The mixture comprising both the two types of nitroxide compound and an aromatic amine for the above mentioned use is novel to the state of the art and it has a synergistic effect experimentally verifiable with respect to the individual and separate use of the two types of compounds.

BACKGROUND OF THE INVENTION The effect of the nitroxide compounds also denominated"free radicals"in the stabilization of the monomers is known. Their use for the stabilization of acrylonitrile is disclosed, e. g., in US Pat. 3747988, whereas the inhibition of the polymerization of vinyl monomers is illustrated in US Pat. 3733326.

The problem of the stabilization of vinyl monomers is also discussed in patents EP 581737, EP 157738, GB 1316341, G3 1316342, US 5254760.

In UK Pat. 1218456 and US Pat. 4670131 the use of nitroxides at various concentrations as stabilizers for olefinic streams is disclosed.

Also aromatic amines in general, and in particular phenylene diamines and diphenylamines, are widely used as

antioxidants and stabilizers.

Recently, it has been observed (US Pat. 5711767) that nitroxides exhibit a particular effectiveness in controlling the formation of gum and in improving the stability to oxidation of pyrolytic gasolines, especially when used in combination with substituted phenylene diamines. This publication, arguably the state of the art closest to the present invention, relates however to a problem (stored gasolines) differing from the one considered in the present invention, wherein processes involving fluid streams containing olefins, aimed just at the treatment of the latter, are tackled.

During the olefin production process by pyrolisis of gaseous or liquid hydrocarbon streams, the formation of polymeric fouling deposits is incurred : such deposits usually generate onto the surfaces of the apparatuses, with a formation mechanism which usually includes a polymerization following the formation of carbon or oxygen radicals, by thermal induction and catalysis of metals deriving from corrosion phenomena. Polymeric fouling reduces thermal exchange, may adversely affect plant performance and stall the production capacity : in some cases the deposit that may be generated is so copious to cause the clogging of the sets of heat exchangers, the transfer pipes, the distillation column plates, and it may cause the unplanned shutdown of the plant itself. Deposits are usually localized in the heat exchanger sets, the bottom column reboilers, on the column bottom itself, onto the plates and in the intermediate pumparounds, as well as onto the top of rectification and extraction columns, in the transfer pipes and on the bottom of containers for the temporary storage of pure monomers.

SUMMARY OF THE INVENTION It has now surprisingly been found, and it is an object of the present invention, that the combination of nitroxide compounds with some aromatic amines allows to

remarkably improve the control of the polymerization reactions which originate fouling in the production, treatment and storage processes of olefinic monomers.

Hence, an object of the present invention is a process for preventing polymeric fouling in petrochemical plants for the treatment of olefinic compounds, in which in the supply stream of the treatment plants an amount of a stabilizing composition effective to prevent said fouling, comprising nitroxides and aromatic amines in a mixture therebetween in a weight ratio of 2 : 1 to 1 : 20, is introduced.

A further object of the invention is such stabilizing mixture and the use thereof to hinder fouling in plants for the treatment of olefinic compounds through petrochemical processes.

DETAILED DESCRIPTION OF THE INVENTION In view of an optimization of the decrease of the polymer fouling deposits which generate in the above stated processes and will hereinafter be detailed, the novelty of the present invention lies in the use of nitroxides in combination with an aromatic amine, carrying out a remarkable synergistic effect causing a surprising increase in the inhibition of the polymerization reaction which leads to fouling.

Nitroxides compounds suitable for use in the present invention are those containing free radicals.

Particularly suitable ones are those selected from the group comprising 4-hydroxy-tetramethyl-piperidino-1-oxy), or a nitroxide having the general formula wherein R, R', and R''represent H, alkyl, cycloalkyl, aryl (Cl-C15), substituted or unsubstituted

with heteroatoms such as N, S, P, halogens and the like, or a dinitroxide, in particular of the type 1- piperidinyloxy-4, 4'- (1, 10-dioxo-1, 10-decanediyl)-bis- 2, 2, 6, 6-tetramethyl and mixtures thereof.

Suitable aromatic amines are in particular diphenylamines (DPA) and phenylene diamines (PDA) or aromatic amines of general formula wherein R, R', R''have the above stated meaning, and mixtures thereof.

