Title of Invention

AN IMPROVED APPARATUS AND PROCESS FOR THE MANUFACTURE OF HIGH PURITY AND FREE FLOWING PHOSPHORUS PENTACHLORIDE

Abstract

An improved apparatus for production of high purity, free flowing Phosphorus pentachloride (99.5 - 99.9% by weight PCI5) by continuous gas phase reaction between phosphorus trichloride and chlorine, having a means for vapourising phosphorus tri-chloride; a means of pre-heating phosphorus trichloride vapours; a means for reacting the preheated phosphorus tri-chloride and chlorine in gaseous form; a means for cooling the reacted gaseous stream; and a means for crystallizing and settling the reaction products; a means for collecting fine particles of phosphorus pentachloride; a means for absorbing waste chlorine gas and a means for collecting and conveying the collected phosphorus pentachloride crystals into drums for packing under dry sealing conditions, characterized in that the improvement comprising: a) a means for providing and maintaining continuously predetermined quantity of phosphorus trichloride vapour and chlorine gas at 1: (1 -1.1) mole ratio; and b) a means for continuously removing traces of impurities from the product to get high purity phosphorus pentachloride.

Full Text

FORM 2 THE PATENTS ACT 1970 COMPLETE SPECIFICATION (See Section 10) TITLE An Improved Apparatus and Process for the Manufacture of high pusity and free flowing Phosphorus Pentachloride APPLICANT SHROFF RAJNIKANT DEVIDAS Uniphos House, Madhu Park, Khar, Mumbai 400 052, Maharashtra. Art Indian National The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- 2 9 MAY 2001 GRANTED 29-5-2001 This invention relates to an improved apparatus and a process for the manufacture of Phosphorus pentachloride. This invention particularly relates to the manufacture of free flowing crystalline Phosphorus pentachloride of high purity. BACKGROUND AND PRIOR ART Phosphorus Pentachloride is white to pale yellow powder or fine granular fuming mass with a pungent odour. Its melting point is 166 to 168° C under pressure and it sublimes. It has a specific gravity of 3.6. It. reacts violently with water. In presence of moisture it decomposes with heat releasing hydrochloric acid, phosphoric acid and phosphorus oxychloride (POCI3). Due to fuming and deliquescent characteristics it is extremely irritating on exposure, corrosive to skin, eyes and mucous membranes. Therefore special procedures are necessary for handling phosphorus pentachloride. Phosphorus pentachloride has a property of caking. On long storage the powder of pentachloride forms a lump. Phosphorus pentachloride is used in organic chemical reactions especially for substitution of CI for OH group. It is a chlorinating agent; it acts as a catalyst for condensations, cyclisation reactions in organic synthesis such as drugs, dyestuffs. The pharmaceutical industry particularly needs high quality Phosphorous pentachloride. It is also used in improving grain structure of light metal castings. Of the various methods of its manufacture the following three are relevant to the present invention, the third being most relevant. US Patent 3995013 (1975) In this patent a process for the preparation of high purity free flowing crystalline powder of Phosphorus pentachloride has been disclosed. In this process Phosphorus trichloride (liquid) is reacted with a deficit of Chlorine, i.e. quantity less than stoichiometric requirement. The reaction involves maintaining reflux condition (preferably at 40 - 110 C) of the reactants and the product formed has about 50% pentachloride in the mixture. The reaction mixture is cooled to 15 - 30 to form a pumpable and filterable crystalline mass containing the pentachloride. This is followed by recovering pentachloride by decantation/filtration/centrifugation etc. Then the pentachloride is purified by removing trichloride that remains with pentachloride crystals, by passing a stream of dry air or nitrogen or exposing it to sub-atmospheric pressure to dry the wet pentachloride crystals to give a powder which flows like sand. Alternately this is done by what is called as post chlorinating, i.e. chlorinating the trichloride that is wetting the pentachloride crystals by passing chlorine gas through the wet pentachloride crystals and converting it into pentachloride. On large - scale preparation this process is not practicable due to difficulties such as: a. phosphorus pentachloride forming a solid crust around the inlet pipe nozzle of chlorine feed pipe; b. separation of pentachloride crystals from the liquid trichloride phase c. increased reaction time due to