Abstract:
Methods for controlling the operation of fractionation columns to avoid column flooding are described. The methods use mass flow meters to measure the mass flow rates of the receiver vapor, and the stripper hydrocarbon liquid or stripper reflux and stripper net overhead. The water from the receiver can be measured with either a volumetric flow meter or a mass flow meter. A computer can be used to determine the dew point from the mass flows, and an alarm can be triggered and/or a process change can be made if the difference between the calculated dew point and the temperature of the overhead vapor stream is less than a predetermined amount.

Description:
STATEMENT OF RELATED CASES 
       [0001]    This application is related to application Ser. No. ______, (Attorney Docket H0035483-8284) filed on even date, entitled APPARATUS FOR MEASUREMENT AND CALCULATION OF DEW POINT FOR FRACTIONATION COLUMN OVERHEADS, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to generally to fractionation columns and more particularly to apparatus and methods for controlling the operation of fractionation columns to avoid column flooding. 
       BACKGROUND OF THE INVENTION 
       [0003]    Many different applications in the hydrocarbon refining and petrochemical industries employ the use of steam strippers to remove lower boiling compounds from liquid streams containing various boiling range compounds. The introduction of steam into a steam stripped fractionation column is beneficial for the separation of different boiling compounds. However, if too much steam is added for the amount of heat available in the column, steam will condense on the stripper trays where water builds up and eventually floods the stripper, causing major operational upsets. The presence of liquid water also leads to increased corrosion of the trays and walls of the stripper column. 
         [0004]    U.S. Pat. No. 6,640,161, which is incorporated herein by reference, describes a computer method for calculating the dew point and providing a warning of operating conditions which may lead to flooding of the column. The total moles of hydrocarbon passing overhead in the steam stripped fractionation column and the total moles of water as steam passing overhead in the steam stripped fractionation column are measured. Using that information, the mole fraction of water as steam passing overhead in the column is continuously calculated. The overhead pressure of the column is measured, and a continuous determination of the partial pressure of water is made by calculating the product of the mole fraction of water as steam passing overhead in the column and the column overhead pressure. In addition, a continuous determination of the dew point temperature of the steam passing overhead in the column is made. The top temperature of the column is measured and provided to the computer wherein the difference between the calculated dew point temperature of the steam passing overhead in the column and the measured top temperature is calculated. As this calculated difference approaches zero, the potential for flooding the column increases. A predetermined value is selected and compared with the calculated difference in order to generate an alarm to alert the operator of unsatisfactory column operation. Once an alarm is detected, the operator may then make the appropriate adjustments to the column in order to avoid flooding the column. 
         [0005]    However, U.S. Pat. No. 6,640,161 does not describe the instrumentation and connections among the instruments needed to make the needed measurements. Without the proper instrumentation, the calculation method will not report useful information and flooding conditions can occur. This leads to expensive repairs and lost production. 
         [0006]    Therefore, there is a need for instrumentation for, and methods of, calculating water dew point in a steam stripped fractionation column. 
       SUMMARY OF THE INVENTION 
