Installation for separating a solvent from a mixture of solvent and hydrocarbons

A method of and system for recovering a solvent from a mixture of solvent and hydrocarbons, the system comprising at least two evaporation flasks successively fed with a charge consisting of said mixture to be separated, at least one steam generator performing the condensation of the solvent, a circuit for conveying the evaporated solvent and connecting the flasks to the generator and a circuit of an intermediate fluid in gaseous phase including a compressor for raising the condensation temperature of this fluid, the latter circuit connecting the generator to heat exchangers arranged upstream of each flask.

BACKGROUND OF THE INVENTION 
The present invention relates essentially to a method of extracting a 
solvent from a mixture of solvent and hydrocarbons without any outer heat 
supply. 
It is also directed to a system or plant for carrying out or practicing the 
method. 
A number of methods of and systems for liquid-liquid extraction with a 
solvent are already known which use solvents for separating families, 
groups or series of hydrocarbons. These methods and plants however are 
very much adversely affected by the power costs since it is necessary to 
subsequently separate the solvent from the extract and raffinate phases. 
This ultimate separation always requires a heat supply from the outside to 
the process or the plant, that supply taking place at a high heat level 
thereby substantially increasing the costs as is well understandable. 
There are thus known methods directed to collecting both extract-solvent 
and raffinate-solvent phases and heating them in a furnace or by an outer 
fluid to provide for the evaporation of the solvent and to bring the 
hydrocarbons to an adequate temperature in order to obtain a viscosity low 
enough to allow the elimination through stripping of the last traces of 
solvent. 
There are also known more performing methods which carry out successive 
evaporations in an order of increasing pressures. With such methods, the 
solvent is flash-evaporated and used to heat the feed of the foregoing 
flash-evaporation so that it is possible to reduce by about 30% the heat 
supply from the outside. 
These methods however exhibit a number of inconveniences. 
They require the use of a source of outer heat (furnace or hot oil) which 
is at a very high temperature and their operation or working is very 
unstable since the least disturbance in the temperature or the output of 
the hot source would be reflected on the plant and put same severely out 
of order. Moreover, these known methods and plants are of a complicated 
practice and use and require stacks of heat exchangers and of columns 
requiring cumbersome or bulky and expensive structures lending themselves 
badly to the reconstruction of old plants. 
SUMMARY OF THE INVENTION 
The object of the present invention is to cope in particular with the 
above-mentioned drawbacks by providing a method of and a system for 
recovering the solvent from solvent-hydrocarbons mixtures which are 
particularly simple, reliable or dependable and cheap in that they do not 
require any heat supply from the outside. 
For that purpose, the invention relates to a method of separating a solvent 
from a mixture of a solvent and hydrocarbons wherein in particular, an 
evaporation by stages of the solvent is carried out for separating it from 
the hydrocarbons, characterized in that the staged evaporation of the 
solvent is performed in a substantially isothermal manner by following an 
order of decreasing pressures and a heat exchanger is effected between the 
evaporated solvent and at least one intermediate fluid to obtain the 
condensation of the solvent and to recover its condensation heat in order 
that the intermediate fluid in gaseous phase may, after a suitable 
treatment, reheat the mixture and itself carry out the evaporation of the 
solvent without any heat supply from the outside being necessary to 
perform this operation. 
It should be pointed out that the treatment of the aforesaid intermediate 
fluid in gaseous phase involves comprising this fluid to raise its 
temperature so as to allow the vaporization of the solvent. 
In other words, the isothermal evaporation process of the solvent is 
coupled with a heat pump which recovers the condensation heat from the 
solvent and raises it to a thermal level high enough so that it may be 
used for the vaporization proper of the solvent. Moreover, the isothermal 
evaporation offers advantages of savings in high level energy thereby 
allowing covering of the needs in heat of this type by the heat due to the 
irreversibility of the compression in the heat pump. 
According to another characterizing feature of the method of the invention, 
the intermediate fluid recovering the condensation heat from the solvent 
is water. 
