HYDROGEN RECOVERY FROM PURGE GAS

(ENERGY SAVING)

Author

Prem Baboo

Sr. manager (prod)

National Fertilizers Ltd. Vijaipur, India

Sr. Advisor & an Expert, www.ureaknowhow.com

Fellow of Institution of Engineers (India)

Abstract Ammonia is continuously condensed out of the loop and fresh synthesis gas is added. Because the synthesis gas contains small quantities of methane and argon, these impurities build up in the loop and must be continuously purged to prevent them from exceeding a certain concentration. Although this purge stream can be used to supplement reformer fuel gas, it contains valuable hydrogen which is lost from the ammonia synthesis loop In order to achieve optimum conversion in synthesis convertor, it is necessary to purge a certain quantity of gas from synthesis loop so as to as to reduce inerts concentration in the loop. Purge gas stream from ammonia process contains ammonia, hydrogen, nitrogen and other inert gases. Among them, ammonia itself is the valuable product lost with the purge stream. Moreover it has a serious adverse effect on the environment.This purge gas containing about 60% Hydrogen was fully utilised as primary reformer fuel. The recovered hydrogen is sent back to the synthesis loop to increase production or save energy, as the quantity of hydrogen produced by steam reforming can be reduced. A cryogenic purge gas recovery unit, designed by M/s L'Air Liquide, France is available in order to recover H2 from it which is recycled back convert it to Ammonia while the by - product tail gas from PGR Unit is burnt as fuel in the primary reformer. The ammonia recovery unit removes and recovers the major part of the remaining ammonia contained in the purge gas, let down gas and inerts vent gas from loop and the refrigeration circuit, respectively. The makeup gas contains small amount of Argon and Methane. These gases are inerts in the sense that they pass through the ammonia synthesis converter without undergoing any chemical changes. Because of the complexity and cost of hydrogen production various processes are employed in the industry to recover hydrogen from tail gases. Specific industries use specific hydrogen separation and purification method based upon their requirement and feed conditions. This process is based on the difference in boiling points of liquid gases in the stream. The basic principle adopted in our refrigeration circuit is employed.

Introduction The use of hydrogen is mainly in refineries petrochemical and fertilizer industries.

Because of the complexity and cost of hydrogen production various processes are

employed in the industry to recover hydrogen from tail gases. Specific industries use

specific hydrogen separation and purification method based upon their requirement

and feed conditions.

The purge gas from synthesis loop of ammonia plant is washed with an aqueous

solution of ammonia and then washed gas having low ammonia content is sent to

absorption unit, where remaining ammonia and water is removed completely. The gas

delivered by the purification unit is cooled by exchange with hydrogen and tail gas

steam coming out of exchangers in cold box unit. The cooled gas is let down to sub

cooled gas where it is partially condensed. The gas and liquid are separated in the

vessel. The gaseous fraction, containing the main part of hydrogen is warmed in this

exchanger and recycled back to synloop to convert it to ammonia. The liquid fraction,

after expansion through a valve is vaporised and warmed in a exchanger and used as a

fuel in Primary Reformer. Ammonia solution from the ammonia washing tower is

concentrated in the distillation unit and fed back to the main refrigeration loop. The

purge gas from synthesis loop of ammonia plant is washed with an aqueous solution of

ammonia and then washed gas having low ammonia content is sent to absorption unit,

where remaining ammonia and water is removed completely. The gas delivered by the

purification unit is cooled by exchange with hydrogen and tail gas steam coming out of

exchangers in cold box unit. The cooled gas is let down to sub cooled gas where it is

partially condensed. The gas and liquid are separated in the vessel.

Description of the Process

Following process are used for recovery of Hydrogen from purge gases.

1. Membrane, 2. Pressure swing adsorption (PSA) processes and, 3. Cryogenics Process

Above all process have all been commercially applied for the recovery of hydrogen

from the purge gas. Purge gas is drawn from the loop after the second cold exchanger

just before addition of the make-up gas. At this point the gas has the maximum content

of inerts. The purge gas is transferred to a purge gas recovery unit for recovery of

hydrogen. Recovered hydrogen is recycled to the process and introduced between first

and second casing of the synthesis gas compressor the choice of separation technology

is driven by the desired purity, degree of recovery, pressure and temperature.

