Abstract:
The invention comprises a method for reducing the amount of carboxylic acids in petroleum feeds comprising the steps of (a) adding to said petroleum feed a catalytic agent comprising an oil soluble or oil dispersible compound of a metal selected from the group consisting of Group VB, VIB, VIIB and VIII metals, wherein the amount of metal in said petroleum feed is at least about 5 wppm, (b) heating said petroleum feed with said catalytic agent in a reactor at a temperature of about 400 to about 800° F. (about 204.44 to about 426.67° C.) and a pressure of about atmospheric to about 1000 psig (about 6996.33 kPa) in the substantial absence of hydrogen, and (c) sweeping the reactor containing said petroleum feed and said catalytic agent with an inert gas to maintain the combined water and carbon dioxide partial pressure below about 50 psia (344.75 kPa).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-In-Part of U.S. Ser. No. 920,556 filed Aug. 08, 1997, now abandoned, which is based on Patent Memorandum 96BR 020. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a method for reducing the Total Acid Number (TAN) of crude oils, a number that is based on the amount of carboxylic acids, especially naphthenic acids, that are present in the oil. 
     BACKGROUND OF THE INVENTION 
     The presence of relatively high levels of petroleum acids, e.g., naphthenic acids, in crude oils or fractions thereof is a problem for petroleum refiners and more recently for producers as well. Essentially, these acids, which are found to a greater or lesser extent in virtually all crude oils, are corrosive, tend to cause equipment failures, and lead to high maintenance costs, more frequent turnarounds than would otherwise be necessary, reduce product quality, and cause environmental disposal problems. 
     A very significant amount of literature, both patents and publications, exists that deal with naphthenic acid removal by conversion or absorption. For example, many aqueous materials can be added to crudes or crude fractions to convert the naphthenic acids to some other material, e.g., salts, that can either be removed or are less corrosive. Other methods for naphthenic acid removal are also well known including absorption, on zeolites, for example. Additionally, one common practice for overcoming naphthenic acid problems is the use of expensive corrosion resistant alloy materials in refinery or producer equipment that will encounter relatively high naphthenic acid concentrations. Another common practice involves blending of crudes with high TAN with crudes of lower TAN, the latter, however being significantly more costly than the former. One reference, Lazar, et al (U.S. Pat. No. 1,953,353) teaches naphthenic acid decomposition of topped crudes or distillates, effected at atmospheric pressure between 600 and 750° F. (315.6 to 398.9° C.). However, it only recognizes CO 2  as the sole gaseous non-hydrocarbon, naphthenic acid decomposition product and makes no provision for avoiding buildup of reaction inhibitors. 
     Additionally, U.S. Pat. No. 2,921,023 describes removal of naphthenic acids from heavy petroleum fractions by hydrogenation with a molybdenum oxide-on-silica/alumina catalyst. More specifically, the process preferentially hydrogenates oxo-compounds and/or olefinic compounds, for example, naphthenic acids, in the presence of sulfur compounds contained in organic mixtures without affecting the sulfur compounds. This is accomplished by subjecting the organic mixture to the action of hydrogen at temperatures between about 450 and 600° F. (232.2 to 315.6° C.), in the presence of a molybdenum oxide containing catalyst having a reversible water content of less than about 1.0 wt %. Catalyst life is prolonged by regeneration. 
     WO 96/06899 describes a process for removing essentially naphthenic acids from a hydrocarbon oil. The process includes hydrogenation at 1 to 50 bar (100 to 5000 kPa) and at 100 to 300° C. (212 to 572° F.) of a crude that has not been previously distilled or from which a naphtha fraction has been distilled using a catalyst consisting of Ni-Mo or Co-Mo on an alumina carrier. The specification describes the pumping of hydrogen into the reaction zone. No mention is made of controlling water and carbon dioxide partial pressure. 
     U.S. Pat. No. 3,617,501 describes an integrated process for refining whole crude but does not discuss TAN reduction. The first step of the process includes hydrotreating a feed, which can be a whole crude oil fraction, using a catalyst comprising one or more metals supported on a carrier material. Preferably the metals are metal oxides or sulfides, such as molybdenum, tungsten, cobalt, nickel and iron supported on a suitable carrier material such as alumina or alumina that contains a small amount of silica. The catalyst can be employed in the form of fixed bed, a slurry or fluidized bed reactor. With regard to slurry operation, no mention is made of catalyst particle size, catalyst concentration in feed or the use of unsupported catalysts (i.e., no carrier). 
     British Patent 1,236,230 describes a process for the removal of naphthenic acids from petroleum distillate fractions by processing over supported hydrotreating catalysts without the addition of gaseous hydrogen. No mention is made of controlling water and carbon dioxide partial pressure. 
