10.2 Natural gas upgrading · 2016. 3. 15. · 10.2.1 Petrochemical intermediates from natural gas...

10.2.1 Petrochemical intermediatesfrom natural gas

Introduction

Natural Gas (NG) is a gaseous mixture consistingmainly of methane, light hydrocarbons (ethane,propane and butane), generally known as Natural GasLiquids (NGLs), and smaller quantities of other so-called condensed hydrocarbons with a molecularweight greater than or equal to pentane. Propane andbutane are also commonly referred to as LiquefiedPetroleum Gas (LPG). Nitrogen, hydrogen, carbondioxide, sulphur compounds (hydrogen sulphide,carbonyl sulphide and mercaptans), aromaticcompounds, water and mercury are also present insmall quantities.

The proven reserves of natural gas – classified asAssociated Gas (AG) or Non-Associated Gas (NAG)in relation to the presence or absence of oil in thesame field – currently amount to more than 170·1012

Nm3, much of which, consisting of so-called strandedgas, is to be found in remote parts of the globe. Thesereserves, mainly of type AG and for the most partsituated in the Middle East, Russia and Africa, aremore or less equal to the proven reserves of oil, thoughthe true figure is probably much higher, given thecurrent tendency to search for new oil deposits ratherthan fields of natural gas of the NAG type.

Most of the methane contained in the natural gasbrought to the surface is currently used to generateelectricity and, to a lesser extent, as fuel for domesticuse; what remains is used as raw material in theproduction of liquid and gaseous hydrocarbons,intermediates in the base petrochemical industry.

Methane is used to obtain what is known assyngas, a mixture of carbon monoxide and hydrogenused in the direct production of liquid hydrocarbons

through the Fischer-Tropsch reaction process, or used in synthesizing methanol, a fundamentalintermediate in the subsequent production of variouschemical compounds, including formaldehyde,methyl-tert-butyl ether (MTBE), acetic acid, methylmethacrylate, various solvents and acetic anhydride.

The use of natural gas in the production ofethylene and propylene, two of the main buildingblocks of the petrochemical industry, is still limited tothe conversion of the NGL component throughsteam-cracking technologies and processes, along thesame lines as the cracking processes of liquidderivatives from oil such as naphtha and diesel, fromwhich most light olefins are obtained today.Nevertheless, the production of light olefins frommethane is one of the most promising functions fornatural gas in the future, particularly with a view to theextraction and exploitation of deposits in remote areas,and as an alternative to the use of oil derivatives in thepetrochemical industry. Light olefins can be obtainedfrom methane by converting it first into methanol, andthen into olefins, through the Methanol To Olefins(MTO) technologies and processes developed in thelast few years up to demonstration plant level thoughnot yet on a large-scale industrial plant.

Transformation of natural gas

The natural gas extracted from the depositsnormally undergoes treatment designed to eliminate orreduce its impurities, after which it is subjected toseparation treatments to obtain methane, ethane, LPGand condensates, the intensity of the treatmentdepending on the subsequent use.

If there are no gas pipelines to transport it directly,the natural gas is usually transformed into LiquefiedNatural Gas (LNG), transported by ship and laterregasified in special terminals.

455VOLUME II / REFINING AND PETROCHEMICALS

10.2

Natural gas upgrading

Natural gas is already an important raw material inthe production of petrochemical intermediates. TheNGL fraction, consisting mainly of ethane and LPG, isused in the production of light olefins, mainly ethyleneand propylene, through steam-cracking processes, and,in the case of the LPG fraction alone, to obtainpropylene through the dehydrogenation of propane andaromatic compounds through the more complextechnologies of dehydrogenation and cyclization.

For the methane contained in natural gas to be usedfor petrochemical purposes, however, it must first betransformed into syngas, which is in turn used in theproduction of methanol and its derivatives. Thegeneration of syngas can be obtained throughtechnologies that make use of various reactions,especially Steam Methane Reforming (SMR):

CH4�H2O��CO�3H2

This endothermic reaction is conducted attemperatures between 800 and 900°C in the presenceof nitrogen-based catalysts and with an excess ofsteam to limit unwanted coking reactions.

Another technology, known as AutoThermalReforming (ATR), combines the steam reformingreaction with the methane oxidation and partialoxidation reactions:

CH4�2O2��CO2�2H2O

CH4�1/2O2��CO�2H2

These are exothermic and can compensate for thehigh endothermic level of the steam reformingreaction. The ratio between H2 and CO can be reduced,thanks to the reaction between CO and water, knownas the CO shift:

CO�H2O��CO2�H2

The choice between the different technologiesdepends on numerous factors, the most important ofwhich is the capacity required and the type ofsubsequent use of the syngas.

