ammonia by coke industry. Since these materials were byproduct, they were sold at whatever price required moving them, and soon becoming major competitors to sodium nitrite.
Synthetic ammonia entered in the competition when the first crude plants were built in the 1890-1925 period. The demand grow so fast there after no byproduct source could keep up, therefore ammonia increased rapidly in importance until by 1970 it supplied over 90% of the worldâ„¢s nitrogen.
THE HABER ERA:
At the beginning of the 20th century, ammonia production technology was at some what of a luring point various methods were known for synthesis of ammonia, but all were too cumber some and expensive to have very bright future. The need was finding process for combination of nitrogen and hydrogen directly without use of sol c or liquid compounds are intermediates.
In 1902, first Haber and his so workers concentrated in fixation of nitrogen both as oxides ammonia. Even at 1000°c only about 0.01% of gas mixture was converted to ammonia. Higher temperatures and use of a catalyst did not produce much important.
In 1901, Le Chatelier in France had synthesized ammonia by using elevated pressure as well as high temperature. In develop that both were right, Haberâ„¢s yield were as high as the reported and Nernst theory was correct except that a fundamental valve involves in applying % theory was wrong. In 1909, Haber reported a concentration of 6.5% ammonia with an osmium catalyst. This was the turning point in the development of practical process. All though the equipment was cumbersome and the catalyst was expensive, yields had been obtain were high enough to make a commercial process possible.
Only a fraction of the H2-N2 mixture could be converted during one pass through the reactor even under extreme operating condition. Haber developed the concept of a recirculation process to solve this problem. At high pressure involve a good parts of ammonia should be condensed cooling the gas leaving the converter, the unconverted gas mixture was then recycled to the converter. Thos was enough for Haber for claim of the patent.
3. PROPERTIES OF AMMONIA
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¦ Structural Properties:
Sr. No. Property Name Characteristic
1. Molecular shape Trigonal pyramid
2. Dipole moment 1.42 D
¦ Physical Properties:
Sr. No. Property Characteristic
1. Molecular Formula NH3
2. Molecular weight 17.036 g/ mol
3. Specific gravity
Gas
Liquid
0.587
0.682
4. Density 0.86 kg/ m3 (at 1.013 bar & at boiling point)
5. Boiling point -33.3°c
6. Freezing point -77.7°c
7. Nature Alkaline
8. Color Colorless
9. Odor Strong pungent odor
10. Solubility Highly soluble in cold water (89.9 gm/ 100 ml at 0°c)
11. Auto ignition temperature 651°c
12. Basicity (pKb) 4.75 (reaction with water)
13. Critical temperature 133°c
14. Critical pressure 5 kg/ cm2
15. Viscosity of saturated liquid 0.25 cp at -15°c
16. Thermal conductivity of saturated liquid 0.502 W/ m °K at 0°c
¦ Chemical Properties:
The aqueous solution of ammonia acts a weak base.
¢ In oxygen atmosphere, it burns rapidly.
¢ At red hot temperature or by passing an electric spark, it decomposes.
2NH3(g) + electric spark N2(g) + 3H2(g)
¢ When a mixture of ammonia and air is passed over Pt gauze at about 800°c Nitric Oxide is formed.
¢ Ammonia acts as Lewis Base because its Nitrogen atom contains a non-bonding electron pair.
¢ It gives colored complex ions with transition metal ions forming a coordinate bond.
¢ It forms salts with acid.
NH3 + HCl NH4Cl
¦ Thermodynamic Properties:
Sr. No. Property Characteristic
1. Standard heat of vaporization 327.619 Kcal/ Kg
2. Specific heat at constant pressure 523.446 Kcal/ Kg
3. Specific heat at constant temperature 471.390 Kcal/ Kg
¦ Hazardous Properties:
¢ When ammonia is stored container NH3 exerts a vapor, which increases rapidly with rising temperature.
¢ NH3 forms explosive mixture with air and oxygen.
¢ Moist NH3 will react rapidly with Cu, Zn and Silver.
¢ The use of Hg in contact with NH3 should be avoided since under certain conditions explosive chemicals may be produced.
¢ Liquid ammonia will cause caustic burn if it contacts the body.
¢ Ammonia is an irritant gas and affects the mucous membrane and eyes.
¦ Physiological Action and Toxicity:
¢ Ammonia is not a poisonous gas but has a very irritating action on the mucous membranes of the eyes, lungs on any skin surface where moisture is present.
¢ Concentration over 300 ppm may cause sudden death by spasm and inflammation of the larynx.
¢ As liquid ammonia vaporizes rapidly and has a great affinity for water, it can severe cold burns on contact with the skin and can cause blindness on contact with the eyes.
Physiological Action and Toxicity of Ammonia
Physiological response Concentration in air (ppm)
Least detectable odour 5
T.L.V. 25
Max. allowable concentration for 3 hrs (S.T.L.V.) 50
Max. allowable concentration to cause irritation to eyes and possible permanent injury 700
Max. concentration to cause coughing 1700
Possible concentration for short exposure 3000
¦ Application of Ammonia
¢ Mainly used as fertilizer and refrigerant.
¢ As a detergent for removing strains up bleaching and ammonia printing and for extracting plant colors and alkaloids.
¢ Manufacturing of nitric acid.
¢ In rubber vulcanization, water treatment, nitrating of steel oil refining, extracting certain metals from ores, solvents and reactions medium in organic synthesis, yeast nutrient sulphate pulp process and as explosive.
¢ A chemical used in water treatment plant for regeneration of weak base anion resins IRA-93.
¢ Used in boiler drums n both offside in ammonia plant to maintain pH.
¢ About 90% of ammonia is used as fertilizers in India. Types of nitrogen fertilizers, produce from ammonia in India are as follows:
Ammonium sulphate
Calcium ammonium nitrite
Nitro phosphate
Urea
Ammonium nitrite
Ammonium phosphate
Di Ammonium phosphate (DAP)
4. METHODS FOR AMMONIUM PRODUCTION
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Before the start of World War 1, most ammonia was obtained by the dry distillation of nitrogenous vegetable and animal products; by the reduction of nitrous acid and nitrites with hydrogen; and also by the decomposition of the ammonium salts by alkaline hydroxides or by quicklime, the salt most generally used by the chloride.
Most process improvement have been made through better catalyst systems, improved reformer and converter designs, and integration of energy needs and heat recovery. New ammonia synthesis catalysts include a ruthenium based system which has an activity 20 times higher than the traditional iron based catalyst. There is a trend towards larger single train plants has developed a flow sheet for a plant that produces over 1m tone/ year.
(1) Haberâ„¢s Process
N2(g) + 3H2(g) 2NH3(g), H = -92KJ/ mol
The purified hydrogen-nitrogen mixture is compressed to 150-350 bar, mixed with recycle feed and fed in to a tubular or multi-bed reactor. The reaction takes place at 450-600°c over a catalyst. The ammonia is condensed out by refrigeration and un-reacted gases are compressed and recycled.
(2) Cyanamide Process
Ammonia is also prepared synthetically by cyanamide process: nitrogen gas combines with calcium carbide (CaC2) at high temperatures to form calcium cyanamide (CaCN2) and carbon; the calcium cyanamide reacts with steam to form calcium carbonate (CaCO3) and ammonia. For use in laboratory, ammonia is prepared by heating an ammonium salt with a strong base. It can also be prepared by reacting a metal nitride with water.
(3) Electrolysis Process
Purified water is used as feed stock in electrolysis process. Potassium hydroxide is added to increase conductivity, but it does not participate in the reaction. Chemical components of water, i.e. hydrogen and oxygen, are obtaining pure state through electrolysis.
H2O H2 + ½ O2
H2 thus obtained is mixed with required N2 from the A.S.U. to get synthesis mixture.
Electrolysis process is very energy intensive process. Typical power consumption is 4.3 KWH/ m3 of hydrogen, which corresponds to about 8600 KWH/ MT of ammonia. Additional energy is required for the air separation plant to produce nitrogen. Energy is also require for compression of H2 and N2 and recirculation of loop gases. The total energy requirement is 10200 KWH/ MT of ammonia. The plants based on this process are located where low cost hydroelectricity is available.
Byproducts of electrolysis process are O2 and heavy water, that have use in other industries.
(4) Partial Oxidation of Hydrocarbons
In this process hydrocarbons (usually naphtha or heavy oil) and O2 are burnt with flame in the top of reactor. Steam is added to the reactor mainly to moderate the temperature. In the flame, a part of hydrocarbon is burnt completely, the feedstock is cracked to shorter chain hydrocarbons and cracked products are reformed. The normal flame temperature is 1300-1500°c, which is high enough to give residual methane content below 0.3% at the pressure of 30 Kg/ cm2.
The exit gas from reactor passes through waste heat boiler, which is designed to avoid clogging by C formed in burner. The C is carried on with gas through a trap & scrubber and may be separated as pellets. The amount of C produced is 1-3 wt% of H2.
Final purification of synthesis gas is done through cryogenic scrubbing, where carbon oxides are removed and methane and argon contents are reduced to low level. So that little or no purging is required. Partial oxidation of hydrocarbon is a simpler and more rugged process than steam reforming. The advantages of the process are:
¢ Prior sulfur removal is not required.
¢ No sensitive catalyst is involved.
¢ All the reforming can be accomplished in one step.
¢ Flexible to any feedstock; almost any liquid or gaseous hydrocarbon can be used from NG to heavy residential fuel oil and the shift from one to another can be made quickly.
¢ Lower total hydrocarbon for process and fuel.
¢ Purer synthesis because of the cold wash purification.
The drawbacks that increase the process cost are:
¢ Oxygen or oxygen enriched gas is required unless there is a nitrogen removal step.
¢ The product gas has much higher CO:H2 ratio than that obtained by steam reforming.
¢ A cold plant is required for air separation, purification or both.
(5) Adiabatic Pre-Reforming
This method in case of vaporized naphtha feed is done to decompose the higher hydrocarbon in to lower once, like CH4 and the other components like H2, CO & CO2.
The vaporized naphtha is mixed with steam and preheated to about 490c. The gas is passed through the pre-reformer containing Ni catalyst. The typical composition of the pre-reforming catalyst is Ni-25%, Al2O3-11%, MgO-balance.
In the pre-reformer the endothermic reforming reactions are followed by the exothermic methanation and the shift reaction. The over all process is normally exothermic.
The gas from adiabatic pre-reformer is sent to primary & secondary reformer for further reforming. The adiabatic pre-reformer reduces the head loss on the primary reformer, thus the life of reformer tubes longer. Pre-reformer also acts as an efficient sulfur guard for the primary reformer catalyst.
(6) Coal Gasification Process
It can be classified according to method of gasification.
(a) Fixed Bed coal gasification process (Lurgi Process)
Lump coal (5-30 mm) is charged at top of gasifier and it descends counter currently to gas stream. As it descends, it is first dried and preheated, then carbonized and finally gasified by the O2 and steam entering at bottom. The coal ash is discharged from the bottom through a gate or as a slag. Because the counter current method result in good heat exchange and very high thermal efficiency, this method requires less heat and hence less O2 than other methods.
This method requires least O2 with respect to the other gasification processes. The requirement of O2 is one half as much as entrained coal gasification. Also less purity O2 of 90% can be used. The gas leaves the top at 450°C and is cooled and washed to remove tar, liquid hydrocarbons, dust etc. the washed gas contains CO, H2, CO2, CH4 and other hydrocarbons. It is treated by a series of steps including steam reforming, co shift conversion, CO2 and H2S removal, liquid N2 wash, steam reforming of CH4 that is separated by N2 wash, N2 addition and compression to produced ammonia synthesis gas.
(b) Fluidized bed coal gasification process (Winkler Process)
In fluidized bed gasification 15 mm size lignite coal is introduced into fluidized bed through feed screws near the bottom. Steam and oxygen are injected near the bottom of the fluidized bed. The fluidized bed is essentially isothermal, consequently there is no tar and gas contains mainly H2 and CO with less than 1% CH4. The hot gas is cooled by waste heat boilers and scrubbed to remove ash and then purified. Disadvantage of the process is the high compression cost.
© Entrained bed coal gasification process (Koppers-Totsek (KT))
In this process coal is dried and finely ground to about 75% through 200 mesh. The powdered coal is picked up by stream of O2 and blown into gasification chamber through two burners facing each other. Steam enters through annular openings around the burners. The gasification is complete is one tenth of second in the temperature range of 1000-1200°C. Part of ash is fused and removed from the bottom of gasifier. The exit gas typically contains 56%CO, 31%H2, 11%CO2 and less than 0.1%CH4. After cooling in waste heat boilers, the ash is removed by wet scrubbing and electrostatic precipitation. The rest of ammonia synthesis gas preparation is similar to partial oxidation.
Disadvantages of the process are the need for fine grinding of coal, operation at low pressure and higher O2 consumption.
(7) Steam Reforming
It is usually carried out in two stages using primary and secondary reformers. Desulfurized naphtha or NG is subjected to steam reforming at about 28-30 kg/cm2 pressure and around 800°C temperature in the primary reformer consisting of a large number of centrifugally cast, high temperature, vertical alloy steel tubes packed with nickel catalyst. The overall reaction is endothermic and requires large amount of heat. The gas leaving the primary reformer containing 5-15% CH4 is sent to an auto thermal secondary reformer. The required amount of N2 is fed to secondary reformer through addition of air to give desired 3:1 H2 to N2 ratio in the synthesis gas. Here CH4 is converted to H2, CO, CO2 over a single bed of catalyst.
