Gas recycle maleic anydride process for high productivity and low carbon emissions

Abstract

Processes and equipment to manufacture maleic anhydride, produced by reacting a mixed gas containing molecular oxygen and a hydrocarbon, normally n-butane, over a suitable catalyst wherein the production of the maleic anhydride in a tubular catalytic reactor, wherein the gas effluent from the reactor, after the recovery of the maleic anhydride product by absorption with an organic solvent and after adequate treatment steps, is recycled back by compression to the inlet of the catalytic reactor.

Description

[0001] GAS RECYCLE MALEIC ANYDRIDE PROCESS FOR HIGH

[0002] PRODUCTIVITY AND LOW CARBON EMISSIONS

[0003] ABSTRACT

[0004] The present invention relates to a process for the production of maleic anhydride by the catalytic oxidation of n-butane , characterized by : a ) Use of pure oxygen as oxidizing medium; b ) Very high yield and productivity; c) Production of a carbon dioxide rich effluent stream which can be applied for carbon capture , utilization and storage ( CCUS ) or enhanced oil recovery ( EOR) .

[0005] FIELD OF DISCLOSURE

[0006] Embodiments disclosed herein relate generally to processes and equipment to manufacture maleic anhydride , produced by reacting a mixed gas containing molecular oxygen and a hydrocarbon, normally n-butane , over a suitable catalyst . More specifically, embodiments relate to the production of the maleic anhydride in a tubular catalytic reactor, wherein the gas effluent from the reactor, after the recovery of the maleic anhydride product by absorption with an organic solvent and after adequate treatment steps , is recycled back by compression to the inlet of the catalytic reactor .

[0007] BACKGROUND OF THE INVENTION AND TECHNICAL PROBLEM

[0008] Maleic Anhydride is an important raw material used in the manufacture of alkyd and unsaturated polyester resins and it is also a versatile intermediate for the chemical synthesis of butanediol (EDO) , dimethyl succinate ( DMS ) , gamma-butyrolactone (GBL ) and tetrahydrofuran (THF) . BDO can then be used as raw material , alone or together with DMS , for the production of biodegradable plastics , as polybutylene adipate terephthalate ( PBAT ) or polybutylene succinate ( PBS ) , respectively .

[0009] It is produced by partial oxidation of hydrocarbons , typically n-butane, over a vanadium- phosphorus-oxygen (VPO) catalyst contained in a tubular fixed bed or in a fluid bed reactor . In both types of reactors , the substantial exothermic heat of reaction is removed with production of steam.

[0010] In the reactor, alongside the conversion reaction of butane into maleic anhydride :

[0011] C4H10+ 3 . 5 O2C4H2O3+ 4 H2O other secondary reactions take place , among which the main ones are the butane combustion reactions forming carbon monoxide and carbon dioxide :

[0012] C4H10+ 4 . 5 02- 4 CO + 5 H20

[0013] C4H1O+ 6 . 5 024 CO2+ 5 H20

[0014] A further minor amount of butane is oxidized to organic acid by-products , specifically acetic acid and acrylic acid .

[0015] All the reactions are strongly exothermic and, in the tubular fixed bed reactors , the reaction heat is suitably removed by circulating a heat transfer medium, usually a mixture of molten salts, which thereafter releases heat to a steam generator.

[0016] In the conventional maleic anhydride process, the oxygen source is compressed air, having an oxygen content slightly higher than 20 mol%.

[0017] To reduce risks of explosivity, the inlet gas to the reactor typically contains only a small amount of raw material, i.e. from 1.5 to 2.0 percent by volume of butane .

[0018] Considering the typical n-butane conversion from 80 to 90% over VPO catalyst, it means that the differential n-butane molar concentration between the inlet and the outlet of the reactor is in the range 1.2 to 1.6% max.

[0019] Non-converted butane is present in the reaction effluent. In the conventional once-through process, after the recovery of maleic anhydride by absorption, the exhaust gas is fed to a thermal oxidizer, where both the not converted butane and the carbon monoxide are burnt to carbon dioxide and, after heat recovery, discharged to the atmosphere.

[0020] Of course, the incomplete conversion of butane causes, besides the higher production cost, a higher release of carbon dioxide to the atmosphere.

[0021] US patent 5,688,970 proposes a mitigation of the above-described drawback, by a partial off gas recycle from the absorber overhead to the reactor inlet through a recycle gas compressor, characterized by use of slightly high pressure in the absorption of maleic anhydride and in the exhaust gas recycle. Although in principle the process described in US 5 , 688 , 970 permits some economic and environmental advantages , its effectiveness is limited by the oxygen content in the air . Considering that each mole of n- butane consumes around 5 moles of oxygen and that , in the conditions of the conventional process , the VPO catalysts lose in efficiency at poor oxygen concentration, the maximum percentage of gas recycle is limited to around 50% of the total gas according to the patent text , or to 30-40% according to the real industrial experience .

[0022] US patent 6 , 040 , 460 overcomes the above limitation by using oxygen or enriched air as oxidizing agent . The patent teaches that , by using butane concentration above the upper explosivity limit , by recovering butane from the purge gas and by using a source of carbon dioxide rich gas , the process of fers superior yield, higher productivity, reduced energy of compression , safer operation and reduced environmental impact . However , in the examples and in claims , the carbon dioxide concentration in the reaction mixture does not exceed 60% by volume .

[0023] The US 5 , 011 , 945 patent also concerns the maleic anhydride process in high yield and catalyst productivity, by using essentially pure oxygen and recycling a gaseous stream rich in carbon oxides , wherein carbon monoxide concentration is higher than carbon dioxide concentration . The process is also characterized by very high butane concentration at the reactor inlet, by low butane conversion per pass and by use of VPO graded catalyst , with minimum reactivity nearest the reactor feed end, and with the addition in the catalyst itself of a co-metal comprising molybdenum.

[0024] In this application the recycling gas contains a high concentration of carbon monoxide and therefore the process presents inherent risks of loss of control and deflagration at the inlet of the oxidation reactor.

[0025] US 4,987,239, US 5,126,463, US 5,179,215, US 5,262,547, US 5,278,319 and US 5,532,384 from BOC Group represent a family of applications dedicated to the use of pure oxygen or air enriched in oxygen for oxidation processes, including the production of maleic anhydride .

[0026] In particular, US 5,126,463 refers to a process for the production of anhydride, wherein the carbon monoxide present in the reactor effluent as a byproduct is oxidized to carbon dioxide and part of the gaseous effluent, comprised mainly of carbon dioxide, is recycled to the reactor. However, the application is based on experimental tests only for the selective oxidation of carbon monoxide to carbon dioxide, while the material balance of the maleic anhydride reaction is based on computer simulations, extrapolating the reaction data from laboratory tests to different reaction conditions, where the data are not valid. In particular, the application ignores the aspect connected to the very low selectivity, mainly forwards the production of light organic acids, in the reaction of n-butane to maleic anhydride, when the oxygen content in the reaction gas is very low. In all the above-described applications and in other applications on the matter, the environmental impact caused by the discharge of large amounts of carbon dioxide connected to the manufacture of maleic anhydride is disregarded or, at best , is only mitigated thanks to a marginal improvement of the efficiency of the proposed process .

[0027] Therefore, it would be desirable to providing new processes or improved processes , in order to permit the industrial production of maleic anhydride from n-butane and oxygen at high yield and catalyst / reactor productivity and, at the same time , limiting or better avoiding any discharge of CO2 to the ambient .

