How Green is the Stamicarbon Urea Process v2 w

8/12/2019 How Green is the Stamicarbon Urea Process v2 w

1/18

Author:

Alfi Soemardji, MSc,Process Engineer

January 2012

Stamicarbon B.V., the Netherlands

HOW GREEN ISTHE STAMICARBON

UREA PROCESS?


2/18

Table of contents Page

1. Abstract ................................................................................................................. 1

2. Preface .................................................................................................................. 1

3. Introduction ............................................................................................................ 2

4. Emission sources in the urea plant ........................................................................ 2

5. Emission reduction in the urea plant ...................................................................... 35.1. In-line solution .................................................................................................... 35.2. End-of-pipe solution ........................................................................................... 3

5.3. Absorber ............................................................................................................ 35.4. Acidic Scrubber .................................................................................................. 75.5. Flare ................................................................................................................... 95.6. Emergency Absorber ....................................................................................... 13

6. Green Stamicarbon urea process ........................................................................ 14

7. Conclusions ......................................................................................................... 14

8. References .......................................................................................................... 15

All technical and other information contained herein is based on general Stamicarbon experience and

within this limit is accurate to the best of our knowledge. However, no liability is accepted therefore

and no warranty or guarantee is to be inferred. Copyright Stamicarbon BV. All rights reserved. No

part of this publication may be reproduced in any form or by any means without the permission of

Stamicarbon BV. You will access its contents solely for your own private use and will comply with all

applicable laws and regulatory requirements relating to your use of this information.


3/18

1

1.AbstractThe Stamicarbon urea process, like other processes, unfortunately emits gaseous(ammonia) emissions due to the fact that the urea reaction is an equilibriumreaction. In principle, emission problems can be solved in two different ways, eitherwith an inline and/or with an end-of-pipe solution. The combination of theequilibrium reaction and the presence of inerts make an in-line solution notfeasible. Therefore end-of-pipe solutions to eliminate ammonia emissions in bothurea melt and finishing plants are still needed to obtain a green andenvironmentally sustainable process.

This paper covers the different available end-of-pipe solutions such as absorbers

(including emergency absorber), acidic scrubbers and flares. These options have awide range of operating windows for further optimization and achieving optimumenvironmental performance. Flaring reduces the ammonia emission by convertingthe ammonia into carbon dioxide and nitrogen oxides (NOx). Therefore, theenvironmental impact evaluation - after implementation of the new end-of-pipesolution - needs to be reviewed, especially when applying continuous emission flarebecause the environmental impact of these new type emissions are not moretolerable than ammonia emission.

Stamicarbon believes that a green urea process combines optimum processconditions with a good choice of end-of-pipe solutions. The available alternatives forend-of-pipe solutions target an optimum Triple P balance, i.e. the balancing

between financial-economical achievements (profit), environmental impact (planet)and public acceptance (people).


4/18

1

2.PrefaceStamicarbon, the innovative, experienced and reliable licensor

Stamicarbon, founded in 1947, is the global market leader in the development andlicensing of patented urea technology.It sells proprietary know-how and delivers services to existing and prospective ureaproducers. It has licensed over 250 urea plants located in over fifty countries.Furthermore it has completed over 90 revamp projects in Stamicarbon and non-Stamicarbon plants.

Stamicarbon grants licenses for new urea plants, providing the urea producer with

all the knowledge and resources to build a reliable, state-of-the-art, low cost andprofitable urea plant. Besides new plants we also take care of revamping existingplants, increasing capacity and improving efficiency with several debottleneckingservices and tools. Stamicarbons is committed to Full Lifecycle Support, helping theurea producer to get maximum return on investment for 30 years and more. Fulllifecycle support comprises from planned inspections and maintenance,troubleshooting, to round-the-clock emergency support. Reliable and durableequipment is integral to the success of urea production. Thats why Stamicarbonsupplies critical high-pressure equipment for an optimum performance for amaximum lifespan.