Membrane separators are widely accepted to recover hydrogen from purge gas and

recover typically 85-90% hydrogen, with a hydrogen purity of 87-90%. The cryogenic

process operates at high pressures (7.0 M.Pa) and can achieve a hydrogen recovery of

90-98%. Pre-treatment of the purge gas to remove ammonia and water is required

for the cryogenic process. Pressure swing adsorption has been used for ultra -high-

purity hydrogen from the purge gas and has a somewhat lower hydrogen recovery

(70-85%) than the membrane and cryogenic processes. No pre-treatment is required

and water and ammonia are removed by PSA in addition to argon and methane.

When comparing membrane separators with cryogenic technology it is noted that

recovery based on cryogenic technology is more energy efficient, whereas the recovery

based on membrane technology requires a lower investment. Which technology to

choose will have to be decided on a case-to-case basis? Various manufacturers supply

different types of hydrogen recovery technologies, including Air Products, Air Liquid,

Linde, and UOP. This measure is applicable to both retrofit and new high pressure

synthesis loop steam reforming plants. In 1980, an Indian plant installed a hydrogen

separation and recovery unit based on the cryogenic process. By recycling the hyd rogen

contained in the purge gas, about 60%, to the synthesis loop, the ammonia throughput

increased by about 40-50 TPD; a 5% increase in plant production. In addition to the

increased production, the energy use decreased by 0.96 GJ/T of NH3. The tail gas from

the purge gas recovery unit containing 30% methane and 15% Hydrogen was used as

fuel in the primary reformer.

1. HYDROGEN RECOVERY WITH MEMBRANE PROCESS

Membrane systems are based on the difference in permeation rates between hydrogen

and impurities across a gas-permeable polymer membrane. Permeation involves two

sequential mechanisms: the gas-phase component must first dissolve into the

membrane and then diffuse through it to the permeate side. Solubility depends

primarily on the chemical composition of the membrane and diffusion on the structure

of the membrane. Gases can have high permeation rates as a result of high solubility,

high diffusivity, or both. The driving force for both solution and diffusion is the partial

pressure difference across the membrane between the feed and permeate sides. Gases

with higher permeability, such as hydrogen, enrich on the permeate side of the

membrane, and gases with lower permeability enrich on the non-permeate side of the

membrane because of the depletion of components with high permeability.

Fig. No-1

The first fraction of the gas to permeate through the membrane consists primarily of the

components with the highest permeability. As a larger fraction of the feed gas is allowed

to permeate, the relative amount of the components with lower permeability increases

in the permeate stream. In hydrogen separations, higher purity product hydrogen is

associated with lower recovery, and lower purity product hydrogen is associated with

higher recovery.

Fig. No.2

The effect of hydrogen purity on recovery is much more dramatic with membrane

systems than with PSA or cryogenics units. A fairly small change in hydrogen purity can

change the recovery significantly.

BENEFITS- Efficient and economical hydrogen recovery: typically 80% to 98% recovery of feed H2 with a purity of 90 volume % to 99 volume %

1. Handles high feed pressures up to 170-175 Bar. 2. Reduce flaring and correct refinery fuel balance 3. Proven performance, long membrane life, many references

Fig. No. -3

Higher hydrogen recovery also requires that more membrane area be provided. The

membrane area required when feed composition and system pressure levels are fixed

increases exponentially at high hydrogen recovery. The performance of a specific

membrane system, that is, the recovery versus the product purity for a given feedstock,

is primarily dependent on the ratio of feed to permeate pressure and is largely

independent of the absolute pressure level. However, the area requirement is inversely

proportional to the feed pressure. Hence, compressing the feed gas rather than the

permeate, even though the permeate flow is smaller, is often preferable when the

objective is to achieve the required pressure ratio.

