WO1999028412A2

WO1999028412A2 - Water based additive for suppression of coke formation

Info

Publication number: WO1999028412A2
Application number: PCT/US1998/025857
Authority: WO
Inventors: Shui Yin Lo
Original assignee: American Technologies Group, Inc.
Priority date: 1997-12-04
Filing date: 1998-12-04
Publication date: 1999-06-10
Also published as: WO1999028412A3; AU2198499A; WO1999028412A9

Abstract

A method for reducing coke formation from pyrolysis of hydrocarbon reactant in a reactor (Figure 1), comprising the steps of forming a hydrocarbon reactant-water-based additive mixture, and introducing the mixture to the reactor (Figure 1). The coke deposited in the reactor (Figure 1) from pyrolysis of the hydrocarbon reactant-water based additive mixture is less than coke deposited from pyrolysis of hydrocarbon reactant in the absence of the water based additive. Also provided is a hydrocarbon reactant-water-based additive mixture, the water based additive comprising a IE crystal structural liquid.

Description

WATER BASED ADDITIVE FOR SUPPRESSION OF COKE FORMATION

Inventor: Shui-Yin Lo

1. Field of the Invention

The invention relates to a method of reducing coke formation from pyrolysis of hydroc-arbon by adding a water based additive to hydrocarbon reactants. The invention further relates to a hydrocarbon reactant- water based additive mixture.

BACKGROUND OF THE INVENTION

2. Description of Related Art The publications and other reference materials referred to herein to describe the background of the invention .and to provide additional detail regarding its practice are hereby incorporated by reference. The most important olefins and diolefms used to manufacture petrochemicals are ethylene, propylene, butylenes, .and butadiene (Matar, S. and Hatch, L.F., "Chemistry of Petrochemical Processes," Gulf Publishing Co., Texas (1994)), Ethylene is one of the most important building blocks of synthetic organic chemistry. It is used in the manufacture of polyethylene .and other products. Ethylene production rate has steadily increased over the years from 29 million pounds in 1985 to 46.7 million pounds in 1995 (Chem. Engin. News, June 24, 1996). The majority of the ethylene produced today is based on the steam cracking or pyrolysis of alkanes, such as ethane, propane and butane, as well as heavier feedstocks such as aphtha and gas oil (Lee, L.K.K., et al., Oil and Gas J. Sept.

10, 1990, p. 60).

The steam cracking of a feedstock is accomplished in the coils of a pyrolysis furnace followed by quenching of the gas in a heat exchanger (Matar, S. .and Hatch, L.F., ibid) or the transfer line exchanger (TLE). A technologically important by-product of steam cracking is coke formation. Because of its accumulative nature, coke deposits build up on reactor walls and influence reactor performance in a number of ways. First, due to coke, the surface temperature of the coils is increased. This adversely affects, i.e. reduces the service life of the coil, and makes it impossible to obtain normal pyrolysis temperatures in the reactor. Second pressure drop is increased due to the reduction of the inner diameter of the coil upon coking which reduces flow rates through the coil, and causes a reduction in heat exchange efficiency. Third, coking may lead to corrosion of the coil due to carbonization.

Consequently, decoking of the reactor coils has to be carried out periodically, which results in loss of production and increased maintenance costs. In eth-ane cracking, commercial reactors must be decoked typically every 20-60 days (Sundaram, K.M. et al. (1981) AICHE Journal 7:946). Sundaram (ibid) studied the thermal cracking of ethane in a nitrogen matrix in the temperature range 750°-870° C in a mixed reactor. Major products reported were ethylene, methane, H^ and C₅+. They found the gas phase decomposition to be first order in ethane concentration with an apparent activation energy of 54.0 kcal/mol in agreement with previous studies in a tubular pilot reactor (Froment, G.F. et al. (1976) Ind. Eng. Che. Process Design Develop., 15:495). Similar results were reported more recently by Fro (Rev.

Chem. Eng. (1990) 6:293) for the steam cracking of ethane. Coke was deposited on an Inconel 500 coupon suspended inside the reactor from the arm of an electrobalance. The rate of formation of coke was found to be time dependent, starting initially at a faster rate and reaching an asymptotic value later in the run. The initial coke formation rate was attributed to catalytic wall effects. Once the coke layer is deposited on the coupon, the rate reaches its asymptotic value corresponding to coke deposition on coke. The estimated activation energy for coke formation base on a kinetic analysis of a reaction model was in the range of 28.3- 49.9 kcal/mole. Gas composition measurements also indicated the rapid formation rate of CO early in the experiments, which leveled off to .an asymptotic value following the coverage of the metal surface by coke. Initial CO production was proposed to be due to metal catalyzed oxidation of hydrocarbon moieties on reactor walls, and subsequent CO formation was attributed to the steam gasification of carbon. These studies also indicated that higher steam dilutions decease coke formation rates.

The decomposition of propane in a nitrogen matrix was studied by Sundaram and Fro (1979) in a mixed reactor in the temperature range 720-870° C. Major products reported were ethylene, methane, andC₃H₆. The disappearance of propane was found to be first order in propane concentration with an activation energy of 49.04 kcal/mol. This is in agreement with the results of Van Damme et al. (AICHE Journal, 21:1065 (1975)) and Fro (1990) in the steam cracking of propane. The activation energy for coke formation was estimated to be 74.97 kcal/mole, again based on the kinetic .analysis of a reaction model. This is in agreement with the experimental results of Trimm et al. ("Fundamental Aspects of the Formation and Gasification of Coke" in Pyrolysis: Theory and Industrial Practice, L.F. Albright et al. Eds., Academic Press, NY, p. 203 (1983)) during the steam cracking of propane in a flow reactor.

Crynes and Crynes (Ind. Eng. Chem. Res. 26:2139 (1987)) also studied the formation of coke during the pyrolysis of alkanes on Incoloy 800 coupons in a flow reactor.

Temperature was maintained at 700° C by mean of an electric furnace. They studied coking during the pyrolysis of ethane, ethane, ethene, propane, propene and isobutane. They found the following order for coking on the coupon: eth.ane < ethene ( propene ( propane ( isobutane, with no coke deposition observed for methane at their experimental conditions. The effects of reactor surfaces on coke deposition rates during the pyrolysis of propane has been studied extensively by Renjun (Fundamental of Pyrolysis of Pyrolysis in Petrochemistry and Technology, CRC Pres, Boca Raton, USA (1993)) in an electrobalance reactor at 850 C. The order of increasing coke deposition rates was found to be nickel ) stainless ) quartz. High coking rates were also observed early on in the experiments, which later reached an asymptotic value upon surface coverage by coke.

At present three mechanisms have been proposed to account for coke formation in hydrocarbon pyrolysis in industrial and laboratory reactors: (1) Coke formation via metal-catalyzed reactions in which metal carbides have been proposed to be intermediates. The resulting coke is filamentous and contains 1-2 wt% metal; the metals are positioned primarily at the tips of the filaments. Filamentous coke has been produced at temperatures from about 400° C up to 1050° C (Albright, et al. (1988) Ind. Eng. Chem. Res. 27:755). This can be one of the coke formation mechanisms on metal reactor surfaces. (2) Coke has also been proposed to form via polycyclic aromatic hydrocarbons (PAH) in the gas phase (see, for example, Wang, H. et al. (1994) J. Phys. Chem. 98:11465; and Gagurevich, I. Ph.D. Thesis, UCLA, 1997 for chemical paths in fuel-rich combustion), their nucleation and condensation into tar droplets followed by adsorption on surfaces where the tar proceeds to dehydrogenate into coke. This mechanism generally results in film or globular coke formation (Albright, L.F. et al. "Importance of Surface Reactions in Pyrolysis Units," in "Pyrolysis Theory and Industrial Practice, Albright, L.F. et al. Eds., Academic Press, New York, p. 233 (1983)). (3) Coke can also grow directly through the reactions of small gas phase species with sites on the coke surface. These species are likely to be acetylene or other olefins, butadiene, and free radicals such as methyl, ethyl, vinyl, phenyl or benzyl radicals. This mechanism should be favored by higher temperatures and with higher concentrations of acetylene in the gas phase (see for example Mauss et al. 1994, for surface growth mechanisms of soot particles in combustion.) The development of coke inhibitors have paralleled the various coke formation mechanisms described above. The techniques commonly used today to reduce coke formation include the pretreatment of feedstocks, changing the materials of construction of the reactor, altering the surface chemistry of the reactor, or the addition of coke inhibitors to the feedstock (Renjun, Z. Fundamentals of Pyrolysis in Petrochemistry and Technology, CRC Press, Boca Raton, US A, 1993 ; and Burns, K.G. et al. ( 1991 ) Hydrocarbon Processing, p. 83). The development and use of additives appears to be the most effective and practical method. Coke inhibitors reported in the literature include salts of alkali metals or alkali-earth metals at ppm quantities which are believed to promote coke gasification by steam. In addition, the use of organic polysiloxane compounds in ppm quantities have been shown to reduce the adhesion of coke to the coil walls. Sulfur compounds have also been used widely to suppress coke formation, especially early on in the pyrolysis process by passivating metal, surfaces (Renjun, 1993). Compounds containing tin, antimony, copper, phosphorous, and chromium were also reported to have a beneficial effect in suppressing coke formation (Renjun, 1993).

SUMMARY OF THE INVENTION The present invention provides a method for reducing coke formation from pyrolysis of hydrocarbon reactant in a reactor. The method comprises the steps of forming a hydrocarbon reactant-water-based additive mixture, and introducing the mixture to the reactor. The coke deposited in the reactor from pyrolysis of the hydrocarbon reactant- water based additive mixture is less than coke deposited from pyrolysis of hydrocarbon reactant in the absence of the water based additive. The water-based additive is a structured liquid comprising I_E crystal structured liquid.

The invention, in another aspect, provides a hydrocarbon reactant- water additive mixture which comprises a hydrocarbon reactant and a water-based additive. It is an object of the present invention to reduce the rate of carbon buildup, i.e. coke buildup that occurs in pyrolysis of hydrocarbons. Another object of the invention is to increase the productive operating period between shutdowns for removal of carbon buildup on the equipment surfaces of an ethane or propane or other hydrocarbon cracking equipment or production plants. Another object of the invention is to extend the life of heat-exchanger surfaces and heat exchanger equipment by reducing the insulating effects of carbon buildup on these surfaces and reducing the surface chemical attack of these surfaces that occurs in the presence of carbon buildup layers. Another object of the invention is the reduction in carbon erosion that is caused by free hard carbon particles in a gas stream impinging on equipment components made from expensive high-temperature alloys and stainless steels. Another object of the invention is the reduction in operating and maintenance costs of a hydrocarbon steam cracking plant.

These and many other features .and attend.ant advantages of the present invention will become better understood by reference to the following detailed description of the invention when taken in conjunction with the accompanying drawings.

FIGURES Figure 1 shows the reactor apparatus used to study the formation of coke during pyrolysis of hydrocarbon reactants.

Figure 2 shows representative data for steam pyrolysis of ethane.

Figure 3 shows coke formation in steam pyrolysis of eth-ane at 830° C and 845° C. Figure 4 shows Arrhenius plots for the rate of formation of coke in the steam pyrolysis of ethane.

Figure 5 shows coke formation in the steam pyrolysis of propane at 820° C and 830° C.

Figure 6 shows Arrhenius plots for the rate of formation of coke in the steam pyrolysis of propane.

DETAILED DESCRIPTION AND MODES OF CARRYING OUT THE INVENTION

The present invention provides a method for reducing coke formation from pyrolysis of hydrocarbon reactant in a reactor. The method comprises the steps of forming a hydrocarbon reactant-water-based additive mixture, and introducing the mixture to the reactor. The coke deposited in the reactor from pyrolysis of the hydrocarbon reactant- water based additive mixture is less than coke deposited from pyrolysis of hydrocarbon reactant in the absence of the water based additive. The water-based additive is a structured liquid comprising I_E crystal structured liquid, as defined and disclosed below.

The invention, in another aspect, provides a hydrocarbon reactant-water additive mixture which comprises a hydrocarbon reactant and a water-based additive. As set forth in the detailed example below, the method and mixture of the invention reduce the rate of carbon buildup, i.e. coke buildup that occurs in pyrolysis of hydrocarbons. As a result, the invention also provides a method to increase the productive operating period between shutdowns for removal of carbon buildup on the equipment surfaces of an ethane or propane or other hydrocarbon cracking equipment or production plants. Further aspects of the invention involve a method to extend the life of heat-exchanger surfaces and heat exchanger equipment by reducing the insulating effects of carbon buildup on these surfaces and reducing the surface chemical attack of these surfaces that occurs in the presence of carbon buildup layers. In still another aspect, the invention reduces carbon erosion that is caused by free hard carbon particles in a gas stream impinging on equipment components made from expensive high-temperature alloys and stainless steels. The invention also provides a method for reducing operating and maintenance costs of a hydrocarbon steam cracking plant. All of these methods are achieved by forming a hydrocarbon reactant-water based additive mixture and introducing the mixture to a reactor under conditions in which coke deposited in the reactor from pyrolysis of the hydrocarbon reactant-water based additive mixture is less th.an coke deposited from the pyrolysis of hydrocarbon reactant in the absence of the additive.

Water Based Additive

In the present invention, the water-based additive comprises a small amount of crystalline structured water with crystals, referred to herein as I_E crystals, in the micron or submicron size range. Growth and formation of these I_E crystalline water structures .and preparation of the water-based additive are described below. Pending U.S. Patent Applications 08/558,330 and 08/799,645, which are incorporated by reference, also disclose

I_E crystalline water structures, solutions thereof, methods for making the I_E crystals, and methods for making concentrated solutions of the I_E water crystals. The type of microscopic crystalline structure, referred to herein is also referred to herein as I_E crystal structured water. Accordingly, the water-based additive of the present invention is .an I_E crystal based additive. I_E crystal structured water is a structured liquid in which the I_E crystal structures are induced in the liquid by strong electric fields from the electric field of an ion or from the dipole moment of molecules. While structured liquids can be formed from a variety of polar solvents, I_E-structured water is a specific case of the general class of structured liquids that is formed from water molecules. By way of explanation, not limitation, the formation of I_E structured water is illustrated as follows: When salt (e.g. NaCl) is dissolved in water, the sodium and the chlorine become ions in the water because of the strong dipole moment of water molecules. Very dilute solutions are considered in which positively or negatively charged ions attract water molecules which have electric dipole moments. However, under these very dilute conditions, one finds that the water molecules surrounding an ion turn into a form of ice, not the ordinary ice where the unit cell has translational invariance, but one in which the crystalline structure of water surrounding the ion has a special symmetry due to the spherical nature of the coulombic force between the ion and the water molecule. The spherical symmetric icy structure surrounding ions is called I_E structure indicating it is an icy structure formed under the effect of an electric field. The I_E structures were observed and recorded under transmission electron microscopy, as disclosed in U.S. Patent Application 08/799,645, and as disclosed in Lo, Shui-Yin (1996) "Anomalous State of Ice," Modern Physics Letters B, 10:909-919; and (1996) "Physical Properties of Water with I_E Structures," Modern Physics Letters B, 10:921-930.

Preparation of the Water-based additive

Generating more I_E structures .and preparation of the water-based additive of the present invention, are described in U.S. Patent Applications 08/799,645 and 08/558,330, and involves forming concentrated crystal solutions of I_E structures. The method involves forming a first structured liquid comprising the I_E structures and/or fragments of I_E structures. This structured liquid comprises a liquid having a dielectric constant greater th.an 1 .and a material having an uneven distribution in charge on the surface of the material. An example of such material is NaCl. The first structured liquid is sufficiently diluted by repetitive dilution to form a second structured liquid. From the second structured liquid, the I_E structures are concentrated to form a concentrated crystal solution. Methods for concentrating I_E solutions are disclosed in a patent application, incorporated herein, which was filed in the United States Patent and Trademark Office by the inventor of the present invention on July 10, 1997 but for which applicant has not received notice of a serial number assigned by the USPTO. The disclosed methods include reverse osmosis, doping a solution with beads that release an I_E nucleating material, gas chromatography, fractional distillation, use of an electrical wire dipole, and dynamic freezing.

