Patent Application
20090188277
This application claims priority from European Patent Application No. 07119862.6, filed on Nov. 2, 2007, which is incorporated herein by reference.
The present invention relates to a method and apparatus for controlling a refrigerant compressor.
Such a method and apparatus may be used in a method of cooling a hydrocarbon stream, such as a natural gas stream. Accordingly, aspects of the invention relate to a method of cooling a hydrocarbon stream, and to a use of the apparatus in a process for cooling, and a use of the apparatus in a process for liquefying, a hydrocarbon feed stream.
Several methods of cooling, usually liquefying, a natural gas stream, thereby obtaining liquefied natural gas (LNG), are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressures.
As an example of liquefying natural gas, the natural gas, comprising predominantly methane, enters an LNG plant at elevated pressure and is pre-treated to produce a purified feed steam suitable for liquefying at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers involving one or more refrigeration circuits with compressors, to progressively reduce its temperature until liquefaction is achieved.
Compressors for gaseous streams are used in many situations, systems and arrangements. Typically there is a vapour recycle or recirculation line around the compressor to avoid surge. A compressor is said to be ‘in deep surge’ when the main flow through the compressor reverses its direction. Normally, this is related to a discharge pressure being lower than the pressure downstream of the compressor outlet. This can cause rapid pulsations in flow, which is generally termed ‘surge’.
Surge is often accompanied by excessive vibration and noise. This flow reversal is accompanied with a very violent change in energy, which causes a reversal of the thrust force. The surge process can be cyclic in nature, and if allowed to continue for sometime, irreparable damage can occur to the compressor.
Where a compressor is dealing with ambient temperature gases or other non-critical situations, recycling of discharge gas via a vapour recycle line to avoid surge is a simple and common operation without complications. Any change in temperature of the compressor flow is not significant.
Compressors used in refrigerant circuits have particular problems associated to them, especially when they are driven by fixed rotational speed drivers, such as a gas turbine. Refrigerant circuits are used in liquefaction systems, facilities and plants, such as for the production of a liquefied hydrocarbon stream such as liquefied natural gas (LNG). In a refrigerant circuit, the refrigerant is evaporated in one or more stages to cool the hydrocarbon stream, and one or more refrigerant compressors are used to recompress the evaporated refrigerant in one or more stages. Refrigerant compressors running at an effectively constant speed require relatively constant inflow of a gaseous stream to their suction side. Where the inflow of a gaseous stream falls for whatever reason below a certain minimum value, surge can occur.
U.S. Pat. No. 4,464,720 discloses a surge control system which utilizes an algorithm to calculate a desired orifice differential pressure, and which compares the calculated result with an actual differential pressure. Pressure and temperature measurements are made on both the suction side and discharge side of a centrifugal compressor, and thus enter a control system so that the actual differential pressure is substantially equal to the desired differential pressure. A suction temperature of gas entering the centrifugal compressor is measured and used. However, the complex algorithm and values required for the calculations in U.S. Pat. No. 4,464,720 do not address any of the problems described above.
The present invention provides a method of controlling a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream, the method at least comprising the steps of:
(a) passing the refrigerant stream through at least one heat exchanger to provide a first at least partially evaporated stream and a second at least partially evaporated stream;
(b) providing a first compressor feed stream from the first partially evaporated stream and a second compressor feed stream from the second partially evaporated stream;
(c) passing the first compressor feed stream through the first refrigerant compressor and the second compressor feed stream through the second compressor to provide first and second compressed streams;
(d) combining the first and second compressed streams at a common pressure; and
(e) either heating the first at least partially evaporated stream or the first compressor feed stream to control the second compressor,
or heating the second at least partially evaporated stream or the second compressor feed stream to control the first compressor.
