ammonia flaring

11th Stamicarbon Urea Symposium19 – 22 May 2008, Noordwijk

Eleventh StamicarbonUrea Symposium 2008

Paper 12

Zero ammonia emission,a PIC Kuwait project

Authors:Luc Dieltjens and Kees de Rijk, Stamicarbon B.V., The Netherlands

Scot Smith, Zeeco Inc., USAS. Raghunathan, PIC, Kuwait


Table of contents (Part 1) Page

1. Introduction.............................................................................................................................12. The PIC Zero Ammonia Emission Project...............................................................................23. Flaring process vapors ...........................................................................................................3

3.1 Flaring process vapors in the A-plant................................................................................ 33.2 Flaring process vapors in the B-plant................................................................................ 53.3 Purging on flare systems .................................................................................................. 53.4 Flaring of liquid ammonia.................................................................................................. 63.5 General arrangement flaring system.................................................................................73.6 Design and safety aspects................................................................................................8

4. Absorbers on process tanks ................................................................................................... 94.1 Off-line urea storage tank ................................................................................................. 94.2 On-line urea storage tank ...............................................................................................11

5. Conclusions.......................................................................................................................... 12

Attached hereto you find Part 2 prepared by Scot Smith, Zeeco Inc. and S. Raghunathan, PIC,Kuwait.

All technical and other information contained herein is based on general Stamicarbon/DSMexperience and within this limit is accurate to the best of our knowledge. However, no liability isaccepted therefore and no warranty or guarantee is to be inferred. Copyright Stamicarbon B.V.


1.

1. Introduction

Reducing emissions from a fertilizer complex is nowadays becoming more and more important.The increasing awareness of the world community that the environment is influenced by humaninterference results in an increasing attention for the emissions of any kind and the drive toreduce these to a minimum. This is valid not only for West Europe but also for all other parts ofthe world.

For urea projects we experience the requirement for offering the best available technology foremission reduction however the form may differ for the individual projects. Stamicarbon hasover the years realized many different configurations designed for controlling emissions of allkinds. The final design for a specific project is however depending on the applicable localauthority requirements and the philosophy of the end customer. In addition also some economicfeasibility is considered in the final selection of the configuration. At this point in time we do notexperience that one uniform configuration for the minimization of all emissions is developing.

In this paper the design for the Petrochemical Industries Company (PIC) Kuwait, Zero AmmoniaEmission Project is presented.


2.

2. The PIC Zero Ammonia Emission Project

Petrochemical Industries Company (PIC), Kuwait is currently operating two Ammonia Plantsand three Urea Plants for the manufacture of liquid ammonia and Urea granules. The totalcapacities of Ammonia plants and Urea Plants are 1880 MTPD and 3150 MTPD respectively.Ammonia Plants are based on M/s Haldor Topsoe technology and Urea Plants are based onStamicarbon Stripping technology. The first Ammonia/Urea plants were commissioned in theyear 1966. In 1970, two more ammonia and two Urea plants were added. With the installation ofanother ammonia plant in the year 1984, the production capacity of PIC's Ammonia and Ureacomplex then became the biggest in the Middle East.

PIC has always been very proactive in working with the community to meet or exceed allenvironmental requirements. The goal of the facility is to always be ahead of any mandatoryrequirements.

Traditionally, the disposal of ammonia containing process gases from emergency relief systems(safety valves or rupture discs) in Urea plants has been done by discharging directly toatmosphere. This was the accepted practice for this type of facility, and continues to be theaccepted practice at many locations around the world. Although direct discharge to atmospherecan be done in a safe way, it causes considerable pollution to direct plant environment.PIC as a socially responsible corporate citizen decided to eliminate ammonia pollution bothinside and outside the PIC complex for such a discharge by implementing the “Zero AmmoniaEmission Project”.


3.

3. Flaring process vapors from safety relief devices

After thoroughly analyzing the pros and cons of absorption and flare systems for the disposal ofNH3-CO2-H2O mixtures discharged from emergency safety relief devices, the flaring wasselected as the appropriate technology.

The design included following steps:

Design and implementation of a major collecting system for the ammonia sources in thefacility.

