Abstract:
A method for mitigating fouling in a wash unit used in a hydrocarbon cracking process wherein the fouling is due to the presence of polymers and deposits thereof formed by condensation of carbonyl compounds contained within a feed stream of the wash unit. In one embodiment, the invention provides a method of mitigating fouling in a wash unit by introducing into the feed stream an effective amount of an additive including: an inorganic salt of dithionite; and an epsilon caprolactam or a 6-amino caproic acid derivable therefrom. The additive scavenges the carbonyl compounds contained within the feed stream and dissolves deposits of the polymers to thereby mitigate fouling in the wash unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation-in-part application claiming priority to International application PCT/IN2002/000195, filed Sep. 30, 2002, which designates the United States of America and claims the benefit of U.S. provisional application 60/391,717, filed Jun. 26, 2002, both of which are incorporated by reference herein to the extent they are not inconsistent with the disclosure herein. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to a method of removing undesired byproducts from pyrolytic cracking of hydrocarbons. The invention more particularly relates to a method for removal of carbonyl compounds not only like acetaldehyde and other carbonyl compounds but also its polymer along with the other acidic gases like H 2 S and CO 2  that are formed when cracked gasses enter a caustic or an amine unit within an ethylene plant where the cracked gases are produced in a pyrolysis heater during the pyrolytic cracking of hydrocarbon such as naphtha, gas oil or ethane, propane, butane and such other hydrocarbons.  
       BACKGROUND OF THE INVENTION  
       [0003]     In pyrolytic cracking operations, feed stocks such as ethane, propane, naphtha, kerosene, gas oil, fuel oil and the like undergo “cracking,” that is the removal of hydrogen, to form unsaturated hydrocarbons. Pyrolytic cracking also tends to produce oxygenated hydrocarbons, including carbonyl compounds such as acetaldehyde. In a typical operation, the cracked effluent stream is quenched, fractionated and compressed. Acidic contaminants such as hydrogen sulfide, carbon dioxide and mercaptans are then typically removed from the effluent.  
         [0004]     During the scrubbing operation of these gases with caustic or amine some oxygenated compounds are removed. At the same time, however, the basic conditions in the scrubber tend to cause base-induced condensation reactions (due to aldol condensation mechanism) of the carbonyl compounds, including aldehydes (e.g., acetaldehyde) and/or ketones, which in turn result in the formation of polymers. These polymers deposit on the internal surfaces of the scrubber. As the mass of polymer grows, it leads to fouling and can eventually obstruct the flow of liquids through the system. This is undesirable, as the operating system must be shut down for a significant amount of time in order to remove the deposited polymer and clean the equipment. This operation itself is very expensive involving many man hours and fmancial loss.  
         [0005]     The prior art systems treat caustic towers with an injection of a compound to inhibit the aldol condensation mechanism. In order to inhibit aldol condensation the prior art systems mostly use additives that are organic in nature and contribute to chemical oxygen demand for caustic used in the scrubbing process.  
         [0006]     In addition, the prior art additives typically require additive to reactant (i.e., carbonyl compound) molar ratios to be of at least about 1:1 for effective performance. Further the adducts of the high molecular weight polymers with these compounds tend to be insoluble in the basic system. Thus, the prior art additives are ineffective for the purpose of maintaining unobstructed flow through the system and reducing significant maintenance time for removing the polymer deposits and cleaning the equipment.  
         [0007]     A current practice in the industry is to treat the weak caustic in the caustic tower with gasoline or another aromatic fraction in order to remove the polymers before sending it to the spent caustic oxidation unit, in order to prevent fouling there. The resulting gasoline-containing stream causes disposal and operational problem, however. Likewise, routing the gasoline-containing stream to other operating units can cause problems due to the presence of the caustic, as it may affect pH, catalyst and other plant parameters.  
         [0008]     Another current practice in the industry is to treat the caustic tower with organic additives and despite the various advances in the art it remains desirable to provide an alternative method to improve the performance of this particular unit in the industry. There have also been shortcomings discussed later when any of these additives have been used concurrently to provide a synergistic effect in such systems.  
         [0009]     In the past, prevention of polymerization of oxygenated compounds, such as carbonyl containing organics in basic solutions, has been attempted by process of inhibition only by adding amine compounds such as hydroxylamine hydrochloride, hydroxylamine sulfate, hydrazine, carbohydrazides and the like. Several patents which relate to methods of inhibiting carbonyl fouling due to polymerization are listed below. It is found that these patents discuss only the removal of carbonyl compounds but fail to address the issue of polymers which are formed as a result of polymerization of unscavenged portion of the carbonyl compounds during the use of inhibiting additive and also of polymers already existing in the system. Thus these patents discuss only the method of inhibition of polymerization.  
