1 Vaporization in Oxidation to Phthalic Anhydride

8/3/2019 1 Vaporization in Oxidation to Phthalic Anhydride

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De Munck et al.(10) Pub. No.: US 201110230667 Al(43) Pub. Date: Sep. 22, 2011

(54) VAPORIZATION IN OXIDATION TOPHTHALIC ANHYDRIDE

(30) Foreign Application Priority Data

(76) Inventors: Nicolaas Anthony De Munck,Barendrecht (NL); Aad Oskam,Rozenburg Zh (NL); Evert Klein,Dorst (NL)

Sep. 28, 2007 (GB) 0718994.7Publication Classification

(51) Int. Cl.C07D 307/89 (2006.01)B01J 19/00 (2006.01)

(52) U.S. Cl. 549/248; 422/224(21) Appl. No.:(22) PCTFiled:(86) PCTNo.:

371 (c)(I),(2), (4) Date:

12/673,420 (57) ABSTRACTSep. 9, 2008 In the production of phthalic anhydride by the oxidation ofortho-xylene with air, the ortho-xylene loading is increasedwithout increasing the likelihood of explosion by insulatingthe system to avoid cold spots to keep the ortho-xylene at atemperature above its dew point; in addition the system maybe electrically interconnected and grounded to reduce the riskof spark initiated explosions or deflagrations.

PCT IEP08/61934

Jun. 7, 2011


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Patent Application Publication Sep. 22, 2011 Sheet 3 of 4 US 2011/0230667 At


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Patent Application Publication Sep. 22, 2011 Sheet 4 of 4 US 2011/0230667 At


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VAPORIZATION IN OXIDATION TOPHTHALIC ANHYDRIDEFIELD OF THE INVENTION

[0001] The present invention relates to a process and appa-ratus for the production of phthalic anhydride, and in particu-lar to the configuration of systems for the generation andhandling of mixtures of ortho-xylene and an oxygen-contain-ing gas, particularly air , as the reactor feedstock for the pro-duction of phthalic anhydride, which mixtures contain morethan 44 grams of ortho-xylene per normal cubic meter of airand are thus flammable and explosive. The invention furtherrelates to the production of phthalate ester and hydrogenatedphthalate ester derivatives.

BACKGROUND OF THE INVENTION[0002] Phthalic anhydride is an important intermediatechemical in the chemical industry. One important use is in theproduction of phthalates such as di-isononyl or di-isodecylphthalates, which are used as plasticisers, typically for poly-vinyl chloride. Phthalic anhydride has been produced on anindustrial scale for many years and has generally been pro-duced by the vapour phase oxidation of ortho-xylene with anoxygen-containing gas, such as air, by passing a mixture ofortho-xylene and the oxygen-containing gas over an oxida-tion catalyst.[0003] A typical plant for the production of phthalic anhy-dride comprises a raw material delivery section, a raw mate-rial mixing section in which a hot mixture of the oxygen-containing gas and ortho-xylene vapour is prepared and amixture delivery section for feeding to a reaction systemcomprising the reactor which typically consists of reactortubes containing catalyst. The components of these sectionsare known as the process equipment. The reaction is exother-mic and the temperature of the reactor tubes is controlled bya temperature control fluid, such as molten salt, flowingaround the tubes.[0004] After the reaction, the crude phthalic anhydride thathas been produced passes to a cooling stage where it is cooled,generally by a gas cooler, passed to optionally a liquid con-denser and finally to a switch condenser. Finally, the con-densed phthalic anhydride is subjected to a purification orfinishing step.[0005] The efficiency of a phthalic anhydride plant is mea-sured in terms of the number of grams of ortho-xylene thatcan be processed for each cubic meter of air that is fed to theraw material section (known as the loading). The greater theamount of ortho-xylene, the greater is the efficiency of thefacility. Considerable attempts have been made over the yearsto increase the loading, and loadings above 80 gram/Nm ' ofortho-xylene in air have been reported. One difficulty in themanufacture of phthalic anhydride is that, at the temperaturesrequired for the reaction of air and ortho-xylene the mixturebecomes flammable and explosive ata loading above 44 gramof ortho-xylene per normal cubic meter of air . Accordingly,great care must be taken to avoid or reduce the likelihood ofexplosions. When an explosion occurs and the flame velocityexceeds the velocity of sound, this supersonic explosion iscalled a detonation. Otherwise, at subsonic flame velocities, itis called a deflagration. By the provision of an adequatenumber of escape ducts, such as chimneys, sealed off byrupture discs, at crit ical locations, the occurrence of a deto-nation is avoided, while the burning gas from a deflagration is

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relieved to a safe location. One or more rupture discs areconveniently located on the ortho-xylene vaporizer, at thereactor inlet and outlet, and on downstream equipment andthe sections of the piping operating within the flammabilitylimits. These rupture discs can be of any suitable design,although reverse buckling or bending rod type are preferred.One of the areas in a phthalic anhydride facility that is proneto a deflagration is the raw material mixing section, where theortho-xylene and the air are mixed. One of the reasons for adeflagration to occur is ifthere is incomplete vaporisation orcondensation in the vapour/air mixture at the time when itreaches the oxidation catalyst. Other reasons can be poormixing of the heated ortho-xylene and the heated air, inho-mogeneity inthe composition of the mixture, discharges fromthe build-up of static electricity, or the decomposition ofperoxides formed from feed impurities like cumene or sty-rene. The present invention is concerned with reducing orminimising the likelihood of a deflagration of an explosionoccurring.[0006] In a typical commercial process the generation of afeed gas mixture has to date been performed as follows.Process air is sucked in from the surroundings through a filterby means of a blower, and compressed to a pressure levelwhich allows the conveyance of the air stream through thephthalic anhydride plant . This process air stream is heated ina heat exchanger disposed downstream ofthe blower. Parallelthereto, l iquid ortho-xylene from a storage tank is brought toa preliminary pressure by means of a pump and passedthrough a basket type filter and a preheater before it is fed toan evaporator, vaporizer drum or spray drum. In the evapora-tor, the preheated ortho-xylene is injected in liquid form intothe heated air stream parallel to the air flow, by means of anozzle system. The fine ortho-xylene droplets completelyevaporate in the air stream, and a further smoothening of theradial concentration and temperature profiles in the gasstream is achieved by means of a homogenisation stage (ahomogenizer section comprising e.g. a static mixer). Thisfeed gas subsequently enters the reactor, typically a tubularreactor comprising of tubes filled with catalyst to provide acatalyst bed, where a part ial oxidation of ortho-xylene withthe oxygen takes place to form phthalic anhydride.[0007] This process for the generation of feed gas has suc-cessfully been used, but with the successive introduction ofhigher ortho-xylene loads inthe air stream (above 80 g ortho-xylene per Nm3 air) the process has shown potential weak-nesses with regard to the explosion safety of the raw materialsection of the plant. The lower explosion limit of a gaseousmixture of ortho-xylene and air is about 44 g of ortho-xyleneper Nm3 of air. Ithas been found that the minimum energyrequired for igniting the mixture is greatly decreased withincreasing ortho-xylene load, and therefore the desire toincrease the ortho-xylene loading increases the possibility ofan explosion. However, to a great extent, the economics of theoverall phthalic anhydride production process depends uponincreasing the load of ortho-xylene per Nm3 air. Itis thereforeof basic importance that plants with a loading in the range of80 g ortho-xylcne/Nm" air to 120 g ortho-xylcne/Nm" airmust be operated safely.[0008] U.S. Pat. No. 6,984,289 B2 relates to a process forthe production of phthalic anhydride by the oxidation ofortho-xylene with air and with a loading of 80 g to 100 g ofortho-xylene per Nm3 of air. This higher loading is said to bemade possible by complete evaporation followed by super-heating of the ortho-xylene prior to admixture with air. U.S.


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Pat. No. 4,435,581 discloses a process wherein naphthalene isfirst completely evaporated before bringing the vapours incontact with the air stream in a reactor containing a fluidisedbed of oxidation catalyst. DE 20 2005012725 Ul provides asystem in which ortho-xylene is sprayed through nozzles intoan air stream in which the flow cross-section of the air feedtube is reduced downstream of the spray nozzles, so thatvapour velocity and turbulence are increased, therebyimproving the mixing of the reaction components, and in thisway the risk of explosion is reduced. DE 20 2005012725 Ulalso provides a cone-shaped perforated screen at either side ofthe spray nozzles to divert the pressure wave from an explo-sion occurring in the evaporation section towards the rupturediscs, thereby protecting the equipment upstream and down-stream from these screens from damage by a shock wave.These screens assist also in homogenising the flow of air andthe flow of the air/ortho-xylene mixture.[0009] U.S. Pat. No. 4,119,645 also relates to a process forthe production of phthalic anhydride by the oxidation of amixture of ortho-xylene with air, but is silent about how themixture is produced and passed to the oxidation reactor. U.S.Pat. No.4, 119,645 is not concerned with the homogeneity ofthe mixture or how to preserve it until it reaches the catalyst.Patents GB 1550036 and GB 1239803 also relate to processesfor the production of phthalic anhydride by the oxidation of amixture of ortho-xylene with air. The processes operate atloadings of ortho-xylene in air that are below or barely abovethe lower explosion limit and much lower than current indus-trial practice. These processes are therefore much less sensi-tive to an inhomogeneity in the ortho-xylene/air mixture. GB1550036 and GB 1239803 are silent about the production ofthe ortho-xylene/air mixture and the passing thereof to theoxidation reactor. These documents are not concerned withthe homogeneity of the mixture or how to preserve it until itreaches the catalyst.[0010] United States patent application US 2003/0013931Al relates to a process and apparatus for producing a homo-geneous mixture of ortho-xylene vapour in air, as feed to anoxidation reactor for the production of phthalic anhydride.US 2003/0013931 Al is concerned with rapid vaporisation ofthe ortho-xylene into the air stream, and employs specialspray nozzles to that effect. The spraying is performed in achamber bounded by side walls heated to a temperature abovethe boil ing point of ortho-xylene, such that droplets of ortho-xylene which impinge on the tube wall are vaporised imme-diately and do not deposit as a liquid film. US 2003/0013931Al is not concerned with avoiding condensation on surfacesin contact with the ortho-xylene/air mixture as it passes to thecatalyst in the reactor. Itis silent about the surfaces betweenthe end of the heatable double-walled tube and the toptube sheet of the reactor. US 2003/0013931 Al is also silentabout any rupture disks that may for safety reasons be pro-vided on the inlet head of the oxidation reactor, in the rawmaterial mixing section or in the section delivering the mix-ture to the catalytic reactor. These rupture disks are safetydevices and in a heatable double-wall version would not bereadily able to perform their critical function. US 200310013931 Al is not aware of the problems of possible conden-sationof ortho-xylene on the internal surfaces ofthese rupturedisks or the flanges and piping connecting thereto. Itis there-fore not concerned with the temperature of internal surfacesof rupture disks or other equipment elements up to the inlet ofthe catalyst bed in the oxidation reactor.

