By Mark Clark, Alky Sulfur Operations Specialist, ConocoPhillips (Sweeny Refinery)
Steve West, Manager, Analytical and Temperature Product Development, Invensys Operations Management
A vital process in petroleum refining is the synthesis of isooctane (i.e., octane) for blending in the dynamic gasoline pool. The most widely used process for octane production is HF Alkylation. In this process, HF (hydrogen fluoride; hydrofluoric acid; "acid") serves as a catalyst for the reaction of isobutane and C4 olefins to form octane or "alkylate." Prior to the development of the alkylation process, these low-molecular weight hydrocarbons were essentially waste. Alkylation turns these former waste products into the most valuable component of gasoline.
There are three main components in HF catalyst. HF is the principal component comprising about 90%. Water is usually present at around the 1% level, and the remainder is acid-soluble organics (ASO). Tight control of these constituent concentrations can save millions of dollars per year in an alkylation unit. Optimization requires monitoring the levels of all three components. In this paper, we describe our experience with a new, simple, safe and reliable system for monitoring HF catalyst composition at the ConocoPhillips refinery in Sweeny, Texas. The Invensys ACA.HF system has enabled optimization of the alkylation process for maximum profitability.
2. Control for Safety and Profitability
Safety and profitability of an alkylation unit depend on fine-tuning the controls and are maximized when the net consumption of HF is minimized. Operating at the lowest possible HF concentration, in regards to octane barrels, without risking an out-of-control phenomenon known as "acid runaway" is the optimal scenario—greatest output of octane barrels at minimum cost. If we can minimize the consumption of HF, the advantages ripple right down the supply chain. There are limitations on how much HF can be safely stored on site and it can take days to bring more in if unexpectedly needed. Accurate monitoring allows us to keep inventory low at the ConocoPhillips Sweeny refinery without risking a shortage.
The definition of a catalyst is: A substance that increases the rate of a chemical reaction without being consumed in the process. Optimal control of the HF acid regeneration operation can make a difference of millions of dollars in profit per year in octane barrels, or lost profit opportunities due to "off-color" alkylate product, non-utilized feed rates, and, worst of all, acid runaways. In acid runaway, HF’s role as a catalyst is compromised. This occurs when the HF concentration falls below a critical concentration threshold and begins participating as a reactant in undesired side reactions—reactions that consume HF and do not produce octane. Knowledge of HF and ASO concentrations are critical to avoiding this condition.
Water concentration is also critical. A small amount of water improves reaction efficiency, but too much water increases the corrosiveness of the catalyst.
In order to avoid acid runaway, an HF concentration margin must be maintained over and above the critical concentration. Obviously, uncertainty around the HF and ASO concentrations at any time requires us to maintain a greater margin and results in poorer efficiency. Accurate, real-time concentration data allows us to run at a tight yet safe margin with confidence.
HF Alkylation Process HF is not consumed; it is recycled.
3. Existing Monitoring and Control Strategies
A prerequisite for control of a process is knowledge of the key process parameters, in this case, the concentrations of HF, ASO and water in the reaction mixture. Two monitoring strategies can be found in the industry today: grab-sample extraction for laboratory analysis and online analysis.
Grab Sampling / Lab Analysis
We have nothing but the highest regard for the chemists and technicians who perform chemical analyses at our ConocoPhillips refinery laboratories. In spite of the challenges described below, the skills and training of these individuals have ensured safe, efficient operation of alkylation units for decades. Nevertheless, the disadvantages of grab sampling and lab analysis of HF cannot be overemphasized. These disadvantages arise on two fronts: safety and accuracy.
The hazardousness of HF is extreme. Not only is it severely toxic through ingestion, inhalation, or contact with skin, it is highly volatile, permeating, and corrosive. Grab samples must be taken in sealed, Monel sampling bombs by personnel wearing full protective gear and respiratory apparatus. Personnel with special training must carry out analyses in a designated, segregated laboratory area with safety showers present in the lab and along the transport route.
HF catalyst is a reactive mixture—removing the grab sample from the process does not stop the reaction. In other words, the clock is ticking on how representative the grab sample is of the process. Removal of the sample from the bomb, and all subsequent handling must be carried out anaerobically using materials and instruments that can withstand exposure to HF. This precludes the use of ordinary laboratory glassware, most metals and plastics. No matter how efficient the analyst or accurate the results, uncertainty grows as time elapses between sampling and the return of the results. This time is at best hours, often a whole day.
The composition of HF catalyst can be determined by the technique known as Fourier-Transform, Near-IR Spectroscopy (FTNIR). This powerful tool analyzes the near-IR absorption spectrum of a mixture and breaks it down into contributions from its components. Using a pre-developed chemometric model, concentration values for target components can then be derived from the IR spectrum. In the 1990s, online FTNIR instrumentation was developed for HF alkylation analysis. Moving the analysis from the lab to the process proved enormously useful. In addition to the advantages that can be enumerated by turning the above-mentioned disadvantages of grab sampling and lab analysis upside down, real-time online analysis allows us to "skate closer to the edge", in other words to narrow the control margin and realize higher efficiency.