The CO content of the gas is converted to CO2 & H2 by passing over catalyst in the presence of steam. Thus generating H2 by water gas shift reaction and is carried out in two stages. The first stage high temperature (HT) shift conversion, is carried over iron-Cr catalyst at 350-430°c, while the second stage, low temperature (LT) shift conversion, is carried over Cu based catalyst at 200-280°c. In the HT shift conversion, level of CO is reduced from 12% to around 3% and in the LT shift conversion, the CO level is reduced to around 0.2%.
The process gas after shift conversion, containing over 18% CO2 and less than 1% CO undergoes further purification in CO2 removal section.
The purified synthesis gas mixture from containing N2 & H2 in the mole ratio of 1:3 is reacted at elevated temperature of the order of 450-500°c and 150-250 kg/cm2 pressure over an activated iron catalyst promoted with potassium and alumina. The gas cooled first by heat exchanger and finally by refrigeration to condense ammonia as liquid. Conversion of synthesis gas to NH3 is about 20-30% per pass. The gas remaining after ammonia condensation is recycled to the converter. The inert gases built up in the synthesis gas are purged.
LINDE AMMONIA CONCEPT (LAC)
(A) INTRODUCTION
The LAC is a breakthrough in the design philosophy of ammonia plants. It is a novel concept of proven process steps. GSFC ammonia-IV plant is first plant in the world based on LAC. The LAC process compared with the conventional process route has the following important features:
Elimination of three catalytic process steps (i.e. secondary reforming, HT shift conversion and methanation) thus reducing g the total catalyst volume to approx 50% of that in a conventional plant.
¢ The quantity of CO2 can be adjusted according to the demand.
¢ Provision of reformed gas purification system by pressure swing adsorption which has a proven and unmatched reliability.
¢ The generation of inert free synthesis gas, resulting in significant savings in synthesis loop and eliminating a purge gas purification step.
¢ Pure hydrogen and pure nitrogen are directly available from process steams. Other potential by products such oxygen, organ, carbon-dioxide, carbon monoxide and methanol can be easily integrated.
¢ The simplified flow-sheet also result in a reduced start up time and important saving in feed stock consumption.
¢ An overall simplication of the classical procreate, resulting in savings in investment costs, construction time, and site area as well as catalyst replacements costs.
(B) THE CONCEPT
The Linde Ammonia concept (LAC) consists essentially of a modern hydrogen plant, a standard nitrogen unit and a high efficiency ammonia synthesis loop. The basic gas generation unit is a modern hydrogen plant with proven and outstanding reliability. The success of this hydrogen process can be judged by the face that today practically no new hydrogen plant is built with the conventions route including HT and LT shift CO2 wash and methanation. The modern hydrogen process using pressure swing adsorption has made this classical concept absolute. The pure hydrogen from the PSA unit is mixed with pure nitrogen from a standard air separation unit (ASU) to give inert free ammonia synthesis gas is fed to a high efficiency ammonia synthesis gas. The loop based on the ammonia CASALE axial radial flow converter. The comparison of the process steps in the LAC and in the conventional process shows clearly the reduction in the process steps achieved by the LAC. The number of temperature change and the temperature levels are lower than in the conventional process. Furthermore, the flow in the conventional plant through the second reformer and all the downstream steps is considerable higher than in the LAC plant due to the aid of process air to the secondary reformer.
© SPECIAL FEATURES
There section of the LAC are exclusive Linde developed technology and are introduced to fertilizer industry through LAC. These are isothermal shift reactor, Linde PSA unit, Linde air separation unit Ammonia CASALE converter represents an equally important contribution to this advance technology.
¢ The Linde Isothermal Shift Reactor (ISR)
The Linde isothermal shift reactor (ISR) is used for the co-shift reaction and allows conversion to below 0.7% CO (dry basis) in a single step. Thus the conventional series of HT and LT shift reactors with heat recovery between the beds is placed by a single reactor with built in steam generation.
Furthermore, the process condensate is far less contaminated than in the conventional process with secondary reformer and HT shift a small amount of methanol is generated in the ISR.
The silent features of ISR are as follows:
¢ The isothermal mode of operation produces the least possible stress on the catalyst and increases its lifetime.
¢ The operation temperature of the reactor is regulated by simple control of the steam pressure.
¢ Under all operating conditions the water cycle ensures absolute temperature stabilization.
¢ A start up heater catalyst reduction without danger of overheating.
¢ Simple and rapid catalyst reduction without danger of overheating.
¢ A start up heater is not necessary the catalyst bed is brought to temperature be steam injection to the water circuit.
¢ The temperature profile in the reactor corresponds to kinetic ideal conditions and gives optimum catalyst performance.
¢ The reactor concept enables the largest possible amount of catalyst to be accommodated per unit reactor volume.
¢ Linde Pressure Swing Adsorption (PSA)
Linde Pressure Swing Adsorption (PSA) technology is widely used for hydrogen purification duties capacities of about 112000 NM3/hr. ultra pure hydrogen products suitable for 1350 MTPD ammonia plant requires a 12 catalytic bed system.
The extremely high reliability of the PSA system results from the use of high quality equipment and the completely automatic switching and monitoring of the unit performance by a Programmable Logistic Controller (PLC).
If there is a disturbance occurs in any of the operating sequences, the faulty item is identified and taken off-line, without interrupting the supply of product.
A 12 bed system can switch automatically to operation on a lower number of beds with only a slight reduction in efficiency. The proven automatic control system given 100% availability in Linde PSA unit.
Furthermore, the PSA unit is able to produce pure hydrogen directly from reformed gas so that disturbances in the front end steps would not interrupt production. This is in sharp contrast to the conventional planner where failure of the CO shift or CO2 removal cause a high temperature trip, on the methanator and compute plant shutdown. Equally, the start up is much faster, since pure hydrogen is produced as soon as reformed gas is available.
¢ Linde Air Separation Unit (ASU)
Linde Air Separation Unit (ASU) is a state of the art low temperature separation process such as is used world wide for the production of the industrial gases. Linde are the inverter of the air separation process and they have good expertise in this field.
¢ Ammonia CASALE Axial “ Radial flow converter
Ammonia CASALE Axial “ Radial flow converter is a development of the classical redial flow type converter.
In the ammonia CASALE converters, as opposed to the pure radial flow converter, the catalyst bed has no top cover and some gas enters axially. The amount of gas flowing axially is controlled by proper design of the perforation pattern and is such that the seal catalyst in the radial flow part of the bed.
