Apparatus for regulating the flow rate of at least one fluid...

Liquid purification or separation – With means to add treating material – Chromatography

Reexamination Certificate

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C210S659000, C585S805000

Reexamination Certificate

active

06332982

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an apparatus and to a process for indirect regulation of the flow rate of at least one fluid, for example an effluent, in a chromatographic zone containing a fixed phase, for example an adsorbent or molecular sieve. More particularly, it enables pressures to be produced which are within a predetermined pressure range upstream and downstream of one or preferably a plurality of regulating valves in a counter-current or co-current simulated moving bed adsorption zone.
It is of particular application to separating at least one xylene isomer, in particular para-xylene from a hydrocarbon feed containing it.
BACKGROUND OF THE INVENTION
The prior art is illustrated in European patent EP-A-0 415 822, U.S. Pat. No. 5,156,736 and French patent FR-A-2 743 002 which describes the most effective implementation.
A simulated moving bed comprises at least three chromatographic zones, advantageously four or five, each of the zones being constituted by at least one column or column section. At least one point between two zones acts to inject a feed to be fractionated and at least one point between two zones acts to inject an eluent or desorbent or solvent. Further, the simulated moving bed comprises at least one point for withdrawing an extract between the eluent injection point and the feed injection point located downstream in the desorbent circulation direction and at least one point for withdrawing a raffinate between each mixture injection point and the eluent injection point located downstream in the direction of desorbent circulation.
The set of columns or sections forms at least one closed loop containing at least one recycling pump, between two sections, which pump is flow rate regulated (between the first and last section, for example).
In general, the injection and withdrawal points are periodically shifted by at least one section in the same direction (upstream or downstream).
It is essential to observe the flow rates of the effluents which circulate from one zone to another and which must remain substantially constant in a given zone. A small variation in flow rate, even in a single zone, can have a very large influence on the separation results.
As an example, consider a counter current simulated moving bed comprising four zones with a recycling pump, two incoming streams, the desorbent and the feed, and two outgoing streams, the extract and raffinate.
Zone
1
is located between the desorbent and the extract; zone
2
is located between the extract and the feed; zone
3
is located between the feed and the raffinate; and zone
4
is located between the raffinate and the desorbent. The flow rates in the different zones are as follows when the pump is in zone
1
, for example:
Flow rate in zone
1
: pump flow rate;
Flow rate in zone
2
: flow rate in zone
1
−flow rate of extract;
Flow rate in zone
3
: flow rate in zone
2
+feed flow rate;
Flow rate in zone
4
: flow rate in zone
3
−raffinate flow rate.
Any errors in the flow rates of the incoming or outgoing streams thus reflect on the recycling flow rate and thus must be controlled with precision.
Each time that one of the incoming or outgoing streams of the loop passes from one to the other side of the recycling pump, for example from a position immediately anterior to a position immediately posterior of the recycling pump when operating in simulated counter-current, two difficulties occur from the point of view of the regularity of the flow rates:
the first difficulty concerns the recycling pump, when it changes zone. It is very important that its flow rate is modified almost instantaneously and that the new flow rate, that of the new zone in which it is to be found, is precisely and stably regulated without the transition from one flow rate to the other being too slow (in the case of damped regulation) or with fluctuations about the new value (in the case of fast response regulation).
This first technical problem is related to changing the zones. It is actually very important that the flow rate passes instantaneously from a first value to a second desired value. As an example, it has been shown that a variation of 0.6% in the recycle flow rate produces a variation of 4.2% in purity. It has been shown that a flow rate regulator which stably regulates the flow rate to which a new set rate is supplied takes a certain time to regulate to the new value. As the transition from one flow rate to another must be very rapid, the gain of the regulator has to be high. In this case, regulation is not very stable. Thus we have to make a choice between rapid but fluctuating regulation or fine and stable regulation but with inertia. Neither of those two solutions is acceptable when regulating a simulated moving bed, which aims to produce high purities.
The second difficulty concerns the flow rate of the stream entering or leaving the unit. The flow rate of that flux must be kept constant, and with very good precision, when its point of injection or withdrawal passes from a low pressure, that of the pump intake, to a high pressure, that of the pump discharge (the pressure difference corresponds to the pressure drop in the group of columns or column sections).
It is very important that these problems are overcome to obtain good separation results.
The solution proposed in the prior art consists of not allowing the regulator to act alone but, by using a computer or any other means which can act on the regulator, to cause that regulator to temporarily stop regulating, simultaneously to cause it to modify its action (pre-positioning the action) such that the new action imposed, such as a percentage valve opening, a current frequency for a motor, a voltage, etc . . . corresponds to the new conditions enabling the flow rate under consideration to be properly regulated and then, but in fact almost immediately, to restart the action of the regulator:
in the case of the recycling pump, to change the flow rate from one zone to another;
in the case of an effluent, to maintain the flow rate in the presence of a large variation in pressure conditions.
Thus good flow rates are obtained without oscillations and almost instantaneously. The ensemble of operations described take between 1/100 and 10 seconds and usually between 1/10 and 5 seconds, depending on the case.
The examples described in the text of the prior art are applicable to a pilot unit comprising 24 beds, for example, and to a single circulating pump.
When the process for regulating the flow rates of the prior art (FR 95/15526) is applied to the unit described in
FIGS. 1A
,
1
B and
2
, it can be seen that residual perturbations subsist, in particular in the raffinate flow rate (see
FIGS. 3A
,
3
B,
3
D,
3
C).
An examination of these figures enables the nature of the problem to be better understood.
FIGS. 1A and 1B
show how the pressures and flow rates in a closed loop of 24 beds constituting the simulated moving bed are regulated. Two adsorbers
1
and
2
each comprise 12 beds 1.3 m in height and 7.6 m in diameter. Two pumps
9
and
10
circulate liquid inside the adsorbers. A flow meter
3
and a flow rate regulation valve
4
control a flow rate of between 1100 and 3200 m
3
/h between adsorbers
1
and
2
with a very high precision (0.2%). A pressure controlled raffinate withdrawal valve
6
maintains a set pressure
5
at the intake of pump
9
. A control valve
8
maintains a set pressure
7
at the inlet to pump
10
. With reference to a cycle of 24 periods where the desorbent is injected into the first bed of adsorber
1
, control valve
6
is in direct communication with adsorber
2
during periods
1
to
3
and
16
to
24
(
FIG. 1
b
) and with adsorber
1
during periods
4
to
15
. Between periods
3
and
4
raffinate withdrawal passes from the bottom of adsorber
2
to the head of adsorber
1
; between periods
15
and
16
, raffinate withdrawal passes from the bottom of adsorber
1
to the head of adsorber
2
. On-off valves enable a given bed to be placed in communication with the withdrawal circuit, ta

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