Patent Publication Number: US-8532830-B2

Title: Method and apparatus for controlling a compressor and method of cooling a hydrocarbon stream

Description:
CROSS REFERENCE TO EARLIER APPLICATIONS 
     The present application is a national stage application of International application No. PCT/EP2009/058317, filed 2 Jul. 2009, which claims priority of EP08161338.2 filed in the European patent office on 29 Jul. 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for controlling a compressor. In another aspect, the invention relates to a method of cooling a hydrocarbon stream. 
     BACKGROUND OF THE INVENTION 
     Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of 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 small volume and does not need to be stored at high pressure. 
     Usually, natural gas comprises predominantly methane. In addition to methane, natural gas usually includes some heavier hydrocarbons such as ethane, propane, butanes, C 5 + hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most if not all known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process. 
     Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGL) from a natural gas stream, generally termed NGL recovery. The NGLs are usually fractionated to yield valuable hydrocarbon products, either as products steams per se or for use in liquefaction, for example as a component of a refrigerant. 
     NGL recovery generally involves an NGL separation column in which the natural gas stream is separated into a bottom stream containing the NGLs and a methane-enriched overhead stream, which is often compressed or recompressed (the natural gas stream may have been depressurized upstream of the NGL separation column) by one or more compressors. 
     Compressors for gaseous streams are used in many situations, systems and arrangements. Usually there is a vapour recycle or recirculation line around the compressor to avoid ‘surge’. Normally, surge is related to a flow to the compressor being too low, which can cause rapid pulsations in flow. 
     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, even with a surge control system damage can occur and the compressor can fail. 
     SUMMARY OF THE INVENTION 
     The present invention provides in a first aspect a method of controlling one or more first compressors at least comprising the steps of: 
     (a) providing a compressor feed stream; 
     (b) passing the compressor feed stream through the one or more first compressors, the or each first compressor having a first inlet and a first outlet to provide one or more first compressed streams; 
     (c) measuring at least one pressure and at least one flow of the group consisting of: the pressure of the compressor feed stream, the flow of the compressor feed stream, the pressure of the first compressed stream and the flow of the first compressed stream, to provide at least two measurement values;
 
(d) providing a first compressor recycle line including an in-line first recycle valve around the or each first compressor;
 
(e) passing the or each first compressed stream through at least one throttling valve downstream of the compressor recycle line to provide a controlled stream;
 
(f) selectively allowing a fraction of the or each compressor feed stream to bypass the or each first compressor and the at least one throttling valve ( 32 ) via a first bypass line; and
 
(g) automatically controlling at least one of the throttling valve(s) using the measurement values of step (c).
 
     In a second aspect, the invention provides a method of cooling an initial hydrocarbon stream, preferably containing natural gas, comprising at least the steps of: 
     (i) passing the initial hydrocarbon stream through a separator to provide a stabilized condensate stream and a mixed hydrocarbon stream; 
     (ii) separating the mixed hydrocarbon stream into a light overhead stream as a compressor feed stream, and a heavy bottom stream; and 
     (iii) passing the compressor feed stream through one or more first compressors and at least one throttling valve, and controlling the one or more first compressors using a method as defined in the method of the first aspect set of the invention as forth above, to provide one or more controlled streams;
 
(iv) passing the or each controlled stream through one or more second compressors to provide one or more second compressed streams; and
 
(v) cooling, preferably liquefying, at least a fraction of the one or more second compressed streams to provide a cooled, preferably liquefied, hydrocarbon stream.
 
     The invention further provides an apparatus for controlling one or more first compressors, the apparatus at least comprising: 
     one or more first compressors to compress a compressor feed stream between a first inlet and a first outlet in the or each first compressor to provide one or more first compressed streams; 
     at least two measurers able to measure at least one pressure and at least one flow of the group consisting of: the pressure of the compressor feed stream, the flow of the compressor feed stream, the pressure of the first compressed stream and the flow of the first compressed stream; to provide at least two measurement values;
 
a compressor recycle line including an in-line first recycle valve around the or each first compressor;
 
at least one throttling valve downstream of the compressor recycle line to received the or each first compressed stream to provide a controlled stream;
 
a first bypass line to allow a fraction of the compressor feed stream to bypass the or each first compressor and the at least one throttling valve; and
 
automatically controlling at least one of the throttling valves using the measurements values of step (c).
 