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[0011] Itis important that a homogenous mixture of ortho-xylene and air is formed for feeding to the reactor and thismay be accomplished by enhancing the rate of ortho-xylenevaporisation. As is described in our co-filed UK applicationreference GB 0718994.7, we have found that this may beaccomplished by employing a particular nozzle system, and aparticular set of conditions within the nozzle, to spray theortho-xylene into the hot air, and in particular GB 0718994.7is concerned with a system for mixing ortho-xylene with anoxygen-containing gas, which system comprises an ortho-xylene evaporator or vaporiser vessel fed with a stream ofoxygen-containing gas and provided with at least one lanceprojecting into the stream of oxygen-containing gas, whichlance is provided with at least one metal spray nozzle adaptedto inject droplets of liquid ortho-xylene into the stream ofoxygen-containing gas concurrently with the direction offlow of the stream of oxygen-containing gas, in which themetal at the surface of the spray nozzle, that in use is incontact with the liquid ortho-xylene, has a hardness expressedas a Vickers hardness number according to ASTM E92-82 ofat least 200, preferably at least 250 and more preferably atleast 600. The spray nozzle is preferably made of hardenedsteel, more particularly surface hardened austenitic stainlesssteel. The desired surface hardness is preferably obtained bynitriding the nozzle surface, more preferably by coldnitridingsuch as by Kolsterising ().[0012] In addition, the applicants co-filed application ref-erence GB 0718994.7 describes a particular sealing system toprevent the leakage ofliquid ortho-xylene at undesired loca-tions from the spray nozzle system. This spray nozzle system,including the sealing system preferably comprising anannealed copper seal ring, is particular useful when used incombination with a specially designed oxygen-containinggas feed system and a particular design of oxygen-containinggas and ortho-xylene mixing system.[0013] In operation, ortho-xylene is preheated to about1400 C. under elevated pressure, flow metered with mass flowmeters, and forced into a spray nozzle configuration for injec-tion into the heated oxygen-containing gas, which istypicallyair . The hot liquid ortho-xylene is thus sprayed as a fine mistinto the hot air upon which the ortho-xylene isvaporised. Thepresent invention is concerned with maintaining the ortho-xylene in the vapour phase. Furthermore, i t is important thatthe ortho-xylene does not coalesce or condense and formliquid deposits within the raw material section of the plant, toreduce the risk of explosion when liquid deposits are formed.When ortho-xylene is allowed to condense on internal sur-faces in the equipment up to the inlet tubesheet of the tubularreactor, relatively large droplets may come loose from thesurface and be entrained by the mixture of ortho-xylenevapour and the oxygen-containing gas. Entrained larger drop-lets may not be totally vaporised by the time they reach theoxidation catalyst bed, and cause a local excessive reaction,increasing the risk for a runaway reaction and possible cata-lyst damage, and for an explosion. When such entrainedlarger droplets become totally vaporised but only just beforethey reach the catalyst bed, they may stil l be causing smallervolumes inthe vapourlgas mixture inwhich the concentrationof ortho-xylene is higher than average. Such an inhomogene-ity may also cause a local excessive reaction when reachingthe catalyst, and trigger an explosion. In order to reduce theexplosion risk it is therefore not only important to rapidlyvaporise the ortho-xylene that is sprayed into the oxygen-containing gas, it is also important to avoid condensation of


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ortho-xylene from the mixture on internal surfaces of theequipment from the production of the vapour/gas mixture upto the inlet tubesheet of the reactor.

SUMMARY OF THE INVENTION[0014] According to the present invention, a process isprovided for the manufacture of phthalic anhydride by thecatalytic oxidation of ortho-xylene, comprising producing ina raw material preparation step, by spraying a heated ortho-xylene liquid feed into a preheated oxygen-containing gasfeed, a mixture of ortho-xylene vapour and the oxygen-con-taining gas, and passing the mixture to a reaction sectioncomprising a fixed bed tubular catalytic reactor for perform-ing the reaction step in which phthalic anhydride is formed,characterised in that the mixture and the surfaces of the pro-cess equipment that are in contact with the mixture are main-tained at a temperature above the dew point ofortho-xylene inthe mixture, during the production of the mixture and thepassage of the mixture to the catalytic reactor.[0015] The manufacture of phthalic anhydride according tothe present invention requires the production of an as homog-enous as possible mixture of oxygen-containing gas andortho-xylene, which is to be maintained at a temperatureabove the dew point temperature of ortho-xylene in the mix-ture, and it is this mixture that is fed to the oxidation reactor.The dew point isthe temperature that is reached when the firstliquid forms within a vapour mixture that is being cooled, andfor the mixture of ortho-xylene and oxygen-containing gasdepends on the prevailing concentration of ortho-xylene inthe mixture, and the pressure to which the mixture is sub-jected. Dew point as used herein is defined in Perry's Chemi-cal Engineers' Handbook 5 th Edition Section 13 page 17. Bymaintaining the surfaces that are in contact with the mixtureup to the inlet tube sheet of the tubular reactor at a temperatureabove the dew point of ortho-xylene in the mixture, conden-sation of ortho-xylene on those surfaces is avoided, and theformation and entrainment of larger liquid droplets with themixture as it is passed to the reactor is avoided. The existenceof cold spots inside the critical equipment is thus avoided, sothat ortho-xylene is not condensed and the risk for localexcessive reaction on the catalyst, runaway reactions, catalystdamage, and possibly also explosions is reduced.

DETAILED DESCRIPTION[0016] The raw material preparation section typically com-prises a raw material delivery section for introducing theoxygen-containing gas, which is usually air, and a separateraw material delivery section for introducing ortho-xyleneliquid. Preferably one, more preferably both of the raw mate-rial delivery sections comprise a preheater for preheating theraw material prior tothe production of the mixture. Further inthe raw material preparation section, inthe raw material mix-ing section, the ortho-xylene is introduced, typically stil l as aliquid, into the oxygen-containing gas, and the mixture issubsequently passed through homogenisers and mixers in themixture delivery section to the reaction section. At least one,and preferably several rupture discs are usually provided atappropriate locations to minimise the effect of any deflagra-tion or explosion that may occur inside the process equip-ment. Observation windows and measuring points may alsobe provided. All these features of the plant or process equip-ment can provide cold spots, where ortho-xylene may con-dense and which thus increase the risk of an explosion or

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deflagration to occur. The present invention therefore pro-vides for thermal insulation of at least some of the externalsurfaces ofthe process equipment and the component parts ofthe raw material section to maintain the mixture and thesurfaces of the process equipment that are in contact with themixture above the dew point of ortho-xylene in the mixtureand thus prevent or reduce the formation of cold spots andthus reduce the tendency for explosions or deflagrations tooccur.[0017] We prefer that, in the oxygen-containing gas supplyor delivery section, at least one, but preferably all of theexternal surfaces of preferably all the piping and equipmenthaving a content above ambient temperature is fully insu-lated, more preferably also including the flanges betweenpiping and connecting the piping with the equipment. Morepreferably insulation is also provided to the oxygen-contain-ing gas blower discharge and continues nntil the locationwhere the oxygen-containing gas enters the vaporizer. Alsohere, we prefer the thermal insulation to be as complete andcontinuous as possible, e.g. including the flanges. We alsoprefer that, inthe liquid ortho- xylene delivery section, at leastone of the external surfaces of the process equipment throughwhich the ortho-xylene liquid feed passes, is provided withthermal insulation.[0018] We also prefer that the insulation starts at the ortho-xylene preheater of the ortho-xylene feed delivery section,which typically is a steam preheater, and continues to thespray nozzle assemblies. We further prefer that the vaporizer,the mixing device, the mixture delivery section and/or thereactor top hood are, more preferably all and most preferablyall fully, insulated, including the vessel flanges and equip-ment nozzles.[0019] As an alternative to thermal insulation, but prefer-ably in combination therewith, heat may be provided to atleast one of the external surfaces or walls of the equipmentthat is in contact with the mixture and/or the mixture compo-nents, particularly the process equipment for the productionof the mixture and for passing the mixture to the reactor. Thisheat may be provided by many possible means, but typicallyis provided by heat tracing, in the form of electrical tracing ortracing by steam or another heated fluid, or by jacketing,typically steam jacketing. With heat tracing, a heating source,such as an electrical resistance or a heating tube is provided atthe external surface or walls ofthe process pipe or equipment.The heating tube is preferably a metal tube, typically copper,which is preferred because it is easy to adapt to the shape ofthe object it is applied to, or stainless steel. The heating tubemay contain a heat transfer fluid, such as a hot oil, but pref-erably contains steam, which provides heat by condensing,upon which the condensate mayor may not be recovered.Steam heating is preferred for tracing or jacketing in aphthalic anhydride facility, because of the ample availabilityof steam generated by the process heat. Electrical tracing maybe easier tobring in place, but may be subject to failure in caseof power failures.[0020] Heat jacketing, typically steam jacketing, isachieved by surrounding the pipe or equipment with a largerdiameter pipe, or by providing an extra envelope aronnd theequipment, or at least parts of it, whereby the heat transferfluid or steam flows between the larger and the smaller pipe,or through the envelope. An alternative is to connect halfdiameter pipe onto the external wall of certain equipmentitems, such as by welding, to form piping with the cross


