Abstract:
A method and apparatus for process control in a distillation column are disclosed. The method allows undesirable interactions among control parameters to be minimized, and results in improved process control and operational stability. Process control of a liquid assisted nitrogen generator using the improved method is disclosed as an illustrative example.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a method and apparatus of process control, and more particularly, to process control of a distillation column.  
         BACKGROUND OF THE INVENTION  
         [0002]    In a typical distillation process, mixtures are distilled and separated into different components by contacting liquid and vapor phases of the mixture on liquid-vapor contact elements contained within a distillation column. The liquid-vapor contact elements can be trays, random packing or structured packing. In an air separation process using cryogenic distillation, a feed air stream is first compressed to a high pressure and pre-purified to reduce water, hydrocarbons and carbon dioxide to acceptable levels. The compressed, pre-purified air stream is then cooled to cryogenic temperature by heat exchange with product and waste streams exiting the distillation column. The cooled air stream is introduced to the bottom of the distillation column where rectification takes place. As vapor rises through liquid-vapor contact elements in the distillation column, it becomes increasingly more concentrated in nitrogen. On the other hand, the liquid phase becomes increasingly more concentrated in oxygen as it descends and accumulates as an oxygen-rich liquid at the bottom of the distillation column.  
           [0003]    The operation of an air separation unit requires the control of three main attributes mass balance, thermal balance and purity. Mass balance is usually controlled by controlling the column pressure, thermal balance is usually maintained by controlling the liquid level inside the column, while product purity is changed by varying the liquid to vapor ratio in the column. In conventional methods of process control, a control measure for one parameter often affects another control parameter, resulting in undesirable variations in overall process stability. Thus, there is an ongoing need for alternative methods of process control with improved process stability.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention provides generally a method and apparatus for process control in a distillation column. According to one aspect of the invention, the method involves the introduction of a feed gas stream to a first inlet of the column and a liquid stream flow into a second inlet located above the first inlet. A liquid indicating controller is used to monitor and control the liquid level inside the column, and a pressure indicating controller is used to monitor and control the column pressure. The purity of a gas product, which is withdrawn from a gas outlet from the top of the column, is controlled at a desired purity level by regulating the liquid stream flow into the column using a liquid flow controller.  
           [0005]    In one embodiment of the invention, a compressed feed air stream is introduced to a first inlet of the column and a liquid nitrogen stream flow is introduced into a second inlet located above the first inlet. A liquid sensor monitors the liquid level inside the column and provides a liquid level signal to a liquid indicating controller. Based on the liquid level signal, the liquid indicating controller controls a valve to regulate an oxygen-enriched liquid stream flow out of a bottom outlet of the column. The pressure in the column is monitored by a pressure sensor, which provides a pressure signal to a pressure indicating controller. Based on the pressure signal, a product gas flow in a product gas conduit and a vent gas flow in a vent gas conduit are regulated by the pressure indicating controller. The purity of a nitrogen-enriched gas product, which is withdrawn from a gas outlet at the top of the column, is controlled at a desired purity level by regulating the liquid nitrogen stream flow into the column using a flow indicating controller in conjunction with an analyzer indicating controller, in which the analyzer indicating controller sends a purity-indicating signal to the flow indicating controller, and the liquid nitrogen stream flow is regulated by the flow indicating controller based on the purity-indicating signal.  
           [0006]    According to another aspect of the invention, an apparatus comprises a distillation column having a feed gas inlet, a liquid inlet located above the feed gas inlet, a liquid outlet at the bottom of the column and a gas outlet at a top of the column. The apparatus further contains a liquid level indicating controller for monitoring and controlling a liquid level inside the distillation column, a pressure indicating controller for monitoring and controlling the column pressure, a liquid source connected to the liquid inlet, a flow indicating controller for monitoring and controlling a liquid flow from the liquid source to the liquid inlet, and an analyzer indicating controller connected at one end to a gas analyzer for monitoring a purity level of a gas product withdrawn from the gas outlet; and the analyzer indicating controller is further connected at another end to the flow indicating controller for adjusting a liquid flow set point for the flow indicating controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    While the specification concludes with claims distinctly pointing out the subject matter that the applicants regard as their invention, it is believed the invention would be better understood when taken in connection with the accompanying drawings in which:  
         [0008]    [0008]FIG. 1 is a schematic diagram illustrating a method of process control according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]    The present invention relates to a method and apparatus of process control for use in a distillation column. More particularly, the method is illustrated in an example of process control in a nitrogen generating plant or nitrogen generator.  