The CASALE design therefore obtains the max. Performance from the catalyst volume charged to the converter.
In addition to the process design advantages of the CASALE converter, the mechanical design features give the following advantages for operation and maintenance compared with competitive designs:
¢ All proven catalyst can be used.
¢ High thermodynamic efficiency due to
- The presence of three beds.
- Optional gas distribution in catalyst beds.
- Min. utilization of the catalyst volume
¢ Maximum utilization of converter vessel volume.
¢ Use of rod baffle internal heat exchangers to improve heat transfer and eliminate vibration problems.
(D) VALUABLE BY PRODUCTS:
Valuable by products can be produced directly from the process streams in the LAC plant or by simple integration of the necessary facilities.
High purity hydrogen and nitrogen are produced within the LAC process and flows from both these streams can be mode available for other consumers, if required.
The potentials for production of oxygen, argon and rare gases from the Air Separation unit can be easily integrated at the design stage.
The installation of the cold box for CO product or of a methanol synthesis and distillation unit can be easily integrated with the LAC process.
(E) OPERATING ASPECTS:
The LAC is a much more direct route to ammonia than the conventional process. This results in rapid start up and important savings in feed stocks consumption per ton of product. Further, the annual production capacity is increased by rapid start up.
The advantages of LAC from the operators point of view arise form the reduction in the number of process steps and ability of the individual parts of the plant to operate independently.
FOR EXAMPLE:
¢ The nitrogen unit (ASU) can be operated to generate inert gas (and other products) for other consumers when the ammonia plant is shut-down.
¢ The isothermal shift reaction can be pre-heated by steam injection to the water circulating through the coils.
¢ The PSA unit can produce pure hydrogen directly from the reformed gas without the CO shift or the CO2 wash. The state up of the PSA unit is automatically performed by the PLC.
The start up of the ammonia synthesis loop can therefore proceed about 2hours after feed is introduced to the reformer.
5. LAC PROCESS DESCRIPTION
________________________________________
¢ PLANT CAPACITY
Liquid NH3 1350 MTPD NH3: 99.5% Wt. Min.
H2O: 0.5% Wt. Max.
Oil: 20 ppm Max.
Carbon Dioxide 1450 MTPD
(33500 Nm3/hr.) CO2: 99 Mol %
H2: 0.8 Mol %
H2O: Saturated
Argon 8880 Nm3/ day Argon: 99.99%
O2: 5 ppm Max
N2: 5 ppm Max.
¢ RAW MATERIALS
Natural Gas Methane 84 to 90%
Ethane 5 to 8%
CO2 1 to 5%
Propane 1 to 4%
i-Butane 0 to 0.2%
n-Butane 0 to 0.2%
i-Pentane 0 to 0.02%
n-Pentane 0 to 0.10%
Hydrogen Traces
Nitrogen 0.1 to 0.7%
Calorific Value Gross : 9800 to 11000 Kcal/NM3
Net : 8500 to 9000 Kcal/ NM3
Density 0.88 to 0.90 g/ml
¢ CATALYSTS
Sr. No. CATALYST USED FOR- CATALYST USED- TYPE AND SUPPLIER QUANTITY CHARGED (m3) CATAYST SIZE
(mm)
1. Reformer
Top Bed
Bottom Bed
Potash
Ni-Cr
Katalco 25-4Q
Katalco 57-4Q
Johnson Matthey
20.6
20.4
OD : 14
L : 7
Hole: 4
2. Shift Converter Copper ICI-83-5
Johnson Matthey 52 OD : 5.4
L : 3.0
3. PSA adsorbers Mol sieves LA-22
LAC-612
C-200-F 25.3
492
501.6 2-5
~ 3
1.6
4. Desulfuriser
Reactor R-0201 A
Reactor R-0201 B ZnO ICI 32-4(ZnO)
ICI UK
ICI 32-4(ZnO)
ICI UK 17
17 3.0-4.7
(granules)
3.0-4.7 (granules)
5. Ammonia Converter Fe-Based ICI 35-4
ICI UK 83.9 1.5-3.0
6. Mol Sieve Adsorbers Mol Sieves 13-X
APG/ Alluminium
Gel
M/s UOP 36.3
6.4
-
¢ UTILITIES
(1) De-mineralized water:
pH : 8-9
Silica : <0.02 ppm
Chlorine : <0.01 ppm
Conductivity : 10 M-mho/ cm max.
(2) Steam:
High pressure (HP) steam : 110 bar g
Medium pressure (MP) steam : 38 bar g
Low pressure (LP) steam : 4.5 bar g
(3) Air:
Dew point: <-80°c
(4) Electric Power:
11 KV - 3.3 KV - 440 V - 230 V
¢ PROCESS DESCRIPTION
The entire process is divided into mainly three sections:
1. HYDROGEN SECTION
(a) If Natural Gas is used:
¢ Sec.100 “ Steam reforming
¢ Sec.200 “ Desulfuizer/ ISR
¢ Sec.300 “ CO2 removal
¢ Sec.400 “ PSA
¢ Sec.800 “ HP steam drum
OR
(b) If Naphtha is used:
¢ Sec.900 “ Naphtha pretreatment
¢ Sec.1500 “ Instrument Air unit
¢ Sec.1600 “ Desulfurizer
¢ Sec.1800 “ Naphtha storage
2. NITROGEN SECTION
¢ Sec.700 - ASU
3. AMMONIA SYNTHESIS SECTION
¢ Sec.500 “ Ammonia synthesis converter
Sec.600 “ Refrigeration unit
Sec.1000 “ Future unit
Sec.1100 “ Cooling water towers
Sec.1200 “ Emergency power unit
Sec.1300 “ Flare
Sec.1400 “ Liquid effluent collection
Sec.2100 “ Fire fighting unit
2
(1) HYDROGEN SECTION
¢ STEAM REFORMER- SECTION 100
NG is received from battery limits. Its side stream is withdrawn to be used as fuel gas whereas main stream goes to a pressure reduction station where pressure is reduced to the feed pressure of 34 bar g.
This compressed gas is preheated to 385°c in e-0105 in reformer convection section & routed to desulfurizer from which sulfur free gas returns to reformer where steam is added to give a steam to carbon ratio of 3.
Further preheating is done in E-0101 by hot flue gas up to 550°c. Exit temperature is controlled via a feed trim cooler.
This preheated reaction mixture is distributed to the top of a Ni catalyst filled reformer tubes in the reformer furnace F-0101.
CH4 + H2O = CO + 3H2
CO + H2O = CO2 +H2 (Shift reaction)
These reactions being endothermic, heat for the reactions is supplied from 3 sources:
(i) By firing purge gas from PSA unit.
(ii) Small fresh gas stream from CO2 removal section & refrigeration section.
(iii) Flow of a controlled amount of NG.