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section of about a half circle. The heat transfer fluid is thentypically passed through the half diameter or half circle pip-ing.[0021] Preferably the exterior of the vaporizer and the mix-ing device have half-pipe steam tracing lines which minimizethe heat loss from this equipment.[0022] When heat is provided to the outer surface and/orwalls of the equipment, it is preferred to have also thermalinsulation provided, to preserve the energy that is provided tothe heating system. Very often, thermal insulation and/or theheat providing systems discussed above are provided to themore accessible external surfaces, such as those of pipes andvessels. The flanges between piping sections and which con-nect piping to equipment, and the equipment nozzles connect-ing vessels and other equipment items to piping, are typicallynot insulated and/or traced or jacketed, because the moreirregular shapes of their external surfaces make these provi-sions more difficult. Another reason for not insulating and/orheating the flanges that connect process equipment isthe riskthat with increased thermal expansion, such as of the bolts,the flange connection may lose its tightness and a leak ofprocess fluid to the outside may develop. The risk of suchleaking increases when the flange connection is thermallyinsulated, and the presence of a leak becomes more difficult toobserve. Wenow have found that, inthe critical sections of thephthalic anhydride plant, i t is strongly preferred to also haveat least one but preferably more than one, most preferably all,equipment nozzles provided with thermal insulation and/orwith external heating provisions. This has been found toreduce the occurrence of cold spots, and to be a significantcontributor to reducing the occurrence of deflagrations. Wefound that only providing insulation and/or heat in the con-ventional way, thereby not covering flanges and/or equipmentnozzles, leads to undesirable cold spots in the equipment incontact with the vapour/gas mixture, and to more frequentdeflagrations.[0023] We have also found that under heavy rain, rain watermay seep into the thermal insulation and reach the hot surfaceof the equipment. Upon evaporation, this may cause a localcold spot to occur in contact with the vapour/gas mixture. Ifthe evaporation is violent, it may also damage the thermalinsulation, which by losing its integrity may increase the riskof further water ingress and hence of cold spot occurrence.We therefore prefer to minimise the ingress of water, hail orsnow into the thermal insulation by appropriate measures,such as appropriate sealing of the insulation outer surfaces,especially those directed upwards.[0024] The one or more rupture discs monnted on the pro-cess equipment for performing the reaction step and/or theraw material preparation step, and/or for passing the mixturefrom the raw material preparation step to the reaction step,preferably also have heating provisions as described before,preferably via tracing and more preferably steam tracing,more particularly along the equipment nozzle connecting thevessel with the rupture disc assembly. The tracing is prefer-ably thermally insulated towards the environment, to con-serve energy. The equipment nozzle towards a rupture disc,and the flange carrying the rupture disc, are typically of arelatively large size, and these provisions therefore have amore important contribution to the desired effect .[0025] A further cause of explosion or deflagration is thatthe ortho-xylene, as vapour but particularly as liquid, has thetendency to accumulate static electrical charge when experi-encing turbulent flow or shear such as by passing through

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restrictions, such as filters, pumps, nozzles, flow meters, con-trol valves and spray nozzles, due to its very low conductivity,which is typically about 0.1 picoSiemens per meter for theliquid phase. The Siemens or Mho is the reciprocal of theOhm. Such a low electrical conductivity implies that ortho-xylene, and especially in its liquid form, has a high resistancefor electrostatic charge relaxation. Itis therefore preferred toretain the liquid ortho-xylene in the process equipment priorto the production of the mixture for a residence time that issufficient for static electric charge relaxation. The appropriateresidence time can be achieved by appropriate equipmentdesign, such as selecting equipment types that provide largerprocess fluid holdups, and during operation by e.g. control-ling the flow of the ortho-xylene from the pump to the spraynozzles.[0026] The residence time that is particularly important, isthe time the liquid ortho-xylene takes for flowing from thelocation where it may still have picked up electrostaticcharge, such as when passing the feed pump, a filter, a flowmeter and/or a control valve, up to the inlet of the preheater,and up to the location where the liquid ortho-xylene issprayed into the oxygen-containing gas, such as in the spraynozzle providing the mixing of the liquid and the gas. Weprefer that the residence time provided for static charge relax-ation is at least 60, more preferably at least 75 and mostpreferably at least 85 seconds.[0027] Even more preferably, we provide such a minimumresidence time already in the preheater only, most preferablyalready in the preheater tubes only. Larger residence timesrequire even bigger equipment, and therefore we prefer theresidence time to be at most 100, preferably at most 95 sec-onds.[0028] In this section of the equipment, we prefer to selectequipment items and designs that provide larger process fluidholdups and/or items that minimise or avoid flow turbulenceof, and/or shear on, the process fluid. It is preferred to avoidhigh shear equipment in particular or more generally anyequipment that is capable of generating static charge, such asa filter, a flow meter or a control valve, in between the pre-heating and prior to the production of the mixture, such as upto the spraying by the spray nozzle or nozzles.[0029] In a preferred embodiment the ortho-xylene steampreheater is a shell and tube heat exchanger, with the steam onthe shell side and the ortho-xylene on the tube side. Illustra-t ive design condit ions are 14.5 barg pressure and 1850 C. forthe steam side, and 22 barg pressure and 1850 C. for theortho-xylene side. In such a system, the ortho-xylene goesthrough a multi-tubular heat exchanger, preferably a TEMAType C exchanger, for example an 18 pass bundle with anoutside tube diameter of about 60 mm with a wall thickness of2 to 3 mm, and we prefer this because of the low liquid flowvelocities through exchanger tubes having such or similardimensions. For a total tube length of about 98 metres in theheat exchanger, a holdup volume of 225 litres will provide aresidence time of 85 seconds at 100 g ortho-xylene/Nm ' airloading and 4 Nrrrvhr/tube air rate, for a reactor having 21000tubes. Measurements for low conductivity or accumulatorfluids, such as ortho-xylene, have shown that at least 20seconds relaxation time is sufficient to disseminate more than63% of an accumulated static electricity charge. Accordingly,the calculated 85 seconds is considered a sufficient holdup forcharge relaxation before entering the spray nozzle to reducethe likelihood of the accumulation of an extra static electricitycharge on the ortho-xylene droplets to cause explosion or


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deflagration. With such residence time, a contribution to theexplosion risk by static charge accumulation in the liquidortho-xylene feed system is essentially eliminated.[0030] In a further aspect, the present invention provides agaseous mixture of ortho-xylene and air containing more than44 grams of ortho-xylene per Nm3 air at a temperature above135 C. in which the ortho-xylene is supplied from a pre-heater and the mixture is passed to an oxidation reactor inwhich the residence time of the ortho-xylene in the preheateris between 60 and 100 seconds preferably between 75 and 95seconds.[0031] It is also preferred to provide adequate electricalgrounding to earth, either directly or indirectly, of metal partsof preferably all piping, instrumentation and process equip-ment in those areas that are in contact with ortho-xyleneliquid and/or vapour, with the oxygen-containing gas andwith the mixture of ortho-xylene vapour and oxygen-contain-ing gas. This grounding preferably starts downstream of thefluid driving means, such as at the outlet of the liquid pumpsand/or the gas blowers or compressors.[0032] It is even more preferred, as an alternate but prefer-ably in combination with electrical grounding, to minimize orto eliminate potential differences between connecting flangesets of this equipment, piping and instrumentation by electri-cal interconnections, such as by providing at least one metalwire cable strongly attached and electrically connected toboth of the flanges of at least one connecting flange set,preferably of all connecting flange sets. For such a connec-tion, a metal plate may be welded to the flange, the metal wirecable may be connected to a metal clip or clamp, and the clipor clamp may be bolted to the metal plate to allow for mini-mum resistance in the electrical connection between themetal cable and the flange. We also found that the cableshould preferably be as straight and short as possible withminimal loops. In order to provide a minimum of slack for incase the distance between the cable connection pointsincreases, a V-shaped cable may be most preferred. Itis mostpreferred to have these connections on every part of the ortho-xylene vaporiser or evaporator vessel, as the available hold upvolume is l imited to allow for sufficient charge dissipationafter the ortho-xylene mist has left the spray nozzles, such asto minimise charge separation in the ortho-xylene mist andbetween the pieces of equipment in contact therewith. Afurther preference is not to have a thermowell in the vaporiserand homogenizer system where ortho-xylene mist is presentbecause the thermowell can act as an antenna prone for build-ing up a static charge.[0033] We prefer that, in the delivery section for the oxy-gen-containing gas, every flange between the gas preheaterand the vaporizer is connected with its counter-flange by aV-shaped connecting cable of minimum length. The gas pre-heater is preferably double grounded to earth and the bottombasket of the vaporizer is preferably also independentlygrounded. Any main block valve in this system preferably hasa separate grounding cable.[0034] Wealso prefer that, inthe ortho-xylene feed deliverysection, every flange starting from the basket filter inlet to thespray nozzle assemblies is connected with its counter-flangeby a V-shaped connecting electrical cable of minimumlength. The fil ters are preferably individually grounded, theortho-xylene preheater is preferably double grounded on theshell side, and single grounded on the tube side. We alsoprefer that the emergency block valves near the vaporizer aresingle grounded independently.

