U.S. patent application number 14/907407 was filed with the patent office on 2016-06-30 for system, method and apparatus.
The applicant listed for this patent is Adrian Graham Alford. Invention is credited to Adrian Graham Alford.
Application Number | 20160187033 14/907407 |
Document ID | / |
Family ID | 49166937 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160187033 |
Kind Code |
A1 |
Alford; Adrian Graham |
June 30, 2016 |
SYSTEM, METHOD AND APPARATUS
Abstract
The present invention relates to an apparatus, system or method
for reducing pressure in a gas flow for a gas let-down system. The
present invention further relates to an apparatus, system or method
for drying gas. A system (10) for reducing pressure in a gas flow
for a gas let-down system comprises an expander (102) driven by gas
at a first pressure expanding to a second pressure, and a
compressor (104) for compressing the gas from the second pressure
to a third pressure, whereby the third pressure is lower than the
first pressure and the third pressure is higher than the second
pressure. By first expanding the gas and then compressing the gas
the intermediate temperature of the gas at the second pressure is
lower than if the gas is expanded directly to the third pressure.
Further, a drying system for drying a gaseous fluid supplying heat
to a heat exchanger comprises a liquid separator, a heat exchanger
downstream of the liquid separator and a cooler for extracting heat
from the gaseous fluid upstream of the liquid separator using the
cold gas downstream of the heat exchanger. By extracting heat from
the gaseous fluid the temperature of the gaseous fluid at the inlet
to the separator can be reduced causing liquid in the gaseous fluid
to condense and be separated in the separator.
Inventors: |
Alford; Adrian Graham;
(Ruscombe, Berkshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alford; Adrian Graham |
Ruscombe, Berkshire |
|
GB |
|
|
Family ID: |
49166937 |
Appl. No.: |
14/907407 |
Filed: |
July 25, 2014 |
PCT Filed: |
July 25, 2014 |
PCT NO: |
PCT/GB2014/052292 |
371 Date: |
January 25, 2016 |
Current U.S.
Class: |
62/498 |
Current CPC
Class: |
F25B 13/00 20130101;
B01D 2257/80 20130101; F17C 1/00 20130101; B01D 53/265
20130101 |
International
Class: |
F25B 13/00 20060101
F25B013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 25, 2013 |
GB |
1313307.9 |
Aug 19, 2013 |
GB |
1314812.7 |
Claims
1. A system for reducing pressure in a gas flow for a gas let-down
system comprising an expander driven by gas at a first pressure
expanding to a second pressure, and a compressor for compressing
the gas from the second pressure to a third pressure, whereby the
third pressure is lower than the first pressure and the third
pressure is higher than the second pressure.
2-40. (canceled)
41. A system according to claim 1, wherein the expander drives the
compressor, preferably directly.
42. A system according to claim 41 wherein the expander drives the
compressor by way of a common shaft.
43. A system according to claim 1, further comprising a heat
exchanger for heating the gas at the second pressure.
44. A system according to claim 43, wherein the heat exchanger is
arranged to provide heat exchange to at least one of: ambient air,
to ground, or water, or an ambient heat source, and a waste heat
source.
45. A system according to claim 43, wherein the heat exchanger is
arranged to provide cooling to a refrigeration load.
46. A system according to claim 43, wherein a secondary circuit is
arranged to transfer heat from a heat source to a heat
exchanger.
47. A system according to claim 1, wherein the expander further
drives an electric generator.
48. A system according to claim 1, wherein the system is arranged
to conduct a portion of gas to the inlet of the expander, and to
conduct a further portion of gas to the outlet of the compressor,
the further portion of gas bypassing the expander and the
compressor.
49. A system according to claim 48, wherein the further portion of
gas drives a further expander that optionally further drives an
electric generator.
50. A system according to claim 1, wherein the system is arranged
to conduct a portion of gas to the inlet of the expander, and to
conduct a further portion to the compressor, the further portion
bypassing the expander.
51. A system according to claim 50, wherein the further portion
drives the compressor.
52. A system according to claim 51, wherein the further portion
drives the compressor by means of a tip turbine.
53. A system according to claim 1, wherein the system is arranged
to conduct a portion of gas to the inlet of the expander, and to
conduct a further portion of gas to an inlet of a further
expander.
54. A system according to claim 53, wherein the further expander
and the expander both drive the compressor.
55. A system according to claim 53, wherein the system is arranged
to conduct the further portion of gas from the outlet of the
further expander to the outlet of the compressor.
56. A system according to claim 1, further comprising a recuperator
for transferring heat from one portion of the gas to another.
57. A system according to claim 43, further comprising a
recuperator for transferring heat from one portion of the gas to
another and, optionally, wherein the recuperator is arranged to
transfer heat from gas upstream of the expander to gas downstream
of the heat exchanger.
58. A system according to claim 56, wherein the recuperator is
arranged to transfer heat from gas downstream of the compressor to
gas upstream of the expander.
