Cokes polished surfaces

Optical microscopy, using polarized reflected light, has been recognized as a very useful tool and has played a major role in studies of carbonization mechanisms and coke quality properties. Examination of a coke polished surface under a microscope reveals the presence of certain parts without optical activity (isotropic coke) and others with optical activity (anisotropic coke). The anisotropic carbon exhibits yellow, blue, and purple areas of various sizes, <1-200 )tm, and shapes (optical texture) with an interchange of color on rotation of the specimen. 

Four of the five comparisons made with Incoloy 800 coupons that were either untreated steam treated or oxygen treated indicated that more coke formed on unpolished coupons as compared to polished coupons (see Table III). The single exception was the coupon on which most of the coke formed on the edges whereas the flat and polished surfaces were relatively free of coke. It seems safe to conclude therefore that less coke formed in all cases on the polished flat surfaces as compared to the unpolished surfaces. 

The results of this investigation help clarify why machining is beneficial relative to coke formation rough areas on the surface act as excellent collection sites for coke or tar droplets formed in the gas phase. Such tar droplets eventually become globular coke. Smoother surfaces hence minimize such collection or surface deposits. In addition. smoothing or polishing of both Incoloy 800 and aluminized... 

The optical anisotropy of cokes gives rise to a characteristic pattern of extinction contours when a polished surface is examined by polarized light microscopy using crossed polars, or reflection interference colours if the polars are parallel and a half wave plate is inserted into the optical system (, 12) ... 

Nomenclature to Describe Optical Texture in Polished Surfaces of Cokes... 

Gasification and Heat Treatment. Two polished surfaces of each of the industrially prepared metallurgical cokes listed above were prepared and examined under the optical microscope. 

Each coke was mounted in resin and a polished surface prepared which was examined and photographed under the optical microscope Fracture surfaces of each coke were examined by SEM ... 

Gasification and Heat Treatment. Examination under the optical microscope showed the Spencer works and Clyde Ironworks cokes to have optical textures mainly consisting of fine- and medium-grained mosaics with some coarse flow anisotropy and isotropic inert material. Of particular interest are the fissures which develop in different types of optical texture and those occurring at the anisotropic-inert interface. SEM examination of these polished surfaces before experimentation shows all of them to be flat and featureless. 

The outside surface of shot coke spheres is coated with a layer of needle coke. The coating gives shot coke its polished-surface appearance. 

For polished Incoloy 800 coupons (see Figure 8)< a mixture of filament and globular coke formed on the oxygen-treated surfaces. Filament coke was however not detected on either untreated or steam treated coupons. The coupon treated with steam at 00°C was covered rather uniformly with globular coke. The coupon treated with steam at 900°C exhibited a rippled layer of coalesced globular coke. 

Figure 8. Coke deposited on various polished Incoloy 800 surfaces. Conditions 5% CtHt 900°C for 1 h.

Much more coke formed on aluminized surfaces that had been sulfided than on any other aluminized surface (see Table III). An appreciable number of coke filaments formed on both the polished and unpolished... 

The results of this investigation greatly clarify several factors that affect coking in pyrolysis coils and transfer line exchangers. In particular, the role of the surface is better defined since significant differences in results were obtained when directly comparing Incoloy 800 versus aluminized Incoloy 800 and polished versus unpolished surfaces. 

Incoloy 800 surfaces acts to produce surfaces with more desirable compositions these new surfaces are more resistant to oxidizing. sulfiding. or surface coking reactions. Of interest. Qregg and Leach (11) had found that less coke was formed on electropolished nickel surfaces as compared to unpolished nickel surfaces. In the case of the aluminized Incoloy BOO. care must be taken not to remove too much of the surface since the aluminum has penetrated only short distances into the surface. The results of the present investigation imply that the method or degree of polishing is important. It is not clear why polished coupons in some cases collected considerable coke near the corners of the coupons. 

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