All species in a solution affect each other directly or indirectly. In the present case, the assumption that conductivity can predict water content and that density can predict HF/ASO content in a ternary solution turned out to be extremely powerful (see figure below). However, corrections for temperature and interactions between the components were built into the ACA.HF model as refinements.
A chemometric model was developed by recording conductivity (Cond), flow (Flow), pressure (P), density (D), and temperature (T) of HF alkylation catalyst in an operating alkylation unit using the online ACA.HF system. The ACA.HF data set was time-correlated to %HF, %H2O, and %ASO from plant records over a period of four months. Using multivariate regression techniques, the data was fit to equations that predicted %H2O and %HF as functions1 of conductivity, density and temperature:
%H2O = ƒ1(Cond) +ƒ1 (Dens) + ƒ1 (Temp)
%HF = ƒ2 (Cond) +ƒ2 (Dens) + ƒ2 (Temp)
ASO was then calculated2,3:
%ASO = 100 - %HF - %H2O
The graphs to the right illustrate the tight agreement between ACA.HF and online FTNIR analyses, demonstrating that the simpler, more robust ACA.HF sensing strategy can deliver high-quality results.
Referring to the system diagram below, the Invensys system consists of a sampling panel located in the hazardous area. All fluid-handling components, sensors and signal transmitters are located at the sampling panel. Signals are transmitted to an Invensys Eurotherm DCS and Invensys Wonderware HMI in the safe area.
¹ Proprietary functions.
² Because of refinery precedent, %ASO was determined by subtraction after calculating %H2O and %HF from the regression model. Determining %ASO by regression and %HF by subtraction is mathematically equivalent.
³ In keeping with industry precedent, results are actually reported on the basis of a 94% rather than 100% total. This assumes a 6% loading of entrained isobutane.
Advantages of ACA.HF over FTNIR
Although an FTNIR spectrometer is an extremely powerful analytical instrument, its adaptation to online operation in an alkylation process is a complex undertaking whereas conductivity and density sensors are extremely simple and reliable—essentially just lengths of pipe. Their use in HF media has long been established. ACA.HF achieves further simplification through elimination of the need to control sample temperature, and through direct installation into the fast loop. Analysis of a continuously flowing sample system obviates the need for a complex sample handling system with a frequently cycling shutoff valves that require maintenance and periodic replacement. ACA.HF outputs its own set of diagnostic information—flow rate, pressure, and temperature. The cost of an ACA.HF system is about half that of online FTNIR and in just over one year at the ConocoPhillips Sweeny refinery, there has been no required maintenance. It was commissioned in less than 2 days, requiring no special skills to install and operate that are not found in every refinery. The components are laid out on an open panel requiring no inert gas purge or HF vapor detection.
At Sweeny, we now perform one lab sample per month, down from two per week prior to the ACA.HF installation. Lab samples were previously taken every Friday and Monday—Fridays so acid levels could be established before leaving for the weekend, and Mondays so planning could be supported in the coming week. Knowing acid and ASO levels allows operation of the HF rerun in a manner that optimizes octane barrels without the risk of acid runaway. Acid runaway not only shuts down the alkylation unit, but also cuts output of the entire refinery by loss of feed to the cat crackers.
Real-time, accurate concentration data helps keep HF down and ASO up. The trouble with FTNIR at high ASO is fouling of the cuvette window, which requires the FTNIR unit to be shut down for cleaning. The ACA.HF system does not have these types of problems.
The advantages of online alkylation process monitoring are clear. Fine-tuning the concentrations of the key parameters—HF, ASO and water—improves profitability and safety. The Invensys ACA.HF Alkylation Solution has demonstrated at the ConocoPhillips Sweeny refinery that continuous monitoring can be achieved with lower installed cost, less maintenance, and higher reliability than ever before. At Sweeny, the ACA.HF system quickly became depended upon as the workhorse for control of the HF alkylation unit.
©2010 Invensys Systems, Inc. All rights reserved. Avantis, Eurotherm, Foxboro, InFusion, Invensys, SimSci-Esscor, Skelta, Triconex, Validation Technologies and Wonderware are trademarks of Invensys plc., its subsidiaries and affiliated companies. All other product names are the trademarks of their respective holders. rel. 08/10 PN FX-0111