Methanol and Ammonia CoproductionMethanol and ammonia coproduced via an integrated process scheme enhance energy

efficiency while utilizing only commercially proven process steps. The key tointegration is an air separation unit which supplies nitrogen to an ammonia unit and

oxygen to a methanol unit.

Edward Cialkowski and Jim LeeThe M,W. Kellogg Company, Houston, TX 77210

An Historical Perspective

Of the many process schemes that havebeen considered for linking ammonia andmethanol production, quite a few have actuallyfound their way into operation, and several havebeen very good investments. The reason forthis, from a process standpoint is simple:conventional methanol plants based on anatural gas feedstock produce excess hydrogen,while conventional ammonia plants with asimilar feedstock must reject carbon dioxide.The two most obvious methods for integratingthe two plants are:

Injection Technology ~ exporting CO2 from astand-alone ammonia plant to improve thenatural gas conversion efficiency of a methanolplant, or

Offgas Technology — treating the H2-richsynthesis loop purge stream from a stand-alonemethanol plant to provide H2 for ammoniasynthesis.

Both concepts have been provencommercially, the latter more recently than theformer, and both concepts result in more

efficient coproduction than independentproduction. Which route is better? Whenconsidering capital outlays, the moreappropriate question may have been: whichstand-alone plant already exists? Typically thetwo plants have not been considered anddesigned together. Because both technologiesmentioned above maintain either a methanol orammonia unit that is independent of thesubsequent plant, coproduction projects havenot improved the energy efficiency of the stand-alone plant in any way, but have resulted in alower specific gas consumption for the add-onplant than if an independent unit were built.

If there is interest in coproduction from theoutset, is there a process scheme for a grassrootsplant that would result in more efficientproduction of both products? There might bequite a few. What if that efficiency were to beachieved without increasing capital cost? Thatwould trim the list substantially. Finally, howmany flowsheets boast substantially improvedenergy efficiency without an increase in capitalcost, while utilizing only commercially provenprocessing steps?

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Truly

The means of integrating methanol andammonia production described in this paper isbased on unit operations that Kellogg hassuccessfully implemented in process facilitiesaround the world. The individual componentsof the processes have proven reliability, andtaken together, result in a methanol andammonia coproduction facility that is moreenergy efficient than either of the twotraditional means of coproduction mentionedabove.

How does it work? Start with the stand-alone methanol plant option where synthesisloop purge gas is used to produce ammonia.The work of Czuppon and Lee (1) describesKellogg's successful implementation of this add-on technology for the Ocelot AmmoniaCompany. Now add oxygen to the front-end ofthe methanol plant and reduce the temperatureseverity of the primary reformer operation whileincreasing the front-end pressure to lowersyngas compression requirements. The result:An integrated flowsheet which improvesfeedstock utilization and overall energyefficiency in the methanol plant whilemaintaining the same high efficiency in the add-on ammonia plant.

Definition of Terms

The more detailed information presentedbelow compares the integrated flowsheet withan add-on process scheme. For ease ofreference, consider a stand-alone ammoniaplant with reject CO2 utilized in a conventionalmethanol plant as Injection Technology, andconsider a stand-alone methanol plant withsynthesis loop purge used to make ammonia asOffgas Technology.

Methanol and Ammonia Chemistry

If a methanol plant is sized to take fulladvantage of the CÔ2 available from a stand-alone ammonia plant (Injection Technology),then the theoretical molar ratio of methanol toammonia production (assuming 100% feedstockutilization) will be about 1.8/1; thus on a massbasis the methanol plant will produce 3.3 tonsfor every ton of ammonia produced. However,when the ammonia plant feedstock is purge gasfrom an independent methanol plant (OffgasTechnology), the ratio of methanol to ammonia

production is not quite as large. Since nohydrogen combustion takes place in this type ofplant, more ammonia can be produced,lowering the theoretical methanol to ammoniaratio to 1.5 mol/mol, or 2.8 ton/ton. In practice,the methanol to ammonia ratios for both casesmay be somewhat different than the theoreticalvalues because, unlike an ammonia plantsynthesis loop, a methanol plant synthesis loopoperates at a conversion efficiency substantiallyless than 100%. On the other hand, for bothcases, as the natural gas becomes richer (lowerH/C ratio) the ratio of potential methanol toammonia production increases.

