Apparatus and process for flushing a simulated moving bed comprising at least two fluid distribution lines

An apparatus and process are described for simulated moving bed separation in which line containing fluid containing the desired product is flushed at least once by a secondary fluid entering each distribution plate or by a fluid leaving each of said plates during at least a portion of a period of time between two successive permutations of the principal supply lines and the principal extraction lines, or during the entirety of said periods, the secondary fluid being selected from the group formed by the solvent, the fluid containing the desired product and the fluid containing the desired product freed of at least a portion of the solvent.

FIELD OF THE INVENTION 
The invention concerns a rinsing or flushing apparatus and process in a 
simulated moving bed separation apparatus comprising at least two 
circulation lines connecting the distribution plates to external fluids. 
BACKGROUND OF THE INVENTION 
The prior art is particularly illustrated in European patents EP-A-0 688 
590, EP-A-0 415 822, EP-A-0 075 611 and U.S. Pat. No. 5,156,736. 
In simulated moving bed separation processes such as those carried out in 
the "Sorbex" series of processes, among them the EX.RTM., MOLEX.RTM., 
SAREX.RTM., OLEX.RTM., and EBEX.RTM. processes, a plurality of beds are 
used which are localised in one or two adsorption columns. Each 
distributor plate situated between two consecutive beds is connected to 
the exterior by means of a single line leading into a rotary valve which 
brings each of the beds in succession into communication with each of the 
streams entering or leaving the adsorption section in sequence. Such 
streams comprise: 
1) the feed to be separated constituted by a mixture of at least two 
products: 
A the most adsorbed in the beds and B the least adsorbed, or retarded, in 
the beds; 
2) the solvent or desorbent which elutes or desorbs the constituents of the 
feed; 
3) the extract, constituted by a mixture of the most adsorbed product (A) 
and the desorbent; 
4) the raffinate, constituted by a mixture of the least adsorbed product 
(B) and desorbent; 
5) the in flush, or in rinse, constituted by a mixture of extract and 
desorbent, which can flush the slug of feed trapped in the common line 
after the feed has been introduced into the adsorber into the interior of 
the adsorber; 
6) the out flush, or out rinse, constituted by a mixture of extract and 
desorbent, which draws the slug of extract which is trapped in the common 
line after the extract has been extracted from the adsorber toward the 
exterior. The in flush and out flush flow rates are equal, and a pump 
places the out flush stream in communication with the in flush. The flush 
flow rate is calculated so that the volume of the longest line connecting 
the rotary valve to the furthest bed is flushed 2 to 3 times during a 
permutation period; 
7) the secondary flush can be constituted either by desorbent, or by 
extract which is depleted in desorbent. Its aim is to flush the extremity 
of the common line so as to flush any impurity which may have lodged there 
by diffusion or exchange into the interior of the adsorber just before 
extracting the extract. 
The disadvantage of this type of process is that each of the common lines 
must be flushed between introducing the feed and extracting the extract 
and between extracting the extract and introducing the desorbent, if a 
high purity of constituent A is desired in the extract. The flush flow 
rate linked to the highest volume of flushed line is far from negligible 
in the light of the feed flow rate, and it has the effect of causing the 
system to operate slightly off the optimum flow rate in the different 
zones. 
A further disadvantage of coupling by means of the rotary valve of the in 
flush and out flush is that this requires a pump, a flow meter and a flow 
rate regulating valve since during a cycle, the pressure of the out flush 
can easily be lower than the pressure of the in flush. 
Further, the flow rate regulating system in the in flush, out flush loop is 
not particularly suitable for a programmable flow rate which varies, for 
example, from zero over a certain portion of the period to a certain 
reference value during another portion of the period, thus allowing 
effective flushing with a minimum displaced volume. 
An alternative technique which is used in the Eluxyl process, for example, 
consists of connecting each distributor plate located between two 
consecutive beds to the exterior by at least two distinct circulation or 
distribution lines. 
It also contains a distinct on-off valve per distributor plate and per 
principal entering or leaving stream (desorbent, extract, feed, 
raffinate). 
In principle, if one of the two lines is used for "clean" fluids (desorbent 
or extract), and the other is used for "dirty" fluids (feed or raffinate), 
flushing each of the two common lines becomes superfluous. If not just two 
lines dedicated to "clean" and "dirty" fluids are used, but four distinct 
lines are used each connecting each of the principal streams to the 
distributor plate, flushing such lines is in principle of no use. 
However, each of such lines leads into the principal stream circulating 
from one bed to the next and the extremity of the two lines (dedicated to 
clean and to dirty fluids) or the extremity of the four lines dedicated to 
extract, raffinate, feed or desorbent may be contaminated by exchange or 
diffusion with the principal fluid. When the purity and yield are to be 
maximised, such contamination becomes deleterious. 
