APPLICATION OF VISBREAKER ANALYSIS TOOLS TO OPTIMIZE PERFORMANCE

CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Patent Application Serial No. 11/456 128 filed July 7  2006  now allowed  which was  in turn  a continuation-in-part of U.S. Patent Application Serial No. 11/178 846 filed July 11  2005 now U.S. Patent 7 394 545.
FIELD OF THE INVENTION
The present invention relates to systems and methods for characterizing and quantifying a dispersive medium; specifically  measuring the concentration of particles or the tendency toward forming a dispersed phase within a fluid sample. The present invention also provides a program which uses these measurements of concentration to monitor and control operation of a process unit in a refinery.
BACKGROUND OF THE INVENTION
Thermal conversion is a process in which  by the application of heat  large hydrocarbon molecules are broken into smaller molecules with a lower boiling point. These operations are carried out in the industry of crude oil refining by plants such as a visbreaker  coker  and hydrocracker for obtaining intermediate or light cuts of higher value  from heavy residues of lower commercial value. As is evidenced by EP Pat. No. 768363  the term “process unit” can also be used in the place of “plant”. The thermal cracking applied in the visbreaker process will also reduce the viscosity and pour point of the heavy residues.
It is well known that the fouling potential of a fluid can be estimated and characterized by the concentration of the dispersed phase  particularly by the concentration of the dispersed phase present in a specific size range. In hydrocarbon systems in particular  it has been recognized that the concentration of asphaltenes (i.e.  carbon particles or opaque species) with linear dimension greater than about 2 microns in visbroken tars is a good indication of the fouling potential of the material.
The VSB process was developed some years ago with the intention of obtaining a viscosity decrease in heavy products in order to reduce the amount of higher valued flux to meet the viscosity specification of the finished heavy fuel product. Today  however  it is managed with substantially different objects  namely with the aim of obtaining a maximum transformation into middle and light distillates to meet the market requirements.
The controlling factor in obtaining a high conversion is the need to obtain a stable residue. In fact an increase of the cracking temperature certainly would involve a higher conversion in light and middle distillates  but it would produce a much more instable tar which would produce a final product outside the required stability specifications.
An increase of the light streams is achieved by increasing the cracking severity through an increase of the outlet furnace temperature of the Visbreaker furnace. While increasing this temperature arbitrarily will serve to drive the conversion rate higher  it also comes at the cost of producing a highly unstable tar as a precipitate in the process  with a high concentration of asphaltene particulates. This particulate matter constitutes a severe fouling threat to the energy recovery devices (i.e.  furnace and heat exchangers) in the process. As such  in order to maximize the profitability a Visbreaker unit  it is desirable to optimize the outlet furnace temperature while maintaining the stability of the produced tar. While it is known that high temperature dispersants and anti-foulants can be introduced into the system to reduce the tendency and rate of fouling  prior art systems have not been entirely satisfactory in providing an automated system for determining an optimum type and/or quantity of chemical dispersants and anti-foulants to be introduced into the visbreaker unit in order to maximize plant profitability. The present teachings will show that if the fouling potential of the tar can be quantified  then the precise level of chemical inhibitor can be dosed to maximize the plant profitability.
It is also known that the fouling tendency of a hydrocarbon feedstock  or blends thereof  can be related to the tendency for insoluble organic materials to precipitate in heat transfer equipment  or other equipment for a refinery process unit.
Therefore  in one aspect the present invention provides a simplified  automated system and method that can easily be used to carry out optical analysis of visbroken tars and other fluid samples in order to characterize and quantify the concentration of particles within the fluid sample with high accuracy and reproducibility. In another aspect  the present invention utilizes these concentration measurements to determine the fouling potential of the visbroken tars  and regulates the introduction of chemical inhibitors into the visbreaker unit to improve the yield of light streams. In yet another or further aspect  a sequence of aliquots are prepared from the same sample at different dilutions to drive phase separation  producing a sequence of concentration measurements correlated to a classical measurement of peptization value (PV)  a qualitative measure of the product quality. In yet another or further aspect  this invention utilizes concentration data to estimate the fouling potential of a feedstock  relates the concentration data to the fouling potential of the feedstock  and provides optimal dosage of chemical treatment to reduce the rate of fouling. These and other aspects of the present invention will become apparent to those skilled in the art upon review of the following disclosure.
SUMMARY OF THE INVENTION
In one aspect  the present invention provides a system and method for estimating a concentration of inhomogeneities contained within a tar byproduct of visbreaker operations. The invention does so by measuring the modulation of transmitted light through a fluid sample. The system uses a strongly convergent optical lens system to focus light onto a prepared sample. In one exemplary embodiment  the optics of a conventional optical microscope are used. A 3-dimensional translation stage is installed downstream of the focusing optics so that the sample can be scanned over a large region  and at a specific focal plane. A photo detector is placed on the opposite side of the stage from the focusing optics to measure the transmitted light through the sample. The photodetector is read-out by an analog-to-digital converter (ADC) in order to provide a digital (i.e.  quantitative) measure of the transmitted light intensity. The translation stages are then moved in a pattern  such that the intensity of the transmitted light is measured over a representative path across the sample. When an opacity  scatterer or opaque particle of a threshold size is encountered in the sample  the intensity of the transmitted light is strongly attenuated. Such change of light intensity is then correlated with the detection of an opaque particle in order to characterize and quantify the concentration of particles within the fluid sample with high accuracy and reproducibility. Data processing algorithms are implemented to determine the background noise level associated with the acquired data and to set a threshold level. As such  a specific signal-to-noise ratio can be specified to define when a detection event is registered. Size discrimination may be achieved according to the physical dimensions of the beam waist of the focused light beam.
In another aspect  the present invention utilizes the concentration measurement data to estimate the fouling potential of visbroken tars in a visbreaker unit in order to regulate introduction of chemical inhibitors into the visbreaker unit and improve the yield of light streams. The invention provides an automated program which allows the user to maximize the production of light streams by modeling the correlation between operational parameters such as feed quality  cracking severity  conversion rate  run length  and fouling rate of the subject exchanger or furnace in order to regulate introduction of chemical inhibitors into the visbreaker unit in accordance with customer specifications and/or production requirements.
