System and method for alkylation process analysis (Invensys)

CATEGORY: ALKYLATION
PATENT
System and method for alkylation process analysis (Invensys)
Publication number
US20130191036 A1
Publication type
Application
Application number
US 13/790,537
Publication date
Jul 25, 2013
Also published as
US7972863, 6 More »
Inventors
W. Marcus Trygstad, Bruce A. Keen, Russell Jackson
Original Assignee
Invensys Systems, Inc.
Abstract
A method and apparatus is provided for determining concentration of components in a liquid hydrocarbon mixture including hydrocarbons and water flowing through an alkylation process. A fluid flow path conveys the liquid continuously from the alkylation process through a first instrument configured for measuring a property of the liquid mixture, and having responsivities to concentration of the components, which are independent of the concentration of the water. A temperature detector generates temperature data for the liquid, and a second instrument measures another property of the liquid mixture. The instruments have mutually distinct responsivities to concentrations of the components. A processor captures data from the temperature detector and instruments, using the data with a model of responsivities of various concentrations of the components at various temperatures, to determine a temperature compensated concentration of the components while the liquid mixture flows continuously through the fluid flow path.
BACKGROUND
1. Technical Field
This invention relates to chemical analysis, and more particularly to alkylation process analysis and control.
2. Background Information
Introduction to the Refining Alkylation Market
Between a quarter and a third of the world's refineries operate alkylation units, which convert relatively low-value byproducts of the crude oil refining process into alkylate, a high octane component used to make gasoline. Among the numerous control variables that determine the economics of alkylation is the composition of the acid catalyst. Globally, the number of alkylation units using hydrofluoric acid (HF) is currently about 125 versus about 90 using sulfuric acid (H2SO4 or SA). Slightly more than half of all alkylation units in the world are located in North America, where gasoline is favored over diesel as a motor fuel for passenger cars and alkylate is accordingly a valued blending component.
Another important application of alkylation technology is in the production of LABs (linear alkyl benzenes), important as a raw material used in laundry detergents. However, the total number of alkylation units in operation to produce LABs, as well as tonnage produced, is rather small compared with the refining industry.
Background: Alkylation Process Control
Alkylate is one of the most important gasoline blending components in the refining industry. Because it has an extremely high octane number, contains virtually no sulfur, and can be produced using olefinic by-products from the fluidized catalytic cracking (FCC) unit, alkylate has been called refiners' gold. Given that the reactants are seldom pure, and that propylene is sometimes mixed with the olefin feed, alkylate in practice comprises a mixture of compounds instead of pure isooctane as depicted in the following idealized equation:
Isobutane+Isobutene→3,3,5-trimethypentane(Isooctane)
Produced in a continuous-flow process, the chemical addition of isobutane and isobutene is effected conventionally through liquid phase catalysis involving strong acids such as hydrofluoric acid (HF) and sulfuric Acid (H2SO4, or SA), although solid phase catalysts are currently under development.
Monitoring and controlling the composition of the liquid acid catalyst, i.e., acid strength and the levels of impurities that dilute the acid, are among the most important challenges associated with the profitable operation of the alkylation process. One important impurity is water, which enters the process with the feed streams. Though present at ppm (parts per million) levels, water accumulates in acid catalyst at percent levels due to feed rates ranging from a few thousand barrels per day (bpd) to tens of thousands of bpd. By contrast, acid soluble oil (ASO, as defined hereinbelow) accumulates in the acid catalyst, a by-product of reactions involving feed impurities that contain sulfur, oxygen, or conjugated double bonds.
HF Alkylation
In the case of HF alkylation (HFA), water generally is controlled at levels below 2% to minimize corrosion of equipment in the unit. Also, total hydrocarbons dissolved in the catalyst are typically held at levels around 11%-16% to yield alkylate of the required quality and maximize process economics. In HFA, HF strength is controlled through acid regeneration within the alkylation unit, which is essentially a distillation process that separates HF from the higher-boiling impurities, H2O and ASO (Acid Soluble Oil, as defined hereinbelow).
