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Oct-2011

Optimal tonnage industrial gas plants

Detailed examination of industrial gas needs is required to develop the most cost-effective industrial gas plant configuration

Wolfgang Schoerner and Goutam Shahani
Linde Engineering
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Article Summary
The refining and petrochemical industries are being hit by fluctuating supply and demand, extreme price volatility and market uncertainty worldwide. Large consumers of tonnage quantities of industrial gases have to make informed investment decisions in this uncertain economic environment. Ideally, manufacturing and engineering companies need to work together as a team to identify the most efficient and economic plant design. This includes an in-depth assessment of capital and operating costs to deliver an optimal solution, taking into account plant reliability and process safety.

It is important to consider all of the industrial gases as well as steam and utility needs at a manufacturing complex, in conjunction with available feedstock and wastes to develop the best possible long-term solution. Oxygen (O2) requirements include oxygen-enriched or oxy-fuel combustion, hydrocarbon oxidation, wastewater treatment, gasification and power generation. Nitrogen (N2) is used for inerting and blanketing, while hydrogen (H2) is used for refining crude oil and various chemical synthesis reactions. All of these industrial gas requirements have to be examined over a long-term horizon in a holistic manner.

Cryogenic air separation technology has been constantly improved since the world’s first air separation plant was built in 1902. For H2 production, plants can be designed to handle the gamut of hydrocarbon feedstocks, from natural gas, LPG, refinery off-gases and naphtha to heavy fuel oil and asphalt.

Turnkey air separation and H2 plants deliver a complete solution so that manufacturers can focus on their core business. By using global procurement and construction capabilities, it is possible to reduce costs and improve project execution, thereby making industrial gas plant procurement simpler and cheaper for the refining and petrochemical industries.

In the petrochemical industry, O2 or O2-enriched air is used in a variety of synthesis processes, H2 is used for hydrogenation, and carbon monoxide (CO) and syngas are used for a variety of carbonylation processes (see Table 1). The process plants that produce these gases are capital intensive. Air separation plants require significant electrical power, while H2, CO and syngas plants require a hydrocarbon feedstock such as natural gas or naphtha. In both cases, utilities such as cooling water are required. Given that these plants are capital intensive, it is important to understand the current and future costs of energy. This is essential to make the appropriate trade-off between capital cost and energy consumption.

Technologies: H2 and CO
Available technologies for the large-scale production of syngas, H2 and CO can use hydrocarbon feedstocks ranging from natural gas to heavy oil, all the way to coal.

In plants for the production of H2 and CO, chemical and physical wash units are used for the removal of hydrogen sulphide and CO2. A low-temperature process is used for product purification, with a downstream adsorption unit for the production of ultra-pure H2.

Syngas (synthesis gas, oxogas), a mixture of H2 and CO, is produced by steam reforming and the partial oxidation of hydrocarbons, or a combination of both processes. The desired H2/CO ratio can be adjusted by conversion or separation. Syngas is utilised in the production of oxo- alcohols, methanol or synthetic fuel (Fischer-Tropsch products).

Lighter feedstocks such as natural gas, liquid gas and naphtha are generally converted by catalytic steam reforming to a raw syngas consisting of H2 and CO. This is then further processed depending on the desired final products. In the case of pure H2, the process includes such features as catalytic conversion of CO and a pressure swing adsorption unit, in which all impurities are removed in a single step.

Heavy oil feedstocks ranging from residual oil to asphalt as well as coal are partially combusted with O2 in non-catalytic partial oxidation to produce a raw gas, which is then further processed into saleable products. Here, such process steps as CO conversion, sour gas removal with a Rectisol wash, and cryogenic separation of H2 and CO are used.

Air separation
The large quantities of atmospheric gases (O2, N2) required in many industrial applications are cost-effectively produced by cryogenic air separation units (ASU). In this process, air is separated into its components by distillation. This necessitates first liquefying air by operating at less than its critical point. In simple terms, air is cooled by reducing pressure, according to the Joule-Thomson effect. For one atmosphere decrease in pressure, air cools by ~0.5° F (0.3°C). The expansion can be carried out across an expansion valve or turbine. To achieve greater energy efficiency, the expansion of a high-pressure stream can be carried out in an expansion turbine or expander. Such machines are more complex than a simple expansion value. The higher capital cost can be justified by lower energy consumption, especially for bigger air separation plants. Typically, heat exchange between feed and products is maximised for high overall energy efficiency.

A simplified schematic of a process that produces both O2 and 
N2 is shown in Figure 1. This process was developed in 1910 and consists of a low-pressure column on top of the pressure column. 
The principle of double-column rectification, combining a condenser and evaporator in the form of a heat exchanger, is still used today.

Modern air separation plants include several important components in addition to the distillation column. These are shown in Figure 2 for a plant that produces gaseous and liquid O2 and N2 (GOX, GAN, LOX and LIN). The main components are air compression, air cooling and purification heat exchange, refrigeration, rectification and internal compression.
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