Lectures 5 8

7/29/2019 Lectures 5 8

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Pharos University جعه فروسFaculty of Engineering كة الندةPetrochemical Department لبوكويت ا قم

LECTURES (5-8)

Ammonia Production Process by steam reforming of Natural Gas

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Steam reforming Process: - 2 Steam reforming concept based on natural gas is considered to

be the most dominating and best available technique for

production of ammonia, as the steam reforming process

accounts for over 80% of the world’s ammonia production.

A-Primary Reforming:

The gas from the desulphurizer is mixed with processsteam, usually coming from an extraction turbine, and

steam gas mixture is then heated further to 500-600°C inthe convection section before entering the primary reformer.

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The primary reformer is a furnace in which amultiplicity of tubes of high-nickel chromium alloy

filled with nickel-containing reforming catalyst in a big

chamber (Radiant box) with burners to provide heat.

The overall reaction is highly endothermic and additional

heat is provided by burning of gas in burners provided for

the purpose, to raise the temperature to 780-830°C at thereformer outlet.

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The composition of gas leaving the reformer is givenby close approach to the following chemicalequilibrium:

CH4 + H2O ↔ CO + 3H2 ∆H = 49.2 kcal/mol

CO + H2O ↔ CO2 + H2 ∆H = -9.8 kcal/mol

The heat for the primary reforming is supplied by

burning natural gas or other gaseous fuels, in theburners of a radiant box containing catalyst filledtubes.

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The flue gas leaving the radiant box has temperature in

excess of 900°C, after supplying the high level heat to the

reforming process.

About 50-60% of fuel’s heat value is directly used in the

process itself. The heat content (waste heat) of the flue-gas

is recovered in the reformer convection section, for

various process and steam duties.

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Primary Reformer f ir ing box

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Secondary Reformer - B The gas leaving the primary reformer usually contains 5-15%

methane (dry basis) and enters the secondary reformer at the bottom.

The object of the secondary reforming step is to complete the

conversion of methane to H2, CO, and CO2 and to supply therequired proportion of N2 for NH3 synthesis.

This is done by adding air in the amount required to give an N:H

atomic ratio of 1:3 in the synthesis gas after the shift conversion step.

The oxygen accompanying the nitrogen in the air burns part of thecombustibles (H2, CO, and CH4) in the partially reformed gas,

thereby raising the temperature high enough or rapid completion of

the reforming.

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The process gas is mixed with the air in the mixing chamber of

secondary reformer then passed over a nickel catalyst that is

supported by a ring-shaped arch made of high-alumina

bricks.

The reformer outlet temperature is around 1000°C, and up to

99% of the hydrocarbon feed (to primary reformer) is

converted, giving a residual; methane content of 0.2-0.3 (dry

gas bases) in the process gas leaving the secondary reformer.The process gas is cooled to 350-400°C in a waste heat boiler

or waste heat boiler/superheater downstream from the

secondary reformer.

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Carbon monoxide conversion process: - 3 A- Water-gas shif t reaction :

The water-gas shift (WGS) reaction is used to convert carbonmonoxide (CO) to carbon dioxide (CO2) and hydrogen (H2)through a reaction with water (H

2O)

CO + H2O ↔ CO2 + H2 ∆H = -41 kJ/mol

The reaction is exothermic, which means the reactionequilibrium shifts to the right and favors formation of the H2 and CO2 products at lower temperatures. At higher temperatures, the equilibrium shifts to the left, limitingcomplete conversion of CO to H2.

The reaction is the basis for most of the industrial H2 produced in the world from methane (CH4) in natural gasthrough steam-methane reforming.

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A conventional water-gas shift reactor then uses a metalliccatalyst in a heterogeneous gas-phase reaction

with CO and steam.

reaction kinetics are faster at elevated temperatures. For thisreason, the catalytic water-gas shift reaction is initially carriedout in a high-temperature shift (HTS) reactor at 350-370°C.

Conversion in the HTS reactor is limited by the equilibriumcomposition at the high temperature.

To achieve higher conversions of CO to H2, the gas leaving theHTS reactor is cooled to 200-220°C and passed throughapproximately 90% of the CO is converted to H2 in the firstHTS reactor and 90% of the remaining CO is converted in theLTS reactor.

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The process gas from the secondary reformer contains 12-

15% CO (dry gas bases) and most of the CO is converted

in the shift section according to the reaction:

CO + H2O ↔ CO2 + H2

In the high temperature shift conversion (HTS), the gas is

passed through a bed of iron oxide/chromium oxide

catalyst at around 400°C, where the CO content is reducedto about 3% (dry gas bases), limited by the shift

equilibrium at the actual operating temperature

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There is tendency to use copper containing catalyst to

increase conversion. The gas from the HTS is cooled and

passed through the low temperature shift (LTS) converter.

The LTS is filled with a copper oxide/zinc oxide-based

catalyst and operates at about 200-220°C. The residual CO

content is important for the efficiency of the process.

Therefore, efficiency of shift step in obtaining the highest

shift conversion is very important.

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Carbon dioxide removal - 4 The process gas from the low temperature shift converter

contains mainly H2, N2, CO2 (≈ 18%) and excess process

steam.