SUMMARY OF THE INVENTION 
The aim of the invention is thus to overcome this disadvantage by carrying 
out flushes where the volumes or flow rates are as small as possible and 
in any case lower than those of processes using a rotary valve and a 
single line which is common to the four principal streams per bed. 
A second aim of the invention is to minimise the flushing volumes by 
increasing their efficiency, by flushing at a very high flow rate for only 
a portion of the period. 
More precisely, the invention concerns a counter-current or co-current 
simulated moving bed separation apparatus which is combined with a line 
flushing apparatus which transports various fluids. In more detail, there 
is provided a simulated moving bed separation apparatus comprising a 
plurality of interconnected chromatographic columns or column sections (2, 
4, 6), a fluid distributor plate (3) between each column section, at least 
two (10, 30) and at most four (10, 20, 30, 40) distinct circulation lines 
connected to the distributor plate (3), each circulation line being 
connected to a different line selected from two supply lines (100, 300) by 
which the feed and the desorbent enter and two extraction lines (200, 400) 
by which a fluid containing the desired product and a fluid containing the 
unwanted product or products leave; in which a first circulation line (10) 
is connected to two lines (100, 200) in which the desorbent and the fluid 
containing the desired product circulate respectively, a second 
circulation line (30) is connected to a feed supply line (300) and a third 
circulation line is connected to an extraction line for fluid containing 
the unwanted product (400); or in which a first circulation line is 
connected to two lines (300 and 400) in which the feed and the fluid 
containing the unwanted product or products respectively circulate, a 
second circulation line is connected to a desorbent supply line (100), and 
a third line is connected to a line for extracting fluid containing the 
desired product (200); or in which a circulation line (10) is connected to 
two lines in which the desorbent (100) and the fluid containing the 
desired product (200) respectively circulate, and the other circulation 
line (30) is connected to two lines in which the feed (300) and the fluid 
containing the unwanted product or products (400) circulate respectively. 
The apparatus is characterized in that the line (10) for circulating the 
fluid containing the desired product comprises a flushing line (250) for a 
secondary incoming fluid (desorbent, fluid containing the desired product 
or fluid containing the desired product depleted in desorbent) or for an 
outgoing fluid (250) (mixture of desorbent and fluid containing the 
desired product). 
At least one other of the circulation lines (30) can comprise a flushing 
line (350) for a fluid (desorbent) entering the distributor plate or for a 
fluid (350) leaving the distributor plate (feed, fluid containing the 
unwanted products). 
In a first variation, the fluid in flush line (250 or 350) comprises at 
least one pressurised chamber or a pump (101 or 253) for supplying said 
fluid respectively connected to a flow rate regulation means (351, 352 or 
256, 257, FIG. 1). 
In a second variation, the fluid out flush line (350 or 250) comprises a 
flow rate regulation means (352, 351 or 256, 257). 
The invention also concerns a process using the apparatus. In more detail, 
a simulated moving bed separation process is provided which is carried out 
in a separation zone or adsorber comprising a plurality of interconnected 
columns or column sections, a fluid distributor plate between each column 
section, at least two (10, 30) and at most four (10, 20, 30, 40) distinct 
circulation lines connected to the distributor plate, each line being 
connected to a different line of the four lines containing the four 
principal streams (fluid containing the desired product, fluid containing 
the unwanted product, feed, desorbent); in which a first circulation line 
(10) is connected to two lines (100, 200) in which the desorbent and the 
fluid containing the desired product circulate respectively, a second 
circulation line (30) is connected to a feed supply line (300) and a third 
circulation line is connected to an extraction line for fluid containing 
unwanted product (400); or in which a first circulation line is connected 
to two lines (300 and 400) in which the feed and the fluid containing the 
unwanted product or products respectively circulate, a second circulation 
line is connected to a desorbent supply line (100), and a third line is 
connected to a line for extracting fluid containing the desired product 
(200); or in which a circulation line (10) is connected to two lines in 
which the desorbent (100) and the fluid containing the desired product 
(200) respectively circulate, and the other circulation line (30) is 
connected to two lines in which the feed (300) and the fluid containing 
the unwanted product or products (400) circulate respectively. The process 
is characterized in that the line (10) containing the fluid containing the 
desired product is flushed at least once with a secondary fluid entering 
each distributor plate or by a fluid leaving each of said plates during at 
least a portion of a period of time between two successive permutations of 
the principal supply lines and the principal extraction lines, or during 
the totality of said periods, the secondary fluid being selected from the 
group formed by the desorbent, the fluid containing the desired product 
and the fluid containing the desired product freed of at least a portion 
of the desorbent. 
In one feature of the process regarding the lines for the "clean" fluids, 
the line (10) containing the fluid containing the desired product is 
flushed by the secondary fluid which is of substantially the same 
composition during at least a portion of the period, said flushing being 
sequential, one plate at a time, all of the plates being flushed 
successively during the course of one cycle. 
