1

DESIGN AND OPERATION OF SOUR GAS TREATING PLANTS FOR H2S, CO2, COS, AND MERCAPTANS

A.F. Carlsson T. Last

C.J. Smit

Shell Global Solutions International B.V. Amsterdam, The Netherlands

ABSTRACT

Natural gas must be purified before it can be sold, liquefied, or fed to a gas-to-liquids process. Growing demand for natural gas is leading to an increase in the production of stranded and contaminated natural gas. In general, the contaminants that must be removed by gas treating are H2S, CO2, COS, and mercaptans (RSH), which are often removed using a combination of chemical solvents, physical solvents, and solid adsorbents – optimal combinations of these were discussed at the 2005 Gastech conference in Bilbao. Amines are commonly used for the removal of H2S and CO2 via chemical absorption and reaction in a trayed or packed column, while the solvent is continuously regenerated at low pressure and high temperature in a separate regenerator column. Molecular sieves can be used in a temperature-swing adsorption process downstream an amine or physical solvent to remove traces of mercaptans. The design and operation of the mol sieve unit involves a number of challenges due to possible BTX co-adsorption and potentially fast deactivation of the sieves. In addition, the transient nature of the temperature-swing adsorption process introduces challenges in the design of the sulphur recovery unit (SRU) and the regeneration-gas treating for the mol sieve unit. Deactivation can be mitigated using a novel regeneration process, while transients in the regeneration gas can be smoothed using a peak-shaving scheme. These and other process and engineering solutions to design and operational challenges encountered by Shell Global Solutions are discussed herein.

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DESIGN AND OPERATION OF SOUR GAS TREATING PLANTS FOR H2S, CO2, COS, AND MERCAPTANS

INTRODUCTION

Natural gas must be purified before it can be sold, liquefied, or fed to a gas-to-liquids process. Growing demand for natural gas [1-3] is leading to an increase in the production of stranded and contaminated natural gas [4, 5]. Significant reserves [6] of natural gas in the Middle East and Central Asia containing H2S, CO2, COS, and mercaptans (RSH) are being developed [4, 7] to meet the growing demand.

First, it is relevant to have a basic idea of what gas compositions might be expected in practice from contaminated fields. Although a wide range of gas compositions are possible, a typical composition containing all the aforementioned contaminants is: 1-4mol% H2S, 1-4mol% CO2, 10-50ppmv COS, and 50-400ppmv RSH.

While the removal of acid-gas components such as H2S and CO2 from natural gas have been well established [8] for decades, the removal of mercaptans and COS from natural gas in combination with H2S and CO2 are not as well established [9]. A number of different gas processing options are possible, and different approaches have been used in various locations [7-11] depending on the source, destination, and composition of the gas.

Amines remove H2S and CO2 from natural gas via chemical reaction in an acid-gas removal unit (AGRU). In the AGRU, impurities of H2S or CO2 are extracted from natural gas with an amine solution in a gas-liquid contactor such as a trayed column, and the corresponding amine salt or carbamate is formed with H2S or CO2. The loaded amine is then routed to a regenerator where H2S or CO2 is liberated from the solution at low pressure and high temperature. Unfortunately, the solubility of mercaptans in aqueous amine solutions is limited, and alternative approaches must be used for mercaptan removal.

One approach is to use a physical solvent for mercaptan removal (figure 1). Rather than using a separate gas-liquid contactor with a physical solvent, a physical-solvent component can be added to the amine unit to form a mixed solvent, such as Sulfinol. An advantage to this approach for mercaptan removal is that large amounts of mercaptans can easily be removed in a continuous and steady process. A disadvantage to this approach is that the physical solvent used for mercaptan removal also co-absorbs hydrocarbons from the natural gas, particularly those of higher molecular weight. Heavy hydrocarbons co-absorbed in the physical solvent leave the process at low pressure from the flash gas or the overhead of the regenerator. While the flash-gas could be re-compressed, the regenerator overheads are sent to the sulphur-recovery unit (SRU), and thus some hydrocarbons cannot be recovered.

Feed Gasfrom slugcatcher

3

Mol sieve unitH2O

Amine &Physical Solvent

Absorber

Sulphur-Recovery Unit

S

H2S + RSH

H2O

NGL-Extraction CH4

NGL

Amine &Physical Solvent

Regenerator

Figure 1. Block scheme for a gas processing plant with full RSH removal via a physical

solvent in the AGRU. The simplicity of scheme comes at the cost of hydrocarbon losses.

An effective solution for minimizing hydrocarbon losses when using a combined chemical/physical solvent in the AGRU is to remove only part of the mercaptans in the AGRU, thus mitigating hydrocarbon co-aborption (figure 2). For example, Sulfinol can be used to remove ~80% of the mercaptans, while 20% of the mercaptans are allowed to slip past the Sulfinol absorber. The balance of mercaptan and hydrocarbon absorption is affected by the fraction of physical solvent in the mixture, the solvent circulation rate and temperature, and the specific column design. The small amount of mercaptans not removed by the AGRU can be removed from the gas stream using a molecular sieve unit.