Replace rupture discs with safety valves. Review and analysis of the relief devices and sources in the facility to determine the impact

of their relieving into a collecting system in lieu of discharge to atmosphere. Analysis of the possible relief scenarios to determine sizing of the relief system. Analysis of the collecting network for both hydraulic and mechanical considerations.

3.1 Flaring process vapors in the A-plantFigure 1:

In case of blowing-off of a safety valve, flashing liquid or vapor enters the blow-off separator. Ifpresent, liquid will be separated from the vapor and discharged into the drain tank. Theammonia containing vapor is sent to the Flare Main Tip after mixing with support (assist) gas inorder to increase its heating value allowing nearly complete destruction of ammonia bycombustion.

PSVs, rupture discs

Flare Main Tip

Other PSVs

Vent stack(existing)

Ammonia water tank

Support gas

N2

Steam

Recovery

Utility Flare Tip 2

N2

Steam

Blow-off separator

Drain tank

Collectingheader

5m


4.

The blow-off separator is a cyclone type with tangential liquid inlet. It is designed for the tuberupture case of the HP Stripper.The liquid from the blow-off separator is discharged to the drain tank via a 5 meter liquid seal toprevent vapor slip into the drain tank. The flare system and interconnecting piping is designedsuch that the maximum pressure in the blow-off separator does not exceed 0.3 barg. Becauseof this overpressure, the liquid from the blow-off separator will flash in the atmospheric draintank, and the vapor will be directed to a dedicated Utility Tip 2 of flare. The pressure drop of thisutility flare is such that the mechanical design pressure of the tank is not exceeded. However inorder to protect the tank from possible over pressure, a manhole water seal is provided on thetank roof.

The volume of the drain tank is designed to accommodate a complete synthesis drain. Thismight happen at tube rupture in the high pressure heat exchangers.When synthesis liquid flashes to atmospheric pressure, its temperature will drop significantlyand will crystallize. To avoid blocking of the liquid outlet line from the separator to the drain tankand to maintain the exhaust systems free of solid carbamate, a continuous water circulation ismaintained. It serves as dilutant for the urea/carbamate solution flowing to the tank.The initial water volume present in the drain tank is adapted to the total liquid synthesis volume.It secures that after a complete synthesis drain no carbamate crystals are present in the draintank.

In case of contamination of the tank with urea and/or carbamate, the liquid can be recovered inthe process by sending it to the rectifying column. During recovery, the minimum liquid level inthe drain tank should be maintained by supplying fresh steam condensate.The system configuration for the A-plant is illustrated in figure 1. The outlet of the existing ventstack is connected to the main flare header, while the synthesis safety valves are connected tothe blow-off separator.


5.

3.2 Flaring process vapors in the B-plantFigure 2:

As can be seen from figure 2, all exhaust lines from existing safety valves in this plant are nowrerouted to the new collecting header. In the original design, all process safety valves (exceptsynthesis PSVs and rupture discs) were connected to the vent stack. The bottom of this ventstack is at grade level, and as such there was no gravity overflow to the ammonia water tank ascompared to the A-plant. Scenarios were possible to overfill the stack and consequently theflare with liquid in case of blowing-off of the safety valves on the carbamate pumps for anextended period of time. A detailed check also indicated that the existing vent stack will not bestructurally sound in case of stack over fill. Hence it was decided to cancel the existing ventstack and reroute all the safety valves to the collecting header including the new safety valvesreplacing the rupture discs.

3.3 Purging on flare systemsTraditionally, the safety valves of the reactor and the synthesis rupture discs are not connectedto the vent stack but have a short exhaust pipe. The main reason for this design is to ensurethat the safety valve outlets are not obstructed by solid carbamate as safety valves might leak.The short exhaust also allows visible inspection.In the new configuration, all safety valves are connected to the collecting header.

Blow-off separator

MIN.

Synthesis PSVs

5m

Support gas

Other PSVs

Drain tank

Circulationpump

Col

lect

ing

head

er

levellevel

N2Steam

Utility Flare Tip 2

Recovery

Flare Main Tip

Dilutingliquid


6.