         [0010]    [0010]FIG. 1 is a schematic diagram showing a method of process control according to the present invention as applied to a cryogenic air separation plant for nitrogen generation. For clarity&#39;s sake, many components such as heat exchangers, valves, expanders and so forth that may typically be found in a nitrogen generator have been omitted. The nitrogen generator comprises a distillation column  100  having a gas inlet  102 , a liquid inlet  104 , a liquid outlet  106 , and a gas outlet  108 . A feed air stream in conduit  112  has been pre-purified, compressed and cooled to a cryogenic temperature suitable for rectification. Such pre-purification, compression and cooling are performed using techniques that are known in the art. The feed air stream, which is introduced at a constant flow rate to a bottom portion of the distillation column  100  via the gas inlet  102 , is subjected to rectification and separates into two components—a nitrogen-enriched vapor and an oxygen-enriched liquid.  
         [0011]    The nitrogen generating plant of FIG. 1 uses a liquid injection scheme in which a liquid nitrogen stream from a liquid nitrogen (LN) source  110  is introduced to a top portion of the distillation column  100  via a conduit  114  and the liquid inlet  104 . The addition of the liquid nitrogen stream provides sufficient refrigeration such that the distillation process can be run without the use of an expander that would otherwise be required.  
         [0012]    A gas product stream (nitrogen-enriched vapor) is withdrawn via the gas outlet  108  at the top of the column  100 . The oxygen-rich liquid at the bottom of the column  100  is withdrawn from the outlet  106  as a liquid stream which passes along a conduit  116  into a condenser  120 . The oxygen-rich liquid stream exchanges heat with a portion of the gas product stream inside the condenser  120 , resulting in vaporization of the oxygen-rich liquid and condensation of the portion of the gas product stream. The vaporized oxygen-rich stream may then be used to cool incoming feed air stream in a heat exchanger (not shown) before exiting the nitrogen generator.  
         [0013]    During operation of the nitrogen generator, several parameters are monitored and controlled in order to maintain desired operating conditions and process performance. These parameters include the liquid level and pressure inside the column  100 , as well as the product gas purity. Process control of the distillation column  100  is accomplished by the use of four controllers—a liquid indicating controller (LIC)  130 , a pressure indicating controller (PIC)  140 , a flow indicating controller (FIC)  150 , and an analyzer indicating controller (AIC)  160 .  
         [0014]    Using a conventional process control scheme, these controllers are connected to the nitrogen generator in a different manner compared to that shown in FIG. 1. Due to the interactions or coupling among different control measures as implemented by the conventional scheme, the control of one process parameter often directly affects another process parameter. For example, an attempt to control product purity may lead to variations in the column pressure, which in turn affects the liquid level control during distillation. Such parameter interactions in the control scheme often result in undesirable instabilities in the operation of the distillation column.  
         [0015]    The method of process control shown in FIG. 1 seeks to minimize the interaction of these control parameters. Liquid level, pressure and product purity are controlled in a way that they are substantially decoupled from each other, so that an attempt at controlling one parameter will not result in undesirable effects on another parameter.  
         [0016]    For example, the LIC  130  in FIG. 1 is connected to a liquid level sensor  134  for monitoring the liquid level at an appropriate location in the column  100 . The LIC  130  is further connected to a valve  142  located in the conduit  116  leading from the outlet  106 . Based on a signal received from the sensor  134  relating to the liquid level, the LIC  130  sends an output signal to the valve  142  in order to maintain the liquid level at a desired operating level by appropriate control of the flow of liquid stream out of the bottom of the column  100 . Thus, depending on whether the liquid level falls below or exceeds a desired level, the valve  142  will be adjusted to reduce or increase the liquid stream withdrawn from the outlet  106 . Although such liquid level control may still affect the purity of the gas product (i.e., by changing the liquid to vapor flow ratio), the purity will be compensated by the liquid reflux flow that is controlled by the AIC  160  and the FIC  150 , which will be discussed below. Under this control scheme, the interaction between product purity and other parameters is reduced compared to the conventional control scheme.  
         [0017]    The PIC  140  is connected to the pressure sensor  144  to receive a signal relating to the pressure in the column  100 . Although the pressure sensor  144  is often located at the top or overhead region of the column  100 , it can also be positioned at other locations appropriate for pressure measurements. Based on the pressure information, the PIC  140  controls valves  152  and  154  in the gas product conduit  118  and the vent conduit  124  accordingly. For example, if the column pressure is too high compared to a desired operating level, then valve  152  and/or valve  154  is opened to increase the flow of gas product out of the column  100 . Depending on the customer&#39;s demand, the flow valve  152  may be set to produce a gas flow rate up to the capacity of the nitrogen generator, while the vent valve  154  is adjusted to allow a portion of the gas product to be vented via the conduit  124  in order to achieve the desired operating column pressure. Pressure control according to this method can be more rapidly achieved than previously attainable.  