Innovations

Innovation drives performance in the fertilizer industry. Stamicarbon maintains itsleading position by its continuous high-quality innovations in close cooperation withresearch institutes, suppliers and customers.This has lead to developing breakthrough innovations, amongst others contributingto investment cost reduction and operating costs reductions: AVANCOREurea process (savings on plant height and equipment) Mega Plant Technology (economy of scale) Urea 2000plus Technology (integrated condenser / reactor and savings on

plant height and piping) Urea Granulation Technology (formaldehyde just 0.3 wt% and run times >100

days) Safurexstainless-steel material (savings on materials, equipment and

maintenance)

Stamicarbon BV

Mercator 2, 6135 KW SITTARD

P.O. Box 53, 6160 AB GELEEN

The Netherlands

Telephone +31 46 4237000

Fax : +31 46 4237001

[email protected]

www.stamicarbon.com


5/18

2

3.IntroductionEnvironmental issues have been high on the agenda in global discussions for sometime, and it is becoming more and more important especially for productionfacilities. Urea production facilities are no exception in this respect and operatingcompanies will have to respond more actively to environmental issues, either as aresult of local or global regulations or as a result of pressure from the commercialor public domain. As one of the leading licensors, Stamicarbon is continuouslyworking on these subjects by making its process greener and to create asustainable green urea process as the ultimate goal.

Stamicarbon urea process, as any other processes, unfortunately emits small

amounts of reactants and dust. In particular ammonia is subjected to more andmore environmental concerns. For the urea process, we offer the best availabletechnology for ammonia emission reduction, the optimal solution differing fromproject to project. The final design for a specific project depends on the applicablerequirements from local authorities, the philosophy of the operating company andof course the economical feasibility.

The ammonia emission reduction in urea plants and compliance with environmentalstandards are becoming an important qualifier for technology licensors, but alsorepresent our own responsibility to strive for a sustainable future. This paper showsa framework on gaseous emissions in urea plants and its prevention, in both meltand finishing plants.

4.Emission sources in the urea plantThe gaseous emissions can be distinguished into two different types, discontinuousand continuous emissions. Discontinuous emissions usually originate from processsafety valves, rupture discs, breathing vents and drain valves. Continuousemissions originate from the fact that inerts are introduced in a urea plant with theraw materials and by using air. Such inerts include nitrogen, hydrogen andmethane. The fact that inerts need to be purged makes emissions (incl. ammonia)inevitable unless these emissions are subjected to treatment.


6/18

3

5.Emission reduction in the urea plantIn principle, there are two types of solution that can be applied here; in-line andend-of-pipe solutions.

5.1.In-line solutionUrea is commonly made by applying the following reactions path.

2 NH3+ CO2 NH2COONH4NH2COONH4 CO(NH2)2 + H2O

2 NH3+ CO2 CO(NH2)2 + H2O

It is an equilibrium reaction, which means that this reaction is reversible and willnot reach 100% completion. Therefore, in a urea plant, all components from thechemical reactions above are present, that is reactants, product and intermediate.Part of the un-reacted (remaining) reactant, especially ammonia, if not subjected tofurther treatment, would be purged with the inerts. It is this combination of theequilibrium reaction and the presence of inerts (at least from the raw materials)that makes an in-line solution not feasible.

5.2.End-of-pipe solution

Some alternative end of-pipe solutions are available, depending on the type of theemission, as shown in Table 1.

Type of emission Melt Plant Finishing Section

Continuous AbsorberAcidic ScrubberFlare

Acidic Scrubber

Discontinuous FlareEmergency Absorber

-

Table 1. Possibilities of End-of-Pipe Solution in a Urea Plant

5.3.AbsorberIn the Stamicarbon melt plant; there are two types of absorbers, the Low Pressure(LP) Absorber and the Atmospheric Absorber. The main difference between thesetwo absorbers is the operating pressure. Both absorbers are used for bringing theammonia from the gas phase to the liquid phase by washing it with water (liquid).


7/18

4

LP Absorber

The LP absorber operates at a low pressure of about 4 bar; it absorbs the ammoniathat still remains in the vent gas from the high pressure scrubber. The column hastwo packed beds, top and bottom beds, as can be seen in Figure 1. The vent gaswill enter at the bottom of the column and be washed directly with cold ammonia-water while passing through the bottom bed. The remaining gas will go further tothe top bed where it will be washed with cold steam condensate. The remainingvent gas, which exists mainly of inerts, will be then released into the atmosphere.The liquid that comes out from this absorber contains some ammonia, but it stilllean enough to be used as wash water for the atmospheric absorber.

