Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02872383 2014-11-26
PROCESS FOR DETERMINING THE INCOMPATIBILITY OF MIXTURES
CONTAINING HEAVY AND LIGHT CRUDES
Description
Technical field
The invention refers to a process for determining the
incompatibility in the heavy and light crudes mixture based on
measurement of dynamic viscosity of crudes mixture using an
electromagnetic viscometer at a constant force in a range of
io temperature of 463 K at room temperature (293 K) and pressure of 0.1
to 68.9 MPa.
Background of the invention
The study of petroleum analysis and its products would not e
complete without considering the incompatibility that generates
changes in its original properties, that is, during and after the blending
process, various secondary products can be formed such as sludge,
semi-solids or solid particles increasing mixture viscosity. The term
incompatibility refers to the formation of a precipitate (sludge,
sediment and deposition of material with asphaltene content) or
separation of phases when two liquids are blended (Speight 1999,
2004).
The phenomenon of incompatibility was firstly used by Martin
(1951) defining it as the tendency of the fuel oil to produce a deposit in
the dilution or blending with other fuel oils. Martin (1951) made a
difference between incompatibility and instability, defining this least as
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the tendency of a residual fuel to produce a deposit of asphaltic or
carbonic material during storage or heating process. The instability
during storage or during heating can be the result of the preparation of
the fuel from incompatible components or can be the result of aging.
The term instability is frequently used referring to color formation,
sediment or bubble gum in the liquid during a period of time; this term
can be used to differentiate the formation of a precipitate in short time
(almost immediately). Nevertheless, the terms incompatibility and
instability are used interchangeably (Speight, 1999).
io The
phenomena of incompatibility and instability of petroleum
and its products are invariably associated with the chemical
composition and physical ratio of the components. In the most of
cases, a certain component in one the crudes reacts with another
component in the crude with which is blended resulting in a chemical
reaction in the formation of a new product that, when it is soluble,
affects the mixture properties and when it is insoluble, it is deposited
as a semisolid or solid matter (Speight, 1999). Normally, the
incompatibility processes increase viscosity of petroleum and its
derivatives, inclusively at low temperatures also causes a change in
viscosity in certain fuels (Speight, 1999). Various studies demonstrated
that blending of different crudes can lead to flocculation/precipitation
of asphaltenes (Wiehe and Kennedy, 2000a, 2000b; van den Berg et
al., 2003; Schermer et al., 2004). This phenomenon, known as crudes
incompatibility, causes much more problems in the transportation and
refining process especially when the economical situation is obliging
many refineries to carry out low cost crudes blending to improve the
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refining margins (van den Berg et al., 2003).
Instead of various studies carried out in the last decades, there
are always important questions in the chemistry and physics knowledge
of the incompatibility phenomenon (Speight, 1999; Wiehe, 2012). It is
well know that this problematic did not lead to a standard method for
determination and quantification of crudes incompatibility. Based on
the above, there are various criteria for determining crudes
incompatibility in literature.
US Patent No. 4,85,337 refers to a blending procedure of liquid
io
hydrocarbons to control incompatibility mentioning that the paraffinic
and condensed liquids can be blended with the crudes meanwhile the
incompatibility of the asphaltenes is controlled; the incompatibility is
expressed as the relation between the aromatics and the content of
asphaltenes of the crudes or liquid hydrocarbon.
Escobedo and Mansoori (1995) show the determination of an
incipient point in asphaltenes flocculation by means of relative
viscosity measurement of a crude diluting it with a precipitating agent
(n-pentane, n-heptane, n-nonane). The phenomenon is graphically
shown with the increase of viscosity during the precipitation of the
crude observing a deviation of the initial behavior during asphaltenes
flocculation.
Asomaning (1997) shows the incompatibility phenomenon
associating it solubility as the mechanism for deposits formation.
Afterwards, Asomaning and Watkinson (2000) introduced an index of
simple colloidal instability (CII) based on the analysis of saturated,
aromatic compositions, resins and asphaltenes (SARA) of the crudes
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mixture, concluding that mixtures with CII>1 tend to be incompatible
and precipitate asphaltenes.
Buckley et al. (1998) associates the incipient point of
precipitation to asphaltenes with solubility that depends on the
refraction index (RI); in this work it is determined that the RI could be
used to predict the incipient point of asphaltene precipitation. See also
Buckley (1999) and Gimenez and Cabeza (2006).
