ASTM D2892 for distillation of crude petroleum (15-Theoretical Plate Column)
3. Terminology
3.1 Definitions:
3.1.1 adiabaticity - the condition in which there is no significant gain or loss of heat throughout the length of the column.
3.1.1.1 Discussion - When distilling a mixture of compounds as is the case of crude petroleum, there will be a normal increase in reflux ratio down the column. In the case where heat losses occur in the column, the internal reflux is abnormally greater than the reflux in the head. The opposite is true when the column gains heat, as with an overheated mantle.
3.1.2 boilup rate - the quantity of vapor entering the column per unit of time.
3.1.2.1 Discussion - It is expressed in millilitres of liquid per hour for a given column or in millilitres per hour per square centimetre of cross-sectional area for comparative purposes. In the latter case, it refers to the test mixture of n-heptane and methylcyclohexane in the efficiency evaluation (see Annex A1) and is measured at the bottom of the column. The maximum boilup of the n-heptane-methylcyclohexane test mixture is that which the column can handle under stable conditions without flooding. In routine adiabatic operation, the boilup rate can be estimated roughly from the takeoff rate multiplied by the reflux ratio plus one.
3.1.3 debutanization of crude petroleum - the removal of the light hydrocarbons up to and including n-butane, and retention of the heavier hydrocarbons.
3.1.3.1 Discussion - In practice, a crude petroleum is regarded as debutanized if the light hydrocarbon cut collected in the cold trap contains more than 95 % of the C2 to C4 hydrocarbons and less than 5 % of the C5 hydrocarbons initially present in the sample.
3.1.4 distillation pressure - the pressure measured as close as possible to the point where the vapor temperature is taken, normally at the top of the condenser.
3.1.5 distillation temperature - the temperature of the saturated vapor measured in the head just above the fractionating column.
3.1.5.1 Discussion - It is also known as the head temperature or the vapor temperature.
3.1.6 dynamic hold-up - the quantity of liquid held up in the column under normal operating conditions.
3.1.6.1 Discussion - It is expressed as a percentage of the packed volume for packed columns so that the data can be compared. For real plate columns, it is expressed in millilitres per plate. The data can only be compared with others of the same diameter because of different tray spacing. Data for packed columns cannot be compared with those of real plate columns except in absolute units of millilitres per theoretical 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.