     This apparatus may form part of a natural gas liquefaction plant or facility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments and examples of the present invention will now be described by way of example only with reference to the accompanying non-limited drawings in which; 
         FIG. 1  is a diagrammatic scheme for a method of controlling a compressor according to one embodiment of the present invention; 
         FIG. 2  is a diagrammatic scheme for a method of controlling a compressor according to a second embodiment of the present invention; 
         FIG. 3  is a diagrammatic scheme of a method of cooling an initial hydrocarbon stream including embodiments shown in  FIGS. 1 and 2 ; 
         FIG. 4  is an exemplary plot of head compression ratio versus capacity for a compressor, showing surge, speed and choke lines; and 
         FIG. 5  is a diagrammatic scheme for a method of controlling two parallel compressors according to a third embodiment of the present invention. 
     
    
    
     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, and a single reference will be assigned to a pressure/flow of a stream as well as to a measurer of that pressure/flow. 
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been found that automatically controlling a throttling valve provided downstream of a compressor using measurement values of at least one pressure of the group consisting of: pressure of the compressor feed stream and pressure of the first compressed stream; and one flow of the group consisting of: flow of the compressor feed stream and flow of the first compressed stream; allows to prevent choking from occurring. Other than surge, a compressor may also be damaged by ‘stonewall’ or choking. Thus, herewith failure and/or damage to the compressor is reduced. 
     Choking of a compressor occurs when there is overcapacity of flow at too low a pressure ratio, so that the compressor ‘chokes’ and is unable to compress the flow of the gas. This causes high vibration which may damage the compressor. 
     The problem of choking can be avoided by the method disclosed herein in which the throttling valve downstream of the compressor is automatically controlled to let down the pressure of the first compressed stream and automatically regulate the pressure of the first compressed stream relative to the pressure of the bypass line. In this way, moving into an operating condition where choking occurs can be avoided. 
     Compressor surge is a phenomenon which occurs in compressors at low volumetric flow rates, and hence limits the minimum capacity of a given compressor. In the operation of a compressor, as the system resistance is increased, the head or compression ratio generated by the compressor increases to overcome this resistance. As the system pressure increases, less flow can pass through the compressor, and this will continue up to the maximum head capacity of the compressor. Limits in the minimum flow form a surge line. Below the surge line the back pressure exceeds that which the compressor is capable of delivering, causing a momentary backflow condition. During backflow the system resistance decreases, causing the back pressure to drop, enabling the compressor to deliver increased flow. If the opposition to flow downstream of the compressor is unchanged, peak head delivery will again be approached and backflow observed, producing a cyclic condition known as surge. Considerable damage can be done to a compressor if it is operated beyond the surge point due to vibration, noise, axial shaft movement and overheating which can produce mechanical damage. 
     The problem of surge can be avoided by the method disclosed herein by automatically controlling the in-line first recycle valve to open and increase the quantity of first compressed stream which is returned to the compressor feed stream along the first compressor recycle line, when the surge line is approached. 
     The present embodiment provides a more efficient method of controlling a compressor based on automatically controlling a downstream throttling valve, which allows control and integration of the compressor in a line-up or system for processing a hydrocarbon stream, for example during start-up and build up of the flow and pressure of the compressor feed stream, or due to any upstream pressure drop. The automation of the control of the compressor enables the determination of the current operating point under which the compressor is operating relative to an acceptable operating window for the compressor by measuring compressor data. The automation of the controller thus allows the operation of the compressor to be altered to reduce the likelihood of compressor problems such as compressor surge and choke. 
     The controlling of the first compressor(s) using the automatic controlling of a downstream throttling valve as described herein, and the apparatus therefore, are of particular usefulness for starting-up of a first compressor. 
     Referring to the drawings,  FIGS. 1 and 2  show various embodiments of methods for controlling a first compressor  12  for compressing a compressor feed stream  10  as part of an NGL recovery system  1 .  FIG. 3  shows a simplified and first general scheme of a liquefied natural gas plant  2  for a method for cooling an initial hydrocarbon stream  100 , including the NGL recovery system  1  of  FIGS. 1 and 2 . 
     An initial hydrocarbon stream may be any suitable hydrocarbon stream such as, but not limited to, a hydrocarbon-containing gas stream able to be cooled. One example is a natural gas stream obtained from a natural gas or petroleum reservoir. As an alternative, the natural gas stream may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process. 
     Usually such an initial hydrocarbon stream is comprised substantially of methane. Preferably such an initial hydrocarbon stream comprises at least 50 mol % methane, more preferably at least 80 mol % methane. 
       FIG. 3  shows an initial hydrocarbon stream  100  containing natural gas, which is cooled by a first cooling stage  104  to provide a cooled and partly condensed initial hydrocarbon stream  110 . 