Methanol is in turn obtained from the syngasthrough the following reactions:

CO�2H2��CH3OH

CO2�3H2��CH3OH�H2O

The strongly exothermic reaction, helped by thehigh pressure and low temperatures, is generallyconducted at temperatures between 180 and 270°C andpressures between 50 and 100 bar, in fixed-bedreactors containing alumina-supported copper and zincoxide catalysts. The use of copper oxide basedcatalysts, which are more easily poisonable than theprevious generation of zinc and chrome oxide based

ones, requires a fairly severe pretreatment stage of themethane to reduce the level of sulphur and chlorineimpurities before they are used in the production of thesyngas. At the same time, the adoption of the presenttype of catalyst has allowed a notable reduction in thetemperatures of the reaction, and thus of the pressure,with a net increase in the thermodynamic limit to themaximum possible yield.

Notable progress in the technology forsynthesizing methanol has made it possible to buildplants of significant dimensions with a capacitysuperior to 5,000 t/d. These plants, particularly if builtin remote areas close to those for extracting methaneand producing syngas, have made it possible toproduce methanol at extremely competitive costs,increasing the economic attractiveness of extractingremote gas and transporting it in the form of methanol,and making the subsequent production of olefins moreprofitable than with standard technologies.

Production of olefins

World production of light olefins (ethyleneand propylene) currently stands at around 108 t/yrof ethylene and around 0.6·108 t/yr of propylene.In the petrochemical industry, they are nowproduced mainly by steam-cracking processes,using raw materials obtained principally fromliquid derivatives of oil such as naphtha (amixture of hydrocarbons containing mainlyparaffins from 5 to 9 atoms of carbon) or diesel,as well as gaseous raw materials drawn fromnatural gas, such as ethane or LPG. A smallerquantity of propylene is also obtained throughthe processes of the dehydrogenation of propaneas well as processes for recovering higherolefins, particularly butenes. The processes usedin the dehydrogenation of propane and for theproduction of propylene from C4 are dealt withbelow. In the petrochemical industry a significantquantity of propylene is obtained at the refinerystage through the processes of Fluid CatalyticCracking (FCC) and Residual Catalytic Cracking(RCC), which are the main ways of producingliquid fuels from the heavy fractions derivedfrom oil distillation. Present-day production ofethylene and propylene, subdivided by sources, isshown in Table 1.

Ethane is the raw material and ethylene the mainproduct in cracking processes fed by gaseoushydrocarbons, while naphtha is mainly used incracking processes fed by liquid hydrocarbons, inwhich the relative quantity of propylene in relation toethylene can be increased by modifying the reactionconditions up to a maximum of about 60%, with a

456 ENCYCLOPAEDIA OF HYDROCARBONS

BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

progressive reduction of the overall yield of lightolefins.

The anticipated annual growth in demand, equal toaround 4% for ethylene and 6% for propylene, couldcreate an imbalance in future supplies, particularly asthe necessary additional capacity seems mainlydirected towards cracking plants supplied by ethane.These are at an advantage given the rising costs of theliquid raw materials extracted from oil (for whichthere is ever greater pressure of demand for use inrefineries), and are particularly cheap in areas withlarge-scale availability of natural gas.

In this scenario, recourse to methane as analternative raw material to satisfy the rising demandfor light olefins, while at the same time avoiding thepotential imbalance caused by the present distributionof the sources of production, offers significantopportunities for the near future and one of the bestprospects for developing and exploiting natural gas,particularly for deposits in remote areas where the unitcost of methane already makes these technologieshighly competitive compared to present ones, whichare based on the use of liquid derivatives from oil.

This way of exploiting the methane in natural gascould also lead to the building of integrated plantscontaining all the transformation stages necessary forproducing polyolefins (Gas To Polyolefins, GTP), asshown in Fig. 1, with the economic advantage of afurther reduction in transport costs.

Olefins from methanol

The development of processes for synthesizinglight olefins from methanol is designed both toimprove the already existing technologies at everystage, and to create industrial units with high

production capacity and the consequent economicadvantages.

The possibility of obtaining various types ofhydrocarbons from methanol has been known for sometime, but, in spite of significant research on thesubject, there has been no real economic advantage indeveloping industrial processes based on the use ofalternative raw materials to oil, which is clearlyavailable at costs that are still competitive.

The reaction forming hydrocarbons from methanolwas discovered in 1977 (Chang and Silvestri, 1987):

CH3OH��[CH2]n�H2O

The hydrocarbons are formed through a firstreaction stage in which the methanol condenses intodimethyl ether, forming an equilibrium mixture ofreagent and products; a second stage in which themixture of oxygenated products is converted into lightolefins; in the final stage the light olefins aretransformed, through oligomerizing, cracking andaromatising reactions, into a mixture of heavierolefins, normal and iso-paraffins, aromatics andnaphthenes.

Technologies are available today to obtain liquidhydrocarbons (Methanol To Gasoline, MTG) andgaseous olefin hydrocarbons (Methanol To Olefins,MTO) from methanol. MTG technology was appliedon an industrial scale in New Zealand in 1986, whilelater MTO technologies remain at present still atdemonstration plant level.

Catalysts MTO technology is catalysed by solid acids,

mainly zeolitic and zeolite-like materials. The type ofcatalyst and the reaction conditions adopted canencourage the formation and distribution of lightolefins, rather than heavier hydrocarbons as in MTGtechnology.

