Process for the production of monomer vinyl chloride by cracking of dichloroethane and system suitable to carry out the process
This is a divisional, of application Ser. No. 897,107 filed Aug. 15, 1986 now U.S. Pat. No. 4,721,604. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process for the production of monomer vinyl chloride by means of the cracking of dichloroethane, and to a system suitable to carry out the process. 
     BACKGROUND OF THE INVENTION 
     According to the technology of the prior art for the production of monomer vinyl chloride by the cracking of dichloroethane, this latter is pre-heated, vapourized, overheated and cracked inside one single piece of equipment heated by combustion (oven) and heat exchange by irradiation and convection from the combustion products towards the stream of dichloroethane. According to this technology, the enthalpy contained in the reaction products at the outlet of the oven, at the typical temperatures of from 450° to 550° C., is no longer used in the plant; the stream of outflowing products is quickly cooled to decrease the reaction rate of undesired reactions, but the enthalpy is not recovered for purposes useful to the system. Furthermore, according to the technology of the prior art, the stream of dichloroethane is vapourized inside the same oven, while being in contact with the combustion products at very high temperature, causing the production of dichloroethane breakdown products, among which coke and coal, which act as precursors of formation of further coke and coal in the irradiation section, which in its turn is quickly fouled up to render the oven no longer operable if not to prejudice of its integrity; the equipment must be hence shut off and submitted to the decoking operation to clean the inner surface of the tubes, with a frequency of about 3-5 times a year. 
     A variant of the known technology consists in vapourizing the charge in a heat exchanger external to the oven, by using steam, the consumption of which becomes however a considerable expenditure item among the plant consumptions, by it being always produced at expense of valuable fuels. 
     It has been surprisingly found that reducing the consumptions of fuel is possible, by using the enthalpy of the efficent from the oven to the purpose of both vapourizing the charge and overheating the combustion air, and that increasing is possible the operating time between two subsequent over decoking operations, by very quickly cooling the efficent from the oven and vapourizing the charge by a high-temperature source at a temperature lower than of the fumes of the known art. 
     A first object of the present invention is a process for the production of monomer vinyl chloride by dichloroethane cracking, comprising feeding a stream of dichloroethane to the convective section of an oven, so to heat it up to a temperature lower than its boiling temperature under the pressure used, vapourizing the dichloroethane stream outflowing from the convective section of the oven in an area external to said oven, by indirect heat exchange with a hot gaseous fluid, selected from the oven combustion products nitrogen and/or air, either alone or as mixtures with one another, heated by indirect heat exchange with the hot dichloroethane breakdown (cracking) products outflowing from said oven, drawing the stream of vapourized dichloroethane from said external area, and introducing it into the radiant section of said oven, the heat in the radiant section being supplied by the combustion of an either liquid or gaseous fuel with a comburent in particular constituted by a portion of the air heated by indirect heat exchange with the hot dichloroethane cracking products outflowing from said oven, if said air has been used as the high-temperature gaseous fluid for indirect heat exchange. 
     A second object of the present invention is an equipment system suitable to carry out the above disclosed process. 
     SUMMARY OF THE INVENTION 
     The system being the object the present invention comprises an oven with a radiant section and a convective section, equipped with heaters for either liquid or gaseous fuels, preferably installed in correspondence of the radiant section, the over being provided in its interior with a first set of tubes in its convective section and with a second set of tubes in its radiant section, the set of tubes in the convective section being connected to pipes for the feed of fresh dichloroethane, and respectively the discharge of dichloroethane heated at a temperature lower than its boiling point, and the set of tubes of the radiant section being connected to pipes for the charging of vapourized dichloroethane, and for the discharging of the dichloroethane cracking products (among which, monomer vinyl chloride), and is characterized in that the tubes for the discharge of heated dichloroethane from the convective section lead to a first heat exchange device for vapourizing the dichloroethane, and that the tubes for the discharge of the dichloroethane cracking products lead to a second heat exchange device, which supplies the heat of the cracking product to a fluid, said fluid supplying in its turn heat to said first heat exchange device through a duct which places in said first and said second heat exchange devices in communication with each other, said duct being equipped with a burner for the initial heating of the fluid (in the absence of the cracking products) and for the possible decoking, and with a fluid circulator, the duct being furthermore provided with a pipe for the possible discharging of the fluid, and connected to a pipe for the delivery of the fluid, if air, as the comburent to the burners of the radiant section of the oven. 
     In a variant of the system of the present invention, the pipes supplying the heated dichloroethane to the first heat exchange device lead first to a separator, by which the dichloroethane vapour is separated from the liquid dichloroethane, which is continuously recycled by a pump through the said first head exchange equipment and through said separator, the separator allowing also the possibly coked products to be separated. 