     The first cooling stage  104  may comprise one or more heat exchangers either in parallel, series or both, in a manner known in the art. The provision of cooling to the first stage cooling  104  is known to the person skilled in the art. The cooling of the initial hydrocarbon stream  100  may be part of a liquefaction process, such as a pre-cooling stage involving a propane refrigerant circuit (not shown), or a separate process. Cooling of the initial hydrocarbon stream  100  may involve reducing the temperature of the initial hydrocarbon stream  100  to below −0° C., for example, in the range −10° C. to −70° C. 
     The cooled initial hydrocarbon stream  110  can be passed into a separator such as a condensate stabilisation column  108 , usually operating at an above ambient pressure in a manner known in the art. The condensate stabilisation column  108  provides an overhead mixed hydrocarbon stream  8 , preferably having a temperature below −0° C., and a stabilized condensate stream  120 . The overhead stream  8  is an enriched-methane stream compared to the cooled initial hydrocarbon stream  110 . 
     The term “mixed hydrocarbon stream” as used herein relates to a stream comprising methane (C 1 ) and at least 5 mol % of one or more hydrocarbons selected from the group comprising: ethane (C 2 ), propane (C 3 ), butanes (C 4 ), and C 5 + hydrocarbons. Typically, the proportion of methane in the mixed hydrocarbon stream  8  is 30-50 mol %, with significant fractions of ethane and propane, such as 5-10 mol % each. 
     The terms “light” and “heavy” are defined relative to each other, and make reference to the overhead stream respectively the bottom stream from the one or more gas liquid separators  14 . The composition of the “light” and “heavy” hydrocarbon streams depends on the composition of the feed gas as well as on the design and operation conditions of the gas liquid separators. 
     The term “heavy hydrocarbon stream” relates to a stream comprising a relatively higher content of heavier hydrocarbons than the light overhead stream. For instance, the heavy hydrocarbon stream could be a C 2 + hydrocarbon stream, which predominantly comprises ethane (C 2 ) and heavier hydrocarbons. The relative amount of ethane is higher than the relative amount of ethane in the feed stream, but a C 2 + stream could still comprise some methane. Likewise, a C 3 + hydrocarbon stream, a C 4 + hydrocarbon stream or a C 5 + hydrocarbon stream is relatively rich in propane and heavier, butanes and heavier, or, respectively, pentanes and heavier. 
     In NGL recovery, it is desired to separate a methane enriched stream from a mixed hydrocarbon stream (for example, for use as a fuel, or to be liquefied in the LNG plant  2  and provided as additional LNG), and to recover at least a heavy stream, optionally one or more of a C 2  stream, a C 3  stream, a C 4  stream, and a C 5 + stream. 
     In  FIG. 3 , at least a fraction, usually all, of the mixed hydrocarbon stream  8  passes into the NGL recovery system  1 . The NGL recovery system  1  usually involves one or more gas/liquid separators such as distillation columns and/or scrub columns to separate the mixed hydrocarbon stream  8  into at least a light stream and one or more heavy streams at a relatively low pressure, for example in the range of 20 to 35 bar. An example of a suitable first gas/liquid separator  14  is a “demethanizer” designed to provide a methane-enriched overhead stream, and one or more liquid streams at or near the bottom enriched in C 2 + hydrocarbons. However, depending on composition of the mixed hydrocarbon feed stream and the desired specification of the light overhead stream, the first gas/liquid separator  14  may be a de-ethanizer, a de-propanizer, or a de-butanizer or a scrub column, instead of a de-methanizer. 
     As the mixed hydrocarbon stream  8  is usually provided from a high pressure initial hydrocarbon stream  100 , for example in the range of 40 to 70 bar, it may need to be expanded prior to the first gas/liquid separator  14 . Such expansion may also cause a reduction in the temperature. As shown in  FIGS. 2 and 3 , the mixed hydrocarbon stream  8  can pass through one or more expanders  52  to provide a reduced temperature and pressure mixed-phase (liquid and vapour) hydrocarbon stream  9 , which then enters the first gas/liquid separator  14  at a suitable height. 
     The first gas/liquid separator  14  is adapted to separate the liquid and vapour phases, so as to provide a light overhead stream (as the first compressor stream  10  subsequently used herein), and a heavy bottom stream  50 . The first gas/liquid separator  14  may include a reboiler and a first reboiler vapour return stream (not shown) in a manner known in the art. 
     The nature of the streams provided by the first gas/liquid separator  14  can be varied according to the size and type of separator, and its operating conditions and parameters, in a manner known in the art. For the arrangement shown in  FIGS. 1-3 , it is desired for the light overhead controlled stream  30  to be methane-enriched. The light overhead stream may still comprise a minor (&lt;10 mol %) amount of heavy hydrocarbons, but is preferably &gt;80 mol %, more preferably &gt;95 mol % methane. The heavy bottom stream  50  can be &gt;90 or &gt;95 mol % ethane and heavier hydrocarbons, and can be subsequently fractionated or otherwise used in a manner known in the art for an NGL stream. 
     The light overhead stream provides one possible source of a compressor feed stream  10 , can now be (re)compressed for subsequent use by at least one or more first compressors  12 . 
       FIG. 1  shows one embodiment of the method disclosed herein comprising: 
     (a) providing the compressor feed stream  10 ; 
     (b) passing the compressor feed stream  10  through the first compressor  12  having a first inlet  13  and a first outlet  16  to provide a first compressed stream  20 ; 
     (c) measuring at least one pressure and at least one flow of the group comprising: the pressure P 1  of the compressor feed stream  10 , the flow F 1  of the compressor feed stream  10 , the pressure P 2  of the first compressed stream  20  and the flow F 2  of the first compressed stream  20 ; to provide at least two measurement values;
 