Very many catalytic materials have been studied,including both zeolites and zeolite-like materials suchas silicoaluminophosphates and other metalaluminophosphates of all classes of porosity (Stocker,1999). These materials have been used in acidic formor variously modified by de-aluminising treatmentwith steam, impregnation, ionic exchange andisomorphic substitution with metals.

So far zeolite ZSM-5 based catalysts, in the classof medium pore materials, and SAPO-34 basedcatalysts in the class of small pore materials, have


NATURAL GAS UPGRADING

Production sources Ethylene Propylene

Ethane 28% –

Propane, butane (LPG) 10%69%

Naphtha, gas oil 60%

Catalytic cracking – 29%

Other 2% 2%

Table 1. Current production sources for ethyleneand propylene (CMAI, 2002)

methane

CH4 CO � H2 CH3OH C2H4 � C3H6 PE, PP

methanol olefinssyngas polyolefinsFig. 1. The transformation of methanol into polyolefins(GTP).

proved to be the most suitable. ZSM-5 gives overallyields of olefins of around 70% on a molar basis,based on methanol, with a large preponderance ofpropylene over ethylene; SAPO-34 affords higheryields of around 90%, with a prevalence of ethylene.The greater selectivity of SAPO-34 is probably due tothe smaller dimensions of the pores and a lower degreeof acidity, which hamper the formation of heavierolefins, and later of naphthenes and aromatics.However, with SAPO-34 there is a much more rapiddeactivation of the catalyst than that observed withZSM-5.

Accordingly, the industrial processes arecharacterized by different technological peculiarities,particularly with regard to the choice of the mode ofconducting the reaction and regenerating the catalyst,which are determined by the specific characteristics ofthe materials used.

MechanismsThere is still considerable scientific debate over the

reaction mechanism behind the formation ofhydrocarbons from methanol, mainly as concerns theformation stage of the first bond between two atoms ofcarbon, with various conjectures.

The hydrocarbon pool mechanism suggests thepresence of a compound, permanently adsorbed on thesurface of the catalyst, with a chemical compositionsimilar to that of coke and generically described withthe formula (CH2)n, responsible for the continualaddition of reagents and the elimination of products,as shown in the scheme in Fig. 2 (Stocker, 1999).

The oxonium-ylide mechanism entails a series ofsubsequent stages, starting from the initial physicaladsorption of a molecule of methanol on the surfaceof the catalyst, in which the molecule of adsorbedmethanol leads, through dehydration, to thesubsequent formation of a methoxyl species linkedto the surface of the catalyst, followed by the rapid

transfer of a methylic proton to the neighbouringoxygen, which bridges the atom of silica and theatom of aluminium in the zeolitic structure, forminga surface ylide species (Hutchings et al., 1999). Thecarbon of the ylide species is sufficientlynucleophilic to react with another atom of carbonpresent in a molecule of free methanol: thesubsequent formation of the first C�C bond is thusa facile reaction of the ylide species with anothermolecule of methanol through a classical SN2mechanism, with the formation of a surface ethoxylspecies, as in the scheme in Fig. 3. The ethoxylspecies frees a molecule of ethylene through thetransfer of a methylic hydrogen in position b to thebridging oxygen atom in the zeolitic structure, orfurther reacts with another molecule of methanol toform propoxyl and butoxyl species, from which thecorresponding molecules of propylene and butenesderive analogously.

The formation of olefins and higherhydrocarbons from light olefins, on the other hand,follows the reaction mechanisms typical of carbene



saturatedhydrocarbons

coke

(CH2)n

C2H4

C3H6

C4H8

CH3OH

Fig. 2. Scheme of the hydrocarbon pool reaction mechanism(Stocker, 1999).

SiO

AlO

Si

H

H3C

CH3

SiO

AlO

Si

CH2

SiO

AlO

Si

�H2O

�H2O

SiO

AlO

Si

H

CH2

� C2H4

HO

SiO

AlO

Si

H

H3CHO

CH2

SiO

AlO

Si

H

CH3

Fig. 3. Scheme of theoxonium-ylide reactionmechanism (Hutchingset al., 1999).

ions in the presence of acid catalysts, includingprotonation/deprotonation, a shift of methyls andhydrides, protonated cyclopropane (PCP) branching,methylation/demethylation, oligomerization and thesplitting of radicals in position b to the charge of thecarbene ion (Alwahabi and Froment, 2004).

The oxonium-ylide mechanism has been verifiedin various ways, both theoretically, demonstratingthat the ylide intermediate is in effect congruent onthe energy level as well as consisting of a carbonatom with tetrahedral symmetry inserted on thebond between the oxygen and aluminium atoms ofthe zeolite, and experimentally, although nodefinitive confirmation of the existence of the ylidespecies has emerged. One of the most problematicaspects of this mechanism concerns the nature of thebasic site, evidently present on the acid catalyst usedin the reaction, which should be able to bring aboutthe required extraction of a proton from themethoxyl species or more generally from thetrimethyloxonium ion, formed by multiple acid-catalysed condensations of the molecules of methanol.

