Abstract:
Coke formation in pyrolysis furnaces is controlled by applying a coating of boron nitride to pyrolysis furnace process equipment surfaces, for instance, parts of the transfer line heat exchanger assembly.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to a method of inhibiting coke or carbon formation and allowing easier cleaning of metal surfaces of processing equipment during high temperature processing of hydrocarbons. More particularly, the invention relates to the coating of certain surfaces of a transfer line heat exchanger with a boron-nitride composition to reduce coke formation and allow easier cleaning of those surfaces. 
       BACKGROUND ART 
       [0002]    In traditional pyrolysis processing using pyrolysis furnaces, mixtures of hydrocarbons and steam flow through long coils or tubes which are heated by combustion gases to produce olefins, such as ethylene and propylene, as well as other valuable by-products. Heat is transferred from the hot combustion gases to the hydrocarbon feedstock passing within the coils. The hydrocarbon feedstock is heated within the coils to temperatures typically in the range of about 750° to 950° C. to form the product stream. 
         [0003]    After passing through the pyrolysis furnace, the product stream is typically cooled or “quenched” in a transfer line heat exchanger (TLE) to both stop the reaction, and to cool for processing and separation. A TLE is designed to recover sensible heat from the hot product stream leaving the pyrolysis furnace. Heat is transferred from the hot product mixture to low pressure steam in the TLE to form high-pressure steam. 
         [0004]    Coke formation is a traditional problem in the TLE as hydrocarbons are dehydrogenated, forming a solid residue on the metal and refractory surfaces of the hot product side of the TLE. Coke formation and collection in the TLE typically results in poorer heat transfer, which in turn results in decreased production of high-pressure steam. Coke formation in the TLE also often results in a larger pressure drop across the TLE. This problem is particularly acute in the inlet cones of the TLE. 
         [0005]    The typical operating cycle for a TLE is to operate for a period of time cooling the product stream from the pyrolysis furnace. During this operation coke forms in the inlet cone plugging up the tubes and tube sheet of the TLE and restricting flow. When the pressure drop becomes too high, the TLE will be hot cleaned (decoking cycle) using steam injected into the inlet cone to remove coke and open up the tubes of the TLE so more product gas can flow and there is less pressure drop. This is only partially effective and after two to four cycles of operation and hot cleaning, the TLE inlet cone must be removed so the coke can be removed through more aggressive methods. This process of removal of the coke on the TLE inlet cone is typically accomplished mechanically, usually entailing hammers and chisels, which also damages the inlet cone refractory and the tubesheet of the TLE. Then, the operation cycle is started over. Because of the damage done to the inlet cone refractory of the TLE by the mechanical removal of coke, the inlet cone refractory must be repaired or replaced at a significant cost. 
         [0006]    In addition to the shutdown and startup process of the pyrolysis furnace, the mechanical de-coking operation of the TLE itself frequently requires several days. De-coking therefore results in increased downtime relative to olefin production time, frequently amounting to a several percent loss of olefin production during the course of a year. De-coking is also relatively expensive and requires appreciable labor and energy. 
         [0007]    Previous methods have been used to control coke formation. For instance, coke inhibitors, i.e., chemical additives, or special coatings of metal surfaces which suppress coke formation have been used. Coke inhibitors/surface coating act to passivate catalytically active metal sites through chemical bonding interactions, and/or forming a thin layer to physically isolate metal sites from coke precursors in the process stream, and/or interfering with those free radical reactions leading to coke formation by blocking active sites on surfaces. Such additives are expensive and may lead to product gas stream quality issues. 
         [0008]    What is needed is a method of controlling the growth and formation of coke, particularly in the inlet cone and tube sheet of the TLE. What is further needed is a method to reduce the damage done to the TLE inlet cone and tube sheet during a de-coking operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a depiction of coke formation on a metal surface of the TLE. 
           [0010]      FIG. 1A  is a pictorial depiction of filamentous coke. 
           [0011]      FIG. 1B  is a pictorial depiction of filamentous and free radical coke. 