     U.S. Pat. Nos. 4,134,825; 4,740,295; 5,039,392; and 5,620,591, all of which are incorporated herein by reference, teach the preparation of highly dispersed, unsupported catalysts, of nominal particle size of one micron, from oil soluble or oil dispersible compounds of metals selected from groups IVB, VB, VIB, VIIB and VIII of the periodic table of elements and application of said catalysts for the hydroconversion upgrading of heavy feeds, including whole or topped petroleum crudes. Hydroconversion is defined in these patents as a catalytic process conducted in the presence of hydrogen wherein at least a portion of the heavy constituents and coke precursors (i.e., Conradson Carbon) are converted to lower boiling compounds. The broadest ranges cited in these references with respect to process conditions include temperatures in the range of 644-896° F. (339.9 to 480° C.) hydrogen partial pressures ranging from 50-5000 psig (446.08 to 34576.33 kPa) and from 10-2000 wppm of catalyst metal based on the weight of the feedstock. These references are directed to the conversion upgrading of heavy feeds and do not recognize that said catalysts can be used to selectively destroy carboxylic acids, e.g., naphthenic acids. 
     Another method for removal of such acids includes treatment at temperatures of at least about 400° F. (204.44° C.), preferably at least about 600° F. (315.56° C.) while sweeping the reaction zone with an inert gas to remove inhibitors indigenous to or formed during the treatment. However, this approach is debited by the volatilization of some of the naphthenic acids, which are found in distillate and light oil fractions that flash during the thermal treatment. Moreover, treatment temperatures may be too high for this method to be used in downstream applications where it is desirable to destroy the acids prior to pipestill furnaces, i.e., at temperatures of about 550° F. (287.78° C.) or below. 
     Thus, there remains a need for eliminating or at least substantially reducing petroleum acid concentration in crudes or fractions thereof that is low cost and refinery friendly. Such technology would be particularly suitable for crudes or fractions where the TAN value is about 2 or above. TAN, determined by ASTM method D-664, is milligrams of KOH required to neutralize the organic acids contained in 1.0 gram of oil. 
     SUMMARY OF THE INVENTION 
     The instant invention is directed to a method for destroying carboxylic acids in whole crudes and crude fractions. The invention comprises a method for reducing the amount of carboxylic acids in petroleum feeds comprising the steps of (a) adding to said petroleum feed a catalytic agent comprising an oil soluble or oil dispersible compound of a metal selected from the group consisting of Group VB, VIB, VIIB and VIII metals, wherein the amount of metal in said petroleum feed is at least about 5 wppm, (b) heating said petroleum feed with said catalytic agent in a reactor at a temperature of about 400 to about 800° F. (about 204.44 to about 426.67° C.), and a pressure of about atmospheric to about 1000 psig (about 6996.33 kPa) in the substantial absence of hydrogen, and (c) sweeping the reactor containing said petroleum feed and said catalytic agent with an inert gas to maintain the combined water and carbon dioxide partial pressure below about 50 psia (about 344.75 kPa). 
     TAN is defined as the weight in milligrams of base required to neutralize all acidic constituents in the oil. 
     Vacuum bottoms conversion is defined as the conversion of material boiling above 1025° F. (551.67° C.) to material boiling below 1025° F. (551.67° C.). 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is the calculated partial pressure for water as a function of reactor pressure and rate of inert gas sweep for the process of the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The instant invention removes or destroys carboxylic acids (e.g., naphthenic acids) from petroleum feeds such as whole crude oils (including heavy crudes) and fractions thereof such as vacuum gas oil fractions, topped crudes, atmospheric resids, vacuum resids, and vacuum gas oil. The instant method reduces TAN by at least about 40% in the petroleum feed. 
     The process is run at temperatures from about 400 to about 800° F. (about 204.44 to about 426.67° C.), more preferably about 450 to about 750° F. (about 232.22 to about 398.89° C.), and most preferably about 500 to about 650° F. (about 260.00 to about 343.33° C.). Pressures range from about atmospheric to about 1000 psig (about atmospheric to 6996.33 kPa), preferably about 15 to about 500 psig (about 204.75 to about 3548.83 kPa), and most preferably about 30 to about 300 psig (about 308.18 to about 2169.83 kPa). The amount of catalyst, calculated as catalyst metal or metals, used in the process ranges from at least about 5, preferably about 10 to about 1000 parts per million weight (wppm) of the petroleum feed being treated. 
     Conveniently, during the process of the instant invention, less than about 30% bottoms conversion occurs, and conveniently less than about 20% wherein vacuum bottoms is defined as hydrocarbon material boiling above 1025° F. (551.67° C.). 
     Catalyst particle size ranges from about 0.5 to about 10 microns, preferably about 0.5 to 5 microns, and most preferably about 0.5 to 2.0 microns. Catalysts are prepared from precursors, also referred to herein as catalytic agents, such as oil soluble or oil dispersible compounds of Group VB, VIB, VIIB, or VIII metals and mixtures thereof. Suitable catalyst metals and metal compounds are disclosed in U.S. Pat. No. 4,134,825 herein incorporated by reference. An example of an oil soluble compound is the metal salt of a naphthenic acid such as molybdenum naphthenate. Examples of oil dispersible compounds are phosphomolybdic acid and ammonium heptamolybdate, materials that are first dissolved in water and then dispersed in the oil as a water-in-oil mixture, wherein droplet size of the water phase is below about 10 microns. 
     Ideally, a catalyst precursor concentrate is first prepared wherein the oil soluble or oil dispersible metal compound(s) is blended with a portion of the process feed to form a concentrate that contains at least about 0.2 wt % of catalyst metal, preferably about 0.2 to 2.0 wt % catalyst metal. See for example U.S. Pat. No. 5,039,392 or 4,740,295 herein incorporated by reference. The resultant precursor concentrate can be used directly in the process or first converted to a metal sulfide concentrate or an activated catalyst concentrate prior to use. 
     Catalyst precursor concentrate can be converted to a metal sulfide concentrate by treating with elemental sulfur (added to the portion of feed used to prepare the concentrate) or with hydrogen sulfide at 300 to 400° F. (148.89 to 204.44° C.) for 10-15 minutes (e.g. see U.S. Pat. Nos. 5,039,392; 4,479,295 and 5,620,591 herein incorporated by reference). 
     The metal sulfide concentrate can be converted into catalyst concentrate by heating at 600 to 750° F. (315.56 to 398.89° C.) for a time sufficient to form the catalyst. (e.g. see U.S. Pat. Nos. 5,039,392; 4,740,295; and 5,620,591). The catalyst of the concentrate consists of nano-scale metal sulfide sites distributed on a hydrocarbonaceous matrix that is derived from the oil component of the concentrate. Overall particle size can be varied, but falls within the range of 0.5 to 10 microns, preferably in the range of about 0.5 to 5.0 microns, and more preferably 0.5 to 2.0 microns. 
     For the present process one may employ the precursor concentrate, the metal sulfide concentrate, or the catalyst concentrate. In each case, the petroleum feed is mixed with the concentrate to obtain the desired concentration of metal in the feed i.e., at least about 5 wppm, preferably about 10to 1000 wppm. When the precursor or metal sulfide concentrates are used, catalyst having a particle size of about 0.5 to 10 microns, preferably 0.5 to 5 microns and most preferably 0.5 to 2.0 microns are formed in the heating step of the process in the TAN conversion reactor. 
     Preferred metals include molybdenum, tungsten, vanadium, iron, nickel, cobalt, and chromium. For example, heteropolyacids of the metals can be used. Molybdenum is particularly well suited to the process of the instant invention. Preferred molybdenum compounds are molybdenum naphthenates, dithiocarbamate complexes of molybdenum (e.g. see U.S. Pat. No. 4,561,964 incorporated herein by reference), phosphomolybdic acid and phosphorodithioate complexes of molybdenum (e.g., MOLYVAN®-L, molybdenum di(2-ethylhexyl) phosphorodithioate, supplied by R.T. Vanderbilt Company. 
     Other small particle catalysts that are useful for the practice of the instant process include metals-rich ash from the controlled combustion of petroleum coke (e.g., see U.S. Pat. Nos. 4,169,038; 4,178,227 and 4,204,943 herein incorporated by reference). Finely divided iron based materials, satisfying the particle size constraints noted herein, such as red mud from the processing of alumina can also be used. 
     The present process, to decrease the amount of organic acids in petroleum feeds, is conducted without the addition of hydrogen. 
     Water vapor and carbon dioxide, resulting from the decomposition of carboxylic acids, act as inhibitors for the decomposition of remaining carboxylic acids. Water is a particularly strong inhibitor. Thus, if the feed to the process contains water, a preflash step may be used to remove substantially all of the water. Moreover, trace amounts of water entering the process with the feed, as well as water and carbon dioxide formed in the course of the destruction of carboxylic acids, must be purged from the process such that the partial pressure of water and carbon dioxide in the reaction zone is held below about 50 psia (about 344.75 kPa), preferably below about 30 psia (about 206.85 kPa), more preferably below about 20 psia (about 137.9 kPa), most preferably below about 10 psia (about 68.95 kPa), and particularly below about 5 psia (about 34.48 kPa). Substantially all of the water as used herein means as much water as can be removed by methods known to those skilled in the art. 
     Though not wishing to be bound by theory, it appears that most of the water formed in the destruction of carboxylic acids under conditions of the present process involves the formation of an anhydride intermediate (a reversible reaction with water), hence the inhibiting effect of water on acid decomposition. ##STR1## 
     As will be illustrated in examples to follow, water can have a strong inhibiting effect on the rate of carboxylic acid destruction. Carbon dioxide is also an inhibitor but to a much lower degree. 
     To illustrate the potential for water pressure buildup resulting from destruction of carboxylic acids under conditions claimed for the process of the present invention, a hypothetical case was assumed where the TAN of a whole crude is lowered from 5.3 to 0.3 by thermal treating within the temperature range set forth in this invention, and that 0.5 mole of water is produced for each mole of acid that is destroyed. Calculated partial pressures for water are shown in FIG. 1 as a function of reactor pressure and of the rate at which an inert sweep gas (e.g. nitrogen, helium, argon, methane) is used. Note that water partial pressures as high as 45 psia (310.28 kPa) can be obtained from acid decomposition alone when operating within the range of pressures claimed for this process, thus emphasizing the preference to start the process with a dry feed and to maintain a sweep gas rate to keep water pressure within specified levels. 
     From a process standpoint, the catalyst may be left in the treated crude (depending on the metal type and concentration) or removed by conventional means such as filtration. 
     The following examples illustrate the invention but are not meant to be limiting in any way. 
     The feedstock that was used in this study was a blend of Kome and Bolobo crudes from CHAD. The blend was desalted and heated to 230° F. (110° C.) with nitrogen purge to remove bulk water. Properties are given in Table 1 . 
     
                       TABLE 1______________________________________TAN (mg KOH/g crude) 5.3Sulfur, wt %         0.2Vacuum bottoms, wt % 49API gravity          18Viscosity, cSt @ 104° F. (40° C.)                1100______________________________________ 
    
     EXAMPLE 1 
     This example was carried out in a 300 cc (300 ml) stirred autoclave reactor. The reactor was operated in a batch mode with respect to the crude that was charged. Gas was flowed through the autoclave to control the concentration of inhibitors in the reaction zone. 
     The reactor was charged with 100 g of the Kome/Bolobo blend, flushed with helium and then heated to 625° F. (329.44° C.) with stirring for 60 minute treatment at 625° F. (329.44° C.). Helium was flowed through the reactor at a rate of 0.1 liters per minute during the run. Upon cooling, the reactor liquid was discharged and measured for TAN content (ASTM D664 where TAN=mg KOH per gram of crude or product oil). 
     EXAMPLE 2 
     Example 1 was repeated except that the reactor was charged with 100 g of Kome/Bolobo blend and 0.62 g. of MOLYVAN®-L (an amount sufficient to give 500 wppm Mo in the reactor feed). This compound, supplied by R.T. Vanderbilt Company, is molybdenum di(2-ethylhexyl) phosphorodithioate that contains 8.1% Mo. 
     EXAMPLE 3 
     Example 2 was repeated except that the reactor product was filtered to recover catalyst solids prior to assay of the liquid. 
     EXAMPLE 4 
     Example 2 was repeated except that water was fed to the reactor to reflect operation with feed that contained 1.0 wt % water. 
     
                       TABLE 2______________________________________Summary of ExamplesExample  1         2         3       4______________________________________Mo wppm   0        500       500     500Temp ° F.    625       625       625     625(° C.)    (329.44)  (329.44)  (329.44)                                (329.44)Time (minutes)    60         60        60      60Reactor  45         45        45      45Pressure (psia)    (310.28 kPa)              (310.28 kPa)                        (310.28 kPa)                                (310.28 kPa)Water (psia)    &lt;1         &lt;1        &lt;1     8.1Filtered no        No        yes     no(yes/no)TAN      3.43      2.71      2.60    3.21Vacuum    0         0         0       0BottomsConversion, %______________________________________ 
    
     The examples of Table 2 illustrate that the rate of TAN destruction under relatively mild thermal conditions can be accelerated by addition of trace amounts of molybdenum, furnished as an oil soluble molybdenum compound, without addition of hydrogen (Compare Examples 2 and 3 with Example 1). Moreover, water is shown to have an inhibiting effect on TAN conversion (Compare Examples 2 and 3 with Example 4).