Abstract:
A method and apparatus for determining the salt concentration of crude oil using a single solvent that allows for improved safety and accuracy. The apparatus can include a pair of electrodes, a solvent storage container, a power source, a temperature sensor, a computer, and a display device. The method includes introducing a volume of the crude oil and a solvent to a mixing zone and mixing them together to form a homogenized mixture. The temperature and conductivity of the homogenized mixture are measured in order to determine the salt concentration of the homogenized mixture, and subsequently, the salt concentration of the crude oil.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and apparatus for measuring the salt concentration of a crude oil sample. 
       BACKGROUND OF THE INVENTION 
       [0002]    Currently, salt content measurement in the crude oil production and refining industry follows either the standard reference method ASTM D3230 or ASTM D6470, both of which are incorporated herein by reference, ASTM D6470 is not practical for on-line measurement as it requires extraction steps. ASTM D3230 requires the utilization of a combination of three chemicals: methanol, butanol, and xylene, to reduce the crude oil resistivity to a measurable range for traditional sensors and common instrumentation. However, these solvents are extremely toxic, volatile, and flammable, which makes them particularly difficult to handle in facilities located in hot environments. 
         [0003]    The principle of the method outlined in ASTM D3230 is to dilute a crude oil sample in an alcoholic mixture in order to allow for conductivity readings and to compare the readings to reference values. This method can be applied either off-line on a collected sample or on-line. Off-line measurements are susceptible to errors due to unequal gas desorption, which can be attributed to time lag between sampling and measurement and/or variations of the external temperature and/or the crude oil temperature, improper mix of chemicals, measurement errors, and data reporting. On-line measurements avoid manual sampling and prevent some potential errors. However, in high temperature environments, the use of volatile solvents is inadvisable because it can lead to fires, explosions, or intoxication. Another disadvantage of ASTM D3230 is the limitation in solvent quantity stored on site due to flammability concerns and risks of explosion. 
         [0004]    Furthermore, xylene, methanol, and butanol must be added in precise proportions, as any minimum alteration in their volumetric ratio, which is even more likely to occur at high temperatures due to their volatile nature, disturbs the homogeneity and causes erroneous measurement readings. This leads to a complicated measurement procedure, thereby introducing potential errors. Moreover, these issues also require bulky equipment with frequent and careful maintenance. 
         [0005]    Therefore, it would be beneficial to provide a method and apparatus that avoids the use of the three listed chemicals, and instead uses a single solvent. It would also be beneficial to provide a solvent that is less toxic, and is more stable under hot weather conditions. It would also be beneficial to provide for a method and apparatus that is capable of measuring the salt concentration on-line. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed to a method and apparatus that provides one or more of these benefits. In one embodiment, the invention provides for a method for determining a salt concentration of crude oil through the steps of feeding a volume of the crude oil to a mixing zone; introducing a volume of a solvent to the mixing zone and mixing such that the crude oil and the solvent form a homogenized mixture; taking a temperature measurement of the homogenized mixture using a temperature sensor; taking a conductivity measurement of the homogenized mixture using a pair of electrodes; determining the salt concentration of the homogenized mixture based on the temperature measurement and the conductivity measurement of the homogenized mixture; and determining the salt concentration of the crude oil based on the salt concentration of the homogenized mixture, the volume of the crude oil, and the volume of the solvent. In one embodiment, the solvent has a normal boiling point that is substantially above ambient air temperature. In one embodiment, the solvent has a boiling point above 140° C., which allows for a relatively low vapor pressure even if ambient air temperature is relatively high, such as 60° C. 
         [0007]    In another embodiment, the solvent can consist essentially of a single solvent with no other solvents present in the mixing zone. In another embodiment, the solvent is operable to solubilize both water and the crude oil into a single phase. In one embodiment, the solvent can exhibit a Relative Energy Difference value with crude oil, as defined by Hansen solubility theory, of less than 1. In another embodiment, the solvent can include a single solvent selected from the group consisting of NMP, ethyl alpha-hydroxy propionate, benzonitrile, dipropylene glycol dimethyl ether, 1-cyclohexyl-2 pyrrolidone, 1-octyl-2-pyrrolidone, 1-dodecyl-2-pyrrolidinone, diethyl glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol butyl ether, propylene glycol butyl ether, di(propylene glycol) butyl ether, propylene glycol propyl ether, tripropylene glycol, and diethylene glycol monoethyl ether. In another embodiment, the solvent can consist essentially of a single solvent selected from the group consisting of NMP, ethyl alpha-hydroxy propionate, benzonitrile, dipropylene glycol dimethyl ether, 1-cyclohexyl-2 pyrrolidone, 1-octyl-2-pyrrolidone, 1-dodecyl-2-pyrrolidinone, diethyl glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol butyl ether, propylene glycol butyl ether, di(propylene glycol) butyl ether, propylene glycol propyl ether, tripropylene glycol, and diethylene glycol monoethyl ether. 
