FR2460149A1 - Sepn. of volatile impurities from ester(s) - via vacuum evapn. and stripping columns, esp. to remove organic solvent and acid from ester used in lubricants - Google Patents

Abstract

Impurities such as an organic solvent and a volatile acid are efficiently removed from a liq., which is esp. an ester or ester mixt. obtd. by esterification. The liq. is first fed through a preheater (i) for heating by indirect heat exchange with a heat transfer fluid (HTF), and then via a pre-evaporator (2) to a plate column (3) followed by an evapn. and stripping column (4), which is indirectly heated via another HTF. At least the two columns (3,4) are operated at a reduced pressure; and the purified liq. ester is removed from the base (33) of column (4). The vapour formed in the pre-evaporator (2) is pref. subjected to progressive condensn. in column (3). Used e.g. for the removal of white spirit and heptanoic acid from a trimethylolpropane triheptanoate ester so the latter has a purity of 99.98%.

Description

 The present invention relates to a process for the separation of volatile impurities, such as organic solvent and volatile acid, in a liquid such as for example an ester or mixture of esters obtained by esterification.

It also relates to installations comprising application of this process.

 The manufacture of the esters used in the lubricating compositions usually comprises an esterification step, a distillation step to separate the ester or mixture of esters prepared and a step of refining this ester or mixture of esters. The distillation is currently carried out by a continuous process, preferably by continuous stripping evaporation, under conditions such that better elimination of the light or volatile fractions, that is to say the solvent and the excess acid used in the esterification step. The evaporator used in this step can be, for example, that described in American patent NG 3 19X 241 (I L. BAIRD) and which used a succession of annular elements and superimposed discs, in alternating arrangement, these annular elements or tubes being heated only 7 by indirect heat exchange with a thermal fluid, the vaporization of the liquid taking place while the latter flows in thin film on the internal wall of said annular elements, the film of liquid of each annular element coming from the disc with upper wall convex located above it and on which a condensation of liquid occurs. Such an evaporation-stripping process allows milder distillation conditions, a saving in calories and a more efficient separation of the volatile fractions.

 However, for certain liquids to be treated, the purification requirements are such that this process is still insufficient in this regard.

On the other hand, the process of the present invention allows, for the same flow rate of liquid, a more thorough purification, that is to say a more complete elimination of the volatile fractions, in particular of the acid constituents while retaining or improving the other advantages of the process known evaporation -stripping. In addition, the method of the invention, which is carried out under reduced pressure, only collects a very small total pressure drop.

 The separation process of the invention, of the type comprising evaporation combined with stripping, is characterized in that it consists in subjecting the liquid to be treated to a conventional prior evaporation by indirect heat exchange with a thermal fluid, in sending the fluid from this prior evaporation in a column of film trays, to collect and condense the vapor phase from this column, to be sent to an evaporation-stripping column, heated by indirect heat exchange with a thermal fluid, the phase not volatilized in this column of film trays, to send into said column of film trays the vapor phase coming from said evaporation-stripping column and to collect the liquid phase, constituted by the purified ester, coming from said column of evaporation-stripping, at least the column of film trays and the evaporation-stripping column working under reduced pressure.

The use of the column of film trays, in addition to the stripping-stripping column, makes it possible to prevent entrainment of fine droplets of liquid within the volatilized phase and, in general, without significantly increasing the pressure drops, to increase the efficiency of the separation of the volatile fractions in the whole apparatus.

In accordance with a preferred embodiment of the present invention, the fluid resulting from the aforementioned prior evaporation is sent to the base of the aforementioned film tray column, which proceeds by fractional condensation, then the aforementioned non-volatilized phase is collected at the base of said column of film trays in the form of a liquid which is drawn off and subjected to a conventional additional evaporation and then the fluid resulting from this new conventional evaporation is returned to the upper part of the evaporation column. stripping.

 The use of this additional evaporator, placed, relative to the path of the liquid phase, between the column of film trays and the evaporation-stripping column, has the effect of producing a new supply of calories independently of that produced in the evaporation-stripping column and to increase the temperature of the fluid feeding this column, which, for a given evaporationstripping column, increases the efficiency of the separation carried out by the latter and makes it possible to reduce the residence time of the fluid in this last 1 and thereby increase the flow of treated liquid.

The stage The aforementioned prior evaporation stage and the additional evaporation stage can be carried out using conventional evaporators, such as containers with a large evaporation surface heated externally and / or internally or , more advantageously, single-pass evaporators, ascending or descending, in which partial or total evaporation is carried out, for example by indirect heat exchange with a thermal fluid circulating in a bundle of parallel pa tubes around which the liquid to be evaporated circulates.

