ASTM D2892 for distillation of crude petroleum (15-Theoretical Plate Column)
10. Procedure
10.1 Charging:
10.1.1 The charge size shall be such that the dynamic hold up as determined in accordance with Annex A2 is between 1 and 4 % of the charge when operating at 75 % of maximum boilup (see Table 1). Chill the flask to a temperature not lower then 0°C.
10.1.2 Insert the stirring device or place some pieces of glass or porcelain into the flask to control bumping.
10.1.3 Determine the density of the sample by Test Method D941, D1217, or D1298.
10.1.4 Calculate to within more or less 5 % the mass of crude petroleum corresponding to the desired volume of the charge. Weigh to the nearest 1 % this quantity of sample into the flask.
10.1.5 Attach the flask to the column and connect the pressure drop measuring device. Install the heating system, stirrer, and support device. (Warning - Poisonous H2S gas is frequently evolved from crude oil and precautions must be taken either to absorb the gas that passes through the cold trap or to vent it to a safe place.)
10.2 Debutanization:
10.2.1 For necessary apparatus refer to 6.1.5 and 6.1.6.
10.2.2 Begin circulation of refrigerant at a temperature no higher than -20°C in the condenser, distillate cooler, and receiver, if so equipped.
10.2.3 Record the barometric pressure at the beginning and periodically throughout the distillation.
10.2.4 Apply heat to the flask at such a rate that vapors reach the top of the column between 20 and 50 min after startup. Adjust heat input so as to achieve a pressure drop of less than 0.13 kPa/m (1.0 mm Hg/m) in packed columns or less than 0.065 kPa (0.5 mm Hg) in real plate columns. Program automated equipment in accordance with the preceding directions. Turn on the stirring device if used.
10.2.5 Allow the column to operate at total reflux until the vapor temperature reaches equilibrium but not longer than 15 min after the first drop of condensate appears in the reflux divider.
10.2.6 Record the vapor temperature as the initial vapor temperature.
10.2.7 Stop the circulation of the refrigerant and observe the vapor temperature. When the vapor temperature reaches 15°C, start the circulation of refrigerant again.
10.2.8 If the vapor temperature drops below 15°C, continue refluxing for at least 15 min. Repeat 10.2.7. If the vapor temperature remains at 15°C or rises, continue with the atmospheric distillation. (Warning - The following three steps should not be done until after the first naphtha cut has been removed to ensure that all the light gases have been recovered.)
10.2.9 Remove and weigh the dry ice traps containing light hydrocarbon liquid after carefully wiping them dry.
10.2.10 Sample the contents of the first dry ice trap using a 10 to 50 mL pressure vessel evacuated to no lower than 26.6 kPa (200 mm Hg). Keep all containers at the temperature of dry ice to ensure no loss of volatiles. The first trap next to the condenser should contain all of the sample. If condensate is found in the second trap, sample both traps or combine the contents before sampling.
10.2.11 Submit the trap sample and gas balloon, if used, for analysis by Test Method D2427 to be reported on a fixed-gas free basis.
10.3 Distillation at Atmospheric Pressure:
10.3.1 Maintain a temperature below -20°C in the lines of the distillate cooler and receiver as well as in the condenser. Turn on the column mantle heat controller and maintain the column jacket temperature 0 to 5°C below the vapor temperature.
10.3.2 Regulate the heat input as necessary to establish and maintain a boilup rate approximately 75 % of maximum. Fig. 3 can be used as a guide for Propak. Rates for other sizes can be estimated by multiplying the boilup rate in Table 1 by the cross-sectional area of the column and dividing by the sum of the reflux ratio +1.
10.3.3 Commence takeoff at a reflux ratio of 5:1 and total cycle time of not over 30 s nor less than 18 s.
10.3.4 Take off distillate in separate and consecutive fractions of suitable size. The recommended size of fraction is that corresponding to 5 or 10°C in vapor temperature. Collect fractions boiling below 65°C in receivers cooled to 0°C or below. When the vapor temperature reaches 65°C, refrigerant in the condenser and related coolers can be discontinued and water at ambient temperature substituted.