In another aspect  the present invention utilizes a method for controlling and optimizing chemical injection into a process unit to control fouling. The method uses an optical device to measure the fouling propensity of the process fluid at various points within the process unit. The measurements are compared with one another and prediction methods are used to evaluate the fouling potential within the unit  and determine the proper chemical dosage. Antifoulant chemical is then introduced into the unit to control the rate of fouling. The method and application continue on a frequent basis to maintain optimal fouling control within the unit.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a view of the scanning apparatus of the present invention  showing the schematic relationship of the various elements;
Fig. 2 illustrates an example of a computer screen displaying a data acquisition interface in accordance with the present invention;
Fig. 3 is a diagram illustrating optics used to convergently focus a light beam to a narrow beam waist;
Fig. 4 illustrates a plurality of spaced apart linear scans compared with a solid block representing an equivalent effective surface area;
Fig. 5 is a graph illustrating raw light transmission data obtained over a single line scan;
Fig. 6 is a graph illustrating the raw data of Fig. 5 after the data has been filtered to remove line noise and gross intensity variations;
Fig. 7 is a graph illustrating decreasing statistical error as a function of overall scan length;
Fig 8 is a graph showing the correlation of sample inhomogeneity  as measured by the instrument to samples with a varying degree of dilution from a fully cracked (i.e.  high asphaltene particle density) sample;
Fig 9 is a schematic of the mechanics of the chemical effect of the dispersants;
Fig 10 is a graph of the relation of PV to the Furnace Outlet Temperature (FOT) with and without chemical treatment;
Fig. 11 illustrates tar stability and conversion as asphaltenes are disbursed in the continuous phase through the peptizing action of aromatics and resins;
Fig. 12 is a graph illustrating raw data obtained from a visbreaker conversion trial;
Figs. 13-16 are graphs illustrating raw data obtained from a conversion enhancement application;
Fig. 17 is a graph illustrating VFM data versus corr. skin temperature;
Fig. 18 is a schematic diagram illustrating exemplary visbreaker process types;
Figs. 19A  19B illustrate Pv measurement with a measurement system of the present invention;
Fig. 20 is a table illustrating VFM particle count data for a particularly asphaltenic crude oil (Crude A) blended at several ratios with a standard crude slate (Crude B);
Fig. 21 is a graph illustrating VFM particle count data for a particularly asphaltenic crude oil (Crude A) blended at several ratios with a standard crude slate (Crude B);
Fig. 22 is a graph illustration actual furnace inlet temperature declines due to fouling as measured in the field;
Fig. 23 is a graph illustrating VFM particle count data for a particularly asphaltenic crude (Crude D) blended into another standard crude slate (Crude E); and
Fig. 24 is a schematic diagram illustrating how the VFM device can be used to control fouling in a process unit preheat exchanger train.

DETAILED DESCRIPTION OF THE INVENTION
Exemplary embodiments and examples describing the present invention will be described below with reference to the accompanying drawings. As shown in Fig. 1  this invention uses an optical system as generally indicated by the number 10  which in the present exemplary embodiment comprises a convergent lens  a light source 12  and a multi-axis translation stage 14. The light source 12 may be implemented  for example  in the form of a solid state visible laser. An infra-red (IR) laser may also be used  and is in some cases preferable owing to the fact that HC solutions are typically much more transparent to IR light  than visible light. The translation stage 14 may be moved horizontally in the x and y directions in response to control signals generated by an associated computer 20 to direct the light beam along a plurality of paths through the sample. The third axis moves the stage vertically  towards and away from the focusing lens. This permits selection of a focal plane within the sample. In another exemplary embodiment  the present invention contemplates providing means for moving the light source 12 with respect to the sample  thereby allowing the light beam to be directed through the sample to achieve the same results. Moreover  the present invention also contemplates usage of a flow cell to receive a flow of sample fluid  wherein the sample fluid flows through the flow cell while the light beam is directed through a portion of the flowing sample. Also implemented is a photodetector 16  for example  a PIN photodiode  located on the opposite side of the stage 14 to detect light 13 being transmitted through the sample volume  which is located on the translation stage. The photodetector 16  in turn  is connected by a connector and cable 17  for example  a twisted pair with BNC connector  to an analog-to-digital (A/D) converter 18 to quantify the transmitted light intensity. As described below  this is done to sample or detect the occurrence of inhomogeneities in light transmission which may be caused by mineral and other inclusions  and agglomerating or stable localized dark matter of various types.
In one exemplary embodiment of the invention  a colloidal fluid sample material of thick viscous tar sampled from a Visbreaker is placed on the translation stage 14. Depending on the conditions in the Visbreaker unit  the sample may or may not contain asphaltene (or carbon based) particles. The asphaltene particles within the tar medium are opaque to visible light. The tar medium is also opaque to visible light when the path length through the medium typically exceeds a linear dimension of about 1 cm. A sample volume is dispensed on a slide  or flow cell 15 such that a typical sample thickness of 10-20 microns is produced. As such  the thickness of sample medium should be made thin enough so as to provide a differential transparency between the viscous tar medium and the asphaltene particles in question. In this exemplary embodiment  in order to optimize light transmission from a low power light source  a solid state laser that produces radiation at about 633 nm is chosen. This provides adequate power at a suitable region in the EM (electromagnetic) spectrum to provide transmission through a thin layer of tar  while the carbide particles remain opaque.
In order to have sensitivity to the specific sized inhomogeneities  appropriate optics should be used to focus the laser light onto the sample. The choice of a monochromatic light source allows the design of the optics to be optimized. As shown in Fig. 3  a highly convergent lens system 200 is used to focus the light beam 100 down to a beam waist of approximately 1 micron. The size of the beam waist determines the minimum cross-dimension an inhomogeneity must have to fully attenuate the laser light. If an inhomogeneity is smaller than 1 micron  it will still allow the transmission of light. As such  the focusing optics define  in part  a threshold size for inhomogeneity detection. An equation for calculating the beam waist is as follows:
W = 0.61? / d
Where W = beam waist (1/e) width
? = wavelength of light
d = numerical aperture
For example  if ? = 633 nm and d = 0.56  then W = 0.7 µm.
Since we are interested in inhomogeneities larger than 1 micron (and smaller than ~20 microns)  we do not use an IR laser  even though the HC solutions are more transparent to IR radiation because the beam waist would increase in size for the given optics. As such  we would reduce the sensitivity of the instrument. Preferably  the wavelength and beam waist are also chosen to minimize interference artifacts that may arise as the concentration of dispersed phase increases or the sample thickness varies (e.g.  under a cover slide.)
The fluid sample 120 thickness is chosen to be about 10 microns. The beam 100 is focused on the slide 104  below a cover slip 102  or a flow cell in the sample volume. The depth and width of focus are constrained by the optical system and the selected light wavelength. In one exemplary embodiment  both dimensions are selected to be approximately 1 micron.
Fig. 2 illustrates an example of a screen display presented by the software of the present invention. The screen display illustrated in Fig. 2 represents a data acquisition interface allowing the operator to specify a variety of scanning acquisition  analysis parameters  operating conditions of the instrument  and results of the measurement. The methods by which the operator selects items  inputs data  and otherwise interacts with the data acquisition interface are conventional  and further discussions of these operations are not provided herein. In an exemplary embodiment of the invention  data acquisition software was implemented via Visual Basic® in Excel® with analysis and signal processing code being implemented in GNU Octave  although those skilled in the art of software programming will appreciate that many other software programming means may be used to achieve the same results.
A testing plan was designed and implemented to validate and measure the scanning performance of an exemplary embodiment of the present invention. In particular  measurement repeatability is validated by analyzing the variation between identical measurements. Reproducibility of the data is examined by analyzing the effects of scanning different regions in the sample. This is complicated by the effects of sample inhomogeneity. Accuracy of the system is tested by comparing the scanning data with visual images and PV (PV = peptization value) of the sample. Precision of results is analyzed for statistical uncertainty with path length and by optimizing sample area  as discussed in more detail below.
Fig. 4 illustrates an example of how the scanning system samples a large region of the sample. The array of linear scans (shown on the right side of Fig. 4) represent the same effective surface area as the small box illustrated on the left side of Fig. 4. For example  an array of 20 linear scans of 15 mm length with a 1 micron wide laser beam effectively samples the same area as does the smaller 0.48 mm x 0.64 mm box. However  by arranging the sampling path to extend over a larger region of the sample  the effects of sample inhomogeneity  local fluctuations in the sample  and sample variation are reduced drastically. As such  the statistical results are much more accurate and reproducible.
To demonstrate the repeatability of our scanning results  five identical 15 mm scans from a same sample  each covering a 0.015 mm2 effective area were measured. The measurement showed that the number of counts per 15 mm line scan were identical within 95% confidence limits. Increasing the sampling region to 20-15 mm scan paths  the same systematic effects were seen. After applying statistical analysis to the results  it was observed that the single line scan measurements are normally distributed  with a standard deviation (s)=1.6 counts on a mean of 8.0 counts. Furthermore  the total integral count of the sample was 159 with a standard deviation of 9 counts. This shows that both the mean inhomogeneity count per path  and the total integral inhomogeneity count were statistically identical and repeatable  over the separate trials  thus demonstrating that instrument stability and repeatability is excellent. It also demonstrates that the fractional error can be reduced by increasing the sampling length. This is due to the fact that independent errors do not add linearly  but in quadrature.
As can be noted from the above data  the system of the present invention is capable of minimizing and quantifying the effects of sample inhomogeneity.
Turning now to Fig. 5  there is shown a graph representing exemplary raw data obtained from a single line scan of 15 mm length taken during a 10 second acquisition window.
In Fig. 6  the raw data of Fig. 5 is processed by a Fourier filtering to remove 50/60 Hz line noise and a median filter is used to remove gross intensity variations to extract the number of counts above a threshold value. This process may be repeated for all line scans (e.g.  20 line scans) to calculate the total homogeneity areal density of the sample under test. In one example  the number of peak counts from a single line scan is calculated as

?1 = (9 ± 3) ÷ (15 mm x 1 µm) = 600 ± 200 mm-2
Repeating this calculation for a measurement spanning over 20 paths  the error decreases as shown below:
?tot = (149 ± 12) ÷ (20 x 15 mm x 1 µm) = 497 ± 40 mm-2
We see that the error decreases according to Gaussian statistics where the error propagates in quadrature which is a well known statistical property.
As shown in Fig. 7  an approximate 5% uncertainty is achieved at 10 line scans of 15 mm length (i.e.  0.15 mm2 effective area). Statistical error is thus shown to decrease with N-0.6  where N is the number of 15 mm path length multiples. From the exemplary data of Fig. 7  it is shown that an overall path length of about 150 mm (10 x 15 mm) would achieve an approximate 5% error.
In order to determine the background noise in the signal as in Figs. 5 and 6  the present invention provides a software algorithm  which automatically computes the background noise and sets a discriminator level or threshold for registering a sample inhomogeneity. A measurement of the light transmission is made when no scanning is occurring. Thus  the signal is an estimate of the nominal noise. Calculating the standard deviation of this signal distribution allows the estimate. The value can be used to determine a fixed signal-to-noise ratio on which to accept inhomogeneities.
In accordance with the present invention  the instrument is capable of quantifying the inhomogeneity of a solution in an automated and timely fashion.
To demonstrate the capabilities of the present invention  the following sample specimens  with various concentrations of asphaltenes were used for analysis and validation:
Specimen A: 9630 Asls  PV = 1.7  low particulate density (highly diluted).
Specimen B: 9630-6  PV = 1.4  intermediate particulate density (partially diluted).
Specimen C: 9630-7  PV < 1.0  high particulate density  heavily cracked sample (slightly diluted).
Specimen D: 9630-mod  13% 9630-7 + 9630 Asls  PV = about 1.35 (partially diluted).
The scanning results from these samples were then compared to photographs of the samples  and a correlation was found between the images and the scanned results. A graph showing the correlation of particle density as measured by the instrument to samples with a varying degree of dilution from a fully cracked (i.e.  high asphaltene density) is shown in Fig. 8.
Overall  the testing results demonstrate that the system of the present invention provides good repeatability and shows correlation with visual image views. It has been shown that a relatively large sample area may be covered with automated operation  thus reducing the effects of local fluctuations in inhomogeneity density. Data can also be assigned an error to quantify precision of results.
We also disclose a program to monitor and control the operation of a Visbreaker unit in a hydrocarbon processing facility (refinery). The program allows the user to maximize the production of light streams (i.e.  usually diesel) while maintaining a highly stable residual tar and reducing the chance that rundown of the tar will foul the preheat heat exchangers.
It is known that the stability of residual visbroken tar and its fouling potential can be measured by the peptization value (PV) and the hot filterable solids (HFT). Note that HFT and PV are two different metrics as HFT is a product specification whereas PV is a characterization of the visbroken tars towards the asphaltene precipitation potential. The optical measurement device (referred to hereinafter as ‘VFM’) of the present invention measures a quantity which is a measure of the opaque filterable solids within a tar sample. The automated program of the present invention utilizes the VFM concentration measurement data to estimate the fouling potential of the visbroken tars. This estimate in turn is used to gauge the needs for optimum feed of chemical treatments.
It is known that high temperature dispersants and anti-foulants are the main components in a chemical regiment used to treat Visbreakers. There are specific chemical families that are particularly effective for use in the Visbreaker for reducing fouling of heat exchanging surfaces (i.e.  exchanger  furnace  etc.) and subsequently stabilizing the produced visbroken tar. The program of the present invention is configured to select the type and quantity of chemistry required to satisfy production requirements. Specific chemical entities include  but are not limited to polyisobutenylphosphonic acids and esters  polyisobutenylthiophosphonic acids and esters  alkylphosphonate phenate sulfides and disulfides that may be neutralized with alkaline earth metals or amines polyisobutenyl succinimides  polyisobutenylsuccinate alkyl esters  magnesium or calcium salts of alkyl or dialkylnaphthelene sulfonic acids as described in U.S. Patent No. 4 927 519 and EP Patent No. 321424B1.
These antifoulant materials have been found to function at low dosages  1-200 ppm  to prevent the undesirable deposition or fouling of surfaces in visbreakers  as well as prevent the carboneceous deposition in visbroken heavy oil products (tar). Fouling in heat exchangers is most generally thought to occur by first generating an unstabilized macromolecular particle that is no longer dissolved in the fluid  or is no longer a stable colloidal species. This occurs due to the thermal stress on the hydrocarbon. Initial deposition occurs  and further destabilized species adsorb onto the site of original deposition. Bigger particles in the hydrocarbon will be more prone to contact and coalesce to the surface. Dehydrogenation of the adsorbed hydrocarbon will be driven by heat and make the deposit more tenacious as crosslinking reactions occur.
The dispersants are generally understood to function by a variety of mechanisms. First  the dispersant materials adsorb to the surfaces of growing insoluble particles and act to keep these particles small; typically less than 1 micron. Thus  the particles are more prone to continue to flow through the system and not settle on heat exchanger or other surfaces. This can be described by Stokes law  which is dependent on the radius of the particles. This is schematically shown in Fig. 9. The dispersants act by a combination of steric stabilization  which acts to repel approaching particles (dramatically increase entropy of local system and drive solvent in between particles)  and blocking of polar sites on the particles which act as a driving force for coalescence. Light scattering evidence exists that shows that dispersant treated thermally stressed fluids generate particles that are up to two orders of magnitude smaller than untreated hydrocarbon fluids.
Even if the particles are not small  the above mechanism explains how the particles will be less prone to coalesce to other particles in solution  or to material already deposited on the surface.
It has also been shown that the nature of the surface plays a role in the ability of thermally stressed fluids to deposit. Metal surfaces with higher roughness  edges  or polarity are more prone to fouling. These dispersants will adsorb to such surfaces and discourage particulate or amorphous insoluble hydrocarbon from sticking to the surface.
The reaction of hydrocarbons at elevated temperatures with oxygen (even very low levels such as <5 ppm) will result in formation of polar functionalities that can drive coalescence of particulate  as well as accelerate the dehydrogenation of adsorbed hydrocarbon  which makes its removal from the surface by turbulent flow much less likely. Dispersant adsorption will block the mass transfer of the oxygen to the surface  and some of these described anti-foulants have antioxidant abilities by interfering with radical reactions.
In addition  the visbroken tar is generally believed to be colloidal in nature  with more highly polar and higher molecular weight asphaltene species being stabilized in the fluid by smaller resin molecules. As the thermal stress disturbs the relationship of the adsorbed resins to asphaltenes  and by driving the conversion of resins to asphaltenes  and by making the asphaltenes more polar  these systems can be described as being more "unstable" or prone to deposition. The dispersants described here are believed to replace the disturbed or destroyed resins and re-stabilize the asphaltene system.
As described herein  the VFM measurement data gives information on the solids content in the residue (tar). Higher amounts of solids will give a higher precipitation potential. The solids might be introduced into the system by the feed (poor feed quality) and/or through the cracking process. The higher the cracking severity the higher the solids content in the residue likely will be.
Based on defining a baseline  which is unit dependent  the VFM data provides information in increasing response to decreasing solids content in the tar. Depending on the main cause of the solids increase (feed or cracking severity) the device can help to optimize the chemical injection rate (if solids are from feed or severity want to be maintained) in order to maintain the fouling rate and thus keeping unit run-length under control. If solids increase is due to cracking severity only  the VFM measurement provides an early warning to potential instability of the tar and cracking severity can be reduced by decreasing the furnace outlet temperature (FOT).