If HF strength drops below about 80%, side reactions can accelerate and lead to a condition called acid runaway, which consumes HF and produces large amounts of ASO. Such runaways rarely occur, as unit operators usually have time to detect the incipient runaway and “pull charge” (withhold olefin feed) to stop the process before the runaway condition actually occurs. However, this action also stops the production of alkylate while the catalyst is regenerated. Furthermore, acid regeneration itself has associated costs including energy required to run the unit, neutralization and disposal of hydrocarbon byproducts, and the addition of fresh, pure HF. Thus, the ability to monitor and control catalyst composition in real time allows refiners to avert runaways, reducing operating costs while also tending to maximize product quality (octane), throughput, and the time between maintenance shutdowns to repair or replace corroded components.
SA Alkylation
SA alkylation (SAA) differs from HFA in that the catalyst generally is not, with rare exception, regenerated on site at the refinery. HF has a relatively low boiling point and can be distilled. By contrast, SA is essentially non-volatile and therefore cannot be purified through distillation. Rather the “spent” acid generally must be shipped by rail car for remote processing. Thus, the high cost associated with off-site regeneration partially offsets the perceived safety advantage of SA over HF, i.e., its low volatility.
Given that the alkylation reaction occurs only when acid strength is sufficiently high to catalyze the reaction of isobutane with olefins, the effectiveness of SA diminishes when its strength falls below a certain level due to accumulation of ASO and H2O— typically around 88%-90%. Thus, the economics of SA alkylation depend on knowing exactly the point where SA becomes too weak and must be taken out of service. For example, taking SA out of service when its strength is 89% may be very costly if good quality alkylate can be produced economically with acid strength≧88.5%.
Traditional Analysis of Acid Catalyst
The composition of acid catalyst is typically determined by manually obtaining a sample for analysis in the local refinery laboratory daily, weekly, or several times each week. In contrast with hydrocarbon samples routinely analyzed in the refinery lab, full analysis of acid catalyst samples tends not to be straightforward due to special requirements for sample handling, preparation, and analysis. Additionally, HF presents a safety hazard due to its volatility and toxicity. With both HF and SA, comprehensive determination of composition is difficult for at least two reasons. First, measurement of water generally depends on a Karl Fischer titration method specially modified to neutralize the strong acid. Second, ASO is not a single compound, but includes a range of chemically-related compounds that have a rather wide range of molecular weights and boiling points, some of which (e.g., “light ASO”) can evaporate rapidly at room temperature.
Analysis Frequency
In consideration of the foregoing difficulties, refiners may test the acid as infrequently as possible to minimize laboratory workload. Some refiners make do with one analysis per week while refineries operating in Los Angeles County, Calif. may be required by regulation to test HF catalyst once every 8 hours. Infrequent analysis may be sufficient to permit process control under stable operating conditions, but not to identify rapid changes caused by occasional surges in feed impurities that lead to generation of ASO.
Analytical Reproducibility and Completeness
Compounding the issue of analysis frequency, laboratory test results may not always be reliable due to the difficulty of obtaining a representative sample when sample volumes are minimized in consideration of safety, as may be done in the case of HF catalyst. This further compounds the difficulty of reproducibly executing the test method itself. And if technicians running the tests do not routinely perform the Karl Fischer water measurement, acid strength measured by titration may be the only parameter known in regard to catalyst composition, severely limiting operators' ability to optimize the process.
Safety
As mentioned, HF is both volatile and toxic. Sampling, sample handling, and testing therefore are executed in accordance with audited procedures carefully designed to ensure the safety of operators and technicians. In the case of HF, testing frequency may be deliberately suppressed to minimize exposure risks.
All of this underscores the undesirability of manual methods for routine analysis. Attempts have been made to replace manual sampling and testing with online measurement techniques, to facilitate the efficient and safe operation of alkylation units. To date, however, these attempts have generally been unsatisfactory, e.g., due to incomplete or inaccurate measurements by simple univariate instruments; or due to excessive complexity, lower-than-desired reliability and/or relatively high costs, such as associated with conventional use of spectrometric technologies. Thus, a need exists for an improved analyzer system for real-time alkylation process analysis and control.