Said line can be sequentially flushed by the secondary fluid, downstream of 
the extract extraction and upstream of the feed supply if the desired 
product is in the extract and downstream of the raffinate and upstream of 
the desorbent if the desired product is in the raffinate. 
Said line (10) containing the fluid containing the desired product can be 
flushed by the fluid containing the desired product or the desorbent or 
said fluid depleted in desorbent, upstream of the feed supply for a 
portion of the period then downstream of the extract extraction during a 
further portion of the period, the two positions thus defined being 
distinct, if the desired product is in the extract. 
In a variation, said line is continuously flushed by the secondary fluid 
over all of the plates at once during all of the periods of the cycle. 
Further, the lines containing a "dirty" fluid (for example feed or 
raffinate) can also be flushed. 
Thus in a first variation, the line (30) containing the fluid containing 
the unwanted product or products is sequentially flushed at least once 
with desorbent entering the distributor plate between the extract 
extraction and the feed supply during at least a portion of the period, 
preferably during the entire period, when the desired product is in the 
extract. 
In a second variation, the line containing the fluid containing the 
unwanted product or products is sequentially flushed at least once with 
the fluid contained in the desorption zone for the desired product which 
leaves a distributor plate between the desorbent supply and the extract 
extraction, preferably near the desorbent supply, during at least a 
portion of the period. 
Finally, in a third variation, the lines containing the unwanted product or 
products from all of the distributor plates are continuously flushed using 
the fluid contained in the adsorber. 
In a further characteristic of the invention, when the distributor plate 
comprises three or four circulation lines, the line transporting the 
desorbent (100) can be flushed by desorbent (line 150). 
The line containing the fluid containing the desired product can be flushed 
with a ratio of the flushing fluid flow rate to the feed flow rate which 
is in the range 0.005 to 0.4, advantageously in the range 0.02 to 0.15, 
and preferably in the range 0.04 to 0.08. 
The same ratio can be used for flushing the line containing the fluid 
containing the unwanted product. 
The invention will be better understood from the figures which 
schematically show embodiments of the invention, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A simulated moving bed separation unit is constituted by at least one 
column 1 separated into a plurality of beds or sections 2, 4, 6. . ., the 
number of beds being in the range 4 to 24. Each bed is filled with an 
adsorbent, for example an X or Y zeolite exchanged with a group IIa cation 
and a group Ia cation, when para-xylene is to be separated from a C.sub.8 
aromatic cut. 
Apart from the bed located at the lower extremity of each column, each bed 
is separated from the bed immediately below it by a distributor 3, 5 . . . 
. This distributor is connected to the exterior of the column by either 2 
circulation lines (10 and 30), see FIG. 1, or by 4 circulation lines (10, 
20, 30, 40), see FIG. 2. 
Referring to FIGS. 1 and 2, a desorbent line 100 supplies each bed via 
valves 21a, 61a. Desorbent is successively sent to each bed via line 100 
by means of a pump 101 and its flow rate is precisely regulated by means 
of a flow meter 102 and a control valve 103. 
An extraction line 200 serves each of the beds via valves 22a, 62a. The 
extract is successively extracted from each bed at a controlled flow rate 
via line 200 by means of a flow meter 201 and valve 202 then directed to 
distillation column 203 where para-xylene, for example, is extracted 
overhead via line 206 while desorbent constituted essentially by 
para-diethylbenzene is extracted via line 205 before being returned to 
pump 101. This column also comprises an extraction plate 204 for 
extracting an extract which is depleted in desorbent. 
A feed line 300 serves each bed via valves 41a, 81a. The feed is 
successively sent to each bed via line 300 by means of a pump 301 and its 
flow rate is precisely regulated by means of a flow meter 302 and a 
control valve 303. Raffinate is successively extracted from each bed under 
controlled pressure via line 400 by means of control valve 401 and 
pressure sensor 402 located on the column. It is directed to a 
distillation column 403 where a mixture of paraffins and naphthenes, 
ethylbenzene, meta-xylene and ortho-xylene, for example, is extracted 
overhead via line 405 while the desorbent constituted essentially by 
para-diethylbenzene is extracted via line 404 towards pump 101. 
Referring to FIG. 1 alone, the distributor plates communicate with the 
exterior via two lines: 10 and 30 for plate 3, 50 and 70 for plate 5, and 
so on. Lines 10, 50 lead into lines 21, 22 and 23 and into lines 61, 62, 
63 respectively. These lines transport the clean fluids, for example 
extract and desorbent if the desired product is in the extract. These 
lines 10, 50 can be flushed either 10 continuously or sequentially by 
means of lines 23, 63 and valves 23a, 63a respectively. When flushing is 
to be continuous, valves which can regulate a constant and substantially 
even flow rate whatever the introduction position are used. In contrast, 
when flushing is to be sequential, on-off valves are used. Valves 23a, 63a 
can place each distributor 3, 5 in communication with a line 250 to either 
extract or inject a flushing fluid into lines 10, 50. The following can be 
injected via line 250: 
either desorbent, in which case valve 251a is opened and the flow rate is 
regulated using flow meter 256 and control valve 257 (valves 254a, 255a 
and 258a are closed); 
or extract, in which case valve 255a is opened. Pump 253, flow meter 256 
and control valve 257 send a regulated flow of extract to line 250 (valves 
251a, 254a and 258a are closed); 
or extract which is depleted in desorbent, in which case valve 254a is 
opened, pump 253, flow meter 256 and control valve 257 send a regulated 
flow of depleted extract to line 250 (valves 251a, 255a and 258a are 
closed). 