N (1, 4Tdimethyl-pentyl)-N'-phenyl-para-phenylene diamine, N (1, 3Tdimethyl-pentyl)-N'-phenyl-para-phenylene diamine and the mixtures thereof are specifically mentioned.

In the stabilizing mixture according to the present invention, the quantitative weight ratio of nitroxide and/or dinitroxide compound with respect to the aromatic amines ranges from 2 : 1 to 1 : 20, preferably from 1 : 7, 5 to 2 : 5.

The mixture is preferably a solution wherein the active substances nitroxide and aromatic amines are dissolved in an extractant. Suitable extractants are, by way of example, aliphatic extractants, xylene, benzene and higher homologous substances.

The stabilizing mixture may preferably be added, with respect to the fluid material to be treated, in an amount of from 0, 5 to 50 ppm.

The combined additive, consisting of the two active substances, may be added in sites which correspond to the plant sites, related to the whole range of steps of production, selective extraction, rectification and storage, that can be liable to fouling due to

particularly disadvantageous thermal conditions or to the presence of metal-containing oxygen or water, or in presence of particularly high concentrations of markedly reactive olefins.

Usually the stabilizing mixture, which is a combination of the two active substances, is added to the load streams. However it may also be supplied at the column bottom reboilers. In the present description the term"column"is intended to mean the distillation apparatus of a hydrocarbon stream containing olefins for the separation of a mixture made of Cn compounds, extracted from the bottom stream which instead is made of Cn+i +-, wherein n is an integer from 2 to 7. Moreover, the stabilizing mixture may be supplied at the bottom reboiler of the quench oil column (cooling of the hydrocarbon stream carried out with quench oil) in the cracking gas compression section, in particular under suction among the 3rd, 4th and 5th stage, in the cuts directed to hydrogenation processes (pyrolysis or cracking gasoline), in the mixtures directed to temporary storage and in the feed and/or in the overhead reflux of the rectification and purification processes of the olefinic compounds.

The use of the mixture according to the invention enables the control of the undesirable polymerization reactions to be increased, as well as the nitroxide amounts required for the antifouling treatment (with the entailed cost reduction) to be decreased.

According to the Applicant, the synergistic effect of the mixture of the invention is substantially carried out in two concomitant stages : 1-decreasing the polymer amount formed in laboratory tests (nitroxide amounts being equal), therefore stopping the chain reaction. with a greater capability.

2-reducing the capabilities as polymerization reaction initiators, exhibited by nitroxides in

laboratory tests, which in certain plant situations might prove extremely hazardous for a possible induction to polymer formation.

The present invention will hereinafter be illustrated by the following examples, which should not be construed as limitative of the object thereof.

EXAMPLE 1 Fouling tendency tests by means of a Alcor HPLS-310 fouling simulator The instrument used to carry out these tests is illustrated in the sole figure 1 of the accompanying drawinq.

The instrument represents one of the most technically sophisticated modules for reproducing fouling situations in any one plant : by controlling the heating of the liquid in the circuit, as well as measuring the AP both at the inlet and at the outlet of the cell provided for the purpose, full information may be obtained, for assessing the tendency of-a certain fluid to deposit organic fouling.

In this case the instrument was used in the AP configuration, using the cell module provided for the purpose and automatically recording over time the pressure exerted onto a filter for collecting the insoluble substances formed upon heating of the processed fluid.

With reference to Figure 1, the operative temperature or skin temperature was set at 120°C. This represents the temperature at which the filament transferring heat to the exchanger 1 is electrically heated. The tested fluid F is run onto the outer surface of the exchanger. The two lines 2, 3 at the top of the exchanger are connected to a transducer which detects the pressure difference Ap between the circuit section upstream of the strain 4 and the downstream one.

The internal pressure of the apparatus, maintained with a nitrogen pressure controlling device, is of about

21 Atm.

A fluid exactly corresponding to that present at the bottom of a depropanizing column (this is the column section wherein the conditions required to form insoluble polymers are most likely to occur, usually under 10-12 atm) would be, under the instrument conditions, in a near-gaseous state.

In view of the difficulties of the instrument to operate with gases, it was selected for testing a cut deriving from an industrial plant for the production of olefins, rich enough in C4 and C5 to be assimilated to the cut circulating at the bottom of the abovementioned column, capable however of remaining mostly fluid under the test conditions.

The gas-chromatography analysis of the cut employed is reported in the following Table 1.

TABLE 1 Cut analysis for-Alcor Test Denomination % by weight Cl-C2 0 C3 0 C4 3, 155 C5 89, 467 MeCPD 0 C6 5, 901 C7-C8 0, 014 DCPD 1, 458 The cut density was set at 0, 65 gr/cm3 : moreover, the mixture employed was further enriched in C4 by scrubbing gaseous butadiene (previously TBC-freed by soda washing) into the cooled liquid (-20°C).

The operative conditions of the instrument were set as follows : Skin temperature : 120°C-Recycling test with 300 cc fluid Prior to testing the reservoir and the sample were

maintained at-15°C Pump capacity : 300 ml/min-Aluminium stirring rod Nitrogen pressure : 300 psi-Testing time : 8 hours Detected APs of the blank sample, diagrammed over time, and of samples to which were added various amounts of polymerization inhibitors, and specifically a phenolic additive, a PDA mixture, a product based on PDA combined with N, N' diethyl-hydroxylamine, are reported in Table 2.

TABLE 2 Time Blank Phenolic PDA PDA (minutes) AP mmHg 20 ppm lOppm hydroxyl AP mmHg AP mmHg amine 2+2 ppm AP mmHg 0 0 0 0 0 10 17, 2 3 2, 8 0, 1 20 39, 6 4 4, 3 0, 1 30 51, 1 6 6, 2 0, 1 40 57, 6 12 7, 4 0, 2 50 58, 7 18 9, 2 0, 2 60 60, 2 30 9, 9 0, 2 70 65, 8 36 16, 4 0, 4 80 67, 4 37 17, 9 0, 3 90 81, 1 39 19, 1 0, 6 100 90, 5 41 20, 1 0, 8 110 99, 6 44 21, 5 0, 9 120 99, 2 48 23, 9 1, 4 130 104, 3 52 23, 2 2 140 106, 1 56 24, 2 5 150 108, 8 59 27, 1 2, 6 160 115, 6 64 26, 4 3 170 116 71 27, 1 3, 4 180 125 85 27, 1 4, 3 190 88 26 4, 6 200 97 27, 2 5 210 106 28, 9 5, 5 220 112 32 6 230 129 35 7 240 37, 9 7, 4 250 44 7, 5 260 50 9 270 57 9, 7 280 64 10, 9

TABLE 2 (cont. d) Time Blank Phenolic PDA PDA (minutes) AP mmHg 20 ppm lOppm hydroxyl AP mmHg AP mmHg amine 2+2 ppm AP mmHg 290 71 13 300 83 310 88 320 92 330 101 340 115 350 128 The APs detected with relation to the use of some of the test nitroxides, 4H-Tempo (200 ppb), Dinitroxide (200 ppb) as well as a mixture of PDA and Dinitroxide (7, 5 : 1) and 4H-Tempo (400 ppb) are reported in Table 3.

TABLE 3 Time Blank 4H-TEMPO Dinitr PDA + 4HTEMPO (min) AP mmHg 200 ppb oxide Dinitrox 400 ppb AP mmHg 200 ppb (7, 5 : 1) AP mmHg AP mmHg 2, 4 ppm+ 320 ppb AP mmHg 0 0 0 0 0 0 10 17, 2 0, 4 0, 4 0, 7 0, 4 20 39, 6 1, 7 3, 2 0, 6 0, 6 30 51, 1 3, 9 6, 2 0, 7 0, 8 40 57, 8 4, 6 7, 3 0, 6 1, 3 50 58, 7 4, 4 7, 3 0, 7 1, 9 60 60, 2 4, 1 7, 7 0, 7 2, 1 70 65, 8 3, 7 7, 7 0, 8 2, 6 80 67, 4 3, 3 7, 6 1 2, 5 90 81, 1 3 7 1, 1 3 100 90, 5 3, 4 6, 5 1, 2 3 110 99, 6 3, 5 6 0, 9 3, 3 120 99, 2 3, 1 6 0, 7 3, 3 130 104, 3 3, 4 5, 5 0, 7 3, 4 140 106, 1 3, 6 5, 8 0, 8 3, 3 150 108, 8 3, 7 5, 1 0, 8 3, 2 160 115, 6 3, 4 5, 1 0, 8 3, 6 170 116 3, 7 5, 2 0, 8 3, 4 180 125 3, 8 5, 5 0, 8 2, 1 190 3, 9 5, 6 0, 8 2, 2

