Patent Application
20090025422
This invention relates to the field of control systems for production of liquefied gas (LG), and more specifically, to a process and system which controls LG production and LG temperature. It has particular but not exclusive application to liquefying natural gas (NG) to produce liquefied natural gas (LNG).
Systems for the liquefaction of natural gas (NG) by refrigeration in heat exchange means, especially using a multicomponent refrigerant, are in use throughout the world. Control of the LNG production process is important to operate a plant efficiently, especially when attempting to meet demands for incremental production for downstream processing or when attempting to adjust to external process disturbances. Essentially simultaneous and independent control of both the LNG production flow rate and temperature is important for LNG plant operation. By fixing and maintaining the LNG production rate, plant operators can adequately plan and achieve desired production levels as required by the product shipping schedule. Maintaining the temperature of the LNG within a specified range is important for downstream processing and the prevention of downstream equipment problems. Once regulatory control is achieved for the key variables, optimization strategies can be properly implemented. However, if regulatory control is not adequate, even standard day to day operation is adversely affected.
In typical NG liquefaction processes, natural gas is fed to the warm end of heat exchange means, having a liquefying section in which the natural gas is liquefied and a subcooling section in which the liquefied natural gas is subcooled, and the LNG outlet stream is withdrawn from the cold end of the heat exchange means. Some refrigeration duty in the liquefying section is provided by flashing a first refrigerant (“MRL”), provided by cooling in the heat exchange means the liquid portion of a phase separation of a multicomponent refrigerant (MR) and refrigeration duty in the subcooling section is provided by flashing a second refrigerant (“MRV”), provided by condensing in the heat exchange means the vapor portion of the MR phase separation. The remainder of the refrigeration duty in the liquefying section is provided by spent MRV from the liquefaction section. The refrigerants exiting the warm end of the heat exchanger means are combined, if not already mixed in the liquefaction section, compressed and precooled before return to the MR phase separation for recycle to the heat exchange means. A process having the aforementioned features is referred to herein as “a typical NG liquefaction process”.
U.S. Pat. No. 5,791,160 (Mandler et at, corresponding to EP-A-0893665) describes a natural gas liquefaction control scheme where LNG product flow rate and temperature are simultaneously and independently controlled by adjusting the amount of refrigeration. In the exemplified embodiments, the control variables (the ones having a set point that can be changed by the operator) of a typical NG liquefaction process include LNG product flow rate and temperature as well as the MRL/MRV ratio. Manipulated variables (the ones that are automatically controlled in response to operator setting of one or more of the control variables) include MR compressor speed and MR/LNG ratio. In this scheme the amount of refrigeration is adjusted after the actual LNG product flow rate has been changed in response to a change in the LNG product flow rate set point.
U.S. Pat. No. 6,725,688 (Elion et al; corresponding to WO-A-01/81845) describes a modification of Mandler et a/with the object of maximizing power utilization. LNG product temperature and MRL/MRV ratio are retained as controlled variables and the manipulated variable is LNG/MRL ratio but LNG product flow rate cannot be independently set.
U.S. Patent Application Publication 2004/0255615 (Hupkes et al; corresponding to WO-A-2004/068049 & EP-A-1595101) describes the use of an advanced process controller based on model-predictive control to control a typical NG liquefaction process. The controller determines simultaneous control actions for a set of manipulated variables in order to optimize at least one of a set of parameters including the production of liquefied product whilst controlling at least one of a set of controlled variables. The set of manipulated variables includes MRL flow rate, MRV flow rate, MR composition, MR removal, MR compressor capacity and NG feed flow rate. The set of controlled variables includes the temperature difference at the warm end of the main heat exchanger, an adjustable relating to the LNG temperature, the composition of the refrigerant entering the MR phase separator, the pressure in the shell of the main heat exchanger, and the pressure and liquid level in MR phase separator.
There is a need to develop a simple and robust control scheme that allows control of LNG product temperature and flow rate without subjecting the heat exchange means to thermal stresses and without the need to manipulate the MR compressor and it is an object of the present invention to meet that need.
A control system for typical NG liquefaction processes has been devised in which the thermal stress on the heat exchange means is limited and the need to manipulate the MR compressor can be avoided by controlling the refrigeration so that variation to reduce any difference between actual and required LNG temperature is initiated before variation of the LNG product flow rate to reduce any difference between actual and required LNG flow rate. Accordingly, refrigeration leads LG production. The invention has particular, but not exclusive, application to a typical NG liquefaction process in which the controlled variables are LNG temperature, LNG flow rate and either heat exchanger warm end temperature difference (“WETD”) or heat exchanger mid-point temperature (“MPT”) and the manipulated variables are MRL and MRV flow rates. However, the invention is not restricted to the control of NG liquefaction processes but is more generally applicable to gas liquefaction, e.g. of hydrocarbon mixtures.