         [0008]    In another embodiment, the step of determining the salt concentration of the homogenized mixture can further include the steps of obtaining a calibration curve of conductivity as a function of temperature for a known mixture having a known salt concentration and comparing the conductivity of the homogenized mixture against the calibration curve to determine the salt concentration of the homogenized mixture. Certain embodiments of the present invention provide a system to determine the salt concentration given the operational parameters of the homogenized mixture, including temperature and volume of crude oil and solvent and the calibration curve or other similar aspect, such as a look-up table. 
         [0009]    In another embodiment, the method is capable of determining salt concentrations of the crude oil in a range from 0 to 50 pounds of salt per thousand barrels (PTB). In another embodiment, the method can be advantageously conducted in an industrial environment that is either on or off-shore. In another embodiment, the solvent does not include an alcohol or xylene. In another embodiment, because the solvent is much less volatile than in methods practiced heretofore, much larger quantities of the solvent may be stored on-site without decreasing overall safety. In another embodiment, the method can include sending a warning signal from the system to an operator if the salt concentration of the crude oil is above a threshold value. In one embodiment, the threshold value is 10 PTB. Examples of warning signals include, without limitation, audible sounds or alarms, blinking lights, vibrations delivered via mobile device such as a pager, phone, or the like, and a display message, which could be displayed on a monitor, pager, phone, or the like. 
         [0010]    In another embodiment, the step of taking a temperature measurement of the homogenized mixture using a temperature sensor includes more than one temperature measurement, and the step of taking a conductivity measurement of the homogenized mixture using a pair of electrodes includes more than one conductivity measurement. In another embodiment, the method can include the step of cleaning the pair of electrodes between successive conductivity measurements. In another embodiment, the step of cleaning the pair of electrodes can be accomplished with a fluid. This fluid can be the same as the chosen solvent, or can be a second solvent that is not the same as the chosen solvent. In another embodiment, the fluid can be crude oil, or a combination of the above. 
         [0011]    Embodiments of the present invention also provide for an apparatus that is useful for determining the salt concentration of crude oil. In one embodiment, the apparatus can include a pair of electrodes, a solvent storage container, a power source, a temperature sensor, a computer, data stored on a readable medium, a program product, and a display device. In one embodiment, the pair of electrodes is adaptable for insertion into a homogenized mixture container, wherein the homogenized mixture container is operable to hold a homogenized mixture. In one embodiment, the homogenized mixture can include crude oil and a solvent, wherein the crude oil contains salt, and the pair of electrodes is operable to measure the conductivity of the homogenized mixture. In another embodiment, the solvent storage container is used for storing the solvent, and the solvent storage container is in fluid communication with the homogenized mixture container. In another embodiment, the power source is in electronic communication with the pair of electrodes, and the power source helps to provide a voltage, electric potential energy, to the pair of electrodes. The temperature sensor is operable to measure the temperature of the homogenized mixture. The computer includes a signal processing device having non-transitory computer memory. The computer is preferably in electronic communication with the pair of electrodes and the temperature sensor such that the computer is operable to receive measured data from the pair of electrodes and the temperature sensor. In one embodiment, the data stored on a readable medium includes conductivity values, temperature values, and corresponding salt concentrations for a known mixture of crude oil and solvent. In one embodiment, the program product can be stored in the non-transitory computer memory. Preferably, the program product includes instructions executable for (1) comparing the measured data from the pair of electrodes and the temperature probe with the data stored on the readable medium, (2) determining the salt concentration of the homogenized mixture, and (3) determining the salt concentration of the crude oil based upon the relationship of volume of solvent, volume of crude oil and the salt concentration of the homogenized mixture. In one embodiment, the display device can be in electronic communication with the computer. The display device is operable to receive the salt concentration of the homogenized mixture from the program product and display said salt concentration in a readable form. 
         [0012]    In another embodiment, the apparatus is useful for detecting the salt concentration of the crude oil in a range from 0 to 50 PTB. In yet another embodiment, the apparatus can be located in an industrial environment. In another embodiment, the apparatus can be located offshore. In another embodiment, the apparatus can also include a warning system that is operable to send a warning signal to the display device if the salt concentration of the crude oil is above a threshold value. In another embodiment, the warning system could also trigger an alarm signal, such as a visual signal and/or an audible signal that could be appreciated by an operator, both on-site or in a remote facility. In one embodiment, the threshold value can be 10 PTB of salt in the crude oil measured. 
