Processes And Systems For Characterizing And Blending Refinery Feedstocks

CATEGORY: CRUDE OIL BLENDING
PATENT
Processes And Systems For Characterizing And Blending Refinery Feedstocks
Pub. No.:    WO/2012/092012    International Application No.:    PCT/US2011/066054
Publication Date:  05.07.2012
Applicants:  Chevron U.S.A. Inc. [US/US]; 6001 Bollinger Canyon Road San Ramon, CA 94583 (US)
Inventors: 
KUSINSKI, Grzegorz, Jan; (US).
DEVINE, Thomas M.; (US) 
Abstract:  
Refinery feedstocks can be characterized based on any of: dissociation of acids in the crude, breakup of naphthenic acid molecular associations, mass changes of samples, and / or dissociation of sulfur compounds in the feedstocks. The characterization is performed as a function of temperature via any of electrical resistivity measurement, crystal microbalance measurements of weight changes, vibrational spectroscopic analysis, voltammetry, electrochemical impedance spectroscopy (EIS) and combinations thereof. The method can be practiced in any of refinery, terminal, and laboratories. It can be used in conjunction with a system comprising models and hardware to optimize the usage of refinery feedstocks in the blending and valuation of the feedstocks. The system can be employed in any of refinery, terminal, and laboratories. In one embodiment, the characterization of refinery feedstocks is via the use of EIS.
TECHNICAL FIELD

[002] The invention relates generally to systems and methods for characterizing crude oils and refinery feedstocks according to their corrosivity. In one embodiment, the invention relates to systems and methods for blending crude oils and refinery feedstocks to produce a final feedstock of desired characteristics.

BACKGROUND

[003] Numerous systems and methods have been disclosed to characterize and treat crude oils or refinery feedstocks that contain acids in several forms. The acids in the feedstocks may be organic acids such as carboxylic or naphthenic or mineral acids such as hydrochloric, phosphoric, hydrogen sulfide and various oxidized forms of hydrogen sulfide such as sulfuric acid. Naphthenic acid is a type of organic acid commonly present in acidic crudes. There are publications teaching the treatment and prevention of acid corrosion in petroleum feedstocks with the demineralization and alkali treatment of crude oil, the use of organic corrosion inhibitors, and selection of equipment and materials for handling petroleum feedstocks by alloying metals with anticorrosive additives, such as Cr, Mo, Ni, etc..

[004] Evaluation of corrosivity of refinery feedstocks has typically been done by a classic model considering the Total Acid Number (TAN) of the feedstocks. The TAN number is computed based on milligrams of KOH required to neutralize one gram sample of the crude. If the feedstock has a TAN greater than 0.5, the crude is usually considered corrosive. One traditional approach has been blending high naphthenic acid crudes with low naphthenic acid crudes to a predetermined TAN number, e.g., below 0.5 for crudes or 1.5 for certain side-cuts, such as vacuum gas oil, or by avoiding refining crudes having relatively high quantities of naphthenic acids. US Patent Application No. 2008/0164137 discloses that naphthenic acid corrosivity can be correlated with the chemical composition of naphthenic acids, especially with respect to the ratio between an alpha fraction and a beta fraction of the naphthenic acids

[005] There is still a need for improved methods and systems to characterize refinery feedstocks by their corrosivity characteristics.

SUMMARY OF THE INVENTION

[006] In one aspect, a method for evaluating the corrosivity of a crude oil is disclosed. The method comprises: withdrawing a representative sample of a crude oil feedstock; performing impedance measurements on the crude oil as a function of temperature to obtain a first electrochemical impedance (EI) spectrum; obtaining a second EI spectrum on a reference crude oil having known corrosion properties, wherein the first and second EI spectra include data for at least three frequencies; and analyzing the first EI data relative to the second EI data to evaluate the corrosivity of the crude oil feedstock, wherein comparing the first EI data with the second EI data includes comparing at least one of a resistance measurement and a capacitance measurement. In one embodiment, EIS is conducted using a two-electrode cell in which one electrode is an ultramicroelectrode and the second electrode is a reference electrode. In one embodiment, both electrodes are composed of platinum.

[007] In one aspect, a linear voltammetric method to characterize refinery feedstocks is disclosed, wherein current passing through the feedstock is measured as a function of the applied DC voltage. As increasing / decreasing voltage is applied at a constant rate with time, oxidation / reduction of corrosive species such as acids occurs, allowing the use of voltammetry to characterize the feedstock with respect to is its corrosion property. In one embodiment, ultramicroelectrodes made of an electrochemically stable conductor such as platinum are used for the procedure.

[008] In another aspect, a method to characterize and / or optimize blends of refinery feedstocks is disclosed with the use of cyclic voltammetry, wherein blends of feedstocks with measured values are characterized, optimized and compared with a pre-determined value of a crude oil with a known corrosion rate, creating an optimized blend. In one embodiment, electrochemically stable ultramicroelectrodes are employed for the cyclic voltammetric evaluation of refinery feedstocks.