US Patent No 5,871,634 refers to a method for blending two or
more petroleum feedstreams, petroleum process streams or
combination thereof, at least one of which includes the steps of
determining the insolubility number, (IN) for each feedstream,
determining the solubility blending number, SBA], for each feedstream
and combining the feedstreams in order of decreasing SBN number of
each feedstream such that the solubility blending number of the
mixture is greater than the insolubility number of any component of the
mix, when the solubility blending numer of any of the feedstreams or
streams is equal or less than the insolubility number of any of the
streams. See also the works of Wiehe and Kennedy (2000a, 2000b).
US Patent N. 5,997,723 refers to a process for blending crudes
with the purpose to avoid incrustation for crudes considering almost
incompatible). See also the works of Wiehe et al. (2001) and Wiehe
(2004).
Gharfeh et al. (2004) show an instrument for detection of diluted
crudes incompatibility with heptane at low temperatures and
atmospheric pressure. The measurement system consists in a titration
container, an infrared laser and a detector for measuring light
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=
transmittance through the container. Initially, the transmittance
increases and when it arrives to a flocculation point, it starts to
decrease, then, the maximum point achieves it considered as the
flocculation point of asphaltenes or the maximum dilution achieved.
US patent No. 7,029,570 refers to a process for determining
incompatibility in the crudes mixtures through the change in length
density of neutrons dispersion at the surface of asphaltene aggregates.
US patent No. US 7,618,822 B2 refers to crudes processing,
mixtures and fraction in refines and petrochemical plants to decrease
asphaltenes flocculation in the pre-heating train interchanger, ovens
and other units of the refining process.
Falkler and Sandu (2010) show a technique for providing
information on crudes stability on its mixtures detecting the incipient
point of the asphaltenes flocculation based on very small changes of
the composition of the mixtures using a source of transmission close to
the infrared. The equipment used in this word has a detection system
of solid able to measure the changes of intensity through the addition
of a precipitate (n-pentane). An inflexion point of a transmittance graph
based on the volume added of precipitate up to the start of flocculation
can be observed. The inflexion point is expressed as the stability index
of asphaltenes that corresponds to the precipitation point of
asphaltenes and provided a relative measurement of the stability in the
crude. Meanwhile, Sedghi and Goual (2010) determine the start of
asphaltenes precipitation through the direct current conductivity
technique in crudes diluted in toluene. Alvarez et al. (2012) use an
ellipsometry technique to evaluate the compatibility of crudes mixtures.
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Mexican Patent application No. MX/2011/003287, publication date
September 27, 2012 refers to a process for measuring dynamic
viscosity of live heavy crude (monophasic samples taken from the well
bottom) at a constant temperature and pressures from 68.9 MPa up to
the atmospheric pressure, including the dynamic viscosity in the bubble
pressure point and under this least; that is, removing the gaseous
phase and measuring liquid phase viscosity up to achieving
atmospheric pressure.
Summary of the invention
io It
is needless to say by the applicant that for effects of solving
the current invention, one aspect of blending heavy and light crudes is
to decrease viscosity of the heavy crudes and to find the optimum
concentration for maintaining the asphaltenes at a level under a
predetermined level, reducing the tendency of asphaltenes formation.
Furthermore, an incipient point of the asphaltenes incompatibility
threshold in the mixture is obtained that is a very important point to
avoid said formation of asphaltene aggregates at a certain
concentration of light crudes; the blending process comprises crudes
with asphaltenes content.
In this sense, we consider convenient to define that a dead crude
is the one that has a sufficiently low pressure, does not contain
dissolved gas or that crude that did not release its volatile
components.
Similarly, the API gravity is a measure of density that describes
how heavy or light the petroleum is compared with water. If the API
grades are more than 10, it is lighter than the water and therefore will
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float on it. The API gravity is also used to compare densities of
fractions extracted from petroleum. For example, is a petroleum
fraction floats on another; it means that it is lighter and therefore its
API gravity is bigger. Mathematically, the API gravity does not have
units; nevertheless, always this number has the name of API grade.
The API gravity is measured with an instrument denominated
densimeter; there are a great variety of these instruments. The API of
crudes oils, generally are in an interval of 47 (for lighter crudes) to
100 (for heavier crudes). Based on this parameter it is possible to
classify the crudes in: extra heavy ( API<10), heavy (10.1< API<22.3),
mean (22.4< API<31.1), light (31.2< API<38.9) and extra light
(39.0< API). It is important to mention that this classification can vary
depending on the considered source.
The applicant wants to mention that the room temperature is the
one where the laboratory is installed without having an external control
of the same, wherein any measurement is carried out, i.e. depending of
the place where it can be found; therefore, subsequently during the
description of the current invention, the room temperature will be
considered between 293.2 and 298.2 K.
By the above, up to now, there is no any reliable technique
destined to determine the crudes mixture incompatibility. Therefore,
the current invention markedly overcomes the previous mentioned
references, since it allows knowing the ratios of the blending wherein
the incompatibility of light and heavy crudes can occur through the
determination of dynamic viscosity.
Therefore, another aspect of the present invention is to provide a
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process for measuring the dynamic viscosity of heavy and light crudes
mixtures through an apparatus containing a sensor based on a novelty
technique; constant electromagnetic force.
In this regard, the apparatus mentioned uses a piston, calibrated
in a determined interval of viscosities, that is submerges in a crudes
mixture to analyze. The piston displacement is hampered by the
viscous pull of the fluid, a characteristic that is used to obtain an exact
measurement of absolute viscosity. The time required for the piston to
go over a given distance is related to the dynamic viscosity of the fluid
confined in a measurement chamber, therefore, as the fluid in the
chamber is more viscous, the piston displacement will be slower.
Another aspect of the current invention is to contribute with a
novel process for determining the cruds mixture incompatibility, due to
the fact that it is possible to measure the dynamic viscosity of said
mixtures at different temperature and pressure conditions.
Brief description of the figures
The figures that accompany the current invention for having a
better understanding of the indicated aspects, without limiting is scope,
are shown below:
Figure 1 shows a cross section of the electromagnetic viscometer
1 used in the current invention.
Figure 2 shows a schematic diagram for measurement of dynamic
viscosity of reservoir fluids.
Figure 3 shows (in logarithmic coordinates in axis y) the typical
behavior of the viscosity of a dead crude based on constant
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temperature and pressure; the viscosity decreases when the
temperature increases. Generally, the laboratory studies measure the
behavior of the dead crude viscosity based on the room temperature up
to the reservoir temperature.
Figure 4 refers to the results of dynamic viscosity obtained for the
mixture {light crude ¨ toluene + n-heptane} (in logarithmic
coordinates), one aspect of the present invention, adding n-heptane in
excess at a constant temperature and 0.1 MPa. In literature, this
behavior of slope change or inflexion point in the viscosity based on
the volume of precipitating agent refers to, as the incipient point of
asphaltenes flocculation.
Figure 5 shows (in logarithmic coordinates) three slopes of
dynamic viscosity (at temperatures of 308.1 K, 313.2 K and 318.4 K
and a constant pressure of 0.1 MPa) for the system {light crude -
toluene + n-heptane}.
Figure 6 shows the results obtained from viscosities of a heavy
and light crude mixture (including viscosities of heavy and light crude),
another aspect of the present invention, at 332.2K and pressure of 0.1
MPa.
Figure 7 shows (in logarithmic scale) dynamic viscosities of the
mixtures {light crude + heavy crude} vs. volume percentage of light
crude added, at different temperatures and pressure of 0.1 MPa.
Detailed description of the invention
The current invention refers to a process based in the
measurement of dynamic viscosities of crudes mixtures using a
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constant force electromagnetic viscometer to determine the
incompatibility of heavy and light crudes in the interval of temperature
of 463 K at room temperature and pressures of 0.1 to 68.9 MPa.
Blending comprises cruds with asphaltene content. The blending
process included a procedure to determine the incipient point of the
asphaltenes incompatibility threshold in the mixture. The apparatus
used in the current invention is particularly easy from the mechanical
point of view and the principle based for determining that viscosity is
effective (Patent document US 6,584,831 B1; US 5,025,656, US
4,864,849, US 4,627,272, MX/2011/003287A). The apparatus is
precise, reliable, and easy to use and with maintenance without any
difficulty. It is based on a simple electromagnetic and reliable principle
that uses only a mobile element (piston containing ferromagnetic
material), at a constant force, submerged in the fluid to be analyzed.
The time required for the piston to go over a given distance is related
exactly to the dynamic viscosity of the fluid confined in a measurement
chamber (Figure 1).
The Applicant would like to repeat that one aspect of this
invention is to measure the viscosity of a heavy crude diluted with a
precipitating agent is to precisely know which concentration of
precipitate has the asphaltenes formation, known as the incipient point
or star of the asphaltenes precipitation to avoid formation or
aggregation of asphaltenes in an industrial application or process in
the field.
Figure 1 shows a cross section of the electromagnetic viscometer
1 used in the current invention. The apparatus consist of a
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measurement chamber 1 where piston 4 is located that makes a voyage
of ahead and return through the alternative driving of electromagnetic
coils 3 (A and B). One of the coils is placed in order for the magnetic
filed occurred, when the current flows through it, to tend to drag the
piston in a direction along the channel. The second coil is placed to
frag the coil along the channel in an opposite direction.