Figure 1. LP Absorber

According to Stamicarbons experience, operating this absorber at 4 bar is theoptimum condition when comparing the investment cost of the equipment and theimprovement on the ammonia emission that can be achieved. The maximumpressure of this absorber will be slightly higher than 4 bar, being about 6 bar. Incase it is needed, operating it at 6 bar will give some further reduction of theammonia emission by approximately 50%. Increasing pressure will result in areduction of ammonia emission until a maximum is reached. However Figure 2shows that beyond a certain pressure emission level is hardly reduced further.

Figure 2. Effect of Column Pressure on the Ammonia Emission from the LP Absorber

AmmoniaEmissionLevel

Column Pressure


8/18

5

The flow and temperature of the steam condensate are two other parameters that

could be used for further reduction of the ammonia emission, as shown in Figure 3.The steam condensate temperature that we chose is around 40oC, since usuallythere is no possibility to cool this condensate to a lower temperature. When there isa refrigeration facility available (usually in the ammonia plant), extra cooling willbring down the ammonia emission further. A higher flow of steam condensate willof course result in better washing of the ammonia into the liquid phase. On theother hand, this extra condensate flow means an extra load going to the wastewater treatment section.

Figure 3. Effect of Steam Condensate Condition on the Ammonia Emission from the

LP Absorber

The main emission from this absorber is the inerts. Reducing the inertconcentration in the system will therefore reduce the emission. The inertconcentration can be reduced by using purer raw materials and by reducing theamount of passivation air. The amount of passivation air can be reduced by using abetter corrosion-resistant material, for example Safurex.

Atmospheric Absorber

This absorber is designed to absorb ammonia from the off gases from the lowpressure section using the effluent from the LP absorber. The low pressure of thesegases determines the pressure of this absorber; therefore pressure is not a

parameter anymore to reduce the emissions.

As in the LP absorber, this column also has a top and a bottom bed. The gas feedenters at the bottom of the column and is washed by the re-circulated and cooledbottom effluent of the same column. The remaining gas is then washed in the topbed with the effluent from the LP absorber, before being released into theatmosphere. The bottom effluent of this absorber contains absorbed ammonia andis sent to the waste water section, where the ammonia will be recovered forrecirculation to the synthesis section.

The temperature and the flow of the bottom effluent circulation are adjustableparameters for this absorber. There is a limit to the improvement these

adjustments can bring though. A way to reduce the emission further is using amore efficient absorbent, for example steam condensate to the top bed. Thedifferent schemes for this absorber can be seen in Figure 4.

15 25 35 45 55

1200 1700 2200 2700

AmmoniaEmissionLevel

Flow

Temperature


9/18

6

a. Without Steam Condensate b. With Steam Condensate

Figure 4. Atmospheric Absorber

Additional steam condensate, as shown in scheme b, can result in a significantreduction of the ammonia emission from this absorber. Figure 5 Shows that a lowertemperature (< 40oC) of the condensate, in contrary, barely reduces the ammoniaemission from this absorber compared to the steam condensate flow. However, alsohere, additional steam condensate flow means an extra load going to the waste

water treatment section.

Figure 5. Effect of Steam Condensate Condition on the Ammonia Emission from theAtmospheric Absorber

15 25 35 45 55

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0 500 1000 1500 2000

Am

moniaEmissionLevel Flow

Temperature


10/18

7

5.4.Acidic ScrubberWhen very low concentrations of ammonia emission are required, acidic scrubbingcan be applied. By adding an acid to the absorbent, usually nitric acid or sulphuricacid, ammonia is extracted from the vapor phase by chemical reaction to anammonium salt and overhead vapor concentrations of < 30 mg ammonia/Nm3canbe reached. However, acidic scrubbing produces an aqueous solution of ammoniumsalt as a by-product.

Acidic Scrubber for Melt Plant

Figure 6 shows an acidic scrubber as applied by Stamicarbon in a melt plant,scrubbing the overhead vapor of the LP absorber and the atmospheric absorber.

