Shona C. Martin

In situ NMR investigation into the thermal degradation and stabilisation of PAN

In situ broadline 1H NMR has been used to assess the low temperature degradation of polyacrylonitrile (PAN) under a variety of atmospheres and temperatures. In combination with conventional analytical techniques (solid state 13C NMR and FTIR), the structural and conformational changes produced in the network have been correlated with its thermal behaviour. Degradation has also been shown to be both temperature and time dependent, irrespective of reaction environment. Moreover, it has been demonstrated why air is the preferred medium for stabilisation in the production of fibre precursors. Enhanced stability and resistance to char formation is observed in the air-modified polymer, which also becomes resistant to resoftening during low temperature carbonisation.

Structural characterisation of catalytic coke by solid-state 13C-NMR spectroscopy

Abstract After reviewing some recent studies on the characterisation of coke deposits on fluid catalytic cracking (FCC) and hydroprocessing catalysts by solid state 13C-NMR, the quantitative structural information that has been obtained through the use of demineralisation of FCC catalysts to provide coke concentrates for analysis will be described. The deactivated catalysts investigated contain only approx. 1% (w/w) carbon and were obtained both from refinery units operating with heavy feeds and from laboratory fluidised-bed tests with n-hexadecane. As for other carbonaceous materials, the use of a low-field field strength in conjunction with the single pulse excitation (SPE or Bloch decay) technique has enabled most of the carbon to be detected and, therefore, NMR-invisible graphitic layers are not thought to be major structural features of the cokes. Although stripping the catalysts gives rise to highly aromatic cokes (aromaticity>0.95), even for n-hexadecane, differences in feedstock composition are still reflected in the structure of the resultant cokes with those derived from n-hexadecane containing less condensed aromatic nuclei than those from heavy feeds.

Development of an Experimental Protocol to Evaluate FCC Stripper Performance in Terms of Coke Yield and Composition

Tests have been conducted in a microactivity test (MAT) and a fluidized bed reactor to develop an experimental protocol to determine how the yield and composition of coke and the associated catalyst surface area vary as a function of stripper conditions in fluid catalytic cracking (FCC). In both reactors, the use of rapid quenching has allowed the relatively short stripping times encountered in FCC units to be simulated. Low sulphur vacuum gas oils (VGO) with a low metal equilibrium catalyst (E-cat) were used for stripping periods of up to 20 minutes. Significant variations occur in the structure of both hard and soft coke during stripping. Although the hard coke becomes more highly condensed with prolonged stripping, the surface area reduction by the hard coke remains fairly constant for stripping periods in excess of ca. 5–10 minutes and is small (10m2 g–1) in relation to the loss of surface area from the soft coke. The use of about 70 g of catalyst in the fluidized bed provides sufficient sample for demineralization to recover sufficient of the hard coke for 13C NMR analysis after the initial extraction of the soft coke. Indeed, it has been found that a further pool of soft (chloroform soluble) coke is physically entrapped within the catalyst pore structure and is only released after demineralization. In fact, this second soft coke fraction is much more highly aromatic than the first and ultimately controls the final coke yield. For the combination of E-cat and VGOs investigated here, typically about half of the final hard coke content of nearly 1% w/w catalyst is derived from this second soft coke fraction by carbonization. The structural information obtained has been used to formulate a model for the stripping process where the soft coke II fraction undergoes cracking in competition with coke formation and evaporative removal from the catalyst.

An experimental protocol to evaluate FCC stripper performance in terms of coke yield and composition

Tests have been conducted in a microactivity test (MAT) and a fluidized bed reactor to develop an experimental protocol to determine how the yield and composition of coke and the associated catalyst surface area vary as a function of stripper conditions in fluid catalytic cracking (FCC). In both reactors, the use of rapid quenching has allowed the relatively short stripping times encountered in FCC units to be simulated. Low sulphur vacuum gas oils (VGO) with a low metal equilibrium catalyst (E-cat) were used for stripping periods of up to 20 minutes. Significant variations occur in the structure of both hard and soft coke during stripping. Although the hard coke becomes more highly condensed with prolonged stripping, the surface area reduction by the hard coke remains fairly constant for stripping periods in excess of ca. 5-10 minutes and is small (10m2 g-1) in relation to the loss of surface area from the soft coke. The use of about 70 g of catalyst in the fluidized bed provides sufficient sample for demineralization to recover sufficient of the hard coke for 13C NMR analysis after the initial extraction of the soft coke. Indeed, it has been found that a further pool of soft (chloroform soluble) coke is physically entrapped within the catalyst pore structure and is only released after demineralization. In fact, this second soft coke fraction is much more highly aromatic than the first and ultimately controls the final coke yield. For the combination of E-cat and VGOs investigated here, typically about half of the final hard coke content of nearly 1% w/w catalyst is derived from this second soft coke fraction by carbonization. The structural information obtained has been used to formulate a model for the stripping process where the soft coke II fraction undergoes cracking in competition with coke formation and evaporative removal from the catalyst.