Tbp Distillation

A Cooke , G. M. and Jameson , B. G. Analytical Chemistry, Vol 27, 1955, p. 1798. B Struck, R. T. and Kinner, C. R. IndustrialandEngineering Chemistry, Vol 42 , 1950, p. 77. C Cannon , M. R. Industrial and Engineering Chemistry, Vol 41, No. 9, 1949, p. 1953. D Bulletin 23, Scientific Development Co. P.O. Box 795, State College, PA 16801. ECooke, G. M. Analytical Chemistry, Vol 39, 1967, p. 286.

FBulletin of Podbielniak Div. of Reliance Glass Works, P.O. Box 825, Bensenville, IL 60106. G Feldman, J., et al, Industrial and Engineering Chemistry, Vol 45, January 1953, p. 214.

"Helipak Performance Characteristics, Begemean, C. R. and Turkal, P. J. (Laboratory Report of Podbielniak Inc.), 1950. 1 Umholtz, C. L. and Van Winkle, M. Petroleum Refiner, Vol 34, 1955, p. 114 for NH:MCH.

Pressure Drop Calculated from data obtained on o- and m-xylene binary. JOldershaw, C. F. Industrial and Engineering Chemistry, Vol 13, 1941, p. 265. KBragg, L. B. Industrial and Engineering Chemistry, Vol 49, 1957, p. 1062. L NA = not applicable.

A Cooke , G. M. and Jameson , B. G. Analytical Chemistry, Vol 27, 1955, p. 1798. B Struck, R. T. and Kinner, C. R. IndustrialandEngineering Chemistry, Vol 42 , 1950, p. 77. C Cannon , M. R. Industrial and Engineering Chemistry, Vol 41, No. 9, 1949, p. 1953. D Bulletin 23, Scientific Development Co. P.O. Box 795, State College, PA 16801. ECooke, G. M. Analytical Chemistry, Vol 39, 1967, p. 286.

FBulletin of Podbielniak Div. of Reliance Glass Works, P.O. Box 825, Bensenville, IL 60106. G Feldman, J., et al, Industrial and Engineering Chemistry, Vol 45, January 1953, p. 214.

"Helipak Performance Characteristics, Begemean, C. R. and Turkal, P. J. (Laboratory Report of Podbielniak Inc.), 1950. 1 Umholtz, C. L. and Van Winkle, M. Petroleum Refiner, Vol 34, 1955, p. 114 for NH:MCH.

Pressure Drop Calculated from data obtained on o- and m-xylene binary. JOldershaw, C. F. Industrial and Engineering Chemistry, Vol 13, 1941, p. 265. KBragg, L. B. Industrial and Engineering Chemistry, Vol 49, 1957, p. 1062. L NA = not applicable.

plate (see Table 1). Dynamic hold-up increases with increasing distillation rate up to the flood point and varies from one kind of fractionator to another.

3.1.7 flood point—the point at which the velocity of the upflowing vapors obstructs the downcoming reflux and the column suddenly loads with liquid.

3.1.7.1 Discussion—Under these conditions no vapor can reach the head and the heat to the distillation flask must be reduced to establish normal operations again. The flood point is normally determined during the efficiency evaluation of a column using the n-heptane-methylcyclohexane test mixture (see Annex A1).

3.1.8 internal reflux—the liquid normally running down inside the column.

3.1.8.1 Discussion—In the case of an adiabatic column when distilling a pure compound, the internal reflux is constant from top to bottom and is equal to the reflux at the reflux divider. When distilling crude petroleum, the fractionation occurring in the dynamic holdup will cause a temperature gradient to be established with attendant greater amount of internal reflux at the bottom of the column.

3.1.9 pressure drop—the difference between the pressure measured in the condenser and the pressure measured in the distillation flask.

3.1.9.1 Discussion—It is expressed in kilopascals (mm Hg) per metre of packed height for packed columns, or kilopascals (mm Hg) overall for real plate columns. It is higher for aromatics than for paraffins, and for higher molecular weights than for lighter molecules, at a given boilup rate.