         [0010]     U.S. Pat. No. 4,673,489 to Roling discloses using hydroxylamine and its salts of hydrochloric acid and sulfuric acid to inhibit polymer formation caused by condensation reactions of aldehydes contained in caustic scrubber units. One disadvantage of the method is that the additive has to be used in almost molar proportion. The other disadvantage is that these chemicals are expensive and must be over fed to the caustic wash unit system. This patent does not disclose a solution to the removal of carbonyl compounds and their polymers that remain unscavenged in the inhibition; process, nor does it provide for removal of already existing polymers.  
         [0011]     U.S. Pat. No. 4,952,301 to Awbrey discloses using ethylenediamines, with the molecular formula NH sub 2 ( CH sub 2 CH sub 2 NH ) sub x H were x is an integer ranging from about 1 to about 10, to inhibit carbonyl based fouling, particularly aldehyde fouling, that often occurs during caustic scrubbing of liquid or gas phase hydrocarbon streams in the base wash unit. This patent similarly does not provide a solution to removal of carbonyl compounds and polymers thereof that remain unscavenged during the inhibition process, nor does it solve the problem of removal of already existing polymers.  
         [0012]     U.S. Pat. No. 5,264,114 granted to Dunbar also discloses the use of amine compounds to inhibit the deposition of foulants during caustic washing of the hydrocarbon gases contaminated with the carbonyl compounds. The method comprises of treating the said hydrocarbon gases with an aqueous amine solution, wherein 2 to about 5000 ppm of amine compound is selected from a group of organic compounds of the formula RNH 2  and R 2 NH, R being selected from the group of alkyl or aryl groups. This patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0013]     Carbohydrazide has been disclosed as useful for inhibiting polymeric fouling deposits during the caustic scrubbing of pyrolytically-produced hydrocarbons contaminated with oxygen-containing compounds in U.S. Pat. No. 5,160,425 to Lewis. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0014]     U.S. Pat No. 5,288,394 to Lewis and Rowe describes a method of inhibiting formation of polymeric fouling deposits after the caustic scrubbing of hydrocarbon stream contaminated with oxygenated compounds. The scrubbing is performed with a basic washing solution having pH more than 7, and comprising at least one hydrazide compound. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0015]     U.S. Pat. No. 5,194,143, granted to Roling describes and claims a method for inhibiting the formation of polymeric based fouling deposits during the basic washings of olefins containing hydrocarbon contaminated with oxygenated compounds comprising adding to the wash about 1 to 10000 ppm acetoacetate ester compound having the formula CH sub 3 COCH sub 2 c sub x H sub, where x is an integer from about 1 to about 8 and y is an integer from about 3 to about 17. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0016]     U.S. Pat. No. 5,220,104 to McDaniel at al. discloses the use of percarbonate salts for inhibition of fouling. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0017]     U.S. Pat No 5,770,041 to Lewis et al. describes the use of certain aldehydic compounds without alpha hydrogen atom or the use of non-enolizable aldehydes like formaldehyde, glyoxal and the like as aldol inhibitor. In this case the inhibitors are to be used at least thrice the molar ratio per mole of carbonyl species. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0018]     U.S. Pat No 5,710,455 to Bhatnagar et al. discloses the use of certain organic amine inhibitors like sulfanilic acid for inhibiting the aldol condensation. Similarly, this patent does not discuss the problems related to removal of already existing polymers, nor does it provide solution for removal of carbonyl compounds and polymers thereof which are unscavenged during the inhibition process.  
         [0019]     All the patents of the prior art discussed above discuss treatment of the caustic with injection of a compound only to inhibit polymer formation by aldol condensation mechanism. They do not, however, solve the problem of removal of the polymers already present in the system.  
         [0020]     Apart from the above mentioned disadvantages there are serious technical problems that exist with the prior art. For instance, one serious technical problem is the extremely rapid polymer formation; it typically takes place within few minutes, rendering impossible the complete scavenging of the carbonyl compounds by any known polymerization inhibition process. Obviously, an important requirement for inhibition of polymerization is that the inhibitor be present in the caustic tower before the carbonyl compounds enter the tower. Delay in supply of the inhibitor or incomplete availability of the inhibitor in the caustic tower will cause the carbonyl compounds to polymerize within few minutes, a process which is very detrimental to the unit leading to fouling. The unscavenged part of carbonyl compounds polymerizes and deposits on the trays, leading to fouling and plugging of the equipment, and eventually to equipment failure.  
         [0021]     Another important technical problem is that the inhibitor used by other researchers can react only with nonpolymerized carbonyl compounds and with very low molecular weight caustic soluble species (2 or 3 repeating units of acetaldehyde), but not with high molecular weight polymers (having greater than 3 repeating units of acetaldehyde). It is precisely the high molecular weight polymers that are insoluble in the caustic system, thereby depositing and fouling the equipment. Hence there is a need to develop a method which that will not only inhibit the formation of polymers, but will also lead to dissolving the polymers already existing in the caustic tower and its downstream units.  
       SUMMARY OF THE INVENTION  
