MODIFIED TUBULAR REACTOR AND PROCESS FOR EFFECTING CATALYTIC REACTIONS INVOLVING THERMAL EXCHANGES IN WHICH IT IS USED The present invention relates to a modified tubular reactor and the process for its use in catalytic reactions involving thermal exchanges carried out in liquid and/or vapour phase .

Tubular reactors generally consist of vertical tube- bundle exchangers, in which the catalyst is kept in the tubes by means of metallic supports whereas the fluid used for releasing or removing heat, flows in the shell side.

This type of reactor is successfully used in both en- dothermic and exothermic reactions. A typical example of an endothermic reaction is the cracking reaction of MTBE (methyl tert butyl ether) , used for the production of high-purity isobutene, in which the reaction heat is supplied by circulating hot oil or steam in the shell side. Tubular reactors however are mainly used in exothermic

reactions and typical examples of application are the eth- erification and/or selective dimerization of branched olefins (iso-olefins) contained in C ₄ and C ₅ hydrocarbon cuts.

Through these reactions, it is in fact possible to convert isobutene and isoamlyenes (2 -methyl -1-butene and 2- methyl-2-butene) , which are the most reactive olefins present in C ₄ and C ₅ streams, into high-octane compounds such as ethers of the MTBE, ETBE (ethyl tert butyl ether) , TAME (methyl tert amyl ether) , TAEE (ethyl tert amyl ether) and/or hydrocarbon streams such as iso-octene (cut containing at least aυ% of C ₈ hydrocarbons) .

Due to the high octane characteristics (both Research Octane Number (RON) and Motor Octane Number (MON) ) , the low volatility, the absence of the most harmful products for the environment (sulfur, aromatic products, benzene, etc.), all these compounds (hydrocarbons and/or oxygenated compounds) fall into the category of compositions which are extremely interesting for obtaining gasolines more compatible with present environmental demands. In order to respond to the ever- increasing request in future gasolines for compounds having these characteristics, there is therefore the necessity of improving their production processes also with the use of simpler and more functional reactors . All of these compounds are synthesized in substan-

tially similar plants as the reactors (tubular and adia- batic) and catalysts (ion exchange acid resins) are practically the same for both etherification and dimerization reactions of branched olefins. Whereas in the case of etherification, it is sufficient to react the iso-olefin with a linear alcohol (preferably methanol and/or ethanol) , in the case of dimerization, in order to succeed in obtaining a high-quality product (content of dimers higher than 80% weight) , it is nec- essary to use moderate quantities of oxygenated compounds (ethers and/or alcohol) capable of moderating the activity of the catalyst and therefore control the reaction rate obtaining the desired selectivity to dimers.

Oxygenated compounds which can be used in the case of C ₄ streams are:

• primary alcohol in a sub- stoichiometric quantity with respect to the isobutene fed in the feedstock (IT- MI95/A001140 of 01/06/1995) ;

• primary alcohol and/or alkyl ether in a sub- stoichiometric quantity with respect to the isobutene fed in the feedstock (IT-MI97/A001129 of 15/05/1997) ;

• tertiary alcohol and/or alkyl ether and/or primary alcohol in a sub- stoichiometric quantity with respect to the isobutene fed in the feedstock (IT-MI99/A001765 of 05/08/1999) ;

• tertiary alcohol (such as tertbutyl alcohol - TBA) in a sub- stoichiometric quantity with respect to the isobutene fed in the feedstock (IT-MI94/A001089 of 27/05/1994) .

Hydrocarbon streams, used in the etherification and/or dimerization reaction, mainly containing isobutane, isobutene, n-butane, n-butenes, isoamylenes and saturated and olefinic C ₅ hydrocarbons.

Although a wide variety of sources is available for providing these streams, the most common relate to those deriving from Dehydrogenation processes of iso-paraffins, FCC units, Steam Cracking units or processes for pure isobutene production, such as the TBA Dehydration or Cracking of MTBE or ETBE; these streams differ from each other in the content of branched and linear olefins. The branched olefin is mainly converted in the reaction area, part of the C ₄ -C ₅ olefins present, however, can also be converted to useful product.

