ASTM D5236 Test Method for Distillation of Heavy Hydrocarbon Mixtures (Vacuum Potstill Method)
6. Apparatus
6.1 Four sizes of apparatus, based upon the internal diameter of the distillation head (25 mm, 36 mm, 50 mm, and 70 mm), are allowed. The apparatus (see Fig. 2) consists of a flask with heating mantles, an upper compensator, and a head containing an entrainment separator. Attached to the head are the vapor temperature sensor, a connection for the vacuum gauge, a condenser, a rundown line, a product receiver(s), and a vacuum pumping line with pump. The parts are connected by vacuum-tight joints to facilitate servicing.

6.2 Distillation Flask:
6.2.1 The sizes specified for flasks are at least 50 % larger than the size of the charge to provide space for suppression of foam and for bubble breaking. The size of the charge for each size of still is determined from the loading factor. The recommended loading factor is between 200 mL and 400 mL of charge per square centimetre of cross-sectional area in the neck of the head. Table 1 shows the range of charge volume that is recommended with each size of apparatus.

6.2.2 Flasks are made of borosilicate glass except those larger than 10 L, which are made of stainless steel for reasons of safety.

6.2.3 The flask is fitted with a thermowell reaching to within 6 mm of the bottom and offset from the center to avoid a stirring bar. In the case of glass flasks, the bottom shall be slightly flattened or slightly concave, but not perfectly flat to facilitate the rotation of the magnetic stirrer. Steel flasks can have a cooling coil for rapid quenching of the distillation in an emergency. Fig. 3 shows a typical example.

6.3 Stirring System - A magnetically driven stirring bar approximately 3 mm diameter and 20 mm long shall be provided for the glass flasks, or 6 mm diameter by 50 mm long for the steel flasks. The edges shall be rounded to minimize grinding the wall of the flask. The external magnetic drive must be capable of rotating the bar in the flask when located directly below and touching the mantle. The drive can be used to support the apparatus above. An adjustable jacking mechanism is recommended for raising and lowering the stirrer.

6.4 Heating System:
6.4.1 The flask shall be heated by means of a nickel-reinforced quartz fabric heating mantle on the lower half so that boiling rates of up to 150 mL/h per cm2 of the cross-sectional area of the neck can be maintained. A heat density of 0.5 W/cm2 is adequate. Usually two or more circuits are used to improve heat control by applying automatic heat to the bottom circuit.

6.4.2 A temperature sensor shall be located between the wall of the flask and the mantle for control of the skin temperature.

6.4.3 The upper half of the flask shall be covered with a mantle to compensate for heat losses. A heat density of 0.2 W/cm2 is adequate.

6.5 Distilling Head:
6.5.1 The head shall conform to the details shown in Fig. 1. It shall be made of borosilicate glass and be totally enclosed in a silvered glass vacuum jacket having a permanent vacuum of less than 0.0001 kPa (0.00075 mm Hg).

6.5.2 The head shall be enclosed in a heat-insulating system such as a glass fabric mantle capable of maintaining the outer wall of the glass vacuum jacket at a temperature 5 °C below the internal vapor temperature in the head. For this purpose, the vacuum jacket shall have a temperature sensor fastened to the outer wall of the jacket at a point level with the vapor temperature sensor and opposite to the outlet arm of the head.

6.5.3 The head shall be fitted with an adapter to support the vapor temperature sensor so that it is held centered in the neck with the top of the sensing tip 3 mm +/- 1 mm below the spillover point. This dimension can be checked by removing the temperature sensor and inserting in its place a copper wire having a short right angle bend at the bottom. By feeling for the spillover point, the distance from the top joint of the adaptor can be found. Laying the wire on the temperature sensor will then permit checking of this dimension.

6.5.4 The vapor temperature sensor shall be either a platinum resistance thermometer, a thermocouple with the junction head fused to the lower tip of the well, or any other device which meets the requirements in 6.5.4 and 6.5.4.1. It shall have a response time of less than 60 s as described in Annex A1.
6.5.4.1 The vapor temperature measuring device shall have an accuracy of 0.5 °C or better and be measured with a resolution of 0.1 °C or better.

6.5.4.2 The vapor temperature measuring device shall be calibrated over the full range of useful temperatures in combination with its associated instrument at the time of first use and at least once per year thereafter as described in A2.2.2. Alternatively, certified sensors may be used, provided the calibration of the sensor and its associated recording instrument can be traced back to a primary temperature standard. Recalibrate when either the sensor or the instrument is repaired or serviced. (Warning - Vapor temperature measurement is one of the two major sources of error in distillation data.)

6.5.4.3 Verification of the calibration of the vapor temperature measuring devices is to be made on a regular basis. Verification at least once a month is recommended. Verification of the calibration of the sensors can be accomplished potentiometrically by the use of standard precision resistance or by distilling a pure compound with accurately known boiling point, as described in A2.2.3.

6.5.5 A head trap as illustrated in Fig. 4 shall be fitted to the adapter described in 6.5.3 for connection to the vacuum sensor. It shall be kept filled with crushed dry ice at all times while in service.

