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Needle coke crystallography
Graphitisation depends on the efficacy of thermal energy to produce slight incremental adjustments to the alignment of the graphene planes (Mochida et al., 1994). The mobility of aromatic molecular planes is fixed when the coalesced mesophase solidifies to form the green coke and thus it is impossible to produce a more highly aligned structure from an isotropic coke by increasing the temperature alone.
As optical microscopy is able to show the basic alignment of planes at various heat treatment temperatures on a micron-scale, X-Ray Diffraction (XRD) is able to follow the growth of crystals on a nanoscale (Murthy et al., 2001). The effect of graphitisation on CTE is more easily understood using XRD, which is able to provide finite values (La, Lc, d002) rather than relying on point counting of aligned planes using Scanning Electron Microscopy (SEM) or optical microscopy, which provides qualitative aspects of the nano/milli-structure. The effect of increasing the temperature on the XRD indices of a petroleum coke is shown in Table 2-2.
Puffing
Puffing may be defined as the irreversible expansion of an electrode during graphitisation. It is caused by the release of bound heteroatoms from the carbon matrix between 1 600 and 2 300 °C, as described by Kawano et al. (1999a). The nature of the heteroatoms released is dependent on the origin of the needle coke precursor. Petroleum-based needle cokes tend to release higher concentrations of sulphur, as described by Orac et al. (1992), while coal-based needle cokes tend to release higher concentrations of nitrogen, as described by Kawano et al. (1999b and 2000).
Graphitisation increases the crystal height (c-axis) and width (a-axis). This densification decreases the space between the graphene layers (d-spacing). Needle coke expands during graphitisation up to 1 500 °C and then from 1 500 to 1 600 °C the coke shrinks. This is followed by rapid expansion from 1 600 to 2 300 °C (dynamic puffing) and then at 2 500 °C secondary puffing occurs (Mochida et al., 1994). The gases released during dynamic puffing include N2, CS2, H2S and COS, depending on the type of needle coke, as described by Mochida et al. (1994).
The initial puffing phase occurs between 1 500 and 1 800 °C as released gases cause a volumetric expansion of the coke on a linear curve as a result of pressure on the coke microstructure due to blocking of pathways. According to Mochida et al. (1994), from 1 700 to 1 800 °C, the d-spacing increases slightly due to the release of these heteroatoms. The extent of puffing in an electrode is also dependent on the ability of the released gases to escape through the carbonised binder matrix. Figure 2-13 shows the expansion and shrinkage curves of needle coke heat-treated to 2 500 °C. The coke shrinks on a linear curve on cooling.
Concluding remarks – The Waxy Oil value chain
Analysis of the Waxy Oil production process and compositional characteristics of the feedstock and calcined coke form a template on which the carbonisation chemistry is determined in later chapters. The two predominant detrimental factors in considering Waxy Oil as a needle coke precursor are the catalyst content and the lack of aromaticity. While catalyst removal would appear a natural process step, it is the author‟s opinion that a determination of the effect thereof on the characteristics of Waxy Oil coke is necessary as an initial investigation. This is especially true given that the catalyst is iron oxide, which is known to both retard the extent of mesophase formation and promote multi-phase graphitisation, as demonstrated by Wang et al. (2001). These effects are further discussed in Chapter 6. The efficacy of graphitisation to sublimate iron from Waxy Oil coke is also discussed in Chapter 6.
However, catalyst removal from heavy residues is not unique to Waxy Oil and thus is more of a general requirement. The “art” will reside in the ability to modify the organic molecular composition of Waxy Oil in such a way as to effect a change in the kinetics of the carbonisation cycle. If the effects of other reactivity promoters (e.g. hydroxyl substituents) are for the moment discounted, the given the fact that the literature provides convincing arguments to indicate that increasing the aliphatic nature of heavy residues is correlated with the production of isotropic carbon (Martinez-Escandell et al., 1999), it may well be argued that the probability of producing highly anisotropic carbon from Waxy Oil is indeed remote. However, the author asserts that the aliphatic nature of previously studied residues may well differ substantially from that of Waxy Oil, with specific reference to its unique origin. Thus it would be myopic to merely discount Waxy Oil without evaluating its molecular composition and carbonisation chemistry. Waxy Oil modification, subsequent carbonisation and determination of the reaction mechanism are discussed in Chapters 7 to 9.