(d) providing a compressor recycle line  22  including an in-line first recycle valve  24  around the first compressor  12 ;
 
(e) passing the first compressed stream  20  through at least one throttling valve  32  downstream of the compressor recycle line  22  to provide a controlled stream  30 ;
 
(f) providing a first bypass line  60  to selectively allow a fraction of the compressor feed stream  10  to bypass the first compressor and the throttling valve  32 ; and
 
(g) automatically controlling at least one of the throttling valves  32  using the measurements values of step (c).
 
     Choking of a compressor occurs when there is overcapacity of flow at too low a pressure ratio, so that the compressor ‘chokes’ and is unable to compress the flow of gas. This causes high vibration which may damage the compressor. The problem of avoiding choking as well as surge in a compressor is not mentioned in U.S. Pat. No. 4,464,720. 
     The selection and/or combination of the pressure and flow measurements taken from the compressor feed stream  10  and/or of the first compressed stream  20  in the presently disclosed embodiment, can be used to determine the operation of the first compressor  12  relative to its choke line. 
     The choke line of a compressor is known to the user of the compressor, and is usually a property of a compressor which is part of the compressor design parameters. The characteristic curves of a compressor, based on the comparisons of the head against the compressor inlet volume flow at different gas conditions (e.g. temperature and molecular weight), are parameters provided by the compressor manufacturer to the user, which provide the user with identification of the compressor&#39;s choke line. An exemplary plot of the characteristic curves of a compressor is provided in  FIG. 4 , showing surge and choke lines in addition to speed lines for 50-110% designed operation in 10% increments. 
     Thus, by determining the operation of the first compressor  12  relative to its choke line by measuring the measurement values of step (c), and controlling the throttling of the compressor in response to these measurements, the choking of the compressor can be avoided. 
     Returning to  FIG. 1 , the automatic control of the throttling valve  32  is based on non-user computation of the pressure and/or flow measurements described herein. Such control can be provided by the use of one or more automatic controllers known in the art, represented in  FIG. 1  as a controller “XC”, able to compute and compare the measurement values provided by step (c) in relation to one or more pre-determined values, and directly provide one or more control instructions to the throttling valve  32  so as to control the discharge pressure of the first compressor  12  depending on the nature and properties of the compressor feed stream  10 . 
     Preferably, the presently disclosed method also comprises automatically controlling the in-line first recycle valve  24  in the compressor recycle line  22  for the same reason, optionally through the same controller(s) such as the controller XC shown in  FIG. 1 . 
     The presently disclosed method and apparatus is not limited by the form of measuring the pressure and/or flow measurement values, or to their nature or number. For instance, measuring the flow of the compressor feed stream or the first compressed stream is not limited to a direct stream flow measurement, such that any parameter from which the relevant stream flow can be derived may be used as the flow measurement value. Consequently, the actual measurement may be of a parameter which indirectly measures flow, such as the pressure change across an orifice, nozzle or venturi, which can then be used to calculate the flow of the compressor feed stream or first compressed stream. Such direct and indirect methods of measuring flow are known in the art. The flow measurement value can be used to determine the operation of the compressor in relation to its choke line. 
     A pressure value can be taken using any suitable pressure measurer such as P 1  and P 2  shown in  FIG. 1 , and a stream flow measurement can be provided by any suitable flow measurer such as F 1  and F 2  shown in  FIG. 1 . Although two flow measurers F 1 , F 2  and two pressure measurers P 1 , P 2  are shown in  FIG. 1 , the method disclosed herein can operate with a single flow measurer and a single pressure measurer. The additional flow and pressure measurers shown provide alternative possible locations for these devices, although the presence of more than one flow meter or pressure meter is included within the scope of this embodiment. The pressure and flow measurers P 1 , F 1 , P 2 , F 2  and the controller XC are not shown in  FIGS. 2 and 3  for clarity purposes only. 
     Preferably, step (c) of the presently disclosed method comprises measuring at least one of the group comprising: 
     (i) the pressure P 1  and the flow F 1  of the compressor feed stream  10 ; 
