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New Gas-Separation Technology Reduces Cost, Weight, and Footprint

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Hydrocarbon processing and treating systems often require large and elaborate surface facilities. When operating in challenging locations, such as deep water or the Arctic, these systems can be expensive. This paper discusses a new adsorption-based gas-separation technology platform to address these challenges.

The technology uses rapid pressure- and temperature-swing cycles to reduce equipment size and weight and the overall gas-treating footprint by approximately 50% compared with conventional technologies. The savings are particularly beneficial in locations where space is limited. Furthermore, the technology is well suited for skid-based deployment, which provides execution efficiencies.

While the technology platform is broadly applicable for a variety of upstream gas separations, development is currently focused on deep dehydration and carbon dioxide (CO2) removal before cryogenic gas processes, such as liquefied natural gas (LNG) or natural gas liquid (NGL) recovery.

Extensive fundamental research, including laboratory and pilot testing, has been completed to demonstrate technical feasibility.

Construction of a field research and demonstration facility is under way as of this writing to demonstrate scaled-up operation over a wide range of operating conditions.

Introduction

In the interest of making oil and gas developments economically viable, oil and gas companies are focused on reducing the cost of treating and processing facilities. Execution and operational efficiency benefits are added incentives to reduce the size, weight, complexity, and footprint of these facilities, particularly for offshore installation. Pursuing these incentives, the operator is developing a new adsorption-based gas separation technology platform targeting a broad range of upstream separation needs.

Adsorption-based separation processes are used in gas conditioning for applications such as dehydration, CO2 removal, and helium recovery. These are often considered batch processes, in which a single bed operates in different substeps at various times.

The sequence of transitioning from one substep to subsequent substeps and back to the first is considered a single cycle. The 4- to 24-hour cycle durations common to conventional adsorption approaches require large adsorption systems. The technology platform discussed in the complete paper is based on an alternative approach using rapid cycles that can result in smaller systems to achieve the same separation.

The new technology intensifies the separation process, resulting in numerous advantages in addition to cost benefits. Smaller footprint can be crucial when operating in challenging locations (Fig. 1). Smaller systems can enable skid-based project execution, which simplifies fabrication, transportation, installation, and startup. Smaller systems have lower hydrocarbon inventories. Additionally, rapid cycles and novel cycle designs can help expand the operating envelope of adsorption processes while retaining their inherent benefits, such as elimination of solvents.

Fig. 1—A comparison of a conventional molecular sieve, right, to the new adsorption technology.

Technology Overview

Typically, in gas adsorption, the molecule that is to be separated is adsorbed from the gas phase preferentially onto an adsorbent bed. Once the bed is saturated with this component, it is taken offline and regenerated by introducing a purge stream while increasing the temperature of the bed, decreasing the pressure over the bed, or a combination of the two. Continuous process flow is achieved by contacting multiple adsorber beds sequentially. In conventional applications, the adsorbers are designed as randomly packed beds of adsorbent pellets, and gas flows through the beds at a slow velocity to facilitate diffusion of the adsorbate into the pellets while limiting pressure drop. These systems generally are large and operate over long cycle times. For example, a conventional molecular sieve used for deep dehydration typically operates over a cycle ranging from several hours to days.

In contrast, the new adsorption technology uses rapid cycles on the order of seconds to a few minutes to shrink the size of the adsorbers dramatically. Gas is introduced at a relatively high velocity into the adsorbers to remove the desired component. This results in a higher mass-transfer rate and rapid adsorption of the desired component. Upon saturation, the adsorber is regenerated rapidly with a pressure or temperature swing to remove the adsorbed component and prepare for the subsequent adsorption step. Because of the relatively high velocity of the gas flowing through the adsorber, an adsorbent with fast kinetics and high selectivity toward the component being removed is chosen. An additional benefit of such a configuration is that other molecules are excluded because of the short residence time.

Rapid cycles are enabled by two mechanical systems: structured adsorbent beds and fast-acting valves. The structured adsorbents facilitate gas access to the adsorbent crystals while keeping the overall pressure drop across the adsorber within acceptable limits. The fast-acting valves switch flows in a single adsorber and between different adsorbers.

For a scaled comparison of conventional molecular sieves and the novel adsorption process in a representative deep-dehydration application, the new adsorption process needs less than 1% of adsorbent used in the conventional technology and the adsorber volume is anticipated to shrink by a factor of almost 90. This process intensification is achieved by reducing the cycle time by a factor of 350. The paper also notes that a relatively cool purge or regeneration temperature is required as compared with the conventional molecular sieve. This opens the possibility of eliminating a fired heater for regeneration. Additionally, the compact beds and absence of solvents minimizes motion sensitivity for offshore applications.

Process Integration

Gas-liquefaction plants require upstream gas treating, including deep dehydration and, potentially, CO2 removal, to prevent freezing and plugging of the cryogenic equipment. The complete paper compares integrating conventional molecular sieves into an LNG process with integrating the new adsorption technology into an LNG process. The paper also compares using the traditional process for low-level CO2 removal with using the new technology for low-level CO2 removal.

Approach to Scaleup and Overview of Field Research Unit (FRU)

Scaleup is a key aspect of development and deployment of this technology at commercial scales. Pilot plant testing demonstrated technical feasibility, but the plant had limitations. Testing was conducted on a single bed, which was operated through the complete process cycle. However, the absence of multiple beds precluded demonstration of the continuous operation of the overall process. Additionally, testing was conducted at relatively small flow rates when compared with typical commercial application.

To overcome these limitations, an FRU is being constructed for a wide range of experiments to test the limits of the technology at sufficiently large scale. Testing will include all mechanical systems, the multibed process configuration, and real fluid streams representative of commercial applications.

The scaleup approach is aimed at ensuring that all operating characteristics for a single channel remain the same, irrespective of the overall scale of operation. Key parameters such as gas velocities, pressures, temperatures, channel hydraulic diameter, channel length, and adsorbent coating characteristics remain the same from pilot plant to FRU to commercial adsorber. Scaleup is achieved simply by increasing the number of channels to process the desired amount of gas.

The first phase of FRU testing will focus on deep dehydration. The dehydration FRU is designed to process 20–40 MMscf/D of natural gas. The mechanical design of the FRU is built to closely resemble commercial designs. The number of adsorbers and the overall bed sequencing is exactly representative of a typical commercial application.

In addition to LNG application, the FRU will demonstrate deep dehydration for cryogenic NGL recovery applications. The complete paper contains a depiction of a 3D model and details of the construction of the dehydration FRU. A skid-based approach has been adopted for fabrication and deployment, leveraging the compact nature of the technology. The skid is designed for a 25-year life.

Testing of the dehydration FRU, at the time of this paper’s writing, was expected to begin in late 2018 and run for 1 year to collect all necessary data. The second phase of FRU testing will focus on CO2 removal and will follow the dehydration test program. During the second phase, the dehydration FRU will continue to operate to provide dry feed gas to the CO2 FRU.

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 29001, “Novel Adsorption-Based Gas-Treating Technology Platform for Upstream Gas Separations,” by William Barnes, Chien-Chiang Chen, Tracy Fowler, SPE, Patrick McMahon, SPE, Ananda Krishna Nagavarapu, SPE, and Donald Shatto, ExxonMobil, prepared for the 2018 Offshore Technology Conference, Houston, 30 April–3 May. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.

New Gas-Separation Technology Reduces Cost, Weight, and Footprint

01 April 2019

Volume: 71 | Issue: 4

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