The contents of lines 10, 50 can be extracted via line 250. Valves 255a and 
258a are opened (valves 251a, 254a are closed), and flow meter 256 and 
control valve 257 extract a regulated flow of extract-desorbent mixture 
and send it to distillation column 203. 
Lines 30, 70 lead into lines 41, 42, 43 and lines 81, 82, 83 respectively. 
These lines transport the dirty fluids, for example the raffinate and feed 
if the desired product is in the extract. These lines 30, 70 can be 
flushed either continuously or sequentially by means of lines 43, 83 and 
valves 43a, 83a. When flushing is to be continuous, valves which can 
regulate a constant and even flow rate whatever the introduction position 
are used. In contrast, when flushing is to be sequential, on-off valves 
are used. Valves 43a, 83a can place each distributor 3, 5 in communication 
with a line 350 to either extract or inject a flushing fluid into lines 
30, 70. 
Desorbent can be injected via line 350, in which case valve 353a is opened, 
valve 354a is closed, and control valve 352 and flow meter 351 regulate 
the injection flow rate. 
A mixture of feed and raffinate can be extracted via line 350 and returned 
to distillation column 403 (valve 354a open and valve 353a closed) at a 
flow rate which is regulated by control valve 32 and flow meter 351. 
Referring to FIG. 2 alone, the distributor plates communicate externally 
with four circulation lines: 
10, 20, 30, 40 for plate 3; 
50, 60, 70, 80 for plate 5. 
Lines 24, 64 are connected to lines 20, 60. Lines 20, 60 exclusively 
transport desorbent from line 100 or desorbent for flushing from line 150 
via valves 24a, 64a. They can be flushed either continuously or 
sequentially by means of lines 24, 64. When flushing is to be continuous, 
valves which can regulate a constant and substantially even flow rate 
whatever the introduction position are used. In contrast, when flushing is 
to be sequential, on-off valves are used. 
Valves 24a, 64a can place each distributor 3, 5 in communication with a 
line 150 to either extract or introduce a flushing fluid into lines 20, 
60. 
Desorbent can be injected via line 150. Valve 151a is opened and the flow 
rate is regulated by means of flow meter 152 and control valve 153 (valve 
154a is closed). A mixture of desorbent and extract can be extracted via 
line 150: valve 154a is opened (valve 151a is closed) and the flow rate is 
regulated by means of flow meter 152 and control valve 153. 
Lines 23, 63 are connected to lines 10, 50. Lines 10, 50 exclusively 
transport extract via valves 22a, 62a (towards line 200) and a flushing 
stream from or to line 250 via valves 23a, 63a. 
They can be flushed either continuously or sequentially. When flushing is 
to be continuous, valves which can regulate a constant and substantially 
even flow rate whatever the introduction position are used. In contrast, 
when flushing is to be sequential, on-off valves are used. 
Valves 23a, 63a can place each distributor 3, 5 in communication with line 
250 to: 
either inject desorbent; in which case valve 251a is opened and the flow 
rate is regulated using flow meter 256 and control valve 257 (valves 254a, 
255a and 258a are closed); 
or inject extract; in this case, valve 255a is opened, pump 253, flow meter 
256 and control valve 257 send a regulated flow of extract to line 250 
(valves 251a, 254a and 258a are closed); 
or inject extract which is depleted in desorbent; in which case valve 254a 
is opened, pump 253, flow meter 256 and control valve 257 send a regulated 
flow of depleted extract to line 250 (valves 251a, 255a and 258a are 
closed); 
or extract the contents of lines 10, 50, in which case valves 255a and 258a 
are opened (valves 251a, 254a are closed). Flow meter 256 and control 
valve 257 extract a regulated flow rate of a mixture of extract and 
desorbent and send it to distillation column 203. 
Lines 44, 84 are connected to lines 40, 80. Lines 40, 80 exclusively 
transport feed via valves 41a, 81a (from line 300) or desorbent for 
flushing the line from line 450 via valves 44a, 84a. 
They can be flushed either continuously or sequentially by means of lines 
44, 84. When flushing is to be continuous, valves which can regulate a 
constant and substantially even flow rate whatever the introduction 
position are used. In contrast, when flushing is to be sequential, on-off 
valves are used. 