3.1.10 reflux ratio, R—the ratio of reflux to distillate.

3.1.10.1 Discussion—The vapor reaching the top of the column is totally condensed and the resulting liquid is divided into two parts. One part L (reflux), is returned to the column and the other part, D (distillate), is withdrawn as product. The reflux ratio ( R = L/D), can vary from zero at total takeoff (L = 0) to infinity at total reflux (D = 0).

3.1.11 static hold-up or wettage—the quantity of liquid retained in the column after draining at the end of a distillation.

3.1.11.1 Discussion—It is characteristic of the packing or the design of the plates, and depends on the composition of the material in the column at the final cut point and on the final temperature.

3.1.12 takeoff rate—the rate of product takeoff from the reflux divider expressed in millilitres per hour.

3.1.13 theoretical plate—the section of a column required to achieve thermodynamic equilibrium between a liquid and its vapor.

3.1.13.1 Discussion—The height equivalent to one theoretical plate (HETP) for packed columns is expressed in millimetres. In the case of real plate columns, the efficiency is expressed as the percentage of one theoretical plate that is achieved on one real plate.

4. Summary of Test Method

4.1 A weighed sample of 1 to 30 L of stabilized crude petroleum is distilled to a maximum temperature of 400°C AET in a fractionating column having an efficiency at total reflux of at least 14, but not greater than 18, theoretical plates.

4.2 A reflux ratio of 5:1 is maintained at all operating pressures, except that at the lowest operating pressures between 0.674 and 0.27 kPa (5 and 2 mm Hg), a reflux ratio of 2:1 is optional. In cooperative testing or in cases of dispute, the stages of low pressure, the reflux ratios, and the temperatures of cut points must be mutually agreed upon by the interested parties prior to beginning the distillation.

4.3 Observations of temperature, pressure, and other variables are recorded at intervals and at the end of each cut or fraction.

4.4 The mass and density of each cut or fraction are obtained. Distillation yields by mass are calculated from the mass of all fractions, including liquified gas cut and the residue. Distillation yields by volume of all fractions and the residue at 15°C are calculated from mass and density.

4.5 From these data the TBP curves in mass or volume %, or both, versus AET are drawn.

5. Significance and Use

5.1 This test method is one of a number of tests conducted on a crude oil to determine its value. It provides an estimate of the yields of fractions of various boiling ranges and is therefore valuable in technical discussions of a commercial nature.

5.2 This test method corresponds to the standard laboratory distillation efficiency referred to as 15/5. The fractions produced can be analyzed as produced or combined to produce samples for analytical studies, engineering, and product quality evaluations. The preparation and evaluation of such blends is not part of this test method.

5.3 This test method can be used as an analytical tool for examination of other petroleum mixtures with the exception of LPG, very light naphthas, and mixtures with initial boiling points above 400°C.

6. Apparatus

6.1 Distillation at Atmospheric Pressure—All components must conform to the requirements specified as follows. Automatic devices can be employed provided they meet the same requirements. A typical apparatus is illustrated in Fig. 1.

6.1.1 Distillation Flask—The distillation flask shall be of a size that is at least 50 % larger than the volume of the charge. The size of the charge, between 1.0 and 30 L, is determined by the holdup characteristics of the fractionating column, as shown in Table 1 and described in Annex A2. The distillation flask shall have at least one sidearm.

6.1.1.1 The sidearm is used as a thermowell. It shall terminate about 5 mm from the bottom of the flask to ensure its immersion at the end of the distillation. When a second sidearm is present, it can be used for pressure drop detection with a nitrogen bleed or for mechanical stirring, or both.

6.1.1.2 If a magnetic stirrer is used with a spherical flask, the flask shall have a slightly flattened or concave area at the bottom on which the magnetic stirrer can rotate without grinding the glass. In this case, termination of the thermowell shall be off center 40 ± 5 mm to avoid the magnetic stirring bar. Boiling chips can be used as an alternative to a stirrer.

