Abstract:
The present invention provides borohydrides that are useful in reducing aldol condensation and subsequent polymer formation in caustic scrubbers. The borohydrides are believed to react with reactive carbonyls yielding more stable alcohols and a salt of the borohydride which remains water soluble, and thus is unlikely to be carried out with the hydrocarbon phase. The borohydrides of the present invention have the potential to reduce reactive carbonyls at a molar ratio as high as about 4:1::carbonyl:borohydride. A preferred borohydride is sodium borohydride (sodium tetrahydroborate).

Description:
FIELD OF THE INVENTION 
     The present invention relates to the use of borohydrides to reduce aldehydes and certain ketones to unreactive alcohols in petrochemical caustic scrubbers, resulting in a reduction of aldol condensation and subsequent polymer formation in these scrubbers. A preferred borohydride is sodium borohydride (sodium tetrahydroborate). 
     BACKGROUND OF THE INVENTION 
     Refineries employ atmospheric and vacuum distillation towers to separate crude oil into narrower boiling fractions. These fractions then are converted into fuel products, such as motor gasoline, distillate fuels (diesel and heating oils), and bunker (residual) fuel oils. Some of the low boiling fractions from various units of the refinery are directed to petrochemical plants, where they are further processed into highly refined chemical feedstocks to be used as raw materials in the manufacture of other types of products, such as plastics and basic chemicals. 
     Within the petrochemical plant, processing of low boiling, mixed olefin streams primarily derived from pyrolytic cracking of hydrocarbons often require that the stream be treated in a caustic scrubber to remove acid gases, such as hydrogen sulfide and carbon dioxide. A caustic scrubber is a vessel containing an aqueous solution of caustic (NaOH, KOH, etc.) through which liquid or gaseous hydrocarbons are passed and mixed to wash out or &#34;scrub out&#34; the acid gases and impurities from the hydrocarbon stream. The hydrocarbon stream entering the caustic scrubber also may contain aldehydes and ketones, their precursors, such as vinyl acetate, or other impurities, that are hydrolyzed or otherwise converted to aldehydes and salts of organic acids in the highly alkaline environment of a caustic scrubber. Such compounds will herein be referred to as &#34;reactive compounds.&#34; These reactive compounds either (a) contain carbonics, or (b) form carbonyls under highly alkaline conditions, that are susceptible to classic aldol condensation reactions. Carbonyls that are susceptible to classic aldol condensation reactions hereinafter will be referred to as &#34;reactive carbonyls.&#34; 
     Under highly alkaline conditions, lower molecular weight aldehydes, such as propionaldehyde (propanal) and especially acetaldehyde (ethanal), readily undergo base catalyzed aldol condensation at ambient temperatures. The result is the formation of oligomers and polymers which precipitate out of the scrubbing solution as viscous oils, polymeric gums, and solids. These precipitates can foul the processing equipment and result in the reduction of processing throughput and costly equipment maintenance or repair. 
     In the past, organic reducing agents or organic and inorganic oxidizing agents have been proposed to prevent such polymerization. These organic agents might successfully retard polymerization in caustic scrubbers; however, the organic agents also tend to undergo other reactions which can reduce their effectiveness as aldol condensation inhibitors. Also, most of the oxidizing and reducing agents in current use only react with reactive carbonyls at a molar ratio of about 1:1 at maximum efficiency. A fewer number of these compounds can only reduce a maximum theoretical ratio of 2 moles of a reactive carbonyl per mole of the inhibitor compound. As a result, a relatively large amount of oxidizing or reducing agent must be added to retard polymerization. 
     A more effective and economical method of retarding aldol condensation in caustic scrubbers would be highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides borohydrides that are useful in reducing aldol condensation and subsequent polymer formation in caustic scrubbers. The borohydrides are believed to react with reactive carbonyls, yielding more stable alcohols and a salt of the borohydride which remains water soluble, and thus is unlikely to be carried out with the hydrocarbon phase. The borohydrides of the present invention have the potential to reduce reactive carbonyls at a molar ratio as high as about 4:1::carbonyl:borohydride. A preferred borohydride is sodium borohydride (sodium tetrahydroborate). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to reactions that cause fouling in caustic scrubbers. Exemplary product streams for use in accordance with the present invention include mixed light olefins, such as ethylene, propylene, butylene, etc., resulting from pyrolytically cracked mixtures of aliphatic hydrocarbons, such as ethane, propane, butane, and naphtha. Without limiting the present invention, it is believed that the red precipitate that forms in caustic scrubbers is the result of several aldol condensation/dehydration steps. As used herein, the term &#34;aldol condensation&#34; is intended to refer to the reactions that ultimately result in the formation of a precipitate in caustic scrubbers. The borohydrides of the present invention are believed to inhibit fouling by inhibiting such aldol condensation. 
     Substantially any borohydride should function in the present invention. Preferably, the borohydride should be reactive enough to reduce the reactive carbonyls in the stream, but not reactive enough to reduce other functional groups in the stream as well. The borohydrides may have the following structure: 
     