This paper compares a new method forUnking methanol and ammonia production withthe existing methods mentioned above. Thebasis selected for evaluating the integratedflowsheet is the commercially proven OffgasTechnology where a nitrogen wash unit,synthesis loop and refrigeration system convertthe methanol synthesis loop purge to anammonia product. The integrated flowsheettakes the add-on technology a few steps furtherby modifying the operation of the methanolplant in such a way as to benefit from somethingthe add-on ammonia plant already requires: anair separation unit (ASU).

Base Case: Offgas Technology

Figure 1 illustrates the basic configurationof the offgas flowsheet which comprises a stand-alone methanol plant, an air separation unit,and an offgas-to-ammonia plant. This flowsheetwas chosen as a basis for three reasons:

1) The new technology subsequentlydescribed is really a further enhancementto the offgas-to-ammonia technology andtherefore, for assessing cost and energyconsiderations, it is more relevant.

2) For stoichiometrically balanced methanoland ammonia production, OffgasTechnology allows more ammoniaproduction for a given methanol plant size(2.8 ton methanol/ton ammonia vs. 3.3ton/ton with Injection Technology).

3) The positive impact of H2 offgasavailability on ammonia plant cost andenergy efficiency is much greater than thecorresponding impact of OX>2 availabilityon a methanol plant. In this sense, offgas-

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to-ammonia technology is a morechallenging basis for comparison.

In the Base Case plant, a hydrogen-richsynthesis loop purge stream from the methanolplant must be wet scrubbed in two separatesteps to remove methanol vapor and carbondioxide. It then enters a nitrogen wash columnwhich removes methane and carbon monoxidewhile adding nitrogen to a controlled H/N ratio.This is all accomplished at the nominal pressureof the methanol plant synthesis loop so theammonia plant make-up gas (MUG) with verylow inert content requires only minimalcompression. The ammonia synthesis loop ismore conventional in nature although the ultra-low inerts concentration in the MUG allowsessentially 100% H2 conversion since thesynthesis loop requires no purge.

Key to Integration: ASU

The make-up nitrogen for the cryogenicwash unit is supplied in vapor form by an airseparation unit. It is the ASU that holds the keyto further integration with the methanol plant.In the add-on plant flowsheet, the ASU rejectsto the atmosphere the oxygen-rich "waste" gaswhich the ammonia plant does not require. Byadding minimal investment to the ASU, it canproduce sufficiently pure oxygen vapor for themethanol plant as well as nitrogen vapor for theammonia plant.

Process Integration Described

Figure 2 illustrates the shared use of theASU in an integrated flowsheet by indicatingthe addition of an oxygen-blown autothermalreformer downstream of the methanol plant'sfired-tube primary reformer. This changes theprocess in two important ways.

1) More methane is reformed, improvingfeedstock utilization and lowering methanecontent in the make-up gas to less than 1%.

2) Lower primary reforming temperature,allowing higher front-end pressure andlower make-up gas compressionrequirements.

Another consequence of oxygen addition isthe combustion of hydrogen within the processwhich reduces the potential production rate ofammonia. By shifting a portion of the

combustion required to supply heat for theendothermic reforming reaction from outsidethe process in the reforming furnace to insidethe process ui the autothermal reformer, theradiant duty in the reformer is reduced by 25%.

Impact on Chemistry

Figure 3 shows that initially, as oxygen isadded to the methanol plant, the heat ofcombustion with hydrogen is used primarily toaccomplish additional reforming of the highresidual methane in the partially reformed gas--exactly what takes place in an ammonia plant'ssecondary reformer. As more oxygen is added,the heat of combustion with hydrogen is used toa greater extent to add sensible heat to thereformed gas than to effect additionalreforming. In any methanol plant it isadvantageous to do this because reducing theexcess hydrogen content of the make-up gas to astoichiometrically balanced level reduces perton compression power requirements andmaximizes feedstock utilization.

Naturally, we don't want to go quite thatfar in the integrated methanol plant because itwould leave us with almost no hydrogen withwhich to make ammonia. A conventionalammonia plant reduces the potential hydrogenin the natural gas feedstock by about 15% withthe addition of process air. The integratedmethanol plant flowsheet reduces the potentialhydrogen in the feedstock about 20% by addingoxygen. So the integrated flowsheet actuallyoperates with a hydrogen consumption in theautothermal reformer that is similar to aconventional ammonia plant. As a result ofoxygen injection then, the integrated offgas-to-ammonia plant capacity is about 20% less thanthat of the add-on ammonia plant.