The aforementioned film tray column implements a section of film trays, of known structure, in which the liquid flows in a thin layer on perforated and very rap horizontal trays close to each other, passing from 'a tray to the one which is immediately located below it and this, by borrowing the perforations of said trays, which perforations are associated with weirs or inverted chimneys. The gas or vapor rises at the level of said chimneys and perforations and circulates between the consecutive plates, participating in maintaining the liquid in a thin layer, the aforementioned perforations being for this purpose offset from a plate to the one that follows it.

 According to another characteristic of the invention, the fluid resulting from the above-mentioned prior evaporation is sent to the base of the aforementioned film tray column, which proceeds by fractional condensation, while the vapor phase coming from the evaporation column said stripping is brought into the column with film trays at a level higher than that to which said fluid is brought.

 Finally, according to another characteristic of the process of the invention, the liquid to be treated is preheated - before being subjected to the aforementioned prior evaporation, this preheating taking place for example in a preheater of the type with a pin structure in which the liquid is preheated by means of a plurality of U-shaped tubes traversed by a heat exchange fluid, the liquid to be preheated circulating outside said tubes.

An installation for implementing the method of the present invention is characterized in that it comprises
(a) a conventional pre-evaporator supplied with liquid to be treated and re
linked by a vaporized liquid outlet pipe to a colon
no film trays, below the lower level tray
said column
(b) a column of film trays connected on the one hand, by a canali
vapor phase outlet from its upper part, at
condenser and, secondly, directly or indirectly, by its
lower part, at the upper part of a stripping evaporation column so that said evaporation-stripping column can receive the non-volatilized phase in the column of film trays and feed directly said column of film trays an vapor phase;
(c) an evaporation-stripping column provided with a support opening
rabies of the purified liquid phase constituting the main product
sought after and provided with indirect thermal heating means by a thermal fluid and, at its lower part, supply means
of an ascending entrainment gas of the vapor phase X and
(d) means for maintaining at least the column under reduced pressure
film trays and the evaporation-stripping column, as well
that means for indirect heating of said pre-evaporator and
said evaporation-stripping column.

In accordance with a preferred embodiment of the invention, this
the installation also includes a conventional complementary evaporator,
connected, on the one hand, by its supply pipe, to the column of trays
at a lower level than that to which the channeling of
outlet of vaporized liquid from the aforementioned pre-evaporator and, on the other hand, by
its outlet pipe, at the top of the evaporator column
tion-stripping.

According to another characteristic of the present invention, the colon of film trays is arranged just above the evaporator column
tion-stripping, the bodies of said columns being in direct communication,
and said film tray column has a first section formed
film trays and a second section located below said first section and rhymed with an annular reservoir of non-volatilized phase,
open at its upper part, and a central vertical chimney, open at its two ends and surmounted by a hat-forming element of larger
diameter than said chimney for the direct passage of vapors from
of the evaporation-stripping column and going into said first
section the outlet pipe of this additional evaporator
advantageously passes through an extreme lower part of the body of the co
lonne of film trays before opening at the top of the
body of the evaporation-stripping column.

Other characteristics, objects or advantages of the invention
will appear during the following description of a real mode
sation, given without limitation, with reference to the accompanying drawings
wherein
- Figure 1 shows a schematic overview of an installation
lation, according to the present invention, comprising application of
method of the invention.

- Figure 2 shows a schematic view, in vertical section mé
diane, of the assembly formed by the column of film trays and by
the evaporation-stripping column of the installation of the figurel
- Figure 3 shows a perspective view of the chimney, provided
from his hat, from the column of film trays of FIG. 2;
- Figure 4 shows an elevational view of a type of preheating
feur can be used in the installation of Figure 1; and
- Figure 5 shows a cross-sectional view, along 5 - 5 ide Figure 4, of this preheater.

The installation shown in Figure 1 is essentially
consisting of a preheater 1, a pre-evaporator 2, a column of film trays 3, an evapcration-stripping column 4, an evapo
additional rectifier 5, a condenser 6 and a reservoir 7 intended
collecting the condensed fractions.

The evaporation-stripping column 4 comprises two sections 4a placed in series one above the other, this evaporation-stripping column being supplied with entrainment fluid at its base (by
example by nitrogen gas) via line 8; references 9 and 10 respectively designate food pumps, the pump 9 being supplied with liquid to be treated by the supply line 11 tan
say that the pump 10 is supplied with a heat exchange fluid by
the supply line 129 the return of this exchange fluid ther
mique, intended to heat the tubular elements as well as the walls of
column 4, being provided by return line 13.

The preheater 1 is heated by a heat exchange fluid
brought by line 14 and returned by line 15. The fluid
heat exchange ensuring prior evaporation in the pre-evaporator
tor 2 enters it via line 16 and leaves it via line
Reading 17. In a similar manner, the thermal fluid supplying the
additional vaporizer 5 enters it via line 18
and out of it through line 19 while, as will be illustrated more
in detail later with reference to Figure 2, the fluid exchange ther
mock heating the tubular elements of the sections 4a and
4b of the evaporation-stripping column 4, enters via line 37a and leaves via line 20 for the section; on the one hand 4a, and
be in line 37b, leaving via line 22 for the
section 4b on the other hand. The coolant used in the
condenser 6 enters it through line 24 and leaves through
line 25; the incondensables escape from the on-
dancer by line 26. References 27 and 28 respectively designate the line for supplying the condensed fractions to the reservoir 7 and for withdrawing said condensed fractions
References 29 and 30 respectively designate the pipes for supplying the preheater 1 and the pre-evaporator 2 with the liquid to be treated. The pre-evaporator 2 is connected, by the vaporized liquid outlet pipe 31 to the column of film trays 3, at a level situated below the lowest film tray of the first section 3a, formed of film trays , of this column 3. The reference 32 designates the vapor phase outlet pipe coming from the upper part of said first section 3, this pipe connecting the column of film trays 3 to the condenser 6. The reference 33 designates the withdrawal pipe of the liquid phase at the bottom of section 4b of the evaporation-stripping column, this liquid constituting the main product sought. It is noted that the column of film trays 3 is located above the section 4a of the evaporation-stripping column 4, so that said evaporation-stripping column can directly feed the column of film trays 3 in the vapor phase. and that it can ~ receive the non-volatilized phase in said column of film trays.

 In the embodiment shown, this non-volatilized phase in the column of film trays is sent to the additional evaporator 5, via line 34, before being brought back, via line 35, into the junction zone of the column with film trays 3 and the evaporation-stripping column 4, which corresponds to the preferred embodiment of the invention. According to another embodiment of the invention, this non-volatilized phase in column 3 could directly feed the upper part of the evaporation -stripping column 4 It is noted that the non-volatilized phase in the column of film trays 3 is withdrawn therefrom, via the pipe 34, at a level lower than that which opens out, in said column 3, the vaporized liquid outlet pipe 31. The installation includes means for maintaining under reduced pressure which are connected to pipe 26 and which allow: partial vacuum of the dish column film waters 3, from the additional evaporator 5, from the evaporationstripping column 4 and from the pre-evaporator 2, the upstream part of the installation being at atmospheric pressure due to the presence of the valve 36.

This installation is furthermore provided with all the conventional means for effective control of continuous operation, namely manual and / or automatically controlled valves, flow regulators, temperature indicators, pressure gauges, pressure gauges, temperature recording, etc.

The evaporators 2 and 5 are conventional evaporators, preferably single-pass evaporators in which partial or total evaporation is carried out, for example by indirect thermal exchange with a thermal fluid, in particular a thermal fluid circulating in a bundle parallel licked tubes running through liquid to be evaporated.

 The column of film trays 4 shown in detail in Figure 2 comprises a first section 3a, consisting of a plurality of film trays such as 38, for example four to twelve trays of a type known per se on which the liquid flowed and streamed like a thin film, using the chimneys. inverted such as 39 to pass from one plate to the next, under conditions such that the gas phase can rise librsmentf, that is to say without having to pass through the liquid, such a system ensuring a flow with a very low loss of Below this first section, there is a second section 3b which is essentially formed by an annular reservoir 40 of non-volatilized phase, open at its upper part, and a central vertical chimney 41 open at two ends and surmounted of an element forming a cap 42. The perspective view of FIG. 3 shows that this cap can be supported by the chimney 41 by support arms 43, for example four in number. Figures 2 and 3, that this cap 42 has a larger diameter than that of the chimney 41, so that the liquid flowing from the lower level plate of the first section 3a and flowing on the convex upper surface of said cap can fall directly into the annular tank 40, without being able to descend into the chimney 41 or lead to the sending of droplets therein; we also note the particular profile of the birth of the pipes 31 and 32, to prohibit the introduction of liquid phase therein. The column of film trays 3 also has a third section which is constituted by its extreme lower part 3c, which is crossed by the outlet pipe 35 of the additional evaporator 5; we note that the section 35a of this pipe which is located inside said extreme lower part 30 comprises, in the wall 44 / upper of its bent part, a lateral opening S i allows the direct release of part of the vapors contained in the tributary of the additional evaporator 5.

We also see, in Figure 2, the inlet pipe
23 of the thermal fluid heating the wall of the section 4b of the evaporation-stripping column 4 (the wall of the section 4a being supplied with thermal fluid by the pipe 21 visible in FIG. 1) and the outlet pipe 13a of the heating fluid of the wall of the section 4a (the outlet pipe 13b of the thermal fluid for heating the wall 4b being visible in FIG. 1).

 The internal structure of each section of the particular evaporation-stripping column illustrated in FIG. 2 is not part of the present invention. The structure illustrated here is that of a colonist not made up, of demaniera per se known, of tubular elements (such as 45) and discs (such as 46) superimposed, in alternate arrangement. Any evaporation-stripping column ensuring a flow of the liquid in the form of a film and consequently a low pressure drop, at the same time as it makes it possible to ensure an adequate passage of a drive fluid to carry out the stripping can be used.

The preheater shown, by way of example, in FIGS. 4 and 5 is a preheater with a cylindrical body traversed, from bottom to top, by the liquid to be preheated which enters through the lateral pipe 29 and exits through the axial pipe 30, and the heating of which is ensured by a plurality of tubular elements, such as 47, inflames with U including the ex trematates. are respectively connected to the inlet and outlet pipes, such as 14 and 15, of preheating thermal fluid; in the example shown, these tubular elements are two in number, 47 and 48, as can be seen in FIG. 5.

Due to the above description, the operation of the installation shown in the accompanying drawings is obvious and will be explained briefly. The liquid to be treated, brought in via line 11, is preheated in the preheater 1 and subjected to prior evaporation or primary evaporation in the pre-evaporator 2 the mixture of liquid phase and vapor phase from this pre-evaporator is brought into the column of film trays 3 in which the vapor phase supplied by the pipe 31 and by the chimney 41 is subjected to progressive condensation; the resulting vapor phase is extracted through line 32; the resulting liquid phase collects in the annular tank 40 and is sent to the additional evaporator 5; the mixture of vapor phase and liquid phase from this evaporator is sent, via line 35, to the top of the evaporation-stripping column 4; the liquid phase resulting from the progressive distillation carried out in this evaporation-stripping column is drawn off through the pipe 33, at the base thereof, this liquid being for ~ e of the least volatile constituent (s), practical ~ ent btale ~ Ent free from volatile fractions which are collected in the tank 7, after condensation in the condenser 6.

By way of example, the treated liquid consists of an ester or mixture of lubricating esters in which the impurities consist of the excess reactants, namely fatty acids and a volatile organic solvent such as whita spirit; the process of the invention makes it possible to effectively separate these impurities from the ester or from the mixture of esters, under conditions such that it is no longer essential to carry out a subsequent refining operation, this separation taking place in particularly favorable conditions and without significant pressure losses in the evaporation apparatus.

A particular example of treatment of an ester is given below, in accordance with the process of the present invention.
In the installation of FIGS. 1 and 2, an impure ester consisting of, by weight, 79% of ethyl 2-triheptanoate, 2-hydroxymethyl, 1,3-propanediol (trimethylolpropane triheptanoate), 9, is treated. 7% heptanoic acid and 11.3 YD of white spirit, which results from the direct preparation of said ester by esterification. the feed rate is 405 liters / hour or 378 kg / h; the pressure provided by the vacuum device connected to the pipe 26 is 2.5 torr; it is 9 torr at the bottom of column 4 while the pressure difference between the bottom and the top of this column is 6 torr.

 The flow of nitrogen gas, used as a training substance in column 4, is 360 l / h (volume measured under normal conditions).

temperatures are as follows
- preheater 1: output 300 C
- pre-evaporate 2: inlet 300 C, outlet 1470 C
- column 3: vapor phase outlet: 1450 C
- additional evaporator 5: input 1210 E; exit 2284 C
- column 4: vapor phase outlet 195D C g withdrawn liquid: 2580 C
The compositions of the various liquid phases are as follows, by weight
- pre-evaporator outlet 2: ester 95%; 3.2ss acid; solvent 1.8 ql;
- additional evaporator 5 output: 99.6% ester; acid 0.4%;
solvent 0%
- base of column 4: ester 99.98%; 0.02% acid; solvent 0%
- liquid outlet from condenser 6: 3% ester; acid 46.3%;
solvent 50.7 days.

Ratio between the liquid phase (purified ester) drawn off at the bottom of
column 4 (W) and the feed-from this column (A): W / A = 0.77
(theoretical maximum: 0.79).