limitation of reaction zone when chlorine gas is passed over liquid trichloride. U S Patent 433 5085 (1982) In this patent another process for the preparation of high purity free flowing crystalline powder phosphorus pentachloride has been disclosed. This is a two stage continuous as well as discontinuous process. The first and the second stage reaction can be carried out in the same reaction container or in two different containers. In the first stage, phosphorus trichloride is combined with chlorine while stirring and distilling off up to 90% of trichloride added thereby being converted into a crystal sludge comprising 50 - 99.9 mole % of pentachloride, remainder being trichloride. In the second stage this sludge is reacted with excess of chlorine to give pentachloride. In a preferred method, in order to avoid gas phase reaction, chlorine is passed into the crystal sludge, while the trichloride is introduced in the mixture of PCL3 and PCL5 from the top for example by being sprayed on. It is thereby ensured that the chlorination predominantly takes place within the reaction mixture, no chlorine escapes into the gas phase in the first stage, and the formation of pentachloride is good. Preferred temperature is 75-90° C. If the temperature goes above 140° C the mixture starts to melt and Solidify again at lower temperature to form lumps. In the second reaction stage, the reaction mixture which is rich in pentachloride, is reacted with excess chlorine. In general, it is necessary to maintain a molar ratio of PCI3 : Cl2 in the range 1:0.1 to 1:2 during the addition of chlorine, to the trichloride and pentachloride remaining at the end of first stage. It is advantageous to keep this ratio within the given limits for each interval of time of 60 minutes. That is means exclusively one reactant is not added over a longer period. It is particularly preferable that phosphorus trichloride and chlorine be added simultaneously and continuously. This again have similar difficulties in operation on large scale, as in the case of US Patent 4335085 mentioned above. Further it is difficult to maintain the ratios of the reactants as required during the reaction time. Ind. Patent No. 172459 (1991) In this patent another process for the production of free flowing dry phosphorus pentachloride has been disclosed. In this process phosphorus pentachloride is prepared by direct reaction between phosphorus trichloride vapours and chlorine, characterized in that phosphorus trichloride is vaporized at a temperature range of 65 C to 85 C. The phosphorus trichloride vapours are subjected to preheating to a temperature range between 100° C to 170° C prior to being fed to a reactor. Chlorine gas is introduced into the reactor for reaction with the pre-heated phosphorus trichloride vapours in the gaseous phase. The weight ratio between phosphorus trichloride and chlorine is maintained within the range of (20:1 to (1:1). Phosphorus pentachloride vapours produced by the said reaction are cooled to a temperature range between 60°C to 756C and passed into a chamber for settling down on the walls of the said chamber in fine crystalline form. Fine particulates of Phosphorus pentachloride are scrapped from the walls of the chamber and collected and packed without being exposed to atmosphere. One common difficulty in carrying out these prior art processes on large-scale production is meeting the exact requirements of maintaining the reactants at the concentrations or relative ratios at various stages of the reaction. None of these patents describe how this is achieved. If for example the required quantity of chlorine was not properly controlled, excess chlorine has to be vented out. It is not only a waste but also a hazard. The excess chlorine has to be vented out of the reactor through a buffer. Particles of Phosphorus pentachloride carried away by the excess chlorine are collected in the buffer and the outlet sometimes gets choked leading to more problems. There is another problem in large-scale production by any of these processes. If the quantitative ratios of the reactants at any stage during the process, batch or continuous, are not within the required limits the Pentachloride yield goes down. The quality suffers due to loss of free flowing powdery nature of the product. Some times there is formation of lumps in the Phosphorus pentachloride. The principal object of the present inventions is to overcome theses difficulties faced in the large-scale production of phosphorus pentachloride SUMMARY OF THE INVENTION: Accordingly, the present invention relates to an improved apparatus