TABLE 3 (cont. d) Time Blank 4H-TEMPO Dinitr PDA + 4HTEMPO (min) AP mmHg 200 ppb oxide Dinitrox 400 ppb AP mmHg 200 ppb (7, 5 : 1) AP mmHg AP mmHg 2, 4 ppm+ 320 ppb AP mmHg 200 3, 7 5, 7 0, 8 3, 1 210 3, 6 6, 3 0, 9 3 220 3, 9 6, 3 0, 9 3, 1 230 3, 3 6, 4 1 2, 6 240 3, 7 6, 6 1 2, 3 250 4, 1 6, 9 0, 9 2, 9 260 4 1 3 270 3, 9 1, 1 2, 8 280 3, 7 1, 1 2, 8 290 3, 6 1, 1 2, 4 300 4 1, 3 2, 7 310 3, 9 1, 3 2, 6 320 3, 6 1, 4 2, 9 330 3, 4 1, 5 2, 3 340 4 1, 3 3, 1 350 4, 2 1, 5 3, 1 360 3, 6 1, 7 2, 9 370 3, 9 1, 6 2, 9 380 4 1, 9 2, 8 390 4 2 3 400 4, 4 2, 2 2, 9 410 4, 3 2, 4 2, 9 420 4, 5 2, 4 3 430 4, 4 2, 6 2, 7 440 4, 7 3 2, 9 450 4, 4 3, 3 2, 7 460 4, 8 4 2, 6 470 4, 7 4, 3 2, 7 480 4, 9 4, 6 2, 5 Two series of measurings were carried out on different cuts sampled within an interval of several weeks : in the second series (Table 4) the blank exhibits a slightly less fouling tendency.

In the first series of measurings (Table 2 and Table 3) an initial AP increase, chiefly with the dinitroxide but also with the 4H-Tempo, is clearly evident. However,

after 200-300 minutes the inhibiting effect on polimerization attained with the nitroxides becomes apparent, whereas the blank and the PDA/Hydroxylamine tests markedly increase the steeping of the resulting curve. Only the tests carried out with the addition of nitroxides reach the 480 minutes with no AP steepenings : moreover, it is evident how the initial increase be completely nullified by adding the aromatic amine together with the nitroxide, attaining, during the 480 minutes, the utmost AP containment.

Concentrations employed in the lst test series (Table 2 and Table 3) : Phenolic : 20 ppm Aromatic amine : 10 ppm PDA + hydroxyl amine : 2 ppm + 2 ppm 4H-TEMPO x 1 : 200 ppb 4H-TEMPO x 2 : 400 ppb Dinitroxide : 200 ppb PDA + Dinitroxide (7. 5 : 1) : 2, 4 ppm + 320 ppb The second test series, (Table 4) carried out with a second sampling of the same cut confirms the preceding inclinations, with the utmost efficiency, both as the initial sinuosity and after 480 min, of the dinitroxide- PDA mixture.

Concentrations employed in the 2nd test series : 4H-TEMPO : 200 ppb Dinitroxide : 150 ppb PDA + Dinitroxide (5 : 2) : 1 ppm + 400 ppb PDA + Dinitroxide (7. 5 : 1) : 1, 2 ppm + 160 ppb