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[0035] In addition, we prefer that each of the equipmentnozzles on the evaporator vessel, through which the spraynozzle assemblies are mounted, is grounded to earth. Thesame applies tothe vaporizer body itself , the top cover with itsrupture disc and the rupture disc vent stack. In our preferredprocess and apparatus, the connecting flanges between thevaporizer, the static mixer and the reactor are connected toeach other by aV-shaped cable and in addition also connectedto earth. Inaddition, we prefer that the reactor top hood flangeis connected with a V-shaped cable to the reactor body flangeand then also connected to earth. We also prefer that the mainreactor body is double grounded. The vent stacks of the reac-tor rupture discs are preferably also grounded to earth. Allreactor instrument connections may also be connected withV-shaped cables across the connecting flanges, and in addi-t ion grounded to earth. Grounding cables may be connectedto earth rails which allow for the connection to groundingcables running to common systems. For instance the ground-ing of the reactor top hood, the mixing device and the vapor-izer can all run to the same grounding rail. In this manner thepotential difference between the various systems is furtherminimised as compared with individual direct earthing. Inorder to improve electrical charge dissipation, electricallyconductive construction materials such as metal are preferredabove non-conductive materials, such as many polymers.[0036] Itis not easy to correlate between gap distance andvoltage required for a spark to occur between two electricallycharged objects, because it is impacted by the conductivity ofthe medium between them, and therefore by the compositionof this medium. Certain trial and error steps may be requiredto determine the minimum acceptable gaps for avoidingsparks however, and as a guideline the breakdown strength ofair-for a spark to occur-between two sharp edges is about28,000 volts per inch. Accordingly, a 3 mm gap between twometal pieces in the reactor inlet system would result in 3304volts maximum allowable voltage potential difference,assuming the mixture of ortho-xylene vapour and oxygen-containing gas behaves like air. Typically 3 mm is the thick-ness of a gasket between two metal equipment pieces such aswith the inlet sieve basket of the evaporator, which ismountedin between two flanges. Similarly the bottom of the outletsieve basket of the evaporator has this kind of distance to thewall of the evaporator. For the sieve basket, the 3304 voltsmaximum potential difference represents a stored energy of0.578 millijoules (mJ) , and which energy is released in thespark when one occurs. This iswell above the literature valuefound for the minimum ignition energy of ortho-xylene atthese temperatures, which is 0.301 mJ at 125 C. and 0.26 mJat 150 C. Even the voltage difference of 2203 volts over a 2mm gap provides sufficient energy to ignite the mixture at atemperature above 150 C. This indicates the importance ofproviding effective grounding and effective electrical inter-connections, particularly for parts ofthe equipment that are inclose proximity to each other.[0037] The present invention further provides an apparatusfor the production of phthalic anhydride which comprises areaction section comprising a fixed bed tubular catalytic reac-tor, and a raw material preparation section comprising[0038] (i) separate raw material delivery sections for a liq-uid and for a gaseous raw material,[0039] (ii) a raw material mixing section comprising a sys-tem for spraying the liquid raw material into the gaseous rawmaterial to form a mixture of vaporised liquid and gas, and


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[0040] (iii) a mixture delivery section for delivering themixture of vaporised liquid and gas to the fixed bed catalyticreactor,[0041] (iv) at least one rupture disk on the raw materialpreparation section and/or on the reaction section[0042] wherein the raw material delivery sections eachcomprise a raw material heater, and wherein the externalsurfaces of the equipment nozzles, which are provided forconnecting individual elements of the process equipment inthe raw material preparation section and/or in the reactionsection up to the inlet tubesheet of the tubular reactor, andwhich are in contact with the mixture of vaporised liquid andgas, are thermally insulated. The apparatus of the inventionmay also contain any single one, or two or more, or even allthe features described herein in relation to the process of thepresent invention. In part icular are meant the following fea-tures:[0043] atleast one rupture disk, on the fixed bed catalyticreactor and/or the raw material preparation section and/or on the reaction section, wherein an insulating blanketmay preferably be provided on the external surface ofthe rupture disc,

[0044] a relief stack on the rupture disc, having a foamcap,

[0045] means for providing heat to external surfaces orwalls of equipment in the raw material delivery sections,in the raw material mixing section, in the mixture deliv-ery section, and/or in the reaction section, such meanspreferably being thermally insulated,

[0046] means for direct or indirect electrical groundingof metal parts of piping, instrumentation and processequipment, in part icular these in contact with the liquidand/or vaporised liquid raw material and/or the oxygen-containing gas and/or the mixture of vaporised liquidand gas,

[0047] at least one metal cable electrically connectingthe two flanges of at least one connecting flange set forconnecting equipment components and/or piping and/orinstrumentation that in use are in contact with the liquidand/or vaporised liquid raw material and/or the oxygen-containing gas and/or the mixture of vaporised liquidand gas,

[0048] at least one spray nozzle system comprising atleast one lance, which lance comprises atleast one spraynozzle for spraying the liquid raw material into the gas-eous raw material to form the mixture of vaporised liq-uid and gas in the raw material mixing section, and

[0049] a metal seal between the spray nozzle and thelance ofthe spray nozzle system, preferably made of halfsoft copper, such as 061 annealed copper according toASTM BIll, because of its softness.

[0050] The present invention is particularly useful in com-bination with the use of the spray nozzle systems for ortho-xylene described in our co-filed application reference GB0718994.7, wherein preferably at least one lance projects intothe raw material mixing section, said lance comprising atleast one spray nozzle for spraying the liquid raw materialinto the gaseous raw material to form the mixture ofvaporisedliquid and gas in the raw material mixing section, and with thepreferred embodiment of that application in which the sealbetween the nozzles and the lance is provided by a metal seal.The use of a metal seal between the typically metal spraynozzle and the typically metal lance provides a system withgood electrical conductivity in which the development of an

Sep. 22, 20116

electrostatic charge is reduced and the risk of explosion isaccordingly reduced. Examples of suitable metals fromwhich the seal may be made include copper and aluminium.061 annealed copper, according to ASTM BIll, is the pre-ferred material. Wealso prefer to provide a conical surface onthe spray nozzle where it contacts the copper seal, such that itis self-centring and improves the contacting of the spraynozzle with the metal seal. Even more preferred is to have aconical surface on the metal seal itself, which preferablymatches the conical surface of the spray nozzle, if this ispresent.[0051] Although the invention is workable with only onelance carrying only one spray nozzle, we prefer a plurality oflances and/or preferably a plurality of spray nozzles to bepresent, thereby foaming a spray nozzle system. The inven-tion includes that the features of the invention are present in atleast one of the elements of these plurali ties, but preferably inmost and more preferably in all of the elements.[0052] In a further embodiment, the ortho-xylene is sup-plied to the lances from the preheater from a single source,through a single line which feeds a supply ring linked to thenozzles around the perimeter or circumference of the vapor-iser, depending on whether or not the vaporiser has a circularcross-section at the nozzle system location. In this way, thepressure at which the ortho-xylene is fed to the spray nozzlescan be maintained substantially constant, and be substantiallythe same to all nozzles if more than one are present, leading tothe production of a similar mixture of oxygen-containing gasand ortho-xylene by each nozzle.[0053] In a further embodiment, the ortho-xylene is sup-plied to the spray nozzles from the preheater at a temperaturebelow the flash point of ortho-xylene at the pressure inside thevaporiser Keeping the ortho-xylene below this temperatureassures that the ortho-xylene may stay in the liquid phaseinside the nozzle. The supply temperature is typically in therange of120 C. to 160 C. The temperature is preferably onlyslightly below the flash point and a temperature in the range135 C. or 137 C. to 142 C., especially 139 or 140 C., isparticularly preferred. Use of this temperature enables theliquid ortho-xylene to be swirled together with oxygen-con-taining gas inside the nozzles, drawing in oxygen-containinggas by the created venturi effect, and is vaporised as it issprayed out from the spray nozzle in the selected spray pat-tern, which is typically a spray cone, as provided by thedesign of the spray nozzle. In addit ion, because the oxygen-containing gas outside the lance( s) is typically hotter than theliquid ortho-xylene on the inside, heat may be transferred tothe ortho-xylene by conduction through the lance walls, andthis temperature control range allows for the ortho-xylene topick up heat before it is sprayed without causing undesirablelevels of cavitation inthe spray nozzle. Wehave found that theuse of these conditions reduces erosion by cavitation of thespray nozzles, improves the homogeneity of the oxygen-containing gas/ortho-xylene mixture and shortens the timerequired to reach complete vaporisation. The selected spraycone of the spray nozzle can also reduce the interferencebetween the spray patterns generated by adjacent spraynozzles, which further reduces the likelihood of coalescenceand condensation of the ortho-xylene. Furthermore, the spraycone can be selected to prevent direct injection of ortho-xylene onto any metal components which could provide coldspots and a location for condensation and/or coalescence.Preferably the direction of the spray nozzle centre axis isparallel to the direction of the oxygen-containing gas flow.


