№6 June 2012Table of contents Issue Archive
№ 5 (May 2008)
The catalytic cracking of vacuum distillate which had not undergone any preliminary desulfurization makes it possible to produce gasoline cuts, containing 0.25- 0.40 mass percent of sulfur and diesel oil cuts, containing 1.5-2.0 mass percent of sulfur
By R.Galiev, V. Khavkin, L. Gulyaeva, B.Solyar
Taking into account toughening ecological requirements to the indicated products (in perspective – not more than 50 ррм – Euro standard 4 and not more than 10 ррм – Euro standard 5), it is necessary to perform deep desulfurization of the initial vacuum distillate or additional hydrotreatment of the produced gasoline and diesel distillates.
In some cases, combination of hydrotreatment of the vacuum distillate and gasoline and diesel distillates of catalytic cracking can be necessary.
About 30 processes of treatment of oil residues and vacuum distillates are known, with use of hydrogen or without it.
The choice depends on the quality of raw stock, required degree of refinement and is determined by the technical and economic parameters.
For vacuum distillate processing, requirements to content of fractions boiling away at the temperature up to 350°С (not more than 5-7 volume percent) are specified, as these fractions do not increase the yield of the target products during the cracking process, but only undergo changes in their chemical composition. Majority of industrial-scale plants in Russia process the raw stock containing about 10-15 volume percent of the mentioned fractions.
There is foreign experience of production of vacuum distillates with end boiling points up to 598 С, and in some cases even to 600-620 С, which increases cracking raw stock resources by 20-25 volume percent, but at the same time requires significant reconstruction of the vacuum plant.
Vacuum Distillate Hydrotreatment
Hydrotreatment is a universal method for catalytic cracking raw stock refining.
Application of vacuum distillate hydrotreatment makes it possible to improve the quality and yield of light oil products, mostly of gasoline, and also to significantly reduce coke formation and SO content in furnace gas of the regenerator, which is of large ecological significance.
The maximum effect is achieved by hydrogenation of raw stock with high content of sulfur, nitrogen, aromatic hydrocarbons, coke-forming components and organometallic compounds: for example, by hydrogenation of high-sulfur gas oils.
Hydrotreatment of raw stock with high content of nitrogen and organometallic compounds results in significant improvement of stability of cracking catalyst performance.
Under standard conditions of hydrotreatment of West-Siberian oil vacuum distillates (pressure up to 5 MPa, temperature 350-400 С, feed space velocity 1.0-2.2 hour-1), average reduction of harmful components amounts to mass percent: sulfur – 60-80, nitrogen – 15-25, Conradson carbon residue – 40-50, heavy metals – 60-70, aromatic hydrocarbons – 10-20.
Table 1 gives data on effect of the raw stock hydrotreatment degree on the results of catalytic cracking, obtained at the commercial unit CCF in the U.S. They indicate that use of desulfurized raw stock ensures not only increase of gasoline and С3 – С4 fraction in the result of cracking, but also facilitates significant reduction of sulfur content in all products.
This makes it possible to produce a gasoline component which needs no further refining. Further refining of light gas oil, diesel fuel component becomes much easier.
The process of vacuum distillate hydrotreatmnet became widespread at oil refineries of Russia and CIS countries (Table 2). However, due to low pressure of hydrogen (3.5-4.0 MPa), sulfur content in the treated product, as a rule, cannot be reduced to the level lower than 0.25 -0.30 mass percent. In the result, catalytic cracking of this raw stock produces gasoline distillate, which contains about 0.05-0.10 mass percent of sulfur, and diesel distillate, which contains 0.3-0.5 mass percent of sulfur. These products need further hydrogenation refining.
The way to solve this problem is to increase the pressure in the process of hydrotreatment to 8.0-10.0 MPa, which allows of reduction of sulfur content in vacuum distillate to the level of 0.1-0.15 mass percent. This technical solution was used at the Ryazan oil-refining company, enabling them to significantly reduce sulfur content in gasoline distillate to 100-200 ррм, in diesel one – to 0.05-0.10 mass percent.
Vacuum Distillate Hydro-cracking
To prepare cracking raw stock, both so-called “soft” hydro-cracking at pressure of 4.0-5.0 MPa, and high-pressure hydro-cracking (about 15-17 MPa) can be used.
Application of high-pressure hydro-cracking is especially advisable for cases of raw stock having high content of sulfur and nitrogen, for example, distillates produced by thermal residue processing. The degree of undesired component removal in this process is much higher compared to “soft” hydro-cracking (Table 3) . Technologies of “soft” hydro-cracking with utilization of both domestic and foreign catalysts are realized in Russia and CIS countries (Table 4)
Selective Hydrotreatment of Catalytic Cracking Gasoline Fractions
VNII NP developed a technology for selective hydrotreatment of catalytic cracking gasoline, which makes it possible to significantly reduce sulfur content with minimal octane number reduction. Peculiarity of the process is preferential hydrogenation of sulfur compounds (by 70-80 mass percent), and also of di-olefinic hydrocarbons without noticeable transformation of mono-olefinic ones.
The latter makes it possible to sharply improve gasoline quality in respect of breakdown time and soluble gum content with preservation of the octane number at the sufficiently high level.
Compared to traditional hydrotreatment, the selective one can be performed at the reduced hydrogen pressure – to 2 MPa. Temperature of the process and feed space velocity are chosen depending on sulfur content in the raw stock and required degree of its hydrotreatment.
To choose the optimal regime of “selective” hydrotreatment of standard catalytic cracking gasoline containing 0.2-0.3 percent (mass) of sulfur (iodine number – 74 g 12,/100 g), VNII NP performed a series of experiments using standard cobalt-molybdenum catalyst.
Experimental results show that at pressure 2 MPa and feed space velocity 5 h-1, sulfur content in hydrogenation product becomes less than 0.1 percent (mass) at 280-3,000 С and less than 0.05 percent (mass) at temperatures exceeding 3,200 С. Motor octane number decreases by 1-2 points.
When feed space velocity increases to 10 h-1, the indicated levels of sulfur content are ensured at 300 and 3500 respectively; motor octane number decreases insignificantly: by 0.5 points. If pressure increases to 2.5 MPa, the required desulfurization degree (to 0.1 mass percent) is achieved at 3h-1. Gasoline octane number decreases approximately by 0.5 points. As one can see in Table 5, at the chosen regime (10 h-1, 3,000 С), 11.9 percent (mass) of unsaturated hydrocarbons are removed from gasoline, including 7.8 percent (mass) of di-olefinic hydrocarbons unstable to oxidation. At harder conditions, i.e. at 3,500 С, di-olefine hydrogenation degree virtually does not change, but hydrogenation degree of relatively stable olefine increases, which is reflected mostly in the motor octane number, which decreases by 1 point.
However, the developed technology does not enable reduction of sulfur content below 0.02-0.03 percent without considerable loss of the octane number.
The technology of separate refining of light (ibp – 700) and heavy (70-2,150) cuts of catalytic cracking gasoline was offered for deeper refining of the gasoline distillate. This technology was successfully realized at TAIF-NK in their commercial project. Cuts (light and heavy) of catalytic cracking gasoline (without hydrotreatment of the initial vacuum distillate) after their separation are characterized in Table 6. It is obvious that the main bulk of sulfur compounds are concentrated in the heavy cut of gasoline, and the main bulk of unsaturated hydrocarbons – in the light cut.
Heavy fraction of gasoline undergoes hydrotreatment sequentially in two reactors with intermediate hydrogen supply for cooling in the presence of GO-70 catalyst.
Conditions of the hydrotreatment process are:
Pressure: 2.5-2.6 MPa
Temperature: Inlet (to the first reactor) – 240 С
Outlet (of the second reactor) – 315 С
Feed space velocity – 2.5 h-1
Hydrogen-bearing gas/feedstock ratio – 200-250 nvolume/volume. In the result, sulfur content in the hydrogenation product drops to 30-50 ррм, research octane number drops on average by 1.5-2.0 points, and motor octane number – by 1.0 point.
Light fraction (initial boiling point - 700) is supplied for de-mecaptanization, where the main bulk of sulfur (in the form of mercaptan) is removed. Hydrocarbon composition of this fraction does not change in this process.
Mixing of the treated light and heavy cuts of catalytic cracking gasoline produces the base stock for production of commercial motor gasoline (motor octane number 80 and research octane number about 90, sulfur content: 20-40 ррм). Production of motor gasoline AI-80, AI-92 and AI-95 is ensured by introduction of supplementary components (MTBE) and a number of additives. Commercial application of the developed technology at TAIF-NK made it possible to considerably expand production of commercial motor gasoline in the Republic of Tatarstan, including gasoline brands complying with the standard Euro-4.