Fig. 10 shows a correlation of FOT versus PV. Increasing the FOT will reduce the PV value up to instability (i.e.  PV=1.0). With the appropriate treatment  the PV will remain higher (i.e.  stable) at the same temperature. Also note that the slope between the treated and untreated curves is different  with the treated curve having a much gentler slope. This provides more security and flexibility to the conversion enhancement objectives as the treatment acts as a buffer to the rate of PV change with FOT. Accordingly  Fig 10 is the correlation of the Furnace Outlet Temperature versus Pv showing that by increasing FOT the Pv will reduce up to instability  and with treatment  the Pv will be higher at the same temperature  but also the slope is different indicating that we provide more security and flexibility to the conversion enhancement objectives. By comparison  other known treatment systems  such as those described in European Patent Nos. 0321424 B1 and 0529397 B1 to Faina  et al.  do not impact Pv in the manner described by the present invention.
Comparing the difference in the VFM measurements from the tar in the inlet of the furnace to measurements in the outlet of the furnace gives a direct measure of the severity of the cracking. When the VFM measures inhomogeneities in the outlet stream  action can be taken on the process side  specific to customer specifications. For example  the simplest action to be implemented is reducing the cracking severity in order to reduce the fouling rate on the furnace  exchangers  columns bottoms or soaker drum. This reduces the risk and rate of fouling deposits  but it also reduces the amount of light hydrocarbon stream produced  so it reduces the profitability of operations. This course of action is accompanied with the feed of high temperature antifoulant chemistry at the rate of approximately 100 ppm. In order to maintain the highest efficiency of conversion and therefore the highest profitability  the goal is to increase the tar stability (increase the P-value) by replacing the converted resins by high temperature dispersant at a higher dose that is up to about 500 ppm of chemical is injected. The economical optimum to provide maximum profitability to the refinery is dependent on the individual refinery operations and objectives and is likely on the order of about 300 ppm. The specific value is determined with the use of the VFM measurements and our quantitative statistical models.
Our MRA models attempt to define a mathematical correlation between the operational parameters such as - feed quality  cracking severity  conversion and the fouling rate of the subject exchanger or furnace. By normalizing the mathematical model  the fouling rate is isolated from the varying operational parameters and the real fouling rate can be demonstrated and quantified. By developing a corrected model which reflects the residuals between the predicted model and the actual measured parameter  statistical process control techniques may be applied to quantify the performance of the chemicals applied to control fouling in the visbreaker unit. Precise determination of the fouling potential in this manner allows a refinery to start treating an opportunity crude and quickly reach an optimum set of operating conditions without incurring fouling  or to quickly change furnace conditions (i.e.  temperature) in order to increase or decrease the amount of specific fractions in the product (i.e.  distribution of components and/or composition of visbroken product) which may be required for immediate production needs  while assuring that operation remains within a safe stability band. In addition to enhanced yield or throughput  it provides enhanced flexibility with minimized risk.
The present invention is adapted to control chemical feed based on VFM measurements to maximize yield of light HC streams in Visbreaker operations. The VFM can also give an estimate of tar stability  which is proportional to HFT measurements. The program of the present invention controls chemical feed based on a predefined furnace outlet temperature  and uses predictive modeling to verify and predict performance based on VFM measurements. The chemical feed rate is then directly tied to customer driven performance measurements such as run length and/or conversion rate. High temperature dispersants can replace the converted resins to maintain tar stability while increasing the cracking severity; or  the system may increase tar stability by maintaining constant cracking severity. Moreover  measuring the tar characteristics with the VFM before and after the furnace indicates the amount of particulates produced directly in the cracking process.
A process for establishing effective visbreaker treatment may be summarized as follows. First  the user clearly defines the problem to be solved. Next  a unit survey or blank test of visbreaker operations is performed. Next  operational data obtained from the unit survey is analyzed  and baseline performance parameters are defined. Next  performance goals are measured in accordance with mutually agreed upon production goals and requirements  and then an appropriate treatment procedure may be designed. Next  the treatment procedure is implemented  monitored and serviced  and finally performance reports and quantity benefits may be provided.
As shown in Fig. 11  tar stability conversion occurs as asphaltenes are disbursed in the continuous phase through the peptizing action of aromatics and resins. It may also be noted from the illustration that cracking modifies the equilibrium so that asphaltenes could cause precipitation – low peptisation value.
Exemplary data recorded from a visbreaker conversion trial is shown in Fig. 12. As it is noted from Fig. 12  the circled regions represent areas to stop chemical injection under the same operating conditions.
Figs. 13-16 illustrate exemplary data obtained during conversion enhancement application. As it can be noted from the illustrated data  an overall +3% conversion increase was achieved. In Fig. 15  Thermoflo 7R630 was injected before preheat: average 300 ppm. It has to be noted that even a conversion increase by 1% in the treated charge has to be considered extremely satisfying in terms of profit.
Fig. 17 illustrates VFM data versus corrected skin temperature over time  and Fig. 18 is a schematic diagram illustrating exemplary visbreaker process types.
The operation described above of path length sampling to develop a measure of concentration of dispersed phase correlates well with a conventional HFT measure of hot filtered tar and may also be used with a suitable protocol to derive the classical peptization value Pv. This allows the VFM to be used to assess the quality of the visbreaker product and efficiently blend or produce various required fuel or other oils. The classical procedure for measuring Pv  in use for decades  involves slowly adding graded amounts of pure n-cetane C16 H34 to a sequence of samples of the product  maintaining each diluted sample in a heated bath for a time (e.g.  thirty minutes) to allow the asphaltenes to agglomerate  and then detecting the concentration of tar. The different samples provide a graph of the product stability  with an abrupt increase in tar separation at the peptization value Pv. The concentration measured by the VFM of the present invention provides an effective tool for performing such a Pv measurement quickly and repeatably.