59. A system according to claim 1, wherein the system is arranged
to conduct a portion of gas from the outlet of the compressor to
the inlet of the heat exchanger.
60. A system according to claim 59, wherein the system is arranged
to conduct a portion of gas from the outlet of the compressor to
the inlet of the compressor.
61. A system according to claim 1, further comprising a sealable
vessel containing system rotative components.
62. A system according to claim 61, wherein the system rotative
components include an output drive shaft of the expander and an
input drive shaft of the compressor.
63. A system according to claim 62, further comprising a gas
bearing supporting the output drive shaft of the expander or the
input drive shaft of the compressor.
64. A system according to claim 62, further comprising a magnetic
bearing supporting the output drive shaft of the expander or the
input drive shaft of the compressor.
65. A system according to claim 1, further comprising a controller
for activating the system when a system inlet gas temperature is
below a pre-defined threshold.
66. A system according to claim 1, wherein the expander comprises a
turbine.
67. A gas let-down station comprising a system according to claim
1.
68. A gas distribution network comprising a system according to
claim 1.
Description
[0001] The present invention relates to an apparatus, system or
method for reducing pressure in a gas flow for a gas let-down
system. The present invention further relates to an apparatus,
system or method for drying gas.
[0002] The present invention relates to a system for reducing
pressure in a gas flow for a gas let-down system. According to one
aspect of the invention, there is provided a system for reducing
pressure in a gas flow for a gas let-down system comprising an
expander driven by gas at a first pressure expanding to a second
pressure, and a compressor for compressing the gas from the second
pressure to a third pressure, whereby the third pressure is lower
than the first pressure and the third pressure is higher than the
second pressure. By first expanding the gas and then compressing
the gas the intermediate temperature of the gas at the second
pressure is lower than if the gas is expanded directly to the third
pressure. The gas is preferably natural gas. The expander is
preferably a turbine. The combination of an expander and a
compressor may also be referred to as a compander.
[0003] Preferably the expander drives the compressor, preferably
directly. This can provide efficiency. By directly is preferably
meant without conversion to another energy form such as
electricity, pressure or heat. Preferably the expander drives the
compressor by way of a common shaft. Optionally a mechanical
coupling may connect the expander and the compressor.
[0004] Preferably the system further comprises a heat exchanger for
heating the gas at the second pressure. This can allow efficient
heating of the gas. Preferably the heat exchanger is arranged to
provide heat exchange to ambient air. The heat exchanger may be
arranged to provide heat exchange to a secondary circuit for
absorbtion of heat from ground or water or other ambient source or
a waste heat source. The heat exchanger may be arranged to provide
cooling to a refrigeration load. The heat exchanger may be arranged
to provide heat exchange to ground, or water, or an ambient heat
source, or a waste heat source. The system may further comprise a
plurality of heat exchangers, with each of the heat exchangers
arranged to provide heat from a different heat source. A secondary
circuit may be arranged to transfer heat from a heat source to the
or a heat exchanger. The cold gas at the outlet of the expander may
transfer energy via a secondary circuit or a heat pipe to provide
cooling to a refrigeration load.
[0005] Preferably the expander further drives an electric
generator. The electricity may be used to drive further electrical
parts such as a fan and/or an electric defrosting heater. The
electricity may be used for exportation to an electrical grid.
[0006] Preferably the system is arranged to conduct a portion of
gas to the inlet of the expander, and to conduct a further portion
of gas to the outlet of the compressor, the further portion of gas
bypassing the expander and the compressor. The further portion of
gas may drive a further expander that optionally further drives an
electric generator. The further portion of gas may be expanded at a
valve. The system may provide sufficient excess heat in the
compander outlet gas stream to be mixed with another stream (such
as the further portion of gas) from a valve or an expander with an
electrical output to produce a mixed gas stream at a suitable
temperature for the downstream requirements.
[0007] Preferably the system is arranged to conduct a portion of
gas to the inlet of the expander, and to conduct a further portion
to the compressor, the further portion bypassing the expander. The
further portion preferably drives the compressor, preferably by
means of a tip turbine. The compressor preferably comprises a tip
turbine. This can provide supplementary drive to the
compressor.
[0008] Preferably the system is arranged to conduct a portion of
gas to the inlet of the expander, and to conduct a further portion
of gas to an inlet of a further expander. Preferably the further
expander and the expander both drive the compressor. Preferably the
system is arranged to conduct the further portion of gas from the
outlet of the further expander to the outlet of the compressor.
[0009] Preferably the system further comprises a recuperator for
transferring heat from one portion of the gas to another.
Preferably the recuperator is arranged to transfer heat from gas
upstream of the expander to gas downstream of the heat exchanger.
This can serve to reduce the gas temperature at the heat exchanger
to a sufficiently low value to enable effective heat exchange with
ambient air (or a secondary circuit for absorbtion of heat from
ground or water or other ambient source or a waste heat source).
This may be appropriate if the temperature drop and pressure drop
across the expander are relatively small.