[0028] As a matter of fact , the issues related to the production of maleic anhydride by oxidation of n-butane are somewhat complex and, particularly when pure oxygen is used as oxidizing medium, some problems and drawbacks are present .

[0029] First , while the industrial experience with use of air is by now largely diffused and consolidated, the knowledge in using different oxidizing gas is still rather restricted, being the industrial production practiced only by single companies and for defined times .

[0030] As a matter of fact , all the conventional processes using once-through air operate with a very large excess of oxygen compared to the overall O2 chemical consumption . A typical oxygen content at the reactor outlet is higher than 12 . 5 mol% . Only 34-35% of the oxygen entering the reactor is consumed during the reaction . It is evident that the VPO catalyst always operates , along the reactor tubes length present in the tubular reactor, with large excess of oxygen . Under the above- described conditions , the oxygen does not limit the reaction . It is only a minor parameter in the definition of the catalyst performances .

[0031] On the contrary, in the operation with gas recycling and use of pure oxygen, due to the cost of the oxygen itself , the operation is normally conduced with minimum excess of oxygen .

[0032] Under the condition of low oxygen concentrations , the inventors observed that the production of light organic acids and particularly of the acetic acid is greatly enhanced .

[0033] The relevant stoichiometry of the reaction is as follows :

[0034] C4H10+ 2 . 5 O2→ 2 C2H4O2+ H2O

[0035] As shown in details in the comparative example 2 , data from a large si ze industrial reactor, having more than 30000 tubes with 21mm internal diameter and 6 meters length, show, at an oxygen concentration at reactor outlet as 2 . 5 mol % , an abnormal production of acetic acid, 4 or more times higher compared to the operation with air and large excess of oxygen .

[0036] A second aspect of the art of production of maleic anhydride under gas recycling condition and at the conditions to produce a purge gas having a high concentration of carbon dioxide , useful for carbon capture , utilization and storage ( CCUS ) , is related to the introduction of a carbon monoxide converter to carbon dioxide .

[0037] All the maleic anhydride plants using VPO catalyst produce a certain amount of carbon monoxide and carbon dioxide , as undesired by-products . In the operation with air, the carbon monoxide production is always higher than carbon dioxide , being the molar ratio CO / CO2 around 1 . 25 .

[0038] Another surprising finding of this application refers that the ratio between the carbon monoxide and the carbon dioxide produced in the maleic anhydride catalytic reactor is almost independent from the absolute concentration of such oxides and also from the ratio of concentration CO / CO2 at the reactor inlet . In other words , the molar ratio of the oxides production does not change between the once through air operation and the recycling operation, whichever is the contribute of pure oxygen to the reaction .

[0039] Of course , based on the above statement, without the introduction of a CO converter, both the recycle gas and the purge gas from the plant should be a mixture richer in carbon monoxide than in carbon dioxide .

[0040] In terms of potential flammability of the reaction gas , the difference between CO and CO2 is substantial . As a matter of fact , the carbon monoxide is flammable , with a very large concentration range , from 12 vol% as lower flammability limit and 70 vol% as upper flammability limit , and with a very low value , equal to 5 . 5 vol% , as Limiting Oxygen Concentration (LOC) , the concentration of oxygen below which combustion is not possible independent of the concentration of fuel ; all above values are referred to atmospheric pressure , 25 °C, and air or air / nitrogen mixtures . The presence of carbon monoxide at the reactor inlet j oins the presence of the butane in the evaluation of the risk of explosion, according to the literature prediction methods for the calculation of flammability of multiple fuel and multiple inert mixture .

[0041] On the contrary, the carbon dioxide is an inert gas ; according to the literature on fire dynamics , the effect of increasing the concentration of inert gas can be understood by viewing the inert gas as a thermal ballast that quenches the flame temperature to a level below which the flame cannot exist . Carbon dioxide is therefore more effective as inert than nitrogen, due to its molar heat capacity more than 50% higher compared to nitrogen .

[0042] Therefore, the introduction of a carbon monoxide selective converter to carbon dioxide , not only reduces the concentration of fuels (C4+ CO) in the mixture , but also introduce a large amount of a compound, the carbon dioxide , with an inertization strength much higher than nitrogen, the main inert gas present in the conventional process for the production of maleic anhydride with use of air .

[0043] As a conclusion, the introduction of a carbon monoxide selective converter is a fundamental component not only to permit the carbon capture from the purge gas , but also to generate a higher catalyst productivity, thanks to the safer reaction conditions obtained by the high concentration of carbon dioxide . On the subj ect of the selective oxidation of carbon monoxide , US2013 / 0131380 remarks the main problem to solve , in the maleic anhydride process under gas recycle conditions , is that of carrying out the conversion of CO to C02 in a stream rich in hydrocarbon (butane ) , without however oxidizing these hydrocarbons . It is possible that the reaction can rapidly run away, with great increase of the reaction temperature and of the conversion of the hydrocarbon present in the stream. Being the heat of combustion of the butane much higher than the carbon monoxide, the start of very dangerous uncontrolled reactions is rather easy .

[0044] The inventors of the above patent propose to solve the problem by using a fluidized bed reactor, that makes it possible an ef ficient heat transfer, a homogeneous temperature of the catalyst in the reactor and a safe operation .

[0045] However, the use a fluidized bed is a complex, expensive and difficult to operate solution .

[0046] On the same subj ect , US2005 / 0032628 proposes the use of a noble metal , Pt or Pd, on a silica support with a continuous coating of molecular sieve material , that permits a substantial increase of the temperature gap between the oxidation of carbon monoxide and the oxidation of butane , up to 250 °C or more compared to only 50 ° C of the conventional catalyst without coating .

[0047] Even if the results are very interesting, the industrial production of such type of catalysts is problematic .

[0048] Accordingly to the above described points , there exists a need for a new process and apparatus , which represents the matter of the present application, for the production of maleic anhydride , in which the plant configuration and the operating parameters permit at the same time a safe operation, a very high yield of reaction and catalyst productivity, together with a production of a carbon dioxide rich effluent stream, which can be applied for carbon capture , utilization ( f . e . its use for the food industry or as a building block for the production of urea ) , and storage (CCUS ) or enhanced oil recovery (EOR) .

[0049] SUMMARY OF THE DISCLOSURE

[0050] It is an obj ect of the present invention to specify an improved and efficient process for producing maleic anhydride from n-butane , characterized by : ( i ) use of pure oxygen as oxidizing medium, ( ii ) high yield and catalyst productivity and ( iii ) a production of a carbon dioxide rich effluent stream which can be applied for carbon capture , utilization and storage ( CCUS ) or enhanced oil recovery (EOR) .

[0051] As described in the background of the invention, in the operation with use of air the catalyst productivity is limited by safety consideration; the dif ferential concentration of n-butane between the inlet and the outlet of the reactor tubes is in the range 1 . 2 to 1 . 6 vol% max .

[0052] To overcome the limitation, various processes of the state of the art propose to operate with butane concentrations above the upper flammability limit , associated with the presence of an inert gas with high specific heat , which expands the safety conditions at the reactor inlet and a better management of the high exothermicity of the reaction .

[0053] In this context , the applicant company has discovered that the reaction operated at high concentration of the reactant butane drives the operation, as logic consequence, to low concentration of oxygen at the reactor outlet , but also to an unexpected enhanced production of the light organic acids , particularly of acetic acid .