Valves 44a, 84a can place each distributor 3, 5 in communication with line 
450 to: 
either inject desorbent, in which case valve 453a is opened, valve 454a is 
closed and the flow rate is regulated using flow meter 451 and control 
valve 452; 
or extract the contents of lines 40, 80, in which case valve 454a is 
opened, valve 453a is closed and the flow rate is regulated using flow 
meter 451 and control valve 452. 
Lines 43, 83 are connected to lines 30, 70. Lines 30, 70 exclusively 
transport raffinate via valves 42a, 82a (to line 400) and a flushing 
stream to line 350 via valves 43a, 83a. 
These lines can be flushed either continuously or sequentially. When 
flushing is to be continuous, valves which can regulate a constant and 
substantially even flow rate whatever the introduction position are used. 
In contrast, when flushing is to be sequential, on-off valves are used. 
Valves 43a, 83a can place each distributor 3, 5 in communication with line 
350 to: 
either inject desorbent, in which case valve 353a is opened, valve 354a is 
closed, and control valve 352 and flow meter 351 regulate the flow; 
or a mixture is extracted from the adsorber via lines 30, 70 and returned 
(valve 354a open, valve 353a closed) to the raffinate distillation column 
403. The flow rate is regulated by flow meter 351 and control valve 352. 
The following examples illustrate the invention: 
Descriptive Section Which is Common to Examples 1 to 19 
A simulated moving bed separation unit constituted by 24 adsorbent beds was 
disposed in two columns, each with twelve beds. The internal diameter of 
each bed was 915 mm. The heights of beds n.degree. 1 to 11 and 13 to 23 
were all substantially the same while the heights of beds 12 and 24 were 
reduced: in accordance with French patent FR-A-2 721 529, the beds located 
near the recycling pumps were shorter to compensate for the effects of the 
dead volume in each recycling loop. 
The average volume of each bed was 0.686 m.sup.3, to which was added an 
average of 0.031 m.sup.3 per bed representing the total dead volume 
(recycle loops and internal volumes of distributors). 
A distributor between every two beds separated the beds and was connected 
to the exterior by two distinct lines. The first of these two lines led 
into a feed valve, a raffinate valve and a flushing valve for the "dirty" 
service line. Each on-off flushing valve was followed by a manual valve 
for regulating the flow rates. The second of these two lines led into a 
desorbent valve, an extract valve and a flushing valve for the "clean" 
service line. 
The feed circulation line was provided with a pump, a flow rate control 
valve and a flow meter, and connected to each of the 24 "dirty" service 
lines of each stage. 
The raffinate circulation line was provided with a pressure control valve 
and a flow meter, and connected to each of the 24 "dirty" service lines of 
each stage. 
The "dirty" flushing circulation line was provided with a flow meter. It 
could be connected either to the intake of the desorbent pump or to the 
supply to the raffinate distillation column. Thus it could carry the in or 
the out flushes. 
The desorbent circulation line was provided with a pump, a flow rate 
control valve and a flow meter, and was connected to each of the 24 
"clean" service lines of each stage. 
The extract circulation line was provided with a flow rate control valve 
and a flow meter, and connected to each of the 24 "clean" service lines of 
each stage. 
The "clean" circulation line was provided with a pump, a flow rate control 
valve and a flow meter. It could be connected either to the intake of the 
desorbent pump or just downstream of the extract control valve, or finally 
to the 25.sup.th plate of the extract distillation column (the first forty 
were in the rectification zone, the last twenty were in the stripping 
zone). This clean flushing distribution line could thus effect in flushes 
of either desorbent or extract, or of extract depleted in desorbent. 
The adsorbent was an X zeolite with barium as the principal compensating 
cation. The desorbent was constituted by 97.9% para-diethylbenzene, 1.6% 
meta-diethylbenzene and 0.5% of about ten different aromatic constituents 
containing 10 carbon atoms. The feed to be separated was constituted by 
3.1% of paraffins and naphthenes, 1.2% of toluene, 11.6% of ethylbenzene, 
21.9% of para-xylene, 39.1% of meta-xylene, 21.9% of ortho-xylene and 0.2% 
of various aromatic constituents containing 9 carbon atoms. 
The unit was operated isothermally at 165.degree. C. The pressure at the 
intake of the two recycling pumps was regulated at 9 bars. The 
compositions of the streams were obtained by the average of analysis of 
five series of samples (desorbent, extract, feed, raffinate) extracted 
every six hours. The flow rates corresponded to an average measurement 
over 24 hours. The purity was calculated with respect to the composition 
of the extract, and the yield with respect to the compositions and flow 
rates of the extract and raffinate. 
The material balances showed a difference of at most 0.3% for the major 
constituents (C.sub.8 aromatics and para-diethylbenzene) and at most 2.6% 
for the minor constituents (paraffins and naphthenes, toluene, C.sub.9 
aromatics, meta-diethylbenzene, other C.sub.10 aromatics). 