[0010] The recuperator may be arranged to transfer heat from gas
downstream of the compressor to gas upstream of the expander. This
can pre-heat the gas entering the expander. The recuperator can
prevent the gas downstream of the expander from excessive cooling
which can cause solid formation and frosting at the heat exchanger,
leading to deterioration of the performance of and potentially
damage to the system. Also, gas exiting the compressor can be
provided at a high enough temperature for effective defrosting of
the heat exchanger in case of solids formation occurring.
[0011] Preferably the system is arranged to conduct a portion of
gas from the outlet of the compressor to the inlet of the heat
exchanger. This can provide additional heating and prevent frosting
of the heat exchanger. Different sections may be defrosted
sequentially at different times. Preferably the system is arranged
to conduct a portion of gas from the outlet of the compressor to
the inlet of the compressor. This can enable adjustment of the duty
position of the system by increasing compressor power when
open.
[0012] Preferably the system further comprises a sealable vessel
containing system rotative components. Preferably the system
rotative components include at least an output drive shaft of the
expander and an input drive shaft of the compressor.
[0013] Preferably the system rotative components include the common
shaft by way of which the expander drives the compressor.
Preferably the sealable vessel contains the generator, expander and
compressor. The sealable vessel can enable the entire rotative
system to operate in an environment at one of the system gas flow
pressures, thus avoiding the requirement for rotating seals in
communication with the external environment, therefore removing the
risk of natural gas leakage and potential explosion.
[0014] Preferably the system further comprises a gas and/or
magnetic bearing supporting the output drive shaft of the expander
and/or the input drive shaft of the compressor. Gas and/or magnetic
bearings can enable particularly low contamination of the gas in
the expander and the compressor, in particular with respect to oil
lubricant, as they require no oil lubricant.
[0015] Preferably the system further comprises a controller for
activating the system when an ambient air temperature is below a
pre-defined threshold. This can enable the system to operate
selectively when the ambient environment makes it necessary, and
otherwise assume another mode of operation such as in a
conventional let-down gas expander. The system can assist when the
ambient temperature (and in particular the temperature of the
incoming gas which is typically at ground temperature) is too low
to permit reducing pressure in a gas flow directly from the first
to the third pressure, as in a conventional let-down gas expander.
When the ambient temperature is high enough to permit reducing
pressure in a gas flow directly from the first to the third
pressure the system can be deactivated. The controller may also
activate the system for a pre-determined period of the year,
preferably during a period of the year that is expected to have an
ambient air temperature below a threshold (for example in winter
months). Alternatively, if the system is operated when an ambient
air temperature is above a pre-defined threshold, then the
generator is able to generate more electricity. In this example the
system may further comprise a controller for exporting surplus
electricity when an ambient air temperature is above a pre-defined
threshold.
[0016] Preferably the system further comprises a drying system as
described below.
[0017] Features of the system may include: [0018] Turbine directly
drives compressor [0019] For use in natural gas let-down of
compressed natural gas [0020] Process gas is used as the operating
fluid for the heat pump
[0021] According to a further aspect of the invention, there is
provided a drying system for drying a gaseous fluid (preferably
air) supplying heat to a heat exchanger, the drying system
comprising a liquid separator (for separating a liquid from the
gaseous fluid); a heat exchanger (for transferring heat away from
the gaseous fluid) downstream of the liquid separator; and a cooler
for extracting heat from the (warm) gaseous fluid upstream of the
liquid separator using the (cold) gaseous fluid downstream of the
heat exchanger.
[0022] By extracting heat from the gaseous fluid the temperature of
the gaseous fluid at the inlet to the separator can be reduced
causing liquid in the gaseous fluid to condense and be separated in
the separator (and thus removed from the gaseous fluid). This can
prevent excessive liquid from entering the heat exchanger, which
can cause frosting and lead to poor performance of and damage to
the heat exchanger.
[0023] Preferably the cooler is a recuperator for transferring heat
from gaseous fluid upstream of the liquid separator to gaseous
fluid downstream of the heat exchanger. A recuperator can enable
efficient transfer of heat.
[0024] Preferably the cooler is a mixer for mixing a portion of
gaseous fluid from downstream of the heat exchanger with gaseous
fluid upstream of the liquid separator. A mixer can provide a
simple implementation of the cooler.
[0025] The system may comprise a controller for controlling the
portion of gaseous fluid conveyed from downstream of the heat
exchanger back upstream of the liquid separator. The controller may
comprise a fan for establishing a flow rate of the conveyed
portion. The controller may respond to a signal from a detector.
The detector may detect frosting or a liquid loading of the gaseous
fluid.
[0026] Preferably the gaseous fluid comprises a component that is
in equilibrium with a liquid at the inlet to the heat exchanger,
and in equilibrium with a solid at the outlet of the heat
exchanger. Condensing and separating liquid from the gaseous fluid
can be particularly beneficial in cases where the gaseous fluid
condition changes due to the heat exchange such that a solid can
form. This can be in particular a change (decrease) of temperature
within the heat exchanger. The liquid may be water and the solid
may be ice. The liquid may be a hydrocarbon.