         [0018]    Since a distillation column is designed to operate at a certain pressure, the process control method of this invention seeks to directly control the column pressure, e.g., using the PIC  140  as shown in FIG. 1. With the column pressure fixed at a desired level, other parameters can be adjusted and controlled relatively independently of each other. This represents an improvement over the conventional control scheme, in which column pressure is only indirectly controlled, and the effect of one control factor or parameter tends to propagate to another control factor in the cycle.  
         [0019]    Different settings of the valves  152  and  154  may be used in conjunction with the PIC  140  for controlling the column pressure. In one configuration, for example, the valve  152  is set at a fully open position, allowing a maximum product flow rate to the customer site. If customer&#39;s demand for the product gas exceeds plant capacity, the pressure in the column  100  may decrease below a desired operating pressure set point. The PIC  140  then closes the vent valve  154  in order to maintain the distillation column  100  within the desired operating pressure range. If this proves insufficient, then valve  152  may be adjusted to help control the column pressure. On the other hand, if customer&#39;s demand for the product gas is less than plant capacity, the column pressure may increase above a desired operating pressure set point. The PIC  140  then sends a signal to open the vent valve  154  in order to maintain the distillation column  100  at the desired operating pressure range.  
         [0020]    In another configuration, the valve  154  may be set initially at a partially open position. Depending on the customer&#39;s demand, the PIC  140  sends appropriate signals to adjust one or both of the valves  152  and  154  in order to maintain the column pressure within a desired operating range. In general, different combinations of settings may be used for valves  152  and  154  to provide the necessary product flow and pressure control in the column  100 .  
         [0021]    Under the process control scheme of FIG. 1, the gas product purity is controlled solely by regulating the liquid nitrogen flow from the liquid nitrogen source  110  into the distillation column  100 . The FIC  150  and the AIC  160  are used in conjunction with each other in a cascade fashion, in which the AIC  160  is the primary controller and the FIC  150  is a secondary controller. In this case, the AIC  160  sets a liquid flow set point for the FIC  150  according to the purity of the gas product from the distillation column  100 . As shown in FIG. 1, the AIC  160  is connected to the gas analyzer  162  that is used to monitor the product gas purity by sampling the product gas stream in the product gas conduit  118 . It is understood that the gas sampling point for the gas analyzer  162  may be located at various positions that are convenient or appropriate for product gas purity measurement.  
         [0022]    Based on the product gas purity, the AIC  160  sends an output signal to the FIC  150  to adjust a set point for the liquid nitrogen flowing through the conduit  114  into the column  100 . The FIC  150  is connected at one end to a liquid flow sensor  136  and at the other end to a liquid flow valve  132 . Both the liquid flow sensor  136  and the liquid flow valve  132  are installed along the conduit  114 . Based on the liquid flow set point (as determined by the AIC  160 ) and the liquid flow measurement by the sensor  136 , the FIC  150  in turn sends an output signal to the valve  132  for regulating the liquid flow accordingly.  
         [0023]    For example, if the gas product purity falls below a desired purity level (i.e., too much impurities in the product), then the liquid flow set point will be adjusted to provide an increased liquid flow into the column  100 . The increased liquid flow will result in an increase of the liquid to vapor flow ratio inside the column  100 , thus increasing the purity of the gas product. Using this control scheme, rapid liquid flow control can be readily accomplished, which in turn results in a fast response for gas purity control. In an alternative embodiment, the flow sensor  136  may be omitted, and the FIC  150  is configured to send a signal to adjust valve  132  according to a liquid flow set point determined by the AIC  160 .  
         [0024]    The process control method of the present invention provides several advantages over the conventional method. For example, by decoupling the control of liquid level, pressure and product purity from each other, a more stable operation of the distillation process may be achieved. The relatively fast response for gas purity control provided by the regulation of liquid stream flow into the column provides a method that is more responsive to customer&#39;s demands. Furthermore, since the product flow control as implemented by the pressure indicating controller is decoupled from the product gas purity, product gas flow to the customer can be supplied at any flow rate up to the plant capacity at desired purity, without being restricted unnecessarily by conditions dictated by gas purity, as may occur in conventional control schemes.  
         [0025]    While the present invention has been described with reference to several embodiments, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention.