4. The Invensys ACA.HF Solution
Although the online FTNIR brought a completely new level of control to an alkylation unit operator, the up-front and ongoing cost of the instrumentation leaves many refineries continuing to rely on lab analysis. The desire to provide the advantages of real-time, online monitoring at a lower cost led to a ConocoPhillips collaboration with Invensys on the development of the ACA.HF system for HF catalyst monitoring. In addition to lower up-front cost and a reduction in maintenance, goals in the effort were simplicity, robustness and the use of proven process analytical hardware.
Products from the Invensys Foxboro Measurements & Instruments (M&I) brand have been used for decades in refineries and many other industries. Simple, robust analytical measurements have an established track record in the characterization of 2-component solutions. Common examples are the use of a density measurement to determine the antifreeze content of automobile engine coolant, or refractive index to measure the sugar content of wine. To characterize binary solutions, one measurement that varies uniquely with solution composition is required, because determination of one component allows the other to be inferred as the remainder to make up a total of 100%. Analogously, a ternary solution can be characterized by two measurements, so long as they vary uniquely with the concentrations of the constituents.
The HF alkylation reaction mixture is composed mainly of HF. Invensys Foxboro electrolytic conductivity sensors are a proven approach to online water determination in HF. Water dissolves in HF undergoing the following dissociation reaction: 2HF + H2
O → HF2
- + H3
The dissolution products are ionized species, which make the solution conductive. In binary HF/water solutions, the concentration of conductive species and therefore the conductivity is determined by the water concentration (see accompanying figure).
Fortuitously, the third major constituent of HF alkylation catalyst, ASO, dissolves without the creation of ions. This means that a conductivity measurement is an uncompromised predictor of water concentration in the reaction mixture.
Conductivity is a highly developed measurement technique with a long legacy in process control. In the alkylation application, an electrodeless, non-contacting conductivity sensor was selected. In this configuration, the process passes through an unobstructed bore of inert material—a simple pipe. External to the bore, a coil called a "drive toroid" generates an oscillating magnetic field, which induces an alternating current in the process fluid. That current in turn generates an oscillating signal in a second external coil called a "sense toroid". The signal in the sense toroid is proportional to process fluid conductivity which is proportional to the water concentration.
Conductivity provides a handle on water, then a second measurement—density—is used to determine the HF and ASO concentrations. Light hydrocarbons have densities as low as 0.6 g/cc, whereas the density of pure HF is very nearly 1.0 g/cc, almost exactly the same density as water. Thus, the relative concentrations of HF and ASO affect the density of the alkylation mixture while water does not. Just as density may be used to determine the composition of a binary mixture, for example, ethanol and water as shown in the accompanying figure, it serves in the ACA.HF system to determine the concentrations of either HF or ASO.
As with conductivity, a proven, process-hardened Invensys Foxboro sensor accomplishes the density measurement. This sensor, known as a Coriolis sensor, measures not only density but mass flow rate, a useful diagnostic output in the ACA.HF system. A coriolis sensor is essentially a coiled tube on which a resonant vibration is induced. Mass flow and density are determined by analysis of phase shift and frequency of the tube vibration, respectively. Again as with conductivity, the fluid passes through nothing but a pipe, but in this case shaped as a coil. With two measurements, conductivity and density, two components of the catalyst are determined, and the third is simply the remainder, since:
%HF + %ASO + %Water = 100%
Accomplishment of the HF catalyst analysis is not quite as simple as described. Two important aspects were not included for simplicity of explanation: the effect of temperature, and the second-order influence of interactions that may occur between the ASO and water.
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: