LNG

Proposed Cluster Liquefied-Natural-Gas Production System Raises Tolerance of CO2

During natural gas liquefaction, CO2 must be removed to prevent icing and plugging in the system. The CO2-removal system may be the most important part of the gas-treatment system.

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Source: Getty Images.

During natural gas liquefaction, CO2 must be removed to prevent icing and plugging in the system. The CO2-removal system may be the most important part of the gas-treatment system. CO2 removal systems require complicated amine contactor and regeneration systems with substantial heat sources. The CO2-tolerant natural-gas-liquefaction system called cluster liquefaction accepts approximately 1% of CO2 for the liquefaction and related systems. The CO2-tolerant characteristics of this liquefaction process will provide a multifold safety margin against CO2 problems in cryogenic systems and valves.

CO2-Tolerant Liquefied-Natural-Gas (LNG) Production System

Considerations. Natural gas liquefaction at higher pressures and equivalent higher liquefaction temperatures has the advantages of reducing liquefaction energy and adopting more-efficient refrigerant. Despite the advantages, the higher-pressure storage requirement for the produced LNG has been a cost burden in overall LNG chains. For that reason, natural gas liquefaction at higher pressures has not been adopted by the industry.

The specific weight ratio (SWR) of natural gas, defined as the natural gas weight divided by the containment system weight, is an important indication of capital expenditures. The typical SWR of conventional LNG is approximately 10–20 times that of compressed natural gas (CNG). Even for the increased-pressure LNG, the SWR is typically 10 times that for CNG. Hence, storage as a liquid after cooling and insulation is far more efficient than CNG.

For increased-pressure LNG, the main cost contributor has been the cryogenic material for the higher pressure. However, if a cost-effective solution for increased-pressure containment is developed, it may become a viable option.

Examples. Intermediate-Pressure LNG (20 bara, 1% CO2). Substantial CO2 can be dissolved in LNG produced at increased pressure. CO2 in the LNG should be liquid, not solid or as two phases. During cool down at a predetermined pressure, the feed gas can pass through the solid formation region in the interim stage. Hence, this region should be avoided by liquefying the natural gas at a higher pressure before reducing the pressure. If the CO2 level is sufficiently lowered during the gas treatment, the solid formation can be avoided regardless of the liquefaction temperatures and processes.

In the case of 1% CO2 in the feed gas, if the pressure is lowered to an intermediate pressure (20 bara) after liquefaction and subcooling at a higher pressure (30 bara), ice formation can be prevented (Fig. 1).

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Fig. 1: Conceptual diagram of CO2-tolerant LNG process.

 

Intermediate-Pressure LNG With Solid CO2 Separation. Solid CO2 formation in a main cryogenic heat exchanger is not preferable because CO2 solid plugging will yield detrimental effects. Nevertheless, if the pressure is reduced in a simple separator or further cooling down is accomplished elsewhere, then ways exist to remove the solid formation mechanically from the LNG. The pure ­liquid part of the LNG without the solid formation may be stored separately as a final product.

Cluster Concept

Natural gas liquefaction at higher pressure has an economic advantage over conventional liquefaction by lowering the unit LNG production cost and, ultimately, the final delivered gas price. Cluster liquefaction is competitive in unit cost of produced LNG, and it has CO2- and N2-tolerant characteristics. Most of the related technologies have been verified through a bench scale pilot plant (BSPP) in many operation conditions and compositions. A comparison of conventional and cluster liquefaction is shown in Table 1.

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Cluster CO2 Test

Information on CO2 solubility in LNG is available in the literature, but actual verifications in cluster conditions are necessary to confirm specific components. For that reason, experiments focused on finding engineering implementation data rather than exact figures. Fig. 2 shows the test facility.

  • The experiments have been carried out in the following four categories:
  • Basic CO2 solidification at different pressures and temperatures (basic CO2 solidification)
  • Derivation of CO2 freezing design data (design parameters for CO2 contents)
  • Verification during actual LNG production by BSPP (demonstration in BSPP)
  • Basic solid CO2 removal by mechanical hydrocyclone (solid CO2 removal)
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Fig. 2: Cluster LNG BSPP.

 

Basic CO2 Solidification.

  • 1, 2, 5, 10, and 20% standard CO2 bottles were prepared.
  • Corresponding pressures and temperatures were applied with external liquid or vapor nitrogen.
  • Differential pressure was observed across the experiment section to check for CO2 ice formation in the experiment coil.
  • LNG was regasified for gas sampling and analysis.

Design Parameters for CO2 Contents.

  • More specific design parameters will be derived in this experiment.
  • Extensive inlet gas compositions will be tested in more accurate temperature and pressure conditions.
  • The test currently is being carried out at the Institut Teknologi Sepuluh Nopember in Indonesia.

Demonstration in BSPP.

  • Many runs of experiments with different CO2 percentages have been performed by injecting CO2 into the BSPP.
  • The differential pressures in the coldbox and elsewhere were monitored.
  • Samples of the regasified natural gas were made for gas-composition analyses to check whether predetermined CO2 has been liquefied properly without solid formation during the liquefaction.
  • Because of some inaccuracies in instruments and some limitations in the BSPP facility, finding exact icing-formation points was difficult. Differential pressures across the coldbox were measured for different CO2 content. Small consistent differential pressures were observed for CO2 in the range of 0.25–0.8%, whereas substantial differential pressures because of CO2 icing were measured at high CO2 percentages (approximately 10%). Through the series of experiments, it was demonstrated that natural gas containing a maximum of 0.8% CO2 could be continuously liquefied without ice formation in the coldbox, piping, valves, or 20-bar LNG storage tanks.

Solid CO2 Removal.

  • A basic solid CO2 removal system was prepared.
  • Higher-CO2-content (approximately 5%) natural gas was used for the liquefaction.
  • Liquefaction pressure in the coldbox was approximately 60 bar, and the temperature was greater than that required for solid formation.
  • After liquefaction, the pressure was reduced to approximately 20 bar in a separator by a pressure-reducing valve before the LNG was sent to storage tanks.
  • CO2 solid formation in the separator and flash gas amount were checked.
  • A hydrocyclone-type removal system was applied to extract CO2 solids from the produced LNG.
  • Produced LNG was sampled to check CO2 content.
  • Additional basic tests are being conducted; however, at this time, only limited results are available. Relatively high-CO2-content natural gas could be liquefied at the coldbox without the formation of solid plugging, and some solid CO2 was found in the separator.
  • Further extensive tests will be conducted, and enhancements of the test facility will be made.

From the basic CO2 solidification tests and actual liquefaction runs in the BSPP, it was seen that natural gas liquefaction for approximately 1% CO2 is possible with the cluster process at 20 bar without forming CO2 solids, which is a CO2 level approximately 200 times greater than that for conventional LNG. The CO2-tolerant characteristics of cluster liquefaction were demonstrated by the CO2-related experiments.

Applications

Onshore LNG Plants. Cluster technology can be applied to land-based LNG plants as well as floating platforms. Although the cluster-produced LNG is transported preferably by ship, it also can be transported by container trucks. The higher pressure of the cluster LNG may not be compatible with existing ambient-pressure LNG storage facilities; nevertheless, no problems exist in providing regasified product to end users such as power plants or with injecting gas into existing pipelines. After flashing and supplying the flashed gas to consumers at the regasification site, the balance of the cluster LNG may be converted to ambient-pressure LNG for other purposes.

Ongoing Cluster Projects. Several onshore cluster projects are undergoing feasibility studies with diverse clients, and some are close to reaching the decision stage. The first small-capacity commercial cluster liquefaction was expected to start operation in the first quarter of 2013. The successful operation of the plant would reconfirm most aspects of the technology, including the cluster LNG containment system, and it would provide additional confidence in technology and business. Because the system adopts a standard design, especially for the containment system, scaling up the system is straightforward, accomplished by adding the required number of standard containment tanks and liquefaction trains.

Path Forward. With the inherent competitiveness of cluster LNG, the prospects for onshore and offshore application are believed to be bright, and several projects are expected to be implemented in the near future. The new flexible LNG model may bring paradigm changes for traditional LNG business. The next phase of design and fabrication certification was in progress at the time the paper was written.

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 23261, “Development of CO2-Tolerant LNG-Production System,” by JungHan Lee, Jeheon Jung, and Kyeongmin Kim, SPE, Daewoo Shipbuilding and Marine Engineering, prepared for the 2012 Offshore Technology Conference, Houston, 20 April–3 May. The paper has not been peer reviewed.