[0054] The problem to be solved by the present invention , consists in the finding a solution which increases the catalyst productivity, limiting, however, the increased production of light acids . Such solution shall also permit the selective conversion of the carbon monoxide to carbon dioxide, at conditions which prevent the start of runaway reaction, with abnormal temperature increase and oxidation of the hydrocarbon present in the gaseous mixture .

[0055] The use of tubular reactor with selective oxidation catalyst coupled with its dilution by inerts material and with the control of the oxygen content at the CO converter inlet , as proposed in the present invention, permits an efficient cooling of the reactor which gives a stable and efficient control of the reaction .

[0056] The obj ect of the invention is achieved by a process including the following steps :

[0057] ( a ) Feeding the reaction gas mixture to a tubular reactor unit containing a vanadium-phosphorus oxide catalyst , wherein the gas mixture includes a vaporized stream rich in n-butane at a concentration higher than the relevant upper flammability limit in a high inertized atmosphere , a stream of high purity oxygen and a gaseous stream rich in carbon dioxide recycled to the reactor through a gas compressor;

[0058] (b ) Producing in the reaction tubes of said tubular reactor unit maleic anhydride as main product , carbon oxides and light organic acids as undesired by products and reaction water;

[0059] ( c ) Cooling the effluent gas , leaving the tubular reactor unit at temperature between 380 and 450 °C, by recovering the heat of reaction for production of steam at a single pressure value or in multiple heat exchangers in series at decreasing values of pressure of the produced steam;

[0060] ( d) Selectively recovering the maleic anhydride contained in the reaction gas in an absorber in form of an high efficiency absorption column, by using an organic solvent as absorption liquid;

[0061] ( e ) Cooling and scrubbing by water the exhaust gas from said absorber in order to remove small amount of maleic anhydride not removed in the absorber and the organic acid by-products , protecting the gas compressor integrity;

[0062] ( f ) Purging part of the water scrubbed exhaust gas , to avoid accumulation of inerts , and sending it to a thermal or regenerative or catalytic first oxidizer , in order to meet the local regulations for the gas emission to the atmosphere as carbon monoxide, hydrocarbons and nitrogen oxides limitations ;

[0063] ( g) Recycling the scrubbed gas mixture by means of a gas compressor increasing its pressure at the reaction value ;

[0064] ( h ) Feeding a portion from 35 to 60 % of the compressed gas to a selective second oxidizer, wherein the use a suitable selective catalyst , together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide , without substantial oxidation of the not reacted n-butane contained in the recycle gas ;

[0065] ( i ) Using the j oined gas mixture rich in carbon dioxide as diluent of the fresh n-butane and pure oxygen to feed the tubular maleic anhydride reactor unit .

[0066] As a preferred embodiment of the invention, rather than purging part of the water scrubbed exhaust gas and discharging it to atmosphere , as described in points ( f ) to ( i ) , in the following points ( f' ) to ( f ' ) it is provided for :

[0067] ( fz) Recycling the scrubbed gas mixture by means of a gas compressor increasing its pressure at the reaction value .

[0068] ( g' ) Feeding a portion from 40 to 60 % of the compressed gas to a selective oxidizer , wherein the use a suitable selective catalyst , together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide , without substantial oxidation of the not reacted n-butane contained in the recycle gas ;

[0069] (h' ) Purging part of the carbon monoxide converter exhaust gas , to avoid accumulation of inerts , and j oining the remaining part to the compressed gas bypassing the CO converter;

[0070] (i' ) (Using the j oined gas mixture rich in carbon dioxide as diluent of the fresh n-butane and pure oxygen to feed the tubular maleic anhydride reactor unit ;

[0071] ( j ' ) Using the purge gas in point (h' ) as a source of carbon dioxide useful , after adequate compression and purification steps , for carbon capture , utilization and storage (CCUS ) or enhanced oil recovery (EOR) wherein, according to peculiar features of the process being described : i . The recycle dilution gas is rich in carbon dioxide at a concentration, before the inj ection of fresh butane and pure oxygen, higher than 85% ; ii . The n-butane content at the inlet of the maleic anhydride reactor unit is controlled within 4 to 6% and its conversion in the reactor is between 35 to 50% ; iii . The oxygen content at the tubular reactor unit inlet is controlled at a value between 12 to 16 mol% ; iv . The combined butane , carbon dioxide , carbon monoxide and water content at the inlet of the tubular reactor unit is outside the flammability limits at its temperature and pressure ; and v . The oxygen content at the tubular reactor unit outlet is controlled to a value lower than the oxygen percentage below which no mixture i s flammable independent of the concentration of fuels (LOC -Limiting Oxygen Concentration) , but not less than 4 . 0 mol% .

[0072] Other aspects and advantages of the present invention will be apparent from the following drawings , the detailed description, the nonlimiting examples and the appended claims . BRIEF DESCRIPTION OF DRAWINGS

[0073] The present invention can be better understood from the attached drawings , which represents simplified schemes of the prior art and of the invention; therefore , the scope of the drawings is only for clarity of explanation and the drawings in themselves are not representative of the entire meaning of the invention .

[0074] Figure 1 represents the conventional process of the maleic anhydride manufacture , in two different configurations , as follows :

[0075] Figure 1A is a block diagram, showing the production of maleic anhydride from n-butane and air , in the common once-through operation;

[0076] Figure IB is a block diagram, showing the production of maleic anhydride from n-butane and from air or enriched air, in the off-gas partial recycle arrangement ; Figure 2 is a block diagram according to the present inventive process , in the operation at full gas recycle , with use of pure oxygen and use of a selective carbon monoxide oxidation reactor, where the purge gas , after treatment , is discharged to atmosphere ;

[0077] Figure 3 is a block diagram according to the preferred embodiment of the present invention, in the operation at full gas recycle , with use of pure oxygen and use of a selective carbon monoxide oxidation reactor, where the purge gas is used for carbon capture . DETAILED DESCRIPTION OF THE INVENTION

[0078] The worldwide demand of sustainable plastics , including the biodegradable plastics , together with the carbon neutrality goal , are changing the scenario of the maleic anhydride market and manufacturing methods .

[0079] First , the maleic anhydride route today has gained a leading position for the production of butanediol , which on turn is one of the most important raw materials for the new bioplastics , in particular the polybutylene adipate terephthalate ( PBAT ) and, together with another maleic anhydride derivative as the dimethyl succinate ( DMS ) , the polybutylene succinate ( PBS ) . The plastics market implies volumes having order of magnitude completely different from the previous market of maleic anhydride . Therefore , the production capacities of the present and, most probably, of the future maleic anhydride proj ects are typically much larger compared to the recent past years .

[0080] A second aspect is connected to the actual carbon neutrality aim, which today has brought unprecedent opportunities for the green or low-carbon transformation of the plastics industry .

[0081] Unfortunately, the state of art technology for the production of maleic anhydride , due to its intrinsic low selective reactions , produces large amounts of carbon dioxide discharged to atmosphere . The present invention enters in the general demand of a new process , which could teach the method for the industrial production of maleic anhydride from n-butane and oxygen at high yield and catalyst / reactor productivity and, at the same time , limiting or better avoiding any discharge of carbon dioxide to the ambient .