Oxygen addition to the methanol plantflowsheet not only makes possible a higher levelof natural gas conversion, but also can improvesteam generation in the reformed gas heatrecovery train. In fact, an integrated steambalance for the two plants provides sufficientsteam to satisfy all the process and utilitydemands for the entire coproduction complex.

Impact on Equipment

O2-Blown secondary

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Integration requires modification to severalpieces of equipment but addition of only oneitem: an oxygen-blown adiabatic reformer. Thegeometry of the vessel itself is ratherstraightforward and Kellogg has had goodexperience repeated many times with itsrefractory-lined, water-jacketed autothermalreformer design. However, the design of theoxygen burner requires special attention sincepure oxygen has been observed to directlycombust some steel alloys above 260 °C.Therefore, Kellogg's proprietary burner designincludes a refractory face for radiant heatshielding and internal water cooling to maintaina low metal skin temperature where contact withoxygen occurs. This particular burner designhas been used successfully in two commercialapplications with over 35 years total experience.

Make-up gas compressor

Two-step reforming in the methanol plantresults in a higher CH4 conversion than simplesteam reforming and allows an increase in thefront-end operating pressure which lowerscompression requirements. Furthermore, evenat the higher reforming pressure, the inertcontent (CH4 & N£) of the make-up gas islower than in a conventional methanol plantwith a single fired reformer operating at a lowerpressure. The make-up gas compression powerreduction is substantial (approx. 40%) and sincethe overall compression ratio is cut almost inhalf, a single-case machine can be used insteadof the 2-case machine normally required with alow pressure front-end.

Primary Reformer

The design of the primary reformer isaffected in two ways which tend to offset eachother: 1) higher operating pressure, but 2)lower process outlet temperature. Notsurprisingly, the operating conditions chosen forthe radiant coil design are quite similar to thoseused successfully in a typical Kellogg ammoniaplant reforming furnace. Because the requiredtube wall thickness is such a strong function oftemperature (compared to pressure), a smalltubesldn temperature reduction more thanoffsets a doubling of the operating pressure. Asa result, by shifting reforming duty from aradiant box to an autothermal reformer, theradiant section of the primary reformer can bedownsized significantly.

Air Separation Unit (ASU)

In the conventional offgas-to-ammoniaplant, the ASU supplies nitrogen to a cryogenicwash column which removes CO and ŒLj. fromthe methanol synthesis loop purge gas. Thewash column also maintains a fixed H/N ratio inthe MUG to the ammonia synthesis loop. Anoxygen-rich waste stream is then rejected fromthe ASU to the atmosphere. By modestlyincreasing investment in the ASU, a sufficientlypure oxygen stream can be produced.

The ASU essentially supplies 2 plants andis the key to integrated methanol and ammoniaproduction. The investment cost and powerrequirement of the ASU can be shared by boththe methanol and ammonia plant, and thereforeboth plants can operate more efficientlytogether than in a non-integrated coproductionfaculty (such as the base case offgas to-ammoniaplant).

Impact on Operation

Reliability

One of the most important aspects ofprocess plant design and operation is reliability.Can the combined plant be operated safely withover 90% availability? The response to thisquestion is crucial to the overall economicperformance of the project. Realizing theimportance of reliability, the integratedflowsheet was assembled using onlycommercially proven components that havebeen demonstrated in successful and reliableoperation for many years.

Flexibility

Furthermore, the integrated plant has thepotential to operate flexibly: efficiently shiftingbetween an emphasis on one product or theother. This simply can't be accomplished to thesame extent with add-on technology. Bydesigning the methanol plant to accommodate ashifting of load from the primary reformer tothe O2-blown secondary, the integrated plantcan produce more methanol (and lessammonia) with almost no change in per tonenergy efficiency. On the other hand, by shiftingload from the secondary to the primaryreformer, more ammonia (and less methanol)can be produced, again with almost no changein per ton energy efficiency.