TABLE 4 Time Blank 4H-TEMPO Dinitr PDA + PDA + (min) AP mmHg 200 ppb oxide Dinitrox Dinitr AP mmHg 150 ppb (7, 5 : 1) oxide AP mmHg 1 ppm+ 1, 2 ppm 400 ppb 160 ppb AP mmHg AP mmHg 0 1 0, 4 0, 3 0, 4 0, 3 10 3, 8 0, 6 0, 5 0, 4 0, 4 20 8,2 1,2 1,2 0,4 0, 4 30 11,5 1,9 2,5 0,5 0, 4 40 13 2, 4 4, 2 0, 5 0, 5 50 14, 7 2, 6 6, 4 0, 7 0, 6 60 14, 7 2, 9 7, 4 0, 9 0, 6 70 14,8 3 7,7 1,1 0, 7 80 15, 2 3, 3 7, 7 1, 3 0, 8 90 15, 5 3, 5 7, 9 1, 8 0, 8 100 15, 7 3, 3 8, 4 2, 2 0, 9 110 14, 8 3, 4 7, 8 2, 8 0, 9 120 14, 8 3, 6 8, 1 3, 3 1, 3 130 15,3 3,5 8,6 2,7 1, 6 140 16,3 3,6 8,9 3 1, 1 150 17, 5 3, 5 8, 6 3 1, 1 160 17, 1 3, 5 8, 4 3, 3 1, 2 170 17, 5 3, 6 8, 4 3, 6 1, 2 180 18,3 3,8 8,9 3,9 1, 1 190 18,1 3,7 8,6 3,6 1, 2 200 18, 7 3, 7 9, 7 4, 3 1, 2 210 19 3,7 101,1 3,5 1, 4 220 19,3 3,7 9,8 4,1 1, 3 230 19, 3 4 10 3, 9 1, 3 240 19 3, 7 10, 5 4, 2 1, 1 250 19,2 3,7 10,3 3,8 1, 5 260 19,4 3,9 10,2 4,2 1, 5 270 19, 5 3, 7 10, 4 4, 3 1, 3 280 20,6 4,1 10,9 4,2 1, 5 290 21,3 3,8 10,7 4,3 1, 6 300 21, 3 4 11, 4 4, 3 1, 5 310 21,6 4,1 11,4 4,3 1, 4 320 21, 2 4, 2 11, 6 4, 3 1, 5 330 21 4, 1 10, 8 4, 4 1, 5 340 22, 3 4, 1 11, 5 4, 4 1, 5 350 23,2 3,9 12,1 4,6 1, 6 360 22,8 4,3 11,5 4,6 1, 6 370 22 4, 2 12 4, 5 1, 5 380 22, 6 4, 3 11, 8 4, 6 1, 8 390 23,3 1, 9

TABLE 4 (cont. d) Time Blank 4H-TEMPO Dinitr PDA + PDA + (min) AP mmHg 200 ppb oxide Dinitrox Dinitr AP mmHg 150 ppb (7, 5 : 1) oxide AP mmHg 1 ppm+ 1, 2 ppm + 400 ppb 160 ppb AP mmHg AP mmHg 400 24 1, 7 410 24, 6 1, 9 420 25, 2 1, 8 430 25, 2 2 440 25 2, 1 450 24, 4 2 460 24, 7 1, 9 470 26 2, 1 480 25, 2 1, 8 490 26, 1 2, 1 500 29 2, 2 The initial sinuosity of the AP curves obtained following the addition with nitroxides and dinitroxides suggests, in the initial 20-40 minutes of the test, a- possible pro-initiatory mechanism of the radicalic reaction by those substances, which is compensated by a radical-stopper action, typical of free radicals, only after 60-80 min.

This phenomenon was observed in all the tests carried out in presence of nitroxides, resulting particularly marked in the tests with a low concentration of the inhibitor, whereas it becomes less evident increasing the amount of free radicals added (Table 5).

The pro-initiatory effect is instead totally absent in the tests carried out with nitroxides to which an aromatic amine was added.

TABLE 5 Time 4H-TEMPO 4H-TEMPO (min) 200 ppb 400 ppb AP mmHg AP mmHg 0 0 0 10 0, 4 0, 4 20 1, 7 0, 6