2. PSA PROCESS

POLYBED™ Pressure Swing Adsorption units for hydrogen purification are based on the ability

of adsorbents to absorb more impurities at high gas-phase partial pressure than at low partial

pressure. Impurities are adsorbed in an absorber at high partial pressure and then desorbed at

low partial pressure. The impurity partial pressure is lowered by swinging the absorber

pressure from the feed pressure to the tail gas pressure and by using a high-purity purge gas.

The driving force for the separation is the difference in impurity partial pressure between the

feed and tail gas. A minimum pressure ratio of approximately 4:1 is required. The feed pressure

is usually in the range of 14 to 35 atm. Optimum tail-gas pressure is as low as possible.

Because vacuum is normally avoided, tail-gas pressures less than 1 atm is typically used when

high hydrogen recovery is desired.

The PSA tail-gas is frequently compressed from this low pressure to fuel-gas pressure.

Hydrogen is essentially not adsorbed in the PSA process and is available at close to feed

pressure: the typical pressure drop between the feed and product battery limits is less than 10

psi. The two key advantages of the PSA process are its ability to remove impurities to any level

and to produce a high-purity high-pressure hydrogen product. The purity of the hydrogen

product from a PSA unit is typically in excess of 99 vol-% and frequently 99.999 vol-%. Removal

of CO and CO2 to a volume level of 0.1 to 10 ppm is common and readily achieved.

3. HYDROGEN RECOVERY WITH CRYOGENIC PROCESS

This process is based on the difference in boiling points of liquid gases in the stream.

The basic principle adopted in our refrigeration circuit is employed here. Ammonia line

1 PGR and IGP is based on this process. The Energy saving is 0.11 Gcal/MT ammonia

with purge gas recovery for 1864TPD ammonia plant and correspondingly 0.06Gcal/MT

urea. Annual profit from PGR after payback period is around Rs 15 crore (USD 2238806).

1. Hydrogen gets liquefied at -253°C atm pressure.

2. Nitrogen gets liquefied at -196°C atm pressure.

3. Methane gets liquefied at -161°C atm pressure.

4. Ammonia gets liquefied at -33°C atm pressure.

PURGE GAS RECOVERY DESIGN BASIS

FACTOR THAT AFFECT HYDROGEN RECOVERY

Sr. No. Process Condition Effect of Hydrogen Recovery

1 Increase no. of separators on-line

Increase

2 Increase feed flow rate

Decrease

3 Increase the hydrogen purity of Increase

the feed

4 Increase permeate pressure

Decrease

5 Increase feed pressure

Increase

FACTOR THAT AFFECT NH3 SLIP FROM SCRUBBER

Sr. No Process condition

Effect on overhead NH3

1 Increase feed flow rate

Increase

2 Increase water flow rate

Decrease

3 Increase the ammonia conc. in feed

Increase

4 Increase feed pressure

Decrease

5 Increase feed or water temperature

Increase

ENERGY AND UTILITY CONSUMPTION

Sr. No.

Utility Supply Condition Normal

Units Quantity

1 LP Steam 3.5 Bar,2500C Kg/hr. 60 2 MP Steam 38 bar,3800C Kg/hr. 1008 3 Cooling Water Supply 3.5 bar,330C Kg/hr. 42993 4 Cooling water return 2.0 Bar,430C Kg/hr. 42993 5 Demineralized water 2.0 Bar,400C Kg/hr. 10(recycled

stripper bottom)1605(100%, fresh feed)

6 Electricity Pump 440 V ,50 Hz ---- KW 6.5

7 Electricity Instruments 115V,AC,50 Hz(Instruments,110 V DC(Solenoid valve)

--- KW Minimal

8 Nitrogen, Dry 6.0 Bar,400C Nm3/Event 175

9 Instrument, air 6.5 Bar, ambient, dew point ≤400C

Nm3/Hr. 50

The refrigeration required for the process is obtained by Joule-Thomson refrigeration,

which is derived from throttling the condensed liquid hydrocarbons. The ammonia

process flow diagram as figure No. 4.

Fig. No.-4

The feed needs to be pretreated to remove water and other components that could

freeze in the system. The pretreated feed at high pressure, 20 to 82 bars, is cooled

against a stream leaving the cryogenic unit to a temperature at which the majority of the

hydrocarbons and ammonia condense.