Heat transfer in the furnace mainly takes place by radiation giving a reformer tube exit temperature 850°c & 8 mol% methane content on dry basis in process gas.
An induced draught fan is used to create vacuum in the furnace so that heat is maintained evenly. Also a number of coffin boxes are provided inside the furnace in order to remove the excess pressure provided by the fan which would otherwise result to crushing of the furnace vertically.
Hot process gas from all tubes is collected & routed to RGC E-0108 where it is rapidly cooled to 500°c by generating HP steam.
After that gas on entering E-0201, cools to 258°c by generating MP steam before entering ISR.
The RGC E-0108 & HP steam generators E-0104 & E-0501 are connected to the HP steam drum D-0101.
Flue gas leaving the furnace passes to the convection section where remaining heat is transferred in:
¦ E-0101 “ Feed Super-heater
¦ E-0102 “ HP steam Super-heater
¦ E-0104 “ HP steam Generator
¦ E-0105 “ Feed Pre-heater
¦ E-0106 “ HP BFW Pre-heater
¦ E-0107 “ Air Pre-heater
After pre-heating the combustion air, the flue gas leaves this section at 120°c via the flue gas blower C-0103 & stack K-0101 to the atmosphere.
Combustion air to the reformer burners is supplied by blower C-0102 & preheated to approximately 210°c in air pre-heater E-0107.
Due to excess steam, basic products are modified to produce CO2 and CH4.
Effect of Variables:
1. Pressure:
High pressure reduces the equilibrium conversion but nevertheless has been adopted due to several process advantages.
a) Since overall reforming reaction increases the gas volume by about 100%. It is more economical to compress feed gases before reforming than after.
b) Any initial pressure of Ng is conserved.
c) The pressure is high enough for Co2 removal without an intermediate compression step.
d) The higher pressure is favorable to the Co shift kinetics and catalyst activity although the reaction itself is essentially independent of pressure.
e) Equipment can be smaller.
f) Steam condensed after the Co converter becomes valuable because of higher pressure.
g) The usual range in pressure is 25 “ 35 bar.
2. Temperature:
Reaction is favored by high temperature as it is highly endothermic. The main drawback of high pressure is the adverse effect on reforming equilibrium which must be offset by higher temperature.
The temperature in reforming tubes usually is in the range of 780 to 8500c (here “ 8300c).
3. Steam/ Carbon Ratio:
Since increase in temperature makes it difficult for the reformer tube to resist the pressure the steam / carbon ratio can be increased to help offset the effect of high pressure.
The usual range is 3.5 to 4 miles of steam per miles of carbon.
A lower ratio could be used but the higher one not only improves conservation but also helps supply the steam need in the CO conversion step. Moreover, excess steam in the reformer is a good insurance against C formation on the catalyst.
4. Carbon formation:
Under normal condition there should be sufficient C formation to interfere with plant operation complete loss of reaction steam, however, may result in massive deposition of carbon and a rapid increase in pressure drop complete catalyst replacement with then be required operation with a small steam deficiency can result in slow carbon deposition from a number of carbon forming reactions.
CH4 C + 2H2
2CO C + CO2
CO + H2 C + H2O
CO2 + 2H2 C + 2H2O
At temperature between 540 to 6500c all the four C forming reactions are capable of producing carbon then only a few seconds in this condition are sufficient to plug tube and break up the catalyst.
Formation of C can be prevented by keeping minimum steam / carbon Thermal decomposition of higher hydrocarbons lead to form sooty carbon which can be removed with steam.
5. Use of Potash Catalyst:
Acidity in the support facilitates cracking and polymerization of hydrocarbon through which C is formed.
An alkali component is introduced in the catalyst which not only neutralizes acidity in catalyst support but also accelerate C removing reaction.
C + H2O CO2 + H2
The most effective alkali is found to be K2O (Potash) which is effective by being mobile on catalyst surface. The most of complex reactions associated with higher hydrocarbons are completed t\in the top half of the catalyst bed with CH4 reforming taking place in the lower port of the tubes, so lower half part can be filled with un-alkalized catalyst.
¢ DESULPHURIZATION/ CO SHIFT CONVERSION - SECTION 200:
Preheated NG enters DS reactors R-0201A/B where H2S is chemically adsorbed in a bed of ZnO catalyst.
ZnO + H2S = ZnS + H2O (Exothermic)
The S-free gas is then mixed with steam generated from DM water plus process condensate E-0201 & R-0202 & is routed to reforming section.
CO reformed gas contains 14 mol% CO on dry basis. Yield of H2 is increased by reaction of CO with excess steam present in gas according to the CO shift reaction.
CO + H2O = CO2 +H2
Maximum conversion is obtained in LINDE ISR R-0202 by removing the reaction heat through coils in the catalyst bed, which keeps catalyst at 245°c throughout.
CO content of gas is reduced to 0.7% in one reactor. The heat of reaction is used to evaporate MP BFW originating from process condensate collected from the downstream gas cooling system plus DMW.
After de-gasification with LP steam in MP de-aerator D-0205, the BFW is treated with hydrazine and morpholine to remove last traces of O2 and to adjust pH respectively.
BFW is then pumped through pre-heater E-0202 and supplied to the common MP process steam drum D-0201 of the reactor R-0202 and the MP process steam generator E-0201.
The generated steam is mixed with the desulfurized feed gas from the reformer. The continuous boiler blow-down is provided to maintain quality in boiler system.
The blow-down water is flashed into D-0204 where the vapor phase is separated and sent to the de-aerator D-0205 while the blow-down water is sent to blow-down drum D-0807.
The gas leaving the ISR is cooled in a series of heat exchangers where energy is recovered. Initially, MP BFW is preheated in E-0202.
The gas then provides heat in the MDEA re-boiler E-0206 followed by condensate pre-heater E-0204 and DMW heater E-0207 before being routed to the first condensate separator D-0202.
Further cooling of gas before supply to MDEA wash unit is carried out in air cooler E-0208. The condensate is then separated in D-0203.
Process condensate from D-0203 is mixed with DMW and preheated in E-0204 before being fed to de-aerator D-0205.
ZnO bed gives outlet sulfur up to <0.1 ppm. Bed is spent when slip increases to 0.2 ppm. It cannot be regenerated and has to be replaced.
Effect of Variables:
1. Temperature:
Efficiency of absorption falls rapidly as temperature is reduced for efficient utilization of the bed needs to be kept > 3500C Bed can be operated up to 4500C without any negative effects on catalyst. However above 4500C, life of catalyst is reduced.
2. Pressure:
Pressure has small effect on adsorption. Adsorption increases with increase in pressure.
3. Space Velocity:
Space Velocity influences the degree of adsorption because it governs the contact time. For a given fixed bed, if space velocity increase the adsorption decreases.