[009] In one embodiment, a two-electrode electrochemical is employed for characterizing the solutions by either linear voltammetry and / or cyclic voltammetry, with the ultramicroelectrode serving as the working electrode and a second electrode having a higher surface area serving as both the reference electrode and the counter electrode.

[010] In one aspect, a method is disclosed for evaluating the corrosivity of a crude oil feedstock by correlating its corrosivity with dissociation of acids in the crude oil. The method comprises: withdrawing a representative sample of the crude oil feedstock, wherein the crude oil sample has a certain amount of acids; detecting the dissociation of the acids in the crude oil feedstock as a function of temperature by obtaining any of impedance measurements, linear voltammograms and cyclic voltammograms over a range of temperature from ambient to 700°F; providing respective impedance measurements, or linear voltammograms, or cyclic voltammograms of a reference oil feedstock having a known dissociation of acids; and comparing the measurements of the crude oil feedstock with the measurements of the reference oil feedstock to evaluate the corrosivity of the crude oil feedstock.

[011] In yet another aspect, a method is disclosed for evaluating the corrosivity of a crude oil feedstock by correlating its corrosivity with the electrical resistivity of the crude oil. The method comprises: withdrawing a representative sample of the crude oil feedstock, wherein the crude oil sample has a certain amount of corrosive species; detecting the corrosive species in the crude oil feedstock as a function of temperature by obtaining the electrical resistivity over a range of temperature from ambient to 700°F; providing electrical resistivity measurements of a reference oil feedstock having a known dissociation of acids; and comparing the electrical resistivity measurements of the crude oil feedstock with the electrical resistivity measurements of the reference oil feedstock to evaluate the corrosivity of the crude oil feedstock. In one embodiment, the four-point probe is employed for the resistivity measurement. The four-point probe is housed in a holder that is relatively chemically inert in the test solutions and which exhibits a high electrical resistance.

[012] In another aspect, a method is disclosed to evaluate the corrosivity of a crude oil feedstock by vibrational spectroscopic analysis as a function of temperature. The method comprises: withdrawing a representative sample of the crude oil feedstock having a certain amount of acids; detecting molecular associations and dissociation of acids in the crude oil feedstock as a function of temperature from ambient to 700°F by vibrational spectroscopic analysis to obtain spectroscopic measurements; and analyzing the vibrational spectroscopic measurements to correlate the molecular associations and dissociation of the acids in the crude oil feedstock to evaluate its corrosivity as a function of temperature.

[013] In yet another one aspect, a method for optimizing blends of refinery feedstock is disclosed. The method comprises: providing a plurality of refinery feedstock samples with each feedstock sample being representative of a feedstock stream to the refinery; obtaining a vibrational spectroscopic measurement as a function of temperature for each of the feedstock samples; providing a database correlating vibrational spectroscopic measurements with known corrosion performance of reference refinery feedstock; using the spectroscopic measurement of the refinery feedstock samples and the database correlating vibrational spectroscopic measurements with corrosion performance of reference refinery feedstock to obtain an optimized feedstock blend having desired vibrational spectra over a temperature range from ambient to 700°F, correlating with an acceptable corrosion performance.

[014] In another aspect, a method to optimize feedstock blends is disclosed. The method comprises: providing a plurality of refinery feedstock samples with each feedstock sample being representative of a feedstock stream to the refinery; obtaining vibrational spectroscopic measurements as a function of temperature for the feedstock blend and the plurality of refinery feedstock samples; blending the feedstock samples in pre-determined proportions to form a feedstock blend; comparing the vibrational spectroscopic measurements of the feedstock blend to pre-determined vibrational spectroscopic measurements; and adjusting the proportions of the feedstock samples so that the vibrational spectroscopic measurements of the feedstock blend are comparable to the pre-determined vibrational spectroscopic measurements.

[015] In one aspect, a system to optimize blends of crude oil feedstock to a refinery to minimize corrosion impact is disclosed. The system comprises: an on-line analyzer for obtaining any of electrochemical impedance measurements, linear voltammograms, cyclic voltammograms, and two-point probe or four-point probe measurements of electrical resistivity as a function of temperature for plurality of refinery feed streams to the refinery; a database correlating the measurements with at least one of molecular break-up of acid molecules in crude oil feed, dissociation of acids in crude oil feed, and dissociation of sulfur compounds into ionic species in refinery feed; and an operator, operatively disposed to receive the measurements from the on-line analyzer and the database correlating the measurements with corrosion characteristics of crude oil feedstock, and wherein the operator modifies a blend of the refinery feed streams in response to the received information. In one embodiment, the on-line analyzer is for obtaining spectroscopy measurements as a function of temperature.
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