[0082] As for the increased catalyst and reactor productivity, the application proposes the operation with a concentration of n-butane at the reactor inlet above its upper flammability limit , in order to obtain a difference of concentration in n-butane between the inlet and the outlet of the maleic anhydride catalytic reactor well higher than in the conventional process . The novel and inventive aspect of the present application consists in an incomplete utilization of the oxygen used for the reaction, with the aim to limit the light organic acids production within economically acceptable values . The control of the oxygen conversion is achieved, according to the usual practice in the high exothermic tubular reactors , by adj usting the temperature of the heat transfer medium, i . e . of the molten salts , usually a eutectic mixture of potassium and sodium nitrates and nitrites , circulating in the shell side of the tubular reactor . In other more precise terms , in the process according to the invention, in consideration of the fact that the operation with high butane concentration and with a concentration of oxygen lower than 20% favors the high conversion of oxygen and that such high conversion produces an undesired high amount of light organic acid, the catalyst temperature is controlled at values lower than in the conventional process , in order to control , on purpose , the oxygen concentration at the reactor outlet at a value lower than the oxygen percentage below which no mixture is flammable independent of the concentration of fuels ( LOG Limiting Oxygen Concentration) , but in any case not less than 4 . 0 mol% .

[0083] The value of the catalyst temperature depends from many factors , including the type of catalyst and, above all , its aging . Under the same type and same aging of the catalyst , the average temperature will result from 4 to 12 °C lower than in a conventional process with low inlet concentration of n-butane . Such lower catalyst temperature represents another advantage of the process , thanks to the general rule of longer catalyst life when operated at lower temperatures .

[0084] As for the need of a process able to avoid the carbon dioxide release to the atmosphere , the new process needs first the use of oxygen rather than air as oxidizing medium, avoiding in this way the introduction in the reaction loop of high amounts of nitrogen . Furthermore , the process includes the use , in the gas recycle loop, of a selective catalytic conversion reactor, in order the selectively oxidize the carbon monoxide produced in the maleic anhydride reactor to carbon dioxide .

[0085] In such a way, ( i ) by avoiding any introduction of nitrogen in the system, ( ii ) by converting the carbon monoxide to carbon dioxide through its selective oxidation, ( iii ) by separating the maleic anhydride in the solvent absorber and ( iv) by separating most of the reaction water and of the light organic acids in a scrubbing column, the circulation gas will contain carbon dioxide at high concentration . Therefore , also the purge gas corresponding to the net production of inerts in the maleic reactor will have a high concentration of C02. The purge gas , rather than to be discharged to the ambient as in the state of art process , can be used, after purification and compression steps adequate to the final scope , for carbon capture, utilization and storage .

[0086] In the state of art processes , the carbon dioxide discharged to atmosphere is partly produced in the maleic anhydride reactor due to undesired side reactions . Another and still more substantial part is produced in the thermal oxidizer as result of the combustion of the residual butane and of the additional fuel gas used for control of combustion efficiency . As shown in the examples 1 and 4 , another advantage offered by the invention is an important reduction of the overall production of carbon dioxide , thanks to the substantial reduction or suppression of the part of the C02produced by the combustion of residual butane and fuel gas . According to the process of the present invention, the control of the oxygen content at the outlet of the maleic anhydride reactor permits the selective conversion of the carbon monoxide to carbon dioxide , at conditions which prevent the start of runaway reactions , with abnormal temperature increase and oxidation of the hydrocarbon present in the gaseous mixture .

[0087] As more in details described in the annexed example 4 , under the controlled oxygen concentration, the CO converter is partially self-protected, since the content of oxygen, even at runway condition, permits only a partial combustion of the residual butane .

[0088] The use of tubular reactor with selective oxidation catalyst , coupled with its dilution by inerts material and with the control of the oxygen content at the CO converter inlet , as proposed in the present invention, permits an efficient cooling of the reactor and a stable and efficient control of the reaction .

[0089] The two main characteristics of the present invention, the control of the oxygen content at the maleic anhydride reactor outlet and the use of a carbon monoxide selective oxidizer, do not simply represent two independent aspects of the process , dictated by different scopes . On the contrary, they shall be regarded in their wholeness and synergetic integration . On one hand, the high carbon dioxide concentration, obtained through the selective CO monoxide oxidizer , offers safer conditions at the maleic anhydride reactor inlet and, therefore , within some extent higher concentration of oxygen and higher catalyst productivity . On the other, the control of the oxygen concentration at the maleic anhydride reactor outlet and in the recycle gas provides to give the minimum conditions for a correct and safe carbon monoxide oxidation .

[0090] The block diagrams in Figures 1 to 3 , show the process of this invention, also as differences with the state of art processes .

[0091] For scope of clarity, the Figures 1A, IB, 2 and 3 use the same nomenclature , as follows :

[0092] The numbers identified by multiples of ten represent a particular equipment or process unit :

[0093] 10 is the gas compressor, used to compress fresh air or a gas recycling stream or a mixture of air and recycle gas ;

[0094] 20 is the maleic anhydride tubular reactor using a VPO catalyst ;

[0095] 30 is the cooling apparatus used to recover the heat of the reactor effluent , at a temperature higher than 400 °C, up to a temperature of 130 to 180 ° C , with production of high pressure and / or medium pressure steam;

[0096] 40 is the absorption / stripping system, or briefly absorber, used for the recovery of the maleic anhydride produced in the reactor and for its purification;

[0097] 50 represents a first oxidizer that is the oxidizer, of the thermal , regenerative or catalytic type , used to burn the residual amount of carbon monoxide and hydrocarbons contained in the exhaust air or in the purge gas , before to be discharged at the atmosphere ; 60 is the water cooling and scrubbing system of the recycle exhaust gas from the absorber in order to remove small amounts of maleic anhydride not removed in the absorber and the organic acid by-products , protecting the gas compressor;

[0098] 70 represents a second oxidizer that is the selective oxidizer, wherein the use a suitable selective catalyst , together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide , without substantial oxidation of the not reacted n-butane contained in the recycle gas ;

[0099] The other numbers represent process streams , as follows :

[0100] 1 is the fresh air used as oxygen source;

[0101] 2 is the make-up of vaporized n-butane

[0102] 3 is the stream of maleic anhydride produced

[0103] 4 is the purge gas from the plant discharged, after treatment , to the atmosphere;

[0104] 5 is the compressor discharge gas , air or recycle gas

[0105] 6 is the maleic anhydride reactor inlet gas

[0106] 7 is the maleic anhydride reactor outlet gas

[0107] 8 is the maleic anhydride reactor cooled effluent gas

[0108] 9 is the maleic anhydride absorber overhead gas

[0109] 11 is the fraction of the gas purged to the oxidizer before its discharge to atmosphere

[0110] 12 is the fraction of the overhead gas recycled back in the operation with partial gas recycle 13 is the recycled gas after the water cooling and scrubbing

[0111] 14 is the gas at the suction of the recycle compressor

[0112] 15 is the fresh oxygen used as oxidizer

[0113] 16 is the gas by-pass of the carbon monoxide converter

[0114] 17 is the CO converter reactor inlet gas

[0115] 18 is the CO converter reactor outlet gas

[0116] 19 is the gas recycled to the maleic anhydride reactor

[0117] 21 is the fraction of the CO converter recycled back to the maleic anhydride reactor

[0118] 22 is the fraction of the CO converter purged and used as C02source

[0119] Figure 1A is a block diagram, showing the production of maleic anhydride from n-butane and air, in the common once-through operation . It represents , in simplified form, the arrangement used in most of the existing worldwide maleic anhydride plants .

[0120] The fresh air 1 enters the compressor 10 , where its pressure is increased to balance the pressure drop of the maleic reactor and of all other equipment and pipes of the system up to the discharge of the exhaust air to the atmosphere . The compressed air 5 j oins , before the reactor, a stream 2 of vaporized and superheated n-butane . The combined stream 6 , with a concentration of C4 near to the lower flammability limit , controlled by on-line analyzer (not shown in the drawing ) , enters the tubular maleic anhydride reactor 20 . Here , the reaction conditions are controlled on the basis of the on-line outlet analyzer (not shown) results and of the catalyst temperature profiles along the tubes , using the molten salt temperature as main variable parameter . The reactor effluent 7 at temperature between 400 and 440 °C enters the cooling system 30 , including multiple heat exchangers in a series / parallel arrangement .

[0121] The cooled reaction gas 8 , including nitrogen, as main component, plus residual oxygen, carbon oxides , maleic anhydride , organic acids and reaction water, enters the absorption / stripping system or absorber 40 used for the recovery of the maleic anhydride produced in the reactor and for its purification . Stream 3 represents the purified maleic anhydride product from the plant . The gases 9 from the absorption section, containing, besides nitrogen, carbon dioxide and water, some amount of unconverted butane , of carbon monoxide and in small concentrations of maleic anhydride and other organic acids , are sent to the oxidizer 50 , of the thermal , regenerative or catalytic type , before to be discharged to the atmosphere ( stream 4 ) .

[0122] Figure IB is a block diagram, showing the production of maleic anhydride from n-butane and air or enriched air, in the off-gas partial recycle operation . It represents , in simplified form, the arrangement used in few of the existing worldwide maleic anhydride plants .

[0123] The fresh air 1 , together with a recycle gaseous stream 13 , enters as stream 14 the compressor 10 , where its pressure is increased to balance the pressure drop of the maleic reactor and of all other equipment and pipes of the system up to the discharge of the exhaust air to the atmosphere . The compressed gas mixture 5 j oins , before the reactor, a stream 15 of high purity oxygen ( optional ) and, afterwards , a stream 2 of vaporized and superheated n-butane . The combined stream 6 , with a concentration of C4near to the lower flammability limit , controlled by on-line analyzer (not shown in the drawing) , enters the tubular maleic anhydride reactor 20 . Here , the reaction conditions are controlled on the basis of the on-line outlet analyzer (not shown) and of the catalyst temperature profiles along the tubes , using the molten salt temperature as main variable parameter . The reactor effluent gases 7 at temperature between 400 and 440 ° C enter the cooling system 30 , including multiple heat exchangers in a series / parallel arrangement .

[0124] The cooled reaction gas 8 , including nitrogen, as main component , plus residual oxygen, carbon oxides , maleic anhydride , organic acids and reaction water, enters the absorption / stripping system, or absorber 40 used for the recovery of the maleic anhydride produced in the reactor and for its purification . Stream 3 represents the purified maleic anhydride product from the plant . The gases 9 from the absorption section 40 , containing some amount of unconverted butane , of carbon monoxide and in small concentrations of maleic anhydride and other organic acids , are partially recycled as stream 12 to the reactor 20 through the water cooling and scrubbing system 60 and the gas compressor 10 ; the remaining part , as stream 11 , is sent to the oxidizer 50 , of the thermal , regenerative or catalytic type , before to be discharged as stream 4 to the atmosphere .

[0125] In particular, the recycle gas 12 enters the water cooling and scrubbing column 60 , where small amounts of maleic anhydride and the organic acid by-products are absorbed in water, protecting the gas compressor from corrosion . The washed gas 13 represents the recycle gas j oining the fresh air at the suction of compressor 10 .

[0126] Figure 2 is a block diagram according to the present improved process , in the operation at full gas recycle , with use of pure oxygen and use of a selective carbon monoxide oxidation reactor, where the purge gas , after treatment , is discharged to atmosphere .

[0127] The recycle gaseous stream 14 , containing mostly carbon dioxide , enters the compressor 10 , where its pressure is increased to balance the pressure drop of the maleic reactor and of all other equipment and pipes of the system up to the discharge of the exhaust air to the atmosphere . Part of the compressed gas 5 , as stream 17 , enters the selective oxidizer 70 , where the use a suitable selective catalyst , together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide , without substantial oxidation of the n-butane , not reacted in the maleic anhydride reactor, contained in the recycle gas . The stream 16 represents the portion of the compressed gas by-passing the selective carbon monoxide oxidizer 70 . As general guideline , any increase of the portion of gas by-passing the CO converter will cause an increment of the concentration of CO in the recycle gas and, therefore , it will make the conversion reaction less stable and more subj ect to run-away condition . On the contrary, the dimensions and capital cost of the converter reactor will increase with the decrease of the by-pass flowrate . According to the process of the invention, the maximum by-pass percentage is 65% of the total recycle gas . The effluent 18 from the CO converter j oins the by-pass stream 16 ; the combined stream 19 j oins , before the reactor, a stream 15 of high purity oxygen and, afterwards , a stream 2 of vaporized and superheated n-butane . The combined stream 6 , with a concentration of C4higher than the upper flammability limit in an atmosphere containing carbon dioxide at high concentration, controlled by an on-line analyzer (not shown in the drawing) , enters the tubular maleic anhydride reactor 20 .

[0128] Here, the reaction conditions are controlled on the basis of the on-line outlet analyzer (not shown ) and of the catalyst temperature profiles along the tubes , using the molten salt temperature as main variable parameter to maintain the outlet oxygen concentration above a prefixed value not lower than 4 vol% . The oxidation reactor operates at a volumetric space velocity GHSV in the range from 1400 and 2500 hr~1, with a value selected for balancing productivity and gas pressure drop . The oxidation catalyst is a Vanadium-Phosphorus-mixed Oxides (VPO) type . Particularly preferred are catalysts developed for minimum production of light organic acids , as for example the Syndane 4122 LA or 4142 LA produced by Clariant . The reactor pressure can be selected between 1 to 5 bar g, with a value chosen to balance capital and operating costs .

[0129] The reactor effluent 7 at temperature between 390 and 440 °C enters the cooling system 30 , including multiple heat exchangers in a series / parallel arrangement .

[0130] The cooled reaction gas 8 , including carbon dioxide , as main component , plus residual oxygen and n- butane , some carbon monoxide , maleic anhydride , organic acids and reaction water, enters at a temperature between 130 and 180 °C the absorption / stripping system or absorber 40 used for the recovery of the maleic anhydride produced in the reactor and for its purification . Here, an organic solvent, typically dibutyl phthalate ( DBP ) , in used in closed loop as absorption liquid selective towards the maleic anhydride . Stream 3 represents the purified maleic anhydride product from the plant; its purity can be controlled at 99 . 8% to 99 . 9% or still higher . The gases 9 from the absorption section at a temperature of 70 to 75 °C, containing some amount of unconverted butane , carbon monoxide and, in small concentrations , of maleic anhydride and other organic acids , are sent to a cooling and scrubbing column 60 , where the organic acids are absorbed in water and part of the reaction water is condensed . The gas 13 from the scrubbing, at a temperature of 35 to 45 ° C, is partially recycled as stream 14 to the reactor by means of the recycle compressor 10 ; the remaining part , as stream 11 , is sent to the oxidizer 50 , of the thermal , regenerative or catalytic type, before to be discharged as stream 4 to the atmosphere .

[0131] Figure 3 is a block diagram according to the preferred embodiment of the present invention, in the operation at full gas recycle , with use of pure oxygen and use of a selective carbon monoxide oxidation reactor, where the purge gas is used for carbon capture .