The use of tubular reactors is fundamental for obtaining the high selectivities which, by using the minimum quantity of catalyst with the same performances, are able to remove the reaction heat, limit the formation of byproducts in the case of etherification and avoid, in the case of dimerization, the initiation of oligomerization and polymerization reactions of olefins which are favoured by high temperatures.

In these reactors, as shown in figure 1, the catalyst is situated in the tubes, whereas the tempered water flows in the shell side, to remove the reaction heat and consequently obtain an optimum temperature profile for an exo- thermic reaction; in this case, in fact, the temperature, as indicated in figure 2, is high in the first part of the catalytic bed to ensure kinetic support for the reaction and as low as possible at the reactor outlet, in order to maximize the thermodynamic conversion and minimize the pro- duction of by-products.

This particular thermal profile also gives to the tubular reactor greater resistance to poisons, because only the initial part of the catalytic bed is effectively involved in the synthesis, whereas the remaining part (used for achieving the equilibrium conversion) practically acts as a catalyst reserve.

In spite of the fact, therefore, that only a part of the catalyst is actually used, this reactor uses, with the same performances, a much lower quantity of catalyst than that of an adiabatic reactor.

In the case of deactivation of the catalyst, it is also possible to use the tempered water for giving thermal support to the reaction.

The tempered water can flow either in co-current or countercurrent with respect to the process fluid, in rela-

tion to the quantity of heat to be removed, and it is re- circulated by means of a pump after being cooled to the desired temperature in an external exchanger.

This reactor has a great operating flexibility because is possible to control the internal temperature and consequently the reaction rate by modifying both the inlet temperature of the reagents and the temperature and flow-rate of the tempered water.

The process fluid flows upwardly through the tubes to limit packing phenomena of the catalyst and significantly reduce pressure drops but occasionally it can also be fed from the top downwards.

The behaviour of this reactor is the same both in the case of very concentrated and also extremely diluted feed- stocks and consequently the tubular reactor can indifferently treat any type of feedstock; for this reason it is normally used as front-end reactor (where most of the conversion is realized ) in etherification and/or dimerization plants in both the first and also in a possible second stage.

A wide variety of solid acid catalysts can be used in this type of reactor, but styrene-divinylbenzene polymeric resins having sulfonic groups as catalytic centres such as, for example, Amberlyst 15 and 35 in the case of etherifica- tion and dimerization, and silicon and/or aluminum oxides

in the case of cracking reactions of alkyl ethers, are preferred.

A wide range of operating conditions can be used; the desired reactions can be carried out in vapour phase (pre- ferred condition for cracking reactions) , in liquid-vapour phase or in liquid phase (preferred condition for etherifi- cation and dimerization reactions) .

The pressure is preferably superatmospheric and generally below 5 MPa, more preferably between 0.2 - 3.0 MPa. The temperature varies in relation to the type of reaction and generally below 300 ⁰ C for cracking and preferably from 30 to 120 ⁰ C for etherification and dimerization reactions.

These reactors can normally be made of carbon steel or stainless steel but it is also possible to use a combina- tion of the two materials; for example it is possible to produce a reactor using stainless steel only for the tubes. This type of reactor is obviously more expensive than a simple adiabatic reactor, but it has so many operational advantages that it is just as competitive. The main limit of this type of reactor, on the other hand, consists in the fact that the loading and unloading procedure of the catalyst is extremely lengthy and onerous; the catalyst is in fact kept inside the tubes, whose number can even reach various thousands, with the use of particu- lar metallic supports, mesh-windings and springs, situated

at both ends of each tube (as shown in figure 1) which must be installed and removed for its load and unload.

A suitably modified tubular reactor has now been found, without metallic supporting devices of the catalyst in the tubes, which is much more flexible and efficient and easier to manage, with respect to the loading and unloading operations of the catalyst.

The tubular reactor, object of the present invention, for catalytic reactions involving thermal exchanges, essen- tially consists of a vertical tube -bundle exchanger whose tubes contain catalyst, having inlet and outlet nozzles for each passage side of the reagents, catalyst and thermal exchange liquid, characterized in that it has one or more metallic supports situated outside the lower tube plate in the lower part of the reactor for sustaining the catalyst so that the same catalyst is contained not only in the tubes of the tube-bundle but also in said lower part outside the lower tube plate and also in the upper part outside the upper tube plate. In particular, in the upper part, the hight of catalyst is preferably higher than 10 mm and more preferably higher than 50 mm with respect to the upper tube plate.

