Process Condensate Purification inAmmonia Plants

Purifying process condensate for reuse as boiler feed water is discussed. Differentconfigurations of the stripping unit with various levels of integration with other units are

described as well as experience from practical operation.

Jörgen MadsenHaldor Tops0e AIS, Lyngby, Denmark

Introduction

During the last two decades, increasingrestrictions have been imposed on the fertilizerindustry with regard to pollution of the environment.As a result, engineering companies have abandonedthe "once-through" condensate stripper system andintroduced new systems, in which both the strippedprocess condensate and the top product from thestripper are recycled to the process.

With the increase in energy prices, therequirement to recover as much energy as possiblefrom the process has become more and more pro-nounced. This has led the engineering companies toincreasingly integrate the stripper systems into theprocess.

To meet the requirements for pollution control,and to obtain savings in cost and energy, majorefforts have been made to reuse the stripped processcondensate as:

• Make-up water to the cooling tower circuit, thusreducing the consumption of fresh make-upwater

• Make-up water to the demineralization unit, thusreducing the consumption of raw water andchemicals

• Make-up water to the circuit for saturation ofnatural gas, thus reducing the consumption ofprocess steam.

In the following, some considerations are givenconcerning the use of stripped process condensate asmake-up water to the demineralization unit, and tothe saturation system for natural gas.

227

Review of Process Condensate Strippers inAmmonia Plants

Process Condensate

The process condensate in ammonia plants hasa significant value as boiler feed water (BFW) make-up. It has a higher purity than raw water and cantherefore be upgraded to BFW at less expense (tochemicals, labour, equipment, etc.) than purificationof raw water. Furthermore, ammonia plants are oftenlocated in remote areas, where water is scarce; insome cases it must even be prepared by distillationof sea water. In such cases fresh water economy isimportant, and reuse of process condensate may beessential.

The process condensate represents the excessprocess steam after the reforming and shift conver-sion section plus condensate from possible lowpressure steam injection in the CO2-removal unitplus condensate after the methanator plus in somecases condensate from compressor interstage coolers(synthesis gas and process air compressors).

In a typical 1000 MTPD ammonia plant, thequantity of condensate is equivalent to about 35% ofthe BFW make-up. Typical analyses of process con-densate separated at different locations in anammonia plant are given in Table 1. The dischargeof such contaminated water is not only unacceptablein many parts of the world, but it is also a waste ofmoney and energy.

Process Condensate Stripper Systems

For the last 15-20 years, design of utility unitshas always included a treatment of the contaminatedprocess condensate. In the following, a short reviewis given of the process condensate stripper systems inorder to illustrate the developments.

228

Conventional Systems

1. In the early seventies, a very simple processcondensate stripper system was adopted. Thissystem, referred to as the "once-through"system, is shown in Figure 1. The processcondensate is routed directly to the top tray,stripping steam is injected at the bottom of thecolumn, and overhead vapour is vented to theatmosphere. The stripped process condensate,containing small traces of contaminants, is usedas BFW or cooling water make-up.

2. A modified design of the above concept isshown in Figure 2. This design makes use of areflux system. In this system the overheadvapour is condensed, and only a very small, butconcentrated purge is vented to the atmos-phere. This purge represents a source ofpollution to the environment compared to thesystem shown in Figure 1, the system consumesenergy for the reflux pumps, and it requirescooling in the overhead system for condensingthe vapour from the PC column. On the otherside, water recovery is improved.

Recent Developments

During the seventies, the requirements forreducing the pollution of the environment increased.This led to modifications in the lay-out. Figure 3shows a system with preflash of the raw processcondensate in a flash vessel, before feeding it to thestripper column. In the flash vessel, approximately45% of the CO2 in the process condensate is flashedoff to the atmosphere. As a result of this preflash,the concentration of ammonia in the reflux stream tothe stripper is increased. The contaminants areremoved with the liquid purge from the reflux drumand used for saturation of the feed to the primaryreformer in the ammonia plant. This concept mini-mizes the pollution problem. The stripped processcondensate is normally used as BFW make-up.

Another development based on the reflux con-cept is shown on Figure 4. This concept makes inten-sive use of process integration in order to utilize "lowlevel" energy as much as possible. Figure 4 showshow low pressure steam is used first as strippingsteam in the deaerator, then as stripping steam in

the process condensate stripper and finally routed toan auxiliary reboiler in the CO2-removal system. Theheat content in the stripped process condensate isused for preheating the raw process condensate fedto the stripper column and for preheating of fuel gas.This concept still has the disadvantage of venting agaseous purge to the atmosphere.

