Soft coke

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

Normal salt (NaCl) and a mixture of salt and soft coke commonly Magnesium sulphate (MgS04)J Copper sulphate (CuSO )... 

In this way, the conjunct polymers serve as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions, but they accept these ions under other conditions. Substantial amounts of the saturated products are supposedly formed via this route with sulfuric acid. In zeolites, species similar to conjunct polymers also form. The heavy hydrocarbon molecules, which deactivate the catalyst by pore blocking or by site blocking, are generally termed soft coke or low-temperature coke , because of the absence of aromatic species. 

Several factors contribute to the effect of coke on deactivation. The amount of hydrocarbons ("soft" coke) entrained to the regenerator depends on the properties of the aged catalyst. 

The amount of soft" coke or hydrocarbons entrained to the regenerator without being stripped (3, 10] will have a significant effect on the overall coke selectivity and will depend on the surface area and pore size architecture of the aged catalyst [10, 49]. 

This statement corresponds to a new concept in classification of polyaromatic coke formed during catalytic reactions that provides a structural basis for the traditional classifications of hard coke and soft coke (d7-(5P). 

Overall, evaluation of catalysts on resid feedstocks requires sophisticated and well integrated catalyst deactivation, catalyst stripping and cracking systems. It is important to determine not only the coke yield, but each of its components (Catalytic coke, contaminant coke, CCR coke and stripper (soft) coke). This paper provides details on how each of the components of the coke yield may be experimentally determined using catalyst metallation by cyclic deactivation, catalyst strippability measurements and modified catalytic cracking techniques. 

Our investigations show that catalyst composition and architecture can have a significant effect on the initial quantity of adsorbed hydrocarbons, i.e. soft delta coke, as well as on stripping rate (table 4). A high zeolite content usually results in a high soft coke make, but not necessarily in a low stripping rate (cat. A vs B). At a constant activity, hard and soft coke make, the stripping rate increases with accessibility ( cat. B vs C). 

Soft coke The contribution of soft coke can be determined in the cyclic deactivation unit. The soft coke make depends both on physical and chemical properties, such as activity and catalyst accessibility. The initial soft coke is expected to increase with the zeolite content, stripping rates are higher for the very accessible catalysts. 

It seems that there are two types of coke on the catalyst, soft or hard coke. The coke deposition c defined as soft coke in Eq. (18) has equivalent characteristics to that defined in Eq. (14). The rate of coke deposition is simulated by Eq. (2) where coke is defined as hydrogenable . This coke is speculated to be adsorbed polyaromatics rather than coke. Hard coke c defined in Eq. (19), on the contrary, is steadily produced with a deposition rate of Eq. (2) this affects the diffusivity of reactant. 

One important difference in activation energies, however, is apparent for Pi, which has been associated with "soft" coke probably adjacent to metal sites, where the activation energy of the cyclohexene-coked catalyst is ca. 15 kJ mok higher than for spent and 1-hexene coked catalysts. This difference may be related to the dynamics of hydrocarbon adsorption. For linear alkenes only one end of the chain may be attached to the catalyst surface, while for small cyclic alkenes, which have a more compact structure, the entire molecule may be adsorbed onto or electronically affected by the catalyst surface. That is, adsorbed cyclohexene which may lie... 

Figure 13. Hard coke and soft coke versus contact time.

Figure 17. Effect of Surface area on soft coke.

In summary, the quantity of soft coke seems to increase with the surface area in the small-pore range (zeolite and matrix), while the stripping rate is determined inversely by the accessibility of the catalyst sites and increases with larger and nonconstrained pore systems. We can conclude that for delta coke limited RFCC catalyst selection it will be essential to assess the diferences in all the factors contributing to commercial delta coke. 

Sachtler72 distinguishes soft and hard coke. These terms are related to the H/C ratio, where soft coke has H/C = 1-1.5, and hard coke has H/C < 0.2, and Jovanovich73 relates these terms to the dispersion of coke, where soft coke is fine-grained. 

Carbon, hydrogen and nitrogen contents of the initial deactivated catalysts, the soft coke fractions and the hard coke concentrates were determined using a Perkin-Elmer 2400 analyser and sulphur contents were measured using the Sulphazo III method. BET measurements were carried out using a Micromeritics ASAP 2000 apparatus on the... 

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