¢ CO2 REMOVAL SECTION “ SECTION 300:
The removal of CO2 by an active agent Methyl Di Ethanol Amine (MDEA) washing is carried out in order to produce a byproduct stream of 31,500 Nm3/Hr, CO2 of 99.0 mol% purity (dry basis).
The separation of CO2 from the process gas is carried out by absorption. This scrubbing process is a BASF, Germany development that has been successfully employed for more than a decade.
The MDEA concentration is about 37 wt%. Additionally, the scrubbing solution also contains 3 wt% piperazine to improve CO2 mass transfer. The reactions are:
R3N + H2O + CO2 = R3NH + HCO3 (Exothermic and reversible)
R2NH + CO2 = R2NH2 + R2N “COO
The scrubbing solution has a low vapor pressure and shows good thermal and chemical stability. Equipment to separate non-regenerated decomposition product from the solution is therefore not necessary. Also MDEA is biologically decomposable and non-corrosive.
¦ Scrubbing:
The CO2 removal from process gas takes place in the CO2 absorber T-0301 at a pressure of 25 bar and an inlet temperature of 70°c. The CO2 content in the feed gas is reduced to less than 0.3 mol% in the purified gas in the two consecutive scrubbing stages (bulk and fine scrubbing sections).
In the lower part of the absorber (bulk scrubber), bulk CO2 of the process gas is removed by semi-lean MDEA solvent supplied from LP flash column T-0303 by pumps P-0301 A/B/R (2 running + 1 standby).
The remaining CO2 is removed in the upper section (fine scrubber) by using lean solvent supplied from MDEA stripper T-0304 and being cooled in the stripper exchanger E-0304 A/B/C and the lean solution cooler E-0302.
The solution feed rates are adjusted by flow control. The contact surface for mass transfer between gas and scrubbing solution is improved by metal packing rings. Additionally, the top of the column is equipped with trays which are fed with MP BFW to re-wash traces of solvent in the process gas.
The two scrubbing stages have been designed in such a way that the CO2 loading of the solution from the fine scrubbing section in T-0301 has roughly the same value as the semi-lean solution fed to the bulk scrubbing part in the column.
The absorption of CO2 causes the scrubbing solution temperature to rise to about 88°c and at this temperature; the rich solution leaves the bottom of the column.
¦ Flashing:
The solution leaving from the bottom of T-0301 is expanded to a pressure of 9 bar a in hydraulic turbine X-0301 and fed into the MP flash column T-0302.
Inerts and some CO2 are thereby flashed from the solution and the flashed gas is sent to reformer fuel gas system.
The flashed solution is further expanded to a pressure of approximately 1.7 bar a and sent into LP flash column T-0303 where it is stripped with water vapor contained in the CO2 overhead fraction entering from MDEA stripper T-0304.
The MP/LP flash drums are equipped with packing to improve mass transfer.
The CO2 released in T-0303; saturated with water vapor is cooled to 43°c in the CO2 cooler E-0303 by cooling water. The condensate is separated from the gas in the knockout drum D-0302 and supplied to the top of the LP flash column T-0303 via pump. Excess condensate is drained to the waste water. The CO2 product from D-0302 is sent to battery limit.
The LP flash column T-0303 acts as a buffer for both scrubber circulation systems and as hold-up volume for the two semi-lean solution pumps P-0301 A/B+R and P-0302+R.
¦ Stripping:
Part of the semi-lean solution is pumped to the top of MDEA stripper T-0304 by means of pump P-0302+R (1 running + 1 standby)
In the stripper exchanger E-0304 A/B/C the semi-lean solution is heated against hot lean solution and sent to the MDEA stripper T-0304, where it is stripped with steam at elevated temperatures.
Thus the CO2 is stripped off from the solution to a residual content of 2 Nm3 CO2 per m3 of solution.
The released CO2 leaves column T-0304 saturated with water vapor. Mass transfer in stripper is improved by packing.
The required stripping steam is generated by evaporation of water from the aqueous solution in the MDEA re-boiler E-0206 which is a kettle type re-boiler with once through circulation of the MDEA solution.
The necessary heat is mainly supplied from condensing LP steam supplies the additional stripping heat.
The lean solution is routed back to the fine scrubber section of the absorber T-0301 via the pump P-0304 + R. The final cooling is performed by cooling water in E-0302.
¦ MDEA Storage and Supply:
The fresh MDEA drums are charged in MDEA slop drum D-0303 and then the solution can be transferred to the LP flash column T-0303 via slop pump P-0306. If DMW addition is required, the same can be carried out similarly.
MDEA storage tank D-0304 will be used to store MDEA solution upon draining of the CO2 removal unit. To refill the unit, pump P-0305 is to be used.
¢ PRESSURE SWING ADSORBERS “ SECTION 400
Adsorption process is designed in such a way that a gas mixture is fed to adsorber at ambient temperature and increased pressure. At higher temperature, a smaller amount of impurities can be adsorbed on any adsorbent. The regeneration at high temperature level T2 reduces the loading L2 of adsorbent at adsorption pressure P1 and temperature T2 and reduced pressure P2.
PSA process works between two pressure levels: (1) Adsorption & (2) Regeneration.
In adsorption phase, adsorption of impurities is carried out at high pressure to increase the partial pressure and thus increase loading of impurities on adsorbent.
The raw feed gas flows through an adsorber in upward direction and the impurities are selectively adsorbed on the surface of the adsorbent in the order- water, hydrocarbons, CO and N2 from bottom to top. The purified product gas leaves the adsorber at the outlet and flows to the product line.
The adsorbing capacity of one adsorber being limited, after certain time the impurities break through at the outlet of the adsorber. First the impurities come in traces, but gradually their concentration increases. To avoid this breakthrough, the adsorption step must be interrupted to regenerate the loaded adsorber. For continuous product supply, at least two adsorbers are necessary. While the first one is regenerated, a second already regenerated one purifies the raw feed gas.
In the regeneration phase, the process is carried out at low pressure at approximately the same temperature. During this phase, the adsorbed impurities are desorbed. The residual impurities loading are reduced as much as possible in order to achieve high product purity and a high H2 recovery.
Regeneration phase consist of several steps;
¦ Expansion to adsorption pressure, where the adsorbed components start to desorb from the adsorbent.
¦ Purging supports this desorption process with some gas having very low partial pressure of impurities.
¦ Pressurization to adsorption pressure.
If more adsorbers are installed, the pressure can be equalized several times to different intermediate pressure levels and gas losses are further reduced.
¢ Temperature effects:
At higher temperature a smaller amount of impurities can be adsorbed on any adsorbent.