A proposed alternative to this mechanism is asubstantially different reaction path with theconjectured existence of an induction period at theend of which there would be the carbene ion 1,3-dimethylcyclopentene, whose existence hasbeen experimentally proven, in equilibrium withthe corresponding diene, easily methylable frommethanol (Haw et al., 2000). Skeletal

isomerization of the methylated cyclic cations canproduce cyclic cations with substituents bearing 2or 3 atoms of carbon, from which ethylene wouldform by elimination. The 1,3-dimethylcyclopentene carbenium ion thus actscontinually as a reserve of cyclic dienes fromwhich the first olefins derive, as shown in thescheme in Fig. 4. Confirmation of the plausibilityof this mechanism comes from the experimentalproof of the absence of the induction period whenthe initial reagents are accompanied by ethylene,whose isotopic carbon is effectively incorporatedin the cation 1,3-dimethylcyclopentene.

The most complete reaction mechanismimaginable for the formation of light and heavyolefins would also include the formation of the ionsdimethyloxonium (DMO�) and trimethyloxonium(TMO�) ions by the protonation and condensationof the methanol, as well as the formation of �CH2

�

species, in accordance with the hydrocarbon poolmechanism described earlier.

ProcessesThere are at the moment two technologies for

producing olefins from methanol: MTO technology,developed by UOP and Norsk Hydro, and MTP(Methanol To Propylene), developed by Lurgi andStatoil. Other companies, particularly ExxonMobil,which already applied MTG technology on anindustrial scale, are involved in developing MTOtechnology.



H3C

CH3 CH3

CH3OH

�

�H�

�H�

�H��H2O

CH3OH

�H2O

CH3

HCH2 CH2�CH2

CH3CH3 C

H

H

CH3 C

H

CH3

CH3

�

�

�

��

CH3

H3C�

�

CH2

H

H3C

H3C

H3C H3C H3C

H3C�

CH3

CH3

�H�

�

CH3 ��

CH3�

CH3�

CH2

Fig. 4. Scheme of theinterconversion reactions of the1,3-dimethylcyclopenteneion in the synthesis of lightolefins from methanol(Haw et al., 2000).

UOP/Hydro MTO technologyThe technology is based on the use of a catalyst

whose active phase consists of SAPO-34. Becauseof the extremely rapid deactivation characteristic ofthis type of catalyst, the reaction is conducted in afluid-bed reactor, from which the catalyst iscontinually removed and sent to a regenerationreactor that effects air combustion of the carbondeposits. The use of fluid-bed reactors requiresspecific characteristics in the catalyst that areobtained through the formulation of the SAPO-34active phase with binders and the subsequentatomisation through spray-drying, producingcatalytic particles of suitable dimensions with highmechanical resistance to limit as much as possiblethe consumption of the catalyst.

The reaction is conducted at pressures from 1 to 3bar and at temperatures varying from 350 to 500°C, inrelation to the desired distribution of ethylene andpropylene, which can be modulated in a fairly wideinterval (from 0.75 to 1.5), acting on the reactiontemperature favouring the formation of ethylene. Tolimit undesired reactions such as oligomerisation andcoking, the methanol is diluted with water beforebeing used to feed the reactor. The process allows aonce-through yield of ethylene and propylene ofaround 80 molar%, and a 90% yield if the fraction ofC4 olefins that can be recirculated to the reactor istaken into account. The remaining methanol isconverted into various by-products, such as saturatedhydrocarbons from 1 to 5 atoms of carbon, carbonoxides and coke.

The UOP/Hydro technology has been developedat demonstration plant level with a capacity of 1 t/dof methanol, producing olefins with the necessarydegree of purity for the subsequent stage ofpolymerization.

Lurgi MTP technologyThe MTP technology developed by Lurgi uses a

zeolite ZSM-5-based catalyst. The morphology of thecatalyst and the nature of the acid sites determine highselectivity towards the formation of propylene, as wellas a relatively low deactivation speed, allowing the useof fixed-bed reactors, with periodic regenerations ofthe catalyst in situ through controlled combustion ofthe coke.

The reaction is conducted in six reactors in series,at temperatures between 400 and 500°C and atpressures from 0.1 to 1 bar, fed by a mixture of water-dimethyl ether-methanol combined with arecycling current containing hydrocarbons from C4 toC6. The effluent contains a mixture of olefins,paraffins, aromatics, naphthenes, and light productssuch as carbon oxides, hydrogen and reaction water,

with an overall molar yield in hydrocarbons of around85% based on the fed carbon.

The overall yield of propylene is around 70%,obtained through the recycling of the ethylene and thehigher olefins produced, with a significant co-production of gasoline and LPG. The scheme of anindustrial unit for synthesizing light olefins frommethanol with Lurgi MTP technology is illustrated inFig. 5. Table 2 shows the relevant mass balance(Liebner, 2005).

