Question

1)

a) Which two processes can be used to stabilize the liquid from the initial separator?

b) How do pressure and flow rate changes affect the design? Can we design the facility based on the initial production condition?

c) List the 6 process modules commonly used in the general scheme for gas processing.

d) What are the three objectives of the gas processing?

Answer 1

Objectives of gas treating and processing

Before the optimum design of any gas treating plant can be decided, at minimum, one must know:

• the raw gas production capability to the plant
• composition of separator inlet gas and condensate
• relative condensate/gas rates; specifications for the residue gas; and rate of gas sales

Consumer expectations

The end user of natural gas needs to be assured of two conditions before committing to the use of gas in a home or factory:

• the gas must be of consistent quality, meeting sales gas specifications
• the supply of gas must be available at all times at the contracted rate

Gas treating facilities, therefore, must be designed to convert a particular raw gas mixture into a sales gas that meets the sales-gas specifications, and such facilities must operate without interruption.

Typical sales gas specifications

Specifications for sales gas describe the required physical properties of the gas such that it can be transported under high pressure through long distance pipelines at ground temperature without forming liquids, which could cause corrosion, hydrates, or liquid slugs into downstream equipment. Limits on the content of certain nonhydrocarbon compounds are also specified. While the specific limits for each item might vary among transmission companies or customers, the overall specifications for sales gas generally include:

• Maximum hydrocarbon dewpoint temperature at a pressure of 800 psig.
• Maximum allowable CO₂ content.
• Maximum allowable H₂S content and total organic sulfur content.
• Maximum allowable water-vapor content.
• Maximum allowable temperature of gas leaving the plant.
• Minimum pressure to enter the gas transmission grid.
• Minimum heating value.
• Free of dust, treating chemicals, and other contaminants from the process plant.

In long distance transmission of sales gas by pipeline, the pressure is usually less than 1,000 psig. It is important that no liquids form in the line because of condensation of either hydrocarbons or water. Hydrocarbon liquids reduce the pipeline efficiency and might hold up in the line to form liquid slugs, which might damage downstream compression equipment. Condensed water can do the same. Additionally, water could form solid complexes (hydrates), which accumulate and block the line. The dewpoint temperature at any pressure is the temperature at which either hydrocarbons or water condense upon cooling of the gas. Thus, the specifications for sales gas include limits on the hydrocarbon dewpoint temperature, as well as limits on the water vapor content of the gas.

Knowing the specifications, and knowing the required sales gas flow rate and the composition of the raw gas and condensate entering the plant, the various process vessels can be designed, and the optimum process conditions of pressure and temperature can be specified.

Depending on the composition of the inlet fluids and the price at the plant gate, other sales products might be recovered in the plant as well. These could be any of the following, which also must meet stringent specifications concerning purity:

• ethane
• propane
• butane and pentanes-plus

The possible processing steps, as illustrated in Fig. 5.1, are:

• inlet separation
• compression
• gas sweetening
• sulfur recovery or acid gas disposal
• dehydration
• hydrocarbon dewpoint control
• fractionation and liquefied petroleum gas (LPG) recovery
• condensate stabilization

Except for gas sweetening, the processing steps involve no chemical reactions. The gas/liquid product specifications are achieved by separating the compounds through changing the physical conditions of temperature and pressure to which the fluids are exposed. Contact with other compounds, such as glycol and absorption oil, affects the relative solubilities of certain compounds, thereby achieving separation from the main gas stream. Exposure to dry compounds, such as silica gel or molecular sieves, separate some compounds from the gas stream by physical adsorption. Distillation is used to separate the various hydrocarbon compounds into liquid fractions on the basis of differences in their volatilities.

the below link contains proper over view off gass processing

http://petrowiki.org/PEH%3AGas_Treating_and_Processing