SUMMARY
According to one aspect of the invention, an apparatus is provided for on-line concentration determination of components in a liquid hydrocarbon mixture flowing through an alkylation process, which liquid hydrocarbon mixture includes an unknown concentration of components including hydrocarbons and water. The apparatus includes a fluid flow path configured to convey the liquid mixture continuously in a downstream direction from the alkylation process. An instrument is located along the fluid flow path and is configured for measuring a property of the liquid mixture, the instrument having responsivities to concentrations of one of the components, substantially independent of the concentrations of the water. A temperature detector is configured to generate temperature data for the liquid mixture, and a second instrument located along the fluid flow path is configured for measuring another property of the liquid mixture. The first and second instruments are configured to have mutually distinct responsivities to concentrations of the components. A processor is configured to capture data generated by the temperature detector and the first and second instruments, and to use the data in combination with a model of responsivities to various concentrations of the components at various temperatures, to determine a temperature compensated concentration of the components in the liquid mixture while the liquid mixture flows continuously through the fluid flow path.
In another aspect of the invention, an apparatus is provided for on-line determination of levels of at least three properties in a liquid mixture which contains unknown levels of the properties. The apparatus includes a fluid flow path configured to convey the liquid mixture continuously in a downstream direction therethrough, and a Raman spectrometer located along the fluid flow path, and configured for measuring a first property of the liquid mixture. A separator is located upstream of the Raman spectrometer along the fluid flow path, to remove hydrocarbon present in a gas or liquid phase distinct from that of the liquid mixture, so that the liquid mixture is conveyed continuously through the Raman spectrometer. The Raman spectrometer is configured to have responsivities to concentration of one of the components, substantially independent of the concentrations of the water. A conductivity sensor is located along the fluid flow path, and configured for measuring conductivity of the liquid mixture. The Raman spectrometer and the conductivity sensor are configured to have mutually distinct responsivities to levels of the components. A processor is configured for capturing data generated by the Raman spectrometer and conductivity sensor and using the data in combination with a model of responsivities to various levels of the properties at various temperatures, to determine levels of the properties in the liquid mixture while the liquid mixture flows continuously through the fluid flow path.
In yet another aspect of the invention, a method is provided for on-line concentration determination of components in a liquid hydrocarbon mixture flowing through an alkylation process, which liquid hydrocarbon mixture includes an unknown concentration of components including hydrocarbons and water. The method includes supplying the liquid mixture in a downstream direction along a fluid flow path to a first instrument configured to have responsivities to concentrations of one or more of the components substantially independent of the concentrations of the water. The acid catalyst is supplied to a temperature detector and to a second instrument configured to have responsivities to concentrations of water, wherein the first and second instruments are configured to have mutually distinct responsivities to concentrations of the components. Properties of the liquid mixture are measured using the first and second instruments. A processor captures data generated by the first and second instruments and the temperature detector, and uses the data in combination with a model of responsivities to various concentrations of the components at various temperatures, to generate a temperature compensated concentration of the components in the liquid mixture, while the liquid mixture flows continuously through the fluid flow path.
In still another aspect of the invention, a method is provided for on-line concentration determination of the composition of a liquid mixture which contains unknown levels of at least three components. The method includes supplying the liquid mixture to at least first and second instruments having mutually distinct responsivities to levels of the three components, at least one of the instruments being a Raman spectrometer and the other being a conductivity sensor. The liquid mixture is supplied to the Raman spectrometer through a separator to remove hydrocarbon present in a gas or liquid phase distinct from that of the liquid mixture, so that the liquid mixture is conveyed continuously through the Raman spectrometer. First and second properties of the liquid mixture are measured using the first and second instruments respectively. Data generated by the first and second instruments is captured and used in combination with a model of responsivities to various levels of the components at various temperatures, to determine levels of the at least three components in the liquid mixture.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.
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