The present invention also provides a method of cooling a hydrocarbon stream such as natural gas, the method at least comprising the steps of:
(a) providing a hydrocarbon feed stream;
(b) cooling the hydrocarbon feed stream by heat exchange against a refrigerant stream to provide a cooled hydrocarbon stream and an at least first and second partly evaporated refrigerant streams;
(c) compressing the first and second partly evaporated refrigerant streams using at least first and second refrigerant compressors in a method as herein defined.
The present invention also provides apparatus for controlling a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream, the apparatus at least comprising:
at least one heat exchanger through which the refrigerant stream passes to provide a first at least partially evaporated stream and a second at least partially evaporated stream;
a first compressor feed stream provided from the first partially evaporated stream;
a second compressor feed stream provided from the second partially evaporated stream;
a first refrigerant compressor through which passes the first compressor feed stream to provide a first compressed stream;
a second compressor through which passes the second compressor feed stream to provide a second compressed stream;
a combiner to combine the first and second compressed streams at a common pressure; and
one or more heaters to either heat the first at least partially evaporated stream or the first compressor feed stream to control the second compressor,
or to heat the second at least partially evaporated stream or the second compressor feed stream to control the first compressor.
The present invention also provides the use of such an apparatus in a process for cooling a hydrocarbon stream, wherein said cooling comprises passing the hydrocarbon stream through the at least one heat exchanger of the apparatus whereby heat is transferred from the hydrocarbon stream to the refrigerant stream.
The present invention also provides the use of such an apparatus in a process for liquefying a hydrocarbon stream, wherein said liquefying comprises cooling of the hydrocarbon stream, said cooling comprising passing the hydrocarbon stream through the at least one heat exchanger of the apparatus whereby heat is transferred from the hydrocarbon stream to the refrigerant stream.
Embodiments and examples of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:
For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.
The present invention is particularly suitable for, but not limited to, controlling two or more connected and multi-stage refrigerant compressors having a plurality of inlets accepting refrigerant at different pressure levels. The refrigerant compressor(s) may be used in one or more refrigerant circuits for cooling, optionally including liquefying, a hydrocarbon stream such as natural gas.
It has been surprisingly found that where two refrigerant compressors have a common post-compression communication, for example a common discharge or common accumulator, the heating of one compressor feed stream, or its main component being its evaporated stream from a heat exchanger, can control the other connected refrigerant compressor. This is because the heating of such a stream affects its subsequent compression, and thus the pressure of the other stream being compressed via their common post-compression communication. This is further discussed hereinafter.
The present invention may provide an improved method of controlling one or more refrigerant compressors, in particular two or more refrigerant compressors, used for compression of a multi-pressure level refrigerant.
The present invention is particularly suitable, but not limited to, where there are two or more refrigerant compressors in the same refrigerant circuit for different compressor feed streams, more particularly where the compressor feed streams are at different pressure levels. When using multiple refrigerant compressors, either as separate compressors or one or more compressors having multiple pressure sections and having multiple inlets for different gas pressures, and optionally multiple recirculation lines, the method of the present invention allows all the refrigerant compressors to be controlled, especially to avoid surge.
The present invention is particularly useful where a refrigerant stream is evaporated at different pressure levels, but requiring each evaporated fraction to be recompressed to a unified pressure for re-use as a refrigerant.
Thus, in one embodiment, the method is for controlling three or more, preferably four, refrigerant compressors, and two, three, four or five compressor feed streams based on two, three, four or five at least partly evaporated refrigerant streams respectively, wherein at least one of the evaporated refrigerant streams or at least one of the compressor feed streams is heated prior to passing through its respective refrigerant compressor.
More preferably, the method of the present invention comprises four compressor feed streams at four different pressures passing through four refrigerant compressors.
The cooling of the hydrocarbon stream may partly or wholly liquefy the hydrocarbon stream so as to provide a liquefied hydrocarbon stream, such as LNG. The cooling of the hydrocarbon stream to provide the cooled hydrocarbon stream may also be followed by further cooling of the cooled hydrocarbon stream to further liquefy and/or sub-cool the cooled hydrocarbon stream.
