Middle-distillate oils limitations

Simple conventional refining is based essentially on atmospheric distillation. The residue from the distillation constitutes heavy fuel, the quantity and qualities of which are mainly determined by the crude feedstock available without many ways to improve it. Manufacture of products like asphalt and lubricant bases requires supplementary operations, in particular separation operations and is possible only with a relatively narrow selection of crudes (crudes for lube oils, crudes for asphalts). The distillates are not normally directly usable processing must be done to improve them, either mild treatment such as hydrodesulfurization of middle distillates at low pressure, or deep treatment usually with partial conversion such as catalytic reforming. The conventional refinery thereby has rather limited flexibility and makes products the quality of which is closely linked to the nature of the crude oil used. 

Once the synthetic crude oils from coal and oil shale have been upgraded and the heavy ends converted to lighter distillates, further refining by existing processes need not be covered in detail except to note the essential character of the products. The paraffinic syncrude from oil shale yields middle distillates which are excellent jet and diesel fuel stocks. The principal requirements are removal of nitrogen to the extent necessary for good thermal stability of the fuels and adjustment of cut points to meet required pour or freeze points, limited by the presence of waxy straight-chain paraffins. The heavy naphtha from shale oil can be further hydrotreated and catalytically reformed to acceptable octane number, but with considerable loss of volume because of the only moderate content of cyclic hydrocarbons, typically 45-50%. On the other... 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented a high conversion operation of vacuum residue, versus a constant desulfurization operation, in the commercial residue hydrodesulfurization unit equipped with fixed-bed reactors, to produce more middle distillates as well as fuel oil with lower viscosity. The catalysts will be replaced when the sulfur content in the product oil reaches the allowable limit. Since we have believed that an increase in the residue conversion decreases the catalyst activity by coke deposition, we have been interested in controlling the coke deactivation to maximize the residue conversion during a scheduled operating period. 

Trace elemental analysis can also be used to indicate the level of contamination of middle distillate fuels, e.g. turbine fuels. Metal contamination can cause corrosion and deposition on turbine components at elevated temperatures. Some diesel fuels have specification limits to guard against engine deposits, however they sometimes employ Mo or Ni as a catalyst for the refining process which eventually ends up in the finished products. There are several sources of multi-elemental contamination in naval distillate fuels. Sea water is pumped into the diesel tanks as ballast to immerse ships and submarines. Some oil transport ships have dirty tanks and contamination and corrosive products can also come from piping, linings and heat exchangers. 

Work carried out at the University of Birmingham (11), in England, by Dr. Emmott on middle distillates has revealed the presence of large amounts of sulfides in virgin stocks. As one ascends the boiling range scale the information on the types of sulfur compounds present becomes more limited. Hoog (26) has examined some Middle East gas oil and has deduced from the results of the several experimental techniques employed that essentially no sulfides, di- or polysulfides, or mercaptans were present and that the sulfur compounds of that specific gas oil were preponderantly of the thiophene and thiophane type. 

The observed behavior in the yield of middle distillates and vacuum gas oil may explain why there is an increase in the total liquid recovery of the product with boiling temperatures above 300°C. It also implies that vacuum gas oil is more feasible to hydrocrack than the middle distillates fraction. The yield of gases does not change substantially at the operating conditions studied and remains almost in the same low limits. This is probably due to the fact that the activation energy to hydrocrack the light fractions such as naphtha has not been reached. 

These factors have akeady led to a substantial investment in fuel oil-desulfurization facilities in several parts of the world, notably Japan, the Middle East, and the Caribbean. With few exceptions these facilities have been based on indirect desulfurization, i,e., vacuum distillation, desulfurization of the vacuum gas oil, and reblending. This technique has limits in a market for fuels below the 1% sulfur level because none of the heavy, high sulfur vacuum residue is processed. To some extent this can be mitigated by preferentially blending this material to bunkers and by using naturally occurring low sulfur residues as blending stocks. 

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