[0132] The recycle gaseous stream 14 , containing mostly carbon dioxide , enters the compressor 10 , where its pressure is increased to balance the pressure drop of the maleic reactor 20 and of all other equipment and pipes of the system up to the discharge of the exhaust air to the atmosphere . Part of the compressed gas 5 , as stream 17 enters the selective oxidizer 70 , where the use a suitable selective catalyst , together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide, without substantial oxidation of the not reacted n-butane contained in the recycle gas . The stream 16 represents the portion of the compressed gas by-passing the selective carbon monoxide oxidizer 70 . As a general guideline , any increase of the portion of gas by-passing the CO converter 70 will cause an increment of the concentration of CO in the recycle gas and, therefore , it will make the conversion reaction less stable and more subj ect to run-away condition . On the contrary, the dimensions and the capital cost of the converter reactor 70 will increase with the decrease of the bypass flowrate . According to the process of the invention, the maximum by-pass percentage is 60% of the total recycle gas . The effluent 18 from the CO converter 70 is split into two parts ; a first part 21 j oins the by-pass stream 16 and the combined stream 19 represents the recycle gas to the reactor; a second portion as stream 22 represents the net purge gas rich in carbon dioxide used for carbon capture and utilization .

[0133] Depending on the final utilization, it shall be subj ect of purification steps , not shown in the drawing, able to reduce the negative impact of some impurities on pipeline transportation, geological storage and / or Enhanced Oil Recovery .

[0134] The combined recycle stream 19 j oins , before the reactor 20 , a stream 15 of high purity oxygen and, afterwards , a stream 2 of vapori zed and superheated n- butane . The combined stream 6 , with a concentration of C4higher than the upper flammability limit in an atmosphere containing carbon dioxide at high concentration, controlled by an on-line analyzer (not shown in the drawing ) , enters the tubular maleic anhydride reactor 20 . Here, the reaction conditions are controlled on the basis of the on-line outlet analyzer (not shown ) and of the catalyst temperature profiles along the tubes , using the molten salt temperature as main variable parameter to maintain the outlet oxygen concentration above a pre-fixed value not lower than 4 vol % . The oxidation reactor operates at a volumetric space velocity GHSV in the range from 1400 and 2500 hr~ with a value selected for balancing productivity and gas pressure drop . The oxidation catalyst is a Vanadium- Phosphorus -mixed Oxides (VPO ) type . Particularly preferred are catalysts developed for minimum production of light organic acids , as for example the Syndane 3122 LA or 3142 LA produced by Clariant . The reactor pressure can be selected between 1 to 5 bar g, with a value chosen to balance capital and operating costs .

[0135] The reactor effluent 7 at temperature between 390 and 440 °C enters the cooling system 30 , including multiple heat exchangers in a series / parallel arrangement .

[0136] The cooled reaction gas 8 , including carbon dioxide, as main component , plus residual oxygen and n- butane , some carbon monoxide , maleic anhydride , organic acids and reaction water, enters at a temperature between 130 and 180 °C the absorption / stripping system 40 used for the recovery of the maleic anhydride produced in the reactor and for its purification . Here , an organic solvent , typically dibutyl phthalate ( DBP ) , in used in closed loop as absorption liquid selective towards the maleic anhydride . In particular , the purge gas stream 22 , which still contains some amount of not converted n-butane, may be sent to an absorption column, not shown in the drawing, where the lean organic solvent coming from the maleic anhydride vacuum stripper included in the process unit 40 , may be used to recover by absorption a significant portion of such n-butane . The solvent from the column is then used in the main maleic anhydride absorber of unit 40 , where , due to the lower operating pressure and to the contact with the exhaust air from the maleic anhydride reactor, the n-butane is desorbed and recovered in the recycle gas stream 9 . In this way, the n-C4absorber reaches a double scope , a reduction of the butane consumption together with a first step of purification of the carbon dioxide rich gas to be utilized or stored .

[0137] Stream 3 represents the purified maleic anhydride product from the plant ; its purity can be controlled at 99 . 8% to 99 . 9% or still higher . The gases 9 from the absorption section at a temperature of 70 to 75 °C, containing some amount of unconverted butane , carbon monoxide and, in small concentrations , of maleic anhydride and other organic acids , are sent to a cooling and scrubbing column 60 , where the organic acids are absorbed in water and part of the reaction water is condensed . The gas 14 from the scrubbing, at a temperature of 35 to 45 ° C, is recycled to the reactor by means of the recycle compressor 10 .

[0138] The invention is illustrated in great detail by the examples below, that anyway shall not be construed as a limitation of the scope of the invention itself or of the manner in which it may be practiced . COMPARATIVE EXAMPLE 1

[0139] An industrial maleic anhydride plant is operated according to the scheme of Fig . IB, with use of fresh air and no oxygen addition, under partial gas recycle conditions .

[0140] The reactor is operated at a gas volume hourly space velocity of 1800 hr-1.

[0141] The reactor tubes have an inner diameter of 21 mm and a length of 6 meters . The reactor cooling and control is made through a circulation of molten salts . The catalyst is of type SYNDANE 3122 LA produced by Clariant .

[0142] The reactor effluent , after cooling, is sent to a high efficiency absorber using dibutyl phthalate as organic selective solvent to recover the maleic anhydride . The off gases from the absorber overhead are partially recycled, after a scrubbing with water, to the reactor through a centrifugal compressor . The part of the off gases not recycled back are sent to a thermal oxidizer, before to be discharged to the atmosphere .

[0143] The gas compositions at the reactor inlet and at the reactor outlet , are as shows in Table 1 .

[0144] Table 1

[0145] The total amount of VPO catalyst is 52 . 8 tons .

[0146] The catalyst productivity is 0 . 106 Kg / h of maleic anhydride per kg of catalyst .

[0147] The amount of carbon dioxide discharged to the ambient from the stack of the thermal oxidizer is around 74000 MT / Y, equivalent to 1 . 64 ton of CO2 per ton of maleic anhydride produced; more than 50% of the carbon dioxide comes from the combustion of the not converted butane and of some additional natural gas needed for reasons of combustion efficiency . The concentration of CO2 in the vent gas is 7 . 7 % by weight . COMPARATIVE EXAMPLE 2

[0148] In a maleic anhydride industrial plant according with Fig . IB, with use of a reduced amount of fresh air with the addition of pure oxygen, the gas recycle ratio, defined as the ratio of the recycle gas flowrate / ( recycle gas + purge gas ) flowrate was 0 . 92 .

[0149] The conditions were not far from the total recycle operation (RR=1 ) , but the reduced amount of fresh air was sufficient to introduce in the system a large amount of nitrogen and to dilute the total carbon oxides ( CO + CO2 ) below 30 vol% .

[0150] The concentration of butane at the reactor inlet was much higher than in the conventional plants . The low oxygen concentrations at the reactor inlet and outlet guaranteed a safe operation .

[0151] The reactor was operated at high gas volume hourly space velocity of 2400 hr-* .

[0152] The reactor inlet and outlet temperatures were 140 and 436 °C, respectively .

[0153] The reactor tubes had an inner diameter of 21 mm and a length of 6 meters . The reactor cooling and control was made through a circulation of molten salts .

[0154] The reactor effluent , after cooling, was sent to a high ef ficiency absorber using dibutyl phthalate as organic selective solvent to recover the maleic anhydride . The off gas from the absorber were scrubbed by water and afterwards were recycled to the reactor through a centrifugal compressor . Part of the scrubbed gas was purged through a thermal oxidizer, before to be discharged to the atmosphere .

[0155] The gas compositions at the reactor inlet and at the reactor outlet , were as shows in Table 2 .

