ASTM D5236 Test Method for Distillation of Heavy Hydrocarbon Mixtures (Vacuum Potstill Method)
9. Procedure
9.1 Determine the density of the sample by one of the following test methods: Test Method D941, D1217, D1480, D5002, or D1298. Refer to Guide D1250 to correct densities to 15 °C.

9.2 Insert the stirring bar.

9.3 From Table 1, determine the volume of the charge and calculate the mass to be charged by multiplying its density by the desired volume.

9.4 Weigh this mass of charge into the flask to the nearest 0.1 %. In the case of flasks too large to handle, the flask can be put in place and the charge drawn in from a container (weighed with its transfer line) using a pressure of 90 kPa to 95 kPa in the still. The charge may need to be warmed to facilitate transfer. Its mass can be determined from the difference.

9.5 Attach the flask to the column (in the case of smaller flasks), and put on all the heating mantles. Put the stirring device in place and turn it on. (Warning - Ensure that the safety shield is in place.)

9.6 A contiguous cutting scheme may be achieved in one of two ways, while remaining within the scope of the key aspects of this method. It may be achieved by gradually reducing the pressure over the course of the distillation (dynamic) or done stepwise, by slowing (or stopping) the takeoff rate to allow lowering of the operating pressure to achieve the final cut temperature. In each case, this must be done keeping in mind the necessity to avoid starving the distillation (due to a slow takeoff rate), while at the same time avoiding entrainment (by reducing too quickly the pressure applied to the system).

9.7 Apply heat to the flask at a rate that will raise the temperature of the charge quickly, but no faster than 300 °C/h (540 °F/h). Do not exceed a skin temperature on the flask of 400 °C (750 °F) or cracking may result on the walls of the flask. (Warning - Some hydrocarbon mixtures cannot tolerate 400 °C for any useful length of time. Reducing the skin temperature may be necessary in these cases.)

9.8 Turn on the head compensation mantle and maintain the outer wall of the glass vacuum jacket at a temperature approximately 40 °C below the temperature of the liquid in the flask.

9.9 Reduce the pressure in the system gradually to a suitable starting pressure. Choose from Table 2 the highest pressure that is consistent with the expected initial boiling point as well as the lowest pressure that is consistent with the maximum cutpoint, using Fig. 7 as a guide. A pressure of 0.133 kPa (1.0 mm Hg) has been found satisfactory for starting a material having an initial boiling point of 343 °C (650 °F) AET, such as residues from Test Method D2892 distillations.

NOTE 2 - Degassing of the charge is sometimes evident before the actual distillation begins. This appears as bubbling at the surface without generation of condensable vapors.

9.10 When distillation begins, evidenced by vapors entering the neck of the flask, reduce the heat input to a level that will maintain the chosen distillation rate from Table 2 (see Note 2). Adjust the heat compensator on the head to maintain the outer wall of the glass vacuum jacket at a temperature 5 °C below the vapor temperature.

NOTE 3 - Although a range of distillation rates is permitted, 80 % of the maximum allowed is recommended.

9.11 In cases in which the observed initial vapor temperature will be 150 °C (302 °F) or lower, it is desirable to refrigerate the first fraction receiver to ensure the retention of light ends. If solid, waxy material appears on the walls, warm the receiver with an infrared heat lamp or hot air gun to liquify the product in the receiver in order to improve the accuracy of the reading. In automatic operation, the receivers must be thermostated at a temperature high enough to ensure that no solidification takes place and low enough to prevent evaporation of light material.

9.12 When using the dynamic method of pressure reduction, calculate a projected final cutpoint using the operating pressure and the differential between the vapor and pot temperature as the operating envelope. Estimating that the difference between the vapor and the pot temperature remain relatively constant, determine if the final vapor temperature can be achieved at this pressure while remaining within the recommended limitations of the flask temperature (see 9.18). If the final cutpoint cannot be achieved at the starting pressure, the pressure should be gradually lowered toward an operating pressure that will allow the final cut to be taken. This must be done bearing in mind the associated takeoff rates for the vacuum pressures indicated in Table 2 and the limitations of the pot temperature. The pressure should be lowered enough to allow the takeoff rate to accelerate briefly while the operator is remaining vigilant to avoid entrainment. The distillation rate at the operating pressure should fall within the recommendations stated in Table 2 and should be allowed to stabilize for at least 2 min before arriving at a cutpoint. Experience has shown that reduced crude oil samples typically run well at 0.133 kPa for up to 25 % to 30 % of the charge volume. Subsequent lowering of the operating pressure, as described above, has yielded satisfactory results. Repeat this procedure throughout the remainder of the distillation until an operating pressure has been attained that will allow the final cut temperature to be reached while remaining within the recommended confines of the maximum pot temperature and temperature/time constraints of flask temperature (see 9.18).

9.13 When the receiver is full, or when a cutpoint is reached, isolate the receiver or move to the next one, as the case may be.
9.13.1 In manual operation, isolate the receiver using the vacuum adaptor and vent it to atmospheric pressure before replacing it with another tared receiver. Apply vacuum, and when the new receiver is at approximately system pressure, reconnect it to the system.

9.13.2 In automatic operation, receivers are changed automatically and do not normally need further attention.

9.14 Record the following observations:
9.14.1 Time in hours and minutes,

9.14.2 Volume of distillate in millilitres,

9.14.3 Vapor temperature to nearest 0.5 °C,

9.14.4 Liquid temperature in the flask in °C,

9.14.5 Pressure in the head to nearest 1 %, and

9.14.6 Atmospheric equivalent temperature by calculation as prescribed in Annex A4.

9.15 Proceed to 9.18.

9.16 Alternatively, the stepwise method can be achieved by initializing the distillation and operating at the pressure at which the distillation stabilizes (see 9.9). Continue taking product and making cuts until the final cutpoint is achieved or until the temperature of the boiling liquid reaches approximately 290 °C (554 °F).

9.17 At this point, if the final cutpoint cannot be achieved before reaching 320 °C (608 °F) in the boiling liquid, reduce the heat input to zero until the distillation slows or stops. This will take 2 min to 10 min depending on the amount of material in the flask. Reduce the pressure slowly to a level that will allow for a reasonable amount of overhead product to evolve at the new pressure level. A pressure reduction by a factor of five or six has been shown to be necessary to produce a viable quantity of overhead at the new pressure level.

9.18 Restore the heat to about 90 % of the previous level and then adjust to give the desired rate at the lower level (see Table 2). Do not take any cuts until the pressure has stabilized at the new level for at least 2 min. Repeat 9.16 and 9.17 until a pressure level has been reached that will allow for achieving the final cutpoint before the boiling liquid reaches a temperature of 320 °C (608 °F).

9.19 Continue taking product as long as there is no indication of incipient cracking. Addition of heat to the flask to maintain product rate should be done with great care. It is recommended to achieve the final cutpoint in less than 1 h after the flask temperature has risen above 310 °C (590 °F).

9.20 The distillation shall be discontinued immediately as soon as signs of incipient cracking are observed (see Note 4).

NOTE 4 - Cracking will significantly affect the quality of the cuts and the residue, for example, the densities and viscosities would be significantly lower than those obtained without cracking.

NOTE 5 - Incipient cracking is usually first observed from a distinct and persistent rise in pressure (for example, >10 % of pressure set point) or an increase of the demand on the vacuum pump capacity. However, automatic vacuum controllers tend to mask these phenomena. Other signs of incipient cracking are the accumulation of thin black deposits on the glassware through the column or the appearance of a smoke-like vapor in the system after the condenser.

9.21 When either the final cutpoint or 90 volume % has been distilled or incipient cracking is observed, discontinue the distillation. Discontinue heat input to the flask and heating jacket at once and slightly raise the pressure of the system by reducing the vacuum pump capacity. Allow the residue to cool while stirring.

NOTE 6 - Beyond 90 volume % distilled, the flask may be too near dryness for safe operation.

9.22 Remove the flask compensating mantle, or in the case of steel flasks, turn on the air in the quench coil.

9.23 When the temperature of the residue has fallen below 150 °C (302 °F), remove and weigh the flask and contents to determine the mass of the residue. For larger stills, the residue can be discharged through the charging line using a positive pressure of about 10 kPa in the still.

9.24 Weigh all overhead fractions to within 0.1 % of the charge mass.

9.25 Determine the relative density of all fractions and convert to 15 °C (59 °F) using Guide D1250 where applicable.

9.26 In the case of the smaller stills, recover the wettage by boiling up a small quantity of solvent such as toluene in a separate flask to wash the head and condenser. Evaporate the solvent in a hood assisted by a stream of air and weigh directly. This wettage may be treated as a separate fraction and its density estimated or blended into the residue before inspections are made. The latter must be done if the residue is to be analyzed for other than density. For larger stills, follow instructions given in Annex A5. Note that the holdup in the latter case includes both the overhead wettage and the wettage of the flask with residue and must be considered a separate fraction. Density must be measured in this case.

10. Calculation
10.1 Calculate the weight recovery by adding the masses of all the fractions plus the holdup or wettage. The total must be between 99.6 % and 100.1 % of the weight of the charge to be acceptable. Show the actual loss on the record, and prorate the loss among all fractions.

10.2 Calculate the volume of each fraction by dividing the mass of each fraction by its relative density.