Gas oils intended for catalytic cracking feedstocks must be very clean and require a good separation between distillate and residuum. These oils should have the minimum amount of Conradson carbon content without sacrificing gas oil recovery. This is necessary to prevent excessive laydown of coke on the cracking catalyst. The metals content, particularly vanadium and nickel, must be strictly limited since these are severe catalyst poisons.
Feedstocks for distillate hydrotreaters and/or hydro-crackers need not necessarily be metal free since these same metals are often found in hydrogénation catalysts. It is imperative, however, that they be as free as possible of carbon and asphaltic materials. If they are not, coking of the catalyst will occur more rapidly, and more frequent regeneration will be required. Product cleanliness is far more important here than in the case of catalytic cracking feedstocks.
Distillate fuel oils which are to be used without further processing are usually specified by API gravity, viscosity, metals content and flash or fire point. Since the latter two properties are general functions of the front end boiling range and the others of total stream yield, atmospheric boiling range, either TBP or ASTM, can be used to define the separation. Metals content, particularly vanadium, is a source of ash and is also harmful to most furnace refractory materials.
Vacuum tower residuum properties can be set in various ways. When distillate production is to be maximized, the amount of gas oil allowed to remain in the bottoms stream must be minimized, and this material, usually 0 to 5 degrees API, is blended into residual fuels. This is called pitch operation. In asphalt operation, some gas oil must be left in the residuum in order to provide the proper degree of plasticity. The gravity of asphalt streams usually falls in the range of 5 to 8 degrees API. Not all crudes can be used to make asphalt since this is a question of whether or not the particular residuum has the proper composition of matter to render it suitable in asphalt applications. This question can only be answered experimentally and is usually contained in a detailed crude assay. Pitch operation can be practiced with any type of crude.
For pitch operation, one can normally set the volume percent of either the whole crude or reduced crude which is to be yielded as vacuum residuum. This is, in effect, specifying a TBP cut point for vacuum distillates. In some cases, the vacuum unit is defined in exactly this way. It can also be defined by determining the amount of distillate yield which will produce a bottoms stream having the desired gravity. More usually, though, the limiting factor is the practical economic limit to how much distillate can be lifted from a given system and is defined in terms of the lowest possible operating pressure and highest allowable oil temperature in the tower flash zone.
For asphalt operation, experimental data is necessary, relating asphalt penetration to residual volume. Using this information, distillate volume can be readily established. Units have been built to make asphalt over the penetration range of 85 down to 10 (approximately 7 to 4 degrees API). The asphalt specifications will most often be set by the owner, and it may be necessary to design for more than one grade of product.
One other method of vacuum tower operation which is rather rare is the production of large volumes of bunker fuels. This operation is usually practiced in foreign refineries in the producing areas where marine fuel requirements are high. These refineries usually produce distillates to satisfy marketing requirements which may vary a great deal and yield the remaining gas oil with the residuum. The resulting residual fuels are slightly lighter than in normal operations but are still quite adequate from the end use standpoint.
In meeting these various product specifications, it must be remembered that the key to satisfactory operations is the maximum volume of clean gas oil, free from contamination by heavier materials. In pitch operation, the quality of the residuum is of little importance other than to minimize its content of recoverable oils. In asphalt operation, gas oil quality must remain high, but the residuum must meet certain stringent specifications as well.
Vacuum towers for manufacture of oils destined either for lube oils or some other type of specialty oils must be designed to provide the same relative degree of fractionation between streams as in the atmospheric tower. Accordingly, the two types of towers are designed and calculated in approximately the same way. Sidestream products are stripped in external towers to control front end properties. Heat recovery is practiced by exchanging hot downflowing tower liquid streams against incoming cool crude oil feed for the atmospheric tower or against other cooler process streams in the unit. This heat recovery can be practiced in two different ways.
Figure 3.1 shows utilization of cooled pumpback reflux from the draw tray to the tray below at all trays except the top sidestream. This liquid is condensed by pumparound heat removal using a grid type material for vapor-liquid contacting. Cooled reflux is pumped back to the tray below the draw tray to provide fractionation between the two light vacuum distillates.
Figure 3.2 shows utilization of cooled pumparound reflux in two sections of the tower. In terms of heat removal and equipment requirements, the two methods are relatively equivalent. Both the external heat recovery and the internal vapor and liquid traffic quantities are essentially equal. The pumpback reflux system has the advantage f f t t 9
Was this article helpful?