10.3.5 At the end of each fraction and at each cut point, record the following observations:
10.3.5.1 Time in hours and minutes,
10.3.5.2 Volume in millilitres,
10.3.5.3 Vapor temperature in °C to the nearest 0.5°C,
10.3.5.4 Temperature of the boiling liquid in °C to the nearest 1°C,
10.3.5.5 Atmospheric pressure in kPa (mm Hg), and
10.3.5.6 Pressure drop in the column in kPa (mm Hg).
10.3.6 If signs of flooding are observed, reduce the heating rate while continuing takeoff until steady conditions are restored. If a cut point is encountered during this period, stop the distillation, cool the charge, and recombine the off-condition cuts. Restart the distillation with a period at total reflux, not to exceed 15 min, to restore operating conditions before continuing takeoff. Do not make a cut within 5°C of startup.
10.3.7 Continue taking cuts until the desired maximum vapor temperature is reached or until the charge shows signs of cracking. Pronounced cracking is evidenced by a fog appearing in the flask and later at the reflux divider. Do not allow the vapor temperature to exceed 210°C nor the temperature of the boiling liquid to exceed 310°C.
10.3.8 Shut off the reflux valve and the heating system. Allow the contents to cool to such a temperature that the distillation can be commenced at 13.3 kPa (100 mm Hg) without flooding. This temperature can be estimated by adding the Delta T between the liquid and vapor temperatures found for the column during atmospheric operation to the expected initial vapor temperature at the reduced pressure, or by subtracting the Delta T from the last recorded liquid temperature.
NOTE 4 - Cooling of the liquid in the flask can be accelerated by blowing a gentle stream of compressed air onto the flask after its heating mantle has been removed. Avoid strong jets of cold air. Alternately, turn on coolant in the quench coil of the flask, if used.
10.3.9 Weigh all fractions and determine their densities.
10.3.10 Submit the first distillate fraction for analysis by gas chromatography.
10.4 Distillation at 13.3 kPa (100 mm Hg):
10.4.1 If further cuts at higher temperatures are required, distillation can be continued at reduced pressures, subject to the maximum temperature that the boiling liquid will stand without significant cracking. This is about 310°C in most cases. Notable exceptions are crude oils containing heat-sensitive sulfur compounds. In any case, do not make a cut within 5°C of the temperature at startup because the column will not be at equilibrium.
10.4.2 Connect a vacuum pumping and control system to the apparatus as shown in Fig. 1.
10.4.3 Start the vacuum pump and adjust the pressure downward gradually to the value of 13.3 kPa (100 mm Hg) or set the pressure regulator at this value. The temperature of the liquid in the flask must be below that at which it will boil at 13.3 kPa (100 mm Hg). If the liquid boils before this pressure is reached, increase the pressure and cool further until the desired pressure can be achieved without boiling.
10.4.4 Apply heat to the boiler and reestablish reflux at any moderate rate in the reflux divider for about 15 min to reheat the column to operating temperature. Momentarily stop heat input and raise the pressure with N2 for 1 min to drop the holdup into the distillation flask.
10.4.5 Reapply heat to the distillation flask and adjust the rate of heating to maintain a constant pressure drop equivalent to the boilup rate of approximately 75 % of the maximum rate for this pressure and begin takeoff without delay. The approximate pressure drops required for this purpose are indicated in Fig. 3. Maintain a column insulation temperature 0 to 5°C below the vapor temperature throughout the operation.