The metallic support is preferably positioned close to or on the lower inlet/outlet nozzle of the reactor. A feeding distributor, which preferably comprises a

series of vertical and/or side holes, can be also present in communication with the lower inlet/outlet nozzle of the reactor.

The distributor should be positioned at a distance from the tube plate preferably greater than 10 mm, more preferably greater than 50 mm. A flow deflector, possibly partially perforated, can be present above the distributor. The reactor can have several feeding nozzles, preferably two or four, equidistant from the vertical axis of the reactor, each in communication with a feeding distributor, above which a flow deflector, preferably perforated, can be positioned.

Said flow deflector positioned above each distributor can be tilted: it is preferably a perforated plate with a tilt angle ranging from 5 to 80° , more preferably from 10 to 70°.

A further central feeding nozzle of the reagent can be present at the bottom of the reactor.

The reactor according to the invention can be made of carbon steel or stainless steel or using a combination of the two materials.

A second object of the present invention relates to the process for effecting catalytic reactions, both exothermic and endothermic, carried out in liquid or vapour phase, involving thermal exchanges, which is characterized

in that it uses the reactor according to the invention.

The process for effecting catalytic reactions, preferably exothermic, in liquid phase, can be characterized by the following further essential aspects: • the process fluid preferably flows upwards;

• the cooling fluid flows in the shell side in co- current or countercurrent with respect to the reagents in relation to the quantity of heat to be exchanged;

• the cooling fluid leaving the reactor is cool down to the desired temperature in a suitable external item before being re-circulating into the reactor.

The catalyst preferably consists of styrene- divinylbenzene polymeric resins having sulfonic groups as functional centres. The linear rates inside the reactor are preferably lower than 4.5 cm/sec.

The cooling liquid is preferably water.

The minimum height of catalyst above the upper tube plate is preferably at least 10 mm, and more preferably at least 50 mm.

The catalytic reactions involving thermal exchanges are preferably etherification reactions between branched olefins and linear alcohol or dimerization reactions of branched olefins. The process for the etherification reaction between

branched olefins and linear alcohol is preferably effected at temperatures ranging from 30 to 120 ⁰ C and at pressures ranging from 0.2 to 3.0 MPa.

The linear alcohol is preferably selected from metha- nol and/or ethanol .

The branched olefin has a number of carbon atoms preferably ranging from 4 to 7.

The process for the dimerization reaction of branched olefins is preferably effected at temperatures ranging from 30 to 120 ⁰ C and pressures ranging from 0.2 to 3.0 MPa.

The branched olefin has a number of carbon atoms preferably ranging from 4 to 7.

The dimerization reaction is preferably carried out in the presence of linear alcohols and/or alkyl ether and/or branched alcohols.

The alkyl ether is preferably selected from MTBE ₇ ETBE, MSBE (methyl sec-butyl ether) , ESBE (ethyl sec-butyl ether), TAME, TAEE or mixtures thereof.

The branched alcohol has a number of carbon atoms preferably ranging from 4 to 7.

The linear alcohol is preferably selected from methanol and/or ethanol .

Other possible olefins present in the feedstock can also react to form high-octane products for both etherifi- cation and dimerization reactions.

The etherification and dimerization reactions of branched olefins can also be carried out contemporaneously. The process for effecting catalytic reactions, preferably endothermic, in vapour phase can be characterized by the following further essential aspects:

• the process fluid preferably flows downwards;

• the heater fluid flows in the shell side in co- current or countercurrent with respect to the reagents in relation to the quantity of heat to be exchanged; • the heater fluid leaving the reactor is reated to the desired temperature in a suitable externa.! item (oven or boiler) before being re-introduced into the reactor.

The heater fluid is preferably steam or hot oil. The endothermic catalytic reactions involving thermal exchanges are preferably cracking reactions of ethers, preferably selected from MTBE, ETBE, MSBE, ESBE, TAME, TAEE or mixtures thereof, or of branched alcohols, preferably with a number of carbon atoms ranging from 3 to 7 , the mast preferable is TBA.