Figure 3:

To ensure that the exhaust systems are free of solid carbamate, each safety valve is providedwith a purging system, see figure 3.Blow-off lines sloping upwards are purged with water, while blow-off lines sloping downwardsare purged with steam. Each purge is provided with a flow meter and a high priority low flowalarm in the DCS.With a view to eliminate problems associated with PSVs passing, the flare header temperature(measured close to flare stack entry point) is maintained at 100oC with injection of auxiliary LPsteam. The injection of this auxiliary steam is regulated with a temperature control loop. Theflare header is also completely steam traced and insulated to minimize LP steam consumption.

For safety reasons, the ingress of oxygen (air) into the flare system should be avoided. Themaximum permissible O2 content in flare header is 8 vol.% to eliminate possibilities of explosivemixture formation in the flare header. This is achieved by installing purge seals in each of thethree tips of the Flare. Nitrogen gas is used as purge medium.

3.4 Flaring of liquid/vapor ammoniaAs might be known, liquid ammonia and water have a strong affinity. A sudden release of liquidammonia into water produces enormous pressure waves. Hence to protect the drain tank,safety valves blowing-off pure liquid/vapor ammonia are treated different as compared toprocess safety valves discharging NH3-CO2-H2O mixtures.

The following safety valves are present at PIC: Safety valves on HP Ammonia pumps Safety valves on Ammonia refrigeration unit of Granulation plant Safety valve on HP Ammonia heater

MIN.

Water flush

Col

lect

ing

head

er

Safety valve

Steampurge

Waterflush

To blow-off separator


7.

In case of blowing-off of the above safety valves, only part of the ammonia will evaporatedownstream the safety valve. The majority of the ammonia will cool down to a temperature of-33°C. The time needed to evaporate all ammonia depends on insulation and the ambienttemperature.

Figure 4:

In order to avoid carry over of liquid ammonia to Flare Utility Tip 1 and to accelerate theevaporation of liquid ammonia, the blow-off lines from the safety valves are connected to aheated ammonia knock-out drum.

The volume of the drum has been optimized taking into consideration the liquid hold-up for eachblowing-off case. However in order to preempt overfilling of the knock-out drum andconsequently the flare header, an interlock has been incorporated to cut-off ammonia feed toUtility Tip 1 of the flare system, which does not require support gas.

3.5 General arrangement flaring systemFor both plants, the flares will be installed on the top of the prilling towers, which are no longerin service. The flare main tip will use natural gas as a support gas for combustion of the ventgases from the blow-off separator. There will be one utility flare tip for the vent gas from theliquid ammonia systems and one for the vent gas of the Drain tank.

To safe location

NH3 pumps PSVs

Utility Flare Tip 1

AmmoniaKnock out drum

NH3 heater PSV

LP steam

N2

NH3 refrigeration PSVs


8.

Figure 5:

3.6 Design and safety aspectsSpecial attention has been given to several safety aspects related to the design and installationof the flare system:

The design shall ensure that there will be no oxygen in the vent gases to the flares. Toavoid oxygen ingress in flare tip, the system shall be kept above 100°C with steam purges.The steam serves not only for inertization but also heats the system to preventcrystallization. The headers and the flare stack are steam traced and insulated. In additionnitrogen is also injected into the flare system to maintain a positive inert gas flow throughthe seal installed at the flare tip.

Introduction of natural gas in the urea plant will require specific measures: Contamination with other media in the urea plant shall be avoided Area classification of the flare area and subsequent compliance of equipment with Ex

requirements. Heat radiation levels have to be taken into account.

Reliable sensors are required to detect the presence of ammonia in the main flare system.They will trigger via interlock the supply of support (assist) gas.

Natural Gas Detectors shall be installed in the Main Tip flare header to monitor thepresence of Natural Gas in the header.

Wide range of possible compositions The vent gasses from the urea plant vary widely in ammonia concentrations and quantities. A thorough evaluation is required for the correct design of the flare. In all cases the connections to the collecting header are of the Y type in order to limit

excessive forces on the piping and minimize frictional pressure drop.

PilotPilot’s

Flare Main Tip

Utility Tip 2Utility Tip 1

Pilot gas

N2

NH3 Vapor fromKO drum

NH3 - N2 - H2O vaporfrom blow off separator

Support gas

N2

NH3 - N2 - H2O mixturefrom drain tank


9.