     The invention is now described with reference to the attached drawings, which are not to be intended as being limitative of the same invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of one embodiment of the apparatus of the invention. 
     FIG. 2, is a schematic view of another embodiment of the apparatus of the invention showing recycling of the dichloroethane stream through a separator means. 
     FIG. 3 is a schematic view of the heat exchangers employed in the apparatus of FIGS. 1 and 2. 
     FIGS. 3A, 3B and 3C are cross-sectional views of a section of pipe employed in the heat exchangers shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 the oven (1) is composed by a convective section (2) wherein the liquid charge (3) of dichloroethane is preheated; this stream is preheated up to a temperature lower than its boiling point under the pressure corresponding to the outlet from the convective section, and hence remains in the liquid phase. The preheating takes place by means of the sensible heat of the products of combustion, which is carried out in the radiant section (4) of the oven to supply to heat for the overheating and the reaction of cracking of dichloroethane (5), which is recycled to the radiant section in the vapour phase. The products of reaction (6) leave the radiant zone at a temperature generally comprised within the range of from 450° to 550° C. The preheating, the overheating and the reaction occur by heat transfer through tubes (7) and (8) suitably positioned in the radiant section and in the convective section. The burners (9) can be installed either on the side walls, or on the crown and/or in the bedplate of the oven. 
     The sensible heat of the products of reaction (6) is drawn in the refrigerator (10) and is transferred to the evaporator (11), wherein the vapourization of the charge previously preheated in the convective section (2) (8) of the oven is made take place. The fluid used to transfer heat from (10) to (11) is air, drawn from the atmosphere, or nitrogen, drawn from the plant system and circulated in system (12) by the fan (13). 
     By the dotted line in FIGS. 1 and 2 indicated has been the possible feed of the combustion products of the oven to the system (12). 
     The exchangers (10) and (11) are similar in structure to each other, and are constituted by the following components, as illustrated at FIG. 3: 
     Tube bundle constituted by finned tubes (1) with either cut or continuous fins having height of up to 50 mm, a thickness comprised within the range of from 0.4 to 1.2 mm, and a density of up to 450 fins per meter, preferably of from 240 to 320 fins per meter; the tubes with nominal diameter of up to 10 inches are preferably connected to each other so to maintain the same number of passages on process side as inside the oven; the connection bends can be welded to the tubes, or they can be connected to the tubes by a flanged connection, which allows the access to the interior of the tubes, to carry out the mechanical cleaning, as indicated in FIG. 3, find number (2). 
     In FIG. 3A, details of types of fins are shown. 
     The tube-containing outer housing is tightly sealed relatively to the inner and to the outer pressure, and can be internally insulated by insulating and refractory materials, or it can be externally insulated by insulating materials. The inner coating is provided for the case in which the cleaning of the inner surfaces of the tube nest is carried out by the operation of decoking by air and steam, in which case the temperature of air surrounding the tube nest in increased up to the values typical for the execution of decoking operation, by means of the burner (14) installed in the fan (13) delivery duct. 
     The burner (14) is preferably of the multiflame gas burner type tubed inside the delivery duct (15) of fan (13), and supplies the heat for the decoking operation and for contributing to the starting up of the system. 
     The exchangers (10) and (11) are provided with a perforated distribution plate, air side, for the uniform distribution of the same air throughout the passage area. 
     The circulating air, which is heated in the exchanging (10) up to typical values of 200°-500° C., releases its heat in exchanger (11) wherein, on tubes side, the vapourization of the dichloroethane charge occurs under less severe temperature conditions than it would find in the convective section of oven (2) and with higher exchange coefficient. 
     This exchanger allows hence a reduction to be achieved in fouling due to the breakdown and production of precursors of coke formation and allows consequently a longer operating time to be obtained between two subsequent cleaning operations. 
     The air circulation duct (12) is provided with a bypass stack (16) to be used during the decoking operations, installed downstream (10) or (11), as required by the cleaning needs. 
     From the same air circulation duct (12), a line (17) is branched, which conveys the preheated combustion air to the oven burners (9), which can be either of natural draught or of forced draught type, and can hence be fed with air under a pressure ranging from atmospheric pressure to some hundred water millimeters, typically 150-250 mm H 2  O gauge pressure. The combustion air is typically at the temperature of 200°-500° C., relatively to the amount circulated, to obtain the closure of the heat balance of exchangers (10) and (11). 
     In FIG. 2 the stream of dichloroethane, heated in (2) in the tube (8), is fed to a separator (18), vapour being obtained which is recycled to the oven through (5), as well as liquid which, through the pipe (19) is fed to pump (20), is then circulated via (21) through the vapourizer (110 and from here is passed through the separator (18), from which the coking products (22) are discharged.