EXAMPLE 1 (comparative, with no line flushing) 
There were 5 beds in zone 1 between desorbent injection and extract 
extraction, 9 beds in zone 2 between extract extraction and feed 
injection, 7 beds in zone 3 between feed injection and raffinate 
extraction, and 3 beds in zone 4 between raffinate extraction and 
desorbent injection. The following flow rates were used for the 
temperature and pressure conditions: desorbent 18.3 m.sup.3 /h, extract 
6.95 m.sup.3 /h, feed 11.8 m.sup.3 /h; raffinate 23.15 m.sup.3 /h. The 
permutation period was 56 seconds, and thus the complete cycle lasted 22 
minutes 24 seconds. 
The composition of the extract was: paraffins and naphthenes 0.009%; 
toluene 1.121%; ethylbenzene 0.055%; para-xylene 35.324%; meta-xylene 
0.095%; ortho-xylene 0.048%; C.sub.9 aromatics 0.017%; meta-diethylbenzene 
1.021%; para-diethylbenzene 62.012%, C.sub.10 aromatics 0.298%. 
The purity was calculated with respect to the paraffins and naphthenes, 
ethylbenzene, meta-xylene, ortho-xylene and the C.sub.8 aromatics. Toluene 
was not included as it was removed in a further distillation column. The 
purity was 99.37%. The composition of the raffinate was: paraffins and 
naphthenes 1.58%; toluene 0.271%; ethylbenzene 5.887%; para-xylene 0.555%; 
meta-xylene 19.903%; ortho-xylene 11.65%; C.sub.9 aromatics 0.097%; 
meta-diethylbenzene 0.952%; para-diethylbenzene 58.82%, C.sub.10 aromatics 
0.30%. The yield was thus 95%. 
EXAMPLE 2: (flushing the clean line with desorbent, in accordance with the 
invention) 
Example 1 was repeated, connecting the "clean" flushing circuit to the 
desorbent pump discharge. The flushing desorbent flow rate was 0.96 
m.sup.3 /h. 
The temperature and pressure were identical to Example 1. As above, there 
were 5 beds in zone 1, 7 beds in zone 3 and 3 beds in zone 4. However, 
there was one bed between the extract extraction and the flush injection 
(zone 5) and there were 8 beds between the flush injection and the feed 
injection (zone 2). 
The flow rates of desorbent, feed and raffinate were strictly identical to 
those of Example 1. The flow rates in zone 1, zone 2, zone 3 and zone 4 
were strictly identical to those of Example 1. 
However, the extract flow rate was held at 7.91 m.sup.3 /h and the flow 
rate in the bed located between the extract extraction and the flush 
injection (zone 5) was reduced by 0.96 m.sup.3 /h relative to the above 
case; this meant that the average recycle flow rate remained 56.06 m.sup.3 
/h. 
Under these conditions, the para-xylene content in the extract was no more 
than 31.072%. The amounts of impurities were: paraffins and naphthenes 
0.002%; ethylbenzene 0.044%; meta-xylene 0.051%, ortho-xylene 0.026%; 
C.sub.9 aromatics 0.008%. The purity was thus 99.58%, and the yield was 
practically unchanged: 95.02%. 
EXAMPLE 3 (flushing the "clean" line with desorbent) 
The conditions of Example 2 were repeated, with the exception that the flow 
rate of the flushing desorbent was reduced from 0.96 m.sup.3 /h to 0.48 
m.sup.3 /h. 
The extract flow rate was then 7.43 m.sup.3 /h, and the average recycle 
flow rate as 56.08 m.sup.3 /h. 
The composition of the extract was: paraffins and naphthenes 0.002%; 
ethylbenzene 0.045%; para-xylene 33.044%; meta-xylene 0.0052%, 
ortho-xylene 0.026%; C.sub.9 aromatics 0.008%. The purity was thus 99.60%, 
and the yield remained 95%. 
EXAMPLE 4 (flushing the "clean" line with desorbent) 
The conditions of Example 2 were repeated, with the exception that the flow 
rate of the flushing desorbent was reduced to 0.24 m.sup.3 /h. The extract 
flow rate was then 7.19 m.sup.3 /h, and the average recycle flow rate was 
56.09 m.sup.3 /h. The purity was 99.59%, and the yield was 94.99%. 
EXAMPLE 5 (flushing the "clean" line with desorbent) 
Example 3 was repeated, changing only the distribution of beds between 
zones 5 and 2. There were 2 beds between the extract extraction and the 
flushing injection. There were 7 beds between the flushing injection and 
the feed injection. The average recycle flow rate changed as there was one 
extra bed in zone 5 and one less bed in zone 2: instead of 56.08 m.sup.3 
/h (Example 3), it reduced to 56.06 m.sup.3 /h. The purity was 99.65%, and 
the yield was 94.99%. 
EXAMPLES 6 TO 8 (flushing the "clean" line with extract) 
The clean flushing circuit was connected downstream of the extract control 
valve. The flushing extract flow rate was 0.48 m.sup.3 /h. 