The MTP technology, too, has been tested in ademonstration plant run by Statoil in Norway, whichconfirmed the expected yield of propylene andreached a duration of 8,000 hours for the catalyst. Theaverage duration of each reaction cycle between twothermo-oxidation regenerations was of around 700hours. Both the propylene obtained and the gasolinewere used for the production of polypropylene and inmotor tests, and proved to be adequate in quality.

ExxonMobil MTO technologyIn the 1970s, Mobil developed the MTG process to

obtain synthetic gasoline from methanol, creating anindustrial plant with a capacity of 600,000 t/yr withfixed-bed ZSM-5-based catalyst technology.

A demonstration plant with fluid-bedtechnology for the MTG reaction was later alsoused in reaction conditions and for sufficient timeto demonstrate the reliability of MTO technology(Keil, 1999). As regards the reaction mechanism,the formation of olefins is an intermediate stage ofthe MTG reaction. Later ExxonMobil continuedresearch activity in this area, registering numerouspatents for improvements in the catalyst and theprocess, not only using the zeolite ZSM-5 but alsozeolite-like materials such as SAPO-34, andannounced that it possessed a technologydeveloped up to demonstration-unit level,including all the transformation stages frommethanol to the polymerization of ethylene andpropylene. However, this technology will not beavailable for some time.

Recovery of higher olefins

The processes for transforming methanol intoethylene and propylene also produce primarilyconsiderable quantities of higher olefins (butenes,pentenes). Higher olefins can also be derived as a by-product in the thermal cracking and, to a lesserextent, catalytic cracking processes. For example, amixture of C4 hydrocarbons, mainly composed ofolefins and diolefins in similar quantities, generallyforms the most quantitatively significant by-product inthermal cracking processes, with variable yields



depending on the type of feeds used. The demand for C4olefins and higher olefins in general is relatively weakand their commercial value low. Inevitably, then, theytend to be transformed into lighter olefins (Tagliabue et al., 2004). The processes are essentially based on twotypes of chemical transformation: metathesis reaction(Mol, 2004), particularly for C4 olefins:

C2H4�2-C4H8��2C3H6

and catalytic cracking reaction involvingoligomerization, disproportioning, cracking and thetransfer of hydrides.

Metathesis reaction is carried out in a gaseous(between 300 and 400°C) or liquid phase (at 30-50°C)in fixed-bed catalytic reactors, where reagents andproducts reach a composition corresponding to thethermodynamic equilibrium. The catalysts are basedon tungsten or rhenium oxide supported on silica oralumina. With a conversion per pass of around 60%,an overall yield of propylene from 90 to 95% isobtained. The catalyst undergoes periodicregenerations to remove the carbon deposits and thecoke formed during the reaction. The main reactionsare accompanied by a series of secondary reactionswith the formation of higher olefins such as pentenesand hexenes.

Catalytic cracking has the advantage of notconsuming ethylene and allows the use of a morevaried spectrum of higher olefins as raw material. Ittakes place at temperatures between 400 and 600°C,using zeolite-based catalysts such as ZSM-5, in fluid-or fixed-bed catalytic reactors with periodicregeneration in situ. The duration of the reaction cyclevaries significantly, according to the process used:from 150-200 hours to around 1,000 hours, whensteam is present. The overall yields vary according to



H2O

H2OHC

DMEC4

�

DMEC4 C3

C7�

C2�

C3�

C5/C6�

vaporizer

gasoline

propylene

C2�

purge (fuel gas)

C5/C6purge

CH3OHrecovery

DME/CH3OH/H2O

H2O

compression4 stages

DMEremoval De - C2

DMEreactor

methanol HC recycle

LPG (C3/C4)

C4 purge

C4 recycle

C4/C5/C6recycledilution

steam

DM

E/C

H3O

Hre

cycl

e

MTPreactor1st stage

MTPreactor

2nd stage

MTPreactor

6th stage

quenchtower De - C4

C3splitter

De - C6

C5/C6 recycle

propane

Fig. 5. Flowchart of the Lurgi MTP technology (Liebner, 2005); DME, dimethyl ether; HC, hydrocarbons.

Feeding Products

Methanol 5,000

Propylene 1,557

Gasoline 429

LPG/fuel gas 162

Water, COx, coke, etc.

2,852

Total 5,000 5,000

Table 2. Lurgi technology: mass balance (t/d). MTP unit with 519 kt/yr capacity of propylene

(Liebner, 2005)

the feeding, but are around 60%, with a prevalence ofpropylene over ethylene and the co-production ofsaturated and aromatic hydrocarbons.

The production of isobutene, a key raw material forthe production of methyl-tert-butyl ether (MTBE), ahigh-octane component of gasolines, requires separateconsideration. In the past, the only source of isobutenewere the C4 fractions of the refineries, from whichmainly butadiene was extracted. However, later plantshave used the dehydrogenation of the isobutaneobtained from the C4 fraction of the field gases.

References

Alwahabi S.M., Froment G.F. (2004) Single event kineticmodeling of the methanol-to-olefins process on SAPO-34,«Industrial and Engineering Chemistry Research», 43,5098-5111.

