US2324088A - Distillation of high boiling esters - Google Patents

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Ju1y13, 1943.y J'. E. JEWETT 2,324,088 l DISTILLATION OF HIGH BOILING ESTERS Filed April 16, 1940 I INVENTOR HMP/f @asf/H dfn/77 BY f,

A TTORNEYN Patented July 13, 1943 AUNITED ,STATESY PATENT OFFICE.

DISTILLATION OF HIGH BOILING ESTERS Joseph E. Jewett, Larchmont, N. Y., assigner to American Cyanamd Company, New York, N. Y., a corporation of Maine Application April 16, 1940, Serial No. 329,849

(Cl. 21R-'52) 1 Claim.

This invention relates to a process for distilling high-boiling organic esters, and more particularly to a process for the vacuum distillation of high-boiling esters of dicarboxylic acids.

During the distillation of high-boiling organic esters, decomposition is likely to occur as a result of heating the product to a high temperature and maintaining it at the high temperature for a prolonged period of time. To avoid this decomposition, it has been attempted in the past to apply vacuum .distillation methods to processes for distilling these high-boiling esters.

'I'he most common method of vacuum distillation, which consists of distilling material under sufllciently reduced pressure to give the required boiling point, is unsatisfactory due chiefly to overheating of the bottom portions offmaterial caused by the hydrostatic head developed in the still. Consequent decomposition is considerable when the decomposition range lies close to the boiling point under the reduced pressure used. Another factor which greatly contributes to the decomposition of the material undergoing distillation is the necessity of maintaining the entire charge at the distillation temperature throughout the entire distillation.

Another method frequently applied provides for introduction of material into a highly heated tube of small diameter under vacuum. Evaporation takes place in the tube and the material is ejected therefrom at a high velocity by expansion of the vapor formed. It is obvious that this method is designed to avoid maintenance of the entire distilland at the distillation temperature throughout the operation. .However, there are two inherent causes of overheating present in this method. First, if the length of the tube (as determined by experiment) is not just exactly right, there will beoverheating of the liquid and/Orsuperheating of the vapor. Second, uid friction in the tube will cause a pressure drop which will, in turn. allow overheating at the feed end since the boiling point along the length of the tube varies with pressure.

lAccording to the present invention, I have found that high-boiling organic esters may be distilled with little, if any, consequent decomposition if an excess of the material isiirst heated to a temperature just below the decomposition temperature and then flashed in a low pressure chamber. Undistilled material is returned, through a cooler if desired, to the charge and recirculated.

It is believed that the process of the present invention can best be understood by reference to the accompanying drawing in which Fig. 1 is a diagrammatic illustration of an apparatus suitable for distilling high-boiling esters of dicarboxylic acids according to the process ov.- lined above.

The charge of ester to be distilled is placed in vtank I and from there is delivered to heater 6 through seal loop 5, by meansof a circulating pump 3 which is driven by motor 4. The heater 6 is a tubular heater of conventional design heated by a medium which is introduced through pipe 21 and leaves through pipe 3|.

After the material is heated to a temperature below that at which vaporization tak'es place, which temperature is regulated by the pressure existing on the system, the rate of flow, and the controller I8 which will be described in detail later, it passes through pipe 3l' into ilash chamber 8 Where it strikes plate 38 and is spread in a sheet or spray, thus affording a large area for flash evaporation. Enough vapor is evolved to cool the liquid to its boiling point under the reduced pressure existng'in flash chamber 8' and the cooled material is returned through pipe. 35, cooler 28, the cooling medium for which enters through pipe 29 and leaves through pipe 30, and pipe 3B to seal chamber 2 from which it# overows into tank I.

Vapors from chamber 8 pass through pipe I I to condenser I 2 which is tted with cooling tubes I2a, the cooling medium for which enters through pipe I3 and leaves through pipe I4. The condensed liquid then flows through pipe 32 and seals 33 and 33a into storage tanks 34 and 34a. Any uncondensed vapors leave the condenser I2 through pipe I5 to vacuum pump I6, from which they are ejected through pipe I'I.

If the material being. distilledis not particularly heat sensitive and not easily decomposed, the cooler 28 may be eliminated, the unvaporized material then returning directly from the ash chamber 8 to tank I. This feature of the apparatus is entirely optional and may 'or may not be used, depending on the material.

Flash chamber 8 may be tted with fractionating plates 9 and reux condenser coil I0 if a.l

' seconds.

on two sides of orifice 39. The boiling point of the material under reduced pressure in the flash chamber v8 is determined, and this temperature applied to one bulb I9 of a dierential temperature controller I8 which holds a Xed temperature difference between bulbs I9 and 20 by throttling the heating medium passing through controller valve 2|.

Increase of pressure in theash chamber may cause the temperature determinedby the absolute pressure in chamber 8 plus the set differential, which sum is the temperature on bulb 20, to go above the predetermined safe limit. In order to avoid consequent decomposition of material being distilled, the same temperature as on bulb is applied to bulb 23 of an electrically operated limit controller 22 which has been set at the appropriate maximum point.Y The contacts of Y this controller 22 are opened, the coil of solenoid valve 24 is de-energized, the valve closes and air prevent reverse operation of valve 25 unti1 it can be reset by hand.

The above-described conventional automatic temperature controller may be omitted and if desired, the control may be manual by reference to appropriate thermometers.

The present process, since it involves a flash distillation, is more rapid than the formerly used vacuum distillation methods and this feature is an advantage of my invention.

It is another advantage of the present invention that there is no overheating of rmaterial or superheating of vapor. Only a small portion of the substance is heated to distillation temperature at one time and it is maintained at that tem perature for-only a very short time, i. e., a few Thus, it will be apparent that decomposition of the material is avoided or appreciably decreased.

It is a further advantage of the invention that the use of an excess of material, coupled with the return of undistilled portionsfor recirculation. prevents the loss of any-substantial amount of material. It is thus unnecessary to maintain a high temperature for more than a very short time since undistilled material is automatically nm through the apparatus again.

Another advantage of the process of the present invention becomes particularly apparent when the ester being distilled is one which is extremely easily decomposed. -In such cases, it is possible to articially cool the undistilled material, which is already somewhat .cooled by the flash evaporation process, on its return to the tank. In many cases where the ester is notl particularly heat sensitive, the cooling attendantupon the flash evaporation itself is suillcient to prevent substantial decomposition of the undistilled portion of the ester without any further cooling by artificial means. i

The process of the present invention described `above in considerable detail is applicable to all dicarboxylic acids which comprises heating a.

continuous, conned stream of the material to be distilled to a temperature below that at which decomposition takes place, rapidly vaporizing the preheated material in a ash chamber maintained under a vacuum, condensing the vaporized portion of the material, cooling the unvaporized portion sufficiently to prevent substantial decomposition of the ester, and returning the cooled unvaporized portion of the material for recirculation.

JOSEPH E. JEWE'I'I.

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