[0027] The separator may be a gravitational separator, a vortex
separator or a plate separator.
[0028] According to a further aspect of the invention, there is
provided apparatus for gas let-down comprising an expander driving
a compressor. Preferably the expander comprises a turbine.
[0029] According to a further aspect of the invention, there is
provided a system for drying a wet gas (preferably air) for
supplying heat to a heat exchanger, the system comprising a liquid
separator upstream of a heat exchanger and a conduit for conveying
a portion of gas from downstream of a heat exchanger back upstream
of the liquid separator, and re-introducing that portion of
gas.
[0030] According to a further aspect of the invention, there is
provided a gas let-down station comprising a system for reducing
pressure in a gas flow as described above.
[0031] According to a further aspect of the invention, there is
provided a gas distribution network comprising a system for
reducing pressure in a gas flow as described above.
[0032] The invention extends to methods and/or apparatus
substantially as herein described with reference to the
accompanying drawings.
[0033] Any apparatus feature as described herein may also be
provided as a method feature, and vice versa. As used herein, means
plus function features may be expressed alternatively in terms of
their corresponding structure.
[0034] Any feature in one aspect of the invention may be applied to
other aspects of the invention, in any appropriate combination. In
particular, method aspects may be applied to apparatus aspects, and
vice versa. Furthermore, any, some and/or all features in one
aspect can be applied to any, some and/or all features in any other
aspect, in any appropriate combination.
[0035] It should also be appreciated that particular combinations
of the various features described and defined in any aspects of the
invention can be implemented and/or supplied and/or used
independently.
[0036] These and other aspects of the present invention will become
apparent from the following exemplary embodiments that are
described with reference to the following figures in which:
[0037] FIG. 1 shows an embodiment of a system for reducing pressure
in a gas flow;
[0038] FIG. 2a shows a further embodiment of a system for reducing
pressure in a gas flow;
[0039] FIG. 2b shows a further embodiment of a system for reducing
pressure in a gas flow;
[0040] FIG. 2c shows a further embodiment of a system for reducing
pressure in a gas flow;
[0041] FIG. 3 shows an embodiment of a system for reducing pressure
in a gas flow with a low let-down pressure drop;
[0042] FIG. 4 shows a further embodiment of a system for reducing
pressure in a gas flow with a low let-down pressure drop;
[0043] FIG. 5 shows an embodiment of a system for reducing pressure
in a gas flow with a high let-down pressure drop;
[0044] FIG. 6 shows an embodiment of a system for reducing pressure
in a gas flow with an upstream separator;
[0045] FIG. 7 shows a drier for a heat exchanger; and
[0046] FIG. 8 shows a further drier for a heat exchanger.
[0047] At a conventional gas let-down station gas (natural gas)
pressure is reduced by expansion across a valve. Following
expansion the outlet temperature can be too low to allow the gas to
be reintroduced to the downstream pipe network, so the gas needs to
be heated to offset the cooling effect from the expansion of gas.
Conventionally gas is burned to provide this heat, with both an
economic and an environmental cost. Water bath heating, as is
conventionally used for heating expanded gas at let-down stations,
uses significant quantities of gas and is expensive to run and
maintain. Heating by burning gas is also environmentally damaging
due to the CO.sub.2 release from combustion.
[0048] It has been proposed to use the expansion energy to generate
electricity and to use this electricity to run a vapour compression
heat pump to heat the gas. This is an expensive and complex way to
heat the gas.
[0049] A gas let-down system with an expander and combined heat
pump that uses the expansion energy from a turbine to directly
drive a compressor which acts as a heat pump is proposed. The
intermediate pressure of the gas, downstream of the turbine and
upstream of the compressor, is lower than the outlet pipeline
pressure. At the same time the intermediate temperature of the gas
is lower than it would be if the gas were brought directly to the
outlet pipeline pressure. The combination of the expander and the
compressor allows the turbine to work over a greater pressure ratio
and increases the temperature drop of the gas. Ordinarily in gas
let-down the outlet gas temperature is too high to allow effective
heat exchange with the environment. Due to the small temperature
difference, in the order of for example only a few degrees Celsius,
the size of the heat exchanger (in the order of magnitude of acres)
would be prohibitive. Heat exchange may not even be possible if the
air temperature is too low. By the combination of the expander and
the compressor a greater temperature difference is available, for
example 20.degree. C. or more. With the increased gas temperature
drop, efficient heat exchange with the environment at the cold
intermediate condition can heat the gas. The size of the heat
exchanger can be in the order of tens of square metres rather than
the prohibitively sized heat exchangers that would be required in
conventional systems. Instead of requiring burning of gas to heat
the gas, heat exchange with the environment can raise the gas
temperature sufficiently.
[0050] This can achieve a number of things: [0051] 1. Increase the
temperature difference with ambient air (or other suitable ambient
or waster heat sources) to allow smaller heat exchangers. [0052] 2.
