Cover Story Improve Energy Efficiency Using Expanders Expanders can take advantage of pressure reductions to drive rotating machines. Information on how to assess the potential benefits of installing expanders is provided here Sebastiano ften in the Giardinella chemical pro- Ecotek Engineering USA cess indus- Edwin Diaz tries (CPI), “a Ecotek Investments Inc. O considerable amount Freddie Sarhan of energy is wasted in and Scott pressure control valves, Sakakura where high-pressure Sapphire Technologies fluids must undergo a pressure reduction” [1]. Depending on various technical and economic IN BRIEF factors, it may be fea- EXPANDER TYPES sible to transform this energy into rotational COMPONENTS mechanical energy that Sapphire Technologies EXPANDER PROCESS can be used to drive FIGURE 1. This image shows a three-dimensional rendering of an inline turboexpander ARRANGEMENT an electrical genera- tor or another rotating EXPANDER PROCESS DESIGN machine. In the case of best-known type of expander (Figure 1), incompressible fluids (liquids), this is accom- there are other types suitable for different EXAMPLES plished by using hydraulic power-recovery process conditions. This article describes CONCLUDING REMARKS turbines (HPRTs; explained in Ref. 1). For the main types of expanders and their com- compressible fluids (gases), expanders are ponents, and summarizes how operations the appropriate machine. managers, consultants or energy auditors in Expanders are a mature technology with various sectors of the CPI can assess the a host of successful applications, such as potential economic and environmental ben- Containment system VSDIN for fluid catalytic cracking (FCC), refrigera- efits of an expander installation. contamination-free and safe drying tion, natural-gas city gate-valve stations, air separation or off-gas venting, to name Expander types of HAPI products a few. Basically, any gas stream subject to There are many different types of expanders, Vacuum drying oven VSD with customized a pressure reduction can be used to drive varying widely in geometry and capability. isolator integration and mobile pressure nutsch an expander, yet “the energy output is pro- The main types are shown in Figure 2, with a System features portional to pressure ratio, temperature and brief description of each given below. Further • GMP-compliant design flowrate of the stream” [2], and the technical details and a chart comparing the operating • Protection of the operator or the product and economic feasibility of implementing regimes of each of these types as a function • Main and lock chamber as well as RTPs for loading and expanders in a process depends on these of specific diameter and specific speed can unloading of the product and other factors, such as the local price of be found in Ref. 3. • Scales and filter millls integrated in the isolator energy and the availability of suitable ma- Piston turboexpander. Piston and rotary- Options chines from manufacturers. piston turboexpanders operate like back- • Drying oven VSD and pressure nutsch made of Hastelloy While turboexpanders (which function in ward combustion engines, taking high- • Wall installation a manner similar to that of turbines) are the pressure gas and converting its stored • Explosion protected version (ATEX) 28 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM APRIL 2021 PINK GmbH Thermosysteme · Am Kessler 6 · 97877 Wertheim · Germany · T +49 (0) 93 42 919-0 · [email protected] · www.pink.de 12_CHE_0421_CS_Expanders_SCJ_p28-39.indd 28 3/22/21 11:42 AM Sebastiano Giardella energy into rotational energy through a Turboexpander generators can crank shaft. be configured with a direct-drive Drag turboexpander. Drag turboex- shaft from the turbine wheel to panders consist of a concentric flow cav- the electric generator or through a ity with scoop-shaped fins fixed to the gearbox, which effectively reduces periphery of the rotating element. These the input speed from the turbine are designed much like a water wheel, wheel to the generator through but the concentric cavity gradually in- a gear ratio. Direct-drive turbo- creases in cross section from inlet to out- expanders are superior in terms let, allowing for expansion of the gas. of efficiency, footprint and main- Radial turboexpander. Radial turboex- tenance costs. Gearbox turbo- panders are designed with axial-flow in- expanders are much heavier and FIGURE 2. Different types of expanders vary widely lets and radial-flow outlets, such that the require a larger physical footprint, in their geometries and their capabilities gas is expanded radially through the tur- auxiliary oil lubrication equipment bine wheel. Similarly, axial-flow turbines and regular maintenance. expand gas through the turbine wheel, Flow-through turboexpanders can be but the direction of flow remains parallel designed with radial or axial turbines. with the axis of rotation. Radial flow-through expanders contain The main focus in this article is on ra- an axial inlet and a radial outlet such dial and axial turboexpanders, discuss- that the gas flow exits the turbine radially ing their various subtypes, components from the axis of rotation. Axial flow tur- and economic benefits. bines allow the gas to flow axially along Turboexpanders recover energy from the axis of rotation. Axial turbines extract high-pressure gas streams and con- energy from the gas flow via inlet guide vert the energy to drive a load. Gener- vanes to the expander wheel, while the ally, the load is either a shaft-coupled cross-sectional area of the expansion compressor or an electric generator. chamber gradually increases to maintain Compressor-loaded turboexpanders constant velocity. compress a fluid in other parts of the process stream requiring a compressed Components fluid, and thus increase the overall plant A turboexpander generator has three pri- efficiency by utilizing otherwise wasted mary components: turbine wheel, spe- energy. Generator-loaded turboex- cialized bearings and generator. panders convert the energy into elec- Turbine wheels. The turbine wheels tricity, which can be used in other plant are typically designed specifically for the processes or returned to the local elec- application, in order to optimize aerody- trical grid for sale. namic efficiency. Application variables Containment system VSDIN for contamination-free and safe drying of HAPI products Vacuum drying oven VSD with customized isolator integration and mobile pressure nutsch System features • GMP-compliant design • Protection of the operator or the product • Main and lock chamber as well as RTPs for loading and unloading of the product • Scales and filter millls integrated in the isolator Options • Drying oven VSD and pressure nutsch made of Hastelloy • Wall installation • Explosion protected version (ATEX) For details visit adlinks.chemengonline.com/80068-28 PINK GmbH Thermosysteme · Am Kessler 6 · 97877 Wertheim · Germany · T +49 (0) 93 42 919-0 · [email protected] · www.pink.de 12_CHE_0421_CS_Expanders_SCJ_p28-39.indd 29 3/22/21 11:42 AM Sebastiano Giardella through an electromagnetic generator, which can be either an induction or permanent magnet (PM) generator. In- duction generators are rated for lower speed, and thus require gearboxes for high-speed turbine applications, but are capable of being designed to match the frequency of the grid, thereby eliminating the need for a vari- able frequency drive (VFD) to transfer the generated electricity. PM genera- tors, on the other hand, can be directly shaft-coupled to the turbine, and can deliver electricity to the grid via a VFD. The generator can be designed to out- put the maximum amount of electricity according to the available shaft power of the system. FIGURE 3. The diagram shows an example of an overall process arrangement for an expander generator Seals. Sealing is also a critical com- ponent in designing a turboexpander affecting turbine wheel design include system. To maintain high efficiency and inlet/outlet pressure, inlet/outlet tem- meet environmental standards, sys- perature, amount of volumetric flow and tems must be sealed to prevent poten- fluid characteristics. Turboexpanders tial process gas leaks. Turboexpanders with several turbine wheels are required can be designed with dynamic or static when the pressure ratio is too large to seals. Dynamic seals, such as labyrinth reduce via a single stage. Both radial seals and dry gas seals, provide seal- and axial turbine wheels can be de- ing around the rotating shaft, usually signed with multiple stages, but the axial between the turbine wheel and the rest turbine wheel has a much shorter axial of the machine where the bearings and length and thus is more compact. A ra- generator are located. Dynamic seals dial multi-stage turbine requires the gas wear over time and require regular to flow from axial to radial, then back to maintenance and inspection to ensure axial again, and thus produces higher they are working properly. Static seals friction losses than the axial turbine. may be used when all components of Bearings. Bearing design is critical for the turboexpander are contained inside SMART TRANSFER SOLUTIONS efficient turboexpander performance. one housing, protecting any lead wires Bearing types associated with turbo- exiting the housing, including leads for FOR CHEMICALS. expander designs vary widely and can generators, magnetic bearing actua- include oil lubrication, fluid film, conven- tors or sensors. These hermetic seals + tional ball and magnetic. Each has its provide permanent protection against