       [0022]     During scrubbing operations in the caustic or amine towers in the chemical industry, condensation reactions of carbonyl compounds lead to formation of polymers, which further leads to fouling and obstructing the flow of liquid through the system. The present invention provides a method to mitigate fouling that occurs due to polymerization of carbonyl compounds. This mitigation is achieved by using certain inorganic salts like sodium dithionite, sodium metabisulphite, certain amino acids like amino caproic acid, sulfanilic acid, and combinations thereof as additives for washes of caustic towers. The compounds disclosed in this invention inhibit polymer formation. The compounds scavenge the carbonyl compounds thereby inhibiting polymerization; in addition, the compounds dissolve polymers formed as the result of the reaction, as well as the polymers already existing in the caustic tower. Furthermore, the present invention can be used along with the caustic in the caustic tower, i.e., the compounds of the invention can be premixed with the caustic used for making the scrubbing solution.  
         [0023]     Accordingly, one objective of the present invention is the inhibition of the formation of polymers of carbonyl compounds in a caustic scrubber; a benefit of this is the inhibition of fouling that occurs due to oxygenated hydrocarbons. Another objective of the invention is to dissolve the polymers in a caustic scrubber which are formed in spite of the inhibitory action, as well as to dissolve the polymers that exist in the scrubber. One objective of the invention is to reduce the concentration of oxygenated hydrocarbons, particularly carbonyl compounds in caustic or amine towers used in the chemical industry, and equipment and products thereof. Yet another objective of the invention is to scavenge oxygenated hydrocarbons without posing polymerization problems and without interfering with plant operations, nor with individual process operations. Still further objective of the invention is to provide an inventive product which can be premixed with the caustic that is used for making the scrubbing solution. Yet further objective is to develop an inventive combination of chemicals that react with nonpolymerized carbonyl compounds, with low molecular weight species, and with high molecular weight carbonyl polymers, such that the reacted adduct is soluble in the caustic solution, thereby preventing plugging and fouling of the equipment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIGS. 1 and 2  depict chemical compounds useful in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The present invention discloses a method of inhibiting polymer formation and also for dissolving polymers formed during reaction, as well as dissolving of existing polymers. The invention is directed toward inhibition of polymerization of carbonyl compounds, both low molecular weight species and high molecular weight polymers. Such polymers lead to deposit formation in caustic or alkaline scrubbers that are used for scrubbing acidic gases (e.g., carbon dioxide, hydrogen sulfide) from the effluent streams formed during pyrolytic cracking of hydrocarbons like naphtha, ethane, and propane. The cracking operations also produce oxygenated compounds such as vinyl acetate or acetaldehyde, which undergo polymerization under the alkaline conditions in the scrubber. Upon hydrolysis under alkaline conditions, vinyl acetate releases acetaldehyde, which further contributes to the buildup of polymeric deposits.  
         [0000]     Use of Inorganic Salts Such as Sodium Dithionite  
         [0026]     The most preferred embodiment of the present invention includes a method wherein certain inorganic salt, like sodium dithionite, is introduced into the feed stream to caustic wash unit system, in order to mitigate fouling. The addition of sodium dithionite causes mitigation of fouling through a dual function. First, sodium dithionate inhibits polymerization, by dissolving the polymers formed during the reaction. If a polymer is formed by escaping the inhibition action or if a polymer already exists in the system before the addition of the additive, sodium dithionite dissolves the same. Second, addition of sodium dithionite mitigates fouling by dissolving the polymers already existing in the caustic scrubber.  
         [0027]     In this most preferred embodiment of the present invention the inorganic salt like sodium dithionite should be added to the alkaline scrubber in an amount wherein a molar ratio of carbonyl compound to inorganic salt is from about 1:0.01 to about 1:25 mole, preferably from about 1:0.05 to about 1:0.005 mole, and more preferably from about 1:1 to about 1:0.01 mole. The preferred amount of additive ranges from about 0.5 to about 1,000,000 parts of additive per one million part of the aqueous scrubbing medium used in the caustic wash unit system; more preferably, the amount of additive ranges from about 25 to about 200 ppm. The sodium dithinoite as an additive can be added as neat product or in any form available commercially, or as a solution in water or alkali.  
         [0028]     Referring to experiments 1 and 2 in Table 1, the use of 0.15 M sodium dithionite yields relative transmittance value T of 68.5%, as compared to the 0.5% T value of the blank without inhibitor. The blank run in the example corresponds to the amount of unscavenged carbonyl low molecular weight species, high molecular weight polymers, and already existing polymer. This example demonstrates the efficiency of sodium dithionite in inhibiting polymer formation.  
         [0029]     Referring to experiments 1 and 2 in Table 3, the T value is 0.5% for the blank and 62.5% for sodium dithionite. This example suggests the effect of addition of sodium dithionite on dissolving of polymer deposits. Those skilled in the art are aware that even a slight delay in the addition of additive leads to formation and deposition of polymers. Sodium dithionite has the ability to dissolve deposited polymers.  