10.4.6 Remove separately, cuts of suitable size as in 10.3.4.
10.4.7 At the end of each distillate fraction and at each cut point, record the following observations:
10.4.7.1 Time in hours and minutes,
10.4.7.2 Volume in millilitres observed at ambient temperature,
10.4.7.3 Vapor temperature in °C to the nearest 0.5°C with correction, if any,
10.4.7.4 Temperature of the boiling liquid in °C to the nearest 1°C,
10.4.7.5 Pressure drop in the column in kPa (mm Hg),
10.4.7.6 Operating pressure measured at the top of the column in kPa (mm Hg) absolute with correction, if any, and
10.4.7.7 AET using the equations given in Annex A8.
10.4.8 Continue taking cuts until the desired maximum point is reached or until the charge shows signs of cracking. Pronounced cracking is evidenced by the evolution of gases as indicated by rising pressure as well as a fog appearing in the flask (see Note 4). Do not allow the temperature of the boiling liquid to exceed 310°C. (Warning - Automatic vacuum controllers could mask a slight rise in pressure due to cracking. Vigilance is required to avoid this.)
10.4.9 Shut off the reflux valve and the heating system. Allow the contents to cool to such a temperature that the distillation can be commenced at a lower pressure without boiling. This temperature can be estimated by adding the Delta T between the liquid and vapor temperatures found for the column during operation to the expected initial vapor temperature at the lower pressure, or by subtracting the Delta T from the last recorded liquid temperature.
10.4.10 Weigh all fractions and determine their densities at 15°C.
10.5 Distillation at Lower Pressures:
10.5.1 If the final cut point has not been reached, distillation can be continued at a lower pressure subject to the same limitation as before (see 10.4.1). Only one pressure level between 13.3 kPa (100 mm Hg) and 0.266 kPa (2 mm Hg) is permitted. Where the maximum cut point is 400°C AET, the minimum pressure is recommended.
10.5.2 Adjust the pressure to the desired level. If the liquid boils before the pressure is reached, increase the pressure and cool further until the desired pressure can be achieved without boiling. Follow the procedure in 10.4.4.
10.5.3 Circulate cooling water in the condenser and liquid cooler either at ambient temperature or warmed to a temperature that will ensure that wax does not crystallize in the condenser or takeoff lines. Alternatively, leave the cooling coils full of water but vented and not circulating, or else circulate a stream of air instead of water as a coolant.
10.5.4 Continue vacuum operation as in 10.4.5 through 10.4.8. During this operation, a reflux ratio of 2:1 is allowed if mutually agreed upon in advance and noted in the report. Correct observed and corrected vapor temperatures to AET using the equations given in Annex A8.
10.5.5 Check periodically that the condensate drips normally in the condenser and that the distillate flows smoothly into the takeoff line. If crystallization is observed, allow the coolant in the condenser to warm as in 10.5.3.
10.5.6 When the final cut point has been reached, or when limits of boiling liquid temperature and column pressure prevent further distillation, turn off the reflux valve and heating system and allow to cool with the vacuum still applied.
10.5.7 When the temperature of the residue in the flask has fallen below 230°C, shut off the vacuum pump. Vent the fractionating unit with nitrogen or other inert gas. Do not use air. (Warning - Air is suspected of initiating explosions in fractionating units that are vented while too hot, such as at the end of a run.)
10.5.8 Stop circulation of coolant in the condenser and ancillary equipment. Disconnect the flask. Recover the static holdup of the column (wettage) by distilling a small quantity of solvent such as toluene in a separate flask to wash the column, condenser, and takeoff system. Evaporate the solvent from the collected residue at 10°C above the boiling point of the solvent, using a small purge of nitrogen. For distillations not involving disagreement, or by mutual consent, the holdup can be estimated using a graph similar to Fig. 4. The density of the holdup is estimated by extrapolation of the density line for the preceding cuts. The static holdup can be treated as a separate small cut or blended into the bottoms before inspections are made. The latter must be done if other analyses besides density are to be performed on the residue.
10.5.9 Weigh all fractions and the residue in the flask and determine their densities at 15°C by Test Method D4052 or by another suitable method. Convert the density to 15°C, if necessary.
NOTE 5 - Heavier flasks, such as those for 50 and 70-mm diameter columns, are not normally removed for weighing. In these cases the residue can be discharged at a temperature not over 200°C into a tared container for weighing. Nitrogen pressure of approximately 6.7 kPa (50 mm Hg) will be sufficient for this. Wettage in these cases will include that of the column and the flask together.
