PROCESS FOR RECOVERING SULPHURATED STYRENE RESIDUES

Abstract: Described is a process for recovering sulphurated styrene residues obtained in the purification of styrene produced by catalytic dehydrogenation of ethylbenzene, which comprises mixing said residues with a gasoil and subjecting the resulting mixture to cracking, fractionated distillation and hydrodesulphurization of the heavy fractions having a boiling point of at least 140 DEG C.

Description: Process for recovering sulphurated styrene residues The present invention relates to a process for recovering sulphurated styrene residues. Particularly, the present invention relates to a process for recovering the sulphurated styrene residues obtained in the purification of styrene produced via catalytic de¬ hydrogenation of ethylbenzene. As is known, styrene obtained by catalytic dehydrogenation of ethylbenzene at high temperatures can be recovered from the crude reaction liquid through fractionated distillation using a set of fractionation columns. Since styrene tends to polymerize at the relatively high distillation temperatures required, it is well known that during this step a certain conversion of the monomer into polymeric materials takes place, resulting in a loss of desired product. To overcome this drawback, a styrene polymerization inhibi¬ tor is generally employed. A very effective and preferably utilized inhibitor is sulphur. In fact this element, as compared with organic compounds such as dinitrophenols, mono- and dinitrophenols containing alkyl substituents in the aromatic nucleus, nitrous phenols, etc., offers consider¬ able technological, process and economic advantages. It is known, in fact, that the above organic compounds, be¬ sides being more expensive, have a high toxicity, can give rise to corrosion of the apparatus due to their acidic resi- due content and can be explosive in the anhydrous state. However, the use of sulphur as styrene polymerization inhi¬ bitor results in the presence of this element in the resi¬ dual material of the final distillation column. Therefore, the residual material obtained from said distillation column contains: - low-boiling hydrocarbons having a boiling point lower than 200°C such as styrene, cumene, alpha-methyl styrene, methyl-ethyl-benzenes, methyl-vinyl-benzenes, butyl-ben¬ zenes, etc. ; - high-boiling hydrocarbons having a boiling point higher than 200°C, generated in the dehydrogenation section in the form of polynuclear aromatic compounds: - polymeric materials such as polystyrene and sulphurated polystyrene; and - sulphur. The sulphur is generally present in a total amount ranging from about 5 to about 30%, preferably from about 10 to about 20% by weight calculated on the total residues. These residues have a very low commercial value and the prior art evidences that their elimination (disposal) causes serious problems. In fact, the combustion of these residual materials involves ecological and corrosion problems, particularly owing to the emission of SO_ . Thus, it is an object of the present invention to provide a process for recovering the residual styrene distillation materials containing sulphur as polymerization inhibitor, which does not show the above drawbacks. It has now been found that this object is achieved by ad- mixing said residual sulphurous materials with a gasoil, subjecting the resulting mixture to a thermal cracking pro¬ cess at a temperature of at least (and preferably higher than) 400°C and subsequent fractionated distillation and hydrodesulphurization of the intermediate fractions having a boiling point ranging from about 140 to about 390°C. Thus, it is an object of the present invention to provide a process for recovering the sulphurous residues obtained from the purification of styrene (obtained by catalytic dehydrogenation of ethylbenzene), which comprises the steps of: a) adding said sulphurous residues to a gasoil; b) subjecting the resulting mixture to a thermal cracking process at a temperature of at least 400°C; c) carrying out a fractionated distillation of the mixture so treated; d) subjecting the distillate fraction having a boiling tem¬ perature ranging from about 140°C to about 390°C to a catalytic hydrodesulphurization process; and e) recovering elemental sulphur from the sulphurated hydro- gen so obtained. The sulphur so obtained can be either re-used as styrene polymerization inhibitor or utilized