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However, to avoid coalescence of the ortho-xylene droplets,especially when a plurality of spray nozzles is employed, aspray nozzle can be located and/or directed at an anglethereto.[0054] In a preferred design, the evaporator vessel is ofsubstantially circular cross section perpendicular to the direc-tion of the gas flow, and a plurality of lances are providedaround the circumference of the evaporator vessel extendingacross the cross section of the vessel. In yet a further prefer-ence, eight lances may be proj ected into the evaporator vessel,from the circumference of the vessel. Preferably with a plu-rality of lances, these are positioned equidistant from eachother around the perimeter of the vessel.[0055] In a further preference, each lance consists of twolimbs of different length, thus extending a different distanceacross the cross section of the vessel .[0056] Each lance is typically provided with a plurality ofspray nozzles, at least one of which may be inclined at anangle of from 0 to 45 to the direction of flow of the streamof oxygen-containing gas, and preferably alternate spraynozzles are inclined relative to each other.[0057] The number of spray nozzles that may be employeddepends upon the capacity of the phthalic anhydride facilityand the diameter of the vaporiser or evaporator unit.[0058] It is preferred that, when the spray nozzles facesideways, they are inclined at different sides of the longitu-dinal axis of the lance(s), forming an alternating pattern,thereby avoiding that the spray from one spray nozzle inter-rupts the spray pattern from an adjacent spray nozzle.[0059] In a further preferred embodiment of a two-limblance, the larger limb of the lance is provided with ten spraynozzles and the shorter limb is provided with six spraynozzles.[0060] In another further preference the spray nozzle clos-est to the wall of the vessel is positioned at least 130 mm fromthe wall of the vessel to avoid the spray cone from the nozzlehitting the vessel wall. Ithas been found that if spray nozzlesare positioned at a distance ofless than 100mm distance fromthe wall the ortho-xylene spray cone can touch the vesselwall, and can be brought below its dew point and wet spotscan occur on the vessel wall increasing the risk of explosionor deflagration.[0061] The lance carrying the spray nozzle(s) is typicallyhollow for the delivery of the liquid ortho-xylene to the spraynozzle(s) and the end of (each limb of) the lance(s) is prefer-ably capped sothat ortho-xylene cannot flow out of the end ofthe lance or limb.[0062] The design of the spray nozzle itself is also impor-tant and we prefer to use a nozzle having a 60-70 spray cone.A vaporising spray nozzle supplied by Schlick-Dusen hol-low-cone type 121V with a 1.3 mm bore diameter is particu-larly useful.[0063] The nozzles are preferably made of hardened stain-less steel to minimise wear and corrosion in order to maintainthe desired spray cone over extended plant runs. Preferablyaustenitic stainless steel is used for making the nozzlesbecause of the better corrosion resistance.[0064] As described in co-filed application reference GB0718994.7, spray nozzles can be damaged by dirt and par-ticles and by any surface treatment, and the resulting damage,in particular of the relatively softer austenitic stainless steel,can be reduced by manufacturing the spray nozzles fromhardened stainless steel. This is conveniently achieved bysurface hardening of the steel spray nozzle.

Sep. 22, 20117

[0065] The surface hardening may conveniently be accom-plished by nitriding, preferably by cold temperature nitridingto maintain dimensional stability, and more preferably byKolsterising ().[0066] Slight surface defects inthe spray nozzle cause cavi-tation and erosion and we have found that the two majorcauses of damage are particles and superheated ortho-xylene.The particles may come from ortho-xylene feed (rust, sand,glass or mineral wool or fibre from tank floating roof seal thaterodes) and hence it is important that, as well as being pre-heated, the feed be fil tered. Itwas found that a 20 micron,basket type filter did not keep all particles out and it is there-fore preferred to use, optionally in addition, a 10 micron sizecartridge filter more preferably a 5 micron size most prefer-ably a 1 micron size cartridge fil ter downstream of the pumpand the optional basket filter.[0067] It is also important to prevent fouling of the spraynozzle with unwanted material which can come from theupstream equipment and valves, and we prefer to use metal,particularly copper seals to avoid the problem when usingTeflon seals, since the Teflon can be softened by the hotortho-xylene so that Teflon fibres are formed which can plugthe spray nozzles. We also have observed that when applyingpolymer tapes such as polytetrafluorethylene like Teflon thesetapes can get damaged by the mechanical forces required formounting the spray nozzle into the lance. Fragments of dam-aged tape frequently end up atthe interior of the spray nozzle.[0068] It is preferred that the liquid ortho-xylene is fed tothe spray nozzles from the ortho-xylene preheater through acommon source to the spray nozzles, so that the ortho-xyleneis injected under substantially equal pressure from all thenozzles. In this way each nozzle can produce a similar mist ofortho-xylene within the hot oxygen-containing gas streamand the mist can have a substantially uniform ortho-xylenedroplet size. In a preferred embodiment ortho-xylene issprayed through a nozzle system to yield a droplet mass-distribution of from 5 to 15%, typically 10%, of part icles thathave a diameter not larger than 40 micron, preferably notlarger than 35 or not larger than 30 micron, more preferablynot larger than 25 or even not larger than 20 micron, from 30to 70%, typically 50% of part icles that have a larger diameterbut not larger than 75 micron, preferably not larger than 65micron, more preferably not larger than 55 or even not largerthan 50 micron, from 20 to 40%, typically 30%, of particleshaving a yet larger diameter but not larger than 110 micron,preferably not larger than 95 micron, more preferably notlarger than 75 micron, and the balance from 0 to 15%, typi-cally 10%, of particles having a even yet larger diameter butpreferably not larger than 200 micron. We have found that thismay be accomplished if the ortho-xylene is sprayed with adifferential pressure of at least 5 bar, preferably at least 7 bar,more preferably at least 9 bar and most preferably at least 10bar, especially 12bar, through a spraying nozzle having a borediameter of at most 3 mm, preferably at most 2.5 mm, morepreferably at most 2 or even 1.5 mm, and most preferably atmost 1.3 mm. In a further preferred embodiment, the liquidortho-xylene is fed from a common source. As one alternativeof such embodiment, a ring system may be provided for thefeed of liquid ortho-xylene to the lances that carry thenozzles. Care should be taken to maintain the ring system andthe spray nozzle system at the desired temperature, whichmay be accomplished by highly effective insulation of theexternal surface of the piping and other metal parts that com-pose the ring system and the spray nozzle system, as well as


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of the piping through which the preheated oxygen-containinggas flow is guided. In addition heat may be provided to thesesurfaces, which may be provided by steam tracing and/orsteam jacketing.[0069] It is also important that there are no leaks of hotliquid into the heated gas stream at undesired locations, suchas where the nozzles are attached to the lance. We thereforeprefer to have an effective seal between spray nozzles and thelances which carry the spray nozzles and we have found thata metal gasket, particularly a heat treated copper gasket,positioned where the spray nozzle connects with the lancesprovides an effective seal.[0070] The oxygen-containing gas which is mixed with thepreheated ortho-xylene is usually air and is typically ambientair , which is fil tered to remove dust, and particularly sea saltbecause that may poison the oxidation catalyst. The gas istypically preheated, preferably to 180-210 C., usually in twostages. Higher temperatures are required for higher ortho-xylene loadings to ensure a high mixture temperature. At thehigher ortho-xylene loadings, of at least 80 grams per Nm3, itis our preference to operate with a reactor inlet temperature of168-172 C., after passing the homogenizer section, to ensurethe completeness of the vaporisation and to reduce the risk ofcondensation. Preferably before the mixing, the hot gaspasses through turbulence reducers ("quieting" vanes). Itsflow rate is preferably measured, e.g. by means of a venturidevice, so that it may be accurately controlled, such as atabout 4 Nm3 lhl tube relative to the number of reactor tubes,which is our preferred flow rate.[0071] We have found that providing an additional step forreducing the turbulence in the hot gas before the flow mea-surement makes it much easier to have an accurate and rep-resentative measurement of the gas velocity, e.g. by means ofa simple and low pressure drop device as a venturi device, andthus of the gas flow. This may be done by a first set of"quieting" or "straightening" vanes upstream of the flowmeasurement, and which can be of any suitable type or formto obtain the turbulence reducing effect. Weprefer, because ofthe simplicity of design and construction, to fill the typicallylarge diameter pipe duct for the hot gas, such as e.g. a 48 inchor 122 cm nominal diameter pipe, over a length of at least 1.5or 2 meter or about 1-2 pipe diameters, as much as possiblewith preferably thin-walled pipes of a significantly smallerdiameter, such as 3 inch (about 8 ern) or 4 inch (about 10 ern)pipes, and preferably using a combination of different diam-eters to obtain a better fi lling of the large pipe duct. The betterthe large diameter pipe duct is fil led, the lesser gas can bypassalong the perimeter of the vanes and the better the overallquieting effect. Many construction materials can be selectedfor fabricating these straightening vanes, but we prefer steel ,typically carbon steel , because the steel pipes can be weldedto each other and the total system can then obtain sufficientstructural integrity. We have found that any welding, solder-ing, brazing or glue applying operation to connect such vanesto each other and/or to the larger pipe duct needs to be per-formed carefully and with sufficient aftercare and cleaning,because we have found that debris from the connecting opera-tion, such as slag from welding, which may remain in theassembly upon commissioning, can be carried with the gasdownstream and disturb operations integrity of the down-stream evaporator, the homogeniser and/or even atthe reactorinlet. When welding is used, we prefer to use longitudinalwelds rather than spot welding because of this reason.