Acidic scrubbing takes place in the bottom bed. In the top bed clean water removesacid from the vapor from the bottom bed (if any). A recycle stream is applied hereto reduce the liquid make-up and the purge of the aqueous solution of ammoniumsalt. This type of scrubbing is very convenient when there is a UAN- or ammoniumsulphate facility on site to discharge the ammonium salt solution to.

Figure 6. Acidic Scrubbing for Melt Plant

Acidic Scrubber for Finishing Section

In the finishing section there are several configurations in which acidic scrubbingcan be applied. Here two examples are given applying sulphuric acid. This acid iseasy to obtain and ammonium sulphate is widely used as a fertilizer, either as aliquid fertilizer or as a solid mixture with urea. In the examples urea dust scrubbingand acidic scrubbing is combined. Other configurations are also possible (likeseparating urea dust scrubbing and acidic scrubbing and using nitric acid) and thechoice of the client is also determined by the opportunities already available on site(like whether or not having a UAN- or ammonium sulphate facility to discharge theammonium salt solution to).

Off gas from

absorbers

Ammonium

salt solution

Cleaned

off-gas

Acid solution

Steam

condensate


11/18

8

Figure 7 shows a Stamicarbon design for acidic scrubbing integrated with urea

ammonium sulphate (UAS) production.

Figure 7. Integrated Acidic Scrubbing and UAS Production

The off-gas from the finishing section is washed in the acidic scrubber usingsulphuric acid. The acid concentration can be varied; lowering the pH of the recyclesolution increases the ammonia reduction efficiency, but at the same time increasesthe hydrolysis of urea. Clean gas goes to the atmosphere via the stack.

Depending on the concentration of the solution from the scrubber, some solidammonium sulphate might need to be added to the mixing tank to obtain a finalUAS melt with a sulphur content of about 5%-w. This solution, with a typicalcomposition of 38%-w urea and 10%-w ammonium sulphate, is concentrated in a

vacuum evaporator to form a melt with water content of about 0.3%-w. This melt isthen shaped in a pelletizer to yield solid UAS pastilles. The vapor from theevaporator is condensed. The off-gas from the condenser is sent to the granulationstack and the condensate from the condenser is returned to the scrubber. The sub-atmospheric pressure in the evaporation section is maintained by an ejector.

Steam

Cond.

Steam

Ammonium

sulphate

Sulphuric acid

UAS product

Cooling

water

Mixing tank

Evaporator

Condenser

Stack

Ejector

Acidic

scrubber

PelletizerCooling

water

Cleaned

off-gas

Off-gasfrom

finishing section


12/18

9

Figure 8 shows a design where in a granulation process, instead of making UAS, the

melt from the evaporator is recycled back to the granulator, resulting in sulphurenriched urea granules.

Figure 8. Integrated Acidic Scrubbing and Sulphur Enriched Urea Production Plant

5.5.FlareWhen flaring is applied in the melt plant distinction is made between continuousand discontinuous flaring.

Continuous Flaring

A continuous flare can be used for all process and breathing vents, for examplefrom the absorbers. The gas streams coming from these vents are collected in ablow-off separator to separate the gas phase from the liquid phase (if any). The gasis then incinerated in the flare and the liquid is recycled back to the ammonia-watertank. Figure 9 shows a scheme for this type of flaring.

During normal operation the continuous emission source will only contain atmaximum about 1%-m ammonia, the rest being mainly nitrogen, oxygen andwater. Due to this oxygen content, a nitrogen stream is added, not only to preventextra oxygen ingress, but also to avoid the occurrence of an explosive mixture in

the system. Therefore quite some support gas is required to incinerate the gasstream coming from this source.


13/18

10

Figure 9. Continuous Flaring System in the Urea Plant

Discontinuous Flaring

Discontinuous flaring can be distinguished by emission source: discontinuousammonia flaring and the discontinuous flaring of other vapors.

The discontinuous ammonia flare is dedicated for pure ammonia emission sources(e.g. from the ammonia pump). The liquid ammonia coming from the pump iscollected and sent to a blow-off separator/evaporator, as can be seen in Figure 10.The bottom part of the separator is heated with steam to release other gases thatare trapped in the liquid and nitrogen is added to the tank to prevent oxygeningress to the system. The gases coming from this separator/evaporator are thenincinerated in the flare and the liquid ammonia can be returned to battery limit.