       [0007]    One aspect of the invention involves a method for controlling operation of a fractionation column. In one embodiment, the method includes measuring the molecular weight or specific gravity, the temperature, and the pressure of an overhead vapor stream from the fractionation column to a receiver; measuring the temperature of a hydrocarbon liquid stream from the receiver; measuring the mass flow rate of a stripper reflux liquid stream, or a reflux hydrocarbon liquid stream and a stripper net overhead hydrocarbon liquid stream; measuring the mass flow rate of a stripper vapor stream from the receiver; and measuring the flow rate of a water stream from the receiver. The total overhead flow is determined using the flow rate of the water stream from the receiver, the mass flow rate of the stripper vapor stream from the receiver, the mass flow rate of the stripper hydrocarbon liquid stream, or the mass flow rate of the stripper net overhead hydrocarbon liquid and the mass flow rate of the reflux hydrocarbon liquid stream. The total overhead moles are determined from the total overhead flow and the molecular weight of the overhead vapor stream. The total moles of water are determined from the water flow from the receiver and the measured temperature of the hydrocarbon liquid stream from the receiver. The partial pressure of water in the overhead vapor stream is determined from the total moles of water, the total overhead moles, and the measured overhead pressure. The dew point temperature is determined at the determined partial pressure of water. The dew point margin is determined from the determined dew point and the temperature of the overhead vapor stream. The calculated dew point margin is compared with a predetermined minimum dew point margin, and an alarm is initiated and/or an operating condition of the fractionation column is changed when the calculated dew point margin is less than the predetermined minimum dew point margin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]      FIG. 1  illustrates one embodiment of the control instrumentation for the steam stripped fractionation column. 
           [0009]      FIG. 2  illustrates another embodiment of the control instrumentation for the steam stripped fractionation column. 
           [0010]      FIG. 3  illustrates the steps of one embodiment of the control method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The present invention helps to prevent undesirable condensation of steam by providing the operator with an alarm that warns of conditions that approach the water dew point so that the appropriate adjustments can be made before the stripper column is upset. It identifies the instrumentation needed and the appropriate calculations to determine the dew point and dew point margin in real time, allowing for proper control and operation of the column, which helps to minimize energy consumption. 
         [0012]    The approach provides simple calculations which are easily configured within common control systems for on-line water dew point margin indication in real time. The overhead flow from the column is determined by measuring receiver vapor, receiver water boot, reflux hydrocarbon liquid, and net overhead hydrocarbon liquid. The receiver vapor, reflux hydrocarbon liquid, and net overhead hydrocarbon liquid are measured using mass flow meters, such as coriolis flow meters. The mass flow meters provide information to the calculation method that is not impacted by differences or changes in specific gravity. Mass flow meters are only used where they are most needed to limit the cost of the flow meters. For example, the water boot mass flow can be found by correcting the volume flow by the actual operating temperature (although a mass flow meter can be used if desired). The molar flow in the overhead is determined by converting the mass flow to molar flow from the molecular weight analyzer or specific gravity (SG) analyzer in the overhead vapor line. The water content of the overhead system is calculated assuming the water content of the overhead is all in the water leaving the receiver water boot, thus directly determining the column overhead water dew point. 
         [0013]    The instrumentation can be used in both new and existing processes. With existing processes, instruments may need to be added and/or different instruments may need to be installed at certain points in the system in order to apply it. 
         [0014]    The control instrumentation for the steam stripped fractionation column is illustrated in  FIG. 1 . A hydrocarbon feed  5  is introduced into the fractionation column  10 . Steam  15  is introduced into the fractionation column  10  and travels upward to strip volatile components from the downward flowing hydrocarbon feed  5 . A hydrocarbon product stream  20  having a reduced concentration of volatile components is removed from the bottom of the fractionation column  10  and recovered. A vapor stream containing lower molecular weight hydrocarbons which have been stripped from the feed and steam is removed from the fractionation column  10 , cooled, and sent to receiver  30  through overhead vapor line  25 . 