The concept shown on Figure 3 results inminimum pollution of the environment while theconcept shown on Figure 4 gives a high degree ofrecovery of "low level" energy. On Figure 5 acombination of these two principles, minimum pol-lution and high degree of energy recovery, is shown.The stripped process condensate is cooled by heatexchange with the incoming process condensate andfurther cooled by heat exchange with demineralizedwater. From this heat exchanger, the demineralizedwater is routed via the demineralized water pre-heaters in the ammonia plant to the partial con-denser in the overhead system of the process con-densate stripper.

Parse Io Alraosphtre

Figure 1. PC stripper on a once-through basis.

229

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Figure 2. PC stripper with reflux system.

Figure 3. PC stripper with reflux system andCO2 flash vessel.

Figure 4. PC stripper with reflux system heatrecovered In the CO2-removal andfuel system.

Figure 5. PC stripping with reflux systemCO2 flash vessel and heat recoveryby preheating demlnerallzed water..

O —d3 «a-™1«;_^n""> MU»

Figure 6. PC stripper using MP processsteam.

It should be noted that this design also includesa modification of the CO2 flash vessel shown inFigure 3. On Figure 5 it can be seen that steam istaken from the stripper column and routed to theCO2 flash vessel as stripping steam. The CO2 flashvessel is furthermore provided with a packing inorder to improve the stripping process.

In Figure 6 a very simple concept is shown. Thestripping process utilizes the process steam (for theprimary reforming process) as stripping steam. Theoverhead vapour consists of the process steam andthe stripped-off contaminants. This stream is routedback to the inlet of the primary reformer. The pol-lution problem is totally eliminated. This concept issimple and the energy requirement corresponds tothe enthalpy difference between the incoming andoutgoing MP steam flow. In addition, electricalenergy is used for the process condensate pump.

231

Contamination of the Process Condensate

The process condensate may be contaminatedby various sources:

• Refractory lining, inert supporting material,catalyst dust, corrosion products, etc.

• Entrained hot potassium carbonate solution(HPC) from the CO2 regenerator

• By-products formed in the reforming system, andin the high and low temperature shift reactors

Catalyst, Inert Material, and Refractory

The catalysts used in today's ammonia plantsdo not cause any contamination of the processcondensate neither during normal operation norduring upset conditions.

Naturally, small quantities of dust will beformed during catalyst loading; the amount de-pending on method of loading. Such dust will becollected in the condensate separators during thestart-up and the initial operation phase.

Likewise, the materials used today as supportand for refractory lining will not, during normaloperation release contaminants to the process gas. Incase of cracks in the inner, high grade, reforminglining, the gas may come into contact with low graderefractory, which may release alkali compoundsand/or silica. This rarely constitutes a seriousproblem of contamination of the process condensate.Actually, such compounds would be more likely toconstitute a fouling problem in the heat exchangersor boilers, because they would mainly be depositedhere.

Corrosion Products in a Process CondensateStripper Unit - An Example

During the initial months of operation of arecently commissioned ammonia plant, an increasingcontent of iron in the HPC solution was reported bylaboratory analysis. It was found that the processcondensate piping downstream the CO2 separator inthe overhead system of the CO2 regenerator wascorroded, cf. Figures 7 and 8. Increasing amounts ofiron corrosion products had entered the HPC systemvia the water balance feed line, and a substantial

amount of corrosion products had ended up into theprocess condensate stripper. The corrosion productshad totally blocked the CO2 flash vessel packing, thusdisturbing the operation of the stripper system itselfby "overloading" the system with CO2, cf. Figure 8.

Corroded Inner Surface of aProcess Condensate Pipe

From photo 1 it appears that severe corrosionhas taken place at the majority of the dark areas onthe photos, except at those at the bottom which arecovered by a FeCO3 layer.

In Appendix 1 a detailed discussion of thecorrosion incident is given.

Based on the actual experience and thediscussion given in Appendix 1, one may concludethe following:

(1) At lower pH values, the concentration of freeCO2 and HCO3" in the condensate is high,whereby its ability to dissolve iron as ironcarbonate increases drastically.

(2) As the rate of corrosion is inversely dependenton the presence of corrosion products, highvelocities have an unfavourable effect on thelifetime of the piping.