Chang C.D., Silvestri A. J. (1987) MTG. Origin, evolution,operation, «CHEMTECH», 17, 624-631.

CMAI (Chemical Market Associates Inc.) (2002) World lightolefins analysis.

Haw J.F. et al. (2000) Roles for cyclopentenyl cations in the

synthesis of hydrocarbons from methanol on zeolite catalystHZSM-5, «Journal of the American Chemical Society»,122, 4763-4775.

Hutchings G.J. et al. (1999) Methanol conversion tohydrocarbons over zeolite catalysts. Comments on thereaction mechanism for the formation of the first carbon-carbon bond, «Microporous Mesoporous Materials», 29,67-77.

Keil F.J. (1999) Methanol-to-hydrocarbons. Process technology,«Microporous Mesoporous Materials», 29, 49-66.

Liebner W. (2005) Lurgi MTP technology, in: Meyers R. A.(editor in chief) Handbook of petrochemical productionprocesses, New York, McGraw-Hill, 10.3-10.13.

Mol J.C. (2004) Industrial applications of olefin metathesis,«Journal of Molecular Catalysis A. Chemical», 213, 39-45.

Stocker M. (1999) Methanol-to-hydrocarbons. Catalyticmaterials and their behavior, «Microporous MesoporousMaterials», 29, 3-48.

Tagliabue M. et al. (2004) Increasing value from steam crackerolefin streams, «Petroleum Technology Quarterly», Winter,145-149.

Gianni GirottiFranco Rivetti

Polimeri EuropaNovara, Italy



10.2.2 Selected technologies fornatural gas upgrading

The MTO processChanging natural gas into olefins is a two-step

process. The first step, which consists of convertingnatural gas to crude methanol, has been available tothe industry for some time. On the other hand, thesecond step, involving the transformation of theresulting methanol into olefins, has recently beenintroduced by UOP (Universal Oil Products) andHydro of Norway.

The UOP/ Hydro MTO (Methanol To Olefins)process provides an economical means to convertnatural gas to olefins. The MTO process primarilyconverts the methanol into ethylene and propylene(Fig. 1). Ethylene and propylene are in increasingdemand worldwide and have significant financialvalue in the marketplace.

Other technologies for indirect conversion ofmethane to higher value products do exist.However, these processes have lower yields thanthe UOP/Hydro MTO process and are, therefore,less economical. The MTO process offers:a) exceptional value for direct conversion ofmethane to polymer-grade ethylene and propylene;b) direct use of ethylene and propylene inchemical-grade products with greater than 98%purity, using a flow scheme that does not requireexpensive ethylene/ethane or propylene/propanesplitters; c) limited production of by-productscompared to a steam cracker, which results in asimplified product recovery section; d ) easyintegration into existing naphtha cracker facilitiesdue to low paraffin yields; and e) flexibility tochange the propylene to ethylene product weightratio from 0.77 to 1.33.

Applications. The MTO process can be utilizedin locations with cheap, abundant natural gasreserves. By integrating the MTO process into a GasTo Olefins (GTO) complex, feedstock prices can beheld down and natural gas can be converted into aform that is more easily transported and is of highervalue. Existing naphtha or ethane-propane crackerfacilities can increase olefin production andfeedstock flexibility by installing an MTO reactorsection and feeding into a revamped cracker

fractionation section in order to minimize capitalinvestment.

The existing fractionation equipment can oftenbe easily debottlenecked to handle the additionalolefins produced, due to the fact that theUOP/Hydro MTO process produces a rich olefiniceffluent containing low quantities of paraffins.

Yet another application would be placeddownstream of an existing methanol plant withexcess capacity, to meet local demands for olefinsand polyolefins.

Description. In the MTO process, methanol isconverted primarily to light olefins (ethylene andpropylene). The process can provide a broad rangeof propylene to ethylene product ratios. Theapproximate ratios of ethylene relative to the totallight olefins (C2�C3) are 0.57 and 0.43, for highethylene and high propylene operating modes,respectively. By simply changing the reactor


drye

r

C1

C4� product

98�%purityethylene

98�%puritypropylene

compressor

aircrudemethanol

H2O

fluegas

Fig. 1. UOP/Hydro MTO process.


operating severity, the MTO process user can changethe operation modes as a function of marketdemands.

Long-term methanol conversion of over 99% andstable product selectivity have been demonstrated atHydro’s large process demonstration plant inNorway. This plant circulates and regeneratescatalyst continuously and uses crude methanol as afeedstock at a rate of more than 0.75 Mt per day. TheMTO commercial process utilizes a fluidized-bedreactor with a continuous fluidized-bed regenerator.This technology is an extension of UOP’sestablished FCC (Fluid Catalytic Cracking)commercial technology.

Feedstocks. Feedstock for the MTO process ismethanol (crude or high purity) usually producedfrom synthesis gas (CO�H2), which is producedfrom the reforming of abundant natural gas.Synthesis gas can also be produced by steamreforming of petroleum products such as naphtha,partial oxidation of natural gas and petroleumproducts, and coal gasification.