in its conventional fields of use. Those skilled in the art will appreciate the importance of the process according to the present invention, which permits to recover the sulphur-containing residues of sty¬ rene without causing atmospheric pollution or other problems which are of considerable technological importance in view of the great amount of styrene produced. In fact, styrene is widely used as monomer for producing resins, plastics, elastomers, synthetic rubbers and the like. The process of the present invention is preferably used for recovering the sulphurous residues coming from the puri¬ fication of styrene which has been obtained through the conventional catalytic dehydrogenation of ethylbenzene; however, its application is not limited to said specific styrene source. In other words, the process of the present invention is applicable to any styrene-containing feedstock which is polluted by various high-boiling polymeric and non-polymeric materials and which, in particular, contains sulphur as polymerization inhibitor. The gasoil utilized in the process of the present invention typically is one produced by means of processes for the vacuum-distillation of oil products and generally has a boiling point ranging from about 390 to about 550 C C. The amount of sulphurous residues obtained from the styrene purification which can be added to the gasoil is not criti¬ cal; generally, amounts exceeding 0.1% by weight, with re¬ spect to the gasoil, can be advantageously utilized; amounts ranging from 0.5 to 10% by weight are preferred. According to the process of the present invention, the mix¬ ture composed of gasoil and residual sulphurous materials is subjected to a thermal cracking process, which preferably comprises heating the mixture to a temperature of about 465 to about 500°C at a pressure of about 10 to about 50 bar in a furnace equipped with an inner heating coil. The residence time of the mixture in the furnace usually varies from about 1 to about 15 minutes. The cracking reaction is preferably completed in a subsequent soaker, where said mixture is kept for about 10 to about 30 minutes. At the soaker outlet the mixture is preferably cooled to a temperature of about 380 to about 400°C and then is fed to a conventional fractionation column, wherein the differ¬ ent product fractions are separated. From the column head separator, a stream of light gases, generally comprising hydrogen, ethane, propane and butane, along with sulphurated hydrogen, is obtained; these light gases can be compressed and conveyed to known aminic scrub- ing systems for the recovery of sulphurated hydrogen and then to the sulphur recovery system. The fractions which boil in the gasoline range(about 70 to 140°C) can be either washed and admixed with other com¬ ponents in order to obtain gasolines or can be sent to the catalytic hydrodesulphurization units where, in the presence of hydrogen and a catalyst based on Co-Mo or Ni-Mo and at a temperature of about 250 to about 350°C and a pressure of about 20 to about 80 bar, they are first freed from foreign matters and then sent to octane conversion units ( isomerization and reforming). The intermediate distillates having a boiling point in the range of from about 140 to about 390°C and comprising kero¬ sene (boiling point 140 to 240 C C) and gasoil (boiling point 240 to 390°C) are subjected to catalytic hydrodesulphuri- zation. The catalytic hydrodesulphurization process is well-known and comprises admixing the intermediate distillates with hydrogen and then heating the resulting mixture to a tem- perature of about 350 to about 420°C at a pressure of about 20 to about 80 bar in the presence of a catalyst, preferably based on Co-Mo, for about 1 to about 60 minutes. After the heating treatment, the unreacted hydrogen and the sulphurated hydrogen formed are separated from the mixture. The obtained residual product is utilized in the mixtures usually sold for being burnt in diesel engines or for heating purposes. The sulphurated hydrogen is preferably recovered by means of aminic washings and converted into elemental sulphur y means of conventional techniques such as, e.g., the Claus process. The residual portion which has not been converted in the thermal cracking process, discharged from the fractionation column bottom and having a boiling point higher than about 390°C, may be subjected to a stripping treatment and utilized