One suitable protocol substitute n-heptanes for cetane in the sample preparation procedure  allowing the dilutions  heating and settling to be performed quickly - on small samples  at lower temperature  and in shorter times. A classical P value is expressed as 1 + Xmin  where Xmin is the maximum dilution before flocculation occurs expressed in number of milliliters of diluent n-cetane per gram of sample. For use with the VFM of the present invention  using n-heptane as the diluent  the sequence of samples with successively increasing dilution may be heated in a water bath at 100°C for fifteen minutes  allowed to cool and stand for fifteen minutes  and then measured with the VFM. This substantially reduces the sample preparation time  and because the VFM requires only a small path sampling procedure  the entire array of samples may be placed on a single slide – for example  a 9-well microsample plate  for the concentration detection step  so measurement is simplified  and made quantifiable and repeatable. Because of the lower molecular weight of the lighter heptane diluent  a correction factor 1/0.443 is applied to the diluent volume Xmin to correct for the different molecular weight of cetane  so that the resulting P value is identical in value to the classical measurement. A series of samples can be placed on the stage. Each sample comprises a small amount of aliphatic hydrocarbon (i.e.  n-cetane  n-heptane  etc.). The more aliphatic compound that needs to be added  the more stable the tar. The light transmission is then measured over a scan path on each individual sample. This allows a functional comparison to be made of optical density to the amount of aliphatic added to each sample.
Fig. 19A illustrates the derived Pv obtained by this procedure for five samples of visbreaker fluid  compared to the P values determined by the classical n-cetane laboratory testing analysis of the samples. The measurements are essentially identical. Fig. 19B graphs the VFM concentration measurement (in arbitrary units) illustrating the onset of instability and flocculation. The value Pv is readily visible as the point at which there is a rapid increase in sample opacity with a relatively small increase in the amount of the aliphatic (heptane) diluent. This abrupt change in the VFM concentration measurement among the tested samples  may be automatically defined as an output with a straightforward software comparison algorithm to provide this measurement of product quality or fluid stability. Other aspects of the sample preparation such as the preparation of a set of different dilutions and loading onto a microsample array for concentration measurement may be fully automated  using various injection  handling and transfer mechanisms that will be familiar from similar tasks performed by equipment used to automate the handling  processing and analysis of chemical  biological  medical or genetic sequencing materials.
Further  the program described above can be adapted to optimize the control of fouling in a heat exchanger network  or other equipment for a refinery process unit (such as a visbreaker unit  coker unit  hydrocracker unit  or crude unit)  based on a-priori detection of particulate agglomeration  size distribution and insolubility. More specifically  fouling caused by asphaltenic instability due to running  or blending  unstable crude diets. The program provides a framework to monitor the fouling propensity of the unit feedstock with regard to optimizing blending ratios  and/or controlling a chemical antifoulant in a feed forward manner. These techniques provide a way to minimize the energy and maintenance costs associated with fouling of heat exchange equipment  as well as to select feed blends that can balance cost savings with fouling control.
It is known that the fouling tendency of hydrocarbon feedstocks  or blends thereof  can be related to the tendency for insoluble organic materials to precipitate in heat transfer equipment. The VFM of the invention measures a quantity relatable to the opaque filterable solids within a hydrocarbon sample. The automated program outlined in the present invention utilizes VFM concentration and particle size distribution data to estimate the fouling potential of a blended feedstock. This  in turn  is used to understand the blending envelope of potential hydrocarbon feeds  relate it to fouling behavior and then to provide optimal dosage of chemical treatment.
It is shown that high temperature dispersants and anti-foulants are the main components in a chemical regiment used to treat process equipment  and thereby reduce the rate of fouling. There are specific chemical families that are particularly effective for use in slowing the rate of fouling in heat exchange equipment and other hot surfaces  such as furnaces. The fouling rates are affected by action of the said chemicals to colloidally stabilize the contained particulates  including asphaltenic materials. The program of the present invention is configured to allow blending stability envelopes to be ascertained prior to their use. Additionally  the program is configured to select the type and quantity of chemistry required to satisfy production requirements. Specific chemical entities include  but are not limited to  poly-isobutenylphosphonic acids and esters  polyisobutenylthiophosphonic acids and esters  alkylphosphonate phenate sulfides and disulfides that may be neutralized with alkaline earth metals  or amines of polyisobutenyl succinimides  polyisobutenylsuccinate alkyl esters  magnesium or calcium salts of alkyl or dialkylnaphthelene sulfonic acids as described in U.S. Pat. No. 4 927 519 and EP Pat. No. 321424B1  herein incorporated by reference.
These antifoulant materials have been found to function at low dosages  1-200 ppm  to prevent the undesirable deposition of insoluble material (fouling) onto surfaces in heat exchange equipment running process fluids. Fouling in heat exchangers is most generally thought to occur by first generating an unstabilized macromolecular particle that is no longer soluble in the bulk fluid  or as a colloidal species. This often occurs due to thermal stress on the hydrocarbon. Initial deposition occurs  and further destabilized species adsorb onto the site of original deposition. Bigger particles in the hydrocarbon will be more prone to contact and coalesce to the surface. Dehydrogenation of the adsorbed hydrocarbon will be driven by heat and time and make the deposit more tenacious as crosslinking reactions occur.
The dispersants are generally understood to function by a variety of mechanisms. First  the dispersant materials adsorb to the surfaces formed by growing insoluble particles and act to keep these particles small; typically less than 1 micron. Thus  the particles are more prone to continue to flow through the system and not settle on heat exchanger or other surfaces. This can be described by Stokes law  which is dependent on the radius of the particles. This is schematically shown in Fig. 9. The dispersants act by a combination of steric stabilization  which acts to repel approaching particles (dramatically increase entropy of local system and drive solvent in between particles)  and blocking of polar sites on the particles which act as a driving force for coalescence. Light scattering evidence exists that shows that dispersant treated thermally stressed fluids generate particles that are up to two orders of magnitude smaller than untreated hydrocarbon fluids.
Even if the particles are not small  the above mechanism explains how the particles will be less prone to coalesce to other particles in solution  or to material already deposited on the surface.
It has also been shown that the nature of the surface plays a role in the ability of thermally stressed fluids to deposit. Metal surfaces with higher roughness  edges  or polarity are more prone to fouling. These dispersants will adsorb to such surfaces and discourage particulate or amorphous insoluble hydrocarbon from sticking to the surface.