for production of high purity, free flowing Phosphorus pentachloride (99.5 - 99.9% by weight PCI5) by continuous gas phase reaction between phosphorus trichloride and chlorine, having a means for vapourising phosphorus tri-chloride; a means of pre-heating phosphorus trichloride vapours; a means for reacting the preheated phosphorus tri-chloride and chlorine in gaseous form; a means for cooling the reacted gaseous stream; and a means for crystallizing and settling the reaction products; a means for collecting fine particles of phosphorus pentachloride; a means for absorbing waste chlorine gas and a means for collecting and conveying the collected phosphorus pentachloride crystals into drums for packing under dry sealing conditions, wherein the improvement comprising: a) a means for providing and maintaining continuously predetermined quantity of phosphorus trichloride vapour and chlorine gas at 1: (1 - 1.1) mole ratio; and b) a means for continuously removing traces of impurities from the product to get high purity phosphorus pentachloride. And also relates to an improved process, using the improved apparatus, the production of high purity, free flowing phosphorus pentachloride (99.5 - 99.9% by weight PCI5) by continuous gas phase reaction between phosphorus trichloride and chlorine having the steps of vapourising phosphorus trichloride at 74 - 78°C; pre¬heating phosphorus trichloride vapours at 80 - 90°C; reacting the preheated phosphorus trichloride with chlorine in gaseous form at 100 - 120°C; cooling the reacted gaseous stream form at to 60 - 65° C; crystallizing the phosphorus pentachloride formed and settling the reacted products; wherein the improved process comprising: a) maintaining continuously the pre-determined quantity of phosphorus trichloride and predetermined quantity of at 1: (1 -1.1) mole ratio; b) continuously removing traces of impurities from the product by passing a hot nitrogen gas at 75 - 80° C counter currently to the flow of product. BRIEF DESCRIPTION OF THE INVENTION: It is noticed from the prior art that in all the three processes described the underlying reaction is PCI3 + Cl2--> PCIS + ΔH Therefore the exact requirement of the reaction is to use 1 mole of Cl2 per 1 mole of PCI3 . That is the stoichiometric requirement. In the event of excess of one of the reactants there is a danger of formation of a sludge containing a mixture of the trichloride and pentachlohde or an appreciable quantity of the pentachlohde being carried away in the waste chlorine stream. Further these materials are hazardous in handling and there have been no proper/good methods to measure and add controlled quantitites in a reaction vessel. In the instant process, the use of a controlled mass flow and density sensors ensure that only the requisite quantities of the reactions are allowed into the reactor. In this process the phosphorous trichloride used is of technical grade containing 98-99% PCI3. The phosphorous pentachlohde obtained is of very high purity having PCI5 content from 99.8% to 99.9 % and even more than that. In this process means for providing and maintaining near exact required amounts, and 1: (1 - 1.1) mole ratio, of the reactants, preheated phosphorus trichloride vapours and chlorine gas is by use of any of the commercially available mass flow density sensor systems. One preferred mass flow and density sensor is a commercially available ELITE CMF025 made by Fisher Rosemount. A preferred mole ratio of the reactants preheated phosphorus trichloride vapours and chlorine is 1.0 - 1.05. s process, an externally cooled reaction chamber controls the temperature of the reaction. In this process the cooling of the reacted gas stream and settling the reaction products in a fine dry crystalline powder form is in a nickel container. However, a glass lined or lead lined chamber having large wall area with external cooling can also be used. In this process means for collecting the pentachloride crystals formed on the walls of said chamber is done by a wall scraper rotating at slow speed inside said chamber The crystallizing chamber cooled externally to 25° - 30° C with a wall scraper operating inside at 40-50 rpm, preferably at 45 rpm. In this process means for heating nitrogen gas is by passing nitrogen gas from a tank through a heating chamber. This step effectively removes marginal and last traces of impurities like chlorine and phosphorus trichloride contaminating the product phosphorus pentachloride. Removal of such trace impurities is essential to ensure long shelf life to the characteristic free flowing crystalline nature