Catalyst. The reaction is catalysed by the MTO-100 silicoaluminophosphate (SAPO) syntheticmolecular sieve-based catalyst. The catalyst hasdemonstrated the degree of attrition resistance andstability required to handle multiple regenerationsand fluidized-bed conditions over the long term. Thecatalyst is extremely selective toward the productionof ethylene and propylene.

The olefin cracking process for ethyleneand propylene production

The Atofina/UOP olefin cracking processconverts C4 to C8 olefins to propylene and ethylenewith a high propylene to ethylene ratio. Followinginitial work by Atofina in the mid 1990s, UOP andAtofina formed an alliance for joint development in2000. The Olefin Cracking Process (OCP) wasdeveloped to utilize low value by-product streamscontaining C4 to C8 olefins from steam crackers,refineries and methanol-to-olefins plants.

The development activities included thesuccessful demonstration of the technology on alarge scale, process design, and catalystmanufacturing development. The demonstration unitwas initiated in 1998 at an industrial facility locatedat Antwerp, Belgium, and processes feedstocks froma commercial operating plant. The demo-plantincludes feed pretreatment, a reactor section,catalyst regeneration facilities, and internal recyclecapabilities.

Description. The olefin cracking process (Fig. 2)is designed to process olefinic feedstocks fromsteam crackers, refinery FCC and coker units, and

MTO units, with typical C4 to C8 olefin and paraffincompositions.

The olefin cracking process features fixed-bedreactors operating at temperatures between 500 and600°C and pressures between 1 to 5 bar. The processutilizes a proprietary zeolitic catalyst and provideshigh yields of propylene. The catalyst exhibits lowsensitivity to common impurities such as dienes,oxygenates, sulphur compounds, and nitrogencompounds. Furthermore, this catalyst minimizesthe reactor size and operating costs by operating athigh space velocities and high conversions andselectivities without requiring an inert diluentstream. A swing reactor system is used for catalystregeneration. Separation facilities depend on howthe unit is integrated into the processing system.

Steam cracker integration. When an olefincracking unit is integrated with a naphtha steamcracker (Fig. 3) the yield of propylene is dramaticallyincreased for the same total naphtha flow-rate. Lowvalue C4 through C6 by-product streams produced inthe cracker furnaces can be charged to an olefincracking unit where additional light olefins areproduced with high P/E ratios (i.e.propylene/ethylene ratios). Internal recycle is usedto optimize conversion of olefins. The OCP lightolefin product streams are sent to the naphthacracker recovery section, while the C4-C6 streams,now depleted in olefins and rich in paraffin, arerecycled to the naphtha cracker furnaces. Casestudies of olefin cracking integration with naphthacrackers have shown 30% higher propyleneproduction compared to conventional crackerprocessing. The integration with olefin crackingallows an overall propylene to ethylene product ratioof 0.8 to be achieved without sacrificing ethyleneproduction.



depr

opan

izer

lightolefinproduct

C4-C8olefins

C4 by-product

C5� by-products

OCPreactor section

debu

tani

zer

Fig. 2. Olefin cracking process.

Refinery integration. When integrated with anFCC refinery (Fig. 4) the OCP converts C4-C8 olefinrich streams from the FCC and coker units to highvalue light olefins, which consist mostly ofpropylene. The gasoline stream is lower in olefinsbut retains virtually the same octane number due tothe small amount of aromatics formed.

MTO integration. MTO offers a new source oflight olefins based on natural gas via methanol.Although the MTO reactions are quite selective, C4�by-product streams are produced. Achieving goodvaluation of these by-products can sometimes bedifficult because MTO projects may be installed inremote locations. By integrating an olefin crackingprocess into an MTO complex, the overall yield withrespect to the feed sent to the complex can begreatly increased (Fig. 5). The yield of methanol feedthat goes to light olefins for an MTO complexaugmented with olefin cracking can approach 90%.

The Oleflex process for propylene productionThe UOP Oleflex process is a catalytic

dehydrogenation technology for the production oflight olefins from their corresponding paraffins. Onespecific application of this technology producespropylene from propane. Propylene is the world’ssecond most important petrochemical commodityand is used in the production of polypropylene,acrylonitrile, acrylic acid, acrolein, propylene oxideand glycols, plasticizer oxoalcohols, cumene,isopropyl alcohol, and acetone. The growth inpropylene production is driven primarily by theindustry demand for polypropylene, which is used insuch everyday products as packaging materials andoutdoor clothing. The growth rate of polypropyleneis expected to be 5% per year for the near future.The Oleflex process provides: a) a dedicated sourceof propylene supply for downstream use; b) increased control over long-term propylene costs;c) high-quality propylene production, which leads to

high-quality polymers; d ) potential for processintegration with downstream technology; and e) continuous on-stream production of propylene.