1 INTRODUCTION
1.1 Background to the investigation
1.2 Feedstock quality
1.3 Problem statement
1.4 Scope of the investigation
1.5 Objectives of the investigation
2 INTRODUCTION TO NEEDLE COKE
2.1 The industrial value chain of graphite electrode production
2.2 The chemistry of needle coke formation
3 THE WAXY OIL VALUE CHAIN
3.1 The production of Waxy Oil calcined coke from Synthol Decant Oil (SDO)
3.2 Concluding remarks – The Waxy Oil value chain
4 REVIEW OF PREVIOUS WORK
4.1 Introduction
4.2 The chemistry of needle coke precursors
4.3 The influence of heteroatoms on needle coke quality
4.4 Interventions to control puffing of needle coke
4.5 The influence of asphaltenes on needle coke quality
4.6 The influence of mineral matter on needle coke quality
4.7 Molecular modification of needle coke precursors
4.8 Concluding remarks – Review of previous work
5 EXPERIMENTAL
5.1 Experimental procedures for Chapter 6
5.2 Experimental procedures for Chapter 7
5.3 Experimental procedures for Chapter 8
5.4 Experimental procedures for Chapter 9
5.5 Concluding remarks – Experimental
6 INFLUENCE OF CATALYST CONCENTRATION ON THE CHARACTERISTICS OF WAXY OIL COKE
6.1 Introduction
6.2 Macrostructure of needle coke
6.3 Macrostructure of Waxy Oil green coke
6.4 Chemical analysis of Waxy Oil green cokes
6.5 Waxy Oil calcined coke
6.6 Waxy Oil pre-graphite
6.7 Graphitisation of Waxy Oil green coke
6.8 Conclusions – Influence of catalyst concentration on the characteristics of Waxy Oil coke
6.9 Recommendations – Influence of catalyst concentration on the characteristics of waxy oil coke
7 WAXY OIL MODIFICATION
7.1 Introduction
7.2 Modification of Waxy Oil
7.3 Hydrogen (1H) Nuclear Magnetic Resonance (NMR) and Aromatic Index (Iar) of modified Waxy Oils
7.4 Influence of Waxy Oil modification on the distillate fractions of Samples C2, E1a and E1b
7.5 Conclusions – Waxy Oil modification
7.6 Recommendations – Waxy Oil modification
8 “STATIC” CARBONISATION OF MODIFIED WAXY OILS
8.1 Introduction
8.2 The role of “static” carbonisation
8.3 Effect of catalyst removal on coke characteristics
8.4 The effect of molecular modification of Waxy Oil on green coke microstructure and yield
8.5 Maximising the value of Waxy Oil modification
8.6 Is the low green coke yield of Waxy Oil carbonisation a potential problem?
8.7 Conclusions – “Static carbonisation” of Waxy Oils
8.8 Recommendations – “Static carbonisation” of Waxy Oils
9 CARBONISATION MECHANISM OF WAXY OIL
9.1 Introduction
9.2 Low-temperature carbonisation of filtered and thermally treated Waxy Oil
9.3 Residue yield and NMP (soluble and insoluble) fractions
9.4 Thermogravimetry Analysis (TGA) and Differential Thermogravimetry (DTG) of the
carbonisation residue
9.5 Aromaticity Index (Iar)
9.6 Typical molecular composition of pre-mesogen molecules produced by lowtemperature carbonization
9.7 Pre-mesogen molecules produced during low-temperature carbonisation
9.8 Proposed reaction mechanism for the formation of pre-mesogens from high molecular weight normal alkanes
9.9 Quantification of one-to six-ring alkylated aromatic and aromatic compounds
9.10 Mesophase development
9.11 Conclusions – Carbonisation mechanism of Waxy Oil
10 CONCLUSIONS
10.1 Introduction
10.2 The role of iron oxide in the carbonisation of Waxy Oil
10.3 The effect of molecular composition on the microstructure of Waxy Oil coke
10.4 The mechanism of Waxy Oil carbonization
11 CONTRIBUTION TO ORIGINAL KNOWLEDGE
11.1 Does the research address a significant challenge?
11.2 Does the research provide an original solution to this challenge ?
11.3 Is the solution provided significant?
11.4 Epitaph
12 REFERENCES AND BIBLIOGRAPHY
12.1 References
12.2 Bibliography
