Method for measurement and calculation of dew point for fractionation column overheads

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.

What is claimed is: 
     
         1 . A method for controlling operation of a fractionation column comprising:
 measuring a molecular weight or a specific gravity, a temperature, and a pressure of an overhead vapor stream from the fractionation column to a receiver;   measuring a temperature of a stripper hydrocarbon liquid stream from the receiver;   measuring a mass flow rate of the stripper hydrocarbon liquid stream, or measuring a mass flow rate of a stripper net overhead liquid stream and a reflux hydrocarbon liquid stream;   measuring a mass flow rate of a stripper vapor stream from the receiver;   measuring a flow rate of a water stream from the receiver;   determining a total overhead flow using the flow rate of the water stream from the receiver; the mass flow rate of the stripper vapor stream from the receiver; and 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;   determining a total overhead moles from the total overhead flow;   determining a total moles of water from the measured water flow rate from the receiver and the measured temperature of the hydrocarbon liquid stream from the receiver;   determining a partial pressure of water in the overhead vapor stream from the total moles of water, the total overhead moles, and the measured overhead pressure;   determining a dew point temperature at the determined partial pressure of water;   determining the dew point margin from the determined dew point and the measured overhead temperature;   comparing the calculated dew point margin with a predetermined minimum dew point margin;   initiating an alarm or changing an operating condition of the fractionation column or both when the calculated dew point margin is less than the predetermined minimum dew point margin.   
     
     
         2 . The method of  claim 1  wherein determining the total overhead flow comprises:
   TOF=WFR+ RVF+HLF    
   or 
   TOF=WFR+ RVF+NOLF+RF    
 Where: 
 TOF=total overhead flow 
 WFR=measured water flow rate of the water stream from the receiver 
 RVF=measured mass flow rate of the stripper vapor stream from the receiver 
 HLF=measured mass flow rate of the stripper hydrocarbon liquid stream from the receiver 
 NOLF=measured mass flow rate of the stripper net overhead hydrocarbon liquid stream 
 RF=measured mass flow rate of the reflux hydrocarbon liquid stream. 
 
     
     
         3 . The method of  claim 2  wherein determining the total overhead moles comprises:
   TOM=TOF/ MWov    
 Where: 
 TOM=total overhead moles 
 TOF=total overhead flow 
 MWov=molecular weight of the overhead stream. 
 
     
     
         4 . The method of  claim 3  wherein the molecular weight of the overhead stream is determined from the measured specific gravity using: 
       
         
           
             
               MWov 
               = 
               
                 
                   
                     ρ 
                     ov 
                   
                    
                   RTa 
                 
                 
                   P 
                    
                   
                       
                   
                    
                   a 
                 
               
             
           
         
         Where: 
         MW ov =molecular weight of the overhead vapor 
         ρ ov =measured specific gravity of the overhead vapor stream 
         R=universal gas constant 
         Ta=absolute temperature of the overhead vapor=measured temperature of the overhead vapor stream+an absolute temperature conversion factor 
         Pa=absolute pressure of the overhead vapor=measured pressure of the overhead vapor stream+an absolute pressure conversion factor. 
       
     
     
         5 . The method of  claim 3  wherein determining the total moles of water comprises:
   TMW=(WFR)/18.015 
 Where: 
 TMW=total moles of water 
 WFR=measured mass water flow rate of the water stream from the receiver 
 18.015=molecular weight of water; 
 or
     TMW =(VFR*ρ)/18.015
 
 
 Where: 
 TMW=total moles of water 
 VFR=measured volume flow rate of the water stream from the receiver 
 ρ=density of water at the measured temperature of the hydrocarbon liquid stream from the receiver 
 18.015=molecular weight of water. 
 
     
     
         6 . The method of  claim 5  wherein determining the partial pressure of water in the overhead vapor stream comprises:
     PPWO =( TMW /TOM)* OP    
 Where: 
 PPWO=partial pressure of water in the overhead vapor stream 
 TMW=total moles of water 
 TOM=total overhead moles 
 OP=measured pressure of the overhead vapor stream. 
 
     
     
         7 . The method of  claim 6  wherein determining a dew point temperature at the determined partial pressure of water comprises:
   WDP=Temperature at  PPWO    
 Where: 
 WDP=water dew point 
 PPWO=partial pressure of water in the overhead vapor stream. 
 
     
     
         8 . The method of  claim 6  wherein determining a dew point temperature at the determined partial pressure of water comprises:
   WDP=0.20+118.084×( PPWO  (psia)) 0.2215  
 
 Where: 
 WDP=water dew point (° F.) 
 PPWO=partial pressure of water in the overhead vapor stream. 
 
     
     
         9 . The method of  claim 1  wherein changing the operating condition of the fractionation column comprises changing the operating conditions of the fractionation column to change the measured temperature of the overhead vapor stream when the calculated dew point margin is less than the predetermined minimum dew point margin. 
     
     
         10 . The method of  claim 1  wherein changing the operating condition of the fractionation column comprises changing a heat input to the fractionation column. 
     
     
         11 . The method of  claim 1  further comprising providing a database containing thermodynamic properties of water. 
     
     
         12 . The method of  claim 1  wherein the measured molecular weight or specific gravity of the overhead vapor stream; the measured temperature of the overhead vapor stream; the measured pressure of the overhead vapor stream; the measured temperature of the hydrocarbon liquid stream from the receiver; the measured mass flow rate of the stripper hydrocarbon liquid stream, or the measured mass flow rate of the reflux hydrocarbon liquid stream and the measured mass flow rate of the stripper net overhead hydrocarbon liquid stream; the measured mass flow rate of the stripper vapor stream from the receiver; and the measured flow rate of the water stream from the receiver are provided to a computer which continuously determines the calculated dew point margin, compares the calculated dew point margin with the predetermined minimum dew point margin, and initiates the alarm or changes the operating condition of the fractionation column or both when the calculated dew point margin is less than the predetermined minimum dew point margin. 
     