Figure 10. Discontinuous Ammonia Flaring System in Urea Plant

Pure ammonia

to B.L.

NitrogenAmmonia

Steam

Cond.

Support gas

Nitrogen

Fromprocess

& breathing vents

To ammonia

water tank

Support gas

Ammonia water


14/18

11

Discontinuous flaring of other vapors is intended for all other discontinuous sources

in the melt plant, such as PSVs. These sources are collected in a large blow-offseparator via a collecting header. This collecting header is equipped with a steaminlet in order to keep the line open. The gas from the separator is incinerated in theflare and the liquid is recovered in a separate drain tank. From this drain tank, theliquid is circulated into the system by means of a circulation pump. The scheme ofthis flaring system can be seen in Figure 11. The nitrogen line to the separator, asmentioned before, is to prevent oxygen ingress to the system.

Figure 11. Discontinuous emergency Flaring System in Urea Plant

In general discontinuous emission contains more ammonia than continuousemission. As indication, depending on the source of the emergency blow-off, it cancontain about 30%-m ammonia. Therefore a discontinuous emission has typically ahigher heating value than a continuous emission. This heating value will affect theamount of support gas needed to incinerate the gas, as can be seen in Table 2.

Table 2. Typical required Mass Ratios of Support Gas and Nitrogen to Ammonia forFlaring Emission Sources in a Urea Plant2

SourceDiscontinuousEmission

Continuous Emission

Support gas toAmmonia

1:10 140:1

Nitrogen to Ammonia Very little, only for purge 600:1

According to Table 2, flaring of a continuous emission will typically require 140 kgof support gas for burning 1 kg of emitted ammonia. This ratio is tremendouslyhigh compared to a discontinuous emission. This justifies reconsideration of flaringof continuous emissions, because flaring produces nitrogen oxides (NOx) and extra

carbon dioxide from the complete burning of ammonia, whereas venting will emitmainly ammonia.


15/18

12

Table 3 gives the environmental benchmark indicators used to compare the impact

of different kind of emissions.

Source/ type of gas

Environmental Benchmark Indicator

Global WarmingPotential (GWP)

AcidificationTroposphereOzone FormingPotential (TOPF)

Ozone DepletionPotential (ODP)

Venting/ ammonia 0 1 0 0Flaring/ nitrogendioxide (NOx)

30 1 1.22 0.017

Table 3. Environmental Benchmark Indicators for Venting and Flaring of Ammonia2

- Global Warming Potential (GWP)4indicates the contribution of a specified gas

mass to global warming by comparison with reference gas carbon dioxide. TheGWP has been defined as the ratio of the time integrated warming effect fromthe instantaneous release of a kilogram of a specified gas, compared to that of akilogram of reference gas carbon dioxide. The GWP value represents the globalwarming potential of the gas over one hundred years.

- Acidification is used to describe the loss of nutrient bases in ground or waterthrough the process of leaching and their replacement by acidic elements. Theacidification is indicated by the acid equivalent. The acid equivalent is a measureto determine the environmental impact of the acidifying substances, which aremainly caused by sulfur and nitrogen based molecules. The equivalent

determines the degree of acidity of a substance per mole.

- Troposphere Ozone Forming Potential (TOFP)7equivalent indicates the formationof ozone in the troposphere, causing radiative forcing. The troposphere is definedas the lowest part of the atmosphere from the surface to about ten kilometer inaltitude in mid-latitudes where clouds and weather phenomena occur. Ozone isformed by two preceding pollutants in the presence of ultraviolet sunlight:volatile organic compounds and nitrogen oxides. TOFP uses ratios to convertemission masses to masses of the TOFP equivalent.

- Ozone Depletion Potential (ODP)3equivalent indicates the ability of a specifiedgas to destroy stratospheric ozone by reaction with the ozone to oxygen. TheODP has been defined as the ratio of the total amount of ozone destroyed due toa kilogram of specified gas and the amount of ozone destroyed by a kilogram ofreference refrigerant gas CFC-11 (CCL3F).