         [0013]    In another embodiment, the apparatus can further include a flow controlling means that is operable to introduce the crude oil and the single solvent continuously into the homogenized mixture container. In another embodiment, the apparatus can further include one or more flowmeters for measuring the flow of the crude oil and single solvent. In another embodiment, the flow of the crude oil and the single solvent is measured by the one or more flowmeters and controlled by the flow controlling means to ensure that proper flowrates of the crude oil and the single solvent are introduced to the homogenized mixture container. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
           [0015]      FIG. 1  is a representation of an apparatus for use with the procedures of ASTM-D3230. 
           [0016]      FIG. 2  is a representation of an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims. 
         [0018]    The various embodiments of the present invention provide a stable solvent which advantageously avoids the problems associated with solvents used in ASTM D3230, discussed earlier in the Background section. Since the solvents described herein have an improved stability, repeatability and accuracy of the measurements are improved, as well as improved ability to store larger amounts of the solvents. Additionally, the solvents used in accordance with various embodiments of the present invention are less toxic than those used in ASTM D3230, and therefore, provide reduced health and safety risks. 
         [0019]    In certain embodiments of the present invention, the method can be practiced in a continuous mode. Additionally, less equipment can be used with the elimination of a multi-solvent system, thereby providing additional capital expenditure savings. Additionally, as only one single solvent is used at one time in embodiments of the present invention, the logistics and operating expenditures are significantly reduced as compared with the three-solvent system described in ASTM D3230. Furthermore, ASTM D3230 uses two polar solvents, which must be in precise proportions, to disperse the crude oil sample homogeneously in a single phase. Xylene is then added to dissolve any considerable amounts of asphaltene material present. In one embodiment of the present invention, a single solvent is used that is capable of solubilizing both water and hydrocarbons in a single phase. The method as taught in ASTM D3230 used xylene to solubilize the crude oil components. However, xylene is relatively insoluble in water. Likewise, butanol and methanol are soluble in water, but not in oil. However, the alcohols are able to dissolve xylene, along with anything that is already dissolved in the xylene. As such, the solvents taught in ASTM D3230 are not operable to dissolve both oil and water components in their individual capacities. In embodiments of the present invention, however, the solvent is selected such that it has the proper solubility in crude oil and is polar, such that the solvent can bring the crude oil and water into one phase. In one embodiment, the water includes produced water. 
         [0020]      FIG. 1  represents a general setup for an apparatus that is in accordance with ASTM D3230. In  FIG. 1 , there are three storage containers, xylene storage  10 , alcohol-mixture storage  20 , and naphtha storage  30 . The xylene and alcohol-mixture (which includes methanol and butanol) are introduced into homogenized mixture container  40  via lines  12  and  22 , respectively. Control valves  15  and  25  are used to help control the flow of each solvent. A precise amount of crude oil sample is introduced into homogenized mixture container  40  via line  38 , and is controlled by control valve  45 . Once all of the liquids are present, the solution is then mixed to form a homogenized mixture. A pair of electrodes  50  and temperature sensor  60  are inserted into homogenized mixture container. Pair of electrodes  50  and temperature sensor  60  are in communication with computer  70 . In ASTM D3230, Naphtha is used to rinse the electrodes following a test. Naphtha is delivered to the pair of electrodes  50  via line  32  by opening control valve  35 . 
         [0021]      FIG. 2  represents an embodiment of the present invention. In  FIG. 2 , solvent storage  110  replaces the three storage containers ( 10 ,  20 ,  30 ) from  FIG. 1 . Similarly, control valve  115  has replaced the need for the three control valves  15 ,  25 ,  35  of the previous set-up. The crude oil sample is introduced into homogenized mixture container  40  via line  38 , and is controlled by control valve  45 . Once the crude oil and solvent are introduced into homogenized mixture container  40 , the solution is then mixed to form a homogenized mixture. A pair of electrodes  50  and temperature sensor  60  are inserted into homogenized mixture container. Pair of electrodes  50  and temperature sensor  60  are in communication with computer  70 . Computer  70  contains data that is used to back calculate the salt concentration of the crude oil sample based on the temperature and conductivity measurements taken. 
         [0022]    Those of ordinary skill in the art will readily understand that new calibration curves should be made for each solvent used, such that when a conductivity reading and temperature reading are taken, computer  70  can calculate the salt concentration in the crude sample. 
       Calibration Method 
       [0023]    Calibration solutions were prepared in conformance with ASTM D3230 standards. In one embodiment, N-methyl-2-pyrrolidone (NMP) was used as the solvent. 15 mL of NMP was added into a dry, 100-mL graduated, glass-stoppered mixing cylinder. 10 mL of neutral oil was added into the 100-mL graduated cylinder using a 10 mL graduated cylinder. The 10 mL graduated cylinder was rinsed with NMP until free of oil. NMP was added until there was 50 mL of the solution. The mixing cylinder was stoppered and shaken vigorously for approximately 1 minute. In accordance with Table I, a quantity of dilute mixed salts solution was added that is appropriate to the range of salt contents to be measured. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Standard Samples 
               