         [0015]    The stream entering receiver  30  includes steam condensate, liquid hydrocarbons, and normally gaseous hydrocarbons. A sour gas stream containing gaseous hydrocarbons is removed from the receiver  30  through receiver vapor outlet line  35  and recovered. Steam condensate is removed from receiver  30  through water outlet line  40  and recovered. A liquid hydrocarbon stream is removed from the receiver  30  through hydrocarbon liquid outlet line  45 , which splits into lines  50  and  55 . A portion of the liquid hydrocarbon stream is sent to the fractionation column  10  through stripper reflux line  50  as reflux. Another portion of the liquid hydrocarbon stream is recovered as net hydrocarbon liquid through stripper net overhead line  55 . 
         [0016]    There is a molecular weight analyzer or a specific gravity analyzer  60  in communication with overhead vapor line  25  to measure the molecular weight or specific gravity of the overhead vapor stream from the fractionation column  10 . The molecular weight analyzer or specific gravity analyzer  60  sends the molecular weight or specific gravity measurements through line  65  to a computer  70 . The computer  70  includes at least a storage unit  75  and a calculating unit  80 . 
         [0017]    Pressure gauge  85 , which is in communication with overhead line  25 , measures the pressure of the overhead vapor stream from the fractionation column  10 , and sends the pressure measurements to the computer  70  through line  90 . 
         [0018]    Temperature gauge  95 , which is in communication with overhead line  25 , measures the temperature of the overhead vapor stream from the fractionation column  10 , and sends the temperature measurements to the computer  70  through line  100 . 
         [0019]    Stripper vapor mass flow meter  105  measures the mass flow of the sour gas stream in receiver vapor outlet line  35 , and sends the mass flow measurements to the computer  70  through line  110 . 
         [0020]    Water flow meter  115  measures the flow of the steam condensate in line  40  and sends the flow measurements to the computer  70  through line  120 . The water flow meter can be a volumetric flow meter or a mass flow meter, as desired. The weight flow of water is needed, but it can either be measured directly with a mass flow meter, or be calculated from a volumetric flow corrected for temperature using the steam table specific gravity. Suitable mass flow meters include, but are not limited to, coriolis mass flow meters. Suitable volumetric flow meters include, but are not limited to, orifice plate flow meters. 
         [0021]    Hydrocarbon liquid outlet temperature gauge  125  measures the temperature of the liquid hydrocarbon stream in hydrocarbon liquid outlet line  45  and sends the temperature measurements to the computer  70  through line  130 . Alternatively, hydrocarbon liquid outlet temperature gauge  125  could be located on either the stripper reflux line  50  or the stripper net overhead line  55 . 
         [0022]    Stripper reflux hydrocarbon liquid mass flow meter  135  measures the mass flow of the liquid hydrocarbon reflux stream in line  50  and sends the mass flow measurements to the computer  70  through line  140 . 
         [0023]    Stripper net overhead hydrocarbon liquid mass flow meter  145  measures the mass flow of the net overhead liquid hydrocarbon stream in line  55  and sends the mass flow measurements to the computer  70  through line  150 . 
         [0024]    Alternatively, as shown in  FIG. 2 , instead of measuring the mass flow of the liquid hydrocarbon reflux stream in line  50  and the net overhead liquid hydrocarbon stream in line  55  separately, the stripper hydrocarbon liquid mass flow meter  160  measures the mass flow of the liquid hydrocarbon stream in the hydrocarbon outlet line  45  and sends the mass flow measurements to the computer  70  through line  165 . 
         [0025]    The control method is illustrated in  FIG. 3 . The various measurements described above are made and sent to the computer  70  in step  200  for use in the calculation of the dew point margin. 
         [0026]    The dew point margin can be determined using the following equations. First, the total overhead flow is calculated in step  205 . This can be done using equation 1a or 1b, depending on whether the mass flow of the mass flow of the liquid hydrocarbon stream  45  is measured, or and the mass flow of the liquid hydrocarbon reflux stream  135  and the mass flow of the net overhead liquid hydrocarbon stream  145  are measured. 
         [0000]      TOF=WFR+ RVF+HLF   (1a)
 