Sep. 22, 20118

[0072] The ortho-xylene ispreferentially also filtered, pref-erably to 1micron, advantageously preheated inthe preheaterto 135-145 C., with a residence time of up to 100 secondspreferably between 60 and 90 seconds, and sprayed into thehot gas through the spray nozzle system as a fine mist cloud,so that the ortho-xylene vaporises in the hot gas. As men-tioned previously, it is our preference to limit the ortho-xylenepreheat temperature to avoid ortho-xylene superheating andflash vaporisation, because this can lead to cavitation insidethe spray nozzle, leading to nozzle damage.[0073] When the oxygen-containing gas is air, the ortho-xylene concentration in air is typically between 32 and 120gram/Nm ' and the reactor may be started for example with aloading of 32 gram/Nm ' and the loading may be graduallyincreased further to a concentration range of 80-120 grams/Nm3, preferably 90-105 gram/Nm". The process of the inven-tion is particularly suitable for processing mixtures contain-ing from 44 to 120, preferably from 75 to 120, morepreferably from 80 or 85 to 110 grams of ortho-xylene perNmof air.[0074] The mixture of ortho-xylene vapour and oxygen-containing gas passes to the reactor inlet through a homog-enizer comprising a perforated screen and a static mixer. Theperforated screen helps to deflect shock waves and protect thedownstream equipment from shock waves originating in thevaporiser. It is preferred that the static mixer comprises twostages in one of which the mixture passes both from top tobottom and from bottom totop in the vert ical plane within themixer and inthe other section it passes from side to side inthehorizontal plane within the mixer. It is further preferred thatthe side to side section is the second section through which themixture passes. In a further preference at least one but pref-erably all metal parts of both the homogenizer and the mixerare electrically earthed.[0075] In a preferred embodiment, as the mixture flowstowards the reactor, it passes through an alternating mixingdevice to which initially the mixture passes through a vertical(up and down) mixing device and subsequently passesthrough a horizontal side to side mixing device. Irrespectiveof the technique that is used itis important that a homogenousmixture of ortho-xylene and oxygen-containing gas (air) ismaintained from the injection zone to the reactor.[0076] The ortho-xylene vapour and oxygen-containinggas mixture then enters a multi-tubular reactor filled withcoated ceramic ring catalyst particles, for reaction at a tem-perature typically of from 350 C. and to 460 C. The inletpressure at the reactor tubes may be in the range of 0.3 to 0.6barg, typically about 0.45 barg, and the pressure drop acrossthe tube may be inthe range of 0.2-0.4 bar, typically about 0.3bar. We prefer to use a catalyst giving a lower pressure dropacross the tube, because this allows to reduce the dischargepressure, and consequently also the energy requirement, ofthe compressor feeding the oxygen-containing gas to theprocess, which is an important energy consumer. A furtheradvantage of this reduced pressure drop is that the reactorinlet pressure may be reduced, which reduces the holdup offlammable material inside the equipment, affecting the sizeand number of rupture disks desired for safe operations, andalso reduces the flammability range of the ortho-xylene andoxygen-containing gas mixture. When replacing a catalystsystem having a higher pressure drop with a catalyst systemhaving a lower pressure drop, we found it advantageous toalso adapt the air or oxygen-containing gas compressor rotor,the gearbox and/or its driver to the new requirements, such


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that the new operating point remains in the highest efficiencyoperating envelope of the equipment, and the energy savingscould be fully realised. The residence time within the catalysttubes may be in the range of 0.5 to 2 seconds, typically abouta second. The tubes are usually arranged vert ically, with thereaction mixture of gas and ortho-xylene passing in a down-ward direction through the tubes.[0077] The oxidation reactor typically is a tubular reactorcomprising a plurality, preferably a multitude of tubes,packed with the oxidation catalyst to form catalyst beds. Thecatalyst beds in the reactor tubes preferably are preceded by alayer of inert rings preferably uncoated ceramic rings with athickness of5-20 cm, which typically isprovided ontop ofthecatalyst. The layer of inert rings can be heated from theoutside of the reactor tube, and the rings preferably are madeof material having high thermal conductivity, to enhance thefurther heat up of the gas ortho-xylene mixture as it enters thetubes, by thermal conduction from the wall of the reactorvessel . A thicker inert layer is less preferred, as it wil l be attheexpense of effective conversion capacity. The inert layer canalso provide further protection against non-vaporised ortho-xylene droplets and can enable additional mixing of theortho-xylene/gas mixture. Preference is given to only packinert material into the upper section ofthe reactor tubes, at theheight of the tubesheet, as it is difficult to remove heat ofreaction if a catalyst were to be present in that location.[0078] The preferred catalyst is a multi-layer catalyst sys-tem composed of a mixture of vanadium pentoxide, t itaniumdioxide, and several other metal, alkali and earth-alkali com-ponents in varying concentrations, typically coated on aceramic ring or hollow cylinder material. Such a hollow cyl-inder may e.g. have 7 mm as the outer diameter (OD) and 4mm as the inner diameter (ID), and have aheight (H) of7 mm.Alternatively the cylinder may have 8x6x5 mm as (ODxHxID) dimensions. The catalyst layers preferably have increas-ing activity along the tube in the direction of flow of theortho-xylene, oxygen-containing gas mixture. Catalystswhich can be used are described, for example, in DE 25 10994, DE 2547624, DE 29 14683, DE 2546267, DE 40 13051, WO 98/37965 and WO 98/37967. Coated catalysts inwhich the catalytically active composition is applied in theform of a shell or coating to the support, such as for examplein DE 1642938 A, DE 17 69 998 and WO 98/37967, havebeen found to be particularly useful. The catalyst may bepre-calcined or may be calcined in-situ in the oxidation reac-tor, although itis preferred that a pre-calcined catalyst isused,as the use of an in-situ calcined catalyst is much more sensi-tive for developing a runaway reaction than a pre-calcinedcatalyst. A runaway reaction is the result of a rapid increase ofthe ortho-xylene concentration, for instance from an inhomo-geneous ortho-xylene/gas mixture. The runaway reactionleads to the formation of a localized hot spot developmentinside the catalyst tube, which can ignite the reaction mixtureand also the upstream ortho-xylene/gas mixture by backfir-ing. In such event, the flame moves counter currently againstthe gas flow direction, and at a velocity that is higher than thegas velocity.[0079] The tubes in the reactor are preferably thin walledtubes, preferably made from mild carbon steel , and are typi-cally of internal diameter from 20 to 30 mill imetres, prefer-ably from 23 to 27 millimetres. It is preferred that the tubesare longer than 2.5 metres and a length of 3.25 to 4 metres ispreferred, more preferably 3.4 to 3.8 metres. Ithas been foundthat use of tubes of this length allows more catalyst to be

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available to the mixture of ortho-xylene and the oxygen-containing gas, allowing a lower temperature to be used,whilst retaining the conversion which is virtually 100%. Theuse of the lower temperature results in a higher reactionselectivity to ortho-xylene, which is typically in the range 80to 85%. The reaction is highly exothermic and the tempera-ture of the reaction is controlled by providing a coolant flow-ing around the outside of the tubes within the reactor, on thereactor shell side. The preferred coolant is molten salt, gen-erally maintained at a temperature in the range of from 320C. to 380 C. The coolant temperature may be increased overthe length of a commercial run to compensate for any deac-tivation of the catalyst.[0080] The reactor product is a gas which leaves the reactortypically at about the cooling salt temperature of for example320-380 C. and typically first enters a gas cooler where it isfirst cooled, for example to about 175 C. at which tempera-ture the product is still a vapour. In the next phase of thecooling stage, the gas is optionally cooled further in a liquidcondenser, preferably to about 138-142 C. Part of the crudephthalic anhydride is condensed in the liquid condenser andpreferably also separated off, whereas the remaining gaseousmaterial flows to one or more switch condensers. The coolingis typically performed against raising steam from condensateon the util ities side of the cooler and the liquid condenser. Wehave found that in order to avoid local condensation, andconsequently fouling, in the gas cooler, it is preferable to feeda condensate for steam generation that is already at a tem-perature of at least 135 C., more preferably at least 150 c.,if needed by a preheater on this condensate stream. If thephthalic anhydride facil ity is in the proximity of a phthalateester production facili ty, the condensate from the steam usedfor heating the phthalate ester facil ity may readily be used forgenerating the steam in the phthalic anhydride facility,optionally after flashing the condensate to a lower pressurelevel. The low pressure steam vent from this flashing may alsobe recovered by condensation to further condensate. Thesteam generated by the phthalic anhydride facility may beintegrated with a steam system supplying other heat consum-ers.[0081] In a switch condenser, the gaseous phthalic anhy-dride is desublimated, typically in the form of needles, on acold surface, a process step that is typically followed by amelting stage. The cooling for the desublimation ispreferablyperformed at about 60 C., typically using a heat transfer oilasthe coolant, whereas the melting isperformed atabout 180C., typically also with the same type of oil. The switching ofthe equipment from desublimation to melting and backcauses thermal stress and fatigue, in particular on the welds,and we have found that the switch condenser life time may beextended by limiting the temperature change rate of the heattransfer oil to at most 50 C. per minute, and preferably alsoby limiting the maximum heat transfer oil temperature to atmost 180 C.[0082] We have also found that acoustic emission monitor-ing may be a suitable tool to monitor the development ofmicrofractures in the steel of the switch condensers, moreparticularly in the welds therein, and to provide an indicationof when these microfractures may be growing or condensingand eventually lead up to a size where a leak is likely to occur.When a leak occurs, hot oil typically leaks into the productphthalic anhydride and contaminates the product to a typi-cally unacceptable degree, such that the leaking switch con-denser must be taken out of service and repaired. We have


