Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-02-26
2001-12-25
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
C062S657000
Reexamination Certificate
active
06332336
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to a method and apparatus for maximizing the production rate of natural gas liquids (NGL) in a gas processing plant. More specifically, this invention relates to the control of a turboexpander that drives a recompressor, with the objective of maximizing NGL production.
BACKGROUND ART
Effective operation of natural gas liquids plants is restricted because turboexpander shaft rotational-speed limits restrict the unit from achieving maximum productivity. Achieving a high level of productivity is necessary because liquid high-molecular weight fractions are more valuable than the lighter gaseous components.
DISCLOSURE OF THE INVENTION
A purpose of this invention is to maximize the production rate of NGL in a gas processing plant by increasing the power that the turboexpander absorbs. This can be achieved at maximum rotational speed and with maximum opening of the turboexpander's adjustable nozzles by opening the recompressor's antisurge (hot recycle) valve or its cold recycle valve, or both of them. Valve opening must be limited to a preset value determined by a curve constructed on the recompressor's performance map. This curve is located such that
moving the compressor's operating point from its surge limit line to the curve along a line of constant speed increases compressor power; and
moving the compressor's operating point (from the above-mentioned curve) further to the right and along a line of constant speed decreases compressor power.
Compressor power in terms of compressor similarity parameters can be expressed
J
p
N
=
f
(
Q
2
Z
R
T
,
R
c
)
(
1
)
where
J=compressor power
p=absolute pressure (in suction or discharge)
N=rotational speed
Q=compressor volumetric flow
Z=compressibility factor
R=gas constant
T=absolute temperature (in suction or discharge)
R
c
=pressure ratio, p
d
/p
s
p
d
=absolute compressor discharge pressure
p
s
=absolute compressor suction pressure
Eq. (1) describes the surface on which the compressor's operating point resides. As in the case with the surge limit, any single curve in this three-dimensional space can be described by any two of these compressor similarity parameters; for instance,
J
p
N
=
f
1
(
Q
2
Z
R
T
)
(
2
)
Using Eq. (2), compressor power must be known to determine which side of such a curve the operating point is on. When compressor power is unknown (as is often the case) the operating point location, relative to this curve, can be determined using another pair of similarity parameters, but not involving the power, J, such as
R
c
=
f
2
(
Q
2
Z
R
T
)
(
3
)
Therefore, adequate information must be available to calculate two similarity parameters; in this case, R
c
and
Q
2
Z
R
T
∝
Δ
p
o
p
where &Dgr;p
o
is a differential pressure across a flow measurement device. This information is all that is required to limit the compressor's operating point to a region to the left of the curve, as described by Eq. (3).
To maintain the operating point to one side of the curve of Eq. (3), a control action can be based on an error calculated as either of
ε
=
R
c
-
f
2
(
Q
2
Z
R
T
)
or
ε
=
f
3
(
R
c
)
Q
2
/
Z
R
T
-
1
However, the invention is not limited to these forms of error calculation.
Maximum NGL production can be realized by two methods.
1. The turboexpander's adjustable nozzles together with the cold and hot recycle valves are manipulated as usual until rotational speed reaches its limiting set point, N
sp
. At this time, the speed controller's output signal opens the cold recycle valve or the hot recycle valve (or both of them) until the recompressor's operating point reaches a preset maximum distance from the surge limit line. The turboexpander's adjustable nozzles are then manipulated to limit rotational speed, N. An advantage of this first method is that the cold recycle valve and the antisurge valve are closed (or minimally opened, when needed) before maximum rotational speed is reached. A disadvantage of the method is if the preset set point is incorrect, or becomes incorrect, a sudden increase in rotational speed may result when the productivity set point is changed to maximum.
2. Both the cold recycle controller and the hot recycle controller (whose set point may be slightly closer to the surge limit line than that of the cold recycle controller) open their corresponding valves to maintain maximum compressor power for the current rotational speed. When maximum turboexpander rotational speed is achieved, maximum plant productivity is also achieved. But when rotational speed is lower than maximum, the antisurge valve (hot recycle) or the cold recycle valve, or both of them, may be open when corresponding controllers operate in a RUN mode. An advantage of this second method is that incorrectness of the preset set point cannot result in sudden rotational speed increase; and the set point may be corrected for the recompressor's maximum power at any rotational speed.
REFERENCES:
patent: 4281970 (1981-08-01), Stewart et al.
patent: 5139548 (1992-08-01), Liu et al.
patent: 5791160 (1998-08-01), Mandler et al.
Mirsky Saul
Zilberman Boris
Capossela Ronald
Compressor Controls Corporation
Henderson & Sturm LLP
LandOfFree
Method and apparatus for maximizing the productivity of a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for maximizing the productivity of a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for maximizing the productivity of a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2583669