TABLE 5 (cont. d) Time 4H-TEMPO 4H-TEMPO (min) 200 ppb 400 ppb AP mmHg AP mmHg 30 3, 9 0, 8 40 4, 6 1, 3 50 4,4 1, 9 60 4, 1 2, 1 70 3,7 2,6 80 3, 3 2, 5 90 3 3 100 3, 4 3 110 3, 5 3, 3 120 3, 1 3, 3 130 3, 4 3, 4 140 3, 6 3, 3 150 3, 7 3, 2 160 3, 4 3, 6 170 3, 7 3, 4 180 3, 8 2, 1 190 3, 9 2, 2 200 3, 7 3, 1 210 3, 6 3 220 3, 9 3, 1 230 3, 3 8 240-3, 7 2, 3 250 4, 1 2, 9 260 4 3 270 3, 9 2, 8 280 3, 7 2, 8 290 3, 6 2, 4 300 4 2, 7 310 3, 9 2, 6 320 3, 6 2, 9 330 3, 4 2, 3 340 4 3, 1 350 4, 2 3, 1 360 3, 6 2, 9 370 3, 9 2, 9 380 4 2, 8 390 4 3 400 4, 4 2, 9 410 4, 3 2, 9 420 4, 5 3 430 4, 4 2, 7 440 4, 7 2, 9 450 4, 4 2, 7 460 4, 8 2, 6

TABLE 5 (cont. d) Time 4H-TEMPO 4H-TEMPO (min) 200 ppb 400 ppb AP mmHg AP mmHg 470 4, 7 2, 7 480 4, 9 2, 5 Finally, it is evident that the highlighted synergistic effect, besides leading to better results in the AP after 480 minutes with respect to the individual substances, prevents an initial AP increase, thus mitigating a phenomenon which might occur in the plant, with a serious danger of polymeric fouling.

EXAMPLE 2 Isoprene polymerization in autoclave.

An amount of isoprene, pure or in a suitable extractant, was injected in a sealed container, capable of withstanding elevated pressures, heated at high temperatures for an appropriate time period. Then the remaining solution was evaporated under nitrogen stream, in order to determine the weight of the polymers formed.

This procedure was repeated without adding additives, as well as with the addition of variable concentrations of nitroxides and aromatic amines.

The isoprene employed is the commercially available one, and, prior of being fed into the autoclave, it was washed in a separatory funnel with repeated soda and water washings, then dehydrated with anhydrous sodium sulfate.

Table 6 reports the weight of the polymer obtained by heating at 90°C a fixed amount of pure isoprene, under the initial pressure of 2 atm obtained with a nitrogen pressure-controlling device.

The actual pressure, after about 1 hour, was settling at about 7 atm.

The test duration was set at 24 hours with constant stirring of 200 rpm : then the residual isoprene left over was evaporated at 100°C, under nitrogen stream.

TABLE 6 Isoprene polymerization in autoclave (first series) Additive Amount Polyisoprene (ppm) mg/100 g Blank 552, 5 Dinitroxide 100 178, 8 4-H-TEMPO 100 156, 86 4-H-TEMPO 50 201, 33 PDA 100 241, 34 4HT.-29 ppm + PDA-71 ppm 197, 07 Table 6 highlights that the result obtained with the combination nitroxide + PDA is improved (taking into account the concentration) with respect to the individual additives.

However, the effect is not completely expressed, probably because under the test conditions the isoprene is almost completely found in gaseous form. In fact, the lack in homogeneity of the system (the additives are in liquid phase) causes a reduced effectiveness of all the products. Operationally acceptable results with the products can be attained only with elevated addition doses thereof.

Table 7 reports other results, always obtained with isoprene, mixed in this case, at the beginning of the test, to an equal amount of isooctane. The solvent allows more isoprene to remain in a liquid state, whereby the effect of the additives, which of course remain dissolved in the liquid phase, becomes more evident. Furthermore, the test temperature was brought to 100°C.

In this case, the mixture of isoprene-solvent-anti- polymerization agents forms a more homogeneous phase, hence the results of polymerization inhibition are markedly improved with respect to the first test series,

notwithstanding a markedly lower concentration of the additive.

TABLE 7 Isoprene polymerization in autoclave (second series) Additive Amount Polyisoprene (ppm) mg/100 g Blank 943, 4 PDA 50 438, 76 4H-TEMPO 50 140, 4 PDA 20 491, 2 4H-TEMPO 20 304, 54 4H-T. 25 ppm + PDA 25 ppm 100, 67 In this case as well the best performance was exhibited by the combination nitroxide-PDA, which, at 25 ppm per each substance, shows a higher decrease with respect to the 50 ppm of 4-Hidroxy Tempo.