Fig. No. 5

The two-phase stream is sent to a separator where the hydrogen-methane vapor stream

is taken overhead and further cooled to a temperature low enough to give the desired

hydrogen purity. The cooled stream is fed to another separator, and the hydrogen

product is taken overhead. Thus, the cryogenic unit typically splits the feed into three

products:

1. A high purity hydrogen stream, 2. A methane rich stream at fuel gas pressure, and 3. Ammonia stream. Nitrogen is also recovered. The Process flow diagram as

follows in figure No. 6

Fig. No. 6

The approximate converter inlet gas composition is shown in below-

1. Hydrogen- 64.2 mole % 2. Nitrogen- 21.4 mole % 3. Ammonia -5.6 ole % 4. Methane – 5.9 mole % 5. Argon -2.9 mole %

The contents of ammonia, Hydrogen and Methane are measured and recorded downstream of

the cold heat exchanger as figure No. 7.

The main inlet flow to the converter is introduced through two inlet nozzles at the bottom. The

inlet gas passes through the annular space between the pressure shell and the insulated basket

and thereby the pressure shell is kept relatively cool.

The temperature profile of the shell may be followed by the eleven thermocouples mounted on

the shell. Nine of the thermocouple are installed at three levels each comprising three elements,

and two are mounted just above the inlet gas nozzle.

The ammonia synthesis takes place in the ammonia synthesis converter, R 3501,

according to the following reaction scheme:

Fig. No-7

3H2 + N2 = 2NH3 + heat

Fig. No.8

The reaction is reversible and only a part of the hydrogen and nitrogen is converted into

ammonia by passing through the catalyst bed. The conversion of the equilibrium

concentration of ammonia is favoured by high pressure and low temperature. In R 3501

only about 30% of the nitrogen and the hydrogen are converted into ammonia.

Fig. No-9

To get maximum overall yield of the synthesis gas, the unconverted part will be recycle

to the converter after separation of the liquid ammonia product.

After the synthesis gas has passed through R 3501, the effluent gas will be cooled down

to a temperature which the main part of the ammonia is converted.

The circulation is carried out by means of the recirculation, which is an integrated part

of the synthesis compressor, K 3431.

As the reaction rate is very much enhanced by high temperature, the choice of

temperature is based on a compromise between the theoretical conversion and the

approach to equilibrium.

The ammonia synthesis loop has been designed for a maximum pressure of 245kg/cm2

g. The normal operating pressure will be 220kg/cm2 g depending on load and catalyst

activity.

The normal operating temperatures will be in range of 360-525°C for the 1st bed and

370-460°C for the 2nd bed.

The heat liberated by the reaction (about 750kcal/kg produced ammonia) is utilized for

high pressure steam production (in the loop waste heat boiler, E 3501) and preheat of

high pressure boiler feed water.

As illustrated in diagram, the converted effluent gas is cooled stepwise, first in the loop

waste heat boiler, E 3501, from 456-350°C. Next step is cooling to about 269°C in the

boiler feed water preheated, E3502. And then the hot heat exchanger, E 3503, where the

synthesis effluent gas is cooled to 61°C by preheating of converter feed gas.

The synthesis gas is cooled to 37C in the water cooler, E 3504 and to 28 C in the heat

exchanger.

The final cooling to 12°C takes place in the ammonia chillers. The condensed ammonia

is separated from the circulated to the ammonia converter through the cold heat

exchanger, the recirculation, and the hot heat exchanger.

The water vapour concentration in the make-up gas is in the range of 200-300 ppm,

depending on the operating pressure in the loop. The water is removed by absorption in

the condensed ammonia. The carbon dioxide in the make-up gas will react with both

gaseous and liquid ammonia, forming ammonium Carbamate:

2NH3 + CO2 = NH4-COO-NH2

Ammonia + Carbon Dioxide = Ammonium Carbamate

The formed Carbamate is dissolved in the condensed ammonia. As the water deactivates

the ammonia synthesis catalyst, the content of carbon monoxide in the make-up

synthesis gas should be kept as low as possible.