     
         13 . The method of  claim 1  wherein the mass flow rate of the stripper net overhead hydrocarbon liquid stream and the reflux hydrocarbon liquid stream are measured. 
     
     
         14 . A method for controlling operation of a fractionation column comprising:
 measuring a molecular weight or specific gravity, a temperature, and a pressure of an overhead vapor stream from the fractionation column to a receiver;   measuring a temperature of a stripper hydrocarbon liquid stream from the receiver;   measuring a mass flow rate of the stripper hydrocarbon liquid stream, or measuring a mass flow rate of a stripper net overhead liquid stream and a reflux hydrocarbon liquid stream;   measuring a mass flow rate of a stripper vapor stream from the receiver;   measuring a flow rate of a water stream from the receiver;   providing the measured molecular weight or specific gravity of the overhead vapor stream; the measured temperature of the overhead vapor stream; the measured pressure of the overhead vapor stream; the measured temperature of the hydrocarbon liquid stream from the receiver; the measured mass flow rate of the stripper hydrocarbon liquid stream, or the measured mass flow rate of the reflux hydrocarbon liquid stream and the measured mass flow rate of the stripper net overhead hydrocarbon liquid stream; the measured mass flow rate of the stripper vapor stream from the receiver; and the measured flow rate of the water stream from the receiver to a computer;   determining a total overhead flow using the flow rate of the water stream from the receiver, the mass flow rate of the stripper vapor stream from the receiver, and the mass flow rate of the stripper hydrocarbon liquid stream, or the mass flow rate of the stripper net overhead hydrocarbon liquid stream, and the mass flow rate of the reflux hydrocarbon liquid stream using the computer using:
   TOF=WFR+ RVF+HLF    
   or 
   TOF=WFR+ RVF+NOLF+RF    
   Where:   TOF=total overhead flow   WFR=measured mass flow rate of the water stream from the receiver   RVF=measured mass flow rate of the stripper vapor stream from the receiver   HLF=measured mass flow rate of the stripper hydrocarbon liquid stream from the receiver   NOLF=measured mass flow rate of the stripper net overhead hydrocarbon liquid stream   RF=measured mass flow rate of the reflux hydrocarbon liquid stream;
 determining a total overhead moles from the total overhead flow and the molecular weight of the overhead vapor stream using the computer using: 
   TOM=TOF/MWov   Where:   TOM=total overhead moles   TOF=total overhead flow   MWov=molecular weight of the overhead stream;
 determining a total moles of water from the measured water flow rate from the receiver and the measured temperature of the hydrocarbon liquid stream from the receiver using the computer using;
   TMW=(WFR)/18.015 
 
   Where:   TMW=total moles of water   WFR=measured mass flow rate of the water stream from the receiver   18.015=molecular weight of water   or
     TMW =( VFR *ρ)/18.015
 
   Where:   TMW=total moles of water   VFR=measured volume flow rate of the water stream from the receiver   ρ=density of water at the measured temperature of the hydrocarbon liquid stream from the receiver   18.015=molecular weight of water;
 determining a partial pressure of water in the overhead vapor stream from the total moles of water, the total overhead moles, and the measured overhead pressure using the computer using;
     PPWO =( TMW /TOM)*OP 
 
   Where:   PPWO=partial pressure of water in the overhead vapor stream   TMW=total moles of water   TOM=total overhead moles   OP=measured pressure of the overhead vapor stream;
 determining a dew point temperature at the determined partial pressure of water using the computer; 
 determining the dew point margin from the determined dew point and the temperature of the overhead vapor stream using the computer; 
 comparing the calculated dew point margin with a predetermined minimum dew point margin using the computer; 
 initiating an alarm or changing an operating condition of the fractionation column when the calculated dew point margin is less than the predetermined minimum dew point margin using the computer. 
   
     
     
         15 . The method of  claim 14  wherein determining a dew point temperature at the determined partial pressure of water comprises:
   WDP=Temperature at  PPWO    
 Where: 
 WDP=water dew point 
 PPWO=partial pressure of water in the overhead. 
 
     
     
         16 . The method of  claim 14  wherein determining a dew point temperature at the determined partial pressure of water comprises:
   WDP=0.20+118.084×( PPWO  (psia)) 0.2215  
 
 Where: 
 WDP=water dew point (° F.) 
 PPWO=partial pressure of water in the overhead stream. 
 
     
     
         17 . The method of  claim 14  wherein the molecular weight of the overhead stream is determined from the measured specific gravity using: 
       
         
           
             
               MWov 
               = 
               
                 
                   
                     ρ 
                     ov 
                   
                    
                   RTa 
                 
                 
                   P 
                    
                   
                       
                   
                    
                   a 
                 
               
             
           
         
         Where: 
         MW ov =molecular weight of the overhead vapor 
         ρ ov =measured specific gravity of the overhead vapor stream 
         R=universal gas constant 
         Ta=absolute temperature of the overhead vapor=measured temperature of the overhead vapor stream+an absolute temperature conversion factor 
         Pa=absolute pressure of the overhead vapor=measured pressure of the overhead vapor stream+an absolute pressure conversion factor. 
       
     
     
         18 . The method of  claim 14  wherein changing the operating condition of the fractionation column comprises changing a heat input to the fractionation column. 
     
     
         19 . The method of  claim 14  further comprising providing a database containing thermodynamic properties of water to the computer. 
     
     
         20 . The method of  claim 14  wherein the mass flow rate of the stripper net overhead hydrocarbon liquid stream and the reflux hydrocarbon liquid stream are measured.