Claims:We claim:
1) A continuous process for preparing dichlorohydrin comprising the steps of:
a) mixing glycerol, anhydrous hydrogen chloride gas and a carboxylic acid catalyst in a first reactor at 80 to 130 oC to obtain dichlorohydrin, the first reactor being coupled to a second reactor wherein the first reactor is operated at a pressure of less than 1 atm;
b) discharging the unreacted hydrogen chloride gas from the first reactor into the second reactor for effective recycling of the hydrogen chloride gas and allowing it to react with monochlorohydrin and glycerol coming into the second reactor from a distillation set-up placed sequentially after a third reactor;
c) discharging the contents of the second reactor into the first reactor and allowing the unreacted contents of the first reactor to continue reacting and then discharging the mixture of chemicals containing dichlorohydrin, monochlorohydrin, concentrated aqueous HCl and catalyst of the first reactor into a third reactor along with addition of fresh glycerol into the third reactor and allowing the same to react in the third reactor to obtain a mixture of chemicals monochlorohydrin, dichlorohydrin and catalyst;
d) discharging the solution of step (c) from the third reactor into a distillation set-up wherein the dichlorohydrin and water are distilled out and remaining chemicals containing monochlorohydrin, glycerol and catalyst are recycled back to the second reactor for reuse therein.

2) The process as claimed in claim 1, wherein the catalyst used is cyclohexane carboxylic acid.

3) The process as claimed in claim 1, wherein the first reactor is a bubble column reactor.

4) The process as claimed in claim 1, wherein the second reactor is a venturi loop reactor.

5) The process as claimed in claim 1, wherein the third reactor is a plug flow reactor.

6) The process as claimed in claim 1, wherein the total yield of dichlorohydrin obtained through distillation is 65% to 70% by weight of the total contents leaving the distillation column.

7) The process as claimed in claim 1, wherein the flow rate of fluids from the second reactor to the first reactor is maintained at 6 to 8 mL /min.

8) The process as claimed in claim 1, wherein the temperature in the first reactor is maintained at 110 oC.
, Description:FIELD OF THE INVENTION:
The present invention relates to a continuous process for preparing dichlorohydrin.