The regeneration at higher temperature level (T2) reduces the loading (L2) of the adsorbent at adsorption pressure (P1) and adsorption temperature (T2) and reduced pressure (P2). Here the operation capacity (C2) for adsorbing the gas corresponds to the difference (L2-L3) which is lower than at the temperature level (T1).
L1 T1
C1
L2
C2
L3
P2 P1
Figure: Adsorption Isotherms
ADSORPTION MATERIAL EFFECT OF GAS SEPARATION BASED ON
ACTIVATED CARBON
SILICA GEL
ACTIVATED ALUMINA (ALUMINIUM OXIDES) BINDING FORCE (INDICATED BY BOILING POINTS AND POLARITY)
ZEOLITES
ZEOLITIC MOLECULAR SIEVES DIAMETER OF MOLECULES, BINDING FORCES (ESP. POLARITY)
CARBONESEOUS MOLECULAR SIEVES DIFFERENT ADSORPTION VELOCITY
(2) NITROGEN SECTION
¢ AIR SEPARATION UNIT (ASU) - SECTION 700:
(a) Composition of Air:
The atmosphere is of compositions which apart form moisture “ fluctuate within narrow limits. The most important constituents of dry air are listed in the following tables :
Medium Chemical Symbol Volume % Weight %
Nitrogen N2 78.1 75.5
Oxygen O2 20.95 23.1
Argon Ar 0.93 1.29
Carbon Dioxide CO2 0.03 0.05
Rare Gases - 0.002 -
99.04% of the air consists of oxygen and nitrogen. The concentration of these gases is more or less the same all over the world. This also applies to argon which participates with 0.93% by volume where as the concentration of hydrogen, carbon dioxide and hydrocarbons vary within certain limits.
The water vapor content of air however differ considerably, this depends upon the temperature which is subject to the degree of saturation, meteorological and local condition which influences the relative humidity.
Water vapor and carbon dioxide possesses properties, which differ considerably from air. These components vaporize at 00c and “ 790c respectively at atmospheric pressure. If there are present, the fine tubes of the heat exchange and the perforated trays in the rectification columns would be blocked. These components must therefore be removed prior to separating the air (rectification).
Hazardous impurities in the air are hydrocarbons. Acetylene, especially, concentrates in the liquid air and liquid oxygen fractions in the A.S.U. (rectification) and could above certain limits cause explosions. Hydrocarbons must therefore be removed. The concentration of C2H2 should not exceed 0.1 ppm in the liquid oxygen.
Rare gases are chemically fairly non reactivity do not interfere with the air separation process because of their very low boiling points, these two gases will always remain in a gaseous state during air separation.
Helium gas pockets forming in the condensers and liquefiers would envelope their heating surfaces and interfere considerably with their functioning. For this reason, heat is continuously vented off from these points through a regulating valve.
The difference in the boiling points of the main components in the air makes it possible to separate air by distillation.
(b) Rectification:
This process of separation ensures a high degree of purity and simultaneously a good output.
Air rectification consists of an oxygen nitrogen exchange between liquid and gaseous phases, the liquid being posed in the counter current to the ascending gaseous O2 - N2 mixture so called rectification plates (e.g. perforated trays) are mostly used for this exchange.
When in equilibrium, the vapor immediately above the liquid mixture (liquid air) has a higher N2 content, due to nitrogenâ„¢s lower boiling points, than the liquid.
The vapor and the liquid fractions passing respectively through and ever the rectification trays attempt to remain in equilibrium by exchanging oxygen and nitrogen when in contact a process which at the same time involves heat exchangers (condensation of oxygen and evaporation of nitrogen). The ascending gaseous mixture will steadily become richer in nitrogen and the descending liquid I oxygen.
That a separation effect must take place when an ascending gaseous fraction is in contact with a descending liquid fraction of a binary mixture can be understood from the following:
Imagine that gaseous air is saturated with its liquid components (O2 and N2) rising in a column is completely liquefied at the upper end (e.g. by the condenser) and then flow back to the sieves trays without change of composition. According to the diagram, a small volume of vapor at the point A is at a higher temperature than the boiling liquid at B of the same composition. The subsequence equilibrium will result in a temperature T3 between T1 and T2 at that point C on the same vertical line. However, at this temperature T3 only a liquid E with a higher oxygen content than in B and a vapor D with a lower oxygen concentrates in the down flowing liquid and nitrogen in the ascending vapor. Finally the liquid reaches the condenser in the form of pure oxygen while a gas consisting mainly of nitrogen escapes at the top of the column.
Process description of ASU:
¦ Oxygen and nitrogen recovery:
The air separation plant is based on a low temperature process, using molecular sieve absorber for cleaning the air before entering the low temperature section.
The process air is going through a suction filter system and is compressed to 5.2 bar by means of the process air compressor which is an electric motor driver. Downstream of the last stage an after cooler is installed.
The further cooling of process air takes place in the process air cooler T 0701. The process air enters the cooler at the bottom part and passes through the packing to the top while it cools down to about +80c and is washed by tricking water. Using the harmful components such as SO2, SO3 and NH3 are removed to a large extent too.
For reaching the design temperature of the process air at the outlet of the cooler, chilled water is used. The cooling water operates in a closed cycle. Chilling of cooling water takes place at the NH3 cooler. The chilled cooling water is fed to the top of the direct contact cooler.
Trickling cooling water and condensed water from the process air collects in the bottom of T 0701. From there, the cooling water goes to the pump P 0701 A/P 0701 B. A small amount of cooling water is drained by the level controller. To substitute this cooling water fresh water is fed to the closed cycle by pump P 0705.
Then this cooled process air passes through one of two molecular sieve absorbers, where carbon dioxide, hydrocarbon and the remaining water vapor are removed. There are two absorbers installed one is in operation while the other one is being regenerated. For the regeneration of the absorbers, impure N2 from the low pressure column, heated up in the steam heater is used.
For cooling down after regeneration the nitrogen bypass the steam heater via valve. The change over sequence of the adsorbers and the regeneration process works automatically by a program.
Downstream of the molecular sieve adsorber, the process air separates into two parts. One part enters the cold box directly, where it cools down to -1730c (nearly liquefaction) in the main heat exchanger. The other parts of the process air warms up to +3800c and further compresses in the air booster compressor to approximately 9.2 bar g. This compressed air pre-cools in heat exchanger to approximately and further cools in the main heat exchanger to approximately -1030c before it expands in the expansion turbine to about 0.37 bar g and mainly goes to low pressure column.
In the pressure column, process air separates into oxygen enriched liquid at the bottom and a pure nitrogen product of the top.
The bottom (sump) liquid sub cools in the warm part of the sub-cooler against the impure nitrogen and low pressure nitrogen and subsequently expands in to the crude argon condenser. From there the liquid is withdrawn and fed to the low pressure column.
