Columns atmospheric distillation unit

Atmospheric distillation separates the crude oil complex mixture into different fractions with relatively narrow boiling ranges. In general, separation of a mixture into fractions is based primarily on the difference in the boiling points of the components. In atmospheric distillation units, one or more fractionating columns are used. 

The first step in the building the atmospheric distillation unit is entering the composition of the crude in order to generate the necessary hypothetical components for model. For the purposes of this simulation, we will consider the crude assays given in Table 2.5 to Table 2.8. It is important to remember that that we may have to remove extraneous details from the distillation curve to avoid unusual column behavior. We use the TB P distillation, density distribution and overall bulk density to define this system in Figure 2.14. 

Modern refineries operate within strong economic, regulatory and process constraints. Many times, the preferable operating mode for the atmospheric distillation unit may not be operating mode that matdmizes the yield of the most valuable product from the distillation unit alone. The atmospheric column operates in concert with many other units in the refinery. Therefore, it is important to understand how the product yield slate changes with different draws of a given cut... 

The atmospheric reduced crude is the feedstock for the vacuum distillation unit. To prevent thermal decomposition (cracking) of the higher boiling point hydrocarbons in the crude oil, the pressure in the vacuum distillation fractionation column is reduced to about one-twentieth of an atmosphere absolute (one atmosphere pressure is 14.7 psia or 760 mm Fig). This effectively reduces the boiling points of the hydrocarbons several hundred degrees Fahrenheit. The components boiling below about 1050°F (565°C) are vaporized and removed as vacuum gas... 

Figure 3-1. Flow diagram of atmospheric and vacuum distillation units (1,3) heat exchangers (2) desalter, (3,4) heater (5) distillation column, (6) overhead condenser, (7-10) pump around streams, (11) vacuum distillation heater (12) vacuum tower.

Atmospheric distillation is least effective in converting heavier products into lighter components. A second distillation column under vacuum is needed to further separate the heavier parts of crude oil into lighter fractions. Some fractions from the vacuum units have better quality than atmospheric distillation cuts because the metal-bearing compounds and carbon-forming materials are concentrated in the vacuum residue. 

In atmospheric or straight-run distillation the crude oil is first pumped into the fractional distillation unit. This is the refinery s tallest unit and some of its columns are used for atmospheric distillation while others are for vacuum distillation. Heated to about 680°F in the gas furnaces, the petroleum reaches the first atmospheric column, which is divided into compartments for fractional distillation. The lighter and more volatile hydrocarbons rise to the upper part. Those that are heavier and less volatile collect in the lower part. While rising, a volatile mass tends to shed its less volatile elements. 

Specialised units are used to simulate complex fractionation processes in petroleum refining. Typical configuration consists of a main column with pump-around and side strippers (Fig. 3.14). Among applications, we may cite pre-flash tower, crude atmospheric distillation, or Fluid Catalytic Cracking (FCC) main fractionator. 

A continuous distillation unit, consisting of a perforated-tray column together with a partial reboiler and a total condenser, is to be designed to operate at atmospheric pressure to separate ethanol and water. The feed, which is introduced into the column as liquid at its boiling point, contains 20 mole% alcohol. The distfllate is to contain 85% alcohol, and the alcohol recovery is to be 97%. 

Therefore, the main succession of petroleum refining includes two main units that of atmospheric distillation and that of vacuum distillation. The lighter products are obtained in the first unit right up to mazut (bottom product of atmospheric column with side strippings) and, in the second unit, the heavier products are obtained right up to tar (bottom product of vacuum column with side strippings). 

In terms of throughput, the biggest unit in most plants is the crude distillation unit Figure 9). Many downstream conversion units also use distillation for production separation. For example, in a coker, hydrocracker, or FCC unit, an atmospheric tower, a vacuiun tower, and a multi-column gas plant may be required. 

This chapter serves as a guide to model atmospheric distillation section of the crude distillation unit. We provide relevant process, operational and modeling details to model the atmospheric column. We also discuss methods to estimate missing data for model development We provide step-by-step instructions to model a particular column in Aspen HYSYS. We discuss how to validate the model predictions with plant data and how to use the model to perform industrially useful case studies. 

The vertical thermosiphon reboiler is a popular unit for heating distillation column bottoms. However, it is indeed surprising how so many units have been installed with so little data available. This indicates that a lot of guessing, usually on the very conservative side, has created many uneconomical units. No well-defined understanding of the performance of these units exists. Kern s recommended procedure has been found to be quite conservative on plant scale units yet it has undoubtedly been the basis for more designs than any other single approach. For some systems at and below atmospheric pressure operation, Kern s procedure gives inconsistent results. The problem is in the evaluation of the two-phase gas-liquid pressure drop under these conditions. 

A modem petroleum refinery in the United States processes between 100,000 and 500,000 barrels/day of crude oil. The incoming cmde is first desalted and then passed through an atmospheric pressure distillation column that separates it into fractions, as shown in Figure 2-12. 

Most atmospheric columns contain from 30 to 50 fractionation trays. For each sidestream desired, about five to eight trays are required, plus additional trays above and below the primary trays. The various sidestreams collected from the distillation column contain lighter boiling products that must be removed. Smaller reboiling units are used to remove lighter products and direct them back into the distillation column as vapor. Also, refluxing units are sometimes employed to condense and remove heavy end products from collected fractions. These condensed heavier products are reintroduced into the lower trays. 

The processes used in industrial air-separation plants have changed very little in basic principle during the past 25 years. After cooling the compressed air to its dew point in a main heat exchanger by flowing counter current to the products of separation, the air feed, at an absolute pressure of about 6 MPa, is separated in a double distillation column. This unit is kept cold by refrigeration developed in a turbine, which expands a flow equivalent to between 8 and 15% of the air-feed stream down to approximately atmospheric pressure. 

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