Molecular sieves effectively concentrate mercaptans from the main process stream into a stream of regeneration-gas. When natural gas is brought into contact with molecular sieves, components from the gas phase such as water and mercaptans adsorb on the internal surfaces of the sieve at low temperature, and can be desorbed from the sieve at high temperature. In practice, a set of 2 or more vessels is used, where each bed cycles through adsorption, heating, and cooling steps such that feed gas is continuously processed. A heated gas stream is used to regenerate a bed at the end of the adsorption step, and the desorbed mercaptans thus concentrate into the regeneration-gas stream, which is typically ~10% of the main gas stream. The bulk of the mercaptans are removed from the regeneration-gas stream using a

physical solvent, and the remaining gas, still containing a small concentration of mercaptans, is recycled to the front end of the mol sieve unit (figure 2).

Feed Gasfrom slugcatcher

4

Mol sieve unitH2O & RSH

Amine absorber

Regen-gasabsorber (RSH)

Sulphur-Recovery Unit

Commonregenerator

S

RSH

H2S + RSH

NGL-Extraction CH4

NGL

Figure 2. Block scheme for a gas processing plant with partial removal of RSH in the AGRU, followed by polishing with mol sieves. A common regenerator is used for the amine

originating from the main amine unit and the regeneration-gas treating unit.

Molecular sieves are particularly well suited for the removal of trace components. Mol sieves typically used for water or mercaptan removal (4A, 5A, 13X) have polar internal surfaces that interact favourably with polar molecules such as water or mercaptans, making it possible to adsorb very low concentrations of mercaptans from the gas phase [12]. In contrast, the uptake of mercaptans in a physical solvent is typically not as favourable [9] at low partial pressures. On the other hand, mol sieves are not well suited for removing high concentrations of contaminants due to the amount of regeneration gas required for large volumes of sieve.

When mercaptan concentrations are low, the combination of an aqueous amine and a mol sieve unit may be considered. When an aqueous amine solution (e.g. MDEA, optional accelerator, and water) is used in the AGRU, a limited fraction of mercaptans are removed from the gas phase, due to the limited solubility of mercaptans in aqueous solutions. The majority of mercaptans are subsequently removed using a mol sieve unit. In contrast to the Sulfinol option, where a single solvent can be used in both the main AGRU and the mol sieve regen-gas treating with a common regenerator [11], a separate regeneration-gas treating unit

is required for the regen-gas of the mol sieve unit when an aqueous amine is used in the AGRU.

Feed Gasfrom slugcatcher

5

Mol sieve unitH2O & RSH

NGL-Extraction CH4

Regen-gasabsorber

Sulphur-Recovery Unit

NGL

SRSH

H2SAmineRegeneratorAmine Absorber

Regen-gasabsorber

Figure 3. Block scheme for a gas processing plant with Removal of nearly all RSH in a mol

sieve unit downstream an AGRU with an aqueous amine. In summary, there are three basic options for the combined removal of mercaptans (RSH)

in combination with the use of amines for H2S and/or CO2 removal: 1. Full RSH removal via a physical solvent in the AGRU (figure 1) 2. Partial removal of RSH in the AGRU, followed by polishing with mol sieves (figure

2) 3. Removal of nearly all RSH in a mol sieve unit downstream an AGRU with an

aqueous amine (figure 3). The economics of these three options, which depend heavily on the gas composition, have

been considered at recent conferences [13, 14]. In this paper, the design and operational issues associated with mercaptan removal in a

molecular sieve unit are discussed. The complexities of mercaptan removal using mol sieves

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can affect the net present value (NPV) of an investment due to operational difficulties resulting in unplanned downtime, or a failure to meet design capacity for a certain period. For example, a single unplanned shutdown of 10 days to replace deactivated mol sieves could easily cost on the order of 10MM$ in lost production and reputation for meeting contractual cargos in the case of LNG production; this sum could have been used to make a more robust design for a unit that may have had a total installed cost of less than 40MM$. The Use of Molecular Sieves for Mercaptan Removal

Molecular sieves (mol sieves) with polar surfaces can be used for mercaptan removal down to very low concentrations because of the affinity of the mercaptan molecule for adsorption on the surface. Zeolites having the general formula Mx/n[(AlO2)x(SiO2)y]zH2O where n is the valence of the cation, and M is the metal ion inside each aluminosilicate cage [15-17], afford polar surfaces for mercaptan adsorption. The type of mercaptan that can fit into the micropores of the zeolite depends on the size of the cation in the zeolite structure. The 3A mol sieve is constructed using K cations, and has a nominal micropore aperture of 3Å, which is too small for mercaptan adsorption. Similarly, the 4A mol sieve is constructed with Na cations, and has a nominal micropore aperture of 4Å, which is also too small for mercaptan adsorption. The 5A mol sieve, with Ca cations, has a micropore aperture of 5Å, which is large enough to admit H2S, CH3SH, C2H5SH, and n-C3H7SH. Mol sieve 13X, with Na cations, has a pore diameter of about 7.4Å, allowing it to adsorb i-C3H7SH as well as the aforementioned mercaptans.