For example, a concentrated I_E solution has been achieved using a reverse osmosis membrane. The exact size and type of reverse osmosis membrane depended on the dipole liquid selected to start with in creating the crystal structure solution. As a result, the reverse osmosis membrane pore size selection and concentration of the structured liquid is achieved according to the physical size of the crystal structures involved. To be specific, a quantity of a dilute or weak I_E crystal structure solution is passed through a reverse osmosis unit which contains a membrane with a pore size of about 1.8 nanometers. This size filter is small enough and intended to allow only the passage of single molecules of water at one time through the pore. The reverse osmosis unit is typical of those commercially available in various sizes and flow capacities and consists of an outside housing, a membrane and sealed end caps with holes for tubing to be connected. A carbonator type vane pump with an electric motor is attached by tubing and valves to the reverse osmosis unit inlet side .and when the motor is turned on, the pump maintains a pressure on the membrane by means of the tubing and valves, and is kept in the range of 100-200 psi by adjusting a valve on the outlet side of the reverse osmosis unit. The key strategy for varying the concentration of the very dilute I_E crystal solution is the use of the reverse osmosis machine in reverse from its intended method by disposing of the output (clean) water and recycling the water that will not pass through the filter pore size selected. It has been determined that the selection of pore size will be dependent on the size of the molecule of the liquid utilized. For water, the membrane pore size selected was just slightly smaller th.an the size of the water molecule, about 1.8 nanometers, but it can vary from 1.0 nanometers to 3.0 nanometers or more depending on the liquid/material system selected. Figure 6 illustrates a reverse osmosis system 10 for concentrating crystal structured water. The weak solution 102 is added to tank 100 then said weak solution is drawn up through pipe 108 by means of pump 118 then pressurized into tube 112 which goes through pressure gage 116 and on through tube 114 into the entry side of the reverse osmosis unit 120. The weak solution then flows through the membrane assembly 122 wherein the single water molecules are driven through said membrane 122 by the pressure created by pump 118 acting against valve 126 and exit through port 128 .and are collected through tube 130 into tank 104 as a weaker solution 106. The crystal structure water, being composed of groups of water molecules, does not go through the membrane 122 and so it flows out of the reverse osmosis unit 120 through port

124. The now more concentrated crystal structure water then flows through adjustable valve 126 which is adjusted to create the membrane back pressure as shown at the valve 116. The crystal structure water then returns through tube 110 to the original tank 100 where it mixes with the weak solution 102 remakiing in tank 100. By constant recirculation around the system described above, the single water molecules are continuously removed from the weak solution and are stored in tank 104 causing the mixture in t.ank 100 to become a stronger concentration of crystal structure water solution. As a result, one can stop the procedure as the desired concentration level desired.

The water-based additive of the invention used in the detailed example disclosed below was prepared by a doping method. In this method, water was used as a dipole liquid. 0.05 moles of platinum chloride was mixed with 100 ml of pure 18 Meg source water, which is a highly pure water. Removal of impurities from the dipole liquid was extremely important. The resulting mixture was called DO. DO was then serially diluted to produce progressively more dilute solutions which were designated, respectively, Dl through D9. For example, Dl was produced by mixing 10 ml of DO with 90 ml of pure 18 Meg source water. Then D2, D3, D4 and so on up to D9 were produced in the same manner as

Dl, that is by adding 10 ml of each dilution to 90 ml of 18 Meg pure source water. Equal volumes of D9 solution .and PVC beads (i.e. 50% v/v) were mixed. The PVC beads were 65 durometer, food grade PVC pellets. The D9-PVC solution was allowed to stand for about two hours, at which time the UV absorbance (wavelength 195 nm) of the solution was, in the various solutions prepared by this method, from about 0.5 to about 2.0. In order to concentrate the I_E structures, this solution was then processed through a reverse osmosis filter and the volume reduced to 1/10th to l/40th of the original volume. This reduced volume had a UV absorbance at 195 nm of about 1.5 to about 3.0 in the various reduced volume solutions prepared by this method. By examination of tr-ansmission electron micrographs of I_E solutions prepared for electron microscopy, it was estimated that the percent by weight of I_E structures in this dipole liquid (i.e. water) was from about 2% to about 10% of the weight of the water. This I_E structured liquid is considered the water-based additive of the invention. As used herein, a percent I_E solution means

(100)weight of I_E structures/(weight of H₂O + weight of I_E structures )

in a given volume. As used in the example below, the water based additive had a UV absorbance at 195 nm of 2.5. For use in the present invention, the concentration of I_E structures in the water-based additive (i.e. the percentage I_E solution) can vary from about 0.2 % to about 20 %. A preferred range of concentrations is from about 0.5% to about 10%. In the industrial cracking of eth.ane to produce ethylene, hot steam is mixed with the gas ethane in a suitable chamber held at the desired temperature and pressure. The cracking process produces free carbon as an unwanted byproduct, which necessitates the shutdown of equipment for periodic maintenance to remove the buildup. In the method of the invention, I_E structured water replaced the water used to produce the steam used in cracking and the resulting reduction of coke formation upon pyrolysis of hydrocarbon reactant-water based additive mixture is one of the applications of the present invention.