INERTS GASES

The make-up gas enters the loop between the two ammonia chillers. This gas contains

small amount of argon and methane. These gases are inert in the sense that they pass

through the synthesis converter without undergoing any chemical changes. The inert

will accumulate in the synthesis loop, and a high inert level, i.e. high concentration of

inert gases will build up in the circulating synthesis gas. The inert level will increase

until the addition of inert gases will make the make-up gas in the same as the amount of

inert removed from the top.

The low temperature outlet of the 1st ammonia chiller means that the partial pressure of

ammonia in the gas phase is relatively low. Only a minor amount of ammonia will be

removed together with the purge gas. The purge gas is further cooled in the purge gas

chillier, E 3511, and the liquid ammonia is separated in B 3512. The liquid ammonia is

sent to B3501.

HYDROGEN/NITROGEN RATIO

By the synthesis reaction, 3 volumes of hydrogen reacts with 1 volume of nitrogen to

form 2 volumes of ammonia.

The synthesis loop is designed for operating at H2/N2 ratio of 3.0, but special conditions

may make it favourable to operate at a slightly different ratio in the range of 2.5-3.5.

When the ratio is decreased to 2.5, the reaction rate will increase slightly, but decrease

again for ratios below 2.5. On the other hand the circulating synthesis gas will be

heavier. Therefore, the pressure drop and ammonia concentration at the inlet of the

ammonia synthesis converter will increase.

AMMONIA CONVERTER R 3501

Flow Pattern in R 3501 At the top of the converter, the gas passes the tube side of the

inter bed heat exchanger, where the inlet gas is heated up to the reaction temperature

of the heated up the reaction temperature of the 1st catalyst bed by the heat exchanger

with gas leaving the 1st catalyst bed. The gas inlet temperature to the 1st bed is adjusted

by means of the so-called “cold shots” which is cold synthesis gas introduced through

the transfer pipe of the centre tube.

The gas, which leaves the 1st catalyst bed, is led through the 2nd bed and into the centre

tube from which it is returned to the ammonia loop.

The two catalyst bed contains a total of 109.3 m3 of KMIR catalyst, which is a promoted

iron catalyst containing small amounts of non-reducible oxides.

Reaction Temperature

At the inlet of R 3501, 1st catalyst bed, the minimum temperature of approx. 360°C is

required to ensure a sufficient reaction rate. If the temperature at the catalyst inlet is

below this value, the reaction rate will become so low that the heat liberated by the

reaction becomes too small to maintain the temperature in the converter. The reaction

will quickly extinguish itself if properly adjustments are not made immediately.

On the other hand, it is desirable to keep the catalyst temperature as low as possible to

prolong the catalyst life. Therefore, it is recommended to keep the catalyst inlet

temperature slightly above the minimum temperature. It is anticipated that the

synthesis gas enters the 1st catalyst bed at a temperature of max. 400°C. As the gas

passes through the catalyst bed the temperature increases to a maximum temperature

in the outlet from the 1st bed, which is normally the highest temperature in the

converter, called the “hot spot”. The temperature of the hot spot is up to 510°C, but

should not exceed 520°C.The gas from the 1st bed is cooled with some of the cold inlet

gas to the 1st bed in order to obtain a temperature of approx. 370°C inlet 2nd bed. The

gas outlet temperature from the 2nd bed is about 455°C.

Catalyst

The catalyst is distributed with 29.0 m3 in the first (upper) bed, and 80.3 m3 in the

second (lower) bed. The particle size of the catalyst is 1.5-3 mm. The smallest particle

size causes a very high overall catalyst activity. Further the radial flow design of the

converter allows small particle without causing a prohibitive pressure drop. The pre-

reduced KMIR catalyst has been stabilizes during manufacturing by superficial

oxidation. The partly oxidized catalyst contains about 2 wt% of oxygen. The

stabilization makes the KMIR catalyst non pyrophoric up to 90-100°C, but above 100°C

the catalyst will react with oxygen and heat up spontaneously.