Table 3 clearly tells us that flaring of ammonia has a more negative effect on theenvironment than direct venting. Flaring contributes to global warming, troposphereozone formation and ozone depletion, while venting does not. Flaring of ammoniaintroduces a new type of emission for urea melt plants in the form of nitrogenoxides. The only advantage of flaring is that it can be a suitable solution to avoidammonia nuisance to the surrounding area of the plant during upset or emergencysituations.


16/18

13

5.6.Emergency AbsorberThe emergency absorber is designed to reduce emissions resulting from a tuberupture in the synthesis. The schematic plot of this absorber is shown in Figure 6.

At the discharge of a PSV/rupture disc safeguarding against the consequences oftube rupture in the synthesis, a vapor/liquid separator is installed to separate theliquid from the vapor phase. The remaining vapor will go through an absorber bed,before being vented into the atmosphere. A large amount of water (approximately1400 m3in 3 hours), is applied counter-currently through the bed. This reduces theammonia emission after tube rupture from about 100 ton to about 10 ton (for a3000 MTPD urea plant).

Figure 6. Emergency Absorber

In spite of the large amount of stand-by (cooling) water needed for this system, thecollected liquid from the absorber cannot be sent to the sewer directly and since theresulting liquid is very diluted, a large collecting tank is required. From that tankthe solution can (gradually) be sent to a biological waste water treatment or can(gradually) be reprocessed to the urea plant.

An emergency absorber system like this probably will never be used throughout thelife-time of the plant, which is the main reason why this option is rarely installed.


17/18

14

6.Green Stamicarbon urea processResearch shows that the biggest ammonia emission contributor is livestock.Livestock is responsible for about 70% of the total ammonia emission to theatmosphere (data from 2002, measured in mid-Atlantic Northeast), which is about5 times higher than the ammonia emission coming from urea plants. So, ureaplants are not the main contributor of ammonia emission. It is our responsibility toreduce this smaller contribution and so to strive for an environmental sustainablefuture. At the end it is about balancing the feasibility between the Triple Psustainability pillars: the financial-economical achievements (profit), environmentalresults (planet) and public acceptance (people). In all cases, there is always asolution available which we can propose to fulfill the customers requirement.

7.Conclusions1. The fact that inerts are introduced in urea production and the fact that

ammonia is always present in a urea solution/melt makes that the inert purgecontains ammonia, usually requiring end-of-pipe type treatment to keep theammonia emission within acceptable limits.

2. Absorption and acidic scrubbing of ammonia are effective treatment methods

for reducing ammonia emission. Both options are easy to apply and have acertain design window within which investment can be balanced with requiredenvironmental performance.

3. Flaring is an alternative end-of-pipe solution. It reduces the ammonia emission,but generates nitrogen oxides (NOx) emission, which is worse from anenvironmental point of view compared to the ammonia emission.


18/18

15

8.References1. Gevers, Bart, Implementation of Stamicarbons Key technology Developments

in Recent Urea Grass Roots Projects, 2ndGPCA Fertilizer Conference, Doha,2011

2. Dobre, Joey, MSc., Stamicarbons Approach in Mitigating the EnvironmentalImpact of a Urea Plant, 2010 IFA Technical Symposium, Sun City, SouthAfrica, 2010

3. Ravishankara, A. R., et al., Nitrous Oxide (N2O): The dominant Ozone-Depleting Substance Emitted in the 21stCentury), Science Vol. 326, p123,2009

4. Forster, P., Ramaswamy V., et al., Intergovernmental Panel on ClimateChange, Fourth Assessment Report, Changes in atmospheric constituents and

in radiative forcing, Ch 2, 20075. Gevers, Bart, Emissions from Urea Plants: An Update, Uhde Fertilizer

Symposium 2006, Dortmund, Germany, 20066. Emission Inventory Improvement Program, Estimating Ammonia Emissions

from Anthropogenic Non Agricultural Sources, Draft Final Report, April 20047. Houghton, J.T., et al., Intergovernmental Panel on Climate Change, Climate

Change 2001: The Scientific Basis, Ch 6, 20018. United nations Industrial Development Organization (UNIDO) & International

Fertilizer development Centre (IFDC), Fertilizer Manual, Kluwer AcademicPublishers, The Netherlands, 1998

How Green is the Stamicarbon Urea Process v2 w

Documents

Transcript of How Green is the Stamicarbon Urea Process v2 w