as heavy fuel, according to conventional techniques. The addition of the residual sulphurated styrenic materials to the gasoil to be subjected to the thermal cracking process results in various surprising and unexpectable advantages. In fact, it has been found that the pyrolysis reactions are enhanced during the cracking step with formation of useful products. Therefore, by adding styrene residues and by operating under otherwise identical temperature and pres¬ sure conditions it is possible to obtain an enhancement of the pyrolysis reaction with a consequent higher yield of useful fractions or, the yield of useful fractions being the same, it is possible to carry out the cracking at lower temperatures (4 T of about 14 to 15 C C), thereby reducing the fouling of the cracking furnace heating coil by 60%, with a consequent increase in the number of operation days without interruptions for cleaning the coil. Another advantage resulting from the use of the sulphurous styrene residues in the gasoil cracking process is the con- version of the high-boiling products contained in said resi¬ dues into oil fractions having a higher added value. Furthermore, the process of the present invention permits to suppress the emission of SO_ connected with the combustion of the styrene residues derived from the styrene production processes, resulting in obvious ecological advantages, and to considerably reduce or even eliminate the consumption of sulphur utilized for the styrene inhibition. To permit a better understanding of the present invention and to reduce the same to practice, the following examples are given hereinafter for illustrative and exemplifying purposes; however, they are not to be construed as a limi¬ tation of the invention. In said examples, reference is made to the attached figure 1 which shows a schematic view of a possible embodiment of the process of the present invention. Example 1 Through a feeding pipeline (1) there were fed 1,200 t/day of gasoil from vacuum distillation, having a boiling range of from 390°C to 522°C, a sulphur content of 2.3% by weight and a density (15/4°) of 0.928 kg/1. To the above charge there were added, through line (2), 3% by weight of styrene residues derived from the production of styrene via catalytic dehydrogenation of ethylbenzene. The styrene residues had the following composition: - hydrocarbons having a boiling point below 200°C 20% - hydrocarbons having a boiling point above 200°C 35% - polymeric products 33% - sulphur 12% The gasoil-styrene residues mixture was heated to about 300°C and fed to a furnace (3) equipped with a heating coil (4). In furnace (3) the mixture was heated to 495°C and kept there for about 7 minutes. The pressure at the outlet of coil (4) was maintained con¬ stant at 15 bar, while the pressure at the inlet, when the operation was started with a clean coil (4), was 25 bar. The mixture leaving furnace (3 ) was then conveyed to a soaker (5) operating at a pressure of about 15 bar, the residence time therein being about 15 minutes. Due to the endothermal cracking reaction the temperature decreased from 495°C at the inlet to about 440°C at the outlet. The mixture was then cooled to about 390°C and fed to a conventional fractionation column (6). The uncondensed light gas leaving the head (7) of the column ( 6) was compressed and sent to an aminic scrubbing unit to recover the sul¬ phurated hydrogen and to subsequently convert it into sulphur, following well-known and conventional processes. The liquid gasoline recovered in the upper portion (8) of column (6), was sent to the gasoline desulphurization unit and then subjected to the well-known isomerization and re¬ forming processes. Kerosene having a boiling point range of from about 14Q to about 240°C, leaving the upper middle portion (9) of column (6) , and gasoil having a boiling point ranging from about 240 to about 390°C, leaving the lower middle portion (10) of column (6), were mixed together and introduced into . a desulphurization reactor (13) after hydrogen (12) had been added thereto in an amount of about 0.3% by weight calculated on the kerosene/gasoil mixture. In the desulphuri¬ zation reactor (13), the kerosene/gasoil mixture was heated to 370°C at a pressure of 57 to 58 bar and in the presence of a Co-Mo catalyst. The sulphurated hydrogen-containing gases leaving reactor (13) through line (14) were subjected to an aminic