LIQUID PHASE CHLORINATION OF CHLORINATED METHANES

Abstract: This invention describes an improved liquid phase chlorination process for the chlorination of methyl chloride to preferentially produce methylene chloride. The chlorine is provided to the reaction mixture in a mole percent, based upon the total moles of chlorine and methyl chloride in the mix, which ranges from 16 to 2 mole percent.

Description: This invention concerns an improved process for the chlorination of chlorinated methanes. More particularly, the process of this invention provides for reacting chlorinated methanes and chlorine in a liquid phase environment and in such relative amounts to yield a highly selective product distribution. The gas phase reaction of chlorine and methyl chloride, methylene chloride and chloroform, is well known in the prior art. The reaction occurs at elevated temperatures, i.e. 400-600°C, and at about atmos­ pheric pressure. While gas phase chlorination has been widely used it has several serious drawbacks, some of which are directly related to the required process conditions. Due to the high temperatures required in the gas phase reaction, side reactions are probable. The most troublesome of the side reactions are those which produce both saturated and unsaturated carbon to carbon bonding. Unsaturated carbon to carbon bonding produces chlorocarbons, such as, vinylidene chloride, trans - and cis - dichloroethylene and tri­ chloroethylene. These unsaturated compounds con­ taminate the product, thereby requiring additional equipment and energy comsumption to effect their removal from desired product. The high process tem­ peratures can also result in the production of tar and carbon which cause equipment fouling. Further, with the gas phase reaction, unreacted methyl chloride must be separated and recycled back to the reactor. This recycle requires a compressor and such compressors are not only expen­ sive but are generally troublesome to keep in service. Even further, the gas phase reaction does not yield highly selective product distribution when the production of methylene chloride is desired. For example, the typical gas phase reaction produces a methylene chloride to chloroform weight ratio of from 3 to 25 depending upon the amount of chlorine fed. In Japanese Patent Application, Kokai Patent No. 55043002 (published March 26, 1980), it is reported that the liquid phase reaction of chlorine and chlor­ inated methane obviates some of the drawbacks which are characteristic of the above discussed gas phase reaction. The reported liquid phase reaction is run at low temperature, i.e. 6 to 150°C, and at pressures from 100 to 430 psig (0.8 to 3.1 MPa). The low reaction temperature avoids or at least lessens the production of carbon-carbon bonding. The high pressure allows for more economical product separa­ tion and eliminates the need for a methyl chloride recycle compressor. Despite these benefits for this reported liquid phase reaction, it is not shown to provide highly selective product distribution when used to produce methylene chloride even though its product distribution is slightly better than that which is obtainable with the gas phase reaction. For example, the reported liquid phase reaction is shown to provide a methylene chloride to chloroform weight ratio which ranges from 0.4 to 4 depending on the amount of chlorine fed to the reaction. This invention provides a liquid phase reaction of chlorine and chlorinated methane which is capable of providing high selectivity in product distribution when methylene chloride is the desired product. The highly selective production of methylene chloride is made possible by the process of this inven­ tion. The process comprises: (a) forming an uninitiated liquid phase reaction mixture containing chlorine and methyl chloride, having said chlorine in an amount from 2 to 16 mole percent based upon the initial total amount of chlorine and methyl chloride; (b) holding the reaction mixture in a closed vessel under pressure within the range of from 100 to 800 psig (0.8 to 5.6 MPa) and at a temperature within the range of from 40 to 175°C during the reaction period; and (c) recover­ ing the methylene chloride formed. By providing the chlorine reactant in the above mentioned amounts to the reaction mix it has unexpectedly been found that the production of methyl­ ene chloride is highly favored over the production of chloroform -- indeed the resultant products from the process of this invention have been found to have a weight ratio of methylene chloride to chloroform of 