[0156] Table 2

[0157] The data from the industrial plant , demonstrate two important points : a . Under operating conditions of low oxygen concentration at the outlet of the reactor, the production of light organic in very high . At 2 . 4 vol . % O2 concentration, the total light acid production was more than 10 mol% of the maleic anhydride production or around 7 % by weight . b . The molar ratio of the produced CO / CO2 was around 1 . 25 , being the value very similar the one with low or without gas recycle conditions .

[0158] The reactor productivity was of 0 . 124 Kg / h of maleic anhydride per kg of catalyst, 17 % higher than comparative example 1 even under conditions of very low oxygen concentrations . Of course , the higher productivity in mostly connected to the higher gas space velocity .

[0159] Fresh air was added to the vent gases to permit the combustion of the not reacted butane and of the carbon monoxide . The amount of carbon dioxide discharged to the ambient from the stack of the thermal oxidizer was around 86000 MT / Y, equivalent to 1 . 94 ton of CO2 per ton of maleic anhydride produced . The concentration of CO2 in the vent gas was 30% by weight . INVENTIVE EXAMPLE 3

[0160] This example refers to the operation in a pilot reaction unit , according to the present invention .

[0161] The reactor consisted in a single tube having the same dimensions of a tube in a typical industrial reactor : 21 mm as inside diameter and 6 meters as length . The tube in surrounded by molten salts circulating under controlled temperature .

[0162] The reactor effluent gases were passed through a water scrubber for the absorption of maleic anhydride and light organic acids . The scrubbed gas , after a partial purge to remove the accumulated inert gas , mostly carbon oxide , were recycled to the reactor through the use of a laboratory diaphragm compressor . At the compressor discharge , the fresh oxygen and the vaporized butane stream were added under flow control , in order to achieve the desired reagent conditions at the reactor unit .

[0163] Being the pilot unit lacking in the carbon monoxide selective converter, the operation at high carbon dioxide concentration was simulated by introducing a stream of high purity ( 95% ) CO2 .

[0164] The test reactor was operated at a gas volume hourly space velocity between 1750 and 2200 hr-1.

[0165] The data in table 3 refer to the operation at 1750 hr~* .

[0166] Table 3

[0167] At the conditions in accordance with the present invention and in particular at a concentration of oxygen at the reactor outlet higher than the Limit Oxygen Concentration, but higher than 4 vol . % , the inventive example 3 shows the following advantages : I ) the differential n-butane concentration was very high : around 2 . 4 vol% , much higher than comparative example 1 (A = 1 . 4 vol% ) and comparative example 2 (A = 1 .7 vol% ) ;

[0168] II ) the high A conversion turns into a very high catalyst productivity, of 0 . 152 Kg / h of maleic anhydride per kg of catalyst . The catalyst productivity increases by 42% in comparison with example 1 .

[0169] It means that the actual intrinsic limitation of the largest maleic anhydride reactor of around 70000 to 75000 MT / Y of maleic anhydride production, under the conditions of the invention can overcome the target of 100000 MT / Y .

[0170] II I ) the production of light acids , even if still higher compared to the once-through operation, is reduced by 40% compared to example 2 and it comes back into values acceptable in consideration of the other advantages offered by this type of operation .

[0171] INVENTIVE EXAMPLE 4

[0172] The experimental data from the pilot unit have been extrapolated to a large si ze industrial reactor, with the introduction of a selective carbon monoxide reactor at the discharge of the recycle compressor, according to figure 3 .

[0173] Considering the present largest size of a maleic anhydride tubular reactor, with 39500 tubes having 25 mm as inside diameter and 5500 mm of catalyst bed, corresponding to 106 , 6 m3of catalyst , a maleic anhydride recovery of 99% in the absorption / stripping / purif ication section, at a gas volume hourly gas velocity of 1830 hr-1, the corresponding maleic anhydride production, having a purity of 99 . 9% , is higher than 12500 Kg / h or 100000 MT / Y . The productivity of a single reactor is 35% higher compared to the state of art processes .

[0174] The plant produces 13600 Kg / h of vent gas at 94 % by weight of carbon dioxide . After compression and purification, the gas can be applied for carbon capture and utilization . The carbon dioxide / maleic anhydride production weight ratio is 1 to 1 . Carbon dioxide production is 40% lower compared to example 1 .

[0175] The gas compositions at the inlet and outlet of the maleic anhydride reactor are shown in Table 4 . The values are very similar to Table 3 under Example 3 , apart from the complete disappearance of the nitrogen, which in example 3 entered the loop as impurity of the carbon dioxide inj ection, and the high reduction of the carbon monoxide , both of them compensated by a higher carbon dioxide concentration .

[0176] Table 4

[0177] More interesting are the conditions at the inlet and outlet of the selective carbon monoxide oxidizer , as shown in table 4A . Table 4 A Carbon monoxide selective converter

[0178] The data are based on use of a Pt / Pd on AI2O3 catalyst within a tubular vertical reactor, with production of low / medium pressure steam . To avoid risk of runway conditions , bringing to the combustion of butane , the reactor has several safety options : first , at the conditions taught by the invention, with control of the oxygen concentration, the converter is partially self-protected, since the content of oxygen, even at runway condition, permits only a partial combustion of the residual butane , being its concentration below the stoichiometric value for the complete combustion of CO + C4. In other and more significant words , the maximum heat of reaction to be removed in case of runway conditions is much lower than the one released under higher oxygen levels . The above condition is fundamental for a proper utilization of the following safety steps :

[0179] • inertization of the catalyst to contain hot spot temperatures ; • use of a heat transfer surface in excess to the minimum required by the volume of catalyst ;

[0180] • use a selective catalyst with start of the butane conversion at temperature much higher by comparison of CO; control of pressure of the steam produced used for reactor outlet temperature control .

Claims

CLAIMS1 . A process for the production of maleic anhydride by the catalytic oxidation of n-butane and pure oxygen, wherein said process comprises the following steps :( a ) Feeding the reaction gas mixture to a tubular reactor unit containing a vanadium-phosphorus oxide catalyst , wherein the gas mixture includes a vaporized stream rich in n-butane at a concentration higher than the relevant upper flammability limit in a high inertized atmosphere, a stream of high purity oxygen and a gaseous stream rich in carbon dioxide recycled to the reactor through a gas compressor ;(b ) Producing in reaction tubes of said tubular reactor unit , maleic anhydride as main product , carbon oxides and light organic acids as undesired by products and reaction water;( c) Cooling the effluent gas leaving the tubular reactor unit at temperature between 380 and 450 °C, by recovering the heat of reaction for the production of steam at a single pressure value or in multiple heat exchangers in series at decreasing values of pressure ;(d) Selectively recovering the maleic anhydride contained in the reaction gas in an absorber in form of high efficiency absorption column, by using an organic solvent as absorption liguid;( e ) Cooling and scrubbing by water the exhaust gas from said absorber in order to remove small amount of maleic anhydride not removed in theabsorber and the organic acid by-products , protecting the gas compressor;( f ) Purging part of the water scrubbed exhaust gas , to avoid accumulation of inerts , and sending it to a thermal or regenerative or catalytic first oxidizer, in order to meet the local regulations for the gas emission to the atmosphere as carbon monoxide , hydrocarbons and nitrogen oxides limitations ;( g) Recycling the scrubbed gas mixture by means of a gas compressor increasing its pressure at the reaction value ;(h) Feeding a portion from 35 to 60 % of the compressed gas to a selective second oxidizer , wherein the use a suitable selective catalyst , together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide without substantial oxidation of the not reacted n-butane contained in the recycle gas ;( i ) Using the j oined gas mixture rich in carbon dioxide as diluent of the fresh n-butane and pure oxygen to feed the tubular maleic anhydride reactor unit ; characterized in that : i . The recycle dilution gas is rich in carbon dioxide at a concentration, before the inj ection of fresh butane and pure oxygen, higher than 85% ; ii . The n-butane content at the inlet of the tubular reactor unit is controlled within 4 to 6%and its conversion in the same reactor is between 35 to 50% ; iii . The oxygen content at the tubular reactor unit inlet is controlled at a value between 12 to 16 mol% ; iv . The combined butane , carbon dioxide , carbon monoxide and water content at the inlet of the tubular reactor unit is outside the flammability limits at its temperature and pressure ; v . The oxygen content at the tubular reactor unit outlet is controlled to a value lower than the oxygen percentage below which no mixture is flammable independent of the concentration of fuels ( LOG -Limiting Oxygen Concentration) , but not less than 4 . 0 mol% .2 . A process according to claim 1 , wherein the catalytic reaction for the oxidation to maleic anhydride is carried out at a space velocity from 1400 to 2500 hr-1;3 . A process according to claim 1 , wherein the catalytic reaction for the oxidation to maleic anhydride is carried out at temperature of 370 and 450 ° C;4 . A process according to claim 1 , wherein a volatile organic phosphorous compound is added to the gaseous rection feed mixture to control the activity of the catalyst ;5 . A process according to claim 1 , wherein the oxidation catalyst is a Vanadium-Phosphorus-mixed Oxides (VPO) type developed for minimum production of light organic acids ;6 . A process according to claim 1 wherein the tubular reactor unit for the production of maleic anhydride has an enhanced heat transfer system, as molten salt circulation or as steam production surface area or as both of them, adequate to the higher productivity of the reactor at the conditions of the present process compared to the state of art process .7 . A process according to claim 1 , wherein the second catalytic selective oxidizer to convert most of the carbon monoxide to carbon dioxide is a fixed bed tubular reactor, under conditions where the temperature is controlled to permit a CO conversion not lower than 80% and a butane conversion not higher than 5% .8 . A process according to claim 7 , wherein the control of the hot spot temperature in the tubes of said second catalytic selective oxidizer is achieved by using a selective oxidation catalyst mixed with inert material .9 . A process according to claim 8 , wherein the exothermic heat of reaction is removed through the circulation in the shell side of said fixed bed tubular reactor of high pressure water or different heat transfer medium or, as preferred solution, with production of steam at controlled pressure .10 . A process for the production of maleic anhydride by the catalytic oxidation of n-butane and pure oxygen, wherein the purge gas is used as a source of carbon dioxide useful , after adequate compression and purification steps , for carbon capture , utilizationand storage (CCUS ) or enhanced oil recovery (EOR) and wherein said process comprises the following steps :( a ) Feeding the reaction gas mixture to a tubular reactor unit containing a vanadium-phosphorus oxide catalyst , wherein the gas mixture includes a vaporized stream rich in n-butane at a concentration higher than the relevant upper flammability limit in a high inertized atmosphere , a stream of high purity oxygen and a gaseous stream rich in carbon dioxide recycled to the same first tubular reactor unit through a gas compressor ;(b ) Producing in reaction tubes of said tubular reactor unit maleic anhydride as main product , carbon oxides and light organic acids as undesired by products and reaction water;( c ) Cooling the effluent gas leaving said tubular reactor unit at temperature between 380 and 450 °C, by recovering the heat of reaction for a production of steam at a single pressure value or in multiple heat exchangers in series at decreasing values of pressure;( d) Selectively recovering the maleic anhydride contained in the reaction gas in an absorber in form of an high efficiency absorption column, by using an organic solvent or water as absorption liquid;( e ) Cooling and scrubbing by water the exhaust gas from the absorber in order to remove small amount of maleic anhydride not removed in theabsorber and the organic acid by-products, protecting the gas compressor;(f) Recycling the scrubbed gas mixture by means of a gas compressor increasing its pressure at the reaction value;(g) Feeding a portion from 40 to 60 % of the compressed gas to a selective oxidizer, wherein the use a suitable selective catalyst, together with a controlled temperature and the controlled content of oxygen promotes the safe conversion of most of the carbon monoxide to carbon dioxide, without substantial oxidation of the not reacted n-butane contained in the recycle gas;(h) Purging part of the carbon monoxide converter exhaust gas, to avoid accumulation of inerts, and joining the remaining part to the compressed gas bypassing the CO converter;(i) Using the joined gas mixture rich in carbon dioxide as diluent of the fresh n-butane and pure oxygen to feed the tubular maleic anhydride reactor unit;(j) Using the purge gas in point (h) as a source of carbon dioxide useful, after adequate compression and purification steps, for carbon capture, utilization and storage (CCUS)or enhanced oil recovery (EOR) ; characterized in that: i. The recycle dilution gas is rich in carbon dioxide at a concentration, before the injection of fresh butane and pure oxygen, higher than 85%;ii . The n-butane content at the inlet of the tubular reactor unit in controlled within 4 to 6% and its conversion in the reactor is between 35 to 50% ; iii . The oxygen content at the tubular reactor unit inlet is controlled at a value between 12 to 16 mol% ; iv . The combined butane, carbon dioxide , carbon monoxide and water content at the inlet of the tubular reactor unit is outside the flammability limits at its temperature and pressure ; v . The oxygen content at the tubular reactor unit outlet is controlled to a value lower than the oxygen percentage below which no mixture i s flammable independent of the concentration of fuels (LOG -Limiting Oxygen Concentration ) , but not less than 4 . 0 mol% .11 . A process according to claim 10 , wherein the catalytic reaction for the oxidation to maleic anhydride is carried out at a space velocity from 1400 to 2500 hr" .12 . A process according to claim 10 , wherein the catalytic reaction for the oxidation to maleic anhydride is carried out at temperature of 370 and 450 ° C .13 . A process according to claim 10 , wherein a volatile organic phosphorous compound is added to the gaseous rection feed mixture to control the activity of the catalyst .14 . A process according to claim 10 , wherein the oxidation catalyst is a Vanadium-Phosphorus-mixedOxides (VPO) type developed for minimum production of light organic acids .15 . A process according to claim 10 , wherein the tubular reactor unit for the production of maleic anhydride has an enhanced heat transfer system, as molten salt circulation or as steam production surface area or as both of them, adequate to the higher productivity of the reactor at the conditions of the present process compared to the state of art process .

16. A process according to claim 10 , wherein the catalytic selective oxidizer to convert most of the carbon monoxide to carbon dioxide is a fixed bed tubular reactor, under conditions where the temperature is controlled to permit a CO conversion not lower than 80% and a butane conversion not higher than 5% .17 . A process according to claim 16 , wherein the control of the hot spot temperature in the tubes of said catalytic selective oxidizer is achieved by using a selective oxidation catalyst mixed with inert material .18 . A process according to claim 17 , wherein the exothermic heat of reaction is removed through the circulation in the shell side of high pressure water or different heat transfer medium or , as preferred solution, with production of steam at controlled pressure .19 . A process according to claim 10 , wherein the process includes an absorption step to recover a significant portion of the unconverted n-butane fromthe purge gas referred to points h) and j ) , by using as absorption liquid the lean organic solvent referred to point d) , wherein the pressure of the C4absorber is higher than the pressure of the maleic anhydride absorber of point d) .

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