Gas separation: processes – Degasification of liquid – Plural successive degassing treatments
Reexamination Certificate
1999-09-24
2001-10-02
Smith, Duane S. (Department: 1724)
Gas separation: processes
Degasification of liquid
Plural successive degassing treatments
C095S254000, C095S261000, C096S158000, C096S163000, C096S167000, C096S196000, C096S216000
Reexamination Certificate
active
06296690
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an alternating compression-pumping system intended for a multiphase fluid having a composition that can vary with time. The composition can successively comprise a large amount of gas, established over a long period of use, but also a low proportion of gas over a period that can lead to choking of a separator arranged upstream from the part of the system whose function is to impart energy to the fluid.
The system according to the invention notably finds applications in the field of petroleum production.
BACKGROUND OF THE INVENTION
Although the term “two-phase pumping” is commonly used to designate energy supply to a fluid consisting of a liquid phase and of a gas phase, we will use in the description hereafter the term “compression”, that is better suited to designate energy transfer to a compressible two-phase fluid, especially when it is characterized by a high gas/liquid volume ratio (GLR under real temperature and pressure conditions).
Various devices, some examples of which are mentioned hereafter, allow to compress a two-phase fluid consisting of a gas and of a liquid, and possibly of solid particles:
a series of single-phase machines (consisting at least of a pump and of a compressor) preceded by a separation system. This production mode leads to bulky and expensive compression installations,
using radial impellers to directly compress a gas-liquid mixture. These impellers are limited to gas ratios generally lower than 20%. This limit can be extended to about 30% by using radio-axial impellers and beyond with axial impellers,
positive-displacement machines (reciprocating, screw, membrane machines) allow to obtain a good compression efficiency for a two-phase mixture. On the other hand, they are very ill-suited to the high volume flow rates that characterize applications with high gas ratios,
rotodynamic devices with helico-axial impellers, such as those described in the claimant's patent FR-2,665,224; the latter are particularly well-suited for compression of a two-phase mixture having a high volume flow rate. On the other hand, the low manometric head produced by each impeller does not allow to obtain very high compression ratios when the GLR is above 20. Furthermore, the efficiency of these impellers is lower than the efficiency of single-phase machines and it tends to decrease when the input pressure decreases,
devices using dynamic separators upstream from a dry gas compressor, such as the gaseous fluid compressor associated with a gas-liquid separator described in patent application WO-87/03,051.
SUMMARY OF THE INVENTION
The present invention relates to a compression system that comprises at least one compression section, capable of accepting gas or liquid and of imparting an energy value to each of these fluids.
The compression system comprises means allowing this compression section to be shifted from gas mode to liquid mode and vice versa.
The present invention relates to an alternating compression-pumping system allowing to impart energy to a multiphase fluid having a composition that can vary with time, for example a variation in the amount of gas phase and in the amount of liquid phase.
It is characterized in that it comprises in combination at least the following elements:
at least one alternating compression-pumping section, suited to impart a pressure value to an essentially liquid fluid or to an essentially gaseous fluid, the compression-pumping section comprising at least one line for delivering an essentially liquid phase, at least one line for delivering an essentially gaseous phase, at least one line intended for discharge of a gas that has acquired a certain energy after passing through the system, and at least one line intended for discharge of a liquid that has acquired a certain energy after passing through the compression-pumping section,
at least one pumping section selected to impart energy to an essentially liquid fluid, the pumping section comprising at least one line for delivering an essentially liquid phase and at least one line for discharge of the liquid phase pumped,
at least one device for separating the various phases that constitute the multiphase fluid, the separation device being connected to a multiphase fluid delivery line and to the line intended for discharge of the liquid coming from the alternating compression-pumping section, the device comprising at least one gas phase discharge line and at least one liquid phase discharge line,
the separation device is provided with means (C
L
) allowing to detect the gas-liquid interface level of the fluid introduced in the separation device,
means (Vgi, Vli) allowing to control the flow rate of the liquid or gas phases in the Bvarious lines,
control means allowing to vary the state of the flow rate control means so as to shift the compression section from an operating mode suited to gas to an operating mode suited to liquid and vice versa.
The compression system can comprise at least one line for recycling at least a fraction of the essentially gaseous fluid from the compression-pumping section to the separation device.
The system comprises for example a line for recycling at least a fraction of the essentially liquid fluid from the pumping section to the separation device.
The separation device can be associated with at least one of the following elements:
a helical line intended to separate the liquid droplets from the gas phase,
a series of disks mounted on the shaft, the shaft extending in the separator.
According to an embodiment, the compression section comprises for example at least one stage allowing to obtain separation of the gas phase and of the liquid phase occurring in the form of droplets.
The present invention also relates to a process allowing to impart energy to each of the phases of a multiphase fluid, the fluid comprising at least a liquid phase and at least a gas phase, knowing that the amount of the essentially liquid phase and the amount of the essentially gaseous phase can vary with time, the gas phase being sent to a compression-pumping section and the liquid phase being sent to a pumping section or to an alternating compression-pumping section, the sections being part of a compression-pumping system.
It is characterized in that it comprises at least the following stages:
a) separating the multiphase fluid into an essentially gaseous phase and an essentially s liquid phase,
b) determining the liquid or liquid-gas interface level L in the separation device,
c) comparing level L with a threshold value L
0
,
if L is greater than L
0
, one acts on a series of means for controlling the flow rate of the liquid and gas phases so as to shift the alternating compression-pumping section of the alternating compression-pumping system from an operating mode P
1
for an essentially gaseous fluid to an operating mode P
2
for an essentially liquid fluid,
by closing nearly totally control means Vg
3
, by opening nearly totally control means Vl
3
so as to drive the liquid towards the compression section and by opening control means Vl
4
,
d) level L is permanently controlled,
as soon as level L becomes lower than a threshold level L
2
, one acts on the flow rate control means so as to shift the compression section from mode P
2
to mode P
1
,
by opening nearly totally control means Vg
3
, by closing nearly totally control means Vl
3
so as to drive the gas towards the compression section and by closing control means Vl
4
.
When shifting from mode P
1
to mode P
2
, the initial rotating speed N
P1
can be varied to obtain a rotating speed N
P2
, rotating speed N
P2
being so selected that the pressure value at the discharge end of the compression section obtained on passage of a gaseous fluid is substantially identical to the discharge pressure value when a liquid fluid flows through the section, and the rotating speed can be conversely varied when shifting from mode P
2
to mode P
1
.
The stage of separation of the liquid droplets from the gas phase is continued in a compression stage arranged in the neighbourhood of t
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Smith Duane S.
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