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In practice, how might this work? Startingfrom a mid-range methanol to ammoniaproduction ratio, lower the primary reformerexit temperature and inject more oxygen in thesecondary reformer. This will accomplish thesame quantity of overall reforming, but consumemore hydrogen in the process. This balancesthe methanol plant MUG to favor methanolproduction and allows more carbon oxides intothe synthesis loop for a given volumetric flowthrough the MUG compressor. Less hydrogenis purged from the loop and subsequently theammonia plant is turned down. The ammoniaproduction efficiency is not significantly affectedbecause its process parameters don't change,only the throughput. The methanol plant energyefficiency actually improves slightly as per tonreformer firing and compression power arereduced, but oxygen consumption (and themethanol plant's share of the ASU's powerload) will increase. The result is almost nochange in per ton variable cost (natural gas plusoxygen) for either methanol or ammonia.

The situation is reversed in moving in theother direction to lower the methanol toammonia production ratio. Oxygenconsumption in the secondary is reduced whilefiring (and hydrogen generation) in the primaryis increased. Again the result is a shift inproduct ratio with almost no impact onproduction efficiency.

The flexibility to efficiently emphasizemethanol or ammonia production in a mannerthat does not sacrifice operating safety orreliability can be a big factor in helping toestablish the overall profitability of a project.The integrated flowsheet presented here wasdeveloped with such a goal in mind.

Side-by-SIde Comparison

The methanol plant technology used as abasis for comparison is the same as thatsuccessfully demonstrated in the Cape HornMethanol project—a world-scale grassrootscomplex constructed in a remote location. Thisis a 2268 MTPD nameplate facility based onconventional steam reforming technology thatwas brought on stream in 1988. The reformerduty, compression requirements, energyefficiency, and estimated ammonia productionare based on the feed composition and siteconditions associated with the Cape Horn plant.

Although integrating methanol andammonia production through shared use of anASU does change the potential capacity of theammonia plant, it actually has very little impacton the operating efficiency of the offgasammonia plant. This is reflected in Figure 4which compares the predicted energyconsumption of the integrated ammonia plantwith the demonstrated energy consumption of acommercial offgas plant and a conventionalgrassroots ammonia plant. Both offgas plantsshow better than a 15% improvement in energyefficiency over the stand-alone ammonia plant.Add to this advantage a substantial capital costsavings in the ammonia plant (where 5 catalyticservices have been removed) and it becomesobvious why offgas ammonia plant projects canbe so profitable.

Methanol Plant Shares Gain

Integrating methanol and ammoniaproduction as proposed above allows themethanol plant operating profitability toincrease as well. The reason for this issummarized in Figures 5 & 6. Total fuel firingin the integrated methanol plant is 20% lessthan a stand-alone plant of the same capacity.This is primarily the result of a 25% reduction inthe primary reformer's radiant duty.

Similarly, the integrated methanol plantenjoys a 12% reduction in total processcompression power requirements. Due tohigher front-end pressure operation the MUGcompression power is cut by almost 40% andthe low-inert, more stoichiometrically balanced,synthesis loop operation requires less recyclecompression. On the other hand, the integratedplant must now provide power for an ASU andan oxygen compressor which, taken together,would offset the power savings in MUG andrecycle compression. However, because boththe methanol and the ammonia plant share theASU, a portion of its power requirement isalready paid for by the ammonia plant. Thisutility sharing makes possible the 12% decreasein process compression power for the integratedmethanol plant.

Table 1 compares the Integrated Case withthe Base Case plant (stand-alone methanol withoffgas-to-ammonia). Integration improves the

r ton energy efficiency of the methanol planta rather substantial 5% over the base case.

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Note that the ammonia plant energy efficiencybased on offgas technology is very good, andthis efficiency is only slightly affected byintegration with a methanol plant. Therefore,integrating methanol and ammonia productionby implementing a shared ASU results in amore efficient methanol plant with little or nopenalty in the ammonia plant energy efficiency.