Allow some pre-heat to be applied to the gas. [0053] 3. Move the
position on the psychrometric chart to reduce the propensity of the
turbine to ice up (in particular control the pressure and
temperature in order to decrease the relative humidity of gas
passing through the turbine). [0054] 4. Provide heat to defrost
external heat exchangers.
[0055] The system harvests heat from the atmosphere (or other
suitable ambient or waster heat sources) and uses this to pre-heat
the gas, avoiding the need for gas combustion and water bath
heaters. The combination of expander and compressor provides outlet
gas at the required pressure and temperature for the downstream
network. The system has aspects of both expander and heat pump,
taking atmospheric heat and boosting it to a temperature suitable
for the efficient heating of gas within the process to acceptable
outlet temperatures.
[0056] FIG. 1 shows a basic system 10 for reducing pressure in a
gas flow 120. The gas flow 120 is directed first to an expander
102, typically a turbine (but other expanders such as a screw
expander are possible), and subsequently to a compressor 104. The
intermediate pressure of the intermediate gas flow 128, downstream
of the turbine 102 and upstream of the compressor 104, is lower
than the outlet pipeline pressure of the outlet gas flow 126. The
intermediate temperature of the intermediate gas flow 128 is lower
than if the gas pressure were at the outlet pipeline pressure. At
the heat exchanger 108 atmospheric air provides heat to warm up the
intermediate gas flow 128. The warmed intermediate gas flow 128 is
directed to a compressor 104 that increases the gas pressure;
downstream of the compressor 104 the system outlet gas flow 126 is
at the outlet pipeline pressure. The outlet pipeline pressure of
the outlet gas flow 126 is below the inlet pipeline pressure of the
inlet gas flow 120; and the intermediate pressure of the
intermediate gas flow 128 is below the outlet pipeline pressure of
the outlet gas flow 126.
[0057] The compressor and turbine are relatively balanced with 1-2
MW shaft power transferred.
[0058] In FIG. 1 the heat exchanger 108 is arranged to warm up the
intermediate gas flow 128 where atmospheric air provides heat. In
an alternative, such an air source heat pump is replaced with a
ground source heat pump, such that a ground source air provides
heat to warm up the intermediate gas flow 128. Other heat sources
for warming up the intermediate gas flow 128 can be used as
available and convenient.
[0059] FIG. 2a shows a variant of the system shown in FIG. 1, where
the cold gas at the outlet of the expander (the intermediate gas
flow 128) is arranged to provide cooling to a refrigeration load by
transferring a portion of the energy away from the cold gas. The
transfer of energy for refrigeration purposes can be arranged via a
heat exchanger 121 to where heat is absorbed from a secondary
circuit 123 as shown in FIG. 2a. Alternative means for the transfer
of energy, such as a heat pipe for example may be used.
[0060] The system can be configured to additionally generate
electricity. The electrical generation is 100 kW in an example,
which provides enough electricity to operate fans for the system.
An electric defrost system (in particular for low temperature
portions of the gas flow) can also or alternatively be operated by
the electricity.
[0061] Systems configured to also be operable during summer months
can convert a significant proportion of the turbine power to
electrical power due to the reduced necessity to use the heat pump
features of the system, with up to 1 MW able to be exported.
[0062] FIG. 2b shows a system 20 for reducing pressure in a gas
flow 120 having an electric generator 110. The turbine 102 drives
both the compressor 104 and also the electric generator 110. The
electricity generated by the electric generator 110 is used to
power a fan 112 that assists the heat exchanger 108.
[0063] Further, a pressure envelope 130 encloses the turbine 102,
the compressor 104 and the generator 110. The pressure envelope 130
includes suitable connections for inlet and outlet of gas streams
and also for electrical connection between the generator 110 and
the electrical load. The pressure in the pressure envelope 130 is
at the intermediate pressure of the intermediate gas flow 128. This
avoids the necessity for shaft seals (on the expander shaft and the
compressor shaft) across a pressure drop (to the external
environment) and the risk of seal failure, and allows the use of
gas bearings for the turbine shaft and compressor shaft. The use of
gas or magnetic bearings is advantageous as oil contamination of
the gas can be minimised. The pressure envelope 130 contains
natural gas, same as the incoming and outgoing streams, and
residual mass transport across the bearings does not cause
contamination of the gas stream.
[0064] The systems disclosed above may include the following
features: [0065] The combination of expander and heat pump using a
single rotating element. [0066] Process gas used as the operating
fluid for the heat pump. [0067] No net electrical output from the
system; a small electrical power output from the generator (ca. 1%
of expander shaft power) can be used to run fans, with the
remainder running the heat pump. [0068] The system can be hermetic
with no seals to ambient, and can run on gas bearings, precluding
the potential for leaks, seal failure or oil contamination of the
gas. [0069] The system can be made available as a road
transportable temporary unit, for use on site when an existing
conventional let-down system with water bath heaters fails, so
removing the requirement for a secondary stand-by unit to stand
alongside unused. This is possible due to the system being
configured to enable the heat exchanger to be appropriately sized
for housing in such a unit yet being able to sufficiently increase
the gas temperature intermediate the expander and compressor.