The gas is cooled and most of the excess steam is condensed

before it enters the CO2 removal section

The CO2 is removed in a chemical or physical absorption

process. The solvents used in chemical absorption process are

mainly aqueous amine solutions Mono Ethanolamine (MEA),activated Methyl DiEthanolamine (aMDEA) or hot potassium

carbonate solutions. Physical solvents are glycol

dimethylethers (Selexol), propylene carbonates and others.

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The main advantage of potassium carbonate solution islower heat requirements for stripping the CO2 from thesolvent

The potassium carbonate system operates mainlyisothermal-CO2 absorption at high pressure and CO2 release at low pressure.

In the absorption step the pressure is typically about 3.0

MPa (reformer pressure minus pressure losses), and thetemperature may be 100°C. The CO2 is absorbedchemically by the conversion of potassium carbonate to

bicarbonate.

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When the solution pressure is reduced to about

atmospheric pressure, part of the CO2 and water vapor

escape. CO2 release is assisted by steam stripping.

The steam is raised in the regenerator reboiler heated by

the gas from the LTS shift converter; thus, some or most

of the heat required by the CO2 removal process is derived

from the heat in the incoming gas.

Reaction involved:

K 2CO3 + CO2 + H2O ↔ 2KHCO3

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During absorption, the reaction proceeds from left to right and

during regeneration from right to left. The heat of reaction

amounts to 340 kcal/Nm3 CO2.

Residual CO2 content are usually in the range (100-1000 ppm),

depending on the process used.

5-Methanation:

The gas leaving the CO2

absorption step still contains about

0.3% CO and 0.1% or less CO2.

These oxides must be removed prior to the ammonia synthesis

step because they would decrease the activity of the ammonia

synthesis catalyst and cause deposition of ammonium

carbamate in the synthesis loop.

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CO + 3H2 → CH4 + H2O, ΔH25C = - 206.1 kJ/mol

CO2 + 4H2 → CH4 + 2H2O, ΔH20C = -164.9 kJ/mol

These reactions are the reverse of the reformer reactions, and a

similar nickel-based catalyst is used.The methanation step is usually carried out with a gas inlet

temperature of 300 - 350°C; therefore, the gas must be preheated to that temperature.

Since the reactions are exothermic, the temperature may rise to320-400°C at the gas outlet, depending on the CO + CO2 content of the gas. A heat exchanger is commonly used to pre-heat the incoming gas and cool the exit gas.

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Compression of synthesis gas-6 The synthesis gas leaving the methanation step typically

contains about 74% H2, 24% N2, 0.8% CH4, and 0.3% , at dry basis. The gas must be compressed to the pressure required by

the synthesis step.centrifugal compressors are now used in most new plants that

have capacities of 600-1,800 tpd. Synthesis pressures in thesenew plants usually are in the range of 15-25 Mpa.

Centrifugal compressors are driven by steam turbines using

high-pressure steam generated mainly from hot process gasleaving the secondary reformer. The steam is exhausted at alower pressure and used in the reforming process and other

process steps.

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Ammonia synthesis:-7The synthesis of ammonia is composed of the following

reversible reaction of hydrogen and nitrogen.

N2 + 3H2 ↔ 2NH3

This reaction is exothermic; the net heat of reaction is about11,000 cal/g mole at 18°C (647 kcal/kg of NH3), assuming NH3 is in the gaseous state.

The metallic iron catalyst is primarily made from magnetite.Fe

3

O4

, that has been promoted using alkali in the form of potash and metals, such as aluminum, calcium, or magnesium

Caution must be taken because the catalyst could undergothermal degradation.

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It could also be permanently poisoned by sulfur, arsenic, phosphorus, chlorine, and heavy hydrocarbons; oxygen-bearingcompounds will cause temporary poisoning, which may be

reversed if the exposure was only for a short while.Synthesis pressure, synthesis temperature, space velocity, inlet

gas composition, and catalyst particle size all affect ammoniasynthesis.

The gas entering the converter consists mainly of gas circulated

in the loop with a relatively small amount of fresh synthesis gascalled “makeup” gas. The gas entering the converter contains

N2 and H2 in a 1:3 ratio plus 10-14% “inerts” and about 2% NH3.

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The “inerts” consist mainly of methane, argon, and sometimes

helium if the natural gas feedstock contains the element.

Since the inert gas concentration tends to increase as the N2 and

H2 are removed, it is necessary to vent a side stream of “purge gas” to keep the inert gas concentration at a tolerable level

Ammonia synthesis converters differ in the type of flow: axial,

radial, or cross flow. The reactor is designed for good gas

distribution throughout the catalytic bed at minimum pressuredrop. The converters also differ in the way temperature control

of the reactants is achieved (quench or indirect cooling) and

how reaction heat recovery is done.

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The gas leaving the converter will contain 12%-18% NH3,

depending mainly on the pressure; conversion per pass

increases with pressure.

The gas is cooled first by heat exchange with the

incoming gas, then by air or water, and finally by

refrigeration to condense most of the ammonia as a liquid.

The unreacted gas is recycled with the addition of freshmakeup synthesis gas, thus maintaining the loop pressure.

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The purge gas is scrubbed with water to remove ammonia

before being used as fuel or before being sent to hydrogen

recovery unit.

Vaporizing ammonia is used as a refrigerant in most

ammonia plants, to achieve sufficiently low ammonia

concentration in the recycled gas. The ammonia vapors

are liquefied by compression in the refrigeration

compressor

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Reactor for Ammonia Synthesis:

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Lectures 5 8