Table 1Efficiency and CostOverall Comparison

Energy Consumption(Gcal/MT)

MethanolAmmonia

Relative Cost(per ton of product)

Methanol UnitAmmonia Unit

OffgasPlant

8.186.17

100%100%

IntegratedPlant

7.786.26

95%100%

Table 1 also shows a relative capital costrange for the integrated plant compared to thebase case facility. If the integrated plant weredesigned for only a single methanol andammonia production rate, the cost of themethanol unit would be approximately 5% lessthan the Base Case plant. Note that the alreadylow per ton capital cost of the ammonia unit isnot affected by integration with a methanolplant. As a further option, if the integratedplant were designed to efficiently vary methanoland ammonia production rates to meet marketdemands, then the initial investment in themethanol unit may increase as much as 5% toaccommodate this flexibility, bringing theequipment cost up to par with the Base Case. Inthis instance, the facility benefits from highmethanol plant efficiency and operationalflexibility.

2) A small reduction in capital cost comparedto non-integrated coproduction.

3) The potential for production rate flexibilitywith little impact on overall efficiency.

4) Reliable operation expected due to provennature of individual process components.

5) Improved return on investment relative tothe Base Case depending on the.relativemarket prices of methanol and ammonia aswell as the cost of energy.

Compared to the Base Case (independentmethanol plant with an offgas-to-ammoniaplant), process integration via shared use of anair separation unit improves overall efficiencyand gives greater product flexibility potentialwith no increase in capital cost. The processingsteps have been commercially proven in plantsall over the world, and together could result in aprofitable project for those interested inmethanol and ammonia coproduction.

Literature Cited

1. Czuppon, TA. and Lee, J.M., "Hydrogen-Rich Offgases Can Reduce AmmoniaProduction Costs," Oil & Gas Journal,September 7,1987, pp. 42-50.

Conclusion

The integrated methanol and ammoniaflowsheet discussed above offers some ratherattractive features for those who may beinterested in making both products:

1) A significant improvement in methanolplant energy efficiency.

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DISCUSSION

Max Appl, BASF, Ludwigshafen, Germany: Howflexible is the integrated system? Often themethanol and ammonia markets developdifferently. Is it possible to increase one product ordecrease the other?Cialkowski: If the plant is designed to operateflexibly you can trade outputs. Oxygen additionallows you to shift duty from the primary to thesecondary reformer. Reducing oxygen providesmore ammonia and adding more provides moremethanol, however, shifting product ratio awayfrom the design presented here may result in amarginal decrease in methanol plant efficiency.Appl: Why did you use such a modest figure for asingle separate ammonia plant? Kellogg is doingmuch better. I believe 6.7, not 7.2.

Cialkowski: I am aware of that. The reason is thatthe presentation is based on a Kellogg study wherewe looked at a plant in a remote location. Thedollar cost of boiler feedwater and cooling watersupplies were converted to a gas equivalent forcomparison purposes.Richard Newland, Tops0e, Houston, TX: Howoften do you change the water cooled burner?Le Blanc: For one plant which operated for 25years the burner was only changed at theturnaround, about every two years, and by having aspare that was no problem. The only repairrequired was to resurface the refractory face of theburner that had been taken out.

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STEAM

NATURAL

GAS

PRODUCT

METHANOL

AMMONIA

Figure 1. Conventional MeOH/Offgas NH3 plant (Base Case).

NATURAL

GAS

STEAM

99 % O2

99.97% N2

METHANOL

AMMONIA

WASTE O2/N2

Figure 2. Integrated MeOH/NH3 plant.

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1000

750 -

Kg-Moles ofHydrogenProducedper Hour

500 -

250 -

MethaneConversion

100

-80Percent ofMethaneConverted

- 60

- 40

r 20

u I I I I I I I I u

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Oxygen to Carbon: Molar Ratio

Figure 3. Oxygen-blown secondary reformer integrated methanol/ammoniaplant.

7.2

6.17 6.26

ConventionalAmmonia

Plant

BasicOffgasPlant

IntegratedOffgasPlant .

Figure 4. Typical ammonia plant energy consumption: Gcal[LHV]/metric ton.

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367 Total Firing

Radiant Duty

295

158

ConventionalMethanol

Plant

IntegratedMethanol

Plant

Figure 5. Comparative fuel firing requirements 2268 MTPD methanol plant:Gcal[LHV]/hour.

30,000 -

I 20,000 -à•x.

10,000 -

41

\ Oxygen--*-»\ Air--»itRecycle — «•

»----MUG »•

/

/

1 1Conventional Integrated

Methanol MethanolPlant plant

Figure 6. Comparative compression requirements 2268 MTPD methanol plant.

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