     (ii) the pressure P 1  of the compressor feed stream  10  and the flow F 2  of the first compressed stream  20 ; 
     (iii) the flow F 1  of the compressor feed stream  10  and the pressure P 2  of the first compressed stream  20 ; and 
     (iv) the pressure P 2  and the flow F 2  of the first compressed stream  20 . 
     A comparison of any of the above two values can provide to a computator a calculation of the operation of the first compressor  12  in relation to its choke line in a manner known in the art. 
       FIG. 1  shows the four measurement values P 1 , F 1 , P 2  and F 2  being passed along dashed signal paths to the controller XC, which computes the measurement values to calculate operation of the first compressor relative to its known choke line, and sends control signals to the throttling valve  32  and optionally the in-line first recycle valve  24  to control their operation, and hence the flows of the first recycle stream  22  and the first compressed continuing stream  25  (discussed below) to avoid choking of the first compressor  12 . 
     A method of controlling the first compressor  12  for any compressor feed stream, especially for one or more hydrocarbons such as an ethane-containing stream, is disclosed herein. 
     The first compressor  12  has a first inlet  13  and first outlet  16  and is able to compress at least a fraction of the compressor feed stream  10  to provide a first compressed light stream  20  in a manner known in the art. 
     Between the first outlet  16  and first inlet  13  of the first compressor  12 , there is the first compressor recycle line  22  which is able to take at least a fraction of the first compressed stream  20  and recycle it back into the path of the compressor feed stream  10 . The first compressor recycle line  22  is added to compressor feed stream  10 . The division of the first compressed stream  20  between a first compressed continuing stream  25  and a first compressor recycle stream  22  may be carried out by any suitable divider or stream splitter known in the art. 
     The division of the first compressed stream  20  may be anywhere between 0-100% for each of the continuing stream  25  and first recycle stream  22  as discussed further hereinafter. 
     The first compressor recycle line  22  is a dedicated line around the first compressor  12 . The first compressor recycle line  22  is preferably uncooled, and thus preferably does not contain a cooler. More preferably the first compressor recycle line  22  only includes one or more control valves  24 , required to change the pressure of the first compressor recycle stream  22  to approximate or equate its pressure to the intended pressure of the compressor feed stream  10  for the suction side of the first compressor  12 . 
     Optionally, the first compressed line  20  providing the first compressed stream  20 , includes one or more coolers, such as one or more water and/or air coolers, to reduce the temperature of at least the compressor recycle stream  22  prior to its re-introduction into the inlet  13  of the first compressor  12 . 
     The first compressed continuing stream  25  then passes through the throttle control valve  32  to provide the controlled stream  30 .  FIGS. 2 and 3  show the option of passing the controlled stream  30  into the one or more second compressors  42 , each second compressor  42  having a second inlet  43  for the controlled stream  30  and a second outlet  44 , to provide a second compressed stream  40  in a manner known in the art. The or each second compressor  42  may be the same or similar to a ‘boost’ compressor, generally having a dedicated driver or drive mechanism separate from the first compressor  12 . 
     Around the or each second compressor  42 , more particularly between the second outlet  44  and second inlet  43  can be a second compressor recycle line  45 , such that the one or more second compressed streams  40  can be divided by a divider or stream splitter known in the art, anywhere between 0-100%, between a final compressed stream  70  and a second compressor recycle stream  45 . The final compressed stream  70  can contain a one-way valve  41 . The second compressor recycle stream  45  includes one or more coolers  46 , such as in-line coolers, preferably one or more water and/or air coolers, known in the art and adapted to reduce the temperature of the second compressor recycle stream  45 . The one or more air coolers  46  are followed by one or more control valves  47  to provide a final recycle stream  48  for re-injection into the main compressor stream in advance of the second inlet  43  of the second compressor  42 . 