The or each refrigerant compressor involved in the present invention may be any suitable refrigerant compressor, optionally having two or more compression stages or pressure sections. Use of the term ‘refrigerant compressors’ herein extends to a single refrigerant compressor having multiple pressure sections in one casing, and able to receive two or more gaseous streams at different pressures. The hydrocarbon cooling or liquefying plant or facility may also involve one or more other refrigerant or other compressors not involved in the present invention.
The first and second compressors may have a vapour recirculation line. Preferably, the first and second compressor feed streams are a combination of at least a fraction of the vapour recirculation stream in the vapour recirculation line and the first and second at least partially evaporated refrigerant streams respectively.
The or each recirculation line useable in the present invention may be any suitable line able to transfer a recirculation stream, which can be liquid, gaseous or mixed phase, from the discharge side of a refrigerant compressor to its suction side or a relevant suction drum. The or each recirculation line may be divided or separated in a manner known in the art, to supply a recirculated stream part or fraction to two or more refrigerant compressors.
A refrigerant stream may comprise a single component such as propane or nitrogen, preferably propane, or comprise a mixture of two or more selected from the group comprising: nitrogen, methane, ethane, ethylene propane, propylene, butanes, pentanes.
The heating of the relevant stream or streams in step (e) can be provided by any method of heating. The heating may involve use of one or more dedicated heaters, optionally using a fuel gas, and/or by heat exchange against a stream capable of releasing heat to the relevant stream of streams, such as a warmer stream or streams, in one or more heat exchangers. In a cooling and/or liquefaction process or plant, there are typically a number of sources of a warmer stream or streams, which could be routed to provide, when required, the heating for step (e).
The common post-compression communication of the first and second refrigerant compressors, which allows combining of the first and second compressed streams at a common pressure, can be any form of combining, either by a dedicated combiner such as a junction or T-piece, or by their common collection, for example in a cooler or accumulator.
In another embodiment, the method of the present invention is able to prevent surge in a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream. Preferably heating the first at least partially evaporated stream or the first compressor feed stream is to prevent surge in the second compressor, and heating the second at least partially evaporated stream or the second compressor feed stream is to prevent surge in the first compressor.
Referring to the drawings,
In
The hydrocarbon feed stream 5 is cooled by heat exchange against a refrigerant stream 40 circulating in a refrigerant circuit 2. The refrigerant stream 40 passes through at least two heat exchangers (32, 34) in the refrigeration zone 42.
In the scheme 1 shown in
Each of the evaporated refrigerant streams 40a, 40b are typically wholly evaporated to achieve maximum cooling in the heat exchangers 32, 34, although partial-evaporation is also a possibility covered by the invention. Usually, the first evaporated stream 40a is at a higher pressure than the second evaporated stream 40b although the present invention is not limited thereto.
Typically, each of the first and second evaporated streams 40a, 40b are passed through first and second suction drums 16, 18 respectively, to provide first and second compressor feed streams 20a, 20b respectively. The first and second compressor feed streams 20a, 20b then pass through first and second refrigerant compressors 12, 14 respectively to provide first and second compressed streams 50a, 50b respectively. The first and second compressed streams 50a, 50b can be combined by a combiner 52 to provide a combined stream 60, which can be first cooled by a first ambient heat exchanger such as one or more water and/or air coolers 54, to provide a cooler compressed stream 60a.
When desired or necessary, the cooler compressed stream 60a can be divided by a divider 56, for instance in the form of a stream-splitter, into a continuing refrigerant stream 60b and a vapour recirculation stream 30 passing through a vapour recirculation line 30. The divider 56 may be any arrangement able to divide a stream into two or more fractions or parts, such as a manifold or dedicated unit, or more simply, a T-piece. The continuing refrigerant stream 60b passes through one or more further coolers 58, one or more accumulators 62, one or more yet further coolers 64, prior to expansion through an expansion valve 66 to provide the refrigerant stream 40 and recirculation through the refrigeration zone 42.