       
    
    
     SUMMARY 
       [0012]    The methods described herein relate generally to the field of reducing and controlling coke formation in the TLE during the olefin production process. 
         [0013]    In one embodiment of the present disclosure, a method of controlling coke formation in pyrolysis furnace process equipment is described, wherein the surface of the process equipment is coated with a layer of boron nitride. 
         [0014]    In another embodiment of the present disclosure, a method of controlling coke formation in a transfer line heat exchanger is described, wherein a boron nitride paint is formed by combining dry boron nitride powder with distilled water. The boron nitride paint is applied to a transfer line heat exchanger tube sheet. 
         [0015]    In still another embodiment of the present disclosure, a boron nitride paint for coating the surfaces of pyrolysis furnace process equipment is described which includes providing a dry boron nitride powder. The dry boron nitride powder contains boron nitride with a hexagonal crystalline structure. Distilled water is provided and the boron nitride powder is mixed with the distilled water to form the boron nitride paint. The boron nitride paint comprises between 20 and 45% boron nitride by weight. 
       DETAILED DESCRIPTION 
       [0016]    While not bound by theory, applicants have determined that coke is classified into two types: Catalytic coke and Pyrolytic coke. Catalytic coke is formed by dehydrogenation of hydrocarbon with catalytic action of metal components on the surface. Metal components, such as nickel and iron, may catalyze a hydrocarbon to reduce or eliminate hydrogen from the olefin, termed “dehydrogenation.” Metal components presenting catalytic activities are generally in the order of Ni&gt;&gt;&gt;&gt;Fe&gt;&gt;Cr, NiO&gt;Ni, FeO&gt;Fe&gt;Fe2O3. These metals and oxides catalyze reaction to form filament and coil type coke by successive dehydrogenation, as shown in  FIG. 1 . 
         [0017]    Catalytic coke tends to be very mechanically hard (“hard coke”) and normally difficult to remove. Hard coke must often be removed from surfaces by mechanical means. The formation of catalytic coke is believed to be most often involved in beginning the coking process of the TLE and is believed to act as trap for pyrolytic coke 
         [0018]    Pyrolytic coke is divided into gaseous and condensation coke. Pyrolytic coke is softer and generally easier to remove than catalytic coke. Gaseous coke is typically formed by dehydrogenation of such light olefinic hydrocarbon as acetylene. Condensation coke is formed by condensation, polymerization, and dehydrogenation of heavy aromatic compounds. Pyroltyic coke can be classified as globular, black mirror, fluffy or amorphous types according to morphology. 
         [0019]    It is believed that the coking process of the TLE begins with the hydrocarbon reacting by catalyst action of metal components on metal surfaces and forms filamentous coke, which grows and provides deposit sites for various types of coke. Free radical coking causes coke filaments to thicken and, as catalytic coke filaments grow, carbon starts to block metal surfaces. Tar is formed as condensation collects in the filaments. The filaments formed by catalytic coking stop growing when metal particles are covered with carbon and, afterwards, radical and condensation coking become dominant. 
         [0020]      FIG. 1  depicts one example of coke formation on typical metal surface  20  of pyrolysis furnace TLE  10 . Filamentous coke filaments  30  grow outwardly (as shown by upward arrow  35 ) from typical metal surface  20 . Free radical coke  40  tends to grow from filamentous coke filaments  30 , causing filaments  30  to grow and thicken. Filamentous coke filaments  30  tend to continue to grow outwardly (as shown by horizontal arrows  45 ) until active metal site  50  is blocked by free radical coke  40 , as indicated by blocked metal site  60 .  FIG. 1A  is a pictorial depiction of filamentous coke during initial formation.  FIG. 1B  is a pictorial depiction of filamentous coke and free radical coke formation. 
         [0021]    In the pyrolysis furnace, the product gas includes the coke precursors. When the product gas is quenched to stop the reaction in the TLE before main fractioner, coking is common on gas entry refractory cone wall, the TLE tube (inside) wall and tubesheet surface. This coking traditionally causes two problems: pressure drop and spalling, i.e., the breaking of the coke into small particles. Either of these problems can result in to reduced product gas flow and ultimately a shutdown for decoking. 
