Research Reality

By Winston K. Robbins, Ph.D.

Most modern refineries are run based on process models. However, underlying these engineering models are laboratory research studies. Between the lab and the refinery, "unit factors" are often applied to make research findings fit the real world. In some cases, the "unit factors" are due to local engineering requirements; however, in others, they arise from chemical limitations inherent in the research. In general, the engineering factors that are required to scale-up lab research are well recognized. On the other hand, the chemical limitations are often overlooked.

Ideally, research would be carried out by following the effects of a process on all components in a refinery stream using multiple analytical tools. Unfortunately, studies that are carried out on whole feeds can quickly degrade into analytical method development projects. As an alternative, research often turns to using "representative" model compounds, synthetic matrix oils, and established detection methods. The limitations of this approach are illustrated in the following examples that include a reality check and potential improvements that are available by using advanced characterization methods.

Lubes Processing

• Background: Solvent extraction of heavy vacuum gas oils (HVGO) is used to remove multi-ring aromatics that degrade lube performance while retaining saturates and mono-aromatics.
• Model Study: Fluorescence measurements of anthracene and pyrene have been studied for the partitioning between iso-octane and polar solvents.
• Reality Check:
  • Model compounds
    • Neither anthracene nor pyrene are found in petroleum HVGO
    • Anthracene and pyrene do not boil in the HVGO temperature range (i.e., 850-1050°F)
    • Most 3+ ring aromatics in HVGO have 2+ sides
    • Some 3+ ring aromatics include fused naphthene rings
    • 3+ ring aromatics in the 850-1050°F range have molecular weights that are equivalent to C25 - C32
  • Matrix oil
    • Iso-octane is immiscible with extraction solvents; HVGO less so
    • Iso-octane has no aromaticity, vs. >20% in HVGO
  • Measurement
    • Fluorescence is sensitive to self-quenching; ppb levels are used for testing
    • Fluorescence is color quenched; so, only pure solvent can be applied
• Advanced characterization: Various methods have shown that HVGO may contain 5 to 20% compounds with three or more fused rings and total aromaticity of over 30%.
  • Appropriate 3-ring structures are phenanthrene and chrysene
  • Appropriate structures are alkylated with two short and one long side chain
  • Extraction evaluation requires wet extraction solvents

Shale Oil Hydro-processing

• Background: Shale oils that are produced as pyrolysis oils from resources located in Colorado and Australia resemble coker liquids but are much richer in N and lower in S.
• Model Study: Shale oil fractions were hydro-treated to reduce olefins under conditions that were developed for coker liquids. Excessive amounts of water and ammonia were detected. Pyrolysis of model amides yields ketones and nitriles.
• Reality Check:
  • Model compounds:
    • Functional groups only identified after the fact
  • Matrix Oil - OK
  • Measurement:
    • Too simplistic; based on elemental analysis, tests for olefins
• Advanced Characterization: HPLC, MS and FTIR demonstrated the presence of ketones, nitriles, and amides in addition to the pyrrole and pyridine benzo-logs normally found in coker liquids.
  • Appropriate model compounds identified
  • HPLC can separate fractions by functional groups
    • Ring types
    • C13 NMR indicates alkylation
  • FTIR can monitor functional groups
  • MS (GC/MS) defines MW

Non-Porphyrin Ni &V Compounds in Vacuum Resid

• Background: Refinery vacuum resid contains varying amounts of Ni & V that is identified as non-porphyrin metal compounds on the basis of Soret and elemental analyses.
• Model Study: Model Ni & V model pophyrins were well resolved from Ni & V compounds in vacuum resid and a variety of other model organometallic Ni & V compounds in toluene by HPLC or GPC techniques.
• Reality Check:
  • Model Compounds:
    • Commercially available Ni & V etio-porphyrins do not resemble native porphyrins
    • No basis for alternative organo-metallics beyond availability
  • Matrix Oil
    • Toluene adequate for dissolving resid
    • Toluene inadequate for dissociating asphaltenes
  • Measurement
    • Soret bands (porphyrin colors) are structure dependent
    • Ni & V porphyrins as pure compounds yield Soret bands
    • Ni & V mixtures of porphyrins overlap Soret bands
    • Vacuum resid color overlaps Soret bands
    • HPLC & GPC not run at sufficient temperature to dissociate asphaltenes (~180°C)
• Advanced Characterization
  • XPS and related techniques demonstrate that all Ni & V are in a tetra-N environment (i.e., in porphyrin-like complexes)
  • Some commercial model compounds are substituted in bridgehead positions, not seen in petroleum
  • Modern MS techniques have identified a large variety of C28 to C32 (i.e., alkyl isomers) of the Ni & V porphyrins

The limitations of a research study can be determined by how adequately the choice of model compounds, matrix material, and methods of detection probe the chemical reaction space. The chemical research space can be envisioned as a balance of three contributing factors (Figure 1).

 

Figure 1 - Chemical Factors in Research Studies

As shown, each of the factors may cover only a portion of the research space, and a given combination may only represent a small portion of the area of interest. It is important to recognize that all three factors bound the results. A combination of two factors may dictate the choice of the third. As a trivial example, if color is to follow a reaction of a model compound, then the matrix oil should dissolve the compound and have no color of its own.

Some of the parameters for each factor are listed below.

 

This list is far from complete. However, it provides a framework for designing or evaluating research studies. As seen in the examples, a poor choice of model compounds, matrix oil, or detection method limits the relevance of the research.

Advanced characterization methods play a vital role in maximizing the benefit of research studies. A thorough characterization of a target process stream defines the range of molecules that must be included as model compounds. The same characterization can identify a composition that meets the solvency of the matrix oil and compatibility with analytical measurement methods. Future articles will expand on the framework described above.