115. Such a high selectivity for the production of methylene chloride occurred with a reaction mix containing 87 weight percent methyl chloride and 3.56 weight percent chlorine (2.82 mole percent chlorine based on methyl chloride and chlorine only). The pre­ sence of carbon tetrachloride in this reaction mix was quite low, i.e. 9.44 weight percent. The chlorination process of this invention is uninitiated. The benefit of uninitiated condi­ tions is that the various initiator compounds known in the prior art add costs to the process and generally will form decomposition products which will require removal from the reaction vessel as bottoms or which may result in production of water in the reaction system causing corrosion problems. Further, these decomposition products may compete with the methyl chloride for the available chlorine in the reaction mix. Initiation with ultraviolet light avoids the problems of initiator compounds but involves the use of vessel light ports or fiber optics to allow entry of the ultraviolet light into the reaction vessel. The high pressures used by the process of this invention could make the reaction vessel design difficult and costly so as to provide the required ultraviolet light entry structures. The methyl chloride, chlorine and any sol­ vent used in the process of this invention should be substantially free of contamination. It is impor­ tant that the presence of dissolved iron, oxygen and water be kept to a minimum as such contaminants adversely affect the reaction rate. Preferably the oxygen and the dissolved iron content should be less than 1 ppm each, while the water content should be less than 20 ppm. The oxygen will most likely be introduced to the reaction mix if liquid chlorine is used as the chlorine portion of the reac­ tion mix. Reduction of the oxygen content in the liquid chlorine can be achieved by conventional means. In the drawings, Figure 1 is a graph of mole precent Cl₂ fed vs weight ratio of methylene chloride to chloroform produced for both a gas phase chlorination process and the liquid phase process of this invention. The methyl chloride reactant used can be best provided by the vapor or liquid phase hydrochlor­ ination of methanol with hydrochloric acid as is well known in the art. After removing the by products and drying, the methyl chloride product is liquified and ready for introduction into the reaction vessel. The chlorine reactant is next charged to the reaction vessel, which vessel is kept at a tem­ perature from 40 to 175°C and a pressure of from 100 to 800 psig (0.8 to 5.6 MPa). The chlorine can be either introduced in liquid form, dissolved in a solvent such as carbon tetrachloride, or can be directly sparged as a gas into the methyl chloride reactant. If the chlorine is fed in liquid form, then, to prevent the existence of a possible explosion hazard, there should be provision for high and quick mixing of the chlorine into the methyl chloride. As mentioned previously the mole percent of chlorine fed is based upon the total moles of chlorine and methyl chloride in the reaction mix and is preferably within the range of from 2 to 16 mole percent, more preferably from 4 to 8 mole percent, to obtain the high preferential production of methylene chloride over that of chloroform. In accordance with the process of this invention, such a quantitative relationship between the methyl chloride and chlorine reactants can yield a weight ratio of methylene chloride to chloroform ranging from about 13 for a 16 mole percent chlorine feed to about 115 for a 2.8 mole percent chlorine feed. The products produced by the process of this invention yield a two phase flow from the reac­ tion vessel and will include hydrogen chloride, methyl chloride, methylene chloride, chloroform and carbon tetrachloride. The products can be separated in any conventional manner. For example, the two phase flow can be first fed to a separator column to split the hydrogen chloride from the balance of the products. The hydrogen chloride can then be recycled to a hydrochlorination reaction for the pro­ dcution of methyl chloride. The remaining products are then sent to a series of rectification columns for separation and purification. Separated methyl chloride is suitable for recycle back to the reaction vessel. The following Examples illustrate preferred embodiments of the process of this invention and should not be construed as limiting the scope of the invention. Examples For all examples, a 1 liter stirred reaction vessel was provided which had feed and discharge lines admitted