         [0000]     Use of Combinations of Inorganic Salt Like Sodium Dithionite and Organic Aliphatic Amino Acid and its Derivatives  
         [0030]     Another embodiment of the present invention includes a method wherein said inorganic salt sodium dithionite is blended in synergistic combination with aliphatic amino acid, including but not limited to 6 amino caproic acid, to mitigate polymerization effects in a caustic wash unit system. The blend of sodium dithionite and aliphatic amino acids mitigates fouling through a dual function, of (i) inhibiting polymer formation and dissolving polymers formed during the reaction, as well as (ii) dissolving polymers already existing in the caustic scrubber.  
         [0031]     In this embodiment of the present invention the blend of inorganic salt (like sodium dithionite) and the aliphatic amino acid (like 6 amino caproic acid) is added to the alkaline scrubber in an amount wherein a molar ratio of carbonyl compound to said blend is from about 1:0.01 to about 1:25 mole, preferably from about 1:0.05 to about 1:0.005 mole, and more preferably from about 1:1 to about 1:0.01 mole. The amount of additive ranges from about 0.5 to about 1,000,000 parts of inhibitor per one million part of the aqueous scrubbing medium used in the caustic wash system, more preferably, the amount of additive ranges from about 25 to about 200 ppm.  
         [0032]     The combination of inorganic salt and the aliphatic amino acids can be added either as blend or as individual components in neat or solution form. The amino acid can be added either as neat product or as an aqueous solution containing from about 0.05 to about greater than about 60 weight percent, preferably from about 18 to about 38 weight percent. Amino acids that are particularly suited for use in the accordance with this embodiment of the invention include, but are not limited to, 6 amino acid such as the amino hexanoic acid made from epsilon caprolactam, glycine, or taurine, or any compound having one of the structures described in  FIGS. 1 and 2 . Also suitable are the derivatives, isomers, and inorganic or organic salts of these compounds. The amino acids mentioned above can be used in its salt form or as pure amino acid or impure form or combinations thereof.  
         [0033]     Referring to experiments 1, 2, and 3 in Table 2, the results further support the advantageous synergistic effect of the compounds used to inhibit polymerization. With 0.1 M sodium dithionite, the relative transmittance value T is 18.5%. With 0.026 M amino caproic acid, the relative transmittance value T is 0.3%; however, the 0.126 M combination of sodium dithionoite and amino caproic acid yields relative T value of 82%. The experiment 4 in Table 2 shows that the amino caproic acid synthesized from caprolactam shows similar behavior. These experiments demonstrate the advantage of combining sodium dithionite and amino caproic acid to inhibit polymer formation.  
         [0034]     Referring to experiments 1, 2, 3 and 4 in Table 3, the combination of sodium dithionite and amino caproic acid also acts synergistically in dissolving the polymer. The T value for individual components is 62.5% for 0.5 M sodium dithionite, and 0.5% for 0.087 M amino caproic acid, but the blend of the two compounds at a total mole ratio of 0.5879 has a T value of 77.05%. This example demonstrates the advantageous effect of combining sodium dithionite and amino caproic acid to inhibit polymerization and to dissolve polymer residues.  
         [0000]     Use of Combinations of Inorganic Salts Like Sodium Dithionite and Lactam and its Derivatives  
         [0035]     Further embodiment of the present invention includes a method, wherein blends of inorganic salt like sodium dithionite and certain lactams, including but not limited to epsilon caprolactam, are used to mitigate the effects of polymerization in the caustic wash unit system. Thus a blend of inorganic salts like sodium dithionite and lactam, particularly epsilon caprolactam, not only inhibits polymer formation but also dissolves the polymers already existing in the caustic scrubber.  
         [0036]     In this embodiment of the present invention the blend of inorganic salt (like sodium dithionite) and the lactam (like epsilon caprolactam) is added to the alkaline scrubber in an amount wherein a molar ratio of carbonyl compound to said blend is from about 1:0.01 to about 1:25 mole, preferably from about 1:0.05 to about 1:0.005 mole, and more preferably from about 1:1 to about 1:0.01 mole. The amount of additive ranges from about 0.5 to about 1,000,000 parts of inhibitor per one million part of the aqueous scrubbing medium used in the caustic wash system; more preferably, the amount of additive ranges from about 25 to about 200 ppm. Inorganic salts and the lactam or sultam can be added either individually or in combination, in neat or solution form. The lactam can be added either as neat product or as an aqueous solution containing from about 0.05 to about greater than about 60 weight percent, preferably from about 18 to about 38 weight percent. Lactams that are particularly suited for use in the accordance with this embodiment of the invention include, but are not limited to, epsilon caprolactam. However, any compound having one of the structures described in  FIGS. 1 and 2  should be effective. Also suitable are the derivatives, isomers, and inorganic or organic salts of these compounds.  
         [0037]     When amino acids or lactams are used along with the inorganic salts, particularly sodium dithionite, they react with unscavenged carbonyl compounds, low molecular weight species, high molecular weight polymers and with already existing polymers. Acting synergistically, the amino acid and the inorganic salt solubilize the polymers formed and prevent precipitation and fouling of the equipment.  