Reducing the iron oxide surface layer to free iron with simultaneous formation of water

activates the catalyst. The reduction is carried out with circulating synthesis gas. The

desired level temperature is obtained by using the start-up heater, H 3501

The use of synthesis gas with hydrogen to nitrogen ratio close to 3:1 for activation of

KMIR has two advantages. The first is production of ammonia starts early, causing a

heat production. The production of heat provides the possibility of circulating more

synthesis gas, which helps in reducing the remaining part of the catalyst.

The second advantage is removal of the formed water from the circulating gas. It will be

dissolved in ammonia and then purify the circulating the synthesis gas. This is

important as water is the catalyst poison. The catalyst activity decreases slowly during

normal operation and the catalyst lifetime, which is normally more than 5 year, its

affected by the actual process conditions, notably the temperature in the catalyst bed

and the concentration of the catalyst poisons in the synthesis gas at the inlet of the

convertor.

Although the KMIR can be used in the range of 530-550°C ,it should be noted that the

lower the catalyst temperature are in operation, the slower the decrease in catalyst

activity will be , and accordingly the lifetime will be prolonged. It is therefore

recommended to maintain the lowest possible catalyst temperature during operation,

especially for the 2nd bed which determines the conversion.

All compounds containing oxygen, such as water, CO and carbon dioxide, are poisonous

gas is clean again. But as some permanent deactivation will take place, high

concentrations of oxygen compounds at the convertor inlet, even of short duration,

should be avoided.

Sulphur and phosphorous compounds are severe poisons, as the catalyst deactivation

will be permanent. A probable source of introduction of such contaminants is the seal

oil.

Refrigeration

The purpose of refrigeration circuit is to carry out the various cooling tasks in the

ammonia synthesis loop. The primary task is to condense the ammonia, which is

produced in the convertor. Other cooling tasks are cooling of make-up gas, purge gas,

let-down gas, and inert gas.

6 chillers operating at three different pressure (3 “chillers levels”),

A refrigeration compressor,

An ammonia condenser,

And an accumulator.

Besides the above-mentioned equipment the refrigeration circuit includes the following.

Three K.O drums (one for each compressor stage), to protect the refrigeration

compressor of droplets of ammonia.

A flash vessel, from where the makeup ammonia is fetched and the spent ammonia is

returned to from the refrigeration circuit.

The first ammonia chillier, E 3506and the make-up gas chillier, E 3514 operate at the

highest level which is a temperature of 18.8°C, corresponding to a pressure of 7.8

Kg/cm2g. The second ammonia chillier, E 3507, operates at the medium level, where the

ammonia boiling temperature is 6.9°C with a corresponding pressure of 4.6Kg/cm2g.

The three chillers: purge gas chillier, E 3511, let down gas chillier, E 3508, and the inert

gas chillier, E 3509, operate at the lowest chillier level, where the ammonia boiling

temperature range from -33 to -30°C and the corresponding pressure is approximately

0.05 kg/cm2g.

Ammonia Wash Section

General

The ammonia wash section removes and recovers the major part of the ammonia

contained in the off gas, let down gas, and the inert vent gas from the loop.

The purge gas steam is taken out from the loop just after the 1st ammonia chiller,

E 3506. Still under pressure the gas is cooled down to -25°C in the E 3511. Some

of the ammonia is separated as liquid. The gas fraction from the separation is

sent to the purge gas absorber, F 3522.

In order to avoid accumulation of inerts in the refrigeration section, a purge flow

of the non-condensable gases is sent to the inert gas chiller. E3509, where they

are cooled further to about -25°C. At this temperature some of the ammonia is

removed as liquid. The gas fraction from the separator is sent to the off gas

absorber, F 3523.

The letdown gas is produced in the letdown vessel. The liquid ammonia from B

3501 is sent to the letdown vessel B 3502. The main part of the dissolved gases

will be released due to the pressure reduction from the loop pressure of

209kg/cm2g to pressure in the letdown vessel of 26kg/cm2g. At this pressure the

gas is cooled to -25°C in E 3308. Some of the ammonia is separated as liquid. The

gas fraction from the separation is sent to the off gas absorber, F 3523.