A test was carried out, as described below, to determine the effect of the water based additive on the pyrolysis of hydrocarbons, in particular, ethane as occurs in the steam cracking of eth.ane. In the normal cracking process the chemical reaction is expressed as:

2C₂H₆ + H₂O + O₂ = 2C₂H₄ + 2H₂O (ethane) (ethylene)

The actual reaction is never 100% complete and partial products of the reaction process are produced such as free carbon, which then deposits in layers, on the walls of the reactor.

In the case of the use of I_E structured water for the production of ste.am, the reaction is enhanced by the presence of the I_E structures which act as catalysts, as shown by the following equation.

2C₂H₆ + H₂O(I_E) + O₂ = 2C₂H₄ + 2H₂O (ethane) (ethylene)

The test was done under controlled laboratory conditions and a diagram of the test apparatus is attached in Figure 1. The test equipment consisted of a quartz reactor which was maintained at 850°C by a furnace. A steam generator was used to heat incoming deionized water to steam. Nitrogen was also mixed with the steam. A second chamber was used to mix the steam, nitrogen and ethane gas to a desired temperature and pressure. The mixture was then fed into the quartz reactor and heated to the 850°C test temperature. Free carbon formed during the cracking process deposits on the surface of a quartz coupon. The coupon was supported in a thermogravimetric analyzer, which measured the change in weight that occurred as the carbon built up on the coupon. The test setup had the following parameters:

Temperature: 830C Qe_thane ^{= 1} ^ Co/Sec

Q_steam = 5.6 cc/Sec Qni_trogen = 2-6 CC/s^βC

Coupon Size: 2x2x0.1 cm

The test parameters chosen were typical of those used in industrial eth.ane and propane cracking plants. It should be understood that the rate at which steam is introduced in relation to the flow rates hydrocarbon reactants can, and the method of the invention is not limited to the rate disclosed herein.

After a specified test time, the ethane or propane was turned off and the system was purged with oxygen. The oxygen quickly oxidized the carbon deposit on the coupon to carbon dioxide which exited the quartz reactor and the weight of the coupon reduced. The rate of deposition of the carbon on the coupon was readily measured over time. The system was then purged with nitrogen to remove .any traces of oxygen and the test was repeated. In the second test which was the I_E structured water test, a sample of I_E structured water was used to replace the deionized water used in the first test and the test was then repeated. In this second test, the rate of carbon deposition was again recorded by thermogravimetric analyzer and it was found to be less than that of the deionized water only. The test was done with ethane and with propane as the main gas. In the tests on eth-ane, the carbon buildup rate using deionized water as the steam source was 0.341 μg/cm²-sec at 830°C. The carbon buildup rate for theI_E structured water as the steam source, was 0.089 μg/cm²-sec. This was a reduction of 74%, a very significant amount.

In the case of propane cracking, the carbon buildup rate using deionized water as the steam source was 0.443 μg/cm²-sec at 820°C. The carbon buildup rate at 820°C for the

I_E structured water as the steam source, was 0.193 μg/cm²-sec. This was a reduction of 56%, also a very significant amount. The carbon buildup values at 830°C fore the propane were 0.514 and 0.331 μg/cm²-sec, a reduction of 36% which was also very significant.

EXAMPLE Effect of Water Based Additive on Rate of Formation of Coke Deposit in Steam Cracking of Eth-ane and Propane

In Figure 1, the experimental apparatus used to study the formation of coke during the pyrolysis of hydrocarbons, and in particular, during the steam cracking of ethane and propane is illustrated. This apparatus is a modified version of the set up used to study coke formation in the pyrolysis and oxidative pyrolysis of methane and methyl chloride (Tran, T. et al. (1994) Ind. Eng. Chem. Res., 33:32). The main component of the experimental system is a Cahn 131 thermogravimetric analyzer (TGA, Madison, WI) that has a detection sensitivity of 1 microgram. The system has an electronic microbalance which continuously measures .and records the mass loss or gain of a substrate material or coupon which was suspended from the balance by means of a 0.0127 cm diameter platinum hang- down wire.

Furnace temperature profile and coupon mass data were acquired and stored by the data acquisition and control system. The data acquisition hardware consisted of an IBM compatible PC and software provided by Cahn Systems. The software allowed for the operation of the furnace for any temperature time history. The coupon material used for these studies was quartz, with dimensions 2 cm wide x 2 cm long x 0.1 cm thick. The coupon was centrally located inside a 3.5 cm i.d. x 32.5 cm long quartz reactor that was vertically placed inside a single zone furnace. The heating elements inside the furnace spanned a distance of about 15 cm, which thereby allowed the establishment of nearly isothermal central zone of about 2 cm in length in which the quartz coupon was placed (Tran, ibid).

Either deionized water or the water based additive, which comprised I_E crystals (Lo, S. (1996)" Anomalous State of Ice," Modern Physics Letters B, 10: 909; Lo, S. (1996) "Physical Properties of Water with IE Structures," Modern Physics Letters B 10:921) was pumped using a high precision metering syringe pump (ISCO-2600 with series D Controller, Lincoln, NE) and was vaporized in an electric furnace maintained at 400° C. Nitrogen gas was introduced into the liquid at the upstream of the steam furnace as a gas carrier. The hydrocarbon reactants, either eth.ane or prop-ane gases, and some additional nitrogen carrier gas were then mixed with the steam to form a hydrocarbon reactant-steam mixture (i.e. absence of water-based IE additive) or a hydrocarbon reactant-water based additive mixture.