of the product. In this process means for collecting carry over fine particles of the phosphorus pentachloride is a conventional catchall system; In this process means for absorbing waste chlorine gas is a conventional washing and alkali scrubbing system. DRAWINGS: A preferred embodiment of the invention will now be described with the help of the accompanying drawing. The drawing is for the illustration only and in no way to restrict the scope of the invention: Fig.1 A schematic diagram of the plant for carrying out the process of manufacture of high purity phosphorous pentachloride of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION: Liquid Phosphorus trichloride from storage tank as shown in the drawing is led through pipe line (1) into one arm of mass flow controller (3A) which goes to the vaporizer (4), which is heated to a preset temperature (4A), which is heated both internally and/or externally to maintain the temperature of vapours at preset temperature. Heating is generally done by electric heaters. Metered quantities of chlorine gas from the cylinder as shown via pipeline (2) is introduced into a reactor (5) through mass flow meter (3B) in a gaseous form. The hot vapours of phosphorus trichloride are reacted with chlorine in a gaseous form in reactor (5) to produce phosphorus penatchloride. The reactor (5) is cooled externally to bring down the temperature of phosphorus pentachloride vapours and to minimize the carry over of the phosphorus pentachloride particles along with the excess chlorine gas. Partially cooled vapours enter a settling chamber (6) which is lead lined, glass-lined, or made of nickel or of the like material and the temperature of the vapours is reduced due to the said cooling. Phosphorus pentachloride, in fine particulate form settles down on the wall surface of the chamber (6) as a fine crystalline powder. A scraper (7) made of nickel or any suitable material is provided within the chamber and is caused to be rotated slowly by a motor (7A) the rotation speed of which can be in a range of 40 to 50 rpm. The scraper, on rotation, scrapes all the material sticking/settled on the walls of the chamber (6). This movement is continuous and the scrapped material keeps collecting at the bottom of the chamber (6). The material dropped at the bottom is collected by a pedal or screw conveyor (8). The screw conveyor is purged with hot nitrogen (about 70°C to 80°C) and carries the pentachloride to the packing container (9). The material is packed immediately, minimizing its exposure to atmosphere. The small quantity excess of chlorine gas is vented out from the chamber (6) through a line (10) made of lead, glass line or nickel to a buffer (11), material of construction for the buffer is either lead, glass-lined, nickel or similar non-corrosive materials. The buffer prevents any carry forward of the fine particulate form of phosphorus pentachloride. The buffer (11) is connected to an absorption system (12) where excess chlorine is washed and neutralized, counter currently, by a dilute alkali, such as sodium hydroxide solution, sodium bicarbonate, lime, soda ash etc. EXAMPLES: The Invention will now be illustrated by way of Examples. The examples are by way of examples only and in no way restrict the scope of the invention. EXAMPLE 1: The preparation of phosphorous pentachloride described in this example was done in the plant as described in the accompanying drawing. The mass flow meter ELITE CMF 025 made by Fisher Rosemount was used to control the exact flow of gases in the reaction chamber.Commercially available phosphorus trichloride, chlorine and nitrogen from a chemical market were used in this experiment. A 2 % w/v solution of sodium hydroxide was used for scrubbing unreacted chlorine gas going out of the reaction system. The phosphorous trichloride was vaporized at 75-76°C and me vapours were heated to a temperature of 85 prior to entering the reaction chamber. The continuous flow of phosphorus trichloride was at the rate of 5.5 kgs per hour and that of chlorine gas at 2.8 kgs per hour. The reaction was controlled by cooling the reaction chamber externally to maintain the reaction temperature at 90°C. After the reaction, the reacted gases were cooled to 60-62°C. The cooled gases were passed into the settling chamber cooled to 30°C; the wall scraper was kept rotating at 40 rpm. The nitrogen gas was passed slowly through the heating coil to get the nitrogen gas heated to 75°C. This was passed from the drum packing point in to the crystallization chamber from where it went out with the excess chlorine stream and