Part 4.3 Gas Processing

Objectives After reading the chapter and reviewing the materials presented the students will be able to: Understand functions of gas processing plants Examine natural gas liquids and cryogenic recovery Analyze absorption and adsorption processes Discuss fractionalization to produce salable products

Introduction Flaring is still a common practice in remotely located oilfields where gas cannot be re-injected into a reservoir for gas lift or used locally as fuel. Natural gas from the well is processed in the field. Natural gas processing includes removal of water, impurities, and excess hydrocarbon liquids as required by the sales contract. It also includes the control of delivery pressure. When it is economical to gather gas from several wells to a central point, an operator may build a gas processing plant. These gas plants dehydrate the gas and remove hydrogen sulfide. In addition they generally separate the hydrocarbons and recover sulfur and carbon dioxide. In recent years manufacturers have developed portable skid mounted plants to provide efficient, relatively inexpensive gas processing for smaller fields.

Recovering NGL Mixtures NGLs (natural gas liquids) include propane, butane, and natural gasoline. 1. Straight refrigeration: The simplest way to produce NGLs is to cool natural gas until it becomes liquid. All of them use refrigerants such as Freon or propane. 2. Cryogenic Recovery: Cryogenic processing uses high pressures and extremely low temperatures to recover most of the ethane as well as all of the propane and the heavier hydrocarbons. The most commonly used cryogenic process is the expander process. After being treated, the dry, clean gas is cooled. Under high pressure, the gas enters the expander-compressor where it expands and cools further. The NGLs condense and are separated from the vapor. The NGLs are piped to a fractionalization facility to be separated into individual components. Cascade refrigeration uses a two stage refrigeration to separate methane from the NGLs.

Recovering NGL Mixtures 3. Oil Absorption: The oil absorption process has three steps: NGL recovery, methane removal, and separation of the absorbent from the NGLs. The absorbent is a hydrocarbon liquid called lean oil. When the lean oil absorbs the heavier hydrocarbons, it is called rich oil. 4. Dry Bed Adsorption: The natural gas passes over a bed of solid desiccant which adsorbs some of the NGLs and separates them from the gas stream. Operators primarily use it to ensure that the NGL content of natural gas meets specifications of the gas sales contract.

Fractionalization of NGLs Fractionalization or fractional distillation separates a mixture of NGLs into salable individual products. NGLs can be separated because they have different boiling points. The hydrocarbons are ethane, propane, isobutane, butane, pentane, and a remaining mixture of heavier hydrocarbons (pentane and higher). The NGLS are separated in a series of fractionalization towers.

Summary Flaring is still a common practice in remotely located oilfields where gas cannot be re-injected into a reservoir for gas lift or used locally as fuel. Natural gas processing includes removal of water, impurities, and excess hydrocarbon liquids as required by the sales contract. It also includes the control of delivery pressure. 1. Straight refrigeration: The simplest way to produce NGLs is to cool natural gas until it becomes liquid. All of them use refrigerants such as Freon or propane. 2. Cryogenic Recovery: Cryogenic processing uses high pressures and extremely low temperatures to recover most of the ethane as well as all of the propane and the heavier hydrocarbons. 3. Oil Absorption: The oil absorption process has three steps: NGL recovery, methane removal, and separation of the absorbent from the NGLs. 4. Dry Bed Adsorption: The natural gas passes over a bed of solid desiccant which adsorbs some of the NGLs and separates them from the gas stream. Fractionalization or fractional distillation separates a mixture of NGLs into salable individual products.

Home Work 1. What does natural gas processing include? 2. What is the simplest way to produce natural gas liquids? 3. What does fractional distillation of NGLs do?