             
          
           
               
                   
                 Salt 
                   
               
               
                 Salt 
                 (lb/1000 bbl of Crude 
                 Mixed Salts 
               
               
                 (g/m3 of Crude Oil) 
                 oil) 
                 Solution (dilute), mL 
               
               
                   
               
             
          
           
               
                 3 
                 1.0 
                 0.3 
               
               
                 9 
                 3.0 
                 1.0 
               
               
                 15 
                 5.0 
                 1.5 
               
               
                 30 
                 10.0 
                 3.0 
               
               
                 45 
                 16.0 
                 4.5 
               
               
                 60 
                 21.0 
                 6.0 
               
               
                 75 
                 26.0 
                 8.0 
               
               
                 90 
                 31.0 
                 9.5 
               
               
                 115 
                 40.0 
                 12.0 
               
               
                 145 
                 51.0 
                 15.0 
               
               
                 190 
                 66.0 
                 20.0 
               
               
                 215 
                 75.0 
                 22.5 
               
               
                 245 
                 86.0 
                 25.5 
               
               
                 290 
                 101.0 
                 30.5 
               
               
                 430 
                 151.0 
                 45.5 
               
               
                   
               
             
          
         
       
     
         [0024]    The NMP and crude oil mixture was then diluted to 100 mL with additional NMP. Again, the cylinder was shaken vigorously for approximately 30 seconds to effect solution, and then the solution was allowed to stand approximately 5 min. The solution was then poured into a dry test beaker. 
         [0025]    A pair of electrodes were quickly placed into the solution in the beaker, making sure that the upper edge of the electrode plates are below the surface of the solution. The voltage was adjusted to a series of values, with the current and voltage being recorded. The electrodes were then removed from the solution, rinsed with NMP, followed by naphtha, and allowed to dry. 
         [0000]    The above procedure was repeated using other volumes of mixed salts solution (dilute solution) as needed to cover the range of chloride contents of interest. In order to develop the calibration curve, the value obtained for the blank measurement was subtracted from the indicated current readings of each standard sample. 
       Experimental Results 
       [0026]    In one embodiment, the three solvents were replaced with a single solution of N-methyl-2-pyrrolidone (NMP). The physical properties of NMP are summarized in Table II below: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Physical Properties of NMP 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Boiling Point (760 mmHg) 
                 204.3° C. 
               
               
                   
                 Freezing Point (760 mmHg) 
                 −23.6° C. 
               
               
                   
                 Flash Point (ASTM-D 93-72) 
                   91° C. 
               
               
                   
                 Ignition Temperature (ASTM-D 286-58 T) 
                   270° C. 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    Various salt concentrations were created in conformance with the calibration procedure outlined above. Equation 1 below represents an equation of the conductivity as a function of temperature and salt concentration for various mixtures of NMP and crude oil: 
         [0000]      Conductivity= a+b *SaltConcentration+ c*T    
         [0028]    Additionally, embodiments of the present invention can also increase electrometric measurement accuracy by increasing the fraction of crude in the tested solution, which also results in reducing overall consumption of the solvent. In one embodiment, the crude content can be increased to 30% by volume of the homogenized mixture, as opposed to the 10% suggested by ASTM 3230. 
         [0029]    Similar results as shown above with regards to NMP were found when using the following solvents: ethyl alpha-hydroxy propionate (VERTECBIO EL™), benzonitrile, and dipropylene glycol dimethyl ether (DPGDME). In another embodiment, a non-exhaustive list of solvents that are useful are shown in Table III below. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Non-exhaustive List of Solvents with their Physical Properties 
               
             
          
           
               
                   
                 Boiling 
                 Flash Point 
               
               
                 Solvent Name 
                 Point (° C.) 
                 (° C.) 
               
               
                   
               
             
          
           
               
                 1-Cyclohexyl-2-pyrrolidone 
                 154 
                 141 
               
               
                 1-Octyl-2-pyrrolidone 
                 171 
                 142 
               
               
                 1-Dodecyl-2-pyrrolidinone 
                 202 
                 113 
               
               
                 Diethylene glycol monobutyl ether 
                 225 
                 99 
               
               
                 Triethylene glycol monobutyl ether 
                 265-300 
                 144 
               
               
                 Ethylene glycol butyl ether 
                 170 
                 67 
               
               
                 Propylene glycol butyl ether 
                 170 
                 59 
               
               
                 Di(propylene glycol) butyl ether, mixture of 
                 225 
                 96 
               
               
                 isomers 
               
               
                 Propylene glycol propyl ether 
                 140-160 
                 48 
               
               
                 Tripropylene glycol 
                 273 
                 113 
               
               
                 Diethylene glycol monoethyl ether 
                 202 
                 96 
               
               
                   
               
             
          
         
       
     
         [0030]    While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed.