Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-07-29
2001-09-18
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S102000, C095S105000, C095S130000, C095S143000, C096S130000, C096S144000
Reexamination Certificate
active
06290751
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and to a plant for processing a fluid comprising at least one or more compounds G
1
having a kinetic diameter greater than or equal to that of methane and at least one or more compounds G
2
having a kinetic diameter less than or equal to that of nitrogen, notably in order to separate them and to improve the recovery ratio of compounds G
1
and/or G
2
.
The method is notably applied to remove the nitrogen contained in a natural gas mainly comprising methane.
BACKGROUND OF THE INVENTION
About 15% of the world's available natural gas reserves comprise appreciable amounts of compounds of low calorific value such as nitrogen. The presence of these compounds can be natural or result from the use of an enhanced recovery process. The presence of such compounds, notably in large amounts (with concentrations typically above 4% mol) reduces the calorific value of the gas. The characteristics of the gas are then incompatible with existing commercial transportation and distribution systems specifications since its calorific value can then be below 960 BTU/SCF which corresponds on average to nitrogen contents above 4% by moles.
Less than 10% of the gas fields containing nitrogen in large amounts (more than 4% mol) are currently developed.
Various methods allowing to remove nitrogen are known from the prior art.
It is thus possible to perform cryogenic distillation. Such a process generally leads to high investment costs.
It is also well-known to use adsorption processes for denitrogenation and a certain number of patents refer to them, based either on thermodynamic selectivities or equilibrium selectivities (U.S. Pat. Nos. 4,578,089, 5,174,796 and 5,536,300) or diffusion selectivities (U.S. Pat. Nos. 4,964,889, 4,935,580 and 2,843,219). The latter originate from the difference between the adsorption kinetics of the molecules to be separated, whereas thermodynamic selectivities originate from a difference in the adsorbed quantity at equilibrium.
These processes are characterized by considerable dead volumes since the interstitial volume (volume between the adsorbent particles) commonly represents 30% of the adsorber volume. In the case where adsorbers are used at high pressures, large amounts of the less retained compounds can lodge themselves in these dead volumes and can thus be produced, not during the production phase, but during the regeneration phase, leading to a decrease in the recovery ratio per pass of these compounds.
SUMMARY OF THE INVENTION
One of the objects of the present invention is to propose a method allowing to increase the recovery ratio per pass of the less retained species, i.e. hydrocarbons in the first section of the process and compounds of low calorific values in the second section.
It is possible to use at least part of a stream produced during the process in order to flush one of the adsorption means so as to recover the compounds that have lodged themselves in the dead volumes of the adsorbers. Advantageously, at least a fraction of stream F
3
is used to flush adsorption zone A
1
.
The object of the invention relates to a method allowing to process at least one fluid comprising one or more compounds G
2
having a kinetic diameter less than or equal to that of nitrogen and one or more compounds G
1
having a kinetic diameter greater than or equal to that of methane.
It is characterized in that it comprises in combination at least the following stages:
sending said fluid to be processed to an adsorption stage (a) after which a first stream F
1
mainly comprising compounds G
1
is obtained, said compounds G
2
being to mainly adsorbed in a first adsorption zone A
1
,
carrying out a desorption stage (b) in first adsorption zone A
1
at a pressure P
2
so as to produce a second stream F
2
, said second stream F
2
being enriched in compounds G
2
in relation to said fluid to be processed,
sending said second stream F
2
at a pressure P
3
to an adsorption stage (c), after which a third stream F
3
mainly comprising non-adsorbed compounds G
2
is obtained, and to a second adsorption zone A
2
enriched in compounds G
1
,
carrying out a desorption stage (d) by lowering the pressure value from P
3
to a value P
4
so as to recover at least a fourth stream F
4
mainly comprising compounds G
1
,
using at least a fraction of third stream F
3
from adsorption zone A
2
at a pressure no substantially equal to P
3
and mainly comprising compounds G
2
to flush adsorption zone A
1
in order to produce a stream comprising compounds G
1
after adsorption stage (a).
Flushing can be performed by cocurrent flow.
According to an embodiment. at least a fraction of fourth stream F
4
from desorption stage (d) is brought to a pressure substantially equal to pressure P
3
and the compressed fraction is recycled to adsorption stage (c).
The fraction of stream F
4
can be recycled to second stream F
2
prior to bringing it to a pressure P
3
.
The fraction of stream F
4
can also be recycled to zone A, after bringing it to a pressure P
3
.
According to a variant, at least a fraction of third stream F
3
mainly comprising non-adsorbed compounds G
2
is for example used to carry out a cocurrent flushing stage in the first adsorption section and to produce a stream comprising compounds G
1
after adsorption stage (a), after bringing the pressure of said third stream F
3
to a pressure required for the flushing stage.
The various adsorption and desorption stages are carried out in order to obtain a purity in compounds G
1
of stream F
4
lower than the purity in compounds G
1
of said stream F
1
.
Stage (a) can be carried out at a pressure P
1
ranging between 0.3 and 3 MPa, preferably between 0.5 and 2.5 MPa.
The value of pressure P
2
and/or of pressure P
4
is for example lower than 0.5 MPa and it preferably ranges between 0.001 and 0.5 MPa.
The value of pressure P
3
can range between 0.2 and 3 MPa, preferably between 0.3 and 2 MPa.
It is possible to work at a temperature ranging between −50 and +100° C., preferably between −30 and 50° C.
The object of the invention also relates to a plant for processing a fluid comprising at least compounds G
1
having a kinetic diameter greater than or equal to that of methane and compounds G
2
having a kinetic diameter less than or equal to that of nitrogen, comprising in combination at least a first adsorption means A
1
and a second adsorption means A
2
, said first means being selected for its diffusion selectivity and its capacity to retain mostly compounds G
2
, said second means being selected for its thermodynamic selectivity and its capacity to retain mostly compounds G
1
, delivery and extraction lines for the various streams, first adsorption means A
1
comprising at least one delivery line for the fluid to be processed, at least one line for discharge of a first stream F
1
mainly comprising compounds G
1
, at least means allowing to extract a second stream F
2
enriched in compounds G
2
in relation to said fluid to be processed, and to send it to said second means A
2
, said second means A
2
comprising at least one line for discharge of a third stream F
3
mainly comprising compounds G
2
and at least a fourth line allowing to extract a stream F
4
mainly comprising compounds G
1
.
The plant is characterized in that it comprises at least means allowing to inject at least a fraction of third stream F
3
comprising compounds G
2
into first means A
1
so as to flush it and to produce a stream comprising compounds G
1
, said line being provided with a device for bringing the third stream to a sufficient pressure.
The plant can comprise means allowing the pressure of stream F
2
to be brought from a value P
2
to a pressure value P
3
.
The plant can also comprise means for recycling at least a fraction of stream F
4
to the inlet of the second adsorption means.
The fraction of stream F
4
is for example recycled upstream from the compression device intended for stream F
2
.
The fraction of stream F
4
can also be recycled to second means A
2
after passi
Jullian Sophie
Ragil Karine
Thomas Michel
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Spitzer Robert H.
LandOfFree
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