Propane deasphalting

Let us briefly consider the propane lube oil refining process shown in figure 7.1. First the compressed liquid propane at 50°C is mixed with the residuum feed in the asphalt settler. Compressed liquid propane dissolves all the constituents of a lube oil feedstock except for the asphalt. Because the 

Before explaining the thermodynamic framework of the propane deasphalting process, we present several paragraphs from the 1936 paper by Wilson, Keith, and Haylett to highlight what was known fifty years ago about near-critical and supercritical fluid processing. 

Unfortunately, during most of this search for solvents, the refiner neglected to look at his own raw materials. In every refinery and in every crude field, millions of tons of propane gas are available, which, by simple compression and liquefaction, can be converted into a solvent with the unique property (under proper conditions of temperature and pressure) of tending to separate every one of the undesirable constituents. Further, assuming recovery facilities are available, propane is the cheapest liquid per gallon available in the refinery with the exception of water it is nontoxic, noncorrosive, and extremely stable. 

If we consider the diverse chemical and physical characteristics of the five undesirable constituents listed, it would seem almost inconceivable to anyone familiar with the theory of fractional solution that any one solvent could possibly throw all of these different compounds out of solution. As a matter of fact, propane cannot be expected to, and does not, separate them all at the same temperature and pressure. It owes its versatility as a precipitant to the fact that its properties change rapidly over the particular temperature range between —44°F [—42°C] and +215°F [100°C]. Over this range it possesses the properties of a series of solvents, any one of which can be obtained by raising or lowering the temperature or changing the pressure or combining those two operations. 

Incidentally, its viscosity and surface tension decrease to nearly negligible values as the critical temperature is approached. Over this range propane changes from a typical liquid to a fluid possessing substantially the properties of gas. As the 220°F [100°C] isotherm indicates, gaseous propane at a pressure of 1000 pounds per square inch [69 bar] is much more dense than liquid propane at 212°F [100°C] and its saturation pressure. Also, as the liquid approaches its critical temperature, it becomes highly compressible. 

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Propane deasphalting uses propane as an anti-solvent for asphaltenes. 

The rotating-disk contactor (RDC), developed in the Netherlands (158) in 1951, uses the shearing action of a rapidly rotating disk to interdisperse the phases (Eig. 15b). These contactors have been used widely throughout the world, particularly in the petrochemical industry for furfural [98-01-1] and SO2 extraction, propane deasphalting, sulfolane [126-33-0] extraction for separation of aromatics, and caprolactam (qv) [105-60-2] purification. Columns up to 4.27 m in diameter are in service. An extensive study (159) has provided an excellent theoretical framework for scale-up. A design manual has also been compiled (160). Detailed descriptions and design criteria for the RDC may also be found (161). 

The early developments of solvent processing were concerned with the lubricating oil end of the cmde. Solvent extraction processes are appHed to many usefiil separations in the purification of gasoline, kerosene, diesel fuel, and other oils. In addition, solvent extraction can replace fractionation in many separation processes in the refinery. For example, propane deasphalting (Fig. 7) has replaced, to some extent, vacuum distillation as a means of removing asphalt from reduced cmde oils. 

Asphalt. This is a distillatioa residuum that can also be produced by propane deasphalting (Fig. 7) (33) and thereafter modified to meet specifications. For example, asphalt (qv) can be made softer by blending hard asphalt with the extract obtained ia the solveat treatmeat of lubricatiag oils. Oa the other hand, soft asphalts can be converted iato harder asphalts by oxidation (air blowiag). 

Propane Asphalt. As noted above, cmde oils contain different quantities of residuum (Fig. 2) and, hence, asphalt. Asphalt is also a product of the propane deasphalting and fractionation process (5,6,21,22) which involves the precipitation of asphalt from a residuum stock by treatment with propane under controlled conditions. The petroleum charge stock is usually atmospheric-reduced residue from a primary distillation tower. 

Propane is usually used in this process although propane—butane mixtures and pentane have been used with some variation in process conditions and hardness of the product. Propane deasphalting is used primarily for cmde oils of relatively low asphalt content, generally <15%. Asphalt produced from this process is normally blended with other asphaltic residua for making paving asphalt. 