[0070] The system can have remote diagnostic and control capability
reducing or preventing site visits.
[0071] A further sub-system to reduce or virtually eliminate the
frosting of the external heat exchanger is a drier that dries
ambient atmospheric air before it comes into contact with the
sub-zero heat exchange surfaces in order to heat the gas stream. A
possible way to achieve this is to use a portion of the cold air
exiting the external heat exchanger to pre-cool the air entering
the heat exchanger, and dropping the temperature of the air
entering the heat exchanger it to just above zero degrees to allow
a large part of the condensate load to drop out. This can then be
separated from the air stream. A system for drying air for a heat
exchanger is described in more detail with reference to FIGS. 7 and
8.
[0072] Optionally, the system for reducing pressure in a gas flow
may only be used if certain conditions are provided, for example
the natural gas at the system inlet being below a certain threshold
(or the ambient air temperature being below a certain threshold).
For example, the system can be used only in winter, and in summer
when the inlet gas temperature (and also the ambient air
temperature) is relatively high the system can be switched into a
summer mode with the gas being expanded directly to the outlet
pipeline pressure (omitting an intermediate condition with lower
pressure and temperature). At the higher temperature the gas is
expanded directly to the outlet pipeline pressure and requires no
heating as it remains above the minimum temperature required by the
downstream system. Alternatively the system can be operated at all
temperatures, and when the temperature is above a certain threshold
surplus electricity can be generated.
[0073] FIG. 2c shows a variant of the system shown in FIG. 1. A
first portion 131 of the gas flow 120 bypasses the expander 102 and
heat exchanger 108 and the compressor 104. A second portion 124 of
the gas flow is directed to the expander 102 and heat exchanger 108
and compressor 104. The first portion 131 of the gas flows via a
pressure reducer 133 (such as a valve or an expander), optionally
with an associated electrical generator for generating an
electrical output, and is reintroduced downstream of the compressor
104. The gas flow at the outlet of the compressor 104 system
provides sufficient excess heat to be mixed with the first portion
131 of the gas to produce a mixed gas stream at a suitable
temperature for the downstream requirements. The second portion 124
is expanded directly to the system outlet (target) pressure, and
mixed with a warmer gas stream treated by the system shown in FIG.
1 in order to ensure the resulting gas stream exiting the system is
sufficiently heated.
[0074] FIGS. 3 to 6 show further versions of the system for
reducing pressure in a gas flow. For the heat exchanger a blast
cooler type heat exchanger is used. A recuperator can provide
supplementary heat exchange. An expander turbine drives the
compressor. Additionally an electrical generator is driven by the
turbine and produces electricity for fans that assist the heat
exchange at the blast cooler. FIGS. 3 and 4 show systems for
reducing pressure in a gas flow with a relatively low let-down
pressure drop, for example with a pressure drop from 44 bar at the
system inlet to 38 bar at the system outlet. The intermediate
pressure is for example 20 to 25 bar. The temperature drop is
around 0.45 to 0.6.degree. C. per bar pressure drop. Due to the
relatively low let-down pressure drop, and assuming the expander
has a suitably high pressure ratio (in order to ensure the gas
temperature is sufficiently low at the heat exchanger), the
compressor needs to provide relatively large compression in order
to bring the gas back to the outlet pipeline pressure. Due to
limited efficiency of the turbine, the turbine alone may not
suffice to drive the compression in this case. Therefore the
systems described with reference to FIGS. 3 and 4 provide
supplemental compression drive, in addition to the drive provided
to the compressor from the expander.
[0075] FIG. 3 shows a system 100 for reducing pressure in a gas
flow 120. In this system 100 there is a relatively low let-down
pressure drop. A first portion 122 of the gas flow bypasses the
expander 102 and heat exchanger 108 and is reintroduced at the
compressor 90. A second portion 124 of the gas flow is directed to
the turbine expander 102 and compressor 90. The compressor 90 is a
compressor with a tip turbine, and the first bypass gas flow
portion 122 drives the compression of the second heated gas flow
portion 124 by means of the tip turbine. In the tip turbine the
first bypass gas flow portion 122 accelerates the compressor blades
and provides supplemental compression drive, in addition to the
drive provided to the compressor 90 from the expander 102.
[0076] A recuperator 106 allows transfer of some of the heat from
the gas upstream of the expander 102 to the gas downstream of the
heat exchanger 108. This can reduce the gas temperature at the heat
exchanger to a sufficiently low value to enable effective heat
exchange. At the heat exchanger 108 atmospheric air provides heat
to warm up the intermediate gas flow 128. The warmed gas is fed
into a compressor 90 and is compressed and combined with the first
bypass gas flow portion 122 to form the system outlet gas flow 126
at a lower outlet pipeline pressure.
[0077] The turbine 102 drives both the compressor 90 and also an
electric generator 110. The electricity generated by the electric
generator 110 is used to power a fan 112 that assists the heat
exchanger 108.
[0078] FIG. 4 shows a further system 200 for reducing pressure in a
gas flow 120. In this system 200 also there is a relatively low
let-down pressure drop. Two expanders 102 202 are combined. A first
portion 222 of the gas flow is expanded in a supplementary expander
202 directly to the outlet pipeline pressure and reintroduced
downstream of the compressor 104. A second portion 224 of the gas
flow is directed to the turbine expander 102 and compressor 104.
The supplementary expander 202 contributes supplemental drive to
the drive provided from the heat pump expander 102 in order to
provide sufficient drive to the compressor 104. This can enable a
sufficiently low intermediate pressure (and hence sufficiently low
gas temperature at the heat exchanger) for efficient heat exchange
with ambient atmosphere air.
[0079] In the systems in FIGS. 3 and 4 the recuperator 106 is
arranged to transfer heat from the incoming gas flow 120 upstream
of the expander 102 to the intermediate gas flow 128, downstream of
the heat exchanger 108 and upstream of the compressor 104. The
purpose of the recuperator 106 is to reduce the gas temperature at
the heat exchanger to a sufficiently low value to enable effective
heat exchange with ambient air or ambient air which has been dried
by cooling.
[0080] For the recuperator 106 to provide efficient transfer of
heat between different gas stream portions, the temperature
difference between the different gas stream portions should be
sufficiently large, typically at least 5 or 10.degree. C. In the
systems with a relatively low let-down pressure drop described with
reference to FIGS. 3 and 4, the temperature difference between the
incoming gas flow 120 and the system outlet gas flow 126 can be
relatively small and insufficient for providing heat to the inlet
side, in which case the recuperator arrangement as shown in FIGS. 3
and 4 and as described above can be appropriate.
[0081] FIG. 5 shows a further system 300 for reducing pressure in a
gas flow 120. In this system 300 there is a relatively high
let-down pressure drop, for example with a pressure drop from 33
bar at the system inlet to 17 bar at the system outlet. The
intermediate pressure is for example 9 to 12 bar. The temperature
drop is around 0.45 to 0.6.degree. C. per bar pressure drop.
[0082] A recuperator 302 allows transfer of some of the heat from
the gas downstream of the compressor 104 to the gas upstream of the
expander 102. This can provide pre-warming of the gas entering the
expander 102. The gas flow 120 is directed to the turbine expander
102 and compressor 104.
[0083] Downstream of the compressor 104 a recirculation gas flow
322 is separated from an outlet gas flow 320. The outlet gas flow
320 passes to the recuperator 302 before progressing to the main
outlet gas flow 126. The recirculation gas flow 322 is split into a
bypass line 324 and a defrost line 326. The flow in the defrost
line 326 is reintroduced upstream of the heat exchanger 108.
Because the flow in the defrost line 326 is relatively warm it can
heat the heat exchanger 108 and thus avoid frosting in the heat
exchanger 108. The flow of the bypass line 324 is reintroduced
downstream of the heat exchanger 108 and upstream of the compressor
104 and can be used to adjust the compressor duty.
[0084] Flow controllers 304 control the speed of the flow in the
bypass line 324 and defrost line 326 based on (for example)
temperature sensing or flow speed sensing. The flow controllers 304
can include an actuated valve.
[0085] In FIG. 5 typical temperatures are indicated for the
different gas flows. Only one defrost line 326 is shown, but more
may be included in the system 300. The defrost line 326 can defrost
both internal and external heat exchanger surfaces. The duty of the
heat exchanger 108 (blast cooler) in FIG. 5 is approximately 1.1
MW. The duty of the recuperator 302 in FIG. 5 is approximately 1.35
MW.
[0086] The recirculation gas flow 322 shown in FIG. 5 can be used
with all system variants.
[0087] FIG. 6 shows a further system 400 for reducing pressure in a
gas flow 120. In this system 400 a separator 402 is included
upstream of the recuperator 302. The separator 402 separates the
gas flow 120 into a liquid stream 424 and a substantially dry gas
stream 422. The liquid stream 424 can include a gas and/or solid
component. The substantially dry gas stream 422 passes to the
recuperator 302 same as the gas flow 120 in the system 300.
Actuated valves 406 control the flow in the liquid stream 424 and
the gas stream 422. If necessary a heat addition 404 to the liquid
stream 424 can be provided, for example by exchange with heated
compressor outlet gas, or by mixing with a hot stream (with
suitable restrictors).
[0088] In FIG. 6 typical temperatures are indicated for the
different gas flows. The defrost line 326 can defrost both internal
and external heat exchanger surfaces. Only one defrost line 326 is
shown, but more may be included in the system 400. The duty of the
heat exchanger 108 (blast cooler) in FIG. 6 is approximately 1.1
MW. The duty of the recuperator 302 in FIG. 6 is approximately 1.35
MW.