vented out of the scrubber. The apparatus was run continuously for 4 hours during which phosphorus trichloride 22 kgs and chlorine gas 11.20 kgs were consumed. This has produced 30.2 kgs of phosphorus pentachloride during the 4 hours of the running. The yield of the product phosphorus pentachloride was more than 99.8% by wt. of the theoretical yield-based on the weight of the PCI3. Purity of phosphorus pentachloride obtained was tested intermittently during the collection in the drums drawing several samples. On an average it was found to be 99.9% pure. It was free flowing powder, devoid of any lumps. Mesh size was 50. EXAMPLE 2: The chemicals used, process adopted and the apparatus used were as described herein before, with the rate of addition maintained very accurately by mass flow meter as in Example 1. The continuous flow of phosphorus trichloride was at the rate of 5.0 kgs per hour and that of chlorine gas at 2.8 kgs per hour. The apparatus was run continuously for 2 hours during which phosphorus trichloride 10.1 kgs and chlorine gas 5.55 kgs were consumed. This has produced 14.7 kgs of phosphorus pentachloride during the 2 hours of the running. The yield of the product phosphorus pentachloride was minimum 98.8% by wt. of the theoretical yield, based on the weight of the PCI3. Purity of phosphorus pentachloride obtained was tested intermittently drawing several samples. On an average it was found to be 99.9% pure. It was free flowing powder, devoid ,of any lumps. Mesh size was 100% passing through 60 mesh. EXAMPLE 3: The description of the process and the apparatus is the same as the previous one and the same mass flow meter. Liquid Phosphorus Trichloride was fed at 10 kilos per hour and chlorine at 5.5 kgs per hour for about 8 hours. Phosphorus trichloride vapour temperature was at a range of 75.5 to 76.5 whereas preheating of phosphorus trichloride vapours is at a range of 80-85°C. Dry nitrogen was introduced at the bottom of the chamber. The reaction mixture of chlorine and phosphorus trichloride which had formed phosphorus pentachloride was brought down by natural cooling to around 70°C. The system was run continuously for 8 hours during which phosphorus trichloride - 80 kgs and chlorine 44.4 were consumed. At the end of the process 120 kgs of phosphorus pentachloride was collected. Purity of PCI5 was 99.8% fine powder Advantages of the Invention : The phosphorous pentachloride obtained by the process of the present invention is more than 99.5% pure. It contains 99.5 to 99.95% PCI5. It is always free flowing and there is no lump formation even in the monsoon period. The quality is excellent for use as reagent in pharmaceutical productions. Reactant quantities being under control operation of the plant is less hazardous. Wastage is less. There are absolutely minimum environmental discharges under breakdown conditions. The yield of the reaction is minimum 99.8% on the phosphorous trichloride used. The purity of the phosphorus pentachloride is high; it is nearly 99.9% pure. The beauty of the process lies in that inspite of using technical grade phosphorous trichloride, the phosphorus pentachloride obtained is of very high purity. This is because the impurities in the phosphorous trichloride remain unvaporised in the vaporiser and pure PCI3 vapour reacts with chlorine gas. The flow of hot nitrogen gas keeps moisture contamination away and the product is a free flowing dry powder. The use of countercurrent flow of hot nitrogen drives away trace amounts of phosphorous trichloride, moisture and chlorine gas if any, wetting the final phosphorous pentachloride crystals. This ensures not only the purity of the product but also its stability and long shelf life. None of the prior art processes manufacture phosphorous pentachloride crystals of such high purity, and with such good yield. Thus the process of the present invention directly gives 99.9 % pure phosphorous pentachloride as a free flowing powder in near quantitative yield starting from technical grade phosphorous trichloride. It is a gas phase reaction, and such good results are achieved by means for providing exact quantities of the reactants, and built in method of its purification or removal of impurities in the final product by means to pass nitrogen gas counter currently. I Claim: 1) An improved apparatus for production of high purity, free flowing Phosphorus pentachloride (99.5 - 99.9% by weight PCI5) by continuous gas phase reaction between phosphorus trichloride and chlorine, having a means for vapourising phosphorus tri-chloride; a means of pre-heating phosphorus