The flow rates in zones 1, 5, 2, 3, 4 and the desorbent, flushing, feed, 
extract and raffinate flow rates were identical to those of Example 3. The 
temperature and pressure conditions were identical to those of Examples 1 
to 5. The number of beds in zones 5 and 2 were varied as shown in Table I 
TABLE I 
______________________________________ 
Average 
recycle 
Beds in Beds in flow rate 
Purity Yield 
Example zone 5 zone 2 m.sup.3 /h 
% % 
______________________________________ 
6 1 8 56.08 99.65 94.82 
7 2 7 56.06 99.70 94.80 
8 4 5 56.02 99.75 94.77 
______________________________________ 
EXAMPLES 9 AND 10 (flushing of "clean" line with extract depleted in 
desorbent) 
The "clean" flushing circuit was connected to the extraction plate of the 
extraction column (25.sup.th plate in the rectification zone). The 
distillation column was regulated so that the concentration of para-xylene 
at this plate was about 65%. 
This figure corresponded to the maximum concentration of para-xylene in the 
adsorber. This maximum was localised in zone 2. 
The flow rates in zones 1, 5, 2, 3, 4 and the desorbent, flushing, feed, 
extract and raffinate flow rates, also the temperature and pressure 
conditions were identical to those of Examples 6 to 8. The number of beds 
in zones 5 and 2 were varied as shown in Table II 
TABLE II 
______________________________________ 
Average 
recycle 
Beds in Beds in flow rate 
Purity Yield 
Example zone 5 zone 2 m.sup.3 /h 
% % 
______________________________________ 
9 2 7 56.06 99.73 94.81 
10 4 5 56.02 99.77 94.79 
______________________________________ 
EXAMPLES 11 AND 12 (flushing with extract in two different positions) 
The clean flushing circuit was connected downstream of the extract control 
valve. The flushing extract flow rate was 0.48 m.sup.3 /h. 
The valves connected to the clean flushing circuit were activated twice 
during the 56 second period. 
During the first part of the period, there were 7 beds between the extract 
extraction and the flushing injection and 2 beds between the flushing 
injection and the feed injection. During the second part of the period, 
there were 2 beds between the extract extraction and the flushing 
injection and 7 beds between the flushing injection and the feed 
injection. 
The flow rates in zones 1, 3, 4, the flow rates of desorbent, flushing, 
extract and raffinate, also the temperature and pressure conditions, were 
identical to those of Examples 6 to 8. 
During the entire period, zones 2 and 5 had no more than 2 beds each and 
their flow rates were identical to those in Examples 6 to 8. There were 
alternately 5 beds in zone 5 during the first part of the period then in 
zone 2 during the second part of the period. To account for this 
particular feature, when the recycle pump was connected to these five 
beds, the set recycle flow rate value was the arithmetic mean of the flow 
rates in zone 5 and in zone 2 (Table III). A sixth zone thus existed, 
exactly as if two clean flushing streams were being permanently injected 
into two different areas of the adsorber. 
TABLE III 
______________________________________ 
Average 
Second recycle 
First part 
part flow rate 
Purity Yield 
Example 
s s m.sup.3 /h 
% % 
______________________________________ 
11 16 40 56.03 99.72 94.75 
12 28 28 56.02 99.79 94.70 
______________________________________ 
These Examples 11 and 12 should be compared with Example 7. 
EXAMPLE 13 (flushing with depleted extract at two different positions) 
The clean flushing circuit was connected to the extraction plate of the 
extraction column. The procedure was exactly as in Example 12, with the 
same conditions of flow rates, the same arrangement of zones 2 and 5, and 
the same division of time between the two parts of the 56 second period. 
The purity was 99.82%, and the yield was 94.68% (compare with Examples 10 
and 12). 
EXAMPLE 14 (for comparison with Example 13) 
The temperature was raised to 175.degree. C. and other conditions were 
used: the mass flow rates were the same as in Example 13, the volume flow 
rates were all increased by 0.9% (in inverse proportion to the density of 
the feed at 165.degree. C. and at 175.degree. C.). The permutation period 
was reduced from 56 seconds to 55.6 seconds. The first and second parts of 
the period were each 27.8 seconds. 
The purity was 99.86%, and the yield was 94.76%. 
EXAMPLE 15 (sequential flushing of the clean line and continuous out flush 
of the dirty line) 
Compared with Example n.degree. 13, the desorbent flow rate was increased 
by 0.24 m.sup.3 /h (i.e., from 18.3 m.sup.3 /h to 18.54 m.sup.3 /h). The 
dirty flushing line was connected to the supply to the raffinate 
distillation column. All of the dirty flush on-off valves were open and 
the flow rates were regulated for each stage so that a continuous flow 
rate of 0.01 m.sup.3 /h was extracted from each distributor. The total of 
the dirty flushes leaving the unit was in total 0.24 m.sup.3 /h. This 
stream was sent to the supply to the raffinate distillation column. 
