GAS AND ALCOHOL RECOVERY PROCESS

Abstract: A process for the recovery of alcohols and hydrogen sulfide from a thiophosphate containing mixture is disclosed.

Description: The present invention describes a process for recovering alcohols from a mixture of thermally sensitive organic phosphorus-containing compounds and alcohols. Hydrogen sulfide is also removed in this process. It is known from United States Patent 4,263,150, that sulfur-containing salts of phosphorus acids may be prepared. Similar processes to obtain dialkylphosphorodithioic acid salts are described in United States Patent 4,308,154 The sulfur-containing salts of dialkylphosphorodithioic acids are typically prepared by contacting e.g. phosphorus pentasulfide with an alcohol corresponding to the alkyl group in the dialkylphosphorodithioic acid. Typically, the alcohol is used in excess and is carried throughout the reaction and eventually recovered following the conversion of the dialkylphosphorodithioic acid to a corresponding metal salt. Hydrogen sulfide is present in the initial reaction mixture as a by-product. The preparation of the metal salts of the dialkylphosphorodithioic acids involves the generation of water. The neutralization of the dialkylphosphorodithioic acid with, for example, zinc oxide results in the generation of water. Steam treatment of the reaction mixture is also employed to assist in the stripping of excess alcohol. However, the water then contaminates the stripped alcohol. The effect of the steam processing is to further introduce water to the reaction mixture. Thus, the neutralization of the dialkylphosphorodithioic acids and the steam treatment amount to about 5 parts of water per 100 parts of the salt. The presence of the water, from whatever source, the alcohol, and hydrogen sulfide present various processing difficulties. Any excess alcohol will be difficult to strip from the water. Where azeotropic conditions exist, it will not be possible without undue difficulty to remove the residual alcohol from the water. Conversely, the problem also exists in that some of the water which is bound to the alcohol is likely to remain in the alcohol following stripping. As the largest use of the dialkylphosphorodithioic salts is in lubricants, it is undesirable to have any water carried into the end product. The lower alcohols which are normally water-miscible and processed as described herein may be directly recycled to the reactor to generate more dialkylphosphorodithioic acid. Water wet lower alcohols may not be reused and are not commercially dryable. Excess hydrogen sulfide gas can result in zinc sulfide being formed thereby causing an inorganic salt to be present as well as loss of conversion to the desired zinc salt. It has been proposed for the recovery of polycyclic alcohols to employ a thin-film evaporator. The foregoing technology is reported in United States Patent 4,263,103. Similar technology with regard to thermally stabilizing alcohols is found in United States Patent 4,198,276. United States Patent 4,209,471, states that a liquid-liquid extraction method is preferable to using a thin film or batch distillation process because of product stability and high vacuum requirements. Various devices are known for use in thin film evaporation of organic materials. Such equipment is described in a reprinted article entitled "Residence Time Control in Thin-Film Evaporators" by Widmer and Giger which was originally printed in Chemical and Process Engineering , November 19, 1970. A further disclosure of evaporation techniques and equipment is found in an article entitled "Agitated Thin-Film Evaporators" originally published in Chemical Engineering , September 13, 1965. Ordinarily, the processing of oil soluble components in a thin-film evaporator is conducted on materials which are extremely viscous or even solid at room temperature (20°C). In the present invention, the dialkylphosphorodithioic acids and excess alcohols which are processed according to the invention are normally liquid materials at temperatures of -40°C. The present invention deals with the separation of excess alcohol from dialkylphosphorodithioic acids prior to the neutralization of the acid to the corresponding salt. Blowing with dry nitrogen to enhance water removal may be avoided. Water contamination of the alcohol is also avoided. Steam treatment to remove higher alcohols is avoided. Hydrogen sulfide gas is separated and may be recovered. The formation of zinc sulfide is avoided herein. Water haze in the end product is also minimized. Throughout the specification and claims, percentages and ratios are by weight, temperatures are in degrees Celsius and pressures are in KPa gauge unless otherwise noted. Ratios and percentages herein are exemplary and may be combined. To the extent that the references cited herein are relevant to the present invention, they are herein incorporated by reference. SUMMARY OF THE INVENTION The present invention describes a process for recovering alcohols from a mixture of the alcohol and a phosphorus compound of the formula: (R¹O)(R²O)P(S)SH where R¹ and R² are each aliphatic or aromatic groups, wherein the mixture is preheated to a temperature of about 20°C to about 115°C, and thereafter rapidly adjusting the mixture to a temperature of about 80°C to about 180°C for a period of time sufficient to substantially separate the alcohol from the phosphorus compound without the substantial generation of hydrogen sulfide. A further aspect of the invention is a process for removing hydrogen sulfide from a mixture of the hydrogen sulfide and a phosphorus compound of the formula: (R¹O)(R²O)P(S)SH where R¹ and R² are each aliphatic groups containing from 1 to about 10 carbon atoms, wherein the mixture is pre-heated to a temperature of about 20°C to about 115°C, and thereafter rapidly adjusting the mixture to a temperature of about 80°C to about 180°C for a period of time sufficient to substantially separate the hydrogen sulfide from the phosphorus compound without the substantial generation of additional hydrogen sulfide. The di-substituted phosphorodithioic acids are prepared in the following manner. To prepare a di-substituted dithiophosphoric acid of the formula (R¹O)(R²O)P(S)SH, a material such as phosphorus pentasulfide is condensed with an alcohol. The moieties R¹ and R² are the hydrocarbyl residues of the alcohol. In the process of the present invention, R¹ and R² are each conveniently aliphatic or aromatic groups or mixtures of the two. In the course of reacting the phosphorus pentasulfide with the alcohol, it is convenient to use an excess of the alcohol to ensure complete formation of the reaction product. Typically, an excess of 10% to 12%, preferably not more than 20% of the alcohol is utilized over the stoichometric weight amount required to prepare the di-substituted dithiophosphoric acid. The alcohol is conveniently removed from a mixture containing 1-20% alcohol. About 80% to 100% by weight of the excess alcohol originally present may be removed. The alcohols which may be utilized may be of any chain length, however, the alcohols are preferably aliphatic, most preferably alkyl. The alcohols as a preferred source of R¹ and R² in the present invention are branched or straight chain alcohols having from 1 to about 10, preferably 3 to about 8 carbon atoms. Particularly preferred values for R¹ and R² are derived from isopropyl alcohol, isooctyl alcohol, amyl alcohol, isobutyl alcohol, methylpentyl alcohol, or 2-ethylhexyl alcohol. Aromatic groups which may be utilized include phenol, cresol or alkylated phenols, typically those containing from about 1 to about 15 carbon atoms in the alkyl portion of the alkyl phenol. In the process of the present invention the mixture of the phosphorus compound as described above and the excess alcohol and/or the hydrogen sulfide is preheated to a temperature from about 20°C to about 115°C, preferably from about 28°C to about 110°C. Thereafter the mixture is rapidly adjusted to a temperature of 80°C to 180°C, preferably about 105°C to about 160°C. In the use of the term "adjusting", it is generally understood that the adjustment of the temperature will be upward and will be done on as rapid a bases as is possible. It is possible, however, to adjust the temperature to a lower temperature and to utilize vacuum conditions to obtain the separation of the alcohol and/or hydrogen sulfide from the phosphorus compound. The preferred equipment for use in the process of the present invention is either a thin film evaporator or a wiped film evaporator. The basic difference between a thin film evaporator and a wiped film evaporator is that the latter scrapes the walls of the evaporator substantially clean of the product whereas the thin film evaporator while using a blade leaves a thin film of the material on the walls of the evaporator. The thin film evaporator is the preferred evaporator in the process of the present invention. Both the thin film evaporator and the wiped film evaporator serve the additional function of agitating the product on the wall of the evaporator to effect a substantially complete removal of the alcohol and the hydrogen sulfide. The excess alcohol and/or the hydrogen sulfide are both valuable by-products. The alcohol may be recycled to prepare additional di-substituted dithiophosphates while the hydrogen sulfide may be utilized for other processes requiring this gas. The following is a description of an agitated thin-film evaporator. Agitated thin-film evaporators are designed to spread a thin layer or film of liquid on one side of a metallic surface, with heat supplied to the other side. The unique feature of this equipment is not the thin film itself but rather the mechanical agitator device for producing and agitating the film. This mechanical agitator permits the processing of high-viscosity liquids, liquids with suspended solids, or those requiring liquid rates too small to keep the thermal surface of, say, a falling-film evaporator