In one of its broadest aspects, the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
setting the predetermined flow rate value for the LG outlet stream and comparing said value with the actual LG flow rate;
setting the predetermined temperature value for the LG outlet stream and comparing said LNG temperature value with the actual LG temperature;
varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences, characterized in that the refrigeration is varied to reduce any LG temperature difference before variation of the LG flow rate to reduce any LG flow rate difference. Thus, this aspect allows the LG flow rate and temperature to be independently set and refrigeration to be correspondingly adjusted to meet the set requirements with limited thermal stress on the heat exchange means. However, the control system concept of the invention is applicable to LG liquefaction processes in which the LG flow rate and temperature requirements are constant but from time to time some variation is required to the actual values in order to compensate for a change in other parameters, such as NG feed temperature and composition, MR composition, ambient air temperature, cooling water temperature, atmospheric pressure etc., that has caused the actual value to deviate from the required value.
The invention also provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
means for setting the predetermined flow rate value for the LG outlet stream and comparing said value with the actual LG flow rate;
means for varying the actual LG product flow rate;
means for setting the predetermined temperature value for the LG outlet stream and comparing said LNG temperature value with the actual LG temperature; and
means for varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences,
characterized in that means for varying the actual LG product flow rate is not adjusted until the refrigeration has been adjusted to reduce any LG temperature difference.
The present invention relates to the control of liquefaction of gas, especially natural gas, in a manner that maintains the LG product at a required flow rate and temperature with limited thermal stress on the heat exchange means even when the LG flow rate and/or temperature requirements have been changed. The invention resides in the manner in which refrigeration is changed by manipulated variables.
In one broad aspect, the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
setting the predetermined flow rate value for the LG outlet stream and comparing said value with the actual LG flow rate;
setting the predetermined temperature value for the LG outlet stream and comparing said LG temperature value with the actual LG temperature; and
varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences,
characterized in that the refrigeration is varied to reduce any LG temperature difference before variation of the LG flow rate to reduce any LG flow rate difference.
In a corresponding apparatus aspect, the invention also provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
means for setting the predetermined flow rate value for the LG outlet stream and comparing said value with the actual LG flow rate;
means for varying the actual LG product flow rate;
means for setting the predetermined temperature value for the LG outlet stream and comparing said LNG temperature value with the actual LG temperature; and
means for varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences,
characterized in that the means for varying the actual LG product flow rate is not adjusted until the refrigeration has been adjusted to reduce any LG temperature difference.
In another broad aspect, the invention also provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
comparing said predetermined LG flow rate value with the actual LG flow rate;
comparing said predetermined LG temperature value with the actual LG temperature; and
varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences, characterized in that the refrigeration is varied to reduce any LG temperature difference before variation of the LNG flow rate to reduce any LG flow rate difference.
In a corresponding apparatus aspect, the invention also provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
means for comparing said predetermined LG flow rate value with the actual LG flow rate;
means for comparing said predetermined LG temperature value with the actual LG temperature; and
means for varying the actual LG product flow rate;
means for varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences,
characterized in that the means for varying the LNG flow rate is not adjusted until the refrigeration has been varied to reduce any LG temperature difference.
The invention has particular application to typical NG liquefaction processes and in a preferred embodiment provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
setting a predetermined value of (i) the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) or (ii) the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) and comparing same with the actual warm end temperature difference or actual mid-point temperature respectively;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates;
varying the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and
varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In a corresponding apparatus aspect, the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
means for setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
means for setting a predetermined value of (i) the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) or (ii) the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) and comparing same with the actual warm end temperature difference or actual mid-point temperature respectively;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates;
means for varying the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and
means for varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
Another preferred embodiment of the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
comparing said predetermined LNG flow rate value with the actual LNG flow rate;
comparing said predetermined LNG temperature value with the actual LNG temperature;
comparing a predetermined value of (i) the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) or (ii) the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with the actual warm end temperature difference or actual mid-point temperature respectively;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates;
varying the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and
varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In a corresponding apparatus embodiment, the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for comparing said predetermined LNG flow rate value with the actual LNG flow rate;
means for comparing said predetermined LNG temperature value with the actual LNG temperature;
means for comparing a predetermined value of (i) the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) or (ii) the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with the actual warm end temperature difference or actual mid-point temperature respectively;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates;
means for varying the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and
means for varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with an embodiment illustrated in
Thus, in accordance with the embodiment illustrated in
setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined warm end temperature differences; and
varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In a corresponding apparatus embodiment, the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
means for setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
means for setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined warm end temperature differences; and
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
Also in accordance with an embodiment illustrated in
comparing said predetermined LNG flow rate value with the actual LNG flow rate;
comparing said predetermined LNG temperature value with the actual LNG temperature;
comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined warm end temperature differences; and
varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In a corresponding apparatus embodiment, the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for comparing said predetermined LNG flow rate value with the actual LNG flow rate;
means for comparing said predetermined LNG temperature value with the actual LNG temperature;
means for comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined warm end temperature differences; and
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with an embodiment illustrated in
Thus, in accordance with an embodiment illustrated in
setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences, the actual LNG flow rate.