When mol sieves are used for mercaptan removal, water must be removed upstream of the mercaptan-removal bed. Water has a small effective molecular diameter compared with mercaptans, and water can adsorb on 3A, 4A, 5A, and 13X mol sieves. Furthermore, the affinity of the 5A and 13X mol sieves for water is greater than the affinity for mercaptans, because the polar interactions of the zeolite surface with water are stronger than the interactions with mercaptans. Thus, the removal of water and mercaptans on mol sieves is often combined in a single vessel, where the first layers of mol sieve remove water, and subsequent layers of mol sieve remove mercaptans. Often, 4A mol sieves are used for water removal because they have a high capacity for water adsorption; furthermore, 4A mol sieves are more resistant to hydrothermal deactivation as compared with 5A or 13X mol sieves. A Shell operated gas plant (NAM-GZI/Emmen) has worked successfully for the past 15 years using successive layers of 4A, 5A, and 13X mol sieves for the removal of water, light mercaptans, and heavy mercaptans, respectively [11]. Kinetics of Adsorption

The first difficulty with the removal of mercaptans using mol sieves is that the kinetics of mercaptan adsorption on mol sieves is generally slow. During operation, the static capacity is not the same as the dynamic capacity that results when gas flows past the adsorbent [15]. The adsorption process consists of a number of mass transport steps [18, 19] before mercaptans are finally bound within the mol sieve. As the concentration front of mercaptans moves through the bed, a gradient in concentration forms due to the kinetics of adsorption on the

mol sieve. In the saturation zone (SZ), the mol sieve is saturated with mercaptans, and in the mass transfer zone (MTZ), where there is a concentration gradient, the mol sieve is only partially saturated. This is shown in the diagram of figure 4. The non-utilized zone (NZ) is the part of the bed that has not come into contact with mercaptans yet because all mercaptans were adsorbed in the SZ and MTZ. If a stream containing mercaptans were allowed to flow through the bed for a long enough time, mercaptans would break through at the end of the bed. It is generally accepted [15, 16] that the capacity in the MTZ is roughly half of the capacity in the SZ, where the adsorption kinetics have reached equilibrium. Because the kinetics of mercaptan adsorption are slower than the kinetics of water adsorption, a relatively large amount of mol sieve is required for mercaptan removal as compared with water removal.

Saturated Zone (SZ)

Mass transfer orchemical reactionzone (MTZ/CZ)

Contaminant conc. in process stream

Directionof the flow

ProductSpec.

FeedConcentration

Non-utilised zone (NZ)

a) The saturated zone (SZ) wherein theadsorbent has achieved its maximumloading and the concentration of thecontaminant in the process stream does notchange.

b) The mass transfer or chemical reactionzone (MTZ/CZ) wherein the adsorbent isonly partially loaded and the contaminantlevel drops from its feed concentrationdown to the product level.

c) The non-utilised zone (NZ) wherein bothconcentration and adsorbent loadingchanges are negligible.

Figure 4. Schematic representation of the adsorption zones in an adsorber vessel.

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Co-Adsorption of Hydrocarbons, CO2, and COS (BTX separately) Mol sieves such as 5A and 13X typically used for mercaptan adsorption also have a

significant capacity for hydrocarbon, CO2, and COS adsorption. While the saturation-zone and part of the mass transfer zone are loaded with water and mercaptans, the non-utilized zone and part of the mass transfer zone (figure 4) become loaded with hydrocarbons, CO2, and COS if these are present in the feed gas. In particular, 5A mol sieves have a capacity of roughly 10 to 15wt% for linear hydrocarbon adsorption [20-23] and roughly 10 to 20wt% for CO2 adsorption at typical feed-gas concentrations.

Depending on the conditions of the mol sieve and the cycle time, a significant portion of the mol sieve can be available for hydrocarbon, CO2, and COS adsorption. Typically, the water capacity at end-of-run conditions is roughly ½ of the capacity available from fresh mol sieves. Using the same number of cycles for water and mercaptan adsorption, the mercaptan capacity is typically much less than ½ of the capacity available from fresh mol sieves. If a mol sieve unit is started with an adsorption time designed for end-of-run conditions, then roughly half the mol sieve volume is available for co-adsorption of hydrocarbons and CO2. During the heating step, the hydrocarbons, CO2, and COS desorb from the mol sieve, resulting in significant concentrations of these components in the regeneration gas. Transients Created by the Mol Sieve Unit

The mol sieve unit creates composition transients in both the regeneration gas and the effluent product due to the cyclical nature of the process. While one or more mol sieve bed(s) are in adsorption mode, other bed(s) are in regeneration mode. All the mercaptans that adsorb on the bed from the feed-gas stream are released into the regeneration-gas stream when the bed is heated during a short period of time. In most cases, the mercaptans must be removed from the regeneration-gas stream – this is often done using a physical solvent or an amine with a component of physical solvent such as Sulfinol. The peaks of mercaptans in the regen-gas translate into peaks in mercaptan concentration in the acid gas being fed to the sulphur-recovery unit (SRU). In addition, co-adsorbed hydrocarbons, CO2, and COS peaks in the regeneration gas are absorbed by the regen-gas absorber, resulting in peaks of these components in the sulphur recovery unit (SRU).

The rapid changes in the oxygen demand introduced in the SRU by varying concentrations of mercaptans can be difficult for the control scheme of the SRU to handle. In the SRU, H2S generated by the AGRU is burned with a sub-stoichiometric amount of oxygen to produce elemental sulphur via the Claus reaction [9], first in a homogeneous burner and then in subsequent catalytic stages. A tail-gas analyser detecting the amount of SO2 in the tail gas typically regulates the amount of oxygen fed to the SRU burner. Because of the residence time of gases in the SRU, the delay in control by the tail-gas analyser makes it difficult to compensate the amount of oxygen fed to the burner for changes in the feed-gas composition. This is particularly difficult when the feed-gas to the SRU contains mercaptans and hydrocarbons, which require several mols of oxygen for the complete combustion of one mol of mercaptan or hydrocarbon. In addition to the transients

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compositions of mercaptans and hydrocarbons originating from the mol sieve unit, the SRU must also contend with the transient concentration of CO2 resulting from co-adsorption in the mol sieve unit if CO2 is absorbed in the regen-gas treating unit.

