Introduction to Crude Oil Distillation Ref 1 R

Introduction to Crude Oil Distillation Ref. 1: R. Smith, Chemical Process Design and Integration, Wiley, 2005, Chapter 11. Ref. 2: R. N. Watkins, Petroleum Refinery Distillation, 2 nd ed. , Gulf Publishing Company, 1979, Chapter 2. 1

Crude Oil Distillation Atmospheric In the first stage of processing crude oil, it is distilled under conditions slightly above atmospheric pressure. A range of petroleum fractions are taken from the crude oil distillation. Designs are normally thermally coupled. Most configurations follow thermally coupled indirect sequence as shown in Figure (a). However, rather than build the configuration in Figure (a), the configuration of Figure (b) is the one normally constructed. Notice that the two arrangements are equivalent. 2

Crude Oil Distillation Atmospheric 3

Unfortunately, a practical crude oil distillation cannot be operated in quite the way shown in Figure (b), because: v If high-temperature reboiling is used, as would be required for the higher boiling products in the lower part of the column in Figure (b), extremely high temperature sources of heat would be required. Steam is usually not distributed for process heating at such high temperatures. v High temperatures in the reboilers would result in significant fouling of the reboilers from decomposition 4 of the hydrocarbons to form coke.

Therefore, in practice, some or all of the reboiling is substituted by the direct injection of steam into the distillation. The steam has two functions: v It provides some of the heat required for the distillation. The reminder required heat is prepared by a furnace. v It lowers the partial pressure of the boiling components, making them more volatile. The steam is condensed in the overhead and is separated in a decanter from the hydrocarbons. 5

Another problem with the arrangement in Figure (b) is that as the vapor rises up the main column, its flow rate increases significantly. This problem can be solved by removing heat from the main column at intermediate points by pumparound. This corresponds with introducing some condensation of the vapor at the top of intermediate columns. 6

Crude Oil Distillation Pumparound In a pumparound, liquid is taken from the column, sub cooled and returned to the column at a higher point. By choosing the most appropriate flow rate and temperature for the pumparound, the heat load to be removed can be adjusted to whatever is desired. The trays between the liquid draw and return in a pumparound have more to do with heat transfer than mass transfer. In addition to returning a sub cooled liquid to the column, mixing occurs as material is introduced to a higher point in the column. 7

Crude Oil Distillation De-salter The crude oil entering the main column needs to be preheated. This is done initially by heat recovery to a temperature in the range 100 to 150 o. C, and the crude oil is mixed with water to remove salt. In the desalting process, the crude oil mixes with water and then separates into two layers, the salt dissolve in the water layer. The desalted crude oil is then heated further by heat recovery to a temperature usually around 280 o. C. 8

Crude Oil Distillation Furnace The crude oil entering the main column needs to be preheated to around 340 -380 o. C. This is down by a furnace (fired heater). Note that this temperature is higher than decomposition limit, but a high temperature can be tolerated in the furnace if it is only for a short residence time. All of the material that needs to leave as product above the feed point (flash zone) must vaporize as it enters the column. In addition to this, some extra vapor over and above this flowrate must be created that will be condensed and flow back down through the column as reflux. This extra vaporization to create reflux is known as overflash. 9

Crude Oil Distillation Pre-fractionation In the preheating train, the crude is under pressure to suppress vaporization. In the case of a light crude, the pressure required to suppress vaporization is too high. The solution is to separate some light components before heating the crude further in the preheat train. In the pre-fractionation design, the light components are separated into two fractions: light naphtha condensed in the condenser of the pre-fractionation condenser and heavy naphtha condensed in the condenser of the atmospheric tower. 10

Crude Oil Distillation Pre-fractionation 11

Crude Oil Distillation Vacuum The residue leaving the atmospheric tower still contains significant amount of valuable oils. These oils cannot be distillated at atmospheric pressure because the temperature required would be so high that severe thermal cracking takes place. The atmospheric residue is usually reheated to a temperature around 400 o. C (750 o. F) or slightly higher and fed to a vacuum column, which operates under a high vacuum (about 5 mm. Hg at top). As shown in the next Figure. 12

Crude Oil Distillation Vacuum 13

Crude Oil Distillation Vacuum column does not have a condenser and does not feature side strippers either, simply because products (LVGO & HVGO), which are usually used as feedstocks for catalytic cracking units, do not have specifications on their light end. However, side strippers can be used in specific cases, when vacuum column is used for lube base oil production. Lighter components are removed from the residue by steam stripping. In addition, coke formation is reduced by circulating partially cooled bottoms to quench the liquid to a lower temperature. 14

Crude Oil Distillation Fraction specifications In the petroleum refinery, two terms are used for presenting the composition and degree of separation between adjacent petroleum fractions. ASTM D 86 boiling range (usually 95%) defines the general composition of the fraction and is usually one of the key specifications for distillation. The second key specification, (5 -95) gap, defines the relative degree of separation between adjacent fractions. It is determined by subtracting the D 86 95% of light fraction from the D 86 5% of heavy adjacent fraction. 15

Crude Oil Distillation Design guidelines The first step in designing the atmospheric distillation column is the column pressure. The minimum pressure is the lowest permissible pressure in the reflux drum. The practical range is 0. 2 to 1. 0 psig. The design pressure drop across the condenser should be set to 5 psi. The allowable pressure drop for trays will be in the range of 0. 1 to 0. 2 psi per tray. Most atmospheric towers have 25 to 35 trays between the flash zone and the tower top. So a pressure drop of 3 to 6 psi can be assumed between the flash zone and the tower top. 16

Crude Oil Distillation Design guidelines A pressure drop of 5. 0 psi between the flash zone and the furnace outlet is recommended. Recommended ranges for the number of trays in various sections of the atmospheric tower are given in the following Table. Separation section Number of trays Light naphtha to heavy naphtha 6 to 8 Heavy naphtha to kerosene 6 to 8 Kerosene to diesel 4 to 6 Diesel to atmospheric gas oil 4 to 6 Atmospheric gas oil to flash zone 3 to 4 Steam and reboiled stripping sections 4 17

Crude Oil Distillation Design guidelines In design of furnace a 1. 0 to 3. 0% overflash and a maximum temperature of 700 o. F is recommended. Volumetric flow of petroleum fractions can be estimated from TBP curve of crude oil based on TBP cut points as shown in the following Table: Petroleum fraction TBP cut points (o. F) Light naphtha or LSR gasoline 90 -220 Heavy naphtha 220 -380 Kerosene 380 -480 Diesel 480 -610 Atmospheric gas oil (AGO) 610 -690 Vacuum gas oil (VGO) 690 -1100 18

Crude Oil Distillation Design guidelines The recommended specifications for petroleum fractions, ASTM D 86 at 95% point, and (5 -95) gap are shown in the following Tables: Petroleum fraction ASTM D 86, 95% (o. F) Naphtha 360 Kerosene 520 Diesel 620 Atmospheric gas oil (AGO) 710 -770 Petroleum fraction (5 -95) gap (o. F) Kerosene - Naphtha ≥ 30 Diesel - Kerosene ≥ 0 AGO - Diesel -10 to -20 19

Crude Oil Distillation Design guidelines The required flowrate of steam in side strippers and main column can be estimated based on the following guideline: lb of steam/bbl of product = 8 to 10 The original purpose of adding pump-around circuits was to reduce vapor and liquid traffic at the top section of the column. Without pump-around circuits, all condensation heat has to be removed from the condenser, which results in a large vapor flowrate at the top trays. 20

Crude Oil Distillation Design guidelines The presence of the pump-around circuit decreases the number of effective ideal trays. The effect can be even more detrimental to separation if the flowrate of a pumparound circuit is increased. These effects can be compensated to a certain extent by increasing the steam rate in the side strippers. Another solution is to increase the number of trays. However, with the constant total number of trays, the relationship between heat recovery and steam consumption can be incorporated into the design procedure. 21