The vapour recirculation stream 30 may then be divided by any suitable divider 31 into two part-recirculation streams 30a, 30b, expanded through suitable control valves 68a, 68b in a manner known in the art, and subsequently combined with the first and second at least partially evaporated streams 40a, 40b respectively to provide first and second combined feed streams 10a, 10b respectively.
It is known that vapour recirculation provides the ability to increase flow through a compressor, more particularly to maintain minimum flow where there is a reduction in the flow of evaporated refrigerant which normally forms the major component of the compressor feed stream.
Every compressor requires a balance between its inlet pressure and outlet or discharge pressure. For a refrigerant compressor in a refrigerant circuit, the inlet and outlet pressures are usually significantly determined by two vapour pressures; firstly from the heat exchangers using the cooling duty of the refrigerant stream, and secondly from the post-compression refrigerant accumulator.
However, there can be occasions or periods where the pressure balance is upset, such that the discharge pressure from a refrigerant compressor can be insufficient compared to the inlet pressure, leading to surge. One example of this is an insufficiency in the provision of an evaporated stream from a heat exchanger. With an insufficient flow of evaporated stream to provide the compressor feed stream, the compressor can go towards surge.
A benefit of the present invention is recognising that where there are two or more refrigerant compressors having a common post-compression communication, such as a common discharge, then the discharge pressure from each refrigerant compressor can affect the other refrigerant compressor. That is, using the post-compression common pressure to allow the action of one refrigerant compressor to control an associated or connected refrigerant compressor.
Referring to the Scheme 1 shown in
The method of the present invention involves heating of the first at least partially evaporated stream 40a and/or the first combined feed stream 10a and/or the first compressor feed stream 20a, to control the second compressor 14. For example,
By heating the first at least partially evaporated stream 40a, there will be the same volume flow in this stream, but the mass flow in the subsequent first compressor feed stream 20a is reduced. With less mass flow in the subsequent first compressed stream 50a, the same amount of cooling duty provided by the first and further coolers 54, 58 increases the amount or degree of cooling of the flow of refrigerant therethrough, such that the temperature of refrigerant in the accumulator 62 is lower than normal, such that there is less vapour pressure created therefrom. With less vapour pressure from the accumulator 62, this feeds back to reducing the pressure on the discharge side of the second compressor 14, such that it is less likely to go into surge.
In the same way, heating of the second at least partially evaporated stream 40b or the second combined feed stream 10b or the second compressor feed stream 20b provides control of the first compressor 12. In scheme 1, there is shown by way of example a heater such as a similar heat exchanger 24 in the path of the second at least partially evaporated stream 40b for this purpose.
A person skilled in the art will be aware of the nature and provision of the first and second heat exchangers 22, 24, which may comprise one or more heat exchangers in parallel, series or both.
The second refrigeration zone 142 can be for withdrawing heat from a stream, for example one or more hydrocarbon streams 100 such as a natural gas stream to be liquefied. Examples of methods for liquefying natural gas are mentioned in U.S. Pat. No. 6,389,844 and U.S. Pat. No. 6,370,910 B1 which are hereby incorporated by reference. In these patent documents, a plant is described for liquefying natural gas wherein the plant comprises a pre-cooling heat exchanger having an inlet for natural gas and an outlet for cooled natural gas, and a pre-cool refrigerant circuit for removing heat from the natural gas in the pre-cooling heat exchanger.
The second refrigeration zone 142 may be equivalent to or an extension of the first refrigeration zone 42 shown in
In the arrangement shown in
Each evaporated stream 140a-d is passed into a corresponding suction drum or gas/liquid separator such as knockout drums 148a-d, from which there are respective overhead gaseous compressor feed streams 120a-d.
The fourth compressor feed stream 120d passes into a first refrigerant compressor 112d to provide a compressed stream 150d, which is combined with the second compressor feed stream 120b to enter a second refrigerant compressor 112b to provide a first final compressed stream 160. The first and second refrigerant compressors 112d, 112b may be separate refrigerant compressors, or may be in one casing, having two inlets and one or two sections to accommodate the different pressure levels of the second and fourth compressor feed streams 120b, 120d.
Similarly, the third compressor feed stream 120c passes into a third refrigerant compressor 112c, and its compressed stream 150c is combined with the first compressor feed stream 120a to pass into a fourth refrigerant compressor 112a and provide a second final gaseous stream 170. As above, the third and fourth refrigerant compressors 112a, 112b may be separate refrigerant compressors, or may be in one casing having two inlets and different sections to accommodate the different pressures of the first and third streams 120a, 120c.
The arrangement of the refrigeration zone 142, the outlets and gaseous streams therefrom, and the refrigerant compressors 112a-d are known in the art, and are shown and described for example in WO 01/44734 A2.
The first and second final compressed streams 160, 170 are themselves combined to form an overall combined compressed stream 180, which is cooled by a first cooler 162 such as an ambient water and/or air cooler known in the art. The first cooler 162 may comprise one or more coolers in parallel, series or both, and provides a cooled compressed stream 190.
In the same manner as described above for the arrangement shown in
The first continuing steam 200 is further cooled and mostly or fully condensed, for example by a second cooler 172 being one or more coolers such as water and/or air coolers, which provides a cooled continuing steam 210. The cooling provided by the second cooler 172 preferably fully condenses the cooled continuing stream 210. The cooled continuing stream 210 passes into an accumulator 166.
The accumulator 166 provides a liquid stream 220 which can be further cooled by a third cooler 168, being one or more coolers such as water and/or air coolers, to provide a reconstituted or reformed generally liquid refrigerant stream ready for passage through a valve 177 and return and use as stream 140 in the second refrigeration zone 142.
In the same manner as described in
Indeed, depending on the arrangement and relationship of the streams and compressors in the second scheme 101 shown in
Similarly, again depending on the arrangement and relationship of the streams and compressors in the second scheme 101 shown in
In a similar arrangement to that shown in
In the same manner as described in
Table 1 shows percentage changes in pressures and volume flows for a working example of the present invention based on the arrangement for refrigerant compressors for a refrigerant circuit shown in
Heating of the first evaporated stream 140a increases the pressure of the subsequent compressor feed stream 120a into the fourth refrigerant compressor 112a (by 7.09%) with a small reduction in its volume flow (−1.73%). The effect of this increase in pressure on the compressor feed stream 120a into the fourth refrigerant compressor 112a is to increase the volume flow through the second refrigerant compressor 112b (by +3.30), such that the second compressor 112b moves away from surge.
Meanwhile, heating the second evaporated stream 140b increases the pressure of the second compressor feed stream (120b) on the second refrigerant compressor 112b (by +6.42), which at least increases the volume flow through the third refrigerant compressor 112c.
Heating the third evaporated stream 140c increases the pressure of the third compressor feed stream 120c (by +1.31), which has the effect of increasing the volume flow through both the first refrigerant compressor 112d (by +0.28) and the second refrigerant compressor 112b (by 0.06), to move them both away from surge.
Lastly, heating of the fourth evaporated stream 140d increases the pressure of the fourth compressor feed stream 120d and the pressure of the first refrigerant compressor 112d, and increases the volume flow (by 0.29%) through the third refrigerant compressor 112c so that the third refrigerant compressor moves away from surge.
There are references hereinabove to various types of “valves” including flow-control valves, recirculation valves and expansion valves. Some valves required in any circuit or process may not be specifically or generally mentioned or referenced herein. The skilled man is aware of the type and arrangement of valve or valves required to affect processing of a line, stream, flow, circuit, etc.
Persons skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07119862.6 | Nov 2007 | EP | regional |