The consequence of transient compositions and poor control in the SRU can be off-specification liquid sulphur. Depending on the type of hydrocarbons present, a certain temperature is required in the SRU burner to ensure complete combustion of hydrocarbons and avoid soot formation. The temperature in the burner depends strongly on how much fuel (H2S, hydrocarbons, mercaptans) and oxygen there is in the feed gas compared with the amount of diluents such as CO2 from the AGRU and mol sieve unit, or N2 from the air supplying oxygen. When the flame temperature becomes too low due to insufficient combustion, soot formation subsequently leads to “black” or off-spec liquid sulphur, which must be disposed of sometimes at a cost, because it cannot be easily sold. For a plant producing hundreds of tons of sulphur per day with only a few hours or days of sulphur storage capacity, the production of off-spec sulphur can become a critical issue in operation. Only a very small amount of soot can contaminate a large volume of bright yellow liquid sulphur. Again, it should be stressed that it is the changes in composition originating from the mol sieve unit that make control and operation of a stable flame in the SRU difficult. Clearly, there would be a benefit to smoothing the transients caused by the mol sieve unit before they are propagated to the SRU. While it would be impractical to smooth the concentrations of the regen-gas or acid-gas streams due to the volumes involved, it is possible to smooth the concentrations of mercaptans in the solvent circulation of the regen-gas treating unit. For example, if Sulfinol is used to remove mercaptans from the mol sieve regen gas, then the loaded solvent can be passed through a large stirred buffer tank before being fed to the regenerator, thus smoothing the concentration of mercaptans over time (see figure 5 for an example). Alternatively, a system of Sulfinol tanks can be used, where the solvent is fed to a one tank during the peak, and to another during the off-peak time, while a steady flow of solvent is drawn from both tanks to create an even concentration of mercaptans flowing to the regenerator. In a third alternative, a control scheme could be used for partial peak-shaving in order to minimize the size of the vessels.

Single stirred tank over several periodsflow = 5m3/min, volume in m3

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000

t [min]

Frac

tion

of in

let

conc

entr

atio

n 1003006009001200

Figure 5. The fractional outlet concentration, Cout/Cin, for a single stirred tank at a flow rate of

5m3/min (300m3/h) for volumes between 100m3 and 1200m3. Several cycles are shown. Larger tank sizes result in a smoother outlet concentration.

Simply optimising the cycle timing of the mol sieve unit can dampen transient

compositions of hydrocarbons, CO2, and COS in the mol sieve regeneration gas. At long adsorption times, more water and mercaptans are adsorbed on the mol sieves compared with short adsorption times; because water and mercaptans displace hydrocarbons, CO2, and COS, the co-adsorption of hydrocarbons, CO2, and COS is minimised at long adsorption times. Naturally, the adsorption time is limited by the breakthrough of mercaptans at the outlet of the bed. As the capacity of the mol sieve to adsorb water and mercaptans diminishes over a number of adsorption and regeneration cycles, the adsorption time must be shortened periodically.

An added advantage of variable cycle time is that mol sieve deactivation is minimized over calendar time. Deactivation of mol sieves used either for water or mercaptan adsorption occurs primarily during the regeneration step at high temperature. Minimizing the number of regeneration steps in a given calendar year thus minimizes the deactivation over that same period. Deactivation

An often-underestimated difficulty with the removal of mercaptans using mol sieves is the deactivation of the mol sieve. The deactivation of mol sieves used for water-removal is well known, and there is abundant operational experience to allow prediction of deactivation rates. In contrast, the removal of mercaptans using mol sieves is a relatively new field in the industry, and deactivation rates have not been well documented. When the mol sieve is heated, mercaptans desorb from the micropores of the zeolite crystals and begin to diffuse out

10

11

,

mol sieves that have been us

much

usual tw

of 3-4

m of the

ansfer zone, further affecting the eff

e of mol

ieve would be required for mercaptan adsorption at the end-of-run conditions.

on of the adsorbed me

ring

, thereby preventing the decomposition of mercaptans leading to coke

the sition of mercaptans by preventing the

formation of reaction intermediates.

through the micropores and macropores of the mol sieve pellet. During the diffusion processthere is an equilibrium between re-adsorption and desorption. When the temperature of the surface becomes high enough, the rate of dissociation of mercaptans on the surface becomes appreciable [24, 25]. Thus, over a number of cycles of adsorption and regeneration, the mol sieve becomes coked with decomposition products. Indeed, spent

ed for mercaptan-removal turn from a white to a black colour. Operational data from the Shell operated gas plant in Emmen, covering mol sieves of

several vendors, has shown that mol sieves used for mercaptan removal deactivate at afaster rate that that observed with mol sieve that is used for water-removal only [26]. Decomposition of mercaptans on the mol sieve at NAM-GZI in Emmen has resulted in a decrease in the capacity for CH3SH from 12 wt%S to 1wt%S at 25oC over the lifetime of 3 years. This twelve-fold decrease in mercaptan-removal capacity starkly contrasts the

o-fold decrease in water-removal capacity often observed over a similar lifetime. Designs of mol sieve units for mercaptan-removal need to account for the conditions at the

end of the mol sieve lifetime. A mol sieve unit is usually designed such that a single batchmol sieve can last for the period of time between planned shutdowns, which is often years. Thus, the unit must be designed with enough mol sieve volume to allow for a minimum regeneration time after going through 3-4 years worth of deactivation. Because the deactivation rate changes over time, operational data is essential to predicting the fordeactivation curve. Furthermore, the kinetics of adsorption change as the mol sieve deactivates, thus changing the characteristics of the mass tr

ective capacity of the mol sieve to adsorb mercaptans. Clearly, a process that retards the deactivation of mol sieves used for mercaptan removal

would be beneficial. For a given schedule of planned shutdowns, a slower deactivation ratewould translate directly to a smaller capital expenditure, because a smaller volums