     The second compressor recycle line  45  provides anti-surge control around the second compressor  42  in a manner known in the art. The second compressor recycle line  45  is a dedicated line around the second compressor  42 . In particular, it is noted that the one or more coolers  46  are only required to cool the percentage of the second compressed stream  40  which is passed into the second compressor recycle line  42 , which percentage is commonly zero or minimal, thus minimising the OPEX of the one or more coolers  46 . 
       FIGS. 2 and 3  show a simplified arrangement of the recompression of a compressor feed stream  10  using a first compressor  12  which has a dedicated first compressor recycle line  22 , (that may not require dedicated or external cooling), and a second compressor  22  with a dedicated second compressor recycle line  45 . Thus, the first and second compressor recycle lines  22 ,  45  are independent, and can be independently controlled. 
       FIG. 1  also shows a first bypass line  60  with a one-way valve  62  around the first compressor  12  so as to be able to take a fraction of the compressor feed stream  10  around the or each first compressor  12  to provide controlled stream  30  which supplies the feed for the or each second compressor  42 . The first bypass line  60  may be used during start-up of the NGL recovery system  1 , especially where there is no driving power for first compressor  12 , (for example where it is mechanically linked to and therefore driven by the expander  52 ). The first bypass line  60  may also be useful where one or more of the first compressors  12  ‘trips’ as further discussed hereinafter. 
     Similarly,  FIG. 2  shows an expander bypass line  80  around the expander  52  having a control valve  82 . In this way, at least a fraction, optionally none or all, of a mixed hydrocarbon stream  8  can be selectively allowed to pass through the expander bypass line  80  to bypass the or each expander  52  and be fed into the mixed-phase hydrocarbon stream in line  9 . This arrangement may occur during start-up of the NGL recovery system  1 , and/or during tripping of one or more of the expanders  52  as further discussed hereinafter. 
     As shown in  FIG. 3 , the final compressed stream  70  may be wholly or partly used as fuel gas  72 , or passed to gas network, or subsequently cooled, preferably liquefied, to provide a cooled hydrocarbon stream such as LNG. The cooling and preferred liquefaction may be carried out by passage along line  71  in the second cooling stage  112 , typically comprising one or more heat exchangers, to provide a liquefied hydrocarbon stream  130 . Suitable liquefaction processes for such second cooling stages are known to the person skilled in the art and will not be further described here. 
       FIG. 3  also shows an embodiment, wherein the expander  52  prior to the first gas/liquid separator  14  is mechanically-linked to the first compressor  12 . Such mechanical-linking may occur by any known linkage, one example of which is a shared or common driveshaft  21 . The mechanical linking of an expander and a compressor, in order to use some of the work energy provided from the expander by the expansion of a gas therethrough, to partly or fully drive a mechanically linked compressor, is known in the art. 
     In this way, operation and performance of the first compressor  12  can be related to the operation and performance of the expander  52  as discussed further hereinafter. 
     The method disclosed herein is particularly advantageous during the start-up of the first compressor  12 . A first by-pass line  60  can be provided around the first compressor  12  to allow a fraction of the compressor feed stream  10  to bypass the first compressor  12  and the throttle valve  32 . The pressure in lines  25  and  30  can thus be regulated. 
     In this way, especially during start-up of a hydrocarbon processing process or treatment, nearly all of the compressor feed stream  10 , such as provided by a first gas/liquid separator  14 , can pass through the first bypass line  60  so as to provide a flow downstream thereof, whilst the flow and/or pressure of the compressor feed stream  10  is increasing. The throttling valve  32  provides automatic control for the integration of the first compressor  12  with a line downstream, by controlling the pressure differential between the first bypass stream  60  and the increasing provision of the controlled stream  30  (based on the increasing fraction of the compressor feed stream  10  being passed into the first compressor  12  and through a one-way valve  31  thereafter). Operation of the throttling valve  32  allows integration of the compressor  12  to proceed in line with diminution of the first bypass stream  60 , without affecting the pressure of the compressor feed stream  10  provided from a separator (such as the first gas/liquid separator  14  shown in  FIG. 1 ). 
     It is a particular advantage of the method and apparatus disclosed herein that the controller XC can provide automatic control of the throttle valve  32  and/or the in-line recycle valve  24  during start-up of the compressor  12  and use of the first bypass line  60 . 