It should be further specified that during the staged evaporation of the 
solvent, the temperature preferably lies between 100.degree. C. and 
200.degree. C. 
The invention is also directed to a plant for carrying out the 
above-mentioned method and of the type comprising at least two evaporation 
flasks or the like successively fed with a charge consisting of a mixture 
of solvent and hydrocarbons to be separated, characterized by a least one 
steam generator providing for the condensation of the solvent, by at least 
one circuit for conveying the evaporated solvent and connecting the flasks 
to said generator and by at least one circuit of intermediate fluid in 
gaseous phase comprising means for raising the condensation temperature of 
this fluid and connecting said generator to at least one heat exchanger 
arranged upstream of each flask. 
It should be specified here that the means for raising the condensation 
temperature of the intermediate fluid in gaseous phase consists of at 
least one compressor. 
According to an exemplary embodiment, a plant according to the invention 
comprises three successive flasks for the evaporation of the solvent and 
is characterized in that the fluxes of vaporized solvent leaving the 
second and third flasks are brought together before reaching a first steam 
generator whereas the flux of vaporized solvent leaving the first flask is 
led to a second steam generator, the fluxes of condensed solvent which 
leave both aforesaid generators being brought together. 
According to still another characterizing feature of this plant, the flux 
of intermediate fluid in gaseous phase produced by both aforesaid 
generators feeds a heat exchanger upstream of the third flask and then 
divides to flow through both heat exchangers upstream of the first and 
second flasks respectively, and again forms a single flux flowing through 
a heat exchanger for reheating the charge introduced into the plant. 
It should be added here that the aforesaid single flux is connected to the 
steam generators.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The system shown on the single FIGURE is for instance the section for 
recovering the solvent in dewaxed oil of a unit for dewaxing lubricants. 
The solvent used may be a (50%-50% by volume) mixture of methylethyl ketone 
and toluene. 
The charge or batch consisting of a solvent-oil mixture is fed to the plant 
for instance at an absolute pressure of 500 kPa and at a temperature of 
39.degree. C. through a pipe line to form the flow or flux 1. The charge 
or batch is divided into two fluxes designated by reference numerals 2 and 
3, respectively and it is preheated in a heat exchange train comprising 
the heat exchangers E.sub.1, E.sub.2 and E.sub.3 arranged in parallel 
relationship and then the heat exchanger E.sub.4. 
In the heat exchanger E.sub.1, the charge or batch is reheated by the total 
flux of condensed solvent 29 and reaches the heat exchanger E.sub.2 
through the pipe line 4. 
In this heat exchanger E.sub.2, the flux 4 is reheated. by a steam flux 111 
to constitute the flux 6. 
In the heat exchanger E.sub.3, the flux 3 is reheated by dewaxed oil 23 
conveyed to storage facilities by the duct 24 and the flux 3 becomes the 
flux 5 which is brought together with the flux 6 to thereby form a single 
flux 7 reaching the heat exchanger E.sub.4. 
In this heat exchanger E.sub.4, the flux 7 is reheated up to the conditions 
prevailing in the evaporator flask or flash-evaporator B.sub.1 by means of 
condensed steam 109. The evaporator flask B.sub.1 operates at a 
temperature of 148.5.degree. C. and under an absolute pressure of 400 kPa 
and allows to vaporize about 40% of the solvent contained in the charge or 
batch passing through the duct 8. 
The flux of vaporized solvent leaves the flask B.sub.1 through the pipe 
line 9 whereas the liquid leaving this flask through the pipe line 10 is 
expanded in a valve V.sub.1 down to the pressure of a second evaporator 
flask B.sub.2 which operates under a pressure of 243 kPa, i.e. lower than 
the pressure of the flask B.sub.1 and at a temperature of 150.degree. C., 
i.e. substantially like that of flask B.sub.1. The mixed or combined phase 
forming the flux 10a after the valve V.sub.1 and leading to the evaporator 
flask B.sub.2 is reheated in heat exchangers E.sub.5 and E.sub.6 up to the 
aforesaid temperature of the evaporator flask B.sub.2. 
In the heat exchanger E.sub.5, the flux 10a is reheated by the flux 9 of 
vaporized solvent issuing from the flask B.sub.1 and this reheated flux 
10a forms the flux 11 which is in turn reheated by the heat exchanger 
E.sub.6 owing to the condensed steam flowing through the duct 107. 
The flash-evaporation in the evaporator flask B.sub.2 occurs as previously 
stated at a lower pressure than that of the flash-evaporation in the flask 
B.sub.1 thereby allowing to practically remove all the remaining solvent 
which issues from the flask B.sub.2 through the duct 13. 
The liquid leaving the flask B.sub.2 is pumped from the bottom of this 
flask and flows through the duct 14 and is reheated by two heat exchangers 
E.sub.7 and E.sub.8 arranged in parallel relationship up to a temperature 
of about 200.degree. C. which is the adequate temperature for carrying out 
the stripping of the hydrocarbons in a column C. 
More specifically, in the heat exchanger E.sub.8 the diverted flux 14a is 
reheated by dewaxed oil issuing from the column C through the duct 22. In 
the heat exchanger E.sub.7, the diverted flux 14b is reheated by the steam 
flowing through a duct 105 and generated by a steam compressor M. 
Upon leaving the heat exchangers E.sub.7 and E.sub.8, both diverted fluxes 
14a and 14b, which are at different temperatures, are blended again to 
form a flux 18 which feeds a flask B.sub.3. 
This flask operates at a temperature of 200.degree. C. and under an 
absolute pressure of 243 kPa like that of the flask B.sub.2. 
The liquid fraction 21 issuing from the flask B.sub.3 is then stripped in 
the column C by the steam 98 so as to remove the last traces of solvent in 
the flux 99. 
The dewaxed oil 22 leaving the column C is, as previously explained, 
carried to the storage facilities by the pipe line 24 after having been 
cooled in the heat exchangers E.sub.8 and E.sub.3. 
The vaporized solvent leaves the flask B.sub.3 through the duct 20 and this 
flux of vaporized solvent is mixed at 20a with the flux of solvent 13 
issuing from the flask B.sub.2 to form the flux of solvent 25 (at an 
absolute pressure of 243 kPa and at a temperature of 154.degree. C.). The 
vapours of the flux 25 are fully condensed and then subcooled after 
passing into a first heat exchanger or steam generator G.sub.1 performing 
the condensation of the solvent and which is fed with liquid water through 
a pipe line 100. The flux of solvent thus condensed forms the flux 26. 
The flux of vaporized solvent 9 leaving the first evaporator flask B.sub.1 
is partially condensed in the heat exchanger E.sub.5 and is led through 
the duct 27 to a second heat exchanger or steam generator G.sub.2 which 
provides for the full condensation and subcooling of the solvent vapours. 
The condensed solvent forms the flux 28 under the same temperature 
conditions as the flux 26. The flux 28 is then expanded in a valve (not 
shown) and then mixed with the flux 26 as seen at 28a to form the 
previously mentioned flux 29 which is cooled in the heat exchanger E.sub.1 
and then carried to the storage facilities through a pipe line 30. 
Now, the heat pump system will be described which consists of both steam 
generators G.sub.1, G.sub.2 fed with liquid water through the pipe lines 
100 and 102, respectively, of the compressor M and of the heat exchangers 
E.sub.2, E.sub.4, E.sub.6 and E.sub.7. 
The saturated steam produced by both steam generators G.sub.1 and G.sub.2 
and resulting from the recovery of the condensation heat of the solvent 
fluxes 25 and 26 passes into the ducts 101 and 103 which are joined 
together to form a flux 104 of saturated steam which is compressed by the 
compressor M. The latter comprises for instance two compression stages and 
the steam is desuperheated between both stages by water as shown by the 
arrow 15. 