For these cracking reactions, the catalyst preferably substantially consists of silicon and/or aluminum oxides.

The minimum height of catalyst above the upper tube plate is preferably at least 10 mm, more preferably at least 50 mm.

The process for the cracking reaction is preferably effected at temperatures lower than 300 ⁰ C and pressures ranging from 0.2 to 3.0 MPa.

A further object of the present invention relates to the use of the reactor according to the invention for catalytic reactions involving thermal exchanges .

In particular, preferred catalytic reactions involving thermal exchanges which require the use of the reactor according to the invention are etherification reactions be- tween branched olefins and linear alcohol, dimerization reactions of branched olefins and cracking reactions of alkyi ethers .

Preferred embodiments are provided for a better illustration of the present invention, with the help of the en- closed figures 3, 5, 6, 7, 8 and 9 which, however, should not be considered as limiting the scope of the invention itself.

Figure 3 shows the scheme of the modified tubular reactor which demonstrates how the catalyst can be easily and rapidly loaded from above and unloaded from the bottom of the reactor analogously to what is effected for an adia- batic reactor. Alternatively, in order to have a better control of the reaction temperature, an inert material can also be used for filling the bottom of the reactor to en- sure that the reaction is only iniziated in the area in

which the thermostat-regulation fluid is present.

In principle, in this reactor configuration, the process fluid can also be fed from above downwards, even if this solution is not recommended as there is a greater pos- sibility of packing of the catalyst.

With this configuration, without support inside the tube plate, the process fluid must obviously be well distributed in the tube bundle and have a maximum rate which is such as to maintain the catalytic bed slightly expanded, avoiding however fluidization and entrainment phenomena of the resin with a consequent emptying of the tubes and the creation of preferential ways.

The rate the fluid must have in the tubes to prevent entrainment depends on the particular geometry of the reac- tor and is preferably lower than 4.5 cm/sec and more preferably lower than 2.5 cm/sec.

It is therefore extremely important for this invention to have of an efficient distribution system of the process fluid which at the same time does not hinder the discharg- ing operations of the deactivated catalyst.

In the case of traditional tubular reactors with supports for the catalyst, the distribution of the liquid is effected through a central tube , as shown in figure 4, which shows a simplified drawing of the reactor bottom. This system, however, cannot be used in a modified tu-

bular reactor according to the present invention, with the catalyst in the bottom of the reactor and above the tube plates and device (mesh winding) in the feeding nozzle to withhold the catalyst in the reactor, as the rate of the fluid in the central part of the reactor would be higher than the minimum fluidization rate; new solutions must therefore be found for improving the distribution of the liquid.

Figures 5 and 6 show two possible modifications of traditional distributors which allow the fluid rate to be reduced in the central area of the reactor.

In figure 5 the central distributor, which is substantially a tube, has been modified creating a series of circular holes so as to allow the liquid to also be discharged laterally whereas, in figure 6 a plate is also introduced into the distributor, in addition to the side holes, as a deflector, positioned at a suitable distance from the central hole, so as to further reduce the central flow in favour of the side flow. This plate can be completely closed or alternatively equipped with a suitable number of holes.

In the case of new reactors, completely different solutions can be used for the distribution of the process fluid.

Figure 7 therefore shows a further possible configura- tion of the reactor which differs from the previous ones in

that it does not have a central nozzle for the inlet of the reagents; in this case, the process fluid enters in the bottom of the reactor from at least 2 nozzles situated below a horizontal plate equipped with at least 2 holes, clearly offset with respect to those of the reagent inlet, in addition to a further central hole used exclusively, however, for discharging the deactivated catalyst.

Alternatively, in the configuration shown in figure 8, it is possible to use at least 2 nozzles for feeding the reaction fluid, situated below a tilted perforated plate from whose centre the deactivated catalyst can be recovered. The angulation of this plate preferably ranges from 5 to 80 degrees and more preferably between 10 and 70 degrees . A further modification of this distribution system, shown in figure 9, also envisages, in addition to at least 2 nozzles situated below a tilted perforated plate, the presence of a small central nozzle for the introduction of a part of the process fluid.