Selecting turboexpanders for PDH

Expander compressors with active magnetic bearings increase reliability and simplify operations with lower maintenance costs in a propane dehydrogenation process.

JOSEPH LILLARD, Atlas Copco Gas and Process

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Article Summary

First commercialised in 1990, the use of propane dehydrogenation (PDH) technology has grown exponentially in the last decade to address the global imbalance in the supply and demand of propylene, particularly in China. In purpose-built PDH plants ranging from 250 000 to 750 000 t/y of propylene production, turboexpanders are used in the cryogenic separation and recovery sections to efficiently produce the low temperature required while minimising the need for external refrigeration. These expanders have generally been configured as expander-gearbox-generators (EGIs). As this article shows, however, expander- compressors (ECs) are alternatives that bring with them several notable benefits, ranging from reliability to maintenance costs. In addition, expanders have been used in butane dehydrogenation (BDH) to produce isobutylene. Modern, high efficiency radial inflow expanders are engineered-to-order and can be designed and built by experienced suppliers in accordance with the oil and gas industry turbomachinery standard API 617, as well as other purchaser specifications.

The cracker imbalance
Since 2011, the production of natural gas liquids (NGL) in the US has doubled, reaching 5 million b/d. The use of shale technologies — the combination of directional drilling and hydraulic fracturing — is responsible for the rise in both oil and natural gas production. Meanwhile, the midstream industry has responded to this increased production by building more than 200 new gas processing plants, which are responsible for the rise in extraction and recovery of NGLs.

NGLs consist of ethane, propane, butanes, and natural gasolines. Ethane is used exclusively as a petrochemical feedstock in steam crackers for ethylene production, and propane is used as a fuel but also as a feedstock by the petrochemical industry. Some ethylene producing plants can process either ethane or propane, or a combination of both, to ethylene, propylene, and other olefins. But as greater amounts of inexpensive ethane are produced through gas processing, the cracking economics favour the use of ethane feedstock rather than propane. As a result, the overall industry cracker yield of ethylene increases, while the yield of propylene decreases (see Table 1).

Nevertheless, the petrochemicals industry has a solution for this imbalance. Abundant propane produced by gas processing is converted to propylene directly in dedicated propane dehydrogenation (PDH) plants (Equation 1). In a similar way, isobutane is converted directly to isobutylene in dedicated butane dehydrogenation (BDH) plants (Equation 2):

C3H8 → C3H6 + H2                            [1]
C4H10 → C4H8 + H2                          [2]

In China, demand for propylene has been rising the fastest: this growth has been driven by the growing need for polypropylene, which is used for films, sheets, fibres, bottles and containers, automobile parts, and various other household and industrial goods. The domestic supply of propane in China, however, is not enough to fill total demand. Therefore, China must import propane from major exporters, such as the United Arab Emirates, Qatar, Kuwait, Saudi Arabia, Nigeria, and from the US (which has only recently become a significant exporter).

Dehydrogenation works by feeding a paraffinic feedstock (in this case, propane or isobutane) into fixed-bed reactors, where it undergoes hydrogen elimination at high temperatures, followed by separation and recovery of the desired olefin: propylene or isobutylene (see Figure 1). The three main commercialised processes are Oleflex (Honeywell UOP), CATOFIN (McDermott Technology), and STAR (ThyssenKrupp Industrial Solutions). In the separation and recovery section downstream from the reactors, isentropic expansion provided by turboexpanders is often used to produce the low temperatures needed while minimising the use of external refrigeration (see Figure 2). For many years, EGIs were favoured for this low temperature stage. New process design innovations, however, mean that nowadays ECs have become highly attractive alternatives.
Turboexpanders: refrigeration and power recovery
‘Turboexpander’ is used here to distinguish the turbines used for process refrigeration from other turbines used only as mechanical drives or electrical power generators, such as steam turbines or gas turbines. The radial inflow turboexpander, or ‘expander’, was developed in the late 1930s and 1940s in Germany, Russia, and the US, replacing less efficient and less reliable reciprocating expanders used in cryogenic air separation before that. By the 1950s, several manufacturers were producing air separation expanders. A decade later, their efficient refrigeration and power recovery meant expanders were being used in natural gas processing plants (NGL recovery), petrochemical plants (ethylene and ammonia production), and refineries (tail gas recovery). The oil and gas industry recognised the high reliability of radial inflow expander-compressors in 2002 with their inclusion in the seventh edition of API 617, an industry standard with design and construction guidelines to provide a minimum service life of 20 years and at least five years of uninterrupted service.

In an integrally geared configuration (EGI), the shaft power from an expander drives an electric generator (see Figure 3). In a single-shaft EC, the shaft directly drives a centrifugal compressor (see Figure 4). Table 2 shows typical expander operating conditions for a typical PDH process. In most applications, two expander stages operating in series are required to efficiently expand the process gas over the available enthalpy drop, denoted HP (high pressure) and LP (low pressure).

High levels of efficiency characterise peak expander performance. Figures 5 and 6 show that high efficiencies are achieved by running at high shaft speeds (above 30 000 rev/min) and at high tip speeds (1200-1400 ft/s, 365-427 m/s). This is determined by a combination of factors, such as outlet volume flow, isentropic enthalpy drop, and the use of high-strength aluminum or titanium-alloy impellers. It is important to note, however, that plant conditions can vary depending on feed composition and operating mode, so a broad and flat performance curve is desirable. The radial inflow expander with variable inlet guide vanes uniquely meets this requirement without inlet throttling (see Figure 7).

Bearings are central to the functioning of EC units and they can either be oil-fed or active magnetic. EC units featuring oil bearings are supplied with a hermetic lube oil system, seal gas system, and appropriate instrumentation and controls for monitoring and protection. The lube oil system uses a pressurised lube oil reservoir, which eliminates the need for mechanical shaft seals and enables recovery and return of seal gas to the process. In contrast, EC units featuring magnetic bearings have the advantage of eliminating lube oil altogether, though they still require filtered process gas as seal gas, which performs three important duties. Firstly, seal gas is a separation gas, keeping the cryogenic process gas separated from the bearings, which are not able to operate in cryogenic temperatures. Secondly, it is a cooling gas, carrying away the windage heat and eddy current heat from the bearing housing. Thirdly, it is a purging gas, keeping the bearing housing under positive pressure to eliminate any possible entrance of air/oxygen that would mix with the process gas, maintaining a non-flammable atmosphere in the bearing housing.

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