Use Analytical Tools to Investigate LNG Molecular Sieve Underperformance

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Use Analytical Tools to Investigate LNG Molecular Sieve Underperformance

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  • Use analytical tools to investigate LNG molecular sieve underperformanceR. Herold, Shell Global Solutions International, Amsterdam, The Netherlands

    In an LNG plant, natural gas from the wellhead must be treated before liquefaction can take place. This treatment happens in dedicated units where contaminants like water, mercury and sour components are removed. These treating facilities are an essential part of LNG plants, helping ensure reliable LNG production.

    Generally, literature places emphasis on the design of future LNG plants, while the operational performance of existing facilities is infrequently discussed. When operational aspects are discussed, the focus is mainly on the liquefaction unit, as this unit consumes most of the energy in the plant.1, 2However, sufficient and structured operational attention to treating facilities is an essential aspect of ensuring LNG plant performance. A case study of molecular sieve deactivation shows that careful analysis is needed to determine and address the cause of operational problems at the dehydration unit.

    LNG process scheme. In a typical LNG plant, a split is made between warm and cold units, as these units can be independently located and operated. The cold units are typically operated at cryogenic conditions. In the treating facilities, front-end processing of the raw natural gas occurs to meet the strict feed gas specifications for liquefaction. The order of connection between the different units is determined by the feed gas requirements and the technology selected for each unit.

    The feed gas from upstream typically arrives in a slug catcher before the gas is routed, via a knockout vessel, to the first gas treating unit, the acid gas removal unit (AGRU). To ensure problem-free operation of the AGRU, it is essential to have proper liquid knockout and to prevent carryover of liquid hydrocarbons to the AGRU.3 In the AGRU, the CO2 content is reduced to the LNG specification, usually by an amine-based solvent. The CO2 is removed from the gas by contacting with the circulating solvent in a high-pressure absorber. The solvent is then regenerated by stripping out the CO2, using steam at low pressure.

    After contacting with the solvent, the treated natural gas is water-saturated. Before liquefaction of the gas can take place, the water content must be reduced below 0.1 ppmv to prevent freezeout in the cryogenic process units.

    Deep removal of water is done by molecular sieve adsorbents in the dehydration unit (DHU). Compared with other adsorbents and water-removal technologies, molecular sieves have a high water capacity at low partial water pressures, which ensures that the tight water specification can be met.

    Generally, LNG plant designs require that the DHU inventory be changed out once every four years. The changeout of the molecular sieve is usually aligned with other plant maintenance, such that a dedicated shutdown is not required and the molecular sieve changeout does not determine plant availability. Mercury-free feed gas is required to prevent mercury-induced corrosion of the main cryogenic heat exchanger (MCHE), which is made of aluminum. Mercury is removed by a non-regenerable adsorbent.

    In contrast with solvent-based systems as applied in the AGRU, adsorbents used in the DHU and mercury-removal unit (MRU) are more unforgiving to process upsets. The DHU inventory cannot be changed out without affecting LNG production capacity.

    To ensure that maintenance planning is not endangered, molecular sieve capacity test runs are executed on a regular basis, whereby a check is made to see if the deactivation pattern of the molecular sieve does not exceed the predicted pattern. In this article, a case is discussed in which test run results suggested that deactivation of the molecular sieves was indeed progressing faster than expected. However, a detailed analysis shows a more complicated picture.

    Molecular sieve dehydration unit. Water removal by adsorbents is a batch process using multiple beds, although the overall DHU is operated in a continuous manner. For a 5-MMtpy LNG train, typically, three beds filled with molecular sieve are installed. Two of the beds are simultaneously in adsorption mode, while the third bed is in regeneration mode. The water-removal capacity of the adsorbent decreases over time due to the number of the high-temperature regenerations to which it was exposed.

    During an adsorption period, the beds adsorb water and are subsequently regenerated using a heated stream of treated gas. The increase in temperature during regeneration is one of the factors that causes water to desorb from the molecular sieve, and the process is called temperature-swing adsorption (TSA). Although TSA is a discontinuous process, the overall DHU behaves like a continuous process because one or more vessels are in adsorption mode while another vessel(s) is in regeneration mode. A sketch of a molecular sieve DHU is shown in Fig. 1.

    Fig. 1. Sketch of a typical DHU.

    Page 1 of 8Use analytical tools to investigate LNG molecular sieve underperformance

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  • The objective of regeneration is to strip off the water and any co-adsorbed impurities from the bed by passing hot gas, and then cold gas, through the bed. Water stripped off by the hot gas is condensed and disposed. The hot regeneration serves two important purposes:

    1. Provide heat for desorption and act as carrier gas to carry away the desorbed impurities, thereby enabling regeneration

    2. Optimize the performance of the DHU (which requires a detailed understanding of its operation).

    During the adsorption period, the amount of water that can be adsorbed on the molecular sieve is dictated by both the capacity that can be reached in the limit of reaching equilibrium, and the capacity that results from the competition between adsorption kinetics and the flow of gas past a molecular sieve pellet.4, 5

    At LNG plants, the DHU is usually operated with a fixed-cycle time. This implies that one dryer bed goes from adsorption into regeneration mode after a fixed operating time. The cycle time is determined by the minimum regeneration time and based on end-of-run (EOR) operating conditions. To account for deactivation, the uptake capacity of a molecular sieve at start-of-run (SOR) conditions could be twice as large as the uptake capacity of a molecular sieve at EOR condition.

    Several mechanisms contribute to molecular sieve deactivation.611 The most prominent ones are caking and coking. Consequently, when confronted with a DHU unit that is operating below its expected performance, there is a tendency to assign this occurrence to the presence of caking and/or coking.

    During regeneration of the water-saturated molecular sieve, hot, dry gas is passed upward through the molecular sieve bed, and water is desorbed. Subsequently, water that was desorbed from the bottom portion of the bed would be carried to the top portion, which would not yet have been heated by the regeneration gas. Since the top portion of the bed would still be cold and saturated with water, the water from the bottom portion of the bed could condense to form liquid water. The formation of liquid water in the bed can cause the clay binder of the molecular sieve to dissolve, subsequently forming cake during the regeneration step.

    Caking due to liquid water formation during regeneration can be avoided by using a well-designed heating profile during the regeneration cycle. If the temperature of the regeneration gas entering the bed is slowly increased, the top portion of the bed can be preheated before much water desorbs from the bottom portion of the bed. Preheating of the top portion of the bed prevents the condensation of liquid water when the water from the bottom portion of the bed is finally desorbed. If a non-ideal temperature profile is used during regeneration, then degradation of the molecular sieve might still occur without the formation of a large solid cake.

    The tendency for coke to form on the molecular sieve depends heavily on the feed composition. When there are heavy components in the feed, they can adsorb in the pores of the clay binder. During regeneration at high temperatures, the heavy components adsorbed on the molecular sieve can subsequently decompose, leaving carbonaceous deposits (coke). It is possible for coke deposits to block not only active sites, but also micropores. In such a case, many moles of active sites can be blocked by a single mole of coke deposit.

    It is also possible for coke to partially block macropores of the clay binder in which the zeolite crystals are contained. In this case, the rate of water uptake on the molecular sieve would be reduced. This would manifest itself in operation in a longer mass-transfer zone, because it is in this zone where the kinetics of water adsorption compete with the rate at which water is carried past particles by the flow.

    Through the blocking of micropores and macropores, the dynamic capacity of the dehydration unit is reduced twofold: first, by the direct blocking of sites by coke; and, second, by the lengthening of the mass-transfer zone, which causes water to break through at the end of the bed sooner than designed.

    DHU test runs. DHU capacity test runs, which provide information on the amount of water that can be adsorbed by the molecular sieve, are essential for evaluating the performance of the molecular sieve and for estimating the remaining lifetime. Once the capacity has been determined from a test run, this can be plotted against the number of regeneration cycles that the bed has experienced. If the results of the test runs are plotted against the expected deactivation curve, one can evaluate wheth