BACKGROUND OF THE INVENTION:
Chlorohydrins are useful for the synthesis of epoxy compounds e.g. epichlorohydrin. Epichlorohydrin is the main raw material for preparing epoxy resins which in turn find applications in automobile, construction, electrical, coating, etc. industries.
A widely known process for epichlorohydrin synthesis involves conversion of allyl chloride to dichlorohydrin with the help of hypochlorous acid. The dichlorohydrin thus produced is then treated with alkali solution to obtain epichlorohydrin. Several publications and patents are also available on direct epoxidation of allyl chloride with hydrogen peroxide to epichlorohydrin.
Some other processes using acrolein, allyl alcohol or acetone as starting materials for epichlorohydrin synthesis are also known but these processes show a tendency to form dichlorohydrin intermediates. It is also known for more than 100 years that glycerol can be hydro-chlorinated to dichlorohydrins. However recently, due to the abundant availability of glycerol and soaring prices of propylene, there is a renewed interest in the manufacturing dichlorohydrin from glycerol.
Various publications in the last decade have described the art of converting glycerol to dichlorohydrin using hydrogen chloride (HCl) gas. An article authored by Gibson G.P., ( Chemistry and Industry, 1931, 20, 949) and Conant et al., (Organic Synthesis CV1, 292) have reported glycerol hydrochlorination with approximately 70 % yield of dichlorohydrins by using large excess of HCl gas for azeotropic removal of water. Few patented methods use the approach of vacuum removal of water (DE1075103) or use of an added azeotropic agent such as n-butyl ether to promote the reactive azeotropic distillation and elimination of water, again using excess HCl (US2144612). However, azeotropic removal of water is time-consuming and at the same time it makes the separation of products difficult.
Other publications also mentions many different method of making dichlorohydrin namely, a batch process with intermittent separation of water during catalytic hydrochlorination of glycerol (DE197308), a series of hydrochlorination reactions in which the water of reaction is removed in an atmospheric or sub-atmospheric process by reactive distillation (WO2005/021476) or a process where glycerol hydrochlorination is carried out in a series of reactors with intermittent removal of water(WO2005/054167). WO 2006020234 also teaches the advantages of leaving water in their process, or that removing the water effects the formation of unwanted chloroethers while WO 2011007931 advocates the use of irradiation technique by using ultrasonic radiation during glycerol hydrochlorination for moisture removal from end-products.
WO2009066327 filed by Conser describes the production of dichlorohydrins. It involves the hydrochlorination reaction in two reactors arranged in series, operating continuously at different pressures. The first reactor operates at 1-4 bar with acetic acid catalyst followed by a second reactor which operates at pressure 5-20 bar with malonic acid catalyst.
A patent filed by Solvay (US7906692) describes the production of dichlorohydrin under super-atmospheric pressure with no removal of water.
Nevertheless, all the known processes or methods make report of a hydrochlorination process where water is removed intermittently from the process or high pressure HCl is used to compensate the negative effect of water on the reaction equilibrium. Intermittent removal of water in the vapour phase suffer from drawbacks such as handling and processing of highly corrosive mixture of water, HCl and chlorohydrins, higher by-product formation, higher consumption of HCl gas and higher capital costs which are required for more number of unit operations. Further, the use of high pressure anhydrous HCl gas has a downside of increased manufacturing costs for pressurizing HCl, higher losses of HCl gas, rise in the corrosive nature of the liquid phase due to higher amount of dissolved HCl because of high pressure and other safety concerns.
Thus, there is a need for a dichlorohydrin manufacturing process on a commercial scale that is user friendly, improves the economy and safety of the process as well as overcomes the shortcomings of the prior art. It is hence desirable to devise a process for manufacturing dichlorohydrin using glycerol and hydrogen chloride which is not only simple and inexpensive to carry out, but also decreases production costs by operation at comparatively lower pressure and without the need for water removal during the process. It is also desirable that the process, efficiently utilizes the raw materials and preferably results in recycling the process by-products.

BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 is a schematic representation of a continuous process for preparing dichlorohydrin from glycerol using a three-reactor system.