[0089] The separator 402 shown in FIG. 6 can be used with all
system variants.
[0090] If the separator 402 is upstream of a pre-heater (where a
pre-heater is used) such as the recuperator 302 shown in FIG. 6,
then on entering the pre-heater the flow has very low or no liquid
loading (as the liquid stream has been removed in the separator),
and is a saturated gas. The heating process in the pre-heater
(recuperator 302 in FIG. 6) moves the gas away from its saturation
line, drying it and therefore reducing the tendency to frost
downstream of the expander 102. If the separator 402 is downstream
of the pre-heater, then liquid is evaporated in the pre-heater
which means that the liquid stream that is removed in the separator
is reduced; also, the gas exiting from the separator would be
saturated, or closer to its saturation line on entering the
expander 102, so more frosting would occur downstream of the
expander 102. Hence locating the separator 402 upstream of the
pre-heater is preferred.
[0091] In the systems in FIGS. 5 and 6 the recuperator 302 is
arranged to transfer heat from the system outlet gas flow,
downstream of the compressor 104, to the incoming gas stream
upstream of the expander 102. In essence, this can transfer some of
the heat gained from the heat exchange with ambient atmospheric air
to the low pressure, low temperature portions of the gas flow in
order to change the gas conditions such that condensation and
especially ice formation is prevented. In some circumstances the
recuperator 302 may not be necessary and can be omitted.
[0092] In a further intermediate pressure version of the system the
features of a supplementary expander 202 as described with
reference to FIG. 4 and a recuperator 302 arrangement as described
with reference to FIGS. 5 and 6 are combined.
[0093] As can be seen from the above arrangements, the gas
pressures at gas let-down stations can vary considerably depending
on a variety of factors and the systems disclosed above are
adaptable to the differing pressures and can be configured as
discussed to efficiently reduce the pressure of a gas to a desired
outlet gas pressure.
Sub-Zero Expander Outlet.
[0094] The sublimation of water vapour to ice is a non-equilibrium
process so does not occur quickly enough for ice crystals to appear
in the expander turbine in a volume or size to cause significant
problems. Ice crystals may form in the gas downstream of the
turbine, and can clog downstream equipment such as heat exchangers.
The system can have defrost units for this eventuality.
[0095] Liquid formation can occur within the turbine and this is
generally hydrocarbon matter that does not freeze at the
temperatures considered here. Condensation may occur in the turbine
wheel and not in the nozzles so erosion is avoided. It may be
necessary to heat the expander shroud/body.
Blast Cooler Drier
[0096] A drier for the blast cooler (the heat exchanger) may be
provided. FIG. 7 shows a system 500 for drying warm air that
supplies heat to a heat exchanger 108 (blast cooler). The drier
system 500 is based on an air recirculation arrangement.
[0097] The incoming air flow 520 is mixed (at Point 1 indicated in
FIG. 7) with a recirculated flow portion 522 of cold air flow
exiting the heat exchanger 108. This partially cools the incoming
air flow 520 to around 0.degree. C., which causes condensate to
form. The condensate can then be separated in the separator 502.
The resulting dried air flow 524 is conveyed to the heat exchanger
108.
[0098] By this arrangement liquid loading in the air is reduced and
detrimental freezing in the heat exchanger 108 is prevented. The
drier system 500 is particularly beneficial if incoming air is
cooled below 0.degree. C. in the heat exchanger 108, as frosting
can cause damage and loss of performance.
[0099] A controller 504 can control the recirculation flow 522, for
example by fan speed control. Temperature sensing can be used by
the controller to regulate the fan speed.
[0100] In FIG. 7 typical temperatures are indicted for different
parts of the flows.
[0101] FIG. 8 shows an alternative drier system 600. Instead of
mixing the incoming air flow 520 at Point 1 indicated in FIG. 7
with a cold recirculated flow portion 522, heat can be transferred
between the two flows 520 522 by means of a recuperator 602
upstream of the separator 502. This has the advantage of avoiding
dilution of the cooling potential of the cold flow 522. Further, if
a recuperator 602 transfers heat between the recirculation flow 522
and the incoming air flow 520, then the cold flow 522 entering the
recuperator 602 can comprise all of the cold air exiting the heat
exchanger 108, and not merely a portion of the cold air exiting the
heat exchanger 108. The alternative with a recuperator 602 does not
require recirculation, so the cooling potential of all of the cold
air exiting the heat exchanger 108 can be harnessed before the air
is discharged back to the atmosphere.
[0102] It will be understood that the present invention has been
described above purely by way of example, and modifications of
detail can be made within the scope of the invention.
[0103] Each feature disclosed in the description, and (where
appropriate) the claims and drawings may be provided independently
or in any appropriate combination.
[0104] Reference numerals appearing in the claims are by way of
illustration only and shall have no limiting effect on the scope of
the claims.
* * * * *