The purity was 99.83% and the yield was 94.27%. 
EXAMPLE 16 (sequential flushing of the clean line and the dirty line) 
Example 13 was repeated, with two flushing valves which were operated 
sequentially. There was one bed between the desorbent injection and the 
dirty flush extraction, and four beds between the dirty flush extraction 
and the extract extraction. The flow rate of the desorbent was 18.54 
m.sup.3 /h and the dirty flush flow rate was 0.24 m.sup.3 /h. One period 
per cycle, when the recycle pump was connected to the bed in zone 7 
(between the desorbent injection and the dirty flush extraction), the rate 
of the pump was increased by 0.24 m.sup.3 /h with respect to the rate in 
zone 1. The purity was 99.84% and the yield was 94.65%. 
EXAMPLE 17 (sequential in flush between the clean and dirty lines) 
The conditions of Example 10 were repeated, with the dirty flushing line 
being connected to the discharge of the desorbent pump. The dirty flushing 
on-off valves were operated sequentially. The dirty flush was injected at 
the same place as the clean flush: four beds after the extract extraction. 
The flow rate in zone 5 dropped by 0.24 m.sup.3 /h, the extract flow rate 
increased by 0.24 m.sup.3 /h (from 7.43 m.sup.3 /h to 7.67 m.sup.3 /h). 
The average recycle flow rate reduced from 56.02 m.sup.3 /h to 55.98 
m.sup.3 /h. The purity was 99.68% and the yield was 95.49%. 
EXAMPLE 18 (flushing in two different positions of the clean line and 
sequential in flush of the dirty line) 
The conditions of Examples 12 and 17 were repeated. The dirty flush was 
injected 7 beds after extract extraction and two beds before the feed 
injection. A flow rate of 0.24 m.sup.3 /h of desorbent was used. The 
average recycle flow rate was 55.95 m.sup.3 /h. The purity was 99.78% and 
the yield was 95.34%. 
EXAMPLE 19 (flushing at two different positions of the "clean" line using 
extract depleted in desorbent and sequential flush out of the "clean" 
line) 
The conditions of Example 14 were repeated, adding the sequential flush out 
described in Example 16. The purity was 99.89% and the yield was 94.67%. 
The following section of the description is common to Examples 20 to 24. In 
the unit described above, the distributors separating the beds were 
replaced by distributors connected to the exterior by four distinct lines. 
The first of these lines led into a feed valve and a flush valve. The 
second of these lines led into a raffinate valve and a flush valve. The 
third line led into a desorbent valve and a flush valve. The fourth line 
led into an extract valve and a flush valve. 
The composition of the feed and desorbent, also the nature of the molecular 
sieve, were identical to those of Examples 1 to 19. 
EXAMPLE 20 (comparative) 
The operating conditions of Example 1 were strictly repeated. The purity 
obtained was 99.19% and the yield was 96.21%. 
EXAMPLE 21 (sequential flushing in two different positions of the extract 
line only) 
The operating conditions of Example 14 were strictly repeated. The purity 
obtained was 99.69% and the yield was 95.95%. 
EXAMPLE 22 (sequential flushing in two different positions of the extract 
line and sequential in flush of the desorbent line) 
The conditions of Example 21 were repeated. In addition, a flushing stream 
constituted by 0.24 m.sup.3 /h of desorbent was injected into the flushing 
valve connected to the desorbent line, issuing 4 beds downstream of the 
extract extraction. The purity obtained was 99.88%, and the yield was 
95.55%. 
EXAMPLE 23 (sequential flushing in two different positions of the extract 
line, sequential in flush of the desorbent line, sequential out flush of 
the feed line) 
The conditions of Example 22 were repeated, increasing the desorbent flow 
rate by 0.24 m.sup.3 /h and extracting a stream of 0.24 m.sup.3 /h via the 
flushing valve connected to the feed line one bed downstream of the 
desorbent injection. 
The out flush stream was sent to the feed addition line. The purity 
obtained was 99.90%, and the yield was 94.97%. 
EXAMPLE 24 (sequential flushing in two different positions of the extract 
line, sequential in flush of the desorbent line, sequential out flush of 
the feed line, sequential out flush of the raffinate line) 
The conditions of Example 23 were repeated, increasing the desorbent flow 
rate by 0.24 m.sup.3 /h and extracting a stream of 0.24 m.sup.3 /h via the 
flushing valve connected to the raffinate line one bed downstream of the 
desorbent injection. The out flush stream was sent to the raffinate 
distillation. The purity obtained was 99.91%, and the yield was 94.62%. 
Examples 2 to 19 and 21 to 24 show that it is essential to rinse the clean 
line or lines to obtain large gains in purity and to separate the 
raffinate line and feed line to obtain a large gain in yield. Flushing the 
dirty lines only results in small gains in purity at the expense of a 
large drop in yield.