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found that acoustic emission monitoring of the switch con-densers may help in forecasting failure risk, inthe planning offurther inspections and maintenance interventions, therebyintroducing a preventive element into the switch condensermaintenance which otherwise would be more strictly reme-diating.[0083] The products from the liquid condenser and theswitch condensers are preferably combined as crude phthalicanhydride, which flows to the intermediate tank for feeding todistillation. The condenser system also purifies the product,the liquid condenser typically reaching 95-98% purity whilethe switch condensers usually give an even higher purity.[0084] The waste gas ofthe switch condensers typically hasa temperature of 65 C. to 75 C. and can conveniently bedisposed off in a catalytic incinerator. The remaining organicsin the waste gas may be combusted at for example 290-350C. over a multi-layer honeycomb catalyst. The off-gas of theincinerator is then preferably discharged to the atmosphere.Alternatively, the waste gas of the switch condenser can bedisposed of into a thermal incinerator orpassed to an aqueousscrubber system to recover some of the organic materialsfrom the waste gas, such as maleic anhydride.[0085] After the condensation section, the condensed anddesublimated liquid phthalic anhydride passes to a final puri-fication step, comprising a thermal treatment step followed bydistillation, typically in one to three stages. The thermal treat-ment dehydrates any phthalic acid that may have been formedduring the oxidation reaction, or downstream thereof in thepresence of even traces of water, and is followed by or com-bined with a treatment with a base, which can neutralise anyacid species, including remaining traces of phthalic acid, thatmay be present. The purification may be performed sepa-rately on the products from the liquid condensers and theswitch condensers, or the materials may be combined prior topurification. The dehydration is typically performed by heat-ing, typically to a temperature inthe range 250 C. to 290 C.,and the neutralisation may be performed with a base such aspotassium hydroxide or sodium carbonate. Sodium hydrox-ide may also be used, but is less preferred because of the riskof stress corrosion of construction materials. These steps maybe performed concurrently in a single vessel or sequentially intwo vessels. Weprefer however to first heat treat, to dehydrateany phthalic acid present, and then to treat with the base toneutralise, as this optimises the yield of phthalic anhydrideand avoids the base converting phthalic acid to the undesir-able impurity benzoic acid. We also prefer that the neutrali-sation be performed with potassium hydroxide.[0086] Finally, after the thermal treatment stage, the purephthalic anhydride can pass to a stabiliser section to removelight components like benzoic acid, followed by a productdistillation section to separate the phthalic anhydride productfrom heavier boiling byproducts, which two sections can becombined in a single section, optionally followed by a heavyby-product concentration section. The phthalic anhydridemay then be provided as an intermediate for subsequent reac-tions either as a melt or as flakes.[0087] Temperature control of the feed mixture and theoxidation reactor are extremely important, and heat inputprovisions, such as electrical tracing and/or steam tracingand/or steam jacketing, and insulation according to thisinvention are preferably employed to prevent condensation ofortho-xylene and reaction products on cold spots, particularlyat the nozzles connecting various sections of the processequipment and the rupture discs. The insulation is preferably

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sealed to prevent water entering the insulation, which mayimpair i ts performance or damage its integri ty. When steamtracing or jacketing is used, it is our preference to providesteam at a pressure of at least 8 barg, even better 12 barg,preferably at least 15 barg, more preferably at least 16 bargand most preferably at least 19 or even 20 barg to the meansproviding heat to the equipment nozzles, particularly impor-tantly those connecting the rupture discs which relieve thereactor contents to the atmosphere upon a deflagration. Afurther preference isto put an insulating mineral wool blanketon top of the rupture disc surface, with a foam cap preferablyof polyurethane being provided on top of the rupture discrelief stack to prevent rain and cold air coming into contactwith the rupture disc surface, so preventing cool down of theprocess. The foam cap protects against ingress of water, hailor snow into the relief stack and collecting on top of therupture disc, and creating possible cold spots on the rupturedisc. The cap is preferably lightweight and disintegrates eas-ily upon blowing the disc and the resulting foam fragmentsare dispersed without being a danger to people who may bepresent. The preferably 3-8 cm thick insulation blanket is laiddown carefully onto the disc so as not to hamper the relievingperformance of the rupture disc.[0088] At start-up, it is preferred to operate at a low loadingbelow the explosive region for an initial period. For example,the facility may operate for 6-24 hrs at 42 g ortho-xyleneloading (below the explosive region) to "bum out" any pyro-phoric compounds that may be present inside the processequipment. Pyrophoric compounds may for instance beformed from the reaction products result ing from the corro-sive action by acids such as maleic acid and phthalic acid.These include iron maleate, iron phthalate, nickel maleate,nickel phthalate, chromium maleate, chromium phthalate andother organic salts of metal present in the steel of the facili tywhere this is in contact with the ortho-xylene feed and/or withreaction products.[0089] Itis important to maintain low levels of impurities inthe ortho-xylene feed. Impurit ies that may occur in the ortho-xylene feed include cumene or isopropyl benzene, styrene,ethyl benzene and methyl ethyl benzene. It is particularlyimportant for operations at high loading that the cumene levelis limited to below 0.3 wt % based on ortho-xylene, becausewe have found that at higher cumene levels, in combinationwith ortho-xylene loadings at and above 85 g/Nm ', the like-lihood of having a deflagration increases significantly. Per-oxides may come from the ortho-xylene. For instance,cumene and oxygen can make hydroperoxide, and this mayalready occur in the feed tank if that is not blanketed with aninert atmosphere. Itmay also occur in a truck or a ship duringtransport of the ortho-xylene. This hydroperoxide is unstableunder vaporising condit ions, and can cause a chain reactionforming more cumene peroxide and ortho-xylene hydroper-oxide, potentially leading to deflagrations. Styrene, ethylbenzene and methyl-ethyl benzene are other possible sourcesof peroxides which may develop in the ortho-xylene storagetank or during shipment. We have found that concentrationsof styrene exceeding 500 ppm may cause polymerisation andlay down of the resulting polymer on the spray nozzle assem-bly, which due to the insulating properties of the polymereventually leads to build up of static charge, which may resultin a deflagration. Furthermore, ortho-xylene and oxygen inthe gas may react to form some ortho-xylene hydroperoxideas an intermediate in the formation of the aldehyde. Generalfouling may also provide a surface for peroxide formation,


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and this can occur with improper air filtration, and/or in adusty environment. Accumulation of peroxide in a layer offouling is known to initiate deflagrations.[0090] The present invention therefore provides animproved process and apparatus for providing and maintain-ing a mixture of the hot oxygen-containing gas and the hotortho-xylene and for the production of phthalic anhydridewhich enables operation at high loading, whilst minimisingthe risk of explosion and/or deflagration while operatingwithin the explosive temperature range.[0091] The present invention is illustrated by reference forthe accompanying drawings in which[0092] FIG. 1 shows curves illustrating the dew point ofortho-xylene at a particular pressure and concentration in air.[0093] FIG. 2 shows the overall layout of the oxidationsection of a phthalic anhydride manufacturing facility, as wellas the parts of the equipment that preferably are thermallyinsulated and/or electrically grounded to earth and/or electri-cally interconnected.[0094] FIG. 3 is a section through the evaporation section ofthe facility shown in FIG. 1.[0095] FIG. 4 shows the location of the lances carryingspray nozzles of the present invention within the evaporatorsection shown in FIG. 3.[0096] FIG. 1 shows curves of how the dew point of ortho-xylene changes with the concentration of the ortho-xyleneand with absolute pressure of a mixture of ortho-xylenevapour and air as the oxygen-containing gas. The concentra-tion of ortho-xylene is for convenience expressed as ortho-xylene loading, expressed as grams per Nm3 of air. The curvesare for absolute pressures ranging from 1300 mbar as thelowest curve, and 1550 mbar for the highest curve shown.These curves may be helpful for quickly determining the dewpoint of the ortho-xylene in the mixture depending on loadingand pressure, without having to do the actual calculations.[0097] FIG. 2 is a schematic illustration of the oxidationsection of a plant for the manufacture of phthalic anhydridefrom a mixture of air and ortho-xylene.[0098] The oxidation section consists in general of the fol-lowing sub-sections, the raw material section (1); the reactionsection (2); the post-reactor section (3); the partial liquidcondenser section (4) and the switch condenser section (5).[0099] In the raw material section (1) air is taken from theenvironment (70), through a filter (71) by a blower (72), andfed to a preheater (73) after which it may be passed through afirst turbulence reducer or "straightening vanes" (not shown)and its flow is measured accurately in a flow meter (74). Theflow meter preferably is a venturi flowmeter, with correctionsfor pressure and temperature to increase accuracy. The heatedair is then fed through a lower sieve basket provided with aturbulence quietening vane system (see FIG. 3)to the mixingzone (13) ofthe evaporator section (11) where it ismixed witha mist of heated ortho-xylene provided from the ring feedsystem (14); the lances for the introduction of the ortho-xylene are not shown inFIG. 2.The ortho- xylene feed is takenfrom its supply point (60) by a pump (61) through a filter (62)to an accurate flow metering and control system (63), prefer-ably comprising a mass flow meter, after which it ispreheatedin a preheater (64) before it is fed as a liquid to the ring feedsystem (14) of the evaporator section (11) for spraying it as amist into the flow of heated air going through the mixing zone(13). The mixture of ortho-xylene and air then passes throughan upper sieve basket (not shown in FIG. 2) into a furthertwo-stage mixing zone (15) of the evaporator where the mix-