6.1.1.3 Warning—While the advantage of visibility in glass distillation flasks is desirable, flasks of glass may become hazardous the larger the charge they contain. For this reason,

Tbp Distillation
FIG. 1 Apparatus

glass flasks of a volume greater than 10 L are not recommended.

6.1.2 Heating System—Heating of the flask shall be provided in such a way that full boilup can be maintained at a steady rate at all pressure levels. An electric heating mantle covering the lower half of the flask and having one third of the heat in an element located in the bottom central area and the remaining two thirds in the rest of the hemisphere is recommended. While proportioning controllers are preferred, heat input can be manually adjusted by use of a variable auto transformer on each circuit, the smaller heater being automatically controlled by an instrument sensing the pressure drop of the column as registered in a differential pressure instrument or alternatively by direct measurement of distillation rate.

6.1.2.1 Minimum wattage required to provide full boilup of crude petroleum is approximately 0.125 W/mL of charge. Twice this amount is recommended for quick heat-up.

6.1.2.2 The heat density in the flask heaters is approximately equal to 0.5 to 0.6 W/cm2. This requires the use of nickel reinforced quartz fabric to ensure a reasonable service life.

6.1.2.3 Immersion heaters can be employed in a similar way and have the advantage of faster response, but they are more fragile and require a specially designed flask to ensure that the heating elements remain immersed at the end of the run. When used, their heat density should be approximately equal to 4 W/cm2.

6.1.2.4 The upper half of the flask shall be covered with a mantle to avoid unnecessary heat losses from the upper surface and shall have an electric heater supplying about 0.25 W/cm2 at full-rated voltage.

6.1.3 Fractionating Column—The fractionating column must contain either particulate packing or real plates similar to those whose performance characteristics are summarized in Table 1 and meet the specifications stated in 6.1.3.1 through 6.1.3.4. Table 2 lists current North American suppliers of suitable packings.

6.1.3.1 The internal diameter shall be between 25 and 70 mm.

6.1.3.2 The efficiency shall be between 14 and 18 theoretical plates at total reflux when measured by the procedure described in Annex A1.

6.1.3.3 The fractionating column shall be comprised of a integral glass column and reflux divider totally enclosed in a highly reflective vacuum jacket having a permanent vacuum of less than 0.1 mPa (~10-6 mm Hg). It shall be essentially adiabatic when tested in accordance with Annex A3.

6.1.3.4 The column shall be enclosed in a heat insulating system, such as a glass-fabric mantle, capable of maintaining the temperature of the outer wall of the glass vacuum jacket equal to that of the internal vapor temperature. To verify this, the vacuum jacket shall have a temperature sensor, such as a thermocouple, soldered to about 6 cm2 of thin copper or brass sheet and fastened to the outer wall of the glass jacket at a level just below the reflux divider.

Note 1—For certain types of columns there is no significant difference in yields and fraction qualities between an uncompensated and a heat-compensated column. In such a case, by mutual agreement between parties concerned, the application of a heated insulating system can be omitted.

6.1.3.5 The adjustable reflux divider shall be located about one column diameter above the top of the packing or topmost plate. It must be capable of dividing the condensate with an accuracy of better than 90 % between the column and the

TABLE 2 North American Sources of Commercially Available Packing Materials

Name

Size

Source

Propak

6 by 6 mm

Scientific Development Co.

State College, PA 16801

Helipak

2.5 by 4 mm

Reliance Glass Works Inc. P.O. Box 825 Bensenville, IL 60106

Perforated plates

25 and 50 mm

Reliance Glass Works Inc. P.O. Box 825 Bensenville, IL 60106 W.A. Sales Inc. 419 Harvester Ct. Wheeling, IL 60090

Knitted wire mesh-

0 0

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