     Thus, the presently disclosed method extends to a method of controlling the start-up of a first compressor  12  using a method of controlling the first compressor  12  as defined herein. 
     It is another particular advantage of the method and apparatus disclosed herein to provide control of the first compressor  12  as a consequence of any upstream pressure drop that affects the pressure of the compressor feed stream  10 , including any sudden or dramatic drop in pressure of the source of the compressor feed stream  10 , or a fraction thereof. 
     An example of this is the ‘tripping’ of an associated or related process, apparatus, unit or device such as a mechanically interlinked expander-compressor string as described hereinafter. In particular, in a multi-stream NGL recovery system, an example of which is shown in  FIG. 5 , the tripping of one expander-first compressor string requires a usually rapid adjustment of the flow of various streams, including the compressor feed stream  10 , through the NGL recovery system so as to maintain continuation of the process whilst the tripped string is re-integrated. Automatic control of a throttling valve  32  allows reintegration of a tripped string back into the main process by controlling the pressure downstream of the or each first compressor whilst full re-pressurisation of one or more compressor feed streams is ongoing. 
       FIG. 5  shows a simplified and second NGL recovery system  3  based on having a first expander and first compressor string A, and a second expander and first compressor string B. 
     In  FIG. 5 , a mixed hydrocarbon stream  8 , such as that provided as shown in  FIG. 3 , is divided by a stream splitter  11  into at least two, preferably two or three, part-feed streams  8   a  and  8   b , which pass into respective expanders  52   a  and  52   b  which are mechanically linked by respective common driveshafts  21   a  and  21   b  to respective first compressors  12   a  and  12   b . The division of the mixed hydrocarbon stream  8  into the part-feed streams  8   a  and  8   b  may be any ratio or percentage, but will generally be equal during normal and conventional operation of the second NGL recovery system  3  wherein the expanders  52   a  and  52   b  have the same capacity. Variations in the size, type, capacity, number and their balance of the expanders  52   a ,  52   b , and in consequence in the size, capacity, type, number and balance of the first compressors  12   a ,  12   b , are known to the skilled man in the art with knowledge of NGL recovery processes, operations and parameters. 
     Each expander  52   a ,  52   b  provides a mixed-phase hydrocarbon stream  9   a ,  9   b  respectively, which can be combined by a suitable combiner such as a T-piece, to provide a single mixed-phase hydrocarbon stream  9  to pass into the first gas/liquid separator  14  as hereinabove described. Optionally, one or more of the mixed-phase hydrocarbon streams  9   a  and  9   b  may pass directly into the first gas/liquid separator  14  without combination with the or all of the other mixed-phase hydrocarbon streams. 
     The first gas/liquid separator  14  provides a light overhead stream, and a heavy bottom stream  50  as hereinbefore described. The light overhead stream can provide the compressor feed stream  10 , which be divided by a stream splitter  36  in a manner known in the art to provide at least two, preferably two or three, part-compressor feed streams  10   a ,  10   b  which pass respectively into the two first compressors  12   a ,  12   b  through their first inlets to provide two respective first compressed streams  20   a ,  20   b.  0-100% of the first compressed streams  20   a ,  20   b  may pass into two respective first compressor recycle lines  22   a ,  22   b  for recycle through respective control valves  24   a ,  24   b  and return to the suction sides of the two first compressors  12   a ,  12   b  as described hereinabove. 
     That fraction of each of the first compressed streams  20   a  and  20   b  not passing into the first compressor recycle lines  22   a ,  22   b  provide first compressed continuing streams  25   a ,  25   b  which can pass through respective one-way valves  31   a ,  31   b  and throttle control valves  32   a ,  32   b  to provide controlled streams  30   a ,  30   b  before being combined by a combiner  53  to provide a combined second compressor feed stream  34  which passes to a second compressor  42  to provide a second compressed stream  40 . As described above, a fraction between 0-100% of the second compressed stream  40  can provide a second compressor recycle stream  45 , which can contain one or more control valves  47 , whilst a final compressed stream  70 , which can be passed through one-way valve  41 , can then be used as described above, for example as one or more of a fuel stream, export stream, or for cooling, preferably liquefying, to provide a liquefied hydrocarbon stream such as LNG. 