4. Absorbers on process tanks

At PIC, a number of atmospheric process tanks are available for intermediate storage of ureasolution and ammonia water. For flexibility reasons, those tanks are connected to both plants.It is common practice that during granulator cleaning the urea production is temporarily stored inprocess tanks. In general, overcapacity in the granulation unit is present to recover the solutionin the tank once the granulation is on-line again.While storing urea solution in process tanks, emission of ammonia is unavoidable due to thebreathing of the tank (continuous rise of liquid level) and hydrolysis of urea to carbon dioxideand ammonia and the formation of biuret.

The possibilities for flaring those vapors were explored but rejected for the following reasons: Some process tanks were located far away from the proposed location of the flare. This

resulted in long lines which had to cross roads etc. The mechanical design pressure of a number of process tanks was limited to 150 mm H2O

which was lower than the required back-pressure from the flare. Since emission of ammonia vapors from the tanks is almost always continuous due to urea

solution transfer to the tanks during plant upsets and / or Granulation plant shutdown,installing a flare to destroy ammonia will not be a meaningful proposition.

Hence it was decided to design “low pressure drop” absorbers to avoid any ammonia emissionto the atmosphere.

4.1 Off-line urea storage tankAs already mentioned before, ammonia emission appears due to hydrolysis and breathing whenurea solution is stored during upsets in back-end section of urea plant and cleaning of thegranulator. In order to make a basis of design for the absorber, a dynamic simulation was madeto calculate the emission of ammonia from the tank.This is a rather complex calculation, as conditions change in time.

For designing the following basis was set: Tank volume 650m3

Initial solution temperature is 100°C Adiabatic conditions (no heat release from the tank)


10.

Figure 6Emission urea storage tank

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60

time (hours)

kg/h

86

88

90

92

94

96

98

100

degr

. C

NH3

CO2

H20

Total

Temp

From figure 6 can be seen that the endothermic hydrolysis reaction causes a continuous drop insolution temperature lowering the reaction rate. Because of the azeotropic behavior of mixturescontaining urea, water, carbon dioxide and ammonia, the emission from the tank shows amaximum as a function of time. Typically, the ammonia emission reaches its maximum afterstorage of about 15 hours.

The design of the absorber system, see figure 7, was a challenge because of the maximumallowable pressure in the tank. The tank is foreseen with a water seal that breaks at an overpressure of 150 mm H2O.

Figure 7:

Urea solution

Processcondensate

Clean processcondensate

Urea solution tank

Vent absorber

LPV

Purge

Circulationloop


11.

By selecting low pressure drop packing and adapting the absorber design, we succeeded toachieve a maximum pressure drop of 35 mm H2O at design load in this absorber system.The expected emission of ammonia at design load is 0.0 kg/h.This absorber system will only be in operation during granulator cleaning and tank recovery.

4.2 On-line urea storage tankDuring process upsets, it appeared that the vacuum pressure in the flash tank could no longerbe maintained resulting in emission of ammonia from the urea storage tank. Especially when thestripping efficiency drops, the ammonia vapor pressure from the flash tank off-gas becomeshigh, and it will be hard to condense the vapor at sub atmospheric pressure. When the liquid inthe flash tank becomes above the atmospheric pressure, flashing will take place in theatmospheric storage tank resulting in ammonia emission.

Figure 8:

The basis for designing the absorber was a complete opening of the liquid level control valve(air failure to open) in the liquid outlet of the rectifying column. In this case, the tank water sealbreaks which is not acceptable for the PIC case. To limit the maximum amount of vapor cominginto the tank, a safety valve with a set pressure of 0.5 barg is installed on the flash tank. Byproviding a 6 meter liquid seal in the liquid outlet of the flash tank to the urea storage tank,direct vapor slip to the storage tank was no longer possible.The maximum vapor from the storage tank is now limited by flashing liquid from 0.5 barg toatmospheric pressure. After absorbing, the ammonia emission to the atmosphere is negligible.

Processcondensate

Desorbercondensate

Absorber

To main flare

6m

LPV

PSV

Flash drum

0.5barg

N.L.L.N.L.L.

Condensers

LPV

Circulationloop

Urea Solution Tank


12.