Fig. 3. Schematic representation of propane deasphalting and fractionation process.

Temperature, solvent ratio, and pressure each have an effect upon the spHt point or yield of the oil and asphalt components (Table 3). Contrary to straight reduction which is a high temperature and low pressure process, propane deasphalting is a low temperature and high pressure process. 

Table 3. Typical Yields and Characteristics of Products Obtained from the Propane Deasphalting Process...

Fig. 4. Comparison of propane deasphalting -with vacuum distillation of asphalt from Lagunillas, Venezuela cmde.

Propane Deasphalting Heater stack gas (CO, SO, NO, HCs and PM), fugitive propane. 

Solvent separation, using the propane deasphalting process, is another procedure by which asphalts of the straight reduced type may be manufactured. This is a physical separation process used to recover high viscosity lube fractions from a given vacuum residuum. When mixed with the residuum, the solvent preferentially dissolves the oil and precipitates the asphalt. 

Since the propane deasphalting operation is primarily directed toward the manufacture of lubes with asphalt as a by-product, the grades of asphalt produced from a deasphalter are usually limited. In some cases considerable blending must be done (frequently with extract oils) to provide the variety of grades required by the consumer. 

On the other hand, liquid propane also has a high affinity for paraffinic hydrocarbons. Propane deasphalting removes asphaltic materials from heavy lube oil base stocks. These materials reduce the viscosity index of lube oils. In this process, liquid propane dissolves mainly paraffinic hydrocarbons and leaves out asphaltic materials. Higher extraction temperatures favor better separation of the asphaltic components. Deasphalted oil is stripped to recover propane, which is recycled. 

Propane deasphalting solvent deasphalting using propane as the solvent. 

Solvent deasphalting is carried out primarily to recover lube or catalytic cracking feedstocks from asphaltic residuals, with asphalt as a byproduct. Propane deasphalting is the predominant technique. The vacuum fractionation residual is mixed in a fixed proportion with a solvent in which asphalt is not soluble. The solvent is recovered from the oil via steam stripping and fractionation, and is reused. The asphalt produced by this method is normally blended into fuel oil or other asphaltic residuals. 

The most outstanding development resulting from these investigations is the use of liquid propane for the selective precipitation of resins and asphalts. The development of the propane deasphalting process is a very important contribution to petroleum technology in the refining of residual oils and provides a method for substantially complete separation of lubricating oils from the asphaltic materials contained in the residua derived from any crude source. 

The theoretical aspects and operating features of the propane deasphalting process have been described fully (12, 14, 89). 

Propane deasphalting is used extensively in the production of lubricating oils from residual stocks, and is almost always applied prior to selective solvent refining, where a single extraction solvent such as phenol or furfural is used. 

Within the past five years, the propane deasphalting process has found a place in the recovery of high boiling, desirable catalytic cracking feed stocks with a relatively low carbon content from petroleum residua (57). 

Processes that have been combined with propane deasphalting include acid treating, cold fractionation of heavy lubricating oils, and selective solvent refining. 

Propane fractionation of heavy lubricating oil combined with propane deasphalting... 

Solvent make-up requirement generally is expressed as per cent of solvent circulation. For furfural and phenol this value will run about 0.03%, both for single solvent extraction and Duosol plants. Propane losses in Duosol plants and in propane deasphalting plants will be from 0.1 to 0.5% of circulation an average of 0.2% would be a reasonable estimate. 

In propane deasphalting, the heavy feedstock and 3 to 10 times its volume... 

The Demex process is an extension of the propane deasphalting process and employs a less selective solvent to recover not only the high quality oils but also higher molecular weight aromatics and other processable constituents present in the feedstock. Furthermore, the Demex process requires a much less solvent circulation in achieving its objectives, thus, reducing the utility costs and unit size significantly. 

Propane fractionation a continuous extraction process employing liquid propane as the solvent a variant of propane deasphalting (q.v.). 

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