         [0038]     Referring to experiments 1, 7, and 8 in Table 2, a highly synergistic effect on inhibition of polymerization is seen between sodium dithionite and caprolactam. Individually used 0.1 M sodium dithionite has a T value of 18.5%; individually used 0.3 M caprolactam has a T value of 0.4%. Shown in experiment eight, 0.25 M caprolactam and sodium dithionite has a T value of 82.9%. This shows the advantage of combining sodium dithionite and caprolactam in inhibiting polymerization.  
         [0039]     Experiments no 2, 6 and 7 in Table 3 show a synergistic effect of sodium dithionite and caprolactam on dissolving of polymers. Used individually, 0.5 M sodium dithionite has a relative T value of 62.5%; used individually, 0.3 M caprolactam has a T value of 11.4%. The combination of both sodium dithionite and caprolactam has a T value of 69.7%. This shows the advantage of combining sodium dithionite and caprolactam in dissolving the existing polymers.  
         [0000]     Use of Combinations of Inorganic Salt Like Sodium Dithionite and Aromatic Amino Acid and its Derivatives  
         [0040]     Yet another embodiment of the present invention includes a method wherein, to mitigate the effects of polymerization in the caustic wash unit system, sodium dithionite is blended in synergistic combination with aromatic amino acids including, but not limited to, sulfanilic acid. The blends of inorganic salt like sodium dithionite and aromatic amino acids, particularly sulfanilic acid, inhibit polymer formation. In this embodiment of the present invention the blend of inorganic salt, like sodium dithionite, and the aromatic amino acid, like sulfanilic acid, are added to the alkaline scrubber in an amount wherein the molar ratio of carbonyl compound to said blend is from about 1:0.01 to about 1:25 mole, preferably from about 1:0.05 to about 1:0.005 mole, and more preferably from about 1:1 to about 1:0.01 mole. The amount of additive ranges from about 0.5 to about 1,000,000 parts of inhibitor per one million part of the aqueous scrubbing medium used in the caustic wash system, more preferably the amount of additive ranges from about 25 to about 200 ppm. The inorganic salt and the aromatic amino acids can be added either as blend or as individual components in neat or solution form. The aromatic amino acid can be added either as neat product or as an aqueous solution containing from about 0.05 to about greater than about 60 weight percent, preferably from about 18 to about 38 weight percent. Aromatic amino acids that are particularly suited for use in the accordance with this embodiment of the invention include, but are not limited to, aromatic amino acid such as sulfanilic acid, or any compound having one of the structures described in  FIGS. 1 and 2 . Also suitable are the derivatives, isomers, and inorganic or organic salts of these compounds. The aromatic amino acids mentioned above can be used in its salt form or as pure aromatic amino acid or impure form or combinations thereof.  
         [0041]     Referring to experiment 1, 5, 6 of table 2, a synergistic effect of inhibition of polymerization is seen when sulfanilic acid and sodium dithionite are used. The relative T value of individually used 0.1 M sodium dithionite is 18%, for 0.16 M sulfanilic acid T is 0.2%, whereas 0.26 M of the blend has a T value of 86.7%. This shows the advantage of combining sodium dithionite and sulfanilic acid to inhibit polymer formation.  
         [0000]     Use of Inorganic Salt Like Sodium Metabisulphite  
         [0042]     Still another embodiment of the present invention includes a method wherein certain inorganic salt, like sodium metabisulphite, is introduced into the feed stream to a caustic wash unit system to mitigate fouling, by inhibiting polymer formation. In this embodiment of the present invention the inorganic salt, like sodium metabisulphite, is added to the alkaline scrubber in an amount wherein the molar ratio of carbonyl compound to inorganic salt is from about 1:0.01 to about 1:25 mole, preferably from about 1:0.05 to about 1:0.005 mole, and more preferably from about 1:1 to about 1:0.01 mole. The preferred amount of additive ranges from about 0.5 to about 1,000,000 parts of additive per one million part of the aqueous scrubbing medium used in the caustic wash unit system, more preferably the amount of additive ranges from about 25 ppm to about 200 ppm. The sodium metabisulphite as an additive can be added as neat product or in any form available commercially or as a solution in water or alkali.  
         [0043]     Also referring to the experiments 10, 14, and 7 in Table 1, 0.125 M of sodium bisulphite has a relative T value of 2.8%, 0.125 M of sodium sulphite has a T value of 0.35%, whereas 0.125 M of sodium metabisulphite has a T value of 80.7%. These experiments demonstrate the superiority of using sodium metabisulphite to inhibit polymer formation.  
         [0000]     Use of Combinations of Inorganic Salts Like Sodium Dithionite and Sodium Metabisulphite  
         [0044]     Yet another embodiment of the present invention includes a method wherein said inorganic salt sodium dithionite is blended in synergistic combination with another inorganic salt like sodium metabisulphite, to mitigate the effects of polymerization in the caustic wash unit system. In this embodiment of the present invention the blend of inorganic salts, like sodium dithionite and sodium metabisulphite, is added to the alkaline scrubber in an amount wherein the molar ratio of carbonyl compound to said blend is from about 1:0.01 to about 1:25 mole, preferably from about 1:0.05 to about 1:0.005 mole, and more preferably from about 1:1 to about 1:0.01 mole. The amount of additive ranges from about 0.5 to about 1,000,000 parts of inhibitor per one million part of the aqueous scrubbing medium used in the caustic wash system, more preferably the amount of additive ranges from about 25 to about 200 ppm. The inorganic salts can be added as a blend or as individual components. The salts can be added either as neat products, as aqueous solutions, or as alkaline solutions or blends thereof.  
         [0045]     Referring to experiment 1, 9, 12 in table 2, a synergistic effect of polymer inhibition is seen between sodium metabisulphite and sodium dithionite. The % T value of the individual components that is sodium dithionite in molar of 0.1 is 18 , for sodium metabisulphite in the mole ratio of 0.0.09 the % T value is 35 where as the blend at the sum total mole ratio of 0.19 has a % T value of 93%. Thus this proves the excellent efficiency of combination of sodium dithionite and sodium metabisulphite in effecting inhibition of polymer formation.  
         [0000]     Premixed Additives for Caustic Wash Unit Systems Serve Carbonyl Scavenging Function  
         [0046]     Yet further embodiment of the present invention includes a method of converting the usual caustic wash unit system, commonly known as caustic tower, into a carbonyl scavenging tower. This conversion can be achieved by premixing the additives described above, either individually or as combinations of compounds, with the caustic solution, before the caustic solution is admitted into the caustic tower. In the current practice, the additives are externally added to the tower by a separate supply unit. One disadvantage of this practice is that the tower may run only with caustic solution without additive in case of failure of the unit that supplies the additive. Those skilled in the art are aware that even a minor delay is detrimental for the unit because the polymer formation of the carbonyl compounds is extremely rapid and takes place within a few minutes.  
         [0047]     To serve the purpose of the invention, the additive should be stable in the caustic solution for reasonably long period of time. Referring to examples 4 and 5, the inorganic salts like sodium dithionite and sodium metabisulphite, once added to the caustic solution, effectively prevent polymer formation for up to 20 days. This is a very economical solution for scavenging of carbonyl compounds in the petrochemical industries.  
         [0048]     For purposes of this invention, low molecular weight species are defined as polymers having 2 or 3 repeating units of acetaldehyde, whereas high molecular weight polymers are defined as polymers having greater than 3 repeating units of acetaldehyde.  
         [0049]     The following Examples are merely illustrative of some embodiments of the present invention and the manner in which it is can be performed, and are not intended to limit the scope of the claimed invention in any way:  
       EXAMPLE 1  
       [0050]     Caprolactam (18 g, 0.1593 mole), sodium hydroxide (7 g, 0.175 mole), and 75.0 g water were added to a clean round bottom flask equipped with a thermometer, stirrer and condenser. The mixture was well agitated and heated to 105-120° C. for a period of six hours. Small samples were periodically withdrawn and checked for conversion using HPLC. The conversion of epsilon caprolactam to six amino hexanoic acid was greater than 75%.  
       EXAMPLE 2  
       [0051]     Twenty ml of 10% NaOH solution were added to a 50 ml stoppered conical flask. Desired inhibitor in solution or in solid form was also added, followed by the addition of 1 ml vinyl acetate. The mixture was shaken well and kept in an oven for 24 hours at 55° C. A blank was prepared wherein all reagents except the inhibitor were added. At the end of 24 hours the contents of the flask were visually checked for clarity or any deposits, and UV readings were measured. The results, as an average of two or free reading, are shown in the Tables below.  
                                                                           TABLE 1                           Use of individual compounds to inhibit polymer formation.                            Transmittance   Relative           Expt           Mole   at 800 nm   absorbance           No.   Compounds   gms   ratio   (T %)   at 720 nm   Observation                     1   Blank   nil   nil   0.5   2.5   Red hazy liquid                               with precipitate        2   Sodium   0.2075   0.125   68.5   0.2575   Red slightly hazy           dithionite                   liquid        3   Sodium   0.4715   0.250   89.55   0.0475   Red clear liquid           dithionite        4   Sodium   0.943   0.5   90   0.042   Faint red clear           dithionite                   liquid        5   Sodium   1.886   1.0   80.35   0.09   Colorless liquid           dithionite        6   Sodium   0.1281   0.0625   0.466   2.54   Red hazy liquid           metabisulphite        7   Sodium   0.2562   0.125   80.7   0.179   Red clear liquid           metabisulphite        8   Sodium   0.549   0.25   86.4   0.113   Red clear liquid           metabisulphite        9   Sodium   2.089   1.0   87.3   0.066   Red clear liquid           metabisulphite       10   Sodium   0.140   0.125   2.8   1.80   Hazy red liquid           bisulphite       11   Sodium   0.338   0.30   86.1   0.133   Red clear liquid           bisulphite       12   Sodium   0.563   0.50   86   0.1035   Red clear liquid           bisulphite       13   Sodium   1.127   1.0   89.1   0.092   Red clear liquid           bisulphite       14   Sodium sulphite   0.170   0.125   0.35   2.653   Red hazy liquid       15   Sodium sulphite   0.682   0.5   83.6   0.130   Red clear liquid       16   Sodium sulphite   1.365   1.0   89.2   0.08   Red clear liquid       17   Sodium sulfate   1.539   1.0   5.9   1.904   Hazy liquid with                               gummy polymer       18   Sodium   1.496   1.0   4.4   1.332   Same as above           hydrogen sulfate                  
 