5. Conclusions

The PIC Zero Ammonia Emission Project is a good example how all the continuous anddiscontinuous ammonia emissions from the urea plant can be captured and disposed off in acontrolled manner making use of either flares and/or absorbers. One should realize that thedesign of the different systems is a rather complicated matter as we have tried to explain in thispresentation.

At the start of such project the requirements of the local statutory authorities and the plantowner will have to be clearly defined. During the development of the design, regular meetingsare required between the parties involved, in order to clarify what is required and to align theopinions on the different subjects that will come across in this phase. Also the required life cyclecost is an issue to be considered in these meetings.

The PIC Zero Ammonia Emission Project proves that flaring is an appropriate technology fordisposal or destruction of ammonia venting when plant disorder happens without disturbing theneighbours.


Eleventh StamicarbonUrea Symposium 2008

Paper 12 (Part 2)

NH3 – CO2 – H2O mixtureflaring to reduce emissions

Authors:Scot Smith, Zeeco Inc., USAS. Raghunathan, PIC, Kuwait


Table of contents (Part 2) Page

1. History....................................................................................................................................12. Ammonia testing..................................................................................................................... 23. Assist gas injection system for main flare tip...........................................................................54. Flaring equipment for PIC....................................................................................................... 6

All technical and other information contained herein is presented by their authors, at their risk andresponsibility. Stamicarbon is not responsible for the contents of any presentation, the correctnessthereof and no right or license as to the application thereof can be deemed to have been granted.However, no liability is accepted therefore and no warranty or guarantee is to be inferred.


1.

1. History

Flares have been used for the disposal of ammonia vapors since the early 1970’s. Many of theammonia storage tanks that were installed in the 70’s, and 80’s were equipped with small flaresthat were designed to combust the vapors that were generated due to breathing and loading ofthese large storage tanks. The flares were generally small, 2 inch to 4 inch in size, and thedesign flow rate was also very small.

During this time, there was little definitive information about the efficiency or effectiveness ofusing flares for the destruction of ammonia vapors. Available testing information generallyindicated that 100% pure ammonia vapors could not be burned in a flare system.Engineering design guideline documents and books on the handling of ammonia vapors forrefrigeration processes almost universally agreed that flaring of 100% ammonia vapors was notpossible, as ignition of the ammonia could not be maintained unless it was premixed with air.

Flare vendors typically approached the flaring of low btu gases, including ammonia, as follows:

1. Design the flare tip barrel diameter and exit area for the available pressure drop andresultant vapor velocity at the tip.

2. Include in the design some type of gas assist injection ring at the flare tip exit point tocreate a fire the ammonia vapors can pass through.

This was the basic design concept for flaring of low btu and ammonia vapors for most flarevendors until the early 1980’s. This is still the concept that is used by some flare vendors today.For destruction of ammonia vapors, this concept has been fully proven to be not effective. Theinjection of assist gas at the flare tip outlet has been shown to actually reduce the destructionefficiency of the ammonia vapors.

During the mid 1980’s there were multiple tests conducted in the USA by the EPA authorities onthe destruction of low heating value gases in flares. These tests were performed on a largevariety of gases, using CO2 and Nitrogen as the inerts. The basic conclusion was gases withlower heating values of 200 btu/scf or higher could be combusted to high efficiency in an openflame flare system.


2.

2. Ammonia testing

The first definitive testing of the combustion of ammonia vapors in a flare tip was performed inthe early 1980’s.The testing was performed for a major chemical company located nearHouston, Texas, USA. The chemical company was expanding their facility, and needed tomodify their permit for flaring to increase the total allowable emissions.

The local environmental authorities would not allow the facility to increase ammonia emissionsfrom the flare. Therefore, to expand the capacity of the facility, the company had to either installincineration equipment for the vented ammonia vapors, or prove the destruction efficiency ofammonia in the flare was actually higher than as shown in the current permit.

The flare system that was installed at the facility was a standard unit with a gas assist ring tocreate turbulence and to create a combustion zone at the flare tip exit point. The chemicalcompany contracted to determine the effectiveness of this flare tip arrangement versus otherpossible options.

Testing was performed on 100% ammonia gases. The flow rate of the gas was varied toevaluate the influence of exit velocity at the flare tip discharge point on the combustionefficiency.

The testing specifics are as follows: Testing was performed on a nominal 12 inch diameter utility type flare tip with a full

flame retention ring. The following were options that were fitted to the tip as part of theevaluations.

o Extended large diameter windshield assembly that enclosed the discharge of theflare tip and the pilots.

o Gas injection assist ring at the flare tip exit point to produce turbulence andincreased air inspiration into the combustion zone.

o Multiple pilots (three maximum) were available to determine the impact of ignitionflames on the combustion process.

The testing included analysis of the performance of the flare tip assembly using various flows ofammonia, using from 1 to 3 pilots, using a gas injection ring, using the extended windshield, andusing combinations of the above. The amount of ammonia present in the plume from the flarewas determined using a heated probe that sampled in a position relative to the measuredtemperature (to ensure the probe was located in the hottest portion of the plume).


3.

See typical ammonia test flame color in the photo below.

The conclusions from the testing were as follows:

Ammonia will burn to technically complete combustion (99% or higher) if the exit velocityat the flare tip discharge point is kept very low. The acceptable velocity is a function ofthe nominal flare tip diameter. See attached graph for guideline exit velocities.

Higher flare gas exit velocities result in the inspiration of too much ambient air into thecombustion zone, which dilutes the ammonia / air mixture to below the combustible limit.Ammonia has a lower explosive / combustible limit that is 16% in air. This is incomparison to most hydrocarbons that have LEL values that are from 1 to 3%. Thismeans the ammonia and air mixture can easily be diluted to a point where the ammoniawill not burn.

Ammonia needs to have a good source of ignition. This is typically provided by a veryreliable pilot flame, and also a sufficient number of pilots around the perimeter of theflare tip. During the testing, if the ignition source was removed, the ammonia would notsustain a stable flame.

A windshield is very useful in limiting the amount or air inspirited into the ammonia flaregas stream to facilitate ignition of the gases in an area that is protected from crosswinds.


4.

Burning of the ammonia vapor eliminates any ammonia smell. This process will alsoproduce NOX. One mole of ammonia will produce one mole of NOX. The temperature ofcombustion in an ammonia flame is actually much lower than in a hydrocarbon flame.The NOX produced will typically be colorless, NO and NO2.

From this testing, it was concluded that ammonia can be burned in a flare system with very highefficiency, if the flare system is designed correctly.

Please note there has been other testing in the industry that has shown different results.Testing that was performed for the US EPA by EER in approximately 1986 in their flarescreening facility showed that ammonia could not be ignited and would not burn. However, thattest was performed on a 1/16 inch diameter orifice with the ammonia exiting the orifice at a highvelocity and with no continuous pilot for ignition. Therefore, this test was not a viable reflectionof the performance of an actual flare tip assembly.

Testing of other mixtures of low heating value gases has confirmed a lower limit ofapproximately 200 btu/scf for the efficient combustion of gas mixtures. The ammonia can bemixed with inerts down to a minimum heating value of approximately 200 btu/scf. If the lowerheating value is not maintained at this minimum level, assist gas must be injected / added to theflare header to ensure the lower heating value is maintained at 200 btu/scf or higher. The assistgas should be controlled to ensure this minimum lower heating value. The gas can be controlledusing a ratio controller as a function of the flare gas flow rate.


5.

3. Assist gas injection system for Flare main tip

Pressure indicators, temperature indicators and ammonia analyzers are installed in the flare gasheader of Main tip to sense the opening of any PSV’s into the flare header. When any one ofthe above process sensors is activated, assist gas will be automatically injected into the flaregas header at an appropriate location to ensure thorough mixing of NH3-CO2-H2O gas mixturebeing relieved and assist gas for complete destruction of ammonia.

On the activation of the above defined interlock, approximately 3000 Kg/hr of assist gas will beinstantaneously injected into flare gas header. The maximum quantity of 3000 Kg/hr of assistgas was required for one of the PSV discharge cases whose lower heating value is lower thanthe threshold value of 200 btu/scf. To facilitate instantaneous injection of assist gas into Flareheader on PSV blow-off, quick opening valves are installed in the assist gas line. In addition, aHCV is also provided in the line to remotely regulate the assist gas flow by the operatormonitoring the color of the flame at main tip with the help of the camera provided for thispurpose. Assist gas will be completely isolated once the operating staffs confirm that there is nomore ammonia flow into the flare gas header.

In order to avoid inadvertent closure of HCV by operating staff when quick shutoff valves remainclosed, an additional interlock has been put in place. This interlock will ensure that HCV remainsfully open when quick shutoff valves remain closed during normal operation by inhibiting theoperation of HCV by operating staff.


6.

4. Flaring equipment for PIC

For the PIC application in Kuwait, there are two (2) separate plants, Urea Plant A and Plant B.Taking into consideration the locations of Urea Plants A & B and pressure drop constraints, itwas decided that each plant would have dedicated flaring systems. Further, in each plant, thereare three (3) collection systems for the ammonia vapors: NH3-CO2-H2O mixtures from PSVs,pure ammonia vapors from PSVs, and NH3-CO2-H2O mixtures from the Atmospheric Drain tank.Each flare system (Plant A and Plant B) consists of three Flare Tips, one tip dedicated for eachcollection system.

Main flare tip for NH3-CO2-H2O mixture from PSV Vent Header Utility Flare Tip 1 for 100% ammonia from PSVs Utility Flare Tip 2 for NH3-CO2-H2O mixtures from the Atmospheric Drain Tank Vent

It was a major challenge for PIC to identify a suitable location for the new Flare System in theexisting Urea Plants, as there is not an obvious location with enough space either within theplant or in the adjoining areas. After analyzing the merits and demerits of the limited alternativesthat were available, PIC made the decision to mount the flare systems on the top of the existingprilling towers that are located in Urea Plant A and Urea Plant B. Prilling towers were used inthe process in the past to produce prills (pellets). The prilling towers are large diameter,concrete structures and are approximately 45-60 meters overall height above grade. Thestructures are approximately 16-21 meters in diameter, and have a flat concrete top.An engineering study was performed and it was determined the prilling towers had sufficientstrength to support flare stacks. By placing the flare stacks on the top of the prilling towers, anyand all possible problems due to radiation heat from the flare flames to personnel at grade waseliminated and in addition the need for an elaborate structure to install a tall self supportingstructure in a grass root location was avoided.

As the prilling towers are quite old, and were originally not designed to have this additionalweight and wind load from flare stacks, the possible height of the flare stacks on top of thetowers was limited. Based on the load stability calculations for the existing Prilling Towersinclusive of the new flare stacks, the heights of the new flare stacks were limited to 10 metersfor Urea Plant A and 15 meters for Urea Plant B. At the top of the prilling tower in Urea Plant B,other process equipment, instrumentation, and electrical equipment are installed, and hencefrequent plant personnel movement will be present. As the flare stack heights on the top of theprilling towers are limited due to structural constraints, the radiation levels at the top of thePrilling towers can be quite high. In addition, depending on the duration of the relief event, theassociated temperatures can also be quite high.

Radiation was analyzed using Zeeco proprietary software, and also using the industry availableFlareSim software. Analysis was performed for several different flaring relief loads, severalcombination of flaring relief loads, and also for several wind speeds. For some flaring cases, theradiation levels at the base of the flare stacks on top of the prilling towers could be as high as3,000 btu/hr-ft2.


7.

Note the recommended limits for radiation from flare systems for personnel access and forequipment are defined in the latest edition of the API RP-521 Recommended Practice “Guidefor Pressure-Relieving and Depressuring Systems”.The latest edition is Edition 5 from January of 2007. The recommended radiation levels fordesign and application of flare systems is as shown in the table below. Note the maximumallowable value for personnel actions lasting only a few seconds is 3,000 btu/hr-ft2. In reality,this predicted radiation level is extremely high, and therefore PIC determined to apply radiationshielding to all areas in the plant including the top of the prilling tower with radiation levels thatcould endanger personnel. In addition, additional protective measures are envisaged for theinstrumentation and electrical items that could be impacted by the flare flame radiation levels orresultant temperatures.


8.