In addition, the Oleflex process utilizes UOP’sproprietary equipment and systems for optimaloperations, including PSA (Pressure SwingAbsorption) Polybed units, modular CCR (CyclicCatalytic Reforming), lock-hopper control, MD(Multiple Downcomer) distillation trays, High-Fluxtubes, and Process Instrumentation Controls (PICs).Integration of these products within the Oleflexprocess results in significant capital and operatingcost savings for the complex and provides an overallguarantee for the Oleflex process and products. Withthe use of CCR catalyst regeneration, the processingunit does not have to be shut down to change thecatalyst.

Applications. The majority of propylene isproduced as a by-product of petroleum refineries(FCC/RCC, Residual Catalytic Cracking) and olefinplant steam crackers. As a result, most propylene is aby-product of other products, specifically gasolineand ethylene. However, when production capacity isnot coupled with a demand for those by-products, asupply/demand imbalance can occur. The Oleflexprocess provides petrochemical producers with acatalytic, intentional means of making propyleneindependent of the demand for gasoline andethylene. The Oleflex process provides producers



furnacesection

naphtha

light olefinsC4-C6

C2productrecovery andpurification

section

OCP unit

C3flue gaspropylene gas

Fig. 3. Integration of an olefin cracking unit with a naphtha steam cracker.

methanol

light olefinsC4-C5 olefins

C2

OCP unit

MTO unitC3

C4�

Fig. 5. Integration of an olefin cracking unitwith an MTO unit.

gas oils

lightolefins

C4-C8 C2olefinrecovery

OCP unit

FCC unit

C3

LPG

gasolinecycle oils

C5�

Fig. 4. Integration of an olefin cracking unit with an FCC unit.

with high quality propylene, which then leads tohigh-quality polymers. This process consumes lesspolymerization catalyst due to the lower impuritylevel in the propylene product and has the potentialto be integrated with existing downstreamtechnology.

Description. The feedstock to a C3 Oleflexprocess unit is propane. Propane is recovered frompropane-rich LPG (Liquefied Petroleum Gas)streams from gas plants. Propane is also available insmaller quantities as a by-product from suchrefinery operations as hydrocracking, FCC and RCCunits.

The UOP Oleflex process (Figs. 6 and 7) isseparated into three different sections: the reactorsection, the product recovery section, and thecatalyst regeneration section.

The reactor section of the Oleflex processconsists of four radial-flow reactors, charge andinterstage heaters, and a reactor feed-effluent heatexchanger. In the product recovery section, thereactor effluent is cooled, compressed, dried, andsent to a cryogenic system to separate hydrogenfrom the hydrocarbon. The net gas is recovered at 85to 93 mol% hydrogen purity. Separator liquid is sentto a selective hydrogenation unit to eliminatediolefins and acetylenes. Then the liquid goes to adeethanizer and Propane-Propylene (P-P) splitter toproduce a chemical or polymer-grade propyleneproduct. Unconverted propane is recycled to thereactor section. The selective diolefin and acetylenehydrogenation step is accomplished with the HülsSHP (Selective Hydrogenation Process) licensed by

UOP. The Oleflex process uses a platinum catalyst topromote the dehydrogenation reaction. The DeH-14catalyst, introduced in 2001, represents the fifthgeneration of this catalyst. Not only does the DeH-14 maintain the high activity and selectivityand low attrition rates required for thedehydrogenation process, it also has lower platinuminvestment than earlier catalysts. A catalystregeneration section burns coke off the catalyst andreturns it to fresh activity.

The Cyclar processThe Cyclar process converts LPG directly

into a liquid aromatics product in a singleoperation. Developed jointly by BP (British



Rx effluentcompressor

turboexpander

off-gas

to propylenerecovery

fresh andrecycle feed

H2 recycleheaters

reactor section CCR section product separation section

Fig. 6. C3 Oleflex process.

deet

hani

zer

depr

opan

izer

P-P

spl

itte

r

H2

C4�

C2�net gas

propanefeed

Oleflexpropylene

SHP

Fig. 7. C3 Oleflex complex.

Petroleum) and UOP, the Cyclar process expandsthe use of LPG to the production of high-valuepetrochemical aromatics. LPG consists mainly ofthe propane and butane fraction recovered fromgas and oil fields and petroleum refiningoperations. The relatively low value andabundance of LPG make it an ideal feedstock forpetrochemical applications.

Benzene, Toluene, and Xylenes (BTX) areproduced primarily by the catalytic reforming ofpetroleum naphtha. However, naphtha is in greatdemand for gasoline and petrochemical production,and the value of naphtha is expected to rise assupplies become tighter. The Cyclar process offers aunique ability to produce petrochemical-grade BTXfrom a lower value feedstock, and can be used in

production fields to convert excess LPG into aliquid product for pipeline transport. A detaileddescription of the Cyclar process technology isreported in Section 10.6.1.

Bibliography

Pujado P.R., Andersen J.M. (1986) Natural gastechnologies, in: Meyers R.A. (editor in chief) Handbookof petroleum refining processes, New York, McGraw-Hill, 15.4.

Jim M. AndersenCopyright 2004 UOP LLC

All rights reservedUsed with permission



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