         M.sup.+- BH.sub.x (OR.sup.1).sub.4-x 
    
     wherein x is between about 1-4; M is selected from the group consisting of an alkali element, a tetraalkylammonium ion or quaternary amine having the structure R 2   4  N +  wherein R 2  is independently selected from an alkyl group having between about 1-10 carbon atoms; and, R 1  is independently selected from an alkyl group having between about 1-10 carbon atoms. Preferred alkali metals are Li, Na, and K. 
     Examples of suitable borohydrides include the following: 
     
         ______________________________________LiBH.sub.4     lithium borohydrideKBH.sub.4      potassium borohydrideNaBH.sub.4     sodium borohydride(CH.sub.3).sub.4 NBH.sub.4          tetramethylammonium borohydride(C.sub.2 H.sub.5).sub.4 NBH.sub.4          tetraethylammonium borohydrideNaBH[OCH(CH.sub.3).sub.2 ].sub.3          sodium triisopropoxyborohydride______________________________________ 
    
     Preferred borohydrides are soluble in hydroxylic solvents such as low molecular weight alcohols or water, of this group, sodium borohydride is preferred. Sodium borohydride is commonly available in powder form under the name VENPURE POWDER® from Morton Performance Chemicals, Danvers, Mass. Cyanoborohydrides are not preferred because they are ineffective in highly alkaline solutions. 
     Although aluminum hydrides should reduce reactive carbonyls, and thus should function in the present invention, aluminum hydrides are very potent reducing agents which tend to react with other functional groups besides reactive carbonyls. Furthermore, the reactivity of aluminum hydrides prohibits dilution using a hydroxylic solvent, such as an alcohol or water, as a delivery vehicle for injection into the caustic scrubber. A non-hydroxylic solvent, such as toluene or hexane, may be used, but is not as desirable. Furthermore, aluminum hydrides tend to react with the water in a caustic solution. Therefore, aluminum hydrides may function, but are not preferred for use in the present invention. As used herein, the term &#34;hydrides&#34; refers to borohydrides and aluminum hydrides. 
     In general, borohydrides have the potential to reduce a molar concentration of reactive carbonyl compounds that is equal to the number of active hydrogens in the hydride compound. For example, NaBH 4  should reduce four moles of a carbonyl compound at maximum efficiency, while NaBH[OCH(CH 3 ) 2  ] 3  is capable of reducing only one mole of a carbonyl compound. 
     Preferably, the borohydride should be introduced into a caustic solution at a rate (if a continuous process) or in an amount (if a batch washing process) to assure that the proper stoichiometric concentration of the borohydride is, at least, equal to or slightly exceeds the molar concentration of all reactive carbonyls present in the caustic solution. Sodium borohydride will inhibit aldol condensation in the caustic scrubber at ambient temperatures. 
     The reactive carbonyl content in the caustic solution may be determined using known analytical techniques (such as spectrophotometric measurements using 2,4-dinitrophenylhydrazine) following neutralization of the caustic solution. The concentration of the borohydride added to the caustic solution may be monitored by plasma emission spectroscopy for boron. In principle, certain analytical methods may be employed on the caustic scrubber solution to measure trace amounts of active, unreacted borohydride. 
