Gas separation: processes – With control responsive to sensed condition – Pressure sensed
Reexamination Certificate
1999-08-03
2001-05-01
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Pressure sensed
C095S025000, C095S096000, C095S116000, C096S116000, C096S117500, C096S130000, C096S143000
Reexamination Certificate
active
06224651
ABSTRACT:
FIELD OF THE INVENTION
The invention related to a process for separation of a gas mixture by pressure swing adsorption, in which a pressure swing cycle comprising a sequence of steps which define adsorption, decompression/regeneration and pressure rise phases is employed for the or each adsorber.
The invention can be implemented with all types of pressure variation adsorption cycles, for example the following cycles:
So-called VSA (vacuum swing adsorption) cycles, in which the adsorption is carried out substantially at atmospheric pressure and the minimum pressure of the cycle is much lower than this atmospheric pressure and typically of the order of 250 to 500 mb. These cycles are generally implemented using units with three adsorbers;
So-called MPSA transatmospheric cycles, which differ from the previous ones by the fact that the adsorption is carried out at a pressure much higher than atmospheric pressure and typically of the order of 1.3 to 2 bar. These cycles are generally implemented using units with two adsorbers;
So-called PSA (pressure swing adsorption) cycles, in which the adsorption is carried out at a pressure much higher than atmospheric pressure, typically of the order of 3 to 50 bars, while the minimum pressure of the cycle is substantially equal either to atmospheric pressure or to a pressure of a few bar.
The latter application will be referred to below and the abbreviation PSA will be used as a generic term to describe all these cycles. Furthermore, the pressures indicated are absolute pressures.
The expression “pressure swing adsorption” or PSA is intended here to mean the various cycles which have been proposed for producing, for example, hydrogen from a steam reforming synthesis gas by substantially isothermal selective adsorption, with the pressure of each adsorber varying between a high pressure and a low pressure. The high pressure of the cycle may be greater than or equal to atmospheric pressure, whereas the low pressure of the cycle may be equal to or less than atmospheric pressure. These processes include various combinations of steps of adsorption, decompression/regeneration and recompression of the adsorbers.
Furthermore, in what follows, the terms “inlet” and “outlet” denote the inlet and outlet ends of an adsorber in the adsorption phase; the expression “cocurrent” denotes the flow direction of the gas in the adsorber during this adsorption phase; and the expression “countercurrent” denotes the reverse flow direction.
BACKGROUND OF THE INVENTION
So-called PSA plants, that is to say plants for implementing the PSA cycles mentioned above, are enjoying increasing success, in particular in the fields of purifying hydrogen, treating natural gas, separating the gases in air, recovering solvents, and fractionating synthesis gas.
This success is leading PSA plant manufacturers to construct plants which, although they perform better, are also becoming increasingly complex.
This can be seen from the fact that the plants comprise a plurality of adsorbers with which a large number of valves are associated.
Thus, for example, a PSA plant described in document U.S. Pat. No. 4,834,780 comprises six adsorbers and thirty-seven valves, and another described in document U.S. Pat. No. 4,475,929 comprises ten adsorbers and sixty-seven valves.
Given that proper running of the cycle depends on the sequences of opening and closing these valves which make it possible to obtain the intended pressure cycle, it can therefore readily be seen that malfunction of one of these valves which are operated on each cycle can cause serious problems in the operation of a PSA plant.
By way of example, two cases of malfunction affecting the valves should in particular be highlighted:
1) A first malfunction may consist in mechanical obstruction of the valve, with the result that the valve remains closed in spite of an opening instruction which is applied (or vice versa), or it opens and closes only very slowly.
Such incidents can be detected by end of travel sensors installed in the valves and, where appropriate, connected to timer systems.
Furthermore, given that this type of malfunction generally causes significant imbalances in the pressure cycle of the various adsorbers, such an incident is relatively easy to identify.
2) A second malfunction may result from a sealing defect of a closed valve, which leads to internal leaks either between the adsorbers or between an adsorber and the production line or the residue line.
Unlike mechanical obstruction, leaks in a closed valve which is not leaktight are difficult to detect on a unit which is in operation.
They nevertheless cause a drop in performance of the PSA plant, due to imbalanced operation of the adsorbers or due directly to a loss of production to the residual gas line.
In order to identify the valves which have a sealing defect in the closed position, periodic maintenance shutdowns are conventionally employed to carry out seal tests.
However, this type of inspection has the drawback that it is time-consuming and laborious. Furthermore, in view of the durations of the plant shutdown, these inspections can be carried out only at distant time intervals.
However, given that this loss of production which is caused by a sealing defect of a valve and leads to a few % drop in the extraction efficiency can last several months, the losses may be significant.
SUMMARY OF THE INVENTION
The object of the invention is to overcome these various drawbacks by providing a process which makes it possible to detect a sealing defect of a valve during an operating cycle of a PSA plant.
To that end, the invention relates to a process for separation of a gas mixture by pressure swing adsorption, in which a pressure swing cycle comprising a sequence of steps which define adsorption, decompression/regeneration and pressure rise phases is employed for the or each adsorber, characterized in that the cycle furthermore comprises at least temporarily at least one step of isolating the adsorber during which the pressure variation in the isolated adsorber is recorded.
The process according to the invention may furthermore have one or more of the following characteristics:
the isolation step is carried out at a pressure intermediate between the high pressure and the low pressure of the cycle,
the recorded pressure variation is compared with a predefined threshold, and a warning is emitted when the pressure variation exceeds the predefined threshold,
the duration of the isolation step is between 0.5% and 5% of the total duration of the cycle,
the duration of the isolation step is more than 5 seconds and preferably between 10 and 20 seconds,
the step of isolating an adsorber is carried out between two steps of balancing pressures between adsorbers,
the isolation step is carried out after a first step of cocurrent decompression of the adsorber and before an elution step, in particular immediately after the first cocurrent decompression step,
the isolation step is carried out after first recompression step and before the final recompression step of an adsorber, in particular immediately before the latter,
the difference between the high pressure and the low pressure of the cycle is greater than or equal to 6 bar, preferably greater than or equal to 10 bar.
The invention furthermore relates to a unit for separation of a gas mixture by pressure swing adsorption, comprising at least one adsorber and means for implementing in it a pressure swing cycle comprising a sequence of steps which define adsorption, decompression/regeneration and pressure rise phases, characterized in that it furthermore comprises means for isolating the or each adsorber and means for recording the pressure variation in the isolated adsorber.
The unit according to the invention may furthermore have the characteristic according to which it furthermore comprises means for comparing the recorded pressure variation with a predefined threshold stored in a memory and, controlled by the comparison means, means for emitting a warning when the pressure variation exceeds the predefined threshold.
The inventi
De Souza Guillaume
Dolle Pierre Olivier
Engler Yves
Monereau Christian
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Spitzer Robert H.
Young & Thompson
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