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ture is homogenised. The mixture first passes through a per-forated screen and then a static mixing device. A preferredmixing device is a Sulzer SMV static mixer with two mixingelements in which the ortho-xylene/air mixture in the firstelement is mixed in the vertical upward and downward direc-tion, followed by mixing in the second element in the hori-zontal directions.[0100] The mixture then passes to the reaction sub-section(2). The reaction sub-section consists of a reactor (22) con-taining a series of vertical tubes (21) packed with catalyst (notshown). The internal temperature of the reactor and the tem-perature of the reaction tubes (21) are controlled by moltensalt circulating on the reactor shell side, and which is pro-vided from a salt pump (25). A major part of the salt flows tothe reaction tubes for cooling. Part of the salt ispassed to a saltbath cooler (26) in which the salt exchanges heat with hotwater to generate steam. The returning cooled salt is mixedwith the returning salt from the reactor before entering the saltpump. A minor part of the salt is passed to a steam superheater(23).[0101] On exiting the reactor, the phthalic anhydridevapour is cooled in the gas cooler (31) and passes to theoptional post-reactor section (3), which may comprise anextra catalyst bed, preferably adiabatic, typically of evenmore active catalyst as compared to the catalysts used in thetubular reactor. Next the cooled gas passes to the optionalliquid condenser section (4) where the crude phthalic anhy-dride is partially condensed, the remaining vapour passing tothe switch condenser section (15).[0102] Itis our preference that the thermal insulation startson the ortho-xylene and the oxygen-containing gas deliverysections at the inlet of the preheaters (64 and 73), indicatedwith points B and C respectively in FIG. 2. We prefer thisthermal insulation to continue up to the top tubesheet of thetubular reactor (22), indicated with point D in FIG. 2. Furtherthermal insulation is preferably also provided from the bot-tom tubesheet of the tubular reactor (22), indicated with pointE in FIG. 2, continuing downstream up to and including theswitch condensers (5), indicated with point F in FIG. 2. Wehave found that cold spots downstream of the catalyst bed areto be avoided because they may lead to pyrophoric materialbuilding up and also leading to deflagrations.[0103] Electrical grounding and interconnections of adja-cent metal parts of the equipment in contact with the processstreams preferably starts on the ortho-xylene supply systemfrom the inlet of the filter(s) (62), indicated with point A inFIG. 2,and from point C on the oxygen-containing gas supplysystem down to point D as defined above.[0104] FIG. 3 is a schematic cross-sectional illustration ofthe apparatus in the evaporator sub-section of the oxidationsection as illustrated in FIG. 2. FIG. 3 shows the air stream(16) passing upwards through a lower sieve basket and thequietening vane system (9) and then shows the hot ortho-xylene (17) being injected into the heated air stream throughthe spray nozzles (18). The ortho-xylene is vaporised in theheated air and the hot vapour mixture passes side-wardsthrough an upper sieve basket (12), which is a perforatedscreen, and a vapour homogeniser such as a static mixingdevice (19) before passing to the reaction sub-section (2).[0105] FIG. 4 is a cross section through A-A' of FIG. 3looking downwards onto a spray nozzle system (18) that maybe employed according to the present invention.[0106] FIG. 4 shows eight lances (181) projecting throughthe wall of the evaporator, each lance being provided with two


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limbs, a longer limb (182) and a shorter limb (183). Eachlance (181) is also provided with means (184) for connectionto the supply of heated ortho-xylene. The limbs of the lancesare provided with nozzles for the spray of the ortho-xyleneinto the stream of hot air , shown in one of the possible nozzlearrangements. In the embodiment illustrated in FIG. 4 thelonger limb of each lance is provided with 7 nozzles and theshorter limb is provided with S nozzles, although the numberof nozzles can be varied. The connection of the lances to theortho-xylene supply system is by a ring system (not shown)common to all the lances and the whole introduction systemis thermally insulated to prevent cooldown of the ortho-xy-lene feed.[0107] The phthalic anhydride produced according to theinvention may be used for esterification with an alcohol or analcohol mixture to produce the corresponding di-ester. Suit-able esterification processes are disclosed in WO 200S1021482, WO 2006/012989 and in pending patent applicationsU.S. Ser. No. 601906732 and U.S. Ser. No. 601906797. Thealcohol may be a secondary alcohol, such as isopropanol, butis preferably a primary alcohol. Suitable primary alcohols areC1-Cl3 primary alcohols, and may be branched orunbranched, such as methanol, ethanol, n-propanol, n-bu-tanol, isobutanol, isohexanol, isoheptanol, iso-octanol,2-ethyl-hexanol, isononyl alcohol, 2,4-dimethyl heptanol,normal decanol, isodecanol, isoundecyl alcohol, 2-propylheptanol, undecyl-dodecyl alcohol, isotridecyl alcohol andmixtures thereof. Dimethylphthalate and diethylphthalate arepreferred products for personal care applications. The phtha-lates with alkyl chains having 4 or more carbon atoms, up to13, are used as plast icizers for polyvinyl chloride (PVC). Theprocess of the invention is suitable for producing all thesephthalates, in particular those produced from alcohols oralcohol mixtures having an average of 8 to 10 carbon atoms,especially those having an average of approximately 9 carbonatoms, such as those designated as DOP, DINP, DIDP andDTDP. Di-isononyl phthalate (DINP) ishighly preferred as aPVC plasticiser, and so is di-isodecyl phthalate (DIDP). Alsosuitable is di-propylheptyl phthalate (DPHP) These highermolecular weight phthalates provide a higher permanence inthe flexible PVC end product compared to the lower molecu-lar weight equivalents such as di-2-ethylhexyl phthalate(DEHP or also called DOP). Di-isotridecyl phthalate (DTDP)is preferred in low volatility applications such as specialpurpose wire and cable manufacture. These phthalate estersmay further be hydrogenated to form their corresponding1,2-cyclohexane dicarboxylic acid esters, such as e.g. di-isononyl dicyclohexanoate, as disclosed inWO 2003/029339The latter hydrogenation step may be performed by tech-niques known in the art, such as by using the processesdescribed in EP 1042273 or WO 2004/046078.[0108] The alcohols used in the esterification may be so-called oxo-alcohols, produced by the hydroformylation ofolefins, when necessary followed by hydrogenation of thealdehyde intermediate. Suitable processes for hydroformyla-tion are disclosed inW0200SIOS8787 or in copending appli-cations PCT IEP2008/0S3 783 and PCT IEP2008/0S3 718, andsuitable processes for aldehyde hydrogenation are disclosedin W0200SIOS8782.[0109] The hydroformylation and hydrogenation processesfor producing the oxo-alcohols, as well as the process forhydrogenating the phthalate ester, need a source of hydrogen.The hydrogen may be supplied from a variety of sources, suchas but not limited to refinery processes, partial oxidation

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(PDX) of various starting materials, steam reforming, auto-thermal reforming (ATR) or the like. One of the potentialsources of hydrogen is a refinery process called catalyticreforming, sometimes also called a Platforming process,wherein a refinery liquid stream, typically a naphtha orequivalent containing primarily naphthenes and/or paraffinsin the C6 to CII range, is converted to a product rich inaromatics over a heterogeneous precious metal chloride cata-lyst. These kind of processes are often known as a "Power-former" or "Powerforming" processes (developed by Exxon),or as Continuous Catalyst Regeneration (CCR) processes (ase.g. offered by UOP and IFP). The hydrogen from such acatalytic reforming processes contains small amounts ofchloride, at a level in the order of 1-10 ppm by volume. It isbelieved that most of this chloride is present as hydrogenchloride, which ismore readily detected by direct gas analysisand ata typical level of 4-8 ppmv. Itishowever suspected that,in addition, also organic chlorides may be present, and pos-sibly even at similar levels as the HCI. Many hydrogen con-suming processes are sensitive to chloride poisoning, and achloride removal step is typically foreseen to remove HClfrom the catalytic reforming hydrogen byproduct, most typi-cally down to a level of at most I ppmv. A typical chlorideremoval step is the adsorption of chloride over activated alu-mina, such as alumina 9139Afrom UOP, CI-7S0andCI-760from BASF, Alcoa 760 from Alcoa, Puraspec from JohnsonMatthey, over ZnO such as members of the Sud-ChemieActisorb Cl series, e.g. Cl 13, and/or over a molecular sieve,such as type CLR-4S4 obtainable from UOP or Unimol typesfrom Unicat.[0110] Some of the typical process steps in the productionof the oxygenates, such as the alcohols, disclosed herein, mayhowever be particularly sensitive to chloride poisoning, suchas a copper chromite hydrogenation catalyst used for alde-hyde hydrogenation. The alcohol production process mayalso employ a hydroformylation catalyst cycle comprising aclosed loop with minimal purge, in particular an aqueousclosed loop, such as with several of the techniques discussedherein and/or disclosed in our co-pending patent applications[attorney dockets PM2006138 and PM2008123]. Organicchlorides may become again converted to HCl in these pro-cesses. The traces of chloride coming with the hydrogen froma source such as a catalytic reforming may therefore build upin any ofthese aqueous loops to levels where corrosion due tochloride may become problematic, and/or where the chlorideacts as a poison to the chemistry of the hydroformylati

Documents

1 Vaporization in Oxidation to Phthalic Anhydride