At the outlet of the compressor M, the steam is at a temperature of about 
220.degree. C. and at an absolute pressure of about 580 kPa and this steam 
flowing through the duct 105 is used to supply high level heat to the heat 
exchanger E.sub.7 upstream of the third flask B.sub.3. Upon leaving this 
heat exchanger, the steam flows in a duct 106 and divides to form both 
ducts 107 and 109 extending through the heat exchangers E.sub.6 and 
E.sub.4 respectively, to heat the supplies of the flasks B.sub.2 and 
B.sub.1 respectively. The steam condensates flowing then through the ducts 
107a and 109a are blended to form the flux 111 and are subcooled down to 
117.degree. C. and then expanded in a valve V.sub.2 down to the absolute 
pressure of 180 kPa for eventually flowing back to the steam generators 
G.sub.1 and G.sub.2 through the pipe lines 100 and 102. 
Reference should now be had to the following table showing the advantages 
of the system which has just been described with respect to known systems 
which use a heat supply from the outside to perform the evaporation of the 
solvent whereas the plant according to the invention does not use it. 
TABLE 
______________________________________ 
Selected example: Dewaxed oil section of a 
solvent-dewaxing unit (capacity: 120,000 t/year of oils) 
Systems 
Known systems 
(3 or 4 System of the invention 
evaporator (3 evaporator flasks with 
Utilities flasks) heat pump) 
______________________________________ 
Utilities consumption 
Fuel (kg/t of solvent 
7.6 0 
in the charge) 
Electricity (kW/t 
1.8 12.9 
of solvent in the 
charge) 
Stripping steam 
7.9 7.9 
(kg/t of solvent in 
the charge) 
Primary energy 
Total (kcal/kg of sol- 
84 33.5 
vent in the charge) 
______________________________________ 
It appears straightforwardly from this table that the gain in primary 
energy represents about 60% with respect to the known systems. 
It has therefore been provided, according to the invention, a method of and 
a system for solvent recovery which exhibit a much higher power yield or 
efficiency and which do not require any heat supply from the outside, 
which heat supply is used in particular to compensate for the 
irreversibilities and losses of the system. Now, in the diagram according 
to the invention, the irreversibilities are minimized and the thermal 
degradation is reduced. In other words, the heat between the process 
fluids and the fluid of the heat pump is transferred with a minimal 
temperature degradation thereby enabling the system to work under optimum 
power conditions. 
It should be also pointed out that the solvent is not heated up to high 
temperatures upon the evaporation and would therefore undergo a lesser 
thermal degradation. 
It should further be noted that the plant of the invention exhibits an 
outstanding operating stability owing to the recovered heat being mixed at 
the heat pump and redistributed in parallel relationship between the 
points of evaporation of the solvent, thereby allowing to separately 
adjust the heat to be supplied to each flash evaporating step. 
As previously explained, the evaporation of the solvent in the flasks 
B.sub.1 and B.sub.2 is performed in an order of decreasing pressures so as 
to allow the evaporation of a very substantial amount of solvent while 
remaining at a substantially constant temperature which may for instance 
lie between 100.degree. C. and 200.degree. C. This still allows to 
minimize the irreversibilities and to have a call or demand for 
concentrated heat within a very narrow range of temperatures, thereby 
being perfectly suitable for the use of a heat pump. 
The invention at last provides a method of and a system for solvent 
recovery which exhibit outstanding results owing to the use of an 
isothermal evaporation scheme of the solvent coupled with a heat pump 
recovering the condensation heat of the solvent and raising it to a 
thermal level high enough to enable the same to be used to provide for the 
vaporization proper of the solvent. 
It should be understood that the invention is not at all limited to the 
embodiment described and shown which has been given by way of example 
only. 
Thus, the method according to the invention may quite well be incorporated 
into old solvent recovery systems. 
This means that it comprises all the technical equivalents of the means 
described as well as their combinations if same are carried out according 
to its gist.