6.5.6 A vacuum sensor shall be connected to the sidearm of the trap. The sensor shall be capable of reading the pressure with a precision equal to or better than 0.00133 kPa (0.01 mm Hg), whichever is greater. Anon-tilting McLeod gauge or other primary reference device can achieve this accuracy when properly used, but a mercury manometer will permit this accuracy only down to a pressure of about 1 kPa and then only when read with a good cathetometer (an instrument based on a telescope mounted on a vernier scale to determine levels very accurately). Also, electronic sensors of the diaphragm type have been found satisfactory. Vacuum gauges based on hot wires, radiation, or conductivity detectors are not recommended.
6.5.6.1 Primary standards, such as the non-tilting McLeod gauge, mercury manometer, or other analogous primary standard pressure devices, can be used without calibration when properly used and maintained. Alternatively, a tensimeter or certified electronic sensors may be used, provided the calibration of the sensor and its associated recording instrument can be traced back to a primary pressure standard.

6.5.6.2 Noncertified gauges shall be calibrated from a non-tilting McLeod gauge, another primary reference device, or a secondary electronic standard traceable to a primary standard. A basic calibration procedure is described in A2.3. Recalibrate when either the sensor or the instrument is repaired or serviced.

6.5.6.3 Verification of the calibration of pressure sensors is to be made on a regular basis. A frequency of at least once a week is recommended. Verification of the calibration of the sensors can be accomplished using the procedures described in A2.3 or against a certified reference system. (Warning - Measurement of vacuum (operating pressure) is one of the two major sources of error in the distillation procedure. It is therefore of prime importance that the instructions on calibration and verification be followed with great care and on a routine basis.)

6.6 Condenser - A condenser made of borosilicate glass shall be connected to the outlet arm of the head (see Fig. 5). It shall have sufficient capacity to condense essentially all vapors and capable of operating at coolant temperatures up to 70 °C to prevent wax buildup.

6.7 Pumping Line:
6.7.1 A pumping line shall be connected from the outlet of the condenser to the vacuum pump. The pumping line can be made of heavy-walled rubber or light metal tubing, but its inside diameter must be greater than half the inside diameter of the outlet of the condenser and less than 2 m long.

6.7.2 A surge tank of a size at least equal to the capacity of the flask shall be inserted in the pumping line adjacent to the pump.

6.7.3 An isolation valve of a diameter at least equal to the diameter of the pumping line shall be connected between the surge tank and the vacuum pump.

6.7.4 A dewar-type trap made of borosilicate glass, such as that illustrated in Fig. 5, shall be placed between the top of the distillation head and the vacuum sensor. It shall be kept filled with crushed dry ice at all times during the distillation to protect the vacuum system from contamination with residual vapors.

6.8 Vacuum Source - A single-stage mechanical vacuum pump capable of maintaining a steady pressure in the system at all operating pressures shall be connected to the pumping line. Automatic or manual control can be used.

6.9 Recovery System:
6.9.1 The recovery system is connected to the lower outlet of the product condenser and consists of a vacuum adapter to permit removal of distillate receivers without disturbing the pressure in the system. A suitable manual device is illustrated in Fig. 6.

6.9.2 Alternatively, either automatic or manual devices can be used to collect part or all of the fractions within the system without disturbing the operating pressure until the end of the run. Heating must be provided when needed to maintain the product in the liquid state.

6.9.3 The product receivers shall be made of borosilicate glass and large enough for the size of the fractions to be collected. They shall be calibrated to the nearest 1 % from the bottom. (Warning - This apparatus operates under high vacuum and high temperature. It is recommended that these stills be kept in an enclosure to ensure that in case of an implosion, the operator and others nearby are protected from flying debris, but that the front, at least, be transparent and removable for access to controls and so forth. Automated stills, which are left unattended for long periods, should be equipped with an automatic fire extinguisher, automatic quench, and alarm.)

7. Sampling
7.1 Obtain the sample for distillation in accordance with instructions given in Practice D4057 or Practice D4177. The sample can also be a residue from Test Method D2892.

7.2 The sample must be in a closed container when received and show no evidence of leakage.

7.3 If the sample looks waxy or has solidified, warm it enough to liquefy it and ensure that it is thoroughly mixed before using.

7.4 If, upon examination, there is evidence of water in the sample, perform a preliminary distillation as described in Annex A3.

8. Preparation of Apparatus
8.1 Clean and dry all glass parts and assemble them with freshly lubricated joints as shown in Fig. 2. In the case of ball joints, use only enough lubricant to produce a thin continuous film. An excess of lubricant can promote leakage. The rings of O-ring joints should be made of Vitron-A, or silicone of equivalent hardness, and be lightly lubricated.

8.2 Tare the receivers to the nearest 0.1 % of the weight of the charge.

8.3 To check for leaks, pump the system down to a pressure of approximately 0.05 kPa (0.4 mm Hg) and isolate it from the vacuum source. If, after 1 min, the rise in pressure is no greater than 0.01 kPa (0.075 mm Hg), the system is acceptable. If the rise in pressure is greater than 0.01 kPa (0.075 mm Hg) in 1 min, the gauge and its connections must be examined and leaks corrected before proceeding.

8.4 Calibrate the temperature and pressure sensors as described in Annex A2.