SUMMARY OF THE INVENTION:
According to an embodiment of the present invention, there is provided a continuous process for preparing dichlorohydrin comprising the steps of:
a) mixing glycerol, anhydrous hydrogen chloride gas and a carboxylic acid catalyst in a bubble column reactor (first reactor) at 80 to 130 oC to obtain dichlorohydrin, the first reactor being coupled to a venturi loop reactor (second reactor) wherein the first reactor is operated at a pressure of less than 1 atm;
b) discharging the unreacted hydrogen chloride gas from the first reactor into the second reactor for effective recycling of the hydrogen chloride gas and allowing it to react with monochlorohydrin and glycerol coming into the second reactor from a distillation set-up placed sequentially after a plug flow reactor (third reactor);
c) discharging the contents of the second reactor into the first reactor and allowing the unreacted contents of the first reactor to continue reacting and then discharging the mixture of chemicals containing dichlorohydrin, monochlorohydrin, concentrated aqueous HCl and catalyst of the first reactor into a third reactor along with addition of fresh glycerol into the third reactor and allowing the same to react in the third reactor to obtain a mixture of chemicals containing monochlorohydrin, dichlorohydrin and catalyst;
d) discharging the solution of step (c) from the third reactor into a distillation set-up wherein the dichlorohydrin and water are distilled out and remaining chemicals containing monochlorohydrin, glycerol and catalyst are recycled back to the second reactor for reuse therein.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION:
The present invention provides a continuous process for preparing dichlorohydrin (DCH) using glycerol and hydrogen chloride (HCl).
The term ‘dichlorohydrin’ is used herein to refer to the isomers 1, 2-dichloro-3-hydroxypropane and 1, 3-dichloro-2-hydroxypropane.
The continuous process for preparing dichlorohydrin comprises the steps of mixing glycerol, anhydrous hydrogen chloride gas along with a catalyst in a bubble column reactor (BCR) (first reactor) to form dichlorohydrin along with unreacted hydrogen chloride gas. Preferably, the temperature in the first reactor is maintained at 110 oC. Preferably, the first reactor is maintained at a pressure of 0.8-0.98 atm.
This unreacted hydrogen chloride is discharged from the first reactor into the venturi loop reactor (second reactor). The contents of the second reactor are discharged again into the first reactor after allowing the unreacted hydrogen chloride gas to continue reacting with monochlorohydrin and glycerol coming into the second reactor from a distillation set-up placed sequentially after a third reactor. Preferably, the flow rate of fluids from the second reactor to the first reactor is maintained at 6 to 8 mL /min.
The process further includes allowing the unreacted reactants in the first reactor to continue reacting with anhydrous hydrochloric acid gas followed by discharging the contents of the first reactor into a plug flow reactor (PFR) (third reactor) along with addition of fresh glycerol in the same reactor to form a mixture of monochlorohydrin, dichlorohydrin, glycerol and catalyst.
The process finally comprises discharging the solution contains monochlorohydrin, dichlorohydrin, glycerol and catalyst into a distillation set-up wherein the dichlorohydrin and water are separately distilled out. The remaining solution containing monochlorohydrin, glycerol and catalyst are recycled back to the second reactor for their re-use in the continuous process.
Figure 1 shows a schematic representation of the continuous process for preparing dichlorohydrin according to the present invention. In accordance with an aspect of the invention, the first reactor used in the present invention is a bubble column reactor (1) while the second reactor used is a venturi loop reactor (2). The bubble column reactor (1) is coupled to the venturi loop reactor (2) to help maintain a pressure of less than 1 atm in the bubble column reactor (1). The venturi loop reactor (2) employed helps for the complete and high conversion of hydrogen chloride gas. Further, at the bubble column reactor (1), a slow reaction between hydrochloric acid and monochlorohydrin takes place to produce dichlorohydrin in the presence of excess HCl gas.
In accordance with another aspect, the third reactor used in this invention is a plug flow reactor (3). The reaction solution discharged from bubble column reactor (1) into the plug flow reactor (2) consists of dichlorohydrin, monochlorohydrin, catalyst and concentrated aqueous HCl. Fresh compensating quantities of glycerol is added into plug flow reactor (3) to obtain dichlorohydrin. Plug flow reactor (3) ensures utilization of dissolved HCl for conversion of glycerol to monochlorohydrin, which is a fast reaction. Further, all the three reactors used in the present invention are glass-lined reactors.
In accordance with an embodiment, the solution after getting discharged from the plug flow reactor (3) undergoes distillation. A vacuum distillation set-up consists of a distillation column (4) operated at a temperature of 50-130oC, more preferably at less than 100 oC for separating dichlorohydrin, water and unreacted HCl. The remaining solution containing a mixture of glycerol, monochlorohydrin and catalyst are fed back into the venturi loop reactor (2) for reuse in the continuous process for producing dichlorohydrin. A total yield of dichlorohydrin obtained through distillation is 65% to 70% by weight of the total contents leaving the distillation column (4). The said dichlorohydrin has purity of 97 to 99%.
In accordance with another embodiment, the catalyst used in the present invention is cyclohexane carboxylic acid. Cyclohexane carboxylic acid has a high boiling point of around 250o Celsius which helps in easy separation of the dichlorohydrin product from the reaction mixture and recycling it in a convenient manner.