It is noted that the entrainment of the ester in the vapors from
of column 3 is weak and that the purified ester collected at the bottom of the
column 4 contains practically no more acid or solvent, at most
traces, so no further rectification is necessary
(changing the respective percentages of acid and pro solvent
comes from a selective ripple effect of the lightest fractions
solvent in the vacuum device;
Of course, the aforementioned example has been given only as a tractive and non-limiting illustration and ous other embodiments, variants or
equivalent means, within the reach of those skilled in the art, are also part
of the present invention.

Claims (10)

HEVENDICATIONS  1. Process for the separation of volatile impurities, tallas as organic solvent and volatile acid, with a high degree of efficiency, in a liquid to be treated, in particular by an ester or mixture of esters obtained by esterification, of the type comprising evaporation and a strip ping or drive, characterized in that it consists in subjecting the liquid to be treated to a conventional prior evaporation by indirect heat exchange with a thermal fluid, in sending the fluid resulting from this prior evaporation into a column of film trays, to collect and condense the vapor phase from this column, to send to an evaporation-stripping column1 heated by indirect heat exchange with a thermal fluid, the phase is volatilized in this column of film trays, to be sent to said column of film trays, the vapor phase coming from said stripping and collecting column and collecting the liquid phase, constitutes by the purified ester, resulting from said column of evaporatlonAstrippinss at least the column with film trays and the stripping evaporation column working under reduced pressure.  2. Method according to claim 1, characterized in that the fluid resulting from the above-mentioned prior evaporation is sent to the base of the aforementioned film tray column which proceeds by progressive condensation, in that the aforementioned non-volatilized phase is collected at the base of said column with film trays under the form of a liquid which is drawn off and subjected to a conventional cement evaporation, and in that the fluid resulting from this new conventional evaporation is returned to the upper part of the column evaporation-stripping.  3. Method according to claim 1 or 2, characterized in that the fluid resulting from the above-mentioned prior evaporation is sent to the base of the aforementioned film tray column which proceeds by progressive condensation and in that the vapor phase coming from the aforementioned evaporation-stripping column is brought into the aforementioned film tray column at a level higher than that to which said fluid is brought.  4. Method according to claim 1, 2 or 3 characterized in that the liquid to be treated is preheated before being subjected to the aforementioned prior evaporation.  5. Installation for implementing the method of claim 1, characterized in that it comprises: (a) a conventional pre-evaporator supplied with liquid to be treated and connected, by a vaporized liquid outlet pipe, to a column of film trays, below the lowest level tray of said column; (b) a column of film trays made on the one hand by a vapor phase outlet pipe coming from its upper part, to a condenser and, on the other hand, directly or indirectly, by its lower part, 8 the upper part of an evaporation-stripping column so that said evaporation-stripping column can receive the non-volatile phase used in the column of film trays and directly feed said column of film trays in phase steam; (c) an evaporation-stripping column provided with an opening for withdrawing the purified liquid phase constituting the main product sought after and provided with means of indirect thermal heating by a thermal fluid and, at its lower part, means of supply of an ascending entrainment gas from the vapor phase; and (d) means for maintaining under reduced pressure at least the column of the film trays and the evaporation-stripping column as well as means for indirect heating of said pre-evaporator and of said evaporation-stripping column.  6. Installation according to claim 5, characterized in that it comprises a conventional complementary evaporator, connected, on the one hand, by its supply pipe, to the column of film trays B at a level lower than that to which opens out the vaporized liquid outlet pipe from the aforementioned pre-evaporator and, on the other hand, through its outlet pipe, to the upper part of the evaporation-stripping column.  7. 7, Installation according to claim 5, characterized in that the column of film trays is arranged just above the evaporation-stripping column> the bodies of said columns being in direct communication, and in that said column of film trays have a first section formed of film trays, and a second section located below said first section, formed of an annular reservoir of non-volatilized phase, open at its top, and of a vertical chimney central, open at both ends and surmounted by a cap element of larger diameter, for the direct passage of vapors from the evaporation-stripping column and going into said first section.  8. Installation according to claims 6 and 7, characterized in that the outlet pipe of the above-mentioned additional evaporator passes through an extreme lower part of the body of the column of film trays precite before opening at the upper part of the body of the aforementioned evaporation-stripping column.  9. Installation according to claim 8, characterized in that the section of the outlet duct of the complementary evaporator which is located inside the extreme lower part of the column with film trays comprises at least one lateral opening of evolution of vapors in this lower part of the film tray column. 10. Installation according to one of claims 5 to 8, charac terized in that the aforementioned evaporation-stripping column comprises at least two sections in series each of which is heated by a separate circuit of thermal fluid, this column preferably being of the type with superimposed tubes and discs, in alternate arrangement.