     Typically, caustic solutions in which aldol condensation occurs will change from colorless solutions to yellow, orange, then red or brown solutions. The color change normally precedes polymer formation. Thus, in the absence of any analytical results for a caustic scrubber solution, the aldol condensation inhibitor should be added at a rate or in an amount, at least, to prevent formation of polymer, but preferably, to prevent further changes or intensification of color in the caustic wash solution. 
     For maximum effectiveness, sodium borohydride may be stabilized against hydrolysis during storage. This can be accomplished in an aqueous or alcoholic solution by maintaining the reaction solution at high alkalinity, preferably at a pH approaching 14, preferably using a quaternary ammonium hydroxide or an alkali metal hydroxide. Generally, the concentration of the sodium borohydride should be between about 0.01%-20% by weight of the alkaline stabilized solution. Caustic (NaOH) solutions at approximately 1 molar concentration may be employed as stabilization solutions for sodium borohydride. A stabilized water solution of 12% sodium borohydride in caustic soda as VENPURE® solution is also available from Morton Performance Chemicals, Danvers, Mass. The stabilized solution of sodium borohydride may be metered into the caustic scrubber units as needed. 
     The invention will be more clearly understood with reference to the following examples. 
     Example 1 
     25.0 ml of NaOH and 32,000 ppm of NaBH 4 , by weight of the final solution, were placed in a two ounce sample bottle, and 100 μl of vinyl acetate was injected into the solution. In a similarly prepared sample bottle lacking the NaBH 4  inhibitor, the vinyl acetate hydrolyzed to give acetaldehyde which, in turn, formed a red precipitate in about one hour from multiple aldol condensations. The NaBH 4  treated solution remained clear and formed no sediment. 
     Example 2 
     Vinyl acetate was dispensed into representative scrubber solutions (100 μl per 25 ml 10% NaOH) predosed with NaBH 4  at 1.1 mole per 1.0 mole of vinyl acetate. The solutions were stored overnight at room temperature. NaBH 4  successfully inhibited both polymer and color formation. Without NaBH 4 , yellow hazy solutions developed with a red precipitate. 
     Example 3 
     Into a clear glass bottle, labelled &#34;A,&#34; were placed equal volumes of two solutions: 
     1 part 10% (w) NaOH(aq) solution, 
     1 part 0.020M acetaldehyde solution in water. 
     The resulting solution yielded a 0.010M acetaldehyde solution in a 5.26% (w) NaOH(aq) solution. (This is approximately equal to 400 ppm-w acetaldehyde in the caustic solution.) After 30 minutes, the solution changed from clear and colorless to clear but yellow. After approximately four hours, the yellow solution became hazy. On the following day (30 hours after mixing), an orange precipitate had formed and settled onto the bottom of bottle &#34;A.&#34; 
     Into another bottle, labelled &#34;G,&#34; were placed equal volumes of the following two solutions: 
     1 part 0.020M NaBH 4  in 10% (w) NaOH(aq) solution, 
     1 part 0.020M acetaldehyde solution in water. 
     As with the previous bottle, the resulting mixture in bottle &#34;G&#34; contained 0.010M acetaldehyde in a 5.26% (w) NaOH(aq) solution. Additionally, the solution contained 0.010M NaBH 4 . With a molar ratio of 1:1::acetaldehyde:NaBH 4 , solution &#34;G&#34; remained clear and colorless without any polymer formation. 
     Into bottles labelled &#34;B&#34; through &#34;F&#34; were placed aliquots of the three stock solutions resulting in mixtures which always yielded 0.010M acetaldehyde in 5.26% (w) NaOH(aq) solution, but having variable concentrations of NaBH 4 . Table 1 summarizes the resulting combinations. 
     
                       TABLE 1______________________________________             Parts     Parts 0.020 M             0.020 M   NaBH.sub.4 in                                Parts  Mole Ratio of             Acetalde- 10% (w)  10% (w)  Acetaldehyde             hyde      NaOH(aq) NaOH(aq)Solution  to NaBH.sub.4             Solution  Solution Solution______________________________________A      --         1         0        1B      6:1        1         1/6      5/6C      5:1        1         1/5      4/5D      4:1        1         1/4      3/4E      3:1        1         1/3      2/3F      2:1        1         1/2      1/2G      1:1        1         1        0______________________________________ 
    
     After 30 hours at ambient temperature, the intensity of any yellow color that developed was measured with a UV/visible spectrophotometer at 425 nm. Any polymer that formed was also noted. Table 2 lists these observations. 
     
                       TABLE 2______________________________________  Yellow Color  AbsorbanceSolution  at 425 nm   Solution Description______________________________________A      0.59        Deep yellow solution with settled and              suspended orange flocculent              precipitateB      0.28        Slightly hazy, yellow solutionC      0.21        clear, yellow solution with no              precipitationD      0.13        Clear, very light yellow solution              with no precipitationE      0.03        Clear, faint yellow solution with no              precipitationF      0           Clear, colorless solution with no              precipitationG      0           Clear, colorless solution with no              precipitation______________________________________ 
    
     The results indicate that sodium borohydride at a molar ratio of 4:1::acetaldehyde:NaBH 4  (sample D) inhibited polymer formation even though some color developed. At a molar ratio of 5:1::acetaldehyde:NaBH 4  (sample C), sodium borohydride had reduced enough acetaldehyde to retard polymer precipitation for 30 hours. 
     Example 4 
     Caustic solution taken from an actual petrochemical plant&#39;s caustic scrubber unit was vacuum filtered to remove particulate matter. The filtered caustic solution was light yellow in color. To a 2-oz. bottle were added 94 mg of a 12% (w) NaBH 4  solution in 1M NaOH(aq) solution, followed by 25 ml of the petrochemical plant&#39;s filtered caustic solution. (This represents 0.30 mmoles of NaBH 4  in the test bottle.) 100 μl (representing 1.08 mmoles) of vinyl acetate were then injected into the test bottle containing the caustic solution with the NaBH 4  inhibitor. The bottle was capped, shaken, then allowed to stand undisturbed for 24 hours. 
     1.08 mmoles of vinyl acetate is equivalent to 1.08 mmoles of acetaldehyde since vinyl acetate yields acetaldehyde following hydrolysis under alkaline conditions, as follows: ##STR1## In the caustic solution, the acetic acid forms sodium acetate while the 1.08 mmoles of acetaldehyde would normally undergo the aldol condensation reaction. 
     At the end of 24 hours, no polymerization nor further discolorization had occurred in the treated solution. A bottle representing no treatment formed a red flocculent precipitate in a red, hazy solution. With this result, it is clear that one mole of NaBH 4  reduces more than one mole of reactive carbonyl compounds, in this case--3.6 moles of acetaldehyde per mole of sodium borohydride. 
     This example highlights two issues. First, it demonstrates sodium borohydride&#39;s potency for reducing nearly its theoretical maximum of 4 moles of reactive carbonyl compounds which would otherwise form oligomers and polymers by base catalyzed aldol condensation. Second, the caustic solution is taken from an actual caustic scrubber unit. Any impurities which it might contain did not deactivate sodium borohydride&#39;s performance. 
     Persons of skill in the art will appreciate that many modifications may be made to the embodiments described herein without departing from the spirit of the present invention. Accordingly, the embodiments described herein are illustrative only and are not intended to limit the scope of the present invention.