In accordance with yet another embodiment, the dichlorohydrin obtained using the continuous process disclosed in the present invention can be further used to form epichlorohydrin by conventionally known methods.
An advantage of the present invention is that the process for preparation of dichlorohydrin disclosed is a continuous process. Process continuity provides less variations in the end-product as that compared to a batch process, is economical, aids in recycling of reaction by-products resulting in decreased manufacturing costs and wastage. Moreover, the present invention operates at a sub-atmospheric pressure, doesn’t require water removal during the hydrochlorination reaction and it uses a catalyst which allows easy separation and recycling. This process brings about effectual and high conversion of starting materials of glycerol and hydrogen chloride gas.
The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1: Conventional Batch Process for Dichlorohydrin Preparation:
The batch process for the preparation of dichlorohydrin was performed in a 2 liter flanged jacketed glass bubble column reactor with an overhead stirrer and a sparger. The sparger is a dip tube with 6 mm opening through which anhydrous HCl gas is bubbled within the reactor. 1200 grams of glycerol along with 12% w/w of cyclohexane carboxylic acid was fed into the reactor and then the contents were heated to the reaction temperature of around 95o to 105o Celsius.
Once the desired temperature is attained, the sparging of HCl gas was started at 1.2 LPM. The reaction is carried out in isothermal conditions for 7 hours. The unreacted HCl from the reactor was neutralized before being directed into the atmosphere. Product samples were withdrawn from the bottom of the reactor at various time intervals to track the progress of the reaction. Finally, the desired product was separated by distillation under vacuum. The samples were analyzed by gas chromatography for organic compounds along with Karl Fischer titration for measuring the moisture content. Table 1 shows the summary of the hydrochlorination reaction (batch process) after completion of 7 hours.
Table 1
DCH Yield (%) Selectivity (%) Glycerol Conversion rate (%) a -MCH Conversion rate (%) ß -MCH Conversion rate (%) HCl Conversion rate (%)
75.7 94.2 100 79.57 13.3 49.8

Example 2: Continuous process for preparation of dichlorohydrin:
A mixture of ~6.2 Kg glycerol and 12% w/w of cyclohexane carboxylic acid, was fed in to a bubble column reactor. The bubble column reactor was connected to a plug flow reactor where fresh glycerol was fed during the steady state operation. The plug flow reactor was immersed in a hot oil bath, the outlet of plug flow reactor goes to the distillation unit where di-chlorohydrin was separated and remaining glycerol, monochlorohydrin, and catalyst was recycled back to the bubble column reactor via a venturi loop reactor. Bubble column reactor was heated to 110 oC and anhydrous HCl gas was purged in to the bubble column reactor at the rate of ~2 LPM. The unreacted HCl from bubble column reactor was received into the venturi loop reactor. The solution from bubble column reactor consisting of dichlorohydrin, monochlorohydrin, catalyst and concentrated liquid HCl then moved into the plug flow reactor where fresh glycerol was fed at a rate of 5 ml/min. A fast reaction between glycerol and concentrated liquid HCl occurred in the plug flow reactor at 80 oC to form monochlorohydrin. Dichlorohydrin, water and traces of HCl were separated by a distillation column and the high boilers consisting of glycerol, monochlorohydrin and catalyst were fed in to the bubble column reactor via the venturi loop reactor. The mixture of solution was then passed to the bubble column reactor at a rate of 8 ml/min from the venturi loop reactor at a steady state. The products were analyzed by Gas Chromatography. The process run was carried out for about 77 hours continuously.
At steady state, the concentrations of the major components from BCR and PFR reactors were noted as shown in Table 2.

Table 2
Glycerol
(%) Alpha-MCH
(%) Beta-MCH
(%) Alpha -DCH
(%) Beta –DCH
(%)
BCR 0.00 15.13 6.31 76.55 1.97
PFR 23.36 15.25 4.76 55.27 1.34

The yield of DCH produced was 65-70% of the total contents leaving the distillation column with 60-64% HCl conversion throughout the process. The said dichlorohydrin had a purity of 97-99%. During the entire operation, degradation of catalyst was not observed.