The let down and inert gas is mixed before entering the ammonia wash section.

Off Gas Absorber, F 3523

The off gas containing ammonia is introduced at the bottom part of the off gas absorber,

F 3523, where it is washed in counter-current with water introduced at the top of the

absorber. The purified off gas leaving the absorber at the top contains approx. 0.02mole

% ammonia, whereas the ammonia water solution leaving the absorber at the bottom

contains approximately 10 mole% ammonia. Operating pressure is 81 kg/cm2g.

The absorber, F 3523 has been provided with two beds, each of 2500 mm height,

containing 1 “pall rings (2*0.22 m3)

Purge Gas Absorber, F 3522

The purge gas containing ammonia is introduced at the bottom part of the purge gas

absorber, F 3522, where it is washed in counter- current with water introduced at the

top of the absorber. The purified off-gas leaving the absorber at the top contains

approximately 0.01 mole ammonia, whereas the ammonia water solution leaving the

absorber at the bottom contains approximately 18 mole % ammonia. Operating

pressure is 15 kg/cm2g.

The absorber, F 3522 is provided with 20 trays, to ensure a good contact between the

liquid and vapour phase.

PURGE GAS RECOVER ADVANTAGES

1. Hydrogen in 15000 Nm³/hr of purge gas is around 9000 Nm³/hr. The equivalent feed saving in primary reformer is around 2.175 T/hr of NG.

2. Or feed can be maintained same increase ammonia production and hence urea

production provided there are no bottlenecks.

3. Consequent to this primary reformer pressure is reduced increasing conversion

and less energy in GV or back pressure is increased saving energy in synthesis

compressor and better absorption in GV.

4. Reduction in firing in primary reforming corresponding to the reduction in feed

and increases in methane slip.

5. Moreover a saving of less than 7 T/h SM steam in reforming countered by loss in

production of HP steam in RG boiler.

PURGE GAS RECOVER DISADVANTAGES 1. Process air to be made up in secondary reformer equivalent to hydrogen

recovered (for feed 15000 Nm³/hr) is around 3.8 KNm³/hr. 2. However process air reduced due to reduction in feed is around 3.5 KNm³/hr 3. So net increase in process air in secondary reformer is around 0.3 KNm³/hr 4. Loss of CO2 around 2.66 KNm³/hr which means under full load, load on CDR

(Carbon Dioxide Recovery) is increased being a costly affair. However energy on GV is also reduced.

5. Although fuel is reduced in primary corresponding to the reduction in feed, fuel is increased to compensate the energy of hydrogen from recovery fuel gas.

6. Overall however there is net energy saving in the PGR project and is considered feasible.

CONCLUSION

In this paper the economic aspects of setting up a Purge gas recovery unit with ammonia production process is analyzed and the benefits that can be obtained by using this small unit is illustrated. The installation of a purge gas converter ensures a very Efficient use with respect to hydrogen and nitrogen balancing. An increased ammonia production, along with hydrogen recovery and environmental regulation, has been

obtained. Furthermore, fixed capital investment for raw gas purification and preparation sections reduces.A cryogenic purge gas recovery unit recovering hydrogen at a pressure of 85 kg/cm2 g improved the efficiency of ammonia synthesis loop. Ammonia recovery unit was provided prior to the hydrogen recovery unit to minimize ammonia loss and finally leading to NOx reduction in the primary reformer. It has been possible to select these operating conditions in view of the high conversion per pass in the S-250 synthesis loop, and also due to the inclusion of a cryogenic purge gas recovery unit. The reformer inlet pressure is 38 kg/cm2 g, a slightly higher operating pressure than used in the past. PRISM Membrane Systems provide operating flexibility when planned or unexpected process changes occur. Some turndown is absorbed by the flexibility of the system and increased capacity requirements are met by the addition of more separators. Additional turndown is accomplished by valving off separators which maintains recovery and purity. Multiple takeoffs from the permeate manifold provide streams of different purities and flow rates. Some applications require feed gas pre -treatment.

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