         [0052]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 Use of blends to inhibit polymer formation 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                 Transmittance 
                 Relative 
                   
               
               
                 Expt 
                   
                   
                 Moles 
                 Moles 
                 Total 
                 at 800 nm 
                 absorbance 
                   
               
               
                 No. 
                 Compounds 
                 Gms 
                 of 
                 of 
                 moles 
                 (T %) 
                 at 720 nm 
                 Observation 
               
               
                   
               
             
          
           
               
                  1 
                 Sodium 
                   
                 0.1 
                 — 
                 0.1 
                 18.5 
                 0.842 
                 Red hazy 
               
               
                   
                 Dithionite 
                   
                   
                   
                   
                   
                   
                 liquid 
               
               
                  2 
                 Amino 
                 0.0372 
                 — 
                 0.026 
                 0.026 
                 0.3 
                 2.571 
                 Closer to 
               
               
                   
                 caproic acid 
                   
                   
                   
                   
                   
                   
                 blank 
               
               
                  3 
                 Sodium 
                 0.1886 
                 0.1 
                   
                 0.126 
                 82 
                 0.183 
                 Red clear 
               
               
                   
                 Dithionite + 
                   
                   
                   
                   
                   
                   
                 transparent 
               
               
                   
                 Amino 
                 0.0372 
                   
                 0.026 
                   
                   
                   
                 liquid 
               
               
                   
                 caproic acid 
               
               
                  4 
                 Sodium 
                 0.1886 
                 0.1 
                   
                 0.126 
                 81.0 
                 0.210 
                 Red clear 
               
               
                   
                 dithionite + 
                   
                   
                   
                   
                   
                   
                 transparent 
               
               
                   
                 Product of 
                 0.09 
                   
                 0.026 
                   
                   
                   
                 liquid 
               
               
                   
                 Example 1 
                 ml 
               
               
                  5 
                 Sodium 
                 0.1886 
                 0.1 
                   
                 0.260 
                 86.7 
                 0.1195 
                 Red clear 
               
               
                   
                 dithionite + 
                   
                   
                   
                   
                   
                   
                 transparent 
               
               
                   
                 sulfanilic acid 
                 0.3 
                   
                 0.160 
                   
                   
                   
                 liquid 
               
               
                  6 
                 Sulfanilic acid 
                 0.30 
                   
                 0.160 
                 0.160 
                 0.2 
                 2.872 
                 Red brown 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 hazy with 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 particles 
               
               
                  7 
                 caprolactam 
                 0.3673 
                   
                 0.3 
                 0.3 
                 0.4 
                 2.783 
                 Dark red 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 slight hazy 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 liquid with 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 some dispersed 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 particles 
               
               
                  8 
                 Caprolactam + 
                 0.1836 
                   
                 0.15 
                 0.250 
                 82.9 
                 0.176 
                 Red clear 
               
               
                   
                 sodium 
                 0.1886 
                 0.1 
                   
                   
                   
                   
                 transparent 
               
               
                   
                 dithionite 
                   
                   
                   
                   
                   
                   
                 liquid 
               
               
                  9 
                 Sodium meta 
                 0.1853 
                   
                 0.09 
                 0.190 
                 93.3 
                 0.067 
                 Faint Red 
               
               
                   
                 bisulphite + 
                   
                   
                   
                   
                   
                   
                 clear 
               
               
                   
                 sodium 
                 0.1866 
                 0.1 
                   
                   
                   
                   
                 transparent 
               
               
                   
                 dithionite 
                   
                   
                   
                   
                   
                   
                 liquid 
               
               
                 10 
                 Sodium 
                 0.1132 
                 0.06 
                   
                 0.123 
                 81 
                 0.285 
                 Red clear 
               
               
                   
                 dithionite + 
                   
                   
                   
                   
                   
                   
                 transparent 
               
               
                   
                 Amino 
                 0.0888 
                   
                 0.0625 
                   
                   
                   
                 liquid 
               
               
                   
                 caproic acid 
               
               
                 11 
                 Amino 
                 0.0888 
                   
                 0.0625 
                 0.0625 
                 0.3 
                 2.872 
                 Same as 
               
               
                   
                 caproic acid 
                   
                   
                   
                   
                   
                   
                 blank 
               
               
                 12 
                 Sodium 
                   
                 0.09 
                   
                   
                 35 
                   
                 Red hazy 
               
               
                   
                 metabisulphite 
                   
                   
                   
                   
                   
                   
                 liquid with 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 polymer 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 particles 
               
               
                   
               
             
          
         
       
     
       EXAMPLE 3  
       [0053]     Twenty ml of 10 % NaOH solution were pipetted into a 50 ml stoppered conical flask. One ml of vinyl acetate solution was added. The mixture was shaken well and kept in an oven for 15 minutes. During this period, the vinyl acetate was hydrolyzed and polymerized to form insoluble products. After 15 minutes the desired amount of inhibitor was added. One control sample was prepared without inhibitor. The flask was shaken well and kept in an oven for 24 hours. After 24 hours, the flask was checked visually for clarity and for any deposits. In some cases, UV transmittance was measured for comparison.  
                                                                                           TABLE 3                           Use of compounds and their blends to dissolve formed polymers                                    Transmittance   Relative           Expt               Moles   Total   at 800 nm   absorbance           No.   Compounds   gms   moles   of   moles   (T %)   at 720 nm   Observation                     1   blank   nil   nil   nil   nil   0.5   2.5   Red turbid                                       liquid with                                       polymer                                       particles        2   Sodium   0.943   0.5   —   0.5   62.5   0.202   Yellow           Dithionite                           clear liquid                                       with few                                       particles        3   Amino   0.125       0.087   0.087   0.5   2.709   Same as           caproic acid                           above        4   Sodium   0.943   0.5       0.5879   77.05   0.1125   Red clear           Dithionite +                           transparent           Amino   0.125       0.0879               liquid           caproic acid        5   Sodium   0.943   0.5       0.5879   85.7   0.07   Red clear           dithionite +                           transparent           Product of   0.3   0.087                   liquid           Example 1   ml        6   caprolactam   0.3673   0.3       0.3   11.4   1.095   Hazy red                                       liquid with                                       particles        7   Sodium   0.943   0.5       0.8   69.7   0.171   Clear red           dithionite +                           liquid with           caprolactam   0.3673       0.3               few particles        8   caprolactam   0.6122   0.5       0.5   64.0   0.387   Dark red                                       liquid with                                       few particles        9   caprolactam   1.224   1.0       1.0   70.0   0.315   Red clear                                       liquid       10   Sodium   1.127   1.0       1.0   10.7       Brown hazy           bisulphite                           liquid with                                       heavy polymer                                       particles       11   Sodium   1.0   1.0       1.0   18.7   0.729   Brown hazy           metabisulphite                           liquid with                                       heavy polmer                                       particles       11   Sodium   1.365   1.0       1.0   4.3   1.389   Brown hazy           sulphite                           liquid with                                       heavy polymer                                       particles                  
 
       EXAMPLE 4  
       [0054]     The stability of the caustic solution was also tested. For this experiment, 0.3 mole strength of sodium dithionite was prepared in 10% NaOH solution. The transparency of this sodium dithionite solution was periodically tested. To 20 ml of the solution, 1 ml vinyl acetate was added. The flask was shaken well and kept in the oven at 55 ° C. for 24 hours. The detailed results are listed in Table 4 below.  
                                                           TABLE 4                           Test of the stability of the caustic solution                    Relative                       transmittance   Relative       Sr.       (T %) at   absorbance       No.   Hours   800 nm   at 720 nm   Observation                    1    24   90.3   0.122   Red clear transparent liquid       2   192   85.3   0.145   Red clear transparent liquid       3   240   87.9   0.140   Red clear transparent liquid       4   360   90.9   0.129   Red clear transparent liquid       5   480   87.3   0.142   Red clear transparent liquid                  
 
       EXAMPLE 5  
       [0055]     The effect of sodium metabisulphite was also tested. In that case, 0.2 M of sodium dithionite was prepared in 10% NaOH solution. The transparency of this sodium dithionite solution was periodically tested. To 20 ml of the solution 1 ml vinyl acetate was added and shaken well. The flask was kept in the oven at 55 deg c for 24 hrs. The details of the result are listed in the Table 5 given below  
                                     TABLE 5                           Effect of sodium metabisulphite.                    Relative                       transmittance   Relative       Sr.       (T %) at   absorbance       No.   Hours   800 nm   at 720 nm   Observation               1    24   90.7   0.095   Red clear transparent liquid       2    72   90.8   0.109   Red clear transparent liquid       3   168   90.3   0.105   Red clear transparent liquid       4   216   88.7   0.118   Red clear transparent liquid       5   336   90.4   0.116   Red clear transparent liquid                  
 
         [0056]     While the present invention has been described herein in terms of various embodiments, one of ordinary skill in the art will recognize that modification to the embodiments can be made without departing from the scope of the claimed invention. While the above description contains many specificities, these should not be construed as limitations in the scope of the invention but rather as exemplifications of different embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated but by the appended claims and their legal equivalents.