After the adsorption step, where mercaptans from the feed gas adsorb on the mol sieve, heated gas is used to regenerate the mol sieve bed. In the traditional process, a slipstream of dry treated gas is heated in a furnace and passed through the bed in regeneration mode [11], causing mercaptans and water to desorb. During the heating step, a fracti

rcaptans decompose on the mol sieve [24, 25], causing deactivation. It has been found [26] that adding a small amount of water to the regeneration-gas stream

during the heating step mitigates the decomposition of mercaptans on the mol sieve duthe heating step. There are at least three possible mechanisms that achieve this result:

1. Water hinders the transient adsorption of mercaptans on the mol sieve at high temperatureformation.

2. Water co-adsorbs with mercaptans at low temperature during the beginning ofheating step, and hinders the decompo

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3. Water displaces adsorbed mercaptans at the beginning of the temperature ramp, preventing them from decomposing simply because they are not adsorbed on the sieve anymore by the time high temperatures are reached.

The implementation of the wet-regeneration scheme in an operating plant is simple. A

small amount of water needs to be added to the regeneration-gas stream at the start of the heating step, and subsequently the addition of water must be terminated at some point prior to the cooling step to avoid pre-loading the bed with water. The first possibility is to inject a small amount of steam into the regeneration gas before it enters the bed in heating mode. A second possibility is to use a slipstream of untreated gas, which is often saturated with water from the acid-gas removal unit. In either case, the valve for the addition of water can be controlled on the timing sequence of the mol sieve unit. Excessive addition of water to the regeneration gas can lead to hydrothermal deactivation of the mol sieve [26] via the mechanism of de-alumination [17, 27, 28].

The wet-regeneration process has been described in detail elsewhere [26], but is summarized here because of its relevance. A comparison of dry and wet regeneration has been made using cycles of adsorption and regeneration in an adsorption test unit (ATU). The ATU is a bench scale unit specially developed to screen mol sieves for its deactivation when operated with rapid adsorption/desorption cycles (Figure 6). Aging using wet regeneration was compared with dry regeneration for 100 cycles for i-C3H7SH adsorption on a 13X sieve. During the “wet” regeneration, 2000Pa of water was added to the regeneration stream. Clearly the sample exposed to the wet regeneration conditions (blue, upper curve) was not deactivated as quickly as the sample exposed to dry regeneration conditions (red, lower curve). The solid lines are best-fits of the function (%wt S) = Ae-Bn where A and B are fitting constants and n is the number of cycles; for wet regeneration A=7.6025 and B=-0.0011, whereas for dry regeneration, A=8.2 and B=-0.0085. Such an exponential decay in capacity with the number of cycles results from first order kinetics, where the rate of destruction of adsorption sites only depends on the number of active sites remaining [26].

0123456789

10

0 200 400 600 800 1000

Number of adsorption/regeneration cycles

i-C3H

7SH

loa

d [%

wt (

S)]

Difference in initial load is caused by the w ater capacity of the sieve at regeneration temperatureWet Regeneration

Dry Regeneration

Comparison of wet and dry regeneration

Pre-aged sieve

Figure 6. Isopropyl mercaptan loading capacity on 13X mol sieve as a function of the number of adsorption / regeneration cycles. Points are actual measurements in the laboratory, and the

lines are fitted exponential-decay equations.

Pre-aged mol sieve samples were used to check the extrapolation of the exponential-decay model to high numbers of regeneration cycles. Pre-aged samples, prepared by exposure to a constant stream of mercaptans at high temperature, were measured for an initial capacity. The effective number of regeneration cycles was then calculated for that initial capacity using the fitted curves in figure 6, and the initial capacities were plotted on the deactivation curves (figure 6, labelled “Pre-aged sieve”). The pre-aged samples were then exposed to cycles of adsorption and regeneration (wet or dry), and the capacity was periodically measured. The number of additional cycles completed was added to the initial effective number of regeneration cycles, and the capacity was plotted. Thus, the pre-aged sieve was used to check the extrapolation of the aging curves.

The wet-regeneration scheme has been applied in practice at the NAM-GZI gas processing plant in Emmen, The Netherlands, since September 2004. The plant in Emmen, operating since 1988, has the capacity to process 2*150 MMscfd [11] of natural gas (currently processing ~95 MMscfd), containing H2S, CO2, CH3SH, C2H5SH, n-C3H7SH, and i-C3H7SH. Sulfinol is used to remove H2S and the bulk of mercaptans while slipping CO2. Downstream of the Sulfinol unit, a mol sieve unit removes water and the remaining mercaptans from the gas before the treated gas is sent to the pipeline.

