Method And Apparatus For Determination Of System Parameters For Reducing Crude Unit Corrosion
Applicants: NALCO COMPANY [US/US]; 1601 W. Diehl Road Naperville, Illinois 60563-1198 (US)
Inventors:
BANKS, Rodney H.; (US).
CIOTA, Steven R.; (US).
WELZ, Sascha; (US)
Abstract:
The invention provides a method and apparatus for determining the amount of various materials in a liquid sample. Because the apparatus is particularly resilient it can be used repeatedly with very harsh liquid samples such as boot water from an oil refinery. The apparatus uses at least one volume and/or concentration independent optical analysis method to determine at least one of: the pH, amount of chloride, and/or amount of iron in the sample. The optical property can be colorimetric, fluorescent or both and result from adding dyes, complexing agents, turbidity inducing compounds, and other optically effecting reagents to the sample. Because the measurements are concentration and volume independent they can be done continuously, quickly, and avoid the inconvenient start and stop procedures in prior art measurement regimens. The method further includes using a BDD cell to oxidize materials (such as sulfoxy compounds) that would otherwise interfere with the optical analysis and/or to sparge the sample with gas.
Background of the Invention
This invention relates generally to methods of reducing corrosion in a crude unit. More specifically, the invention relates to methods of optimizing system parameters in a process stream of a crude unit to reduce corrosion in the crude unit. The invention has particular relevance to sampling dew point water and accumulator boot water to measure system parameters and respond to such measurements to reduce corrosion and/or corrosion byproduct deposition in the crude unit.
In a crude oil refinery, generally the oil is pumped from a storage tank to a crude unit for processing. The crude unit cleans the oil through water washing in a desalter and then splits the oil into fractions in an atmospheric distillation tower. These fractions are pumped to various processing units downstream of the crude unit (e.g., coker, catalytic cracker, hydrotreater etc.). Though corrosion and corrosion byproduct deposition (the latter sometimes referred to herein as fouling) occur in many areas of a crude unit, the most severe corrosion and fouling typically take place in the overhead condensing system of an atmospheric distillation tower system.
Refinery crude unit processing has becoming increasingly difficult in recent years and is predicted to become even more challenging and complex for several reasons. For example, significant increases in crude oil prices have caused refiners to aggressively pursue "opportunity" or "challenging" crudes that are obtainable at discounted prices. The lower price is linked to a crude property such as high acid or high solids content that makes it less desirable than the light, sweet benchmark crudes.
Refiners switch crude slates more frequently than in the past due to minimum on-hand crude oil inventory combined with increased crude oil variety. A crude slate switch typically upsets the steady state condition of a crude unit for up to several hours. Generally, about eighty percent of the corrosion and fouling occurs during these switches or disruptions, which normally last about twenty percent of the time. If fouling and corrosion issues are severe enough, the refiner will discontinue processing the crude oil or blend of crudes causing the problem. However, these challenging crudes are available to the refiner at a discount thus making them more profitable. Discontinuing such problematic crudes is accordingly not a very popular option.
In efforts to reduce corrosion, a crude unit may be serviced two or three times per week, or in some cases daily. Daily service at best provides a snap shot view of a dynamic crude unit system. Crude type and/or raw crude storage tanks are switched several times per week, sometimes daily. The contents of each tank are different from the others, so each switch causes a change of feed quality to the crude unit, many times upsetting the steady state status and causing disruptions in the system. Preheating, desalting, and distilling operations shift with the new crude, sending products and/or effluent water sources off specification. Many adjustments over several hours (in some cases days) normally take place to return the crude unit to steady state operation.
The most common current industry practice to control such disruptions and optimize crude unit operation is to provide enough manpower and man-hours. For instance, each crude unit may have an operating crew from three to ten people, depending on size and complexity of the unit. This crew may spend their day gathering various samples for wet chemistry lab testing, and measuring and making adjustments for temperature and flow to keep the unit running within specification. Such practice is typically geared towards keeping the unit operating properly with respect to fractionation quality cut points and end points, with minimal attention being paid to a specialty chemical corrosion control program. If a disruption is severe, changes may be made to the process chemicals and/or changes in levels, flows, or temperatures may be recommended around the crude unit to keep the dynamic system in as optimum a condition as possible.
