Applied Chemistry Spans Molecules, Models, and Refinery Reality

By Winston K. Robbins, Ph.D.

Modern refineries operate on the basis of complex optimization models that are derived from basic research.  Optimization model inputs include refinery configuration, operating conditions, physical properties, and target product qualities.  Process parameters are adjusted to maximize the return for a given blend of crudes and target products.  The sophistication that has been reached in producing control models has made refinery practice distant from the chemical knowledge underlying the fundamental molecular models (Figure 1).

 

Operations run smoothly for conditions where the models adequately manipulate the molecular information with kinetic and thermodynamic conditions adapted for specific refinery hardware configurations.  Refinery operation emphasizes process control parameters (e.g., heat, hydrogen, pressure, etc.); modeling combines engineering with high-level computation; molecular characterization draws on research analytical methods. Success continues as long as the integrity of the molecular data and assumptions of the models are maintained. That is, knowledge flows along the diagonal from molecules to refinery.

Refinery operators, model builders, and research analysts often apply chemistry to a narrow task; while each uses chemistry, none may have a broad enough chemical knowledge to troubleshoot upsets. When upsets occur, help from an applied chemist with practical knowledge of the underlying petroleum chemistry may be needed.  Drawing on experience, broad chemical knowledge, an understanding of processes, and the assumptions underlying the model development, the applied chemist provides molecular insights to a real refinery problem.  In troubleshooting teams, the applied chemist may suggest analyses, question assumptions, and propose solutions that might be overlooked by the specialist.

Contributions of applied chemists are often overlooked, but can be found throughout the industry.

For example, consider the following incident:

A refinery hydro-finishing wax for food-related applications, routinely sent samples to an analytical lab for FDA compliance testing.  Although this product had a long history of complying easily, a sample failed.  Requests for additional tests confirmed that the product was off-spec and the product was diverted to other applications.  An applied chemist was asked to determine the cause of the failure.

The FDA test, developed for screening products for polynuclear aromatic hydrocarbons (PAH), combined DMSO extractions with pass/fail UV measurements at four wavelengths.  The applied chemist reviewed previous compliance tests at the lab and found that while most samples had passed easily, the batch prior to the rejected lot exhibited a marginal pass.  When asked about this, the refinery responded that there had been no apparent change in the hydroprocessing conditions that might coincide with the change in UV performance.

At the applied chemist’s request, additional tests were run on retains of the “easy,” “marginal,” and “rejected” lots.  These tests replicated the earlier data, confirming that the analytical procedure was being performed properly.  In observing the analytical test, the applied chemist used a “black-light” to examine the extracts from each test type as a quick check for PAH (many fluoresce under black light excitation).  The “reject” sample glowed brightly, the “marginal” glowed dimly and the “pass” sample remained dark.  With this observation, he returned to the lab and inspected the spectra of the “rejected” and “marginal” tests.  These spectra exhibited sharp peaks characteristic of individual PAH.  Upon his request, more sophisticated analytical procedures were used to isolate and identify perylene as the PAH responsible for the rejection.

The applied chemist recognized perylene as a PAH that can be formed in hydroprocessing when operated under hydrogen starvation conditions.  With perylene identified, the refinery again checked its records on operations.  This investigation revealed that just prior to sampling of the “marginal” batch, the wax had gone “off-color.”  Suspecting a pin-hole leak in the feed heat exchanger, the operator had increased the hydroprocessing temperature 550F to 575F and brought the color back in spec.  Over time, temperature was increased further to hold color, but according to the readings, the temperature never exceeded 650F (a temperature where hydrogen starvation would begin).  Because the applied chemist had provided such strong evidence of over-temperature, the thermocouples were checked and the controlling thermocouple was found to be 50F.  As a result of this work, the wax hydroprocessing unit was shut down, the pin-hole fixed, and all thermocouples recalibrated.  Once lined out, the unit was back on stream producing food-grade wax that easily met the FDA requirements.

In this case, the applied chemist provided the link between the hydroprocessing process and the analytical laboratory.  His knowledge of PAH, the choice of analyses, and the effect of process conditions on hydroprocessing chemistry, allowed him to guide the refinery to a successful solution of its upset.

In many cases, the input of the applied chemist is overlooked because his skill doesn’t fit into a “routine” job description.  In a refinery, an experienced person (senior engineer, lab head, etc.) may serve as an applied chemist when needed, as in troubleshooting.  At other locations, outside resources (e.g., company research group, engineering service provider, or consultant) may also be called upon to provide this capability.  Ideally, every refiner should have a network of applied chemists identified for their specific operations.  Do you know who your applied chemists are?