Up to this point in the book, we have looked at distillation columns that separate specific chemical components. In the refining of crude oil, mixtures of many thousands of components must be handled. These components vary from quite light hydrocarbons (methane, ethane, propane, etc.) to very high-molecular-weight components that boil at extremely high temperatures. Petroleum refineries have units that separate (by distillation) and transform (by a variety of reactions) these mixtures.
Crude oil is produced in hundreds of locations around the world. It is found underground, sometimes under high pressure and sometimes requiring pumping. A vast system of pipelines and huge supertankers transport the crude oil to refineries in which it is processed to make a large number of important products, such as gasoline, heating oil, jet fuel, asphalt, and wax. Most of the raw feedstocks for the chemical industry are produced in refineries, including ethylene, propylene, and benzene.
The initial separation of crude oil into several "cuts" is achieved in a very large distillation column called a "pipestill" or "atmospheric crude distillation" unit. These cuts have different boiling point ranges. Low-molecular-weight gas comes off the reflux drum as a vapor product from a partial condenser. The liquid product from the reflux drum is a light low-boiling naphtha. Products of jet fuel, heating oil, and a heavy high-boiling gasoil are removed as sidestreams from the column. The effluent from a catalytic cracking reactor is also a mixture of petroleum fractions, which are separated in a distillation column called a "cat fractionator." Many other units in a refinery must deal with these complex mixtures.
Crude oil as it comes from the ground is usually a mixture of saturated hydrocarbons: paraffins and naphthenes. The effluent of a cat cracker contains both saturated and unsa-turated hydrocarbons: aromatics and olefins. These differences in the type of components affect the density and average molecular weight of the petroleum fraction for the same boiling point range.
Distillation Design and Control Using Aspen™ Simulation, By William L. Luyben Copyright © 2006 John Wiley & Sons, Inc.
As is true with all naturally occurring feedstocks, the composition or boiling range of crude oil varies greatly from production field to production field. This variability results in a very significant dynamic control problem in a refinery that feeds crude oil from a variety of sources, which is often the case.
Nelson1 provides a thorough discussion of many aspects of the petroleum industry, including types and sources of crude, characterization of petroleum fraction, and types of refinery operations.
The following section is a brief introduction to how petroleum fractions are characterized and quantified so that petroleum fractionators can be designed and their control studied. In the next section we will discuss the details of setting up a steady-state simulation using the specified properties of the crude oil to a simple "preflash" column. Then we will look at the dynamic control of this column. In a later section we expand the process considered to include a pipestill (atmospheric crude unit). This large complex column has three sidestreams, which are withdrawn from sidestream strippers using open steam to remove the light hydrocarbons. It also has two "pumparounds" at intermediate locations up the column to remove heat. This complex system presents challenging design problems and challenging control issues.
Was this article helpful?