As shown in figure 7, there has been very little deactivation of the mol sieves with the use of the wet-regeneration scheme over time, as compared with normal dry regeneration. The

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exponential-decay aging curves for wet and dry regeneration were calculated as follows. The test of the wet-regeneration scheme began when the mol sieve had already deactivated from 800 cycles of adsorption and regeneration. At this point, the capacity of the mol sieve was 0.163wt% mercaptans, and the pre-exponential factor (A) in the model (%wt) = Ae-Bn was set to 0.163wt%. The exponential factor, B, was set to the value found in the laboratory for wet or dry regeneration, respectively. The data from Emmen were determined during capacity test-runs, where it was found that the dynamic capacity was roughly a linear function of mercaptan concentration in the feed gas. Thus, a linear fit to the data was used to calculate a best-fit for a single feed-gas concentration, at 15mg/Nm3. These points are plotted in figure 7.

As shown in figure 7, the test runs conducted using the wet-regeneration scheme show slower deactivation than that predicted by either the dry regeneration or the wet regeneration models. In fact, the capacity of the mol sieve appeared to be increasing slightly, although the increase was not considered statistically significant. In any case, it is clear that the wet-regeneration scheme mitigates the deactivation of mol sieves used for mercaptan removal in actual operation. The capacity of the mol sieves for mercaptan-removal is low due to the history of the beds, which have experienced many cycles of adsorption and regeneration, and the low concentration of mercaptans in the feed gas. The success of the wet-regeneration scheme in Emmen has allowed the operating company to postpone a shutdown for the change-out of the mol sieve material.

Since the first publication of this data in March 2005 [26], the wet regeneration scheme has continued to perform successfully, though changes in operating conditions preclude a straight forward comparison over a longer period than that shown in figure 7.

Mitigated deactivation at NAM-GZI Emmen

0.1

0.12

0.14

0.16

0.18

0.2

790 810 830 850 870 890 910

Number of cycles

Load

ing

[wt%

]

Emmen Data; fit for 15mg/Nm3 RSH

Prediction - wet regenPrediction - dry regen

Figure 7. Demonstration of the wet-regeneration scheme in operation at NAM-GZI Emmen. The wet-regeneration scheme was started when the mol sieve had already experienced 800

cycles of adsorption/regeneration. 14

15

Another solution for mitigating the deactivation of mol sieves is to use a gradual temperature ramp during regeneration. When the mol sieves are heated to a temperature below the temperature where mercaptans begin to react [24], mercaptan molecules desorbed from the internal surfaces of the sieve have a good chance of diffusing out of the mol sieve pellet before re-adsorption followed by reaction can occur. At sufficiently low temperature, mercaptan molecules in re-adsorption and de-sorption equilibrium with the internal surfaces of the sieve have a low probability of surmounting the activation energy barrier to form reaction products which can cause deactivation. H2S Formation during Regeneration

The mechanism of deactivation of mol sieves leads to another design and operational complexity: the formation of H2S in the regeneration gas from the mol sieve unit. The specification on H2S at the outlet of the AGRU is typically <5ppmv, while the concentration of mercaptans may be in the range of hundreds of ppmv. Despite this large difference, significant amounts of H2S may be observed in the regeneration gas relative to the total mercaptan concentration. The evolution of H2S along with mercaptans during regeneration can be illustrated with the following example from the laboratory.

After loading the mol sieve with n-C3H7SH until breakthrough was detected, a mol sieve sample was regenerated and the effluent gases were measured during regeneration, as shown in figure 8. Under these conditions, no H2S was adsorbed. During the early stages of regeneration, a sharp peak of H2S desorption was observed, followed by a roughly an exponential decay in H2S concentration. The sharp spike in H2S evolution suggests that H2S is formed from a reaction within a small temperature range, and the exponential decay may result from the diffusion of produced H2S out of the macropore structure of the mol sieve pellet.

The point at which H2S suddenly begins to form corresponds to a temperature of 200C. This is exactly the temperature at which differential scanning calorimetry (DSC) measurements have shown the onset of a steep exotherm in the usually endothermic desorption of mercaptans [24]. In other words, the observed evolution of H2S probably corresponds to a reaction on the surface. Possible reactions are [25, 29]:

3 R-SH => R-S-S-R + H2S + RH (with zeolite and heat) ½ O2 + 2 R-SH => R-S-S-R + H2O (with zeolite and O2)

R’-CH2-CH2-SH => R’-CH=CH2 + H2S (with zeolite)

The reaction mechanism converting mercaptans to the corresponding alkene and H2S is relevant for C2H5SH, n-C3H7SH, i-C3H7SH, and butyl mercaptans, but not CH3SH.

0100020003000400050006000700080009000

10000

15 20 25 30 35 40

Run time [hours]

Con

cent

ratio

n [p

pmv]

0

50

100

150

200

250

300

350

Tem

pera

ture

[°C

]

Desorption of n-C3H7SH from 5A mol sieves(in N2 at 40bara, 40- 290 °C)

Temperature

n-propyl RSH

H2S

Figure 8. The desorption of n-C3H7SH from 5A mol sieves after the adsorption of n-

C3H7SH from the feed gas of N2. The feed concentrations were 9075ppmv n-C3H7SH in a balance of N2 at 40bara and 40C during adsorption. The regeneration gas was dry nitrogen.

The evolution of H2S during regeneration of mol sieves used for mercaptan removal

suggests that the regeneration-gas treating unit should be capable of quantitatively removing H2S from the regeneration gas. A mixed solvent containing an amine and a physical solvent component, such as Sulfinol, is perfect for this task, combining both chemical H2S absorption and physical mercaptan absorption. The consequence of not removing H2S from the regen-gas is that regen-gas would need to be recycled to upstream the AGRU, thus resulting in a larger AGRU, in addition to the regen-gas treating unit. H2S Conversion to COS

Aside from deactivation, a further complexity in the design and operation of a mol sieve unit is the formation of COS from H2S and CO2. In general, an amine unit for H2S and/or CO2 removal precedes a mol sieve unit used for mercaptan removal, with a typical H2S specification of a few ppmv. The amount of CO2 in the feed gas to the mol sieve unit depends on the destination of the treated gas. However, the concentration of CO2 often exceeds the concentration of H2S in feed to the mol sieve unit.

COS is formed on the mol sieve by the following reaction: H2S + CO2 ⇔ COS + H2O

16

]][[]][[

22

2

COSHOHCOSKp =

Table I. The equilibrium constant for COS formation at various temperatures.

Temp (degrees C) Kp

20 1.38 x 10-6

100 3.16 x 10-5

200 3.64 x 10-4

300 1.8 x 10-3

400 5.4 x 10-2

As shown in the table above, the equilibrium lies far to the left at normal adsorption

temperatures (25C), and even at regeneration temperatures (300C) [30]. However, when the mol sieve removes water, the equilibrium is shifted to the right. The rate of COS formation is dependent on the type of mol sieve – i.e. the conditions of reactive site where COS is formed. It has been noted [30] that more basic sites contribute to a faster rate of COS formation. COS formation is relatively fast on the 4A sieve, which has mono-valent Na ions in the cavities. In contrast, COS formation is slower on the 5A sieve, because the di-valent Ca ions in the cavities reduce the effect of the basic sites. The disadvantage of using a 5A sieve for water removal is that it is somewhat more prone to deactivation compared with the 4A mol sieve [26].

Test runs by Zeochem [30] have shown that up to 100% of a 15ppmv H2S stream in natural gas with 1% CO2 can be converted to COS on a 4A sieve at 27oC and 49bara. Thus, as a conservative estimate, it can be considered that all H2S is converted to COS under adsorption conditions in the presence of CO2.

In addition to the COS formed on the mol sieve, some COS in the feed-gas may slip through the amine unit. The total COS level in the product gas from the mol sieve unit must be taken into consideration in the design of the NGL-extraction unit, because COS concentrates into the LPG phase, where it can be removed using amines and/or molecular sieves.

The following solutions can be considered when dealing with COS formation on mol sieves:

1. Some mol sieve vendors offer “COS-minimizing” molecular sieves, which presumably have a proprietary additive in the binder.

2. A 3A molecular sieve could be considered for water removal, because H2S adsorption is excluded from the 3-Ångstrom pore opening of the sieve. However, the 3A sieve is best regenerated at ~240C, whereas 5A and 13X molecular sieves used for mercaptan removal require temperatures above this for full regeneration.

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3. A 5A mol sieve can be used for combined water and mercaptan removal, with the compromise that faster deactivation of the 5A mol sieve can be expected when used for water removal.

18

4. COS formation from a small H2S concentration can be accepted, and the formed COS can be removed in the extracted LPG downstream of the mol sieve unit.

In any case, some degree of uncertainty must be accounted for in the COS concentration

expected downstream the molecular sieve unit – only a few ppmv of uncertainty in COS concentrations can make a significant difference in the basis of design for the COS-removal unit in the LPG treating section. BTX Co-Adsorption

A final complexity in the design and operation of a mol sieve unit for mercaptan removal is the co-adsorption of BTX. The affinity of 13X mol sieves for BTX (benzene, toluene, and xylenes) is slightly greater than their affinity for mercaptans [29]. An amount of BTX in the feed gas roughly equal to the amount of mercaptans in the feed gas severely hinders mercaptan adsorption. Furthermore, a high concentration of BTX in the feed gas relative to the concentration of mercaptans results in almost negligible adsorption of mercaptans on the 13X mol sieve (shown by example below). In contrast to 13X sieves, the adsorption of BTX is not an issue for 5A mol sieves, because BTX molecules are too large to fit into the 5A zeolite structure.

An example of the co-adsorption of toluene and i-C3H7SH measured in the laboratory is shown in figure 9. In this experiment, a micro-flow reactor was filled with 13X mol sieve and heated to 300C to regenerate the mol sieve. A flow of nitrogen at 5bara and 40C containing 99ppmv i-C3H7SH and 1842ppmv toluene was then fed to the reactor, and the concentrations of i-C3H7SH and toluene were measured using a gas chromatograph (GC) at the outlet of the reactor. From the period of 0 hours to 10 hours, both toluene and i-C3H7SH adsorbed on the mol sieve, and thus no toluene or i-C3H7SH was detected at the outlet of the reactor. At 10 hours, both toluene and i-C3H7SH began to break through at the outlet of the reactor. As the concentration of toluene rose to a value equal to the feed concentration, the concentration of i-C3H7SH rose far above the concentration of 99ppmv in the feed gas. Finally, the concentration of i-C3H7SH returned to a value equal to the concentration of i-C3H7SH in the feed gas.

The breakthrough experiment shown in figure 9 suggests that toluene displaces i-C3H7SH adsorbed on the 13X mol sieve. During the initial adsorption period (0-10h), both toluene and i-C3H7SH adsorbed on the mol sieve. However, when both components began to break through, toluene began to displace i-C3H7SH from the mol sieve, causing the concentration of i-C3H7SH in the effluent to rise above the inlet concentration. Only when nearly all the i-C3H7SH had been displaced by toluene did the toluene concentration reach a value equal to the concentration of toluene in the feed gas.

Adsorption of iso-propyl mercaptan and toluene on 13X mol sieve (pressure = 5 Bara, Temperature 40 °C)

0

500

1000

1500

2000

0.0 5.0 10.0 15.0

Adsorption time [hour]

conc

entr

atio

n [p

pmv]

isopropyl RSHtoluene

Feed concentration i-C3H7SH = 99ppmv

Feed concentration toluene = 1842ppmv

Figure 9. The concentration profiles of i-C3H7SH and toluene measured at the outlet of the reactor during a dynamic adsorption experiment. The feed concentrations were 99ppmv i-

C3H7SH and 1842ppmv toluene in a balance of nitrogen at 5bara and 40C.

One solution for dealing with the combined removal of BTX and mercaptans is to use Sulfinol in the main amine unit, with only 5A sieves in the mol sieve unit. The solubility of BTX is generally greater in Sulfinol than in aqueous amines, resulting in some BTX removal in the amine unit upstream the mol sieve unit. Furthermore, Sulfinol removes the heavy branched mercaptans that generally require a 13X sieve for removal in the mol sieve unit more easily that it removes light mercaptans such as methyl or ethyl mercaptan. Thus, when using Sulfinol, it may be possible to use a 5A mol sieve for mercaptan removal without a 13X mol sieve, thus avoiding the problem of BTX co-adsorption altogether.

A second solution for dealing with the combined removal of BTX and mercaptans is to use Sulfinol or an aqueous amine in the main amine unit, with both 5A and 13X sieves in the mol sieve unit, accounting for some BTX co-adsorption. In this case, BTX co-adsorption in the mol sieve unit would be greater when using an aqueous amine as compared with Sulfinol due to relative solubilities. BTX desorbed during the regeneration step may be absorbed by the physical solvent in the regeneration-gas treating unit and subsequently routed to the SRU. BTX destruction in the SRU requires high flame temperatures and accurate control in order to avoid black sulphur formation – if possible, it is desirable to avoid BTX in the SRU. If BTX is not removed in the regen-gas treating unit, there is a danger of building up BTX in the mol sieve regeneration gas loop, thereby displacing mercaptans from the 13X mol sieve.

A third solution for dealing with the combined removal of BTX and mercaptans is to use an aqueous amine or Sulfinol with mercaptan slip, and use only a 5A mol sieve for mercaptan removal. In this case, branched mercaptans slip past the mol sieve unit and report to the LPG and/or condensate streams from the NGL extraction unit, where they must be removed.

19

20

Advantages of Integrating Regen-Gas and Acid-Gas Treating Units

Using the same solvent for both the main acid-gas removal unit (AGRU) and the mol sieve regeneration-gas treating has the advantage that only two separate absorbers are necessary rather than two separate solvent systems. The regenerator, tanks, pumps, and perhaps even flash vessels can be shared between the two units. It may be more effective to use a separate absorber for the mol sieve regen-gas and main AGRU rather than to recycle the mercaptan-laden regen-gas to the front end of the AGRU, because solvent rates and tray layouts for optimal mercaptan-removal may be different from the optimal tray layout and solvent rates in the AGRU.

CONCLUSIONS

Molecular sieves are particularly well suited for the trace removal of contaminants from natural gas, such as water and mercaptans. Depending on the mercaptan concentration, it can also be effective to use a physical solvent for mercaptan removal, although this comes with the trade-off of hydrocarbon co-absorption.

Complexities in the design and operation of a mol sieve unit for mercaptan removal include the following.

1. Co-adsorption of hydrocarbons, COS, and CO2 can cause unexpected transients in concentrations of these components in the regeneration gas. Optimising the cycle time of the mol sieve unit can help to mitigate co-adsorption.

2. The transient nature of the mol sieve unit creates transients in the regen-gas treating unit and sulphur-recovery unit, which cause difficulties in the control and operation of these units. Transients in composition may also be caused downstream the mol sieve unit. Transients can be mitigated by controlling the mol sieve cycle time and using a peak-shaving scheme in the regen-gas treating unit.

3. Mol sieves deactivate rapidly when used for mercaptan removal, as compared with dehydration. Various approaches have been proposed to mitigate deactivation, including temperature ramps and the wet-regeneration scheme.

4. During the regeneration step, mercaptans decompose to form H2S, which subsequently must be removed. The production of H2S during regeneration can make a combined amine and physical solvent an ideal choice for regeneration-gas treating.

5. H2S slip from the AGRU is converted to COS in the presence of CO2 on the mol sieve, when the mol sieve removes water. COS can subsequently be removed from extracted natural gas liquids, or COS-minimizing mol sieves can be used.

6. BTX (benzene, toluene, xylene) is adsorbed more strongly than mercaptans on 13X mol sieves. Physical solvents can be used to remove BTX upstream the mol sieve unit. Alternatively, a 5A sieve can be used for linear mercaptan removal, because BTX does not fit in the 5Å pore opening.

21

Optimal gas treating solutions depend on the gas composition and economics [13, 14], but also on the operability of the unit. The complexities involved in the design and operation of a mol sieve unit used for mercaptan removal presented herein suggest that potential lost production or difficulty in meeting design capacity needs to be carefully considered when choosing the optimal process line-up.

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