         [0022]    In certain embodiments of the present disclosure, a coating is applied to the TLE tubesheet and inlet cone refractory and other elements to reduce the amount of catalytic hard coke formed during pyrolysis furnace operation. In at least some embodiments of the present invention, the coating applied to the TLE tubesheet and inlet cone refractory is resistant to temperatures of 1600° F. (870° C.) or higher, is easy to apply to existing installations (retrofitability). In these embodiments, the coating adheres well to metal and/or refractory surfaces and is effective as a thin film or layer. In certain embodiments of the present disclosure, the coating is water based; in other embodiments the coating is an organic solvent based material. In certain embodiments, the coating can be readily sprayed onto surfaces at room temperature. It is preferable that the coating be easy to dry and easy to cure developing good adhesion or bonding to metal and/or refractory surfaces. Typically, the coating requires minimum surface preparation. 
         [0023]    In certain embodiments of the present invention, the coating material is boron nitride. Boron nitride has a number of crystalline structures, includes hexagonal, cubic, and wurtzite. In certain embodiments of the present disclosure, the TLE coating includes hexagonal boron nitride. 
         [0024]    The hexagonal crystalline structure of boron nitride is often lubricious, exhibiting some of the same properties of solid lubricants as graphite and molybendum disulfide, with low shear strength, low abrasivity, a good adherence of solid lubricant film and superior thermal stability. Hexagonal boron nitride typically has an oxidation threshold of approximately 1562° F. (850° C.) in an oxidizing atmosphere and up to 1832° F. (1000° C.) in a reducing atmosphere. In certain circumstances, hexagonal boron nitride can be used in inert or vacuum atmospheres at temperatures of approximately 3632° F. (2000° C.). Hexagonal boron nitride tends to have a high thermal conductivity with a low thermal expansion. Hexagonal boron nitride typically has a relatively high thermal shock resistance compared to other oxide based refractory compounds. and tends to be chemically inert to the compounds to which it is exposed in a pyrolysis furnace. It therefore tends to have a relatively high resistance to chemical attack compared to other oxide based refractory compounds. and is resistant to chemical corrosion. Boron nitride tends to have an excellent parting plane compared to oxide based refractory compounds and reduces sticking in glass forming applications. 
         [0025]    As those of skill in the art will appreciate, the thickness of the coating on the TLE inlet cone components and tubesheet depends on a number of factors including the cost of the coating material and the porosity and/or roughness of the surface to which it is applied. In certain embodiments of the present disclosure, the thickness of the coating applied to the TLE is in the range of about 25 microns to about 100 microns; in certain other embodiments of the present disclosure, the thickness of the coating is between about 40 microns and about 60 microns. In still other embodiments the thickness of the coating material is about 50 microns. In certain embodiments of the present disclosure, the thickness of the coating is not uniform, but varies across the tubesheet or inlet cone refractory surface, depending on the characteristics of the surface and the expected level of coke formation. 
         [0026]    Application of the boron nitride coating may be accomplished by traditional coating application methods. In certain embodiments, a water-based boron nitride paint is made from dry boron nitride powder and water, such as distilled water. In particular embodiments, the water-based boron nitride paint is at 20 to 40% solids concentration by weight; in other embodiments, a 20 to 35% concentration is used, although as those of ordinary skill in the art will understand with the benefit of this disclosure, a greater or lesser concentration of solids may be used. 
         [0027]    In certain embodiments of the present disclosure, the boron nitride coating is accomplished by use of a spray gun, such as a DeVilbiss Compact Pressure Spray Gun, although this example is non-limiting and any suitable spray gun may be used. 
         [0028]    When applying the boron nitride coating, it is preferable to have the surface to which the coating is to be applied to be clean, dry, and as free from grease or oil as is practical. In certain embodiments of the present disclosure, the TLE inlet cone and tubesheet have not been previously used, i.e., they have not been exposed to process chemicals. In such embodiments, it may be necessary to roughen the surface of the area to which the coating is to be applied in order to assist in physical adherence to the surface. Non-limiting examples of surface roughening include those specified by SSPC-SP6 and NACE 3-Commercial Blast Cleaning. 
         [0029]    In certain other embodiments, such as when the TLE has been previously used in a pyrolysis furnace, the residual coke on metal surfaces such as the TLE tubesheet inlet surface and Intrabody Flow Diverter surface are first removed and cleaned by a hydroblasting procedure, typically at about 10,000 psi then allowed to air dry. In those embodiments, typically the residual coke on the TLE inlet cone refractory is removed and the inlet cone refractory surface is dusted to remove residual coke. 
         [0030]    The boron nitride paint may be applied in a single coat or in multiple coats to achieve the desired thickness. In particular embodiment of the present disclosure, when applying multiple coats of the boron nitride paint, it may be necessary to allow drying of the boron nitride paint between coats. The drying may be performed at ambient temperature or at an elevated temperature, depending on need. The boron nitride paint may then be allowed to cure after completion of the application of all coats. Curing, like drying, may be accomplished at ambient temperature or at an elevated temperature depending on need. Typical curing times are between 60 and 120 minutes, although more or less time may be necessary depending on such factors as the thickness of boron nitride paint, the relative humidity, and the ambient temperature. 
         [0031]    Following the application of the boron nitride paint, the TLE may be reassembled and placed in the discharge line of the pyrolysis furnace. It has been determined by the applicants that the boron nitride coating often allows for increased run times of the pyrolysis furnace between steam decoking. Further, it has been observed by applicants that run times between mechanical cleaning of the TLE can be significantly extended as compared to run times of TLEs with uncoated tubesheets and inlet cones. While not bound by theory, applicants believe that the boron nitride coating acts to inhibit formation of catalytic coke by reducing or preventing contact between the hot product stream and the metal catalysts by coating the metal surfaces. While pyrolytic coke continues to form, it softer than the hard catalytic coke and may be more easily cleaned by steam decoking. It is further believed that the boron nitride coating provides a surface with less friction than the uncoated surface, weakening coke adhesion compared to the uncoated surface. 
         [0032]    While this disclosure has focused on the use of the boron nitride coating of the TLE tubesheet and inlet cone, those of ordinary skill in the art, with the benefit of this disclosure, will recognize that the boron nitride coating may be used with process equipment having metal surfaces where coke forms and deposits, and where the temperature does not exceed 1832° F. (1000° C.) in oxidation environment and 3632° F. (2000° C.) in a vacuum or inert atmosphere. Such equipment would include, but not be limited to the pyrolysis furnace, the furnace tubing, the transfer piping from the furnace to the TLE, the piping between the primary and secondary TLEs and the tubes inside the TLEs. This method could also apply to any similar process where coke is formed under similar high temperature process conditions. 
         [0033]    This disclosure will now be further illustrated with respect to certain specific examples which are not intended to limit the invention, but rather to provide more specific embodiments as only a few of many possible embodiments. 
       EXAMPLE 1 
       [0034]    A previously used TLE was removed from a pyrolysis furnace process and disassembled to allow access to the inlet tubesheet surface and TLE cone internal components. 
         [0035]    Coke was removed and the tubesheet and TLE inlet cone were cleaned by hydroblasting. The TLE components were allowed to dry and remaining dust was removed. 
         [0036]    A boron nitride paint was made by combining distilled water with a boron nitride powder with a hexagonal crystalline structure. Sufficient boron nitride powder was added to reach a concentration of 28% concentration by weight. 
         [0037]    A commercial spray gun was pressurized to between 20 and 40 psi using compressed air filtered to remove moisture and particulate. The boron nitride paint was introduced into the spray gun. A 50 micron thick coating of boron nitride was applied to the TLE tubesheet and inside surfaces of the TLE inlet cone. The application was made in two stages with each stage applying a 25 micron coating. The boron nitride coating was allowed to dry approximately 30 to 60 minutes between coats at room temperature, The final boron nitride coating was cured for 60 to 120 minutes at room temperature.