uniformly wet (this condition is called surface starving). The agitation produces a complex liquid-flow pattern when compared to "standard" evaporators. The usual flow resulting from gravity or the pumping mechanism is greatly complicated by the action of agitator blades plowing through the liquid film. The film thickness is conveniently about 1.0 mm to about 5 mm, preferably about 1.25 mm to about 2.5 mm. The evaporator is typically constructed such that there are four non-scraping blades (attached to a rotation shaft on the unit) rotating at a speed of 9.1 to 12.2 meters/second (30 to 40 ft./sec.), pushing a bow wave having a zone length determined by product properties and the vertical liquid flow rate. Zone lengths of 2.5 cm to 10.1 cm (1 to 4 in.) are typical. Adjacent to the bow wave is a highly turbulent "squeeze zone" followed by a "tranquilizing" zone; this three-stage cycle is repeated by the next blade attached to the vertical rotor. Energy induced by the rotation blade is mainly consumed by internal friction or turbulence within the liquid. In commercial equipment, this energy is approximately 0.16w/square cm to 0.32w/square cm (150 to 300 w./sq. ft.) . This is equivalent to the pressure drop of a liquid flowing at 3.66 to 7.31 meters/second (12 to 24 ft./sec.) through a 0.635 cm (1/4 inch) I.D. tube with a surface of 929 square cm (1 sq. ft.). These comparable velocities are considerably higher than actual velocities in tube heat exchangers and explain the high heat-transfer values in agitated thin-film evaporators. Agitation has benefits in thin-film equipment other than liquid turbulence. The blades assure even distribution of the liquid over the metal heat-transfer surface; they eliminate any channeling of liquid as the liquid flows down the evaporator; the considerable shearing effect decreases the apparent viscosity of the product, thus improving internal heat and mass transfer. Heat-transfer rates between the liquid and the wall, in most situations, determine the size and effectiveness of thin-film evaporator. In many applications, the low wall temperatures due to high film-side heat transfer rates is important. With temperature differences between the heating medium and the liquid of 80°C to 100°C the product-side wall temperature is usually only 10°C to 20°C above the liquid boiling temperature. By comparison, the falling-film evaporator may have a product-side temperature 20°C to 40°C above the boiling point in a similar situation with the possibility of hot-spots developing on the heat-transfer wall. In many applications of the agitated thin-film evaporator, mass transfer, not heat transfer, determines size and capacity of the evaporator. Deodorization, low-boiler stripping, and dehydration are mass-transfer-controlled processes. A volatile component has to be transported within the film to the interface, then vaporized. Transport of the volatile component within the film is accomplished by molecular diffusion or by eddy diffusion. Molecular diffusion, the only possibility in laminar flow, is extremely slow and decreases linearly with increasing viscosity of the film liquid. Eddy diffusion can be influenced and increased by generating turbulence in the film. Values of diffusivities in agitated thin-film evaporators are on the order of 10⁻⁶ sq. m./sec. or 1,000 to 10,000 times greater than molecular diffusivities. The high treat-transfer rates in agitated thin-film evaporators is an important advantage when carrying out chemical reactions in the evaporator. The high specific heat-transfer area permits accurate control of the reacting temperature. Also, the high rates of mass transfer favor reactions that form a volatile reaction product since removal of the product shifts the reaction equilibrium toward further conversion to the desired product. Countercurrent liquid-vapor flow in an agitated thin-film evaporator can produce a "reflux effect" that will increase stripping or fractionation efficiency. Cocurrent liquid-vapor flow is considered to eliminate this effect entirely. It should be noted that with an internal separator and condenser, the vapor and liquid streams are separated completely and this eliminates the reflux effect. Should there be entrained concentrated liquid in the vapor stream, the separator is designed to discharge it with the balance of the concentrated material. The "fractionation effect" of countercurrent liquid-vapor flow is approximately 0.25 to 0.5 theoretical plates better than the 1-plate efficiency of a straight distillation pot. The residence time for the mixture of the alcohol and the phosphorus compound and/or hydrogen sulfide at the temperature at which the separation of the alcohol and/or hydrogen sulfide is effected is typically from about 5 seconds to about 10 minutes, preferably from about 10 seconds to about 1 minute, and most preferably from about 12 seconds to about 25 seconds. In the foregoing time at the later described pressure conditions typically from about 80% to 100% by