Journal of Natural Gas Chemistry 20(2011)471–476
Comparison of three methods for natural gas dehydration ∗
Michal Netusil ,
Pave Pavell Ditl Ditl
Department of Process Engineering, Czech Technical University, Prague 6, 166 07, Czech Republic
[ Manuscript received April 6, 2011; revised May 23, 2011 ]
Abstract This paper compares three methods for natural gas dehydration that are widely applied applied in industry: (1) absorption by triethylene glycol, (2) adsorptio adsorption n on solid desiccants desiccants and (3) condensat condensation. ion. A comparison comparison is made according according to their energy demand demand and suitabilit suitability y for use. The energy calculations are performed on a model where 105 Nm3 /h water saturated natural gas is processed at 30 C. The pressure pressure of the the gas − 10 C at gas varies from 7 to 20 MPa. The required outlet concentration of water in natural gas is equivalent to the dew point temperature of − pressure of 4 MPa. ◦
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Key words gas reservoir; underground gas storage; natural gas; gas dehydration
1. Introduction
The theme theme of natural natural gas (NG) dehydr dehydratio ation n is closel closely y conconnected with the storage of NG. There are two basic reasons why storingNG storingNG is an interes interestingidea. tingidea. First, First, it can decrea decrease se the dependency dependency on supply. supply. Second, Second, it can exploit the maximum capacity of distribution lines. NG is stored in summer periods when there is lower demand for it, and is withdrawn withdrawn in winter periods when significant amount s of NG are ar e used for heating. Underground Underground Gas Storages Storages (UGSs) are the most most advantageou advantageouss option for storing large volumes volumes of gas. Nowadays Nowadays there are approximately approximately 130 UGSs inside the European Union. Union. Their total maximum technical storage capacity is around 95 bcm. According According to the latest update, over 70 bcm of additional storstorage capacity will come on stream in Europe till 2020 [1]. There are three types of UGSs: (1) aquifers, (2) depleted oil/gas oil/gas fields fields and (3) salt cavern cavern reservoirs reservoirs.. Each Each of these types possesses possesses its own physical physical characteristics. characteristics. Generally Generally,, the allowable pressure of stored gas inside a UGS is up to 20 MPa. MPa. The inside pressure pressure increases increases as the gas is injected injected and decrease decreasess when there is a withdra withdrawal wal.. The output output gas pressure depends on further pipeline distribution. Distribution sites sites normally normally begin begin at 7 MPa. MPa. The tempera temperature ture of the gas usually ranges from 20−35 C. The exact temperature temperature varies varies with the location of UGS and with the time of year. A disadvantage of UGSs is that the gas becomes saturated by water vapors vapors during during the storage. storage. In the case case of depleted depleted oil field field ◦
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UGSs, vapors of higher hydrocarbons also contaminate the stored gas. The distribution distribution specification specification sets the allowable allowable water concentration in NG by specifying a dew point temperature (T dew) of NG. T dew is usually usually taken to be −7 C for NG at 4 MPa [2]. This value value is equivalent equivalent to roughly 5 gH2 O /m3 NG at 4 MPa. The water content in NG at saturation is dependent on the temperature and pressure within the UGS. This is well well presente presented d in Figure Figure No.20, No.20, Chapte Chapterr 20, in the GPSA GPSA Data Book (12th Edition). The average average value of H 2 O in NG is five times higher than that of required. A dehydration step of NG from UGS is therefore essential before the gas is distributed. This paper compares industrially applied dehydration methods according to their energy demand and suitability for use. ◦
2. Dehydration methods
2.1. Absorption First dehydration method is absorption of H 2 O. Absorption is usually performed using triethylene glycol (TEG). Absorption proceeds in a glycol contactor (a tray column or packet bed) with countercurrent flows of wet NG and TEG. During the contact, contact, TEG is enriched by H 2O and flows out of the bottom bottom part of the contact contactor or.. Enriche Enriched d TEG then then contin continues ues flowinginto flowinginto the interna internall heat heat exchan exchanger ger,, which which is incorpo incorporate rated d at the top of the still column. It then flows into the flash drum,
Corresponding author. Tel: +420-2243522714; Fax: +420-224310292; E-mail:
[email protected] This work was supported by the Inovation and Optimalization of Technologies for Natural Gas Dehydration (No. FR-TI1/173).
Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60218-6
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where the flash gases are released and separated from the stream. TEG then runs to the cold side of the TEG/TEG heat exchanger. Just afterwards, warmed TEG is filtered and sprayed into the still column. From there, TEG runs into the reboiler. In the reboiler, H2 O is boiled out of TEG. The inside
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temperature should not exceed 208 C based on the decomposition temperature of TEG. Regenerated TEG is then pumped back through the hot side of the TEG/TEG and NG/TEG heat exchanger into the top of the contactor. The entire process is depicted in Figure 1 [3].