trichloride vapours; a means for reacting the preheated phosphorus tri-chloride and chlorine in gaseous form; a means for cooling the reacted gaseous stream; and a means for crystallizing and settling the reaction products; a means for collecting fine particles of phosphorus pentachloride; a means for absorbing waste chlorine gas and a means for collecting and conveying the collected phosphorus pentachloride crystals into drums for packing under dry sealing conditions, characterized in that the improvement comprising: a) a means for providing and maintaining continuously predetermined quantity of phosphorus trichloride vapour and chlorine gas at 1: (1 -1.1) mole ratio; and b) a means for continuously removing traces of impurities from the product to get high purity phosphorus pentachloride. 2) The improved apparatus as claimed in claim 1, wherein the means for providing and maintaining continuously pre-determined quantity of phosphorus tri-chloride vapours and chlorine gas at 1: (1 - 1.1) mole ratio by using a mass flow density sensor system. 3) The improved apparatus as claimed in claim 1, wherein the said means for continuously removing traces of impurities from the product is a provision for supplying nitrogen gas at 75 - 80° C from a nitrogen tank connected to a heating coil, so as to let nitrogen gas at 75 - 80° C enter the system at the output end of the apparatus and flow counter currently to the flow of the product. 4) The improved apparatus as claimed in claim 2, wherein mass flow and density sensor system used is ELITE CMF025 made by Fisher Rosemount. 5) The improved apparatus as claimed in claim 1, wherein the said means for reacting the pre-heated phosphorus tri-chloride and chlorine in gaseous form (at 100 -120° C) is an externally cooled reaction column. 6) The improved apparatus as claimed in claim 1, wherein the said means for crystallizing and settling the reacted products is a nickel chamber or a glass lined or a lead lined chamber having large wall area cooled externally at 25-30° C. 7) The improved apparatus as claimed in claim 1 wherein, the said means for collecting fine particles of phosphorus pentachloride carried over from the said chamber of claim 6 is a buffer system. 8) The improved apparatus as claimed in claim 1, wherein the said means for collecting and conveying the collected phosphorus pentachloride crystals into drums for packing are provision of a slow speed 30 - 60 rpm wall scrapper inside the said chamber as claimed in claim 6, and a screw conveyor for conveying. 9) The improved apparatus as claimed in claim 1, wherein the said means for absorbing the waste chlorine gas is a washing and alkali scrubbing system. 10) An improved process, using the improved apparatus as claimed in claim 1, the production of high purity, free flowing phosphorus pentachloride (99.5 - 99.9% by weight PCI5) by continuous gas phase reaction between phosphorus trichloride and chlorine having the steps of vapourising phosphorus trichloride at 74 - 78°C; pre-heating phosphorus trichloride vapours at 80 - 90°C; reacting the preheated phosphorus trichloride with chlorine in gaseous form at 100 - 120°C; coollig the reacted gaseous stream form at to 60 - 65° C; crystallizing the phosphorus pentachloride formed and settling the reacted products; characterized in that the improved process comprising: a) maintaining continuously the pre-determined quantity of phosphors trichloride and predetermined quantity of at 1: (1 - 1.1) mole ratio b) continuously removing traces of impurities from the product by passing a hot nitrogen gas at 75 - 80° C counter currently to the flow of product. 11) The improved process as claimed in claim 10, the phosphorus trichloride used is of technical grade containing 98 - 99% phosphorus trichloride. 12) The improved process as claimed in claim 10, wherein the said ratio of reactants phosphorus trichloride vapours and chlorine gas is 1:1.05. 13) An improved apparatus for the production of high purity free flowing phosphorus pentachloride (99.5 - 99.9% by weight PCI5), by continuous gas phase reaction between phosphorus trichloride and chlorine as exemplified in the text, examples 1-3 and the accompanying drawing. 14) An improved process for the production of high purity free flowing phosphorus pentachloride (99.5 - 99.9% by weight PCI5), by continuous gas

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501-mum-2001-correspondence 1(17-07-2002).pdf

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501-MUM-2001-SPECIFICATION(AMENDED)-(5-6-2012).pdf

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