Attempts to compensate for periodic or sometimes prolonged lack of human involvement include installing online pH meters on atmospheric distillation towers overhead accumulator water boots; however, due to a high rate of fouling of the pH sensor only a small percentage of these meters operate correctly for any length of time. Online instrumentation, such as pH meters, requires routine maintenance and calibration.
Moreover, online pH merely tracks the pH and sends an alarm to the operator when the pH is outside the control limits. Often, poorly calibrated and/or fouled pH meters cause frequent alarms. This frequency tends to minimize the effectiveness of the alarm system.
Due to the lack of industry success with online pH metering and other monitoring efforts refiners have not pursued more exotic and effective online instrumentation for process chemical programs. There thus exists an ongoing need for more sophisticated and effective online and/or automatic methods for monitoring parameters and reducing corrosion in crude units.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is "prior art" with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. § 1.56(a) exists.
Brief Summary of the Invention
At least one embodiment of the invention is directed towards a method of measuring at least one property of a predominantly liquid sample. The method comprises the steps of: 1) adding at least one chemical reagent to the sample, the chemical reagent capable of inducing a measurable optical effect when added to the sample that is directly related to the property to be detected, 2) measuring the optical effect, and 3) deducing the value of the property by comparing the measured optical effect to pre-determined values associated with the property to be determined. The relationship between the measured optical effect and the property to be determined is independent of the volume of the liquid sample and independent of the volume of the reagent added to the sample.
The measured property may be one item selected from the list consisting of: pH, iron concentration, chloride concentration, and any combination thereof. The measured optical effect may be a colorimetric effect, turbidity effect, or a fluorescent effect. The reagent may be thoroughly mixed with the sample. The optical effect may be measured by determining an absorbance level at a particular wavelength whose measurement is recognized as an isosbestic point for all values of the property, detecting at least one other absorbance level for one other wavelength, comparing the two absorbance levels with pre-determined data, and correlating the two absorbance levels to the known absorbance levels of a particular value of the property. The reagents may be selected from the list consisting of: bromcresol purple, fluorescein, PTSA, TPPTSA, calcein blue, Ferrozine, silver nitrate, thioglycoiic acid, ammonia, pH buffer, ferric iron reductant, fluorescent dye, lucigenin, and any combination thereof.
The optical effect may be measured by the reagents being at least two fluorescent dyes, one of the dyes' fluorescence at a first wavelength is affected by the value of the property and one of the other dyes' fluorescence at a second wavelength is unaffected by the value of the property. The method may further comprise the steps of measuring the ratio of the fluorescence intensities of the first and second wavelengths in the sample, comparing that ratio to the ratio of the fluorescence of the first and second wavelengths in a control having a known value of that property, and correlating the proportional change in the two ratios to the property value. The optical effect may be measured by the reagent's absorbance and fluorescence where the absorbance is unaffected by the val ue of the property and the fluorescence is affected by the value of the property, by comparing the ratio of the fluorescence to absorbance to a control having a known value of the property, and correlating the proportional change in the two ratios to the property. The reagent may form a complex with a compound that causes the property, the absorbance of the complex at a pre-determined wavelength is directly related to the amount of that compound present and not to the amount of reagent added.
The sample may be positioned within an apparatus. The apparatus comprises at least one reagent source constructed and arranged to feed the reagent into a chamber where it is mixed with the sample and the sample is moved past an optical sensor that measures the optical property. The apparatus may further comprise a light source which may be positioned in line or perpendicular to the optical sensor. The light source may also be in line or perpendicular to a BDD cell through which the sample passes before the reagents are added. The BDD cell may be constructed and arranged to oxidize sulfoxy compounds. The light source may also be in line or perpendicular to a vertically angled sensor flow path through which the sample flows whereby measured light passing to the optical sensor passes horizontally through the sample. There may be at least two optical sensors and the sensors are positioned along a horizontal plane relative to the vertical flow path. The apparatus may further comprise a tube downstream from the sensor, at least a portion of the tube is higher than the sensor and is horizontally angled, the tube is constructed and arranged to facilitate the migration of gas bubbles away from the sensor. The tube may be inverted U-shaped. The apparatus may further comprise a gas source upstream from the sensor, the gas source constructed and arranged to sparge undesired materials away from the sample. The apparatus may be interfaced with a control system governing at least some of the operations of a chemical process stream from which the sample was taken, the measured data resulting in the control system implementing a counter-measure in response to the property.
Free Full Text Source: http://patentscope.wipo.int/search/en/WO2012075076
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