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The Sorbead™ Quick-Cycle Process For Simultaneous Removal of Water,
Heavy Hydrocarbons and Mercaptans from Natural Gas

Presented at the
Laurance Reid Gas Conditioning Conference
February 25-27, 2007

Michael Mitariten, P.E.
Guild Associates, Inc.
Dublin, OH
www.moleculargate.com
614-760-8013

Dr. Waldemar Lind
Engelhard Process Chemicals GmbH
Hannover, Germany
www.basf-catalysts.com
+49-511-2886-762

Abstract

Used for decades, BASF Sorbeadä oil-drop silica gel is a premium silica gel-based adsorbent that allows the single step removal of both heavy hydrocarbons and water from natural gas (‘quick-cycle units’) as well as for natural gas dehydration.

Recent developments in the quick-cycle process allow the removal of heavy hydrocarbons, mercaptans, water and, through integration with amine treating, enables H2S / CO2 removal without an additional dehydration unit.  The benefits of the improved quick-cycle process can include amine plant operations with reduced foaming, improved sulfur recovery rates, longer Claus catalyst lifetimes, simplification of propane and LPG mercaptan treating, and the ability to integrate non-regenerable mercury traps at an earlier stage of processing and the production of dehydrated product. The integrated process can be attractive for gas conditioning, LNG and GTL facilities.

Introduction

Natural gas always contains contaminates or other unacceptable components that can include heavy hydrocarbons, water, mercaptans, mercury and the acid gases of H2S and CO2. Conditioning natural gas for pipeline, LNG or GTL generally requires the removal of these contaminants.  Historical arrangements of the process units for their removal are optimized on an individual plant basis but fundamentally removal of the contaminants as early as possible in the processing train is desirable.

BASF Sorbead™ oil-drop silica gel has been applied for decades to remove water and heavy hydrocarbons from natural gas in "quick-cycle" units.  These units operate on the basis that water and heavy hydrocarbons are attracted through molecular scale forces to the surface of the Sorbead adsorbent with a higher adsorption capacity than that of lighter hydrocarbons. 

As a general rule, the adsorption capacity is directly related to the molecular weight and boiling point of the gas adsorbed (with the exception that water is very strongly adsorbed).  This means water and higher molecular weight components are adsorbed more strongly than lighter components.  In this manner, BASF Sorbead adsorbents act much like a giant gas chromatograph with the adsorption strength as demonstrated in Figure 1.

Sorbead Process - Relative Adsorption Strength

Examination of Figure 1 indicates that the flow is downward, which is typical, and, after C5, the C6 component is nearest the product discharge side of the bed.  This means that if additional feed were sent into the adsorbent bed the C5 components would be the first to appear in the product gas followed by C6.  Note that water is most strongly adsorbed and that it is removed at the entrance to the bed of adsorbent.  The mercaptan species are also removed and are adsorbed more strongly than C6.  As with the other components the higher molecular weight mercaptans are removed first.

Once saturated with the targeted impurities, the bed is thermally regenerated by passing a hot stream of feed gas through the bed which upon heating causes a decrease in the adsorbent affinity for the impurities and they are released into the regeneration stream.  This heating step is followed by cooling and the cycle repeats.


The Quick-Cycle Process for the Removal of Hydrocarbons and / or Mercaptans

Most process engineers, in the natural gas industry, have at least a passing familiarity with thermal swing adsorption (TSA) systems used for dehydration.  These systems generally consist of two or more vessels of adsorbent where, at any point in time, one vessel is removing vapor phase water from the wet natural gas feed stream while the second vessel is first heated and then cooled - often with the product gas.  Adsorbents used in this service include BASF Sorbead oil-drop silica gels, granular gels, aluminas and molecular sieves.  While many configurations and adsorbents are used, they mostly share the trait of long adsorption and regeneration times, often in excess of 8 hours each.  Long regeneration times allow the vessels to use external insulation.

Given that the quick-cycle process must remove not only the strongly adsorbed water components but also some of the C5+ hydrocarbons as well as heavy components, long cycle times of 8 hours are economically impractical due to the adsorbent quantity required.  Instead, the system is held to an economical cost by operating in a quick-cycle mode with typical adsorption time of 0.5-2 hours. 

This quick-cycle imposes several design constraints on the system.  First, and most importantly, the adsorbent will see many times the number of adsorption and regeneration cycles experienced as compared to conventional TSA dehydration.  Since adsorbent life is related to the number of cycles, it is imperative to employ an adsorbent that can withstand the large number of cycles without premature deactivation.  It has been demonstrated, at many plants, that conventional silica gels can rapidly deactivate requiring recharges, in some cases within weeks of start-up.  However, the oil-drop manufacturing process used in the Sorbead production imparts a higher strength and easy regeneration that allows years of operation, generally 3-5 years or more with some units in operation for over 10 years.

The quick-cycle design also requires that the vessels be internally insulated.  This internal insulation allows the rapid heating and cooling of the adsorbent beds without having to heat and cool the entire vessel with the associated demand for excessive levels of regeneration gas and the dilution of the heavy hydrocarbon concentration in the regeneration stream.  Internal insulation also reduces mechanical stresses due to the thermal temperature swings on the adsorber vessels.

The quick-cycle design consists of three basic steps and generally at least three adsorber vessels (more typically four adsorber vessels) - all with a down flow direction and all from the feed end of the vessel.  The first step is the adsorption step as demonstrated in Figure 1.  The saturated vessel is then regenerated in two steps - first by heating and then by cooling.  The bed that has terminated heating is cooled with raw feed gas by passing a slipstream through the bed in the same direction as feed.  This cooling step lowers the adsorbent temperature and also transfers heat from the adsorbent bed to the regeneration stream. Since the regeneration effluent from this vessel is partially heated, a higher heating efficiency results.

The partly heated effluent is then further heated, often with a gas fired heater, to about 500°F (260°C) and passed in the same direction as feed into the vessel to be heated.  This heating is the initial step of regeneration and is directed into a bed saturated with the previously adsorbed heavy components.  It is during this heating step that the heavy components desorb into the regeneration stream and are enriched in concentration.  By cooling and condensing this effluent stream a liquid product is recovered.

After heavy components are condensed, the uncondensed regeneration vapor stream is now wet and contains a residual amount of heavy hydrocarbons and is recycled back to the raw feed.  In general, this recycle rate is 15-30% of the raw feed rate and the system must be sized to accommodate this recycle flow.

The regeneration steps are demonstrated in Figure 2, on the following page.

Sorbead Process - Regeneration Steps

The adsorption and regeneration of the quick-cycle unit is typically from 450 to 1500 psig (30 to 100 bar g) with product at about 30 45 psi (2-3 bar) lower than the feed pressure. The regeneration is also at high pressure and no compression is required in the system (the penalty being the 2-3 bar g pressure drop).

A typical simplified PFD is in Figure 3 and a photo of a 4-vessel design is shown in Figure 4.

Sorbead Process - Simplified Quick-Cycle PFD


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