In a corresponding apparatus embodiment, the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
means for setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
means for setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences, the actual LNG flow rate.
Also in accordance with an embodiment illustrated in
comparing said predetermined LNG flow rate value with the actual LNG flow rate;
comparing said predetermined LNG temperature value with the actual LNG temperature;
comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio determined by the difference between the actual and predetermined LNG temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for comparing said predetermined LNG flow rate value with the actual LNG flow rate;
means for comparing said predetermined LNG temperature value with the actual LNG temperature;
means for comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio determined by the difference between the actual and predetermined LNG temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences, the actual LNG flow rate.
In accordance with an embodiment illustrated in
Thus, in accordance with an embodiment illustrated in
setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences multiplied by a value dependent on the actual MRL flow rate, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
means for setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
means for setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences multiplied by a value dependent on the actual MRL flow rate, the actual LNG flow rate.
Also in accordance with an embodiment illustrated in
comparing said predetermined LNG flow rate value with the actual LNG flow rate;
comparing said predetermined LNG temperature value with the actual LNG temperature;
comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences multiplied by a value dependent on the actual MRL flow rate, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for comparing said predetermined LNG flow rate value with the actual LNG flow rate;
means for comparing said predetermined LNG temperature value with the actual LNG temperature;
means for comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences multiplied by a value dependent on the actual MRL flow rate, the actual LNG flow rate.
In accordance with an embodiment illustrated in
Thus, in accordance with an embodiment illustrated in
setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
setting a predetermined value of the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) and comparing same with the actual mid-point temperature;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates and also, when the difference between actual and predetermined warm end temperature differences exceeds a threshold value, to said difference between warm end temperature differences, MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined mid-point temperatures; and
varying, by an amount corresponding to the difference between the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
means for setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
means for setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing same with the actual warm end temperature difference;
means for setting a predetermined value of the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) and comparing same with the actual mid-point temperature;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates and also, when the difference between actual and predetermined warm end temperature differences exceeds a threshold value, to said difference between warm end temperature differences, MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined mid-point temperatures; and
means for varying, by an amount corresponding to the difference between the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
Also in accordance with an embodiment illustrated in
comparing said predetermined LNG flow rate value with the actual LNG flow rate;
comparing said predetermined LNG temperature value with the actual LNG temperature;
comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
comparing a predetermined value of temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with the actual mid-point temperature;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates and also, when the difference between actual and predetermined warm end temperature differences exceeds a threshold value, to said difference between warm end temperature differences, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined mid-point temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for comparing said predetermined LNG flow rate value with the actual LNG flow rate;
means for comparing said predetermined LNG temperature value with the actual LNG temperature;
means for comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
means for comparing a predetermined value of temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with the actual mid-point temperature;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates and also, when the difference between actual and predetermined warm end temperature differences exceeds a threshold value, to said difference between warm end temperature differences, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined mid-point temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with an embodiment illustrated in
Thus, in accordance with an embodiment illustrated in
setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing said warm end temperature difference value with the actual warm end temperature difference;
comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and calculated mid-point temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for setting the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;
means for setting the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;
means for setting a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) and comparing said warm end temperature difference value with the actual warm end temperature difference;
means for comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and calculated mid-point temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
Also in accordance with an embodiment illustrated in
comparing said predetermined LNG flow rate value with the actual LNG flow rate;
comparing said predetermined LNG temperature value with the actual LNG temperature;
comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and calculated mid-point temperatures; and
varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
In accordance with a corresponding apparatus embodiment, the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
means for comparing said predetermined LNG flow rate value with the actual LNG flow rate;
means for comparing said predetermined LNG temperature value with the actual LNG temperature;
means for comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) with the actual warm end temperature difference;
means for comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;
means for varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and calculated mid-point temperatures; and
means for varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
Referring to
The flow of LNG product is controlled by a valve 120 and the flows of the refrigerant portions to the heat exchanger are controlled by valves 132 and 138 respectively.
