Crude oil as produced in the oil field is a complex mixture of hydrocarbons ranging from methane to asphalt, with varying proportions of paraffins, naphthenes, and aromatics. The objective of crude distillation is to fractionate crude oil into light-end hydrocarbons (Ci-C4), naphtha/ gasoline, kerosene, diesel, and atmospheric resid. Some of these broad cuts can be marketed directly, while others require further processing in refinery downstream units to make them saleable.
The first processing step in the refinery, after desalting the crude, is separation of crude into a number of fractions by distillation. The distillation is carried out at a pressure slightly above atmospheric. This is necessary for the following considerations:
- To raise the boiling point of the light-end carbons so that refinery cooling water can be used to condense some of the C3 and C4 in the overhead condenser.
- To place the uncondensed gas under sufficient pressure to allow it to flow to the next piece of processing equipment.
- To allow for pressure drop in the column.
Crude oil is preheated in exchangers and finally vaporized in a fired furnace until approximately the required overhead and sidestream products are vaporized. The furnace effluent is flashed into the crude column flash zone, where the vapor and liquid separate. The liquid leaving the flash zone still contains some distillate components, which are recovered by steam stripping. After steam stripping, the bottom product, also known as reduced crude, is discharged from the tower. The bottom temperature is limited to 700-7500F to prevent cracking.
The atmospheric resid is fed to a furnace, heated to 730-770oF and next to a vacuum tower operated at a minimum practical vacuum (80-110 mm Hg). The operating conditions are dictated by cracking and product quality required. The objectives of vacuum distillation is generally to separate vacuum gas oil (VGO) from reduced crude. The VGO may become feedstock for FCCU or hydrocracker units or used to make lube base stocks. Depending on the end use, there may be one or more sidestreams. The bottom stream from the vacuum distillation unit may be used to produce bitumen or used for fuel oil production after mixing it with small amounts of cutter stocks (in the diesel/kerosene range).
If the crude contains very high percentages of light-ends, a flash drum or a prefractionator with an overhead condensing system is added ahead of atmospheric tower. The prefractionator is designed to recover most of the light-ends and a part of the light naphtha. The bottom stream from prefractionator becomes feed to atmospheric tower.
The following variables are important in the design of crude columns:
- The nature of the crude—water content, metal content, and heat stability. The heat stability of the crude limits the temperature to which crude can be heated in the furnace without incipient cracking.
- Flash zone operating conditions—flash zone temperature is limited by advent of cracking; flash zone pressure is set by fixing the reflux drum pressure and adding to it to the line and tower pressure drop.
- Overflash is the vaporization of crude over and above the crude overhead and sidestream products. Overflash is generally kept in the range of 3-6
LV% ( = Liquid Volume). Overflash prevents coking of wash section plates and carryover of coke to the bottom sidestream and ensures a better fractionation between the bottom sidestream and the tower bottom by providing reflux to plates between the lowest sidestream and the flash zone. A larger overflash also consumes larger utilities; therefore, overflash is kept to a minimum value consistent with the quality requirement of the bottom sidestream. LV
- In steam stripping, the bottom stripping steam is used to recover the light components from the bottom liquid. In the flash zone of an atmospheric distillation column, approximately 50-60% of crude is vaporized. The unvaporized crude travels down the stripping section of the column containing four to six plates and is stripped of any low boiling-point distillates still contained in the reduced crude by superheated steam. The steam rate used is approximately 5-10 lbs/bbl of stripped product. The flash point of the stripped stream can be adjusted by varying the stripping steam rate.
- Fractionation is the difference between the 5% ASTM curve of a heavy cut and the 95% point on the ASTM curve of a lighter cut of two adjacent side products. A positive difference is called a gap, and a negative difference is called an overlap.
The design procedures used for atmospheric and vacuum distillation are mostly empirical, as crude oil is made of a very large number of hydrocarbons, from methane to asphaltic pitch. The basic data required, refinery crude distillation column, and a brief overview of the design procedures follow.
TRUE BOILING POINT CURVE
The composition of any crude oil sample is approximated by a true boiling point (TBP) curve. The method used is basically a batch distillation operation, using a large number of stages, usually greater than 60, and high reflux to distillate ratio (greater than 5). The temperature at any point on the temperature-volumetric yield curve represents the true boiling point of the hydrocarbon material present at the given volume percent point distilled. TBP distillation curves are generally run only on the crude and not on petroleum products. Typical TBP curves of crude and products are shown in Figures 1-1 and 1-2.
For petroleum products, a more rapid distillation procedure is used. This is procedure, developed by the American Society for Testing and Materials (ASTM), employs a batch distillation procedure with no trays or reflux between the still pot and the condenser. The only reflux available is that generated by heat losses from the condenser.
EQUILIBRIUM FLASH VAPORIZATION
In this procedure, the feed material is heated as it flows continuously through a heating coil. Vapor formed travels along in the tube with the remaining liquid until separation is permitted in a vapor separator or vaporizer. By conducting the operation at various outlet temperatures, a curve of percent vaporized vs. temperature may be plotted. Also, this distillation can be run at a pressure above atmospheric as well as under vacuum. Equilibrium flash vaporization (EFV) curves are run chiefly on crude oil or reduced crude samples being evaluated for vacuum column feed.
The complete and definitive analysis of a crude oil is called crude assay. This is more detailed than a crude TBP curve. A complete crude assay contains some of the following data:
- Whole crude salt, gravity, viscosity, sulfur, light-end carbons, and the pour point.
- A TBP curve and a mid-volume plot of gravity, viscosity, sulfur, and the like.
- Light-end carbons analysis up to C8 or C9.
- Properties of fractions (naphthas, kerosenes, diesels, heavy diesels, vacuum gas oils, and resids). The properties required include yield as volume percent, gravity, sulfur, viscosity, octane number, diesel index, flash point, fire point, freeze point, smoke point, and pour point.
- Properties of the lube distillates if the crude is suitable for manufacture of lubes.
- Detailed studies of fractions for various properties and suitability for various end uses.
PROCESS DESIGN QF A
A very brief overview of the design steps involved follows:
- 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.
- 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, 'Maxwell,' or computer methods.
- 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
%, mid vol% vs. SG, molecular weight, mean average boiling point, and enthalpy curves for crude and various products. LV
- 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).
- The deviation or gap can be correlated with an F factor, 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 600F) of the hot reflux below the draw offplate of the lighter product divided by the volume of liquid products (at 600F) 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 (5-10) and reflux ratios (1-5) normally used. The procedure is made clear by Example 1-1.
NUMBER OF TRAYS
Most atmospheric towers have 25-35 trays between the flash zone and tower top. The number of trays in various sections of the tower depends on the properties of cuts desired from the crude column, as shown in Table 1-1. The allowable pressure drop for trays is approximately 0.1-0.2 psi, per tray. Generally, a pressure drop of 5 psi is allowed between the flash zone and the tower top. Flash zone pressure is set as the sum of reflux drum pressure and combined pressure drop across condenser and trays above the flash zone. A pressure drop of 5 psi between the flash zone and furnace outlet is generally allowed.
FLASH ZONE CONDITIONS
The reflux drum pressure is estimated first. This is the bubble point pressure of the top product at the maximum cooling water temperature. The flash zone pressure is then equal to reflux drum pressure plus pressure drop in the condenser overhead lines plus the pressure drop in the trays.
Before fixing the flash zone temperature, the bottom stripping steam quantity and overflash are fixed. The volume percentage of strip-out on crude is calculated using available correlations.8 If D is the sum of all distillate streams, V is percent of vaporization in the flash zone, OF is overflash, and ST is strip out, then
V = D + OF – ST
From the flash curve of the crude, the temperature at which this vaporization is achieved at flash zone pressure is determined. This temperature should not exceed the maximum permissible temperature. If it does, the quantity of overflash and stripping steam are changed until a permissible temperature is obtained.
The temperature at which a crude oil begin to undergo thermal decomposition varies from crude to crude, depending on its composition (naphthenic, paraffinic, or aromatic base) and the trace metals present in the crude. Decomposition temperature can be determined only by actual test runs. For most paraffinic and naphthenic crudes, it is in the range of 650-6700F.
COLUMN OVERHEAD TEMPERATURE
The column top temperature is equal to the dew point of the overhead vapor. This corresponds to the 100% point on the EFV curve of the top product at its partial pressure calculated on the top tray. A trial and error procedure is used to determine the temperature:
- The temperature of reflux drum is fixed, keeping in view the maximum temperature of the cooling medium (water or air).
- Estimate a tower overhead temperature, assuming steam does not condense at that temperature.
- Run a heat balance around top of tower to determine the heat to be removed by pumpback reflux. Calculate the quantity of pumpback reflux.
- Calculate the partial pressure of the distillate and reflux in the overhead vapor. Adjust the 100% point temperature on the distillate atmospheric flash vaporization curve to the partial pressure.
- Repeat these steps until the calculated temperature is equal to the one estimated.
- Calculate the partial pressure of steam in the overhead vapor. If the vapor pressure of steam at the overhead temperature is greater than the partial pressure of steam, then the assumption that steam does not condense is correct. If not, it is necessary to assume a quantity of steam condensing and repeat all steps until the partial pressure of steam in the overhead vapor is equal to the vapor pressure of water at overhead temperature. Also, in this case, it is necessary to provide sidestream water draw-off facilities.
- To calculate overhead gas and distillate quantities, make a component analysis of total tower overhead stream consisting of overhead gas, overhead distillate, pumpback reflux, and steam. Next make a flash calculation on total overhead vapor at the distillate drum pressure and temperature.
- The overhead condenser duty is determined by making an enthalpy balance around the top of the tower.
To determine the amount of liquid to be vaporized by the stripping steam in the bottom of the tower, it is necessary to construct the flash curve of this liquid (called the initial bottoms). The flash curve of the reduced crude can be constructed from the flash curve of the whole crude. It is assumed that the initial bottom is flashed in the presence of stripping steam at the pressure existing on top of the stripping plate and at the exit temperature of liquid from this plate. Approximately 50-60% of the crude is vaporized in the flash zone of the atmospheric tower. The unvaporized crude travels down the stripping section of the tower, containing four to six plates, and is stripped of any remaining low-boiling distillates by superheated steam at 6000F. The steam rate used is approximately 5-10 lb/bbl of stripped product. The flash point of the stripped product can be adjusted by varying stripping steam rate.
Distillate products (kerosene and diesel) are withdrawn from the column as sidestream and usually contain material from adjacent cuts. Thus, the kerosene cut may contain some naphtha and the light diesel cut may contain some kerosene-range boiling material. These side cuts are steam stripped using superheated steam, in small sidestream stripper columns, containing four to six plates, where lower-boiling hydrocarbons are stripped out and the flash point of the product adjusted to the requirements.
In normal distillation columns, heat is added to the column from a reboiler and removed in an overhead condenser. A part of the distillate condensed in overhead condenser is returned to the column as reflux to aid fractionation. This approach is not feasible in crude distillation because the overhead temperature is too low for recovery of heat. Also the vapor and liquid flows in column increase markedly from bottom to top, requiring a very large-diameter tower. To recover the maximum heat and have uniform vapor and liquid loads in the column, intermediate refluxes are withdrawn, they exchange heat with incoming crude oil before entering the furnace and are returned to the plate above in the
column (Figure 1-3).
The flash curve of the product stream is determined first. This product is completely vaporized below the sidestream draw-off plate. Therefore, the 100% point of the flash curve is used. To determine the partial pressure of the product plus reflux vapor, both of which are of same composition, the lighter vapors are considered inert.