to its interior. The interior of the reaction vessel, transfer lines, and valves together with the stirrer blades and shaft were coated with polytetra­ fluoroethylene. It is believed that such coating would diminish or prevent the formation of the CCl₃ radical from the reaction of CCl₄ and the equipment metal. The CCl₃ radical will produce, in the presence of a hydrocarbon, CHCl₃ which is not the product sought. It is also believed that the above coating will not be necessary for larger, commercial size process equipment as the ratio of the metal surface subject to attack to the volume provided is much smaller than is the case for the instant 1 liter reaction vessel of these Examples. The reaction vessel was maintained at the desired reaction temperature by the use of an electri­ cal heater built for Parr reactors. CCl₄ when used in a particular example, was dried by passage through an activated molecular sieve. The dried CCl₄ was fed into a N₂ flushed tank. The CCl₄ was then sparged with N₂ for 15 minutes to remove any dissolved O₂. Cl₂ was then bubbled through the CCl₄ and pumped to the reaction vessel to obtain a chlorine concentration as per the following Table. The CCl₄/Cl₂ solution was introduced into the reaction vessel and brought to a temperature generally 10 to 50°C above the desired reaction temperature. Liquid methyl chloride was then pressured in the reaction vessel. The reaction vessel was run at the pressures shown in the Table. Stirring of the resultant reaction mix was continuous. The reaction mix was cooled by the liquid methyl chloride introduction but was main­ tained at reaction temperature by the electrical heater. If the Cl₂ was not dissolved in CCl₄, it was sparged into the liquid methyl chloride in the reaction vessel. The liquid methyl chloride was constantly stirred during the sparging. Liquid samples were drawn from a display in the reactor into a Valco liquid sample valve with a 3 microliter sample size. The samples were injected into a HP 5710 gas chromatograph with a TC detector. A 12 foot (3.6 m) fluorolube column was used to separate the products. The chlorine used in the Examples was liquid chlorine containing less than 25 ppm oxygen. Figure 1 graphically shows a plot (solid line) of the results in the Table and a plot (dashed line) of standard results from a gas phase chlorination scheme. As can be seen from the plots, the dashed line plot yields a weight ratio of CH₂Cl₂ to CHCl₃ ranging from 6 to 26 while the solid line plot has weight ratios within the range of from 12 to 115 for the same mole percent chlorine fed. In fact, for the same mole percent chlorine fed, the solid line plot shows at least about twice the preference for methylene chloride production than that for the dashed line plot.

Claims: 1. A process for the preparation of methyl­ ene chloride by the liquid phase chlorination of methyl chloride, which process comprises: (a) forming an uninitiated liquid phase reaction mixture containing chlorine and methyl chlor­ ide, having said chlorine in an amount from 2 to 16 mole percent based upon the initial total amount of chlorine and methyl chloride; (b) holding the reaction mixture in a closed vessel under pressure within the range of from 100 to 800 psig (0.8 to 5.6 MPa) and at a temperature within the range of from 40 to 175°C during the reaction period; and (c) recovering the methylene chloride formed. 2. The process of Claim 1 wherein said chlorine is provided to said reaction mixture as gaseous chlorine which is solubilized in the liquid methyl chloride in the reaction mixture. 3. The process of Claim 1 wherein said reaction mixture consists of chlorine and methyl chloride. 4. The process of Claim 1 wherein said chlorine portion of said reaction mixture is introduced thereto by sparging gaseous chlorine into the methyl chloride portion of said reaction mixture. 5. The process of Claim 1 wherein the reac­ tion mixture contains less than 1 ppm of oxygen. 6. The process of Claim 1 wherein the reaction mixture contains less than 1 ppm of dissolved iron. 7. The process of Claim 1 wherein the reaction mixture contains less than 20 ppm of water. 8. The process of Claim 1(b) wherein the temperature is from 100 to 175°C. 9. The process of Claim 1(a) wherein the mole percent of chlorine is from 4 to 8. 10. The process of Claim 1 wherein the weight ratio of methylene chloride to chloroform ranges from 115 to 12.

Inventors: HOLBROOK, MICHAEL T., MORRIS, THOMAS E.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C07C17
IPC codes: C07C67, C07C19, C07C17, B01J19