This method is based in viscosity measurements of a crude that is
diluted with a precipitating agent (i.e. n-pentane, n-heptane, etc.) at a
defined concentration (interval of 0.01 to 99.9% volume), with a volume
of 25 mL of sample, viscosities in the interval of 0.2 to 10000 cP, at a
temperature up to 463 K and pressures of 0.1 to 68.9 MPa. The
incipient point of flocculation/precipitation of asphaltenes is detected
for a remarkable increase of viscosity (relative) of the suspension
where the asphaltene particles aggregation occurs (Escobedo and
Mansoori, 1995).
It can be observed in figure 4 that the behavior of the mixture
dynamic viscosity {light crude ¨ toluene + n-heptane} shows (in
logarithmic scale) a decrease when it is added to the precipitating
agent; the value of viscosity has a minimum value fro precipitate
volume fraction in the interval of 2-35%, After this minimum value, the
dynamic viscosity of the mixture increases in
the
flocculation/precipitation threshold up to obtaining a maximum value in
this region.
Based on the above description, it can be concluded that the
incipient point of flocculation/precipitation of asphaltenes corresponds
to the immediately previous point when viscosity increases, that is, the
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35% volume of n-heptane in the mixture.
It can be observed in figure 5 that the incipient point of
asphaltenes flocculation/precipitation coincides in the same relation
(35% of volume) of n-heptane in the mixture at three different
temperatures, i.e. 308.1 K, 313.2 K and 318.4 K and a constant
pressure od 0.1 MPa.
Figure 6 shows the dynamic viscosities measured at 333. K in
various mixtures {light crude + heavy crude} between 0 and 100%
volume of light crude. At low volume fractions of the added light crude
(up to ¨ 15%), viscosity gradually decreases and the has a
considerable increase of viscosity when the volume fractions of light
crude exceed 26.6&; this increase of viscosity achieves a maximum
value and then the viscosity of the mixture gradually decreases.
Based on the previously described criteria and that is another
aspect of the current invention to prescribe, the incipient point of
asphaltenes flocculation/precipitation or the incipient point of
flocculation threshold (or in this case, the incipient point of
asphaltenes incompatibility threshold) was determined in 26.6% of light
crude in the mixture.
It can be seen in Figure 7 (in logarithmic scale) that the dynamic
viscosity slopes vs. volume fraction of light crudes showed the same
behavior at different temperatures, that is, firstly a decrease of
viscosity until obtaining a minimum value afterwards an important
increase in the asphaltenes incompatibility threshold. This figure shows
the incipient point of the asphaltene incompatibility measured or
mixtures of these two crudes (heavy and light) at four different
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temperatures and 0.1 MPa. In the temperature range of 293.2-353.2 K,
the incipient point starts when the fraction of light crude in the mixture
is of 26.7%. The shadowed area in figure 7 shows the incompatibility
threshold for each temperature.
It is evident that the asphaltenes are maintained in the crude in a
delicate balance (Speight, 1999) and this balance can be easily
disturbed by the addition of saturated and removal of resins and
aromatic (Wiehe and Kennedy, 2000b; Wiehe, 2012); therefore, the
crudes blending can greatly change the global concentration of these
molecular types altering this balance and flocculation/precipitating the
asphaltenes.
The following example is shown to illustrate the operation of
the best novel process known by the applicant for the determination of
heavy and light crudes mixtures incompatibility. Of course, it must not
be considered as a limitation of what is claimed herein but that it only
describes the method through which the operation process, reason of
the current invention is described:
EXAM PLE
Before including an example, it is important to mention that in
order to guarantee that our determination of dynamic viscosities are
reliable, we previously calibrated the piston to be used, as well as the
pressure transducer and system temperature gauge. The calibration
and verification of the piston was carried out with S20, N4, S6
standards) provided by Cannon Instrument Company, ASTM S2162)
and it involves the measurement of a standard fluid that can be
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identified at a stable temperature and that adjust the calibration
parameters related to the selected piston to reproduce dynamic
viscosity (with a mean absolute deviation of +1.0%) corresponding to
the known value of said calibration standards to the established
temperature.
The following examples show the operation of the process and
apparatus described herein to determine the incompatibility in the
crudes blending (heavy, light) in an interval of temperature of 463 K
and room temperature and pressure of 0.1 MPa (See Figure 2).
Figure 2 shows a schematic diagram for measurement of
dynamic viscosity of reservoir fluids. A small volume quantity of sample
contained in the high-pressure stainless steel container is requested to
carry out viscosity measurement at different temperature and pressure
conditions. The temperature inside the measurement chamber is
measured with a temperature gauge 17 connected to a digital indicator
23. A pressure transducer together with a digital indicator 16 is
connected to viscometer 18 to monitor the pressure in the
measurement system. A serial interface RS-232 allows the
communication of viscometer 18 with a computer. In order to generate
pressure in the system, a positive displacement pump 1 is used,
meanwhile for temperature generation in the apparatus; a recirculating
bath 22 is used (MX/2011/003287A).
Step A. Mixtures of heavy and light crudes of 0%, 25-35%, 40-
50%, 60-75% and 100% volume of light crude were prepared.
Step B. The heavy and light crude mixture 1 is loaded in a high-
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pressure stainless steel container 10 and connected to the
measurement circuit through the valves 7 and 11. The high-pressure
stainless steel container 10 contains a high-pressure stainless steel
piston 9 inside that freely floats through the stainless steel container
10 separating the mixture 8 of the pressurization fluid 4. In order to
maintain a homogeneous temperature in the measuring system, the
high-pressure stainless steel container 10 is heated with a heating
resistance. The stainless steel pipelines that integrate the measuring
circuit are also heated with heating tapes.
Step C. The temperature in the system is established through the
recirculating bath 22. The temperature in the apparatus is measured by
a temperature detector 17 that is connected to a digital indicator 23.
The pressure in the system is generated and controlled by a positive
displacement pump 1 that used a mineral oil 4 as pressurization fluid.
The pressure in the system is monitored by a pressure transducer
connected to pressure digital indicator 16. When the temperature in the
apparatus 18 is close to the measurement temperature, the apparatus
18 is vertically placed and is connected to a vacuum pump 15 by the
valve 14. The valves 12, 13, 14 and 21 must be open during the
vacuum process; meanwhile the valves 11 and 19 must be maintained
closed. The measuring circuit is emptied up to obtaining an appropriate
vacuum (generally, after 20 minutes approximately), close the valves
14, 12, 13 and 21. Establish the required pressure in the positive
displacement pump 1 and open slowly the valves 3, 7, 11, 12, 13 and
21. The valve 2 must be maintained closed; meanwhile the valve 5
must be open. In order to ensure that the system was filled with the
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mixture, purge a small volume quantity for the valves 14 and 19. Close
slowly the valve 21 and place the apparatus 20 in measuring position
(45 C).
Step D. When the mixture is stabilized at a temperature and
pressure of 0.1 MPa, the values of dynamic viscosity and measuring
temperature were recorded. Afterwards, temperature in the system is
increased through a recirculating bath 22; when the analysis
temperature is newly stabilized, the viscosity values for temperature
and pressure of 0.1 MPa are registered. Repeat Step D up to the
io temperature of 463 K or any other temperature.
Step E. Monitor the behavior of the mixture viscosity based on the
light crude added at a constant temperature for experimental
determination of the incipient point of asphaltenes incompatibility
threshold in crudes mixture through the mixture viscosity behavior
based on the light crude added at a constant temperature; i.e. through
the graphic observation of the slope change of behavior vs. light crude
added (%).
Step F. If the behavior of the viscosity vs. percentage of volume
added of light crude, is not the typical behavior shown in Figure 3,
mixtures of heavy and light crudes are prepared with volume
percentages of light crude less than the inflexion point found in Step E
and steps B, C, D and E are repeated.
Step G. Step F is repeated until the slope change in the dynamic
viscosity behavior of the mixture corresponds to the minimum
percentage of added light crude (as observed in Figure 6); the
immediately previous point to the slope change (or inflexion point) of
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viscosity is considered as the incipient point of the asphaltenes
incompatibility threshold.
Based on the above described, we want to emphasize that it is
well known that the asphaltenes is a crude fraction that can be
precipitated when it is blended with non-polar hydrocarbon (n-pentane,
n-heptane) or when two or more crudes are blended, causing
tamponades in the pipelines, tanks, heat interchanger, etc.
The present invention consists of determining the concentration
when the precipitation starts, that is, when they are incompatible. The
to incompatibility can be determined from the viscosity measurement
based on the concentration, at a given temperature that is an aspect of
the current invention.
The viscosity results shown in Figure 6 clearly indicate where
incompatibility occurs in a mixture of heavy and light crude showing the
inflexion point in the graphic.
It is important to have an adequate mixing due to the fact that
small quantities of flocculated material loose energy due to little
transfer of heat. Moderated quantities of asphaltenes causes pressure
drops ad interfere in the equipment operation, provoking an inefficient
process. At last, big quantities of asphaltenes cause intolerable
tamponades and cause the process to stop until the pipelines are
clean. That is why it is important to find the incompatibility point (the
optimum mixture) to avoid the above-mentioned damages.
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