Modeling, Control, and Optimization of Natural Gas Processing Plants

William A. Poe, Saeid Mokhatab

• Publisher: Gulf Professional Publishing
• Year: 2016
• ISBN:
  9780128029817
• Topic: Technologie et ingénierie
• Subtopic: Génie mécanique

Modeling, Control, and Optimization of Natural Gas Processing Plants presents the latest on the evolution of the natural gas industry, shining a light on the unique challenges plant managers and owners face when looking for ways to optimize plant performance and efficiency, including topics such as the various feed gas compositions, temperatures, pressures, and throughput capacities that keep them looking for better decision support tools.

The book delivers the first reference focused strictly on the fast-growing natural gas markets. Whether you are trying to magnify your plants existing capabilities or are designing a new facility to handle more feedstock options, this reference guides you by combining modeling control and optimization strategies with the latest developments within the natural gas industry, including the very latest in algorithms, software, and real-world case studies.

• Helps users adapt their natural gas plant quickly with optimization strategies and advanced control methods
• Presents real-world application for gas process operations with software and algorithm comparisons and practical case studies
• Provides coverage on multivariable control and optimization on existing equipment
• Allows plant managers and owners the tools they need to maximize the value of the natural gas produced

Chapter 1

Introduction to Natural Gas Processing Plants

Abstract

Natural gas produced from the well contains hydrocarbons, carbon dioxide, hydrogen sulfide, and water together with many other impurities. Raw natural gas after transmission through a network of gathering pipelines therefore must be processed in a safe manner and with minimal environmental effect before it can be moved into long-distance pipeline systems for use by consumers. Although some of the required processing can be accomplished at or near the wellhead (field processing), the complete processing of natural gas takes place at a processing plant, usually located in a natural gas–producing region. The objective of a gas-processing plant is to separate natural gas, associated hydrocarbon liquids, acid gases, and water from a gas-producing well and condition these fluids for sale or disposal. This chapter describes the scope of natural gas processing and briefly reviews the function and purpose of each of the existing process units within the gas-processing plants.

Keywords

Operations; Optimal design; Processing plant; Raw natural gas; Separation; Treatment units

1.1. Introduction

For natural gas to be available to the market, it must be gathered, processed, and transported. Natural gas produced from the well contains hydrocarbons, carbon dioxide, hydrogen sulfide, and water together with many other impurities. Raw natural gas after transmission through a network of gathering pipelines therefore must be processed in a safe manner and with minimal environmental effect before it can be moved into long-distance pipeline systems for use by consumers. Although some of the required processing can be accomplished at or near the wellhead (field processing), the complete processing of natural gas takes place at a processing plant, usually located in a natural gas producing region. The objective of a gas processing plant is to separate natural gas, associated hydrocarbon liquids,¹ acid gases, and water from a gas producing well and condition these fluids for sale or disposal. The processing philosophy depends on the type of project being considered and the level of treating required, i.e., the difference between the feed gas and product specifications. This determines what components will need to be removed or recovered from the gas stream.

This chapter describes the scope of natural gas processing and briefly reviews the function and purpose of each of the existing process units within the gas processing plants.

1.2. Natural Gas Processing Objectives

Raw natural gas stream must be treated to comply with emissions regulations and pipeline gas specifications. Typical pipeline gas specifications are shown in Table 1.1. The specifications are to ensure gas qualities and provide a clean and safe fuel gas to the consumers. The product gas must meet the heating values or Wobbe Indexes² specifications, which are required to ensure optimum operation of gas turbines and combustion equipment to minimize emissions. Pipeline operators also require the product gas to be interchangeable and similar in properties with existing pipeline gas.

Table 1.1

Typical pipeline gas specifications

Characteristic	Specification
Water content Hydrogen sulfide content Gross heating value Hydrocarbon dew point Mercaptan content Total sulfur content Carbon dioxide content Oxygen content Nitrogen content Total inerts content (N2 + CO2) Sand, dust, gums, and free liquid Typical delivery temperature Typical delivery pressure	4–7 lbm H2O/MMscf of gas 0.25–1.0 grain/100 scf 950–1200 Btu/scf 14–40°F at specified pressure 0.25–1.0 grain/100 scf 0.5–20 grain/100 scf 2–4 mol% 0.01 mol% (max) 4–5 mol% 4–5 mol% None Ambient 400–1200 psig

When the gas is high in heavy hydrocarbon contents, they must be removed to meet the heating value specification. The removed natural gas liquids (NGLs) can typically command a higher value than natural gas for the same heating value. Ethane can be used as feedstock to petrochemical plants. Propane and butane can be sold as liquefied petroleum gas (LPG). The C₅₊ components can be exported to refineries as a blending stock for gasoline. The characteristics of various types of NGL products can be found in GPSA Engineering Data Book (2004). The C₂₊ NGL, which is termed “Y-Grade” NGL, shall meet the specifications given in Table 1.2. The Y-Grade liquids must be free from sand, dust, gums, gum-producing substances, oil, glycol, inhibitor, amine, caustics, chlorides, oxygenates, heavy metals, and any other contaminants or additive to the product used to enhance the ability to meet specifications.

Note should be made that sometimes a slight change on the product specifications may have significant impacts on the processing options, which will affect the cost and complexity of the gas processing plant.

Table 1.2

Y-grade NGL specifications (Mokhatab et al., 2015)

Characteristics	Product specifications
Composition
Methane, maximum	Not to exceed either 0.5 vol% of total stream or 1.5 vol% of ethane content
Aromatics, maximum	1 wt% in total stream or 10 vol% in contained natural gasoline
Olefins, maximum	1 vol%
Carbon dioxide	500 ppmv or 0.35 liquid volume % of ethane
Corrosiveness	Copper strip at 100°F—1A/1B pass
Total sulfur	150 ppm wt
Distillation: End point at 14.7 psia	375°F maximum
Free water	None at 35°F
Product temperature	60–100°F

1.3. Gas Processing Plant Configurations

The gas processing plant configuration and complexity depend upon the feed gas compositions and the levels of treating and processing required in meeting product specifications and emission limits. Liquid values can also be the drivers for process complexity, which determines the levels of NGL components to be recovered. Fig. 1.1 shows two simplified gas processing plant schematics. The first scheme is to remove condensate, sulfur, and the heavier components to meet sales gas specifications. The second scheme is to process the feed gas for recovery of the NGL components to increase plant revenues. The residue gas is typically recompressed to a sales gas pipeline. It can also be sent to a natural gas liquefaction plant for liquefied natural gas (LNG) production for use as fuel gas to power plants or as a feedstock to petrochemical plants.

Figure 1.1 Two different schemes of gas processing plants (Mokhatab et al., 2015). LNG, liquefied natural gas; NGL, natural gas liquid.

There are various technologies, conventional or proprietary, for configuring the gas pro...