     The combination of the first expander  52   a , the mechanically linked first compressor  12   a , and their associated lines, provide the first string A, whilst the combination of the second expander  52   b , the mechanically linked first compressor  12   b , and their associated lines, provide the second string B. 
     In this way, the user of the second NGL recovery system  3  is able to have greater options and flexibility concerning the flow of the mixed hydrocarbon stream  8  through the second NGL recovery system  3 , in particular operations and flows through the expanders  52   a ,  52   b  and first compressors  12   a ,  12   b . As well as providing operational advantages during normal and/or conventional running of an NGL recovery system, this arrangement further provides two further advantages. 
     As discussed hereinbefore, should any string of a multi-string NGL recovery system not be able to run normally, either by accident or design, the continuance of the NGL recovery is possible through one or more of the other strings. In particular, where a string should ‘trip’, then the or each other string is able to continue operation of the NGL recovery, even if the volume and/or mass of the mixed hydrocarbon feed stream continues at the same level, or continues at a significant level. 
     The ‘tripping’ of an expander-compressor string can occur for a number of reasons, and/or in a number of situations. Common examples include ‘overspeed’, for instance where the driver produces more power than that required by the compressor and ‘vibration’ when the compressor is operating beyond the flow envelope and the flow angle with respect to the vane angle is incorrect. 
     A second particular advantage of the second NGL recovery system  3  shown in  FIG. 5  is during start-up of the NGL recovery. By providing two or more strings, each string can be separately started at a different time, and optionally with different starting parameters than each other strings. Thus, the user has greater options and control over the start-up of all the strings prior to full and normal operation of the overall NGL recovery system  3 . 
     As an example, at the start-up of an NGL recovery system, the mixed hydrocarbon feed stream  8  is usually passed through an expander bypass stream  80  to bypass the first expanders  52   a ,  52   b  to provide the mixed-phase hydrocarbon stream  9  because the pressure in the mixed hydrocarbon feed stream  8  may already be at a low level, such that expansion in first expanders  52   a ,  52   b  is unnecessary, or would result in too low a pressure in mixed-phase hydrocarbon stream  9 . This provides a higher pressure compressor feed stream  10  to first compressors  12   a ,  12   b  than would otherwise occur. 
     Similarly, the compressor feed stream  10  can pass through the first bypass line  60 , and one-way valve  62  to bypass the first compressors  12   a ,  12   b , especially where these are not provided with power or otherwise driven by the first expanders  52   a  and  52   b  which are being similarly by-passed. 
     It is a particular advantage of the method and apparatus disclosed herein that through pressure and flow control of each bypass stream and each part-stream, as the flow and/or pressure of the mixed-phase hydrocarbon stream  9  increases during start-up, one or more strings of a multi-string NGL recovery system can be separately started and brought up to normal operation as a controlled procedure. Thus, the two throttle control valves  32   a ,  32   b  in the paths of the first compressor continuing streams  25   a ,  25   b , allow control of the introduction of each compressor feed stream  10   a ,  10   b  into the first compressors  12   a ,  12   b  in calculation with reduction of the flow of the first bypass stream  60 . The two throttle valves  32   a ,  32   b  can control the pressure at the discharge of each of the first compressors  12   a ,  12   b , especially near stonewall of each first compressor  12   a ,  12   b , which most usually can occur during start-up and following any tripping of a string. 
     In this way, the pressure of the stream in the first bypass line  60  does not hinder the start-up of each of the first compressors  12   a ,  12   b , either together or independently. This arrangement seeks to ensure maximum forward flow through the or each first compressor, (and hence no overheating), without operating in the stonewall region. 
     It is a further advantage of a multi-string NGL recovery system that one or more of the first compressors  12   a ,  12   b  can be isolated from the or each other first compressors, so as to reduce interaction between the first compressors  12   a ,  12   b.    
     A person 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.