scrubbing, and from said gases sulphurated hydrogen was recovered and re-converted into elemental sul¬ phur in a Claus plant. The desulphurized mixture leaving the reactor (13) through line (15) was stored. The residues at the bottom (11) of the fractionation column (6) were utilized as fuels. During the test, the pressure at the inlet of furnace (3) gradually rose until reaching a value of 38 bar after 61 days of operation. At day 61, the run was discontinued and the coil (4) was cleaned, following conventional modalities of decoking operat¬ ions. The obtained yields are reported in the following Table I. Example 2 (comparison test) Example 1 was repeated without addition of 3% of styrene residues. The obtained yields are reported in the following Table I. TABLE I EXAMPLE 1 EXAMPLE 2 YIELDS AT THE COLUMN OUTLET A) Gas % by weight 1.4 1.0 B) Gasol ine % by weight 4.1 3.1 C) Kerosene % by weight 6.5 5.0 D) Gasoil % by weight 23.1 14.9 E) Residues % by weight 64.9 76.0 DAYS OF OPERATION OF COIL (4) 61 85 TOTAL B+C+D PRODUCED DURING THE OPERATION (t) 25,400 23,460 From the above data it is apparent that under identical operating conditions the presence of styrene residues results in a considerable increase in the plant productivity. Example 3 Following the procedure of example 1, the gasoil-styrene residues mixture was treated in furnace (3) at 485°C and at the same inlet and outlet pressures. The obtained results are reported in the following Table II. Example 4 (comparison test) Example 3 was repeated without addition of 3% of styrene residues. The obtained results are reported in the following Table II. TABLE II EXAMPLE 3 EXAMPLE 4 YIELDS AT THE COLUMN OUTLET A) Gas % by weight 1.1 0.8 B) Gasoline % by weight 3.4 2.4 C) Kerosene % by weight 4.9 3.8 D) Gasoil % by weight 17.6 13.2 E) Residues % by weight 73 79.8 DAYS OF OPERATION OF COIL (4) 106 140 TOTAL B+C+D PRODUCED DURING THE OPERATION (t) 33,930 32,600

Claims: il . CLAIMS 1. Process for recovering sulphurous residues derived from the purificationof styrene, comprising the following con- secutive steps: a) adding said sulphurous residues to a gasoil; b) subjecting the resulting mixture to a thermal crack¬ ing process at a temperature of at least 400°C; c) carrying out a fractionated distillation of the mix- ture to treated; d) subjecting the distillate fraction having a boiling temperature ranging from about 140°C to about 390°C to a catalytic hydrodesulphurization process; and e) recovering the elemental sulphur from the sulphurated hydrogen so obtained. 2. Process according to claim 1, wherein the gasoil has been produced by vacuum distillation of oil products, having a boiling point ranging from about 390 to about 550°C. 3. Process according to claim 1, wherein the amount of sulphurous residues obtained from styrene purification and added to the gasoil is at least 0.1% by weight, calculated on the gasoil. 4. Process according to claim 3, wherein the amount of sulphurous residues is from 0.5 to 10% by weight. 5. Process according to claim 1, wherein the thermal crack¬ ing is carried out at a temperature of from about 465 to about 500°C and at a pressure of from about 10 to about 50 bar. 6. Process according to claim 5, wherein the thermal crack¬ ing is carried out for about 1 to about 15 minutes in a furnace equipped with a heating coil, and then is completed in a soaker within about 10 to about 30 minutes. 7. Process according to claim 1, wherein the mixture sub¬ jected to thermal cracking is cooled to a temperature of about 380 to about 400°C before being fed to a fract¬ ionation column. 8. Process according to claim 1, wherein in step d) hydro¬ gen is added to said distillate fraction which is then heated to a temperature of about 350 to about 420 C C at a pressure of about 20 to about 80 bar in the presence of a catalyst for about 1 to about 60 minutes. 9. Process according to claim 8, wherein the catalyst is based on Co-Mo. 10. Process according to claim 1, wherein sulphur is re¬ covered from the sulphurated hydrogen by means of the Claus process.

Inventors: BALLABIO CARLO, BORGHI ITALO, CARRETTONI GIOVANNI, DISPINSIERI FILIPPO, MASSEI MARIO, VESCOVI PAOLO, BALLABIO, CARLO, BORGHI, ITALO, CARRETTONI, GIOVANNI, DISPINSIERI, FILIPPO, MASSEI, MARIO, VESCOVI, PAOLO
Applicants: MONTEDIPE SRL, MONTEDIPE S.R.L.
Application year: 1991
Main classification: C08C7
IPC codes: C07C7, C07C15, C10G9, C10G69