Introduction

The most common pieces of equipment in the process industries are separation devices, which remove one or more components from a stream and concentrate them in another stream. Mixing of components occurs regularly in nature (e.g., minerals dissolve in rainwater as the water flows down a creek bed), but to separate components requires separation equipment that uses energy and materials to accomplish the separation. It is well known that the value of products can greatly increase when the key component in a product is taken from a dilute solution by a separation device and transformed into a highly concentrated form (e.g., pharmaceutical products). Therefore, separation technologies can provide significant economic advantages for processing companies. This chapter deals with the description of multiphase systems that are used in the development and design of various types of separation systems. Examples of the application of separation systems include

• Drinking water from seawater: One way drinking water can be produced from seawater is by boiling the seawater to produce water vapor, which is then condensed, yielding drinking water. This process of boiling and condensing is a simple example of distillation.
• Gasoline from crude oil: Part of the gasoline produced by an oil refinery comes directly from the distillation of crude oil. The crude is distilled into a number of products, each with a different boiling point range; one of these products is gasoline.
• Removal of pollutants from effluent streams: Plants that discharge water and gas streams into the environment are required to reduce the concentration of pollutants to specified levels, a step that usually requires the application of a separation system(s). As an example, when coal is burned in a power plant, SO₂ is produced from the sulfur in the coal. SO₂ in the atmosphere is converted to sulfite, which forms acid rain. Therefore, coal-fired power plants are required to remove SO₂ from their flue gas (i.e., the combustion gases after most of the thermal energy has been removed) before discharging it to the atmosphere. Many power plants pass their flue gas through a scrubbing process, which exposes the flue gas to a lime-water mixture to absorb the SO₂. The column that accomplishes this contacting for SO₂ removal is known as an absorber because the lime-water mixture absorbs the SO₂ from the flue gas.
• Pharmaceuticals: Certain prescription drugs are produced by concentrating a dilute solution of the desired product, which was produced in a bioreactor, using an extraction process. An extraction process uses a liquid that has a much greater affinity for the desired product than the components in the reactor effluent. Thus, an extraction process is able to produce a very nearly pure product.
• Typical chemical plant: In a typical chemical plant, a reactor produces a mixture of products and unconverted feed that is fed to a separation train (i.e., a series of separation equipment) that concentrates the products into salable form and returns the unreacted feed to the reactor. For most chemical plants, the separation train is primarily composed of distillation columns with some absorbers and extractors.