Process Design Of A Crude Distillation Tower
A very brief overview of the design steps involved follows:
1. Prepare TBP distillation and equilibrium flash vaporization curves of the crude to be processed. Several methods are available for converting TBP data to EFV curves.
2. Using crude assay data, construct TBP curves for all products except gas and reduced crude. These are then converted to ASTM and EFV curves by Edmister,5 'Maxwell,'6 or computer methods.
3. Prepare material balance of the crude distillation column, on both volume and weight bases, showing crude input and product output.
Also plot the physical properties, such as cut range on TBP and LV%, mid vol% vs. SG, molecular weight, mean average boiling point, and enthalpy curves for crude and various products.
4. Fractionation requirements are considered next. Ideal fractionation is the difference between the 5% and 95% points on ASTM distillation curves obtained from ideal TBP curves of adjacent heavier and lighter cuts. Having fixed the gaps as the design parameter, the ideal gap is converted into an actual gap. The difference between the ideal gap and actual gap required is deviation. Deviation is directly correlated with (number of plates x reflux).
5. The deviation or gap can be correlated with an F factor,7 which is the product of number of plates between two adjacent side draws offstream and internal reflux ratio. Internal reflux is defined as volume of liquid (at 60°F) of the hot reflux below the draw offplate of the lighter product divided by the volume of liquid products (at 60°F) except gas, lighter than the adjacent heavier products. This implies that the reflux ratio and the number of plates are interchangeable for a given fractionation, which holds quite accurately for the degree of fractionation generally desired and the number of plates (510) and reflux ratios (15) normally used. The procedure is made clear by Example 11.
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