Nearly all of the pure nitrogen top product (except seal and purge gas) liquefies in the main condenser against vaporizing oxygen on the low pressure side.
One part serves as reflux for the rectification in the pressure column. Another part sub cools in sub cooler and expands into the low pressure column.
Excess liquid nitrogen will be branched off and goes the liquid storage tank (LIN product).
In the low pressure column, oxygen enriched liquid form the bottom T 0702 separator into pure oxygen bottom product and the pure nitrogen / impure nitrogen top products. The pure nitrogen product and the impure nitrogen warm first in the sub-cooler and second in the main heat exchanger against air.
The impure nitrogen is used as regeneration gas for the molecular sieve absorbers liquid oxygen product from the bottom of the main condenser goes to the storage tank after sub-cooling in sub-cooler. The GOX product warms up in the main heat exchanger.
The main refrigeration requirement of the process is covered by the expansion of process air via the expansion turbine. The expansion turbine is connected to the booster which compresses part of the process air to a higher pressure. This air cools down in the main heat exchanger, goes through the turbine expands and is blown into the low pressure column.
¦ Crude and Pure Argon Recovery:
The argon recovery takes place in the crude argon and pure argon column. The crude argon column is separated into two sections due to cold box eight limitations. Both crude argon columns as well as the pure argon column are equipped with structured packing.
Enriched argon oxygen gas is withdrawn from the low pressure column and fed in to the crude argon column. Here the gas is separated into an oxygen enriched bottom and argon enriched top gas.
The liquid in the bottom goes back to the low pressure column. The crude argon top gas goes to the column and mainly liquefies in the condenser against the oxygen enriched liquid air coming from the pressure column. This liquid serves a reflux for the crude argon column.
The liquid in the sump of T 0704 B will be pumped to the crude argon column T 0704 A by pump P 0704 A / B and is used there is a reflux too.
Part of the oxygen free crude argon is withdrawn form the condenser E 0716 and goes further to the pure argon column.
In the pure argon column, the nitrogen which still remains in the crude argon is separated from the argon and released to the atmosphere.
The top gas of the pure argon column liquefies in the condenser against the oxygen enriched liquid of the crude argon condenser and serves as reflux for T 0705. Part of this steam is vented to atmosphere to remove the N2.
Liquid argon in the sump of T 0705 evaporates in the evaporator against the oxygen enriched liquid and the gas rises up in the column to maintain the rectification.
The liquid goes as product to storage tank.
(3) AMMONIA SYNTHESIS SECTION “ SECTION 500
(a) Theoretical principal of process:
The synthesis of H2 and N2 takes place in presence of ion catalyst at H2:N2 ratio of 3:1 and at a high temperature and pressure as per following reaction.
N2 + 3H2 2NH3 H298 = -52.1 KJ / mol
The synthesis of ammonia from gaseous hydrogen and nitrogen is an exothermic reaction, which is accompanied by considerable volume decreases at constant pressure. Equilibrium consideration dictates that the synthesis reaction must be carried out at high pressure to obtain an economic yield. According to Le Chatelierâ„¢s principles, thermodynamic equilibrium data show that the highest yield is obtain at low temperature. As the synthesis reaction proceeds, the exothermic reaction causes the temperature to rise which increase rate of reaction. But at the same time the equilibrium becomes less favorable and the synthesized ammonia increased the rate of the reversed reaction.
(b) Ammonia CASALE Converter:
General Description:
Gas flow path in the converter.
The converter has three catalyst beds of the axial / radial type with intermediate cooling by heat exchanger with incoming cooled gas in order control temperature at the inlet of beds. The feed gas entering the converter at the bottom passes along the angular space between the converters, pressure shell and catalyst cartridge which is thermally insulated. This way the converter HP shell is protected against high temperature and maintained at the temperature of the feed gas.
An extra connection at the top of the converter is used during startup operation to introduce hot gas from the fire starter heater from heating reducing and activating catalyst.
© Catalyst bed description:
In each bed two cylindrical walls, one external near the cartridge wall and one internal are provided.
According to the CASALE “ Patented designed each wall is made of:
¢ A Perforated wall
¢ A wire-net lining in contact with the catalyst.
The perforated wall has the function of securing an even gas distribution, supporting the mechanical strength and creating the necessary gas space to improve gas distribution.
Process description of Ammonia Synthesis Unit:
In ammonia synthesis unit, 99.99% pure hydrogen from PSA unit and 99.99% pure nitrogen from ASU unit are mixed in 3:1 ratio at 23.50 Brag to form synthesis gas.
The synthesis gas is compressed to about 135 Brag pressure in first three stages of synthesis gas compressor and after mixing with recycle gas at 135 Brag pressure it is further compressed to 143 Brag pressure in four stage centrifuge compressor is driven by high pressure steam turbine.
Density controller and H2 controller accurately control the ratio of H2 and N2. The density of synthesis gas 7.763kg / mt3 is controlled at the suction of compressor.
Hydrogen flow is controlled by flow controller. Any excess hydrogen is vented through flow controllers of hydrogen of PSA unit. Nitrogen flow is controlled by flow controller. At the discharged 74% H2 is controlled.
Since synthesis gas compressor is centrifugal type compressor, to antis urge are provided at make up stage and recycle stage to protect compressor against surging. These controllers at make up stage and recycle stage fulfill the minimum flow requirements of compressor by operating flow valves. Thus part of synthesis gas is recycled to the compressor via heat exchanger.
Synthesis gas is preheated in hot exchanger and is routed to ammonia synthesis converter at 1850c via HV. The temperature of synthesis gas is controlled by TCO0574 at the inlet of R0501. The ammonia synthesis converter R0501 is a thru bed catalytic reactor with two inter bed exchangers and have axial 1 radial flow path. Ammonia synthesis reaction is an exothermic reaction takes place in presence of Fe “ based catalyst.
N2 + 3H2 = 2NH3 H298 = -52.1 KJ / mol
After conversion to ammonia, heat recovery is done at various stages as follows:
Hot gas at 430°c enters the BFW heater which on exchanging heat cools down to about 330°c heating up BFW which is used in various stages.
This gas from BFW heater goes to heat exchanger E-0503 and cools down to 220°c on exchanging heat with fresh synthesis gas from compressors unit prior to feeding to the synthesis converter.
The outlet gas from E-0503 passes to a heat exchanger where heat is exchanged with water to achieve an outlet ammonia temperature of about 100°c. The outlet gas then passes into a cold exchanger where its temperature further reduces to about 25°c.
Further chilling is done in a set of two chillers using ammonia from ammonia collector as a heat exchanging medium. Outlet gas from cold exchanger goes to 1st chiller where its temperature falls down to 10°c which further reduces to -2.8°c on passing through a 2nd chiller.
Thus after extracting