         [0000]      TOF=WFR+ RVF+NOLF+RF   (1b)
 
       Where: 
       [0027]    TOF=total overhead flow (mass flow)
 
WFR=measured water flow rate of the water stream from the receiver (either measured as mass flow or converted to mass flow—from water flow meter  115 )
 
RVF=measured mass flow rate of the stripper vapor stream from the receiver (from stripper vapor mass flow meter  105 )
 
HLF=measured mass flow rate of the hydrocarbon liquid stream from the receiver (from stripper hydrocarbon liquid mass flow meter  160 )
 
NOLF=measured mass flow rate of the stripper net overhead hydrocarbon liquid flow (from stripper net overhead hydrocarbon liquid mass flow meter  145 )
 
RF=measured mass flow rate of the reflux hydrocarbon liquid stream (from stripper reflux hydrocarbon liquid mass flow meter  135 ).
 
         [0028]    Next, the total overhead moles are calculated in step  210 . This calculation can be performed using equation 2. 
         [0000]      TOM=TOF/ MWov   (2)
 
       Where: 
       [0029]    TOM=total overhead moles
 
TOF=total overhead flow (mass flow from equation 1)
 
MWov=molecular weight of the overhead stream (from molecular weight analyzer  60  or calculated from equation 3).
 
         [0030]    MWov can be calculated using equation 3 if a specific gravity analyzer  60  is used. 
         [0000]    
       
         
           
             
               
                 
                   MWov 
                   = 
                   
                     
                       
                         ρ 
                         ov 
                       
                        
                       RTa 
                     
                     
                       P 
                        
                       
                           
                       
                        
                       a 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
       Where: 
       [0031]    MW ov =molecular weight of the overhead vapor
 
ρ ov =density of the overhead vapor (from specific gravity analyzer  60 )
 
R=universal gas constant
 
Ta=absolute temperature of the overhead vapor (from temperature gauge  95 +absolute temperature conversion factor)
 
Pa=absolute pressure of the overhead vapor (from pressure gauge  85 +absolute pressure conversion factor).
 
         [0032]    The absolute temperature conversion factor for temperature measured in ° F. is 460° F. The absolute pressure conversion factor for pressure measure in psia is 14.7 psia. Those of skill in the art can determine the appropriate absolute temperature and pressure conversion factors for other temperature and pressure units. Next the total moles of water are determined in step  215 . This can be calculated using equation 4a or 4b, depending on whether a mass flow meter or a volumetric flow meter is used. 
         [0000]      TMW=(WFR)/18.015  (4a)
 
       Where: 
       [0033]    TMW=total moles of water
 
WFR=measured mass flow rate of the water stream from the receiver (from water flow meter  115 ).
 
         [0000]      TMW=(VFR*ρ)/18.015  (4b)
 
       Where: 
       [0034]    VFR is the volume flow rate in consistent units
 
ρ=density of water at the measured temperature of the hydrocarbon liquid stream from the receiver (from temperature gauge  125 )
 
18.015=molecular weight of water.
 
         [0035]    Next, the partial pressure of water in the overhead stream is determined at step  220 . This can be calculated using equation 5. 
         [0000]        PPWO =( TMW /TOM)* OP   (5)
 
       Where: 
       [0036]    PPWO=partial pressure of water in the overhead vapor stream in psia
 
TMW=total moles of water from equation 4a or 4b
 
TOM=total overhead moles from equation 2
 
OP=measured pressure of the overhead vapor stream (from pressure gauge  85 ) in psia.
 
         [0037]    The saturation temperature (water dew point) is determined at step  225 . It can be determined according to equation 6 using the steam tables stored in the computer. 
         [0000]      WDP=Temperature at  PPWO   (6)
 
       Where: 
       [0038]    WDP=water dew point, in ° F.
 
PPWO=partial pressure of water in the overhead vapor stream, in psia.
 
         [0039]    Alternatively, the dew point can be calculated using equation 7, which can be programmed into the computer. Equation 7 has been verified for multiple points, and it is accurate to within 0.5° C. (1° F.). The error decreases at saturation temperatures above 150° C. (302° F.). 
         [0000]      WDP=0.20+118.084×( PPWO  (psia)) 0.2215   (7)
 
         [0040]    Where: 
         [0000]    WDP=water dew point (° F.)
 
PPWO=partial pressure of water in the overhead vapor stream in psia (from equation 5).
 
         [0041]    Next, the dew point margin is determined at step  230 . It can be calculated using equation 8. 
         [0000]      DPM=OT−WDP  (8)
 
       Where: 
       [0042]    DPM=dew point margin
 
OT=measured operating temperature (from temperature gauge  95 )
 
WDP=water dew point from equation 6 or 7.
 
         [0043]    At step  235 , the DPM is compared to a predetermined minimum dew point margin. The predetermined minimum dew point margin is selected for safe operation of the column. If DPM is less than the predetermined dew point margin, an alarm  155  is triggered by the computer  70  at step  240  or an operating condition is changed, or both. The change in operating condition can be performed by the computer or by the operator or both. Changes in operating condition can include, but are not limited to, changing an operating condition of the fractionation column to change the measured temperature of the overhead vapor stream, such as changing the heat input to the fractionation column. 
         [0044]    Desirably, the measurements and calculations are continually performed by the apparatus. However, it is within the scope of the invention to take measurements and/or perform the calculations at regularly set intervals, e.g., every sec, every 30 sec, every min, every 5 min, etc., or irregularly set intervals, e.g., every 5 min, and if the DPM decreases past a pre-set limit, increasing the interval to every 30 sec, for example. 
         [0045]    The apparatus eliminates a total overhead liquid flow meter and uses reflux and net overhead liquid mass flow meters instead. 
         [0046]    The calculations are simplified because molar flow rates can be calculated directly without having to convert volumetric flow rates. 
         [0047]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.