A typical radiation graph output from the FlareSim software for one of the ammonia flaring caseswith injection of assist gas at a nominal 14 meter per second wind speed is shown above. Eventhough ammonia has a very low heating value, and a low emissive value, the radiation levelscan be very high.

As noted before, the flare systems for each plant consist of three separate flare tips.The actual equipment that is being supplied for each plant consists of:

Flare Tip Assemblies with Pilots and Windshields.o Nominal 78 inch diameter MAIN flare tipo Nominal 48 inch diameter UTILITY 1 flare tipo Nominal 14 inch diameter UTILITY 2 flare tip

Flare support structure suitable for three (3) flare tips. Purge gas flow reduction seal devices.

o Molecular type seal device for the MAIN flare tipo Velocity type seal devices for the UTILITY flare tips

Utility piping and supports along the flare stack to flare stack base Retractable thermocouple assemblies from each pilot to the stack base Flare pilot ignition and status monitoring rack assembly located at grade at the base of

the prilling tower. The FFG type pilot ignition system is designed to control all of thepilots on all flare tips.


9.

The support structure was designed to support all three (3) flare tip assemblies. In addition, thesite was reviewed and the wind rose information for each site was obtained to ensure theorientation of the flare tips provided the least amount of flame impingement from one flare tip tothe adjacent flare tip. The three (3) flare tips are mounted on the stacks in a line, and thatcenterline is perpendicular to the predominant wind direction for the jobsite. See the picturebelow, both elevation view and top view, to understand the arrangement of the stack.

The flare stacks are equipped with ladders and platforms for access to the flare tips and pilots,aircraft warning lights and heat shields for same to warn aircraft in the area, and also withthermocouples that are retractable to the base of the stack for easy maintenance andreplacement. The flare stacks including flare tips are identical in all respects except the height.As already noted, the height above the prilling tower is 10 meters for Urea Plant A and15 meters for Urea Plant B.

Purge gas flow rate reduction devices are included in each of the flare tip assemblies.The MAIN flare tip has a diffusion or molecular type purge gas seal device. The UTILITY flaretips each have a velocity type purge gas seal device. These devices are applied as a function ofthe operation of the flare headers, the flare gas relief frequency, the required protection level foreach flare, the amount of purge gas that is required for the nominated flare tip assembly, cost ofpurge gas, etc.


10.

Purge gas seal devices are described in detail in the latest edition of API RP-521 in section4.4.3.4.2. The velocity seal devices are designed as an integral part of the individual flare tipassemblies. The diffusion seal device for the MAIN flare is designed as an integral part of thesupport flare stack structure. These will not be discussed in detail in this paper.

The seal device types are depicted below:

The flare tip assemblies are designed with extended windshields, lifting lugs for handling, fullflame retention ring for flame stability, and with multiple pilot assemblies for ignition.


11.

See the diagram below.

The Zeeco pilot assemblies are designed to remain ignited and burning in a wind speed up toand including 67 meters per second, or in a combination of a wind speed of 50 meters persecond simultaneous with a rain storm that is producing rain at the rate of 100 millimeters perhour. The Zeeco pilot assemblies are designed and fabricated from stainless steel materials andcastings that are very suitable for the anticipated temperature extremes likely from a large flaresystem. Each pilot is monitored by a type K thermocouple in a cast thermowell.


12.


13.

The pilots are ignited and monitored from a pilot ignition rack assembly that is located at gradeat the base of the prilling tower. The ignition rack assembly provides for the manual ignition ofthe pilots, and also monitors the pilot status and will automatically relight any pilot that givesindication of not burning. The ignition rack assembly includes controls for regulating the pilotfuel gas pressure. The rack assembly also includes the controls for the aircraft warning lightsthat are mounted on the flare stacks. The rack assembly includes a sunshield for operatorprotection and to keep the direct sun off the control enclosures and control components. In theevent all power is lost to the FFG rack assembly, there is also a piezoelectric spark device thatcan be used to create a spark and generate a flame front that would travel up to the pilots toprovide ignition.


14.

Zeeco Automatic / Manual FFG pilot ignition Rack Assembly

ammonia flaring

Documents

aircraft warning lights

flare tip assemblies

complete synthesis drain

main flare tip

flare main tip

flaring relief loads

zeeco pilot assemblies

existing vent stack