Figure 1. Scheme of absorption dehydration
The purity of the regenerated TEG and the circulation rate (LTEG /kgH2 O ) limit the obtained output T dew of NG. Gas stripping can be implemented to enhance TEG regeneration. Proprietary design DRIZO , licensed by Poser-NAT, COLDFINGER and Gas Conditioners International, have been patented as an alternative to traditional stripping gas units. The Drizo regeneration system utilizes a recoverable solvent as the stripping medium. The patent operates with isooctane solvent, but the typical composition is about 60%
aromatic hydrocarbons, 30% naphthenes and 10% paraffins. Water separator of the three-phase solvent is crucial for this method. The Coldfinger regeneration system employs a cooling coil (the “coldfinger”) in the vapor space of the surge tank. The cooling taking place there causes the condensation of a huge amount of vapors. The condensate is a water rich TEG mixture, which leads to a further separation process [4]. Enhanced regeneration systems are depicted in Figure 2.
Figure 2. Scheme of enhanced TEG regeneration systems
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2.2. Adsorption The second dehydration method is adsorption of H2 O by a solid desiccant. In this method, H2 O is usually adsorbed on a molecular sieve, silica gel or alumina. A comparison of the physical properties of each desiccant is shown in Table 1 [5,6]. Table 1. Comparison of the physical properties of desiccants used for NG dehydration
Properties
Silica gel
Alumina
Molecular sieve
2
750−830
210
650 −800
3
Pore volume (cm /g)
0.4−0.45
0.21
0.27
˚ Pore diameter (A)
22
26
4 −5
Design capacity
7−9
4−7
9−12
721
800 −880
690−720
920
240
200
Regeneration temperature ( C)
230
240
290
Heat of desorption (J)
3256
4183
3718
Specific area (m /g)
(kgH2 O /100 kgdesiccant) Density (kg/m3 ) Heat capacity (J·kg
1
−
K 1 )
·
−
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Source: Tagliabue (2009), Gandhidasan (2001)
The amount of adsorbed H2 O molecules increases with the gas pressure and decreases with its temperature, which are taken into account when the process parameters are designed. Adsorption dehydration columns always work periodically. Minima of two bed systems are used. Typically, one bed dries the gas while the other is being regenerated. Regeneration is performed by preheated gas, as depicted in Figure 3.
specified by time borders A, B, C and D with appropriate border temperatures T A , T B , T C and T D . Regeneration starts at point A. The inlet regeneration gas warms the column and the adsorbent. Around a temperature of 120 C (T B ), the sorbed humidity starts to evaporate from the pores. The adsorbent continues warming more slowly, because a considerable part of the heat is consumed by water evaporation. From point C, around the temperature of 140 C ( T C ), it can be assumed that all water has been desorbed. Adsorbent is further heated to desorb C5+ and other contaminants till point D. The regeneration is completed when the outlet temperature of the regeneration gas reaches 180–190 C ( T D ). Finally, cooling proceeds from point D to E. The temperature of the cooling gas should not decrease below 50 C, in order to prevent any water condensation from the cooling gas [7]. Part of the processed NG is sometimes used as the regeneration gas. Then it is cooled, and water condensed when it is separated. After H2 O separation, the regeneration gas is added back into the processed stream. So-called LBTSA (Layered Bed Temperature-Swing Adsorption) processes are an upgrade of TSA method. Here, the adsorption column is composed of several layers of different adsorbents. Hence, the properties of the separate adsorbents are combined in one column. For example, a combination of silica gel with alumina is used in NG dehydration. Alumina has better resistance to liquid water, so it is put in the first place to contact the wet NG. This ordering prolongs the lifetime of the silica gel, which is placed below the alumina layer. ◦
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2.3. Condensation
Figure 3. Scheme of the temperature swing adsorption dehydration process
The heater for TSA can be realized as an ordinary burner or as a shell and tube heat exchanger warmed by steam or hot oil. The regeneration gas flows through the adsorbent into a cooler (usually using cold air) and then further into the separator. Most of the desorbed humidity from the adsorbent is removed there. A downstream flow of wet NG through the adsorption column is usually applied. In this way, floating and channeling of an adsorbent is avoided. The regeneration is performed by countercurrent flow in order to provide complete regeneration from the bottom of the column, where the last contact of the dried NG with the adsorbent proceeds. The typical temperature course for regeneration of molecular sieves is presented by Kumar (1987) [7]. The shape of the curve representing the course of the outlet regeneration gas temperature is typically composed of four regions. They are
The third dehydrationmethod employs gas cooling to turn H2 O molecules into the liquid phase and then removes them from the stream. Natural gas liquids (NGLs) and condensed higher hydrocarbons can also be recovered from NG by cooling. The condensation method is therefore usually applied for simultaneous dehydration and NGL recovery. NG can be advantageously cooled using the Joule-Thompson effect (JT effect). The JT effect describes how the temperature of a gas changes with pressure adjustment. For NG, owing to expansion, the average distance between its molecules increases, leading to an increase in their potential energy (Van der Waals forces). During expansion, there is no heat exchange with the environment or work creation. Therefore, according to the conservation law, the increase in potential energy leads to a decrease in kinetic energy and thus a temperature decrease of NG. However, there is another phenomenon connected with the cooling of wet NG. Attention should be paid to the formation of methane hydrate. Methane hydrate is a solid in which a large amount of methaneis trapped within the crystal structure of water, forming a solid similar to ice. The hydrate production from a unit amount of water is higher than the ice formation. Hydrates formed by cooling may plug the flow. This is usually prevented by injecting methanol or monoethylenglycol (MEG) hydrate inhibitors before each cooling. Figure 4 depicts a dehydration method utilizing the JT effect and hydrate inhibition.
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The wet NG is throttled in two steps inside the flash tanks. The lower temperature (due to the JT effect) of the gas stream in the flash tanks leads to partial condensation of H2 O va pors. The droplets created are removed from the gas stream by a demister inside the flashes. In cases when cooling by the JT effect is insufficient (the usable pressure difference between the UGS and the distribution network is insufficient), the air precooler and the external cooler are turned on. Since dehydration is normally applied to large volume of NG, the external coolers need to have good performance, so this type of cooling is very energy expensive. However, if the usable pressure difference is large, the JT effect inside the flashes is so strong that internal heating of the flashes is required to
defreeze any ice that may form. A condensation method is ap plied when suitable conditions for the JT effect are available. Each of the methods presented here has its advantages and disadvantages. Absorption by TEG is nowadays the most widely used method. Outlet T dew around −10 C is usually reached. Indeed, with improved reboiler design (Vacuum stripping, Drizo, Coldfinger), the outlet T dew is even 2−3 times lower. However, TEG has a problem of sulfur or gas contaminated with higher hydrocarbons. The TEG in the re boiler foams, and with time it degrades into a “black mud”. BTEX emissions (the acronym for benzene, toluene, ethyl benzene and xylenes) in the reboiler vent are a further disadvantage. ◦
Figure 4. Scheme of dehydration method utilizing the JT effect and hydrate inhibition
Adsorption dehydration can obtain very low outlet water concentration of T dew<−50 C, and contaminated gases are not a problem. Even corrosion of the equipment proceeds at a slow rate. However, adsorption requires high capital investment and has high space requirements. The adsorption process runs with at least two columns (some lines use three, four or as many as six). Adsorption columns are taller and heavier than absorption contactors. The allowed flow velocity for TEG contactors is approximately three times higher than that for adsorption. This results in an approximately 70% larger diameter of the adsorption column for the same amount of processed gas. Industrial experience indicates that the capital cost for an adsorption line is 2−3 times higher than that for an absorption one [8]. Expansion dehydration is the most suitable method in cases when a high pressure difference is available between UGS and the distribution connection. However the difference decreases during the withdrawal period and becomes insufficient, so that an external cooling cycle is needed. A cycle for hydrate inhibitor regeneration from the condensate separated inside the flashes is also required. ◦
3. Experimental
The energy demand for the methods presented here was compared on the basis of a model, where a volume of 105 Nm3 /h NG from UGS was processed. The NG was H 2 O saturated at 30 C. The pressure of the gas was varied from ◦
7 to 20 MPa, but in the case of the condensation method the pressure range started at 10 MPa. The required outlet concentration of H2 O in NG was equivalent to the dew point temperature of −10 C at gas pressure of 4 MPa. The calculation of TEG absorption was based on GPSA (2004) [9]. The results were compared with the paper by Gandhidasan (2003) [8] and with industrial data provided by ATEKO a.s. The total energy demand was composed of heat for TEG regeneration in the reboiler, energy for the pumps, filtration and after-cooling the lean TEG before entering the contactor. Enhanced regeneration was not considered. The basic parameters for the calculation were: regeneration tem perature of 200 C, concentration of lean TEG of 98.5% and circulation ratio of 35 LTEG /kgH2 O . For calculating adsorption dehydration, molecular sieve 5A was considered to be the most suitable adsorbent. The total energy demand was directly connected to the regeneration gas heater, and no other consumption was assumed. The calculations