The temperature of the LNG product is compared in temperature indicator controller (“TIC”) 114 against the required product temperature determined by an operator set point (SP). A signal proportionate to the difference in actual and required temperature is sent from the TIC 114 to a flow indicator controller (“FIC”) 116, which in turn adjusts the position of the product valve 120 to maintain the required temperature. At constant refrigeration, an increase in product flow will reduce the actual product temperature and a decrease in product flow will reduce the actual product temperature. The product flow rate is monitored by the FIC 116 and a signal proportionate to the actual value (“PV”) of the flow is sent from the FIC 116 to a FIC 122 for comparison with a set point value determined by the operator.
A signal proportionate to the difference between the actual and required product flow rates is sent to FIC 126, which compares the actual flow rate of the MRL with a required value set by that signal. The MRL control valve 132 is adjusted in response to differences between the actual and required flow rates in order to adjust the refrigeration in heat exchanger 112.
A signal proportionate to the difference between actual and required MRL flow rates is sent to a flow ratio indicator controller (“FRIC”) 140 where it is compared with a signal from flow indicator (“FI”) 136 measuring actual MRV flow rate in order to determine the actual MRV/MRL flow ratio. The actual MRV/MRL flow rate is compared with a set point value determined by a signal received from the temperature differential indicator controller (“TDIC”) 142. A signal proportionate to the difference between the actual and required MRV/MRL flow ratios adjusts flow valve 138 and the corresponding refrigeration provided to the heat exchanger 112.
The TDIC 142 compares the actual temperature difference between the spent refrigerant in line 144 and the MRL in line 124 with a set point value determined by the operator. The set point signal provided by the TDIC 142 to the FRIC 140 is proportionate to that difference in temperature.
The TDIC 142 could measure temperature difference between the spent refrigerant and either the MRV in line 134 or the natural gas feed in line 100 instead of the difference with the MRL as shown in
FI 136 could be located upstream instead of downstream of the heat exchanger 112. Similarly, the FIC 126 also could be located upstream instead of downstream of the heat exchanger 112.
It will be apparent that following operator change to the required LNG product flow rate, required LNG product temperature and/or warm end temperature difference (“WETD”), there will be resultant changes to valves 132 and 138 determined by the extent to which the flow rate, temperature and/or WETD have been changed. This will change the amount of refrigeration provided to the heat exchanger 112 and thereby change the difference between the actual and set LNG product temperature values. That change will adjust the valve 120 and hence the actual product flow rate. The change of the actual product flow rate will result in further adjustment valves 132 and 138 controlling the refrigeration supplied to the heat exchanger 112 and provide a corresponding change in the actual LNG product temperature.
Essentially simultaneously with actual change in product temperature, there will be a corresponding change in the WETD detected by TDIC 142 that will result in a corresponding change in the required MRV/MRL flow ratio of FRIC 140. Further, also essentially simultaneously with the change in actual product flow rate, there will be a corresponding change in product temperature that will cause, via the change in difference between actual and required product flow rates, change in refrigeration. Thus, the differences between actual and required product temperatures, actual and required LNG product flow rates and actual and required WETDs will automatically incrementally change in order to achieve the required combination of LNG product flow rate, LNG product temperature and WETD. Further, the control system will automatically change the refrigeration provided to the heat exchanger to maintain the set values if there is any change in LNG product flow rate, LNG product temperature or WETD arising from changes to any of those parameters not occasioned by changes to their required values, such as changes in NG composition, NG flow rate, partial condensation refrigeration duty for 104, ambient air temperature, cooling water temperature, or atmospheric pressure.
The control system of
The control system of
The control system of
The control system of
The control system of
It is a common feature of all of the exemplified embodiments that there is no change in LNG product flow rate except in response to changes in refrigeration duty for the heat exchanger 112.
Each of the embodiments of
The process of
The process of
The process of
The process of
Although the increased production disturbance was easily achieved by the exemplified embodiments of the invention, there still remained a question as to how the systems would respond to a truly unattainable production disturbance. Accordingly, the process of
The system tracked the LNG production to the new set point, however, the LNG temperature continued to rise driving the MRV valve 138 fully open. The LNG temperature finally settled out approximately 4° F. (2.2° C.) warmer than the desired LNG temperature. The mid point temperature between the middle and cold bundles of the heat exchanger 112 warmed up by close to 20° F. (11° C.). The increased MRV flow almost doubled the cold bundle pressure drop. The gas turbine reached full power and then bogged down reducing its speed by approximately 1%. The results showed that the control system had no checks to prevent it from reaching an unacceptable new operating point.
The process of
Other embodiments and benefits of the invention will be apparent to those skilled in the art from a consideration of the specification and from practice of the invention disclosed herein. It is intended that this specification be considered as exemplary only with modifications and variations being within the scope and spirit of the invention as defined by the following claim. In particular, any of the exemplified embodiments could be used for liquefaction of gases other than natural gas and the tube and shell heat exchanger 112 could be replaced by two or more individual heat exchanges arranged in series and/or by any of the heat exchanger types known in the art.
