◆ ATMOSPHERIC RESIDUE VACUUM DISTILLATION ◆

1. PURPOSE AND PRINCIPLE OF THE INSTALLATION

The vacuum distillation installation is designed to separate atmospheric residue into several petroleum cuts. The actual fractionation depends on the composition of the atmospheric residue and the ultimate use different cuts obtained.

There are currently three principal uses:
   - bitumen production
   - production of VGO for upgrading by catalytic caracking or hydrocracking
   - production of base lube oils.

The cuts required for the first two uses can be simultaneously obtained in a single vacuum distillation installation. However the cut for the third use requires its own especially sized unit called an oil vacuum distillation unit. When a production of the gas oil cuts is left in the atmospheric residue after atmospheric fractionation, a vacuum gas oil is obtained at the lightest cut. Examples of cuts corresponding to the two different types of vacuum distillation of atmospheric residue are shown below.

In both cases, fractionation is carried out in a single multiple draw off distillation column, called a vacuum distillation column because it operates at a pressure well below atmospheric pressure. By this operation distillation temperatures can be lowered, thus avoiding thermal degradation of high boiling temperature hydrocarbons. Temperatures can be maintained below 400 °C level which, in this unit, is approximately the limit for hydrocarbon stability.

A section device for residue gases at the top of the column creates a vacuum, the vacuum gasoil and the vacuum distillates are withdrawn as sidestreams and the vacuum residue is obtained at the bottom of the column. Quality control of the products obtained is generally based on the following standard tests:

2. EXAMPLE OF A UNIT : OPERATION AND OPERATING CONDITIONS

2.1 Description

A vacuum distillation flow scheme is shown in Figure 5. The installation is integrated in the atmospheric distillation unit and is designed to produce a VGO for conversion (VGO1), and possibly a bitumen vacuum residue from a suitable crude. When the installation is operating under bitumen-producing conditions, specifications for VGO1 (particularly metal content) and for vacuum residue (penetrability) have to be simultaneously met which means obtaining an intermediate cut (VGO2) which is generally used a base fuel. Withdrawing VGO2 as a sidestream is only justified when the specifications for both cuts, VGO1 and vacuum residue, are incompatible.

2.2 Atmospheric Residue Inlet

Atmospheric residue is fed in at the base of the vacuum column and its partial vaporization creates the vapor flux necessary for distillation. As in the atmospheric column, the vapor flow rate corresponds more or less to the flow rate of all the sidestreams withdrawn higher up the column, i.e. heavy gas oil+VGO. This degree of vaporization is obtained by:
   - heat supplied in the vacuum furnace
   - the considerable expansion of the atmospheric residue owing to the very low pressure in the vacuum column.

The residue inlet temperature is generally between 390 °C and 430 °C.

2.3 Vacuum System and Heavy Gas Oil Side-Streaming

Gas oil is condensed at the top of the column by a circulating reflux which is injected cold at the top of the column and acts as a genuine integrated condenser. The small amount of vapors that enter this section are sucked up by the steam ejectors which are arranged in 3 stages separated by intermediary condensers. Any noncondensable material is ejected at a pressure slightly higher than atmospheric pressure and can thus be sent to the unit furnace for incineration. The pressure thus obtained is about 60 mbar at the top of the column, and allowing for pressure drop, this gives a pressure of around 90 mbar pressure in the feed inlet section. Nowadays some units are even working under a 10 mbar pressure at the top of the column.

A total sidestream tray located below this section is used at a temperature of 170 °C to withdrawn the gas oil circulating reflux, the heavy oil sidestream and the internal reflux which is reinjected just below the tray to separate the heavy gas oil and VGO.

2.4 VGO1 and VGO2 Sidestream

The total VGO1 sidestream is withdrawn in the same manner at a temperature of around 300 °C. VGO1 and VGO2 are in this case separated by reinjection of the internal reflux below the sidestream tray. The VGO2 cut sidestream is withdrawn just below the feed inlet section at a temperature of 360 °C.

2.5 Column Bottom Stripping

The purpose of the column bottom section is to steam strip the liquid part of the feed to obtain vacuum residue. Incidentally, by recirculating the cooled vacuum residue the temperature at the bottom of the column can be lowered, thereby avoiding thermal degradation of the hydrocarbons present. Some fractionation columns operate without steam injection and the process is then called dry vacuum distillation.

2.6 Liquid-Vapor Contact

Liquid and vapor are brought into contact by trays and packed beds (pall rings, grid packing, etc.). The latter are increasingly used in this type of column because they combine very effective liquid - vapor contact and minor pressure drops, thereby maintaining low pressure throughout the column. A metal wire mesh can be used to prevent droplets of heavy liquid from being entrained in the flux of ascending vapors.

3. VACUUM DISTILLATION OF LUBE OILS

The simplified flow scheme below shows an oil vacuum distillation unit. The higher quality required for separations chiefly involves:
   - a greater numbers of trays
   - steam strippers for each sidestream on the column.