232

CO,Pn«Ju« rx . _ .

tULJ

X

Figure 7. OH condensing system In a CO2regeneration unit.

Figure 8. Process condensate strippersystem.

Contamination of Process Condensate withBy-Products

Contamination of process condensate with by-products shall be mentioned because of the effect onthe stripping process, and because the resultingdamages in pH value make an influence on thecorrosivity of the process condensate.

By-product formation takes place in thereforming system, in particular in the secondaryreformer, where ammonia is formed, and in the shiftsystem, where a number of components are formedamong which methanol is the most dominant.

In the reforming system ammonia is formedaccording to the chemical reaction

N2 + 3H2 *• 2NH3 + heat

The ammonia content exit the primaryreformer is usually low, due to low nitrogen contentof the feed. At the outlet of the secondary reformer,the ammonia concentration is close to equilibrium asdetermined by the partial pressures of nitrogen andhydrogen and by the temperature and pressure -typical values are 180-300 ppm by volume. Anincrease in temperature leads to a lowering of theammonia content at the exit since the reaction isexothermic.

In the shift reactors methanol is formedaccording to the following reaction:

CO2 + 3H2 •*• CH3OH + H2O + heat

The methanol synthesis reaction is also exo-thermic, which means that the higher the tempera-ture, the lower the chemical equilibrium concen-tration will be.

Methanol is formed both in the high- and lowtemperature shifts (HTS and LTS, cf. Table 1). Dueto the high temperature level in the HTS, themethanol synthesis reaction is fast, and the methanolformed corresponds to the chemical equilibrium atthe exit of the HTS. The methanol concentration israther insignificant. For further reference see Figure9.

Vol. ppm MeOH

20 -

IS

10 •

HTS Inlet Conditions:

S/DG Ratio = 0.57CO/CO2 = 12,8/7.8 mole% dryInlet Pressure = 31 kg/cmzg

340 350 360 370 380 390 'C

Figure 9. Equilibrium methanol concentrationIn wet gas outlet HTS at S/C=3.5versus HTS Inlet temperature.

233

In the LTS, there is a potential for formationof substantially larger amounts of methanol than inthe HTS (see Figure 10). However, since the tempe-rature level in the LTS is 150-200" C lower than inthe HTS, the formation of methanol is controlledkinetically rather than thermodynamically. Thismeans that the formation of methanol in the LTS isdetermined by the LTS operating conditions and thecharacteristics of the catalysts used.

An example of methanol formation over theHTS and LTS catalysts is shown in Table 1, page 4.

Vol. ppm MeOH

2000 :

1500 -

1000 -

500

LTS Inlet Conditions:

S/DG RatioCO/CO2 =Inlet Pressure =

0.433.0/16.0 mole% dry30 kg/cro'g

190 200 210 220 230 -CFigure 10. Equilibrium methanol concentra-

ten In wet gas outlet LTS at S/C=3.5versus LTS Inlet temperature.

Besides the above mentioned "main" by-products a number of "minor" by-products areformed (see Table 1, page 4).

From Table 1 the following may also be seen:

(1) Methanol is formed in both the HTS and LTS,but the bulk formation takes place in the LTS

(2) Formic and acetic acids are synthesized in theHTS. Acetic acid undergoes a chemical reationin the LTS (the acetic acid is beinghydrogenated to ethanol)

(3) A small fraction of the methanol reacts withammonia to form methylamines in the LTSreactor. Table 1 shows where the amines endup. A major part (about 60%) of the amines gointo the bottom product of the process conden-sate stripper.

HP Boiler Feed Water Requirements

The process gas waste heat boiler, waste heatsuperheater and the auxiliary boilers of today's largeammonia plants are very important, integrated partsof the plant. Therefore, it is very important to giveproper attention to the boiler feed water preparationand to the proper application of the chemicaltreatment programs.

Furthermore, it is also important that the steamproduced is of high quality to avoid deposit onturbine blades and to avoid catalyst contamination.Maintenance of high quality steam requires correctoperation of the demineralizer, condensate polisherand chemical injection systems.

When specifying the waste heat boilers and theauxiliary boiler of an ammonia plant, the designercompany will normally follow the codes given in thedesign basis from the client, or his own standard.

A commonly applied code is the German VGBGuidelines for Boiler Feed Water, Boiler Water andSteam for High Pressure Turbines.

The VGB code specifies the quality require-ments of boiler feed water, boiler drum water andsteam quality. The code furthermore gives guidelinesfor the following modes of operation:

BFWtype

Mode of Operation

Alkaline(conditioning withalkalizing agent)

Neutral(conditioning withoxidizing agent)

Salt-free

Salt-containing

The alkaline mode of operation is characterizedby the utilization of a volatile alkalizing agent likeammonia, hydrazine and the formation of a protec-tive layer of magnetite. In this operating mode,dosing with solid alkalizing agents like tri-sodiumphosphate is often used. In case of circulation boilersand steam boiling pressure below 160 bar, the com-bined use of volatile and solid alkalizing agents is thestandard method applied in today's ammonia plants.

234

The neutral mode of operation is characterizedby the addition of an oxidizing agent to the boilerfeed water and by the formation of a protective layerof haematite. Hydrogen peroxide or gaseous oxygenis often used as the oxidizing agent. The oxidizingagent present in the liquid phase will cause forma-tion of a protective layer in the equipment. Theneutral mode of operation is most often used in theonce-through boiler type.

Following the guidelines of the VGB or ABMAcodes (the American Boiler Manufacturers Asso-ciation) will ensure trouble-free continuous operationof both the high pressure steam waste heat boiler aswell as the steam turbines.

Engineering companies and plant owners oftenmake use of their own standards and experience,combining them with the code requirements. Thespecification given in Table 3 gives a comparisonbetween extracted standard values from the VGBcode for salt-free boiler feed water and an actualboiler feed water specification recently applied in anammonia plant.

By closely monitoring the boiler feed waterpreparation according to the project specificationshown in Table 3, the plant obtained the followingexcellent results for the BFW and boiler drum wateras shown in Table 4.

Table 3

BFW Requirementsfor HP Steam Prod.

Conductivity at 25° C,liS/cm 3)

Total kon (Fe), mg/kg

Total copper (Cu), mg/kg

Sodium (Na) + potassium(K), mg/kg

Silica (SiCy, mg/kg

Chloride (Cl), mg/kg

pHat25°C

Dissolved oxygen, ppm byvol

Organic substances

ProjectSpecification

<0.20

< 0.020

<0.003

<0.01

<0.020

<0.020

8.5-9.5

<0.005

-

VGBGuidelines

<0.20

< 0.020

<0.003

.

< 0.020

-

>9

0.020

1) 2)

1) Organic matter expressed as potassiumpermanganate consumption <5 mg/1

2) Oil content <0.5 mg/13) Before injection af ammonia and hydrazine

Table 4

pHat25°C

Conductivity,|iS/cm

Hydrazine, N2H4,ppm w

Phosphate, PO4,ppm w

SiO2, ppm w

BFW ExitDeaerator

8.3-8.5 1)

1-1.3 2)

0.10

.

< 0.001

Boiler DrumWater

9.8

19-24

-0.02

5-6

0.08 - 0.10

1) Before injection of ammonia2) Hydrazine (N2H4) added upstream

measuring point.

Reuse of Process Condensate

The silica analysis are of particular interest, asthey show what can be obtained on a sustained basiswith timely regeneration of the demineralizationresins. The effect on catalyst life in the reformer andshift is significant.

The stripped process condensate from thereferred plant is being routed to a polishing unit,consisting of a cation exchanger, a degasifier and amixed bed filter. Typical analysis of the strippedprocess condensate inlet and exit the polishing unitis given in Table 5.

235

Table 5

pH at 25° C

Conductivity,u-S/cm

Ammonia, NH4,ppm w

Chloride, Cl",ppm w

Silica, SiO2,ppm w

Sulphate, SO4~,ppm w

Total iron, Fe,ppm w

HCO3" + CO3~ asCaCO3, ppm w

Copper, Cu, ppm w

Sodium + potas-sium, Na + K,ppm w

Methanol, CH3OH,ppm w

Organic matter

Inlet

9.6

32

1.22

0.05

nil

nil

0.01

14.2

nil

0.7

18

1)

CationOutlet

4.8

8.9

0.14

0.05

nil

nil

nil

4.4

nil

nil

12

-

Mixed BedInlet/Outlet

6.7/6.0

7.2/0.09

0.83/nil

0.24/nil

nil/nil

nil/nil

nil/nil

4.3/1.4

nil/nil

nil/nil

27/24

-A

1) Not measured in the plant.

In the present case, the make-up water flow tothe deaerator consists of approximately 65% demine-ralized water from the demineralization unit and35% polished process condensate.

It should be noted, that even with the aboveexcellent quality of the stripped process condensate,even a partial by-pass of the polishing unit is notpermissable, as the conductivity of the mixed water(i.e. process condensate and demineralized water)will exceed the VGB limit of 215.000 kcal/hm2 forpeak heat flux.

Utilization of Process Condensate forSaturation of Natural Gas

The principles of saturation of a gas has beenknown for many years. In ammonia plants based ongasification of coal or partial oxidation of fuel oil,the CO-conversion section is often provided with apacked saturation tower upstream and a desaturatordownstream the high temperature shift converter.

The design of the saturator systems used duringthe last 5-10 years has normally been based on oneof the following principles:

(1) Saturation of natural gas with processcondensate in a packed tower (see Figure 11).

(2) Vaporization of process condensate in thenatural gas feed preheater coil (see Figure 12).The principle of vaporization of processcondensate and thus the saturating of a gasmay be adopted at other points in theammonia plant, where heat can be recovered.An example is the controlling of the inlettemperature to a CO-converter (HTS or LTS)by quenching with process condensate (seeFigure 13). In this concept the CO-leakage isreduced due to the increased content of steamin the gas. As the CO is converted into CH4 inthe methanator, less inerts will be introducedinto the ammonia synthesis loop by thisconcept.

From Figure 11 and Figure 12, it can be seenthat untreated process condensate is recycledback to the process in form of steam, thusreducing the amount of process condensate tobe treated. At the same time, low level heatfrom the reformer flue gas is recovered byproduction of process steam.

Integration of one of these principles into theammonia plant will result in the followingadvantages:

• Reduction on the process steam requirement.• Less process condensate to be treated in the

process condensate stripper thus reducingthe amount of stripping steam to the processcondensate stripper.

• Reduction in usage of chemicals for the

236

regeneration process in the polishing unitdue to reduction of the amount of processcondensate.Reduced emission to the environmentbecause pollutants are recycled to theprocess.

Coils Located In [he Waste HealSection of the Primär? Reformer

Figure 11. Saturation of natural gas by usinga packed tower.

Figure 12. Saturation of natural gas byvaporization of processcondensate.

Figure 13. Temperature control by quen-ching with process condensate.

Appendix 1

Discussion on Carbon Steel Corrosion

Case Study

Introduction

Under the conditions existing in the OHcondensate system of the CO2 regenerator, carbonsteel would normally be sufficient for that service.However, it requires that the stripped CO2 from theHPC regenerator column is alkalized with f.inst.ammonia. Sufficient ammonia should be present inthe vapour phase to give a pH value in the conden-sate of approximately 7. In the actual case, thecorrosion of the piping was so severe, that in someparts of the piping the wall thickness was reduced bymore than 60% compared with the original thickness(ultrasonic measurement test). The corrosion pro-ducts deposited in the stainless steel packing of theCO2 flash vessel were analyzed by the XRPDmethod and were found to consist mainly of FeCO3and some Fe3O4.

Whether carbon steel piping will be protectedby a carbonate scale or not depends on a number offactors, such as temperature, pH value and turbu-lence. Excessive turbulence may be a critical factor,preventing formation or retention of the protectivecarbonate layer.

237

A change in the pH value of the condensatehas a critical impact on the iron carbonate layer. Forexample, when the front-end is operated during aircompressor trips in order to produce CO2 for ureaproduction, or when the back-washing system to thedemister in top of the CO2 regenerator, whichnormally is used intermittently has been incontinuous operation, none or very little ammoniawill be present in the OH process condensate. Theprocess condensate will be saturated with CO2, andthe pH value will drop to about 3.9. Under theseconditions the iron carbonate will be dissolvedaccording to the following reaction:

FeC03 + H+ —

Theoretical Background

+ HCO,

Assuming that no ammonia is present in theOH system the following equilibria may be con-sidered:

(C02)

(C02)aq

HCO,-

H0

(C02)aq

HC03- + H+

C0~ + H+

(1)

(2)

(3)

For the above 3 equilibria the followinggeneral expressions apply:

(1) Y ,x*P T = a , x H ,

When applying the general expressions (I andH) to the weak eletrolyte system given by theequilibrium equations (1), (2) and (3) above, and atthe same time realizing that these weak electrolytesin the OH system of the CO2 regenerator will bestrongly diluted, then the above expressions may bereduced to the following:

(3) P. = m . x H ,

(4) K,

where:

P- =m,Hi

m+, m_ =

x m_m,

Partial pressure of component i, atMolality of component iHenry's constant for component i,kg x at/moleMolality of dissociation productsof component i

Applying (lu) and (IV) to the actual system(1), (2) and (3) above this gives:

(5)

(6)

co2

(HC03-)(H*)

(CO,,) aq

(2) K,

where:

Y,

$

PT

a+, a.

a+ x a.a,

Mole fraction in the gas-phaseof component iVapour-phase fugacity coefficientTotal pressure, atHenry's constant of component i,(kg) x (at)/moleDissociation equilibrium constant(molality) for component iActivity of component iActivity of dissociation products ofcomponent i

(7)(CQTKH*)

(HC03-)

where:

PCO2 = Partial pressure of CO2, at

CO2 = Henry's constant for CO2, kg x at/mole

(C02)a? (H+), (HC03-), C03-) = molelity of therespective components.

The equations VI and VU may be rearrangedand by taking the logaritm to both sides thefollowing expressions may be derived:

238

(8) Log(C02)

= PH-PK1

aq

(9) 108

K, and K2 are first and second dissociationconstants for dissociation of carbonic acid.

From these expressions and by varying the pH,the curves on Figure 14 may easily be derived. Thefollowing may be seen from Figure 14:

(1) At high pH value, say above 7, practically alldissolved CO2 exists as HCO3' and CO". Tosatisfy the solubility product equation forFeCO3 i.e. [Fe++] x [CO3~] with highconcentration of CO3~ the concentration ofFe + + will be low.

(2) At low pH value, below 7, practically alldissolved CO2 will exist as free CO2 and HCO3"and virtually no CO3~. The equilibriumconcentration of iron in the process condensatewill be high, and FeCO3 will dissolve.

In the actual case the following processconditions have been reported:

Temperature, °C 40CO2 partial pressure, at 1.56pH range 3.9 - 7.1

The following constants may be found in theliterature:

Henry's constant, = 38.09 at x kg/mole

Dissociation constants for carbonic acid at 40° C:

First (KO = 4.94 x 10'7 mole/1Second (K2) = 6.05 x 10'11 mole/1Solubility productfor FeCO3 = 34.53 x 10'12 (mole/1)2

From the formulas described above, andfrom the solubility product of FeCO3, Figure 15showing the concentration of dissolved iron as afunction of pH value can be derived.

10 11 12 13 14 pH

Figure 14. Conversion of Dissolved CO2 as afunction of pH (at 40° C).

From Figure 15 it can be seen that athigher pH values (above 7), the iron concentrationbecomes practically nil. At lower pH values (below6-7), the iron concentration becomes increasinglysignificant, and the protecting iron carbonate layerwill be dissolved. At normal operation, the measuredpH value in the OH process condensate is approxi-mately 6.8. By using this value and the previousinformation (see Table 1) it is possible to make thefollowing comparison between calculated andmeasured value in the OH system:

Measured Calculated

Total Carbon in PCas (CO3-) 0.2 l%w 0.24%w

Iron Content, ppm w <0.20 0.16

During operation without the air compressor,the pH value of the OH condensate drops toapproximately 4. At this value the iron concentrationwill increase dramatically to approximately 10000ppm (cf. Figure 15), and severe corrosion will occur.

239

Iron ppm w

3 4 5 6 7 8 9

Figure 15. ppm (weight) Iron In processcondensate in equilibrium withC03 2-.

Jörgen Madsen

References

1. G. Butler, M.A., Ph.D. and H.C.K. ISON,A.I.M., "Corrosion and its Prevention inWaters".

2. Ghemlins Handbuch Der AnorganischenChemie Part B - Iron Compounds.

3. Methanol By-Product Formation over HTS andLTS Catalyst by Haldor Tops0e A/S.

4. W.M.T. Body and O.S. Sort, "Water TreatmentData".

5. BETZ Handbook of Industrial WaterConditioning, 7th Edition 1976.

6. VGB Guidelines for Boiler Feed Water, BoilerWater and Steam for Water Tube Boilers witha Pressure of 64 bar and Higher. October 1980Edition.

7. Boiler Water Requirements and AssociatedSteam Purity for Commercial Boilers.

8. Ammonia Plant Safety 29 (1988).

240