The reaction of hydrocarbons at elevated temperatures with oxygen (even very low levels such as <5 ppm) will result in formation of polar functionalities that can drive coalescence of particulate  as well as accelerate the dehydrogenation of adsorbed hydrocarbon  which makes its removal from the surface by turbulent flow much less likely. Dispersant adsorption will block the mass transfer of the oxygen to the surface  and some of these described anti-foulants have antioxidant abilities by interfering with radical reactions.
In addition  hydrocarbon feed stocks  such as crude oils and their heavier distillate fractions  are thought to contain asphaltenic materials that are colloidal in nature. These colloids have highly polar and high molecular weight asphaltene species that are stabilized in the fluid by smaller resin molecules. These systems can be rendered “unstable” or prone to deposition either by thermal stress  which changes the critical ratio of the adsorbed resins to asphaltenes required to maintain a stable colloidal. The dispersants described here are believed to replace the disturbed or destroyed resins and re-stabilize the asphaltenic systems.
As described herein  the VFM measurement data provides information regarding the concentration and particle size distribution of insoluble solids in the hydrocarbon feed and/or its blends and their resultant distillate fractions. Fluids having higher amounts of solids will give a higher precipitation potential. Additionally  fluids having particle size distributions that present substantial fractions greater than a critical size threshold will also exhibit increased fouling potential. The critical size threshold varies  in a practical sense  depending upon mechanical properties of the fluid and its flow  such as viscosity  velocity and shear stress etc. Furthermore  the nature of the particles making up the distribution will also affect the fouling propensity. Finally  the feed  the particular blending ratios of individual feedstocks comprising the blend  thermal destabilization  and dehydrogenation mechanisms can all introduce solids into the system  affect the size distribution and also and the nature of the particulate matter.
Baseline performance parameters  such as fouling potential  can be established by performing a unit survey or blank test of the process unit operations and analyzing the operational data obtained from the unit survey. Based on defining a unit dependent baseline for a particular feedstock  the VFM data provides information regarding the increasing or decreasing fouling potential of the incoming feed. This information can then be used to alter chemical antifoulant injection rates. Additionally  crude blending practices are often fraught with uncertainty and significant non-linearity can occur with respect to blending ratios when considering the stability of contained asphaltenic materials and the associated fouling rates. The VFM data  when run across the desired blending envelope(s)  can define regions of significant fouling potential. This can allow rational decisions to be made in real-time to adjust blending strategies to minimize fouling and/or maximize run-length.
Fig. 20 and Fig. 21 show VFM particle count data for a particularly asphaltenic crude oil (Crude A) blended at several ratios with a standard crude slate (Crude B). The chart additionally shows the same blending curve with the addition of a chemical antifoulant. This set of fluids was chosen because this blend has caused severe pre-heat fouling rate increases  as well as desalter upset conditions  on multiple occasions in the past. Several important points can be noted from this data. First is that when considering the pure Crude A and Crude B data  the asphaltenic crude oil (Crude A) has much lower particle count than the normal slate (Crude B). This observation shows that the VFM can be used to evaluate incoming crude feeds to ascertain their relative fouling propensity. By performing the VFM test on these crudes after the desalting process  the intrinsic fouling propensity can be related to asphaltenic components or other inorganic contaminants which can be removed in the desalting process. Crude feedstocks and blends can be ranked as low  medium or high fouling potential feedstocks. Also notable in Fig. 21 is that upon mixing the Crude A and Crude B at varying ratios  the particle count dramatically rises for blend ratios between 40% and 60%. This shows that this blend has a tendency to exhibit a significant non-linear increase in insoluble particulate. This is likely due to the destabilization of colloidal asphaltenes as outlined above. Fig. 22 shows actual furnace inlet temperature (hereinafter “FIT”) declines due to fouling as measured in the field. The FIT declined severely during several timeframes while processing crude diets having blend ratios in this problematical regime. Fig 21 also shows that the number of insoluble particles can be dramatically affected by the addition of small amounts of chemical antifoulant. Fig. 23 shows a similar set of data for another asphaltenic crude (Crude D) blended into another standard crude slate (Crude E). The second data set is indicative of fairly widespread experience with these issues and shows that this is not something particularly related to narrow sets of crude oils.
The VFM data shown above forms the basis for the current invention in which a program is outlined to use the VFM instrument to proactively control fouling behavior in heat exchange equipment  or other pieces of process equipment prone to fouling  before problematical and costly impacts are actually experienced. By using the VFM to screen incoming crude oil  changes to the incoming particle count can be used to control chemical feed rates in combination with statistical models. By utilizing the combination of the VFM  chemical antifoulants and statistical process control methodologies  the overall program can be managed to provide an improved ability to achieve unit runtime  maintenance and economic goals. Additionally  blending strategies can be proactively developed such that great flexibility to meet overall economic goals is achieved  while preserving critical heat transfer efficiency and runtime.
An exemplary method of the invention is described  wherein the VFM device is used to control fouling in a process unit preheat exchanger train.
A process fluid enters a crude unit after being blended in a tank farm 201. It then enters a preheat system composed of a cold exchanger train 202  a desalter 203  a hot exchanger train 204  a furnace 205  and a fractionating unit 206. Fluid samples are taken at points 207 such that characteristics of the native fluid  before chemical injection  can be compared to the characteristics at various points in the system during the heating process and after chemical has been added. Fluid samples are passed to the VFM 208 and fouling propensity data is determined and compared at the various sampling points 207. VFM data is then transmitted to a processing and control mechanism 209 where mathematical analysis takes place to determine antifouling chemical demand. The analysis of the data consists of using predictive modeling techniques to evaluate the impact of chemical on fouling propensity  as measured by the VFM  and also to project theoretical run length based on the measured rate of fouling. Based upon the derived statistical parameters  necessary chemical demand is computed to maintain the fouling rate within specified limits. Instructions are then transmitted to the chemical feed pump 210  which delivers chemical from the chemical storage tank 211. Based upon the data analysis and control algorithms  an optimized dosage of antifouling chemical is injected by the injection pump 210 into the process line at injection point 212.
Other schemes can be envisioned  and this embodiment is not meant to be limiting to a specific piece of process equipment  data acquisition methodology  feed configuration  or control mechanism. One preferred embodiment might be envisioned to additionally collect data and/or inject chemical prior to tankage when raw process feed is offloaded from transportation to the refinery.
Pieces of process equipment at which a scheme similar  but not limited to  that described above could be envisioned by those skilled in the art are: fractionating unit preheat exchanger trains in the crude unit and hydrotreating unit; FCCU slurry loop exchangers and steam generators; heat exchanger equipment in delayed coking units  furnaces and reboilers throughout the refinery.
While the disclosure has been illustrated and described in typical embodiments  it is not intended to be limited to the details shown  since various modifications and substitutions can be made without departing in any way from the spirit of the present disclosure. As such  further modifications and equivalents of the disclosure herein disclosed may occur to persons skilled in the art using no more than routine experimentation  and all such modifications and equivalents are believed to be within the scope and spirit of the disclosure as defined by the following claims.

WHAT IS CLAIMED IS:
1. A method to reduce and optimize the fouling rate in a process unit comprising the steps of:
estimating a concentration of foulant materials contained within a fluid of the process unit;
determining the fouling potential based on said concentration;
determining the impact of antifoulant chemical on the fouling potential within the unit;
determining an acceptable baseline fouling potential;
comparing said baseline parameter to said concentration of foulant materials measured during subsequent unit operations; and
regulating input of said anti-fouling chemical into said process unit during running of unit operations based on said comparison so as to achieve one or more unit goals.
2. The method of claim 1  wherein said unit goals include improved yield  improved quality  and reduced fouling related maintenance.
3. The method of claim 2  wherein said estimating step includes estimating a distribution of foulant materials contained within said fluid; and said fouling potential is determined by said concentration and said distribution.
4. The method of claim 3  wherein said regulating step includes selecting and controlling the type or quantity of said anti-fouling chemical being input into the unit.
5. The method of claim 4  further comprising the step of improving run-lengths compared to previous un-treated conditions.
6. The method of claim 5 further comprising the use of predictive modeling to derive the impact of chemical on the fouling rate  and the amount of necessary chemical to provide adequate run length  according to planned maintenance schedules.
7. The method of claim 6  further comprising the use of multiple measurements  either within the unit or its feed steams  to determine the impact of introduced chemical upon foulant concentrations.
8. The method of claim 7  further comprising the use of multiple chemical injection locations inside and/or outside unit boundaries.
9. The method of claim 1  wherein said estimating step is performed using an apparatus comprising:
an optical lens system comprising a stage adapted to receive a sample of fluid;
a light sources for focusing a beam of light onto said sample;
means of directing said light beam along a plurality of path lengths within a predetermined area of said sample;
means of detecting light transmitted through said sample along each path length;
means for quantifying an intensity of said transmitted light; and
means for correlating said quantified transmitted light to a concentration of said foulant particles in said samples.
10. The method of claim 9  further comprising means for determining the type or quantity of antifouling chemical injected into the process unit.
11. The method of claim 9  further comprising means for estimating the impact of injected anti-fouling chemical on foulant particles and run length predictions.
12. The method of claim 9  further comprising the usage of measured data to generate predictive models for use in controlling chemical injection.

13. The method of claim 9  further comprising the usage of multiple chemical injection locations inside and/or outside the units boundaries.
14. A method to improve a hydrocarbon fluid stream in a refinery comprising the steps of:
estimating a concentration of foulant materials contained within a hydrocarbon fluid stream in a refinery;
determining the fouling potential based on said concentration;
determining the impact of antifoulant chemical on the foulant materials;
determining an acceptable baseline fouling potential;
comparing said baseline parameter to said concentration of foulant materials measured during subsequent refinery operations; and
regulating input of said anti-fouling chemical into said fluid stream during running of refinery operations based on said comparison so as to achieve one or more refinery goals.
15. The method of claim 14  wherein said refinery goals include improved yield  improved quality  improved run-lengths  and reduced fouling related maintenance.
16. The method of claim 15  wherein said estimating step includes estimating a distribution of foulant materials contained within said fluid; and said fouling potential is determined by said concentration and said distribution.
17. The method of claim 16  wherein said regulating step includes selecting and controlling the type or quantity of said anti-fouling chemical being input into the fluid stream.
18. The method of claim 17 further comprising the use of predictive modeling to derive the impact of chemical on the fouling rate  and the amount of necessary chemical to provide adequate run length  according to planned maintenance schedules.
19. The method of claim 18  further comprising: the use of multiple measurements  either within the refinery or its feed steams  to determine the impact of introduced chemical upon foulant concentrations; and the use of multiple chemical injection locations.
20. The method of claim 19  wherein said estimating step is performed using an apparatus comprising:
an optical lens system comprising a stage adapted to receive a sample of fluid;
a light sources for focusing a beam of light onto said sample;
means of directing said light beam along a plurality of path lengths within a predetermined area of said sample;
means of detecting light transmitted through said sample along each path length;
means for quantifying an intensity of said transmitted light; and
means for correlating said quantified transmitted light to a concentration of said foulant particles in said samples.
21. A system to reduce and optimize the fouling rate in a process unit comprising:
an optical lens system comprising a stage adapted to receive a sample of fluid;
a light sources for focusing a beam of light onto said sample;
means of directing said light beam along a plurality of path lengths within a predetermined area of said sample;
means of detecting light transmitted through said sample along each path length;
means for quantifying an intensity of said transmitted light; and
means for correlating said quantified transmitted light to a concentration of said foulant particles in said samples.
means for determining the fouling potential based on said concentration;
means for determining the impact of antifoulant chemical on the fouling potential within the unit;
means for determining an acceptable baseline fouling potential;
means for comparing said baseline parameter to said concentration of foulant materials measured during subsequent unit operations; and
means for regulating input of said anti-fouling chemical into said process unit during running of unit operations based on said comparison so as to achieve one or more unit goals.