weight of the alcohol present in the mixture, preferably from about 85% to about 100% of the alcohol mixture which has been unreacted are removed. Substantially complete removal of the hydrogen sulfide is also effected in the processing of the present invention. It is, of course, not desirable in the present invention to break down the phosphorus compounds to generate alcohol and/or additional hydrogen sulfide. The application of a reduced pressure is typically in the range of about 0.26 KPa to 13.3 KPa (2 mm Hg-100 mm Hg), preferably from about 0.93 KPa to 4 KPa (7 mm Hg-30 mm Hg). It has been found that the application of the reduced pressure substantially increases the yield of the hydrogen sulfide and the unreacted alcohol. The use of reduced pressure allows the alcohol and hydrogen sulfide to be removed without further degradation of the phosphorus compounds. There are five major producers of agitated thin-film evaporator in the U.S.: Buflovak Div. of Blaw-Knox, Votator Div. of Chemetron, Luwa, Kontro, and Pfaudler. The present invention is illustrated by the following example. EXAMPLE I A feed stream comprising diisooctyl dithiophosphoric acid, isooctyl alcohol, and hydrogen sulfide gas as previously described is used. The components are in a respective weight ratio of 91.8:8:0.2. This reaction mixture is maintained at a temperature of about 110°C. As evaporator a Luwa™ thin-film evaporator, model number L210-1600-15, is employed. The separation between the walls of the still and the rotor blade is set for 1.8 mm. The evaporator is set for a relatively even heat distribution system of 166°C. The evaporator is equipped for overhead take off for hydrogen sulfide and excess isooctyl alcohol. The diisooctyl dithiophosphoric acid, the excess isooctyl alcohol and the hydrogen sulfide are then charged to the evaporator at a rate of 3.3 kg/minute. The hydrogen sulfide is observed to distill rapidly from the 1.8 mm thick falling film of the reaction mixture. Substantially all of the hydrogen sulfide is removed prior to the reaction mixture flow front reaching the midpoint of the still flow path. The excess isooctyl alcohol is substantially completely removed by the time the product reaches the sump in the evaporator. The evaporator is designed in such a manner that the diisooctyl dithiophosphoric acid may be removed at the bottom and drawn off for further processing. At the point at which the diisooctyl dithiophosphoric acid is removed from the sump, it is found to be at a temperature of about 166°C and is immediately cooled. No degradation of the diisooctyl dithiophosphoric acid is observed. Substantially similar results are obtained with dialkyl dithiophosphoric acids prepared from 2-ethylhexyl-, isobutyl-, amyl- or methylpentyl alcohol. A further variation of the process of the present invention is to utilize a wiped film evaporator wherein there is substantially no space between the still wall and the scraping blade.

Claims: A process for recovering alcohols from a mixture of the alcohol and a phosphorus compound of the general formula: (R¹O)(R²O)P(S)SH where R¹ and R² are each aliphatic, or aromatic groups, wherein the mixture is preheated to a temperature of 20°C to 115°C, and thereafter rapidly adjusting the temperature of the mixture to a temperature of 80°C to 180°C for a period of time sufficient to substantially separate the alcohol from the phosphorus compound without the substantial generation of hydrogen sulfide. The process of claim 1 wherein R¹ and R² each contain from 1 to 10 carbon atoms The process of claim 1 wherein the adjusting is rapid heating which is accomplished in from 5 seconds to 10 minutes. The process of claim 1 wherein the preheating is to 28°C to 110°C. The process of claim 1 wherein R¹ and R² each contain from 3 to 8 carbon atoms. The process of claim 1 wherein the rapid heating step is conducted on a falling film of the mixture. The process of claim 6 wherein the falling film is 1.25 to 5 millimeters in thickness. The process of claim 1 wherein the alcohol removed from the mixture during the rapid heating step is 80% to 100% by weight of the unreacted alcohol originally present in the mixture. A process for removing hydrogen sulfide from a mixture of the hydrogen sulfide and a phosphorus compound of the general formula: (R¹O)(R²O)P(S)SH where R¹ and R² are each aliphatic groups containing from 1 to 10 carbon atoms, wherein the mixture is pre-heated to a temperature of 20°C to 115°C, and thereafter rapidly adjusting the temperature of the mixture to a temperature of 80°C to 180°C for a period of time sufficient to substantially separate the hydrogen sulfide from the phosphorus compound without the substantial generation of additional hydrogen sulfide. The process of claim 9 wherein R¹ and R² each contain from 3 to 8 carbon atoms.

Inventors: VAN LIER FRANK M, VAN LIER, FRANK, M.
Applicants: LUBRIZOL CORP, THE LUBRIZOL CORPORATION
Application year: 1988
Main classification: C07F9
IPC codes: C07F9, C01B17, C07C29