The water based additive of the invention replaced the water used as a source for steam. As steam, the water based additive was added to the eth.ane or prop-ane to form a hydrocarbon reactant-water based additive mixture. Hydrocarbon reactant-water based additive reduced coke deposited in the reactor. The relative levels or flow rates of hydrocarbon reactant and steam for forming hydrocarbon reactant-water based additive mixture that find use in the invention r-ange were: Q_ethane ⁼ 1.8 cc/sec, Q_steam = 5.6 cc/sec, Q_ni_trogen ⁼ 2.6 cc/sec. It should be understood that in the step of forming the hydrocarbon reactant-water based additive mixture that the rate at which steam is introduced in relation to the flow rates hydrocarbon reactants can vary, and that the method of the invention is not limited to the rate disclosed herein.

The hydrocarbon reactant-water based additive mixture was then introduced to the reactor through electrically heated lines. All the gas flows were regulated by high accuracy rotameter (Mathes on, Cucamonga, CA) that were calibrated before the experiments. The weighing components of the TGA were protected from the reaction products by passing helium purge gas through the chamber. The gases used were obtained from Mathes on (Cucamonga, CA) unless otherwise indicated and had the following purities: He:99.99%; ■C₂H₆:99.9%; C₃H₈:99.99%; N₂:99.999%, and O₂:99.9% (Liquid Air Co.).

All the studies were conducted at 1 atm pressure and for 1 hour reaction time. Before each run, the reactor was purged with N₂ for about 10 minutes and then decoked using 15% O₂ (balance N₂) mixture to assure that the reactor and the coupon were coke free. This was accomplished both by visually observing the appearance of the coupon through an observation hole in the furnace and by monitoring the weight of the coupon during the decoking process. If the appearance of the coupon was transparent and nonluminous, -and its weight did not decrease with time and retained its original (coke-free) value, the coupon was assumed to be coke free. The reactor was then purged again with N₂ for about 10 minutes after which a mixture was formed between the hydrocarbon reactants and steam (either deionized water or water-based additive), and the mixture was introduced to the reactor. The primary reason for nitrogen purge before and after the decoking studies was to minimize the accumulation of potentially explosive mixtures in the reactor. Each run was repeated at least five times to ensure reproducibility and to assess the range of experimental errors associated with the experiments.

Results

Since the TGA had a sensitivity limit in the microgram level, it was necessary to determine the optimum gas flow rates that did not result in excessive noise, yet allowed the acquisition of reliable coking data over the r-ange of concentrations and temperatures which were used during the experiments.

Following the initial scoping studies, a total gas flow rate of about 2.5 cmVs, measured at STP, was determined suitable. Higher flow rates resulted in the establishment of undesirable flow patterns in the reactor that caused lateral movement of the hangdown wire and resulted in its contact with the baffle inside the reactor. It should be noted that at 2.5 cmVs, the flow regime in the reactor would have been laminar and would have corresponded to a nominal residence time of 15 s and about 1.5 s to cross the quartz coupon. This residence time was determined by taking into account the volume occupied by the baffle (Tran, T., (1992) MS Thesis, UCLA Chemical Engineering). Overall reactant conversions, measured separately by gas chromatography at the exit of the reactor were generally in the range 2-5%. However, because the quartz coupon occupied a small fraction of the reactor volume, it was subjected to a nearly constant gas composition along the flow direction due to the differential conversion of the reactants within the 1.5 s reaction time. Consequently, one would have expected uniform coke formation along the coupon if diffusion limitations were also absent. If diffusion limitation were present, the variation of the boundary layer thickeness along the coupon would have led to non-uniform coke depostion. Coke formation appeared to be uniform along the coupon as determined by SEM in previous studies (Tran and Senkan, 1994), indicative of the absence of tr-ansport limitations under the study conditions investigated. In Figure 2, a representative set of raw data obtained by the TGA is shown for the steam pyrolysis of ethane. As seen from this figure, the reproducibility of the experiments was excellent, well within 15% from one set to -another. A close inspection of the individual experiments showed that cooking rates, i.e. the slope of the weight vs. time lines, were generally initially higher, but leveled off to an approximately constant value. The latter rate, corrected for the baseline shift due to the loss or gain of coke on the hang-down wire after the decoking process, was designated as the specific coke formation rate, R_TGA in micrograms/min units. High initial coking rates were consistent with the results of other investigators (Sund-aram, K.M. et al. (1979) Chem. Eng. Sci. 34:635; Renjun, Z. (1993) Fundamentals of Pyrolysis in Petrochemistry and Technology," CRC Press, Boca Raton,

USA; Froment, G.F. (1990) Rev. Chem. Eng. 6:293; Tran, T. et al. (1994) Ind. Eng. Chem. Res. 33:32).

The physical meaning of the weight ch.ange measured by the TGA was considered. As evident from the experimental system described above, the TGA simply measured the weight change experienced by the quartz coupon. This weight change could have been affected directly by molecular events, e.g. chemical reactions that resulted in the growth .and/or destruction of molecular entities on the surface, or by macroscopic events, such as soot, tar particle collisions with the quartz coupon. Clearly, TA measurements could not distinguish between these two types of mech-anisms. Consequently, these lumped sets of events, as detected by TGA, are referred to herein as the coke formation process.

The specific coke formation rate, r_c, μg/cm²-min) was then determined from the following equation: r_c=R_TGA/A (1) where A is the surface area of the coupon. The specific coke formation rate can also be represented by the following phenomenological expression: r_c=k₀exp(-E/RT)f()C) μg/cm²-min (2) where k₀ is the specific rate constant for coke formation, E is the apparent activation energy, and f(C) is a functional dependency of coke formation on the composition of the gas phase.

This type of a rate expression has often been used to model coke formation kinetics (see for example Sundaram, K.M. et al. (1979) Chem. Eng. Sci. 34:635; Renjun, Z. et al. (1987) Ind.

Eng. Chem. Res. 26:2528; Froment, G.F. (1990) Rev. Chem. Eng. 6:293; Tran, T. et al. (1993) Ind. Eng. Chem. Res. 33:32). As evident from the above expression, under differential conversions that should be observed along the 1 cm long quartz coupon, f(C) would be nearly constant. The determination of f(C) was not the subject of this study.

In Table I, the experimental conditions were investigated and summarized. As evident from this table, coke formation rates were determined not only at fixed C₂ H₆ , C₃H₈ and H₂O concentrations but over a range of temperature ranges both in the absence and presence of the I_E additive. It should be noted that the temperature ranges studied were different for different mixtures because of differences in the decomposition temperatures of C₂H₆ and C₃H₈ . Consequently, all the experiments conducted did not correspond to identical residence times because of differences in gas velocities caused by different temperatures. In addition, changes in number of moles caused by the reaction process would have also altered residence times. These issues, however, should have had a relatively small effect on the results provided here. For example, differences in reactor temperatures should have introduced a variation in residence times no larger than about 2.3% between the lowest and highest temperature experiments, i.e. 100 x (840-820)/(820+273) = 2.3%. This uncertainty was well below the measurement errors associated with these types of experiments. Similarly, percent change in the total number of moles would have been extremely small due to small conversions involved and the presence of ste.am and nitrogen dilution.

Table I. Operating Conditions of the Experiments (cmVs at reaction conditions)

Ethane

Ethane Steam Nitrogen Steam/Ethane Ratio

830°C 0.724 1.472 1.235 2.03

840°C 0.716 1.459 1.224 2.03

845°C 0.714 1.453 1.218 20.3

Propane

Propane Steam Nitrogen Steam Ethane Ratio

820°C 0.750 1.486 1.246 1.98

825°C 0.746 1.479 1.240 1.98

830°C 0.743 1.472 1.235 1.98

In Figure 3, the weights of coke deposited on the quartz coupon are presented as a function of on-stream time for the steam pyrolysis of ethane at 830° C and 845° C, both in the absence (solid lines) and presence (dashed lines) of the I_E additive. The specific coke formation rates, determined from the slopes of these lines by the least squares fit method and the surface area of the coupon are presented in Table II. As is evident from this table, the amount of coke deposited on the coupon steadily increased with increasing time and reaction temperature. These results were totally consistent with previous studies (Froment 1990; Renjun et al. 1987; Tran and Senkan 1994). What is important, however, was the significant and consistent reduction in coke deposition when a hydrocarbon reactant-water based additive mixture was introduced in the feedstream. For example, at 830° C, coke formation rate decreased from a high value of 0.341 μg/cm -min in the absence of the I_E solution to a low value of of 0.0893 μg/cm²-min , representing a factor of 3.81 decrease or reduction in coke formation when a hydrocarbon reactant-water based additive mixture (comprising the I_E crystal solution) was formed and the mixture was introduced to the reactor. Similarly at 845° C, the coke formation rate decreased from 0.489 to 0.225 μg/cm -min, corresponding to a factor of 2.17 improvement.

Table fJ. Specific Coke Formation Rates. r_c. P g/cm²-min

Species W Wiitthhoouutt AAddddiittiivvee W Wiitthh AAddddiittiivvee Ratio

Ethane

830°C 0.341 0.0893 3.82

840°C - 0.158

845°C 0.489 0.225 2.17

Propane

820°C 0.443 0.193 2.29

825°C - 0.266

830°C 0.514 0.332 1.55

In Figure 4, the Arrhenius plots for the specific coke formation rate (r_c) in the steam pyrolysis of C_jHg -ire presented, again, in the absence and presence of the I_E crystal solution in accordance with equation (2) presented above. The slope of these lines, which correspond to apparent activation energies, were 58.9 and 149 kcal/mole, without and with the I_E crystal solution, respectively. These activation energies are significantly high, thus are indicative of the absence of transport limitations. If coke formation rates were limited by transport phenomena, the measurements would have been less sensitive to temperature and the apparent activation energies would have been in the range 1-5 kcal/mole. The specific coke formation rates reported in Figure 4 were also analyzed with regard to the wall collision frequency of C₂H₆ with the quartz coupon at the process conditions. These calculations indicated that coke formation rates measured by TGA were several orders of magnitude below the maximum limit set by the collision theory. It is important to note that the slopes of individual data sets presented in Figure 4 are different, suggesting that the mechanism of coke formation was different in the absence .and presence of the I_E crystal solution. In Figure 5, the .amount of coke deposited on the quartz coupon are presented as a function of on-stream time for the ste.am pyrolysis of prop.ane at 820 and 830° C. A comparison of the results with those for ethane (Figures 2 and 3) clearly showed that propane had a greater propensity for coke formation. The specific coke formation rates in the absence of the additives were 0.443 and 0.514 μg/cm²-min at 820° and 830° C, respectively. The latter rate was 50% higher than the rate of coke formation in ethane pyrolysis at the same reaction temperature. As evident from Figure 5, the presence of the I_E crystal solution also reduced coke formation in the steam pyrolysis of propane. Coking rates were reduced by factors of 2.29 and 1.55 at 820 and 830° C, respectively (Table IT). In Figure 6, the Arrhenius plots for the specific coke formation rates in the steam pyrolysis of C₃H₈ are presented. The apparent activation energies were 35.4 and 129 kcal/mole, in the absence and presence of the I_E additive, respectively. Although these values were lower than those observed for eth.ane, they were still high -and indicated that intrinsic reaction kinetics, not transport limitations, controlled coke formation rates in the studies reported herein.

Based on bond dissociation energy considerations, propane was expected to undergo pyrolysis at lower temperatures, and thus produce more coke than ethane at a given temperature. The experimental results presented above are consistent with this picture, and are presented below by way of illustration, not limitation. The decomposition of the hydrocarbon reactant was initiated by unimolecular decomposition or the scission of the weakest bond in the molecule. For eth.ane .and propane these paths would be:

C_jHg + M = 2CH₃ + M (ΔH_T=90 kcal/mole) (3)

C₃H₈ + M = CH₃ + C₂H₅ = M (ΔH_T=87 kcal/mole) (4)

The following C-H bond dissociation reactions are energetically more difficult:

C₂H₆ + M = C₂H₅ + H + M (ΔH_T=100 kcal mole) (5)

C₃H* + M = I-C₃H₇ + H + M (ΔH_T=95 kcal/mole) (6)

Once radical species are generated, they accelerate reactant destruction leading to unsaturated C₂, C₃, -and C₄ species such as C₂H₂, C₂H₃, C₃H₃ , C₄H₂ These small species can then polymerize and result in the formation of aromatics which are believed to be precursors to soot and coke (Wang, H. et al. (1994) J. Phys. Chem. 98:11464; Miller, J.A. et al. (1992) Combust. Flame 91:21) For example, feasible reaction sequence resulting in molecular growth and leading to benzene are as follows:

C₂H₃ + C₂H₂ = C₄H₄ + H (7) n-C₄H₅ (8)

C₄H₄ + H = n-C₄H₃ + H₂ (9)

followed by:

n-C₄H₃ + C₂H₂ = lV tøhenyl) (10) n-C₄H₅ + C₂H₂ = CA (11)

or via direct C . ,3HXJ.,3 recombination:

C₃H₃ + C₃H₃ = CA (12)

Once the first aromatic ring is formed, molecular weight growth leading to polycyclic aromatic hydrocarbons (PAH) can occur by either even carbon route (Wang, H. et al, 1994) or odd carbon route (Miller, J.A. et al., 1992; Colket, M.B. et al. (1995) Proceed. Of 2th Symposium (IntT.) On Combustions, p. 1205); Marinov, N.M. et al. (1997) Combust. Sci. Tech., in Press) or more likely a combination of both routes. Gas phase polymerization subsequently leads to tar, soot and ultimately coke as a consequence of series of PAH condensation and dehydrogenation reactions.

In the studies reported herein, since coke formation decreased or reduced in the presence of the I_E crystal solution, one can postulate several mechanism to explain this phenomenon. These explanations are presented as illustrations and should not be construed as limitations of the present invention. First, the I_E crystals may have preferentially adsorbed on the quartz surface and retarded the adsorption of coke precursors or tar droplets. Second, the I_E crystals may have chemically interfered with the surface reaction processes thus preventing buildup of coke by suppressing the following type of coke buildup reactions:

coke*_! + C₂H₂ ^~~ coke*_I+1 + H

where coke*_r represents an activated radical site on the coke surface with molecular weight I.

It should be understood that these findings show that the method of the invention in which a hydrocarbon reactant-water based additive mixture was formed and then introduced to a reactor for pyrolysis achieves reduction in coke formation in the reactor compared to pyrolysis of the hydrocarbon reactant in the absence of the water based additive. This hydrocarbon reactant-water based mixture of the invention also finds use in reducing coke formation from pyrolysis of hydrocarbons. The method and mixture of the inventions are intended for use in industrial production facilities, although other practical applications are also contemplated for the method and mixture where reduction of coke formation is an object. The application of I_E crystal solution or structured water to the pyrolysis hydrocarbons has been demonstrated herein to show a significant reduction in carbon deposit rates. This reduction in carbon deposit rate will allow longer use of industrial steam cracking plants between shut downs for carbon removal and repair of carbon eroded components, thus reducing plant operating costs.

Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the .art that the disclosures herein are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments as illustrated herein, but is only limited by the following claims.

Claims

What is claimed is:

1. A method for reducing coke formation from pyrolysis of hydrocarbon reactant in a reactor, said method comprising the steps of:

(a) forming a hydrocarbon reactant-water based additive mixture; and

(b) introducing said mixture to said reactor

whereby coke deposited in the reactor from pyrolysis of said hydrocarbon reactant-water based additive mixture is less th.an coke deposited from the pyrolysis of said hydrocarbon reactant in the absence of the additive.

2. The method of claim 1 wherein said water based additive comprises I_E crystals having a concentration from about 0.2 % to about 20%).

3. A hydrocarbon reactant-water based additive mixture comprising: (a) hydrocarbon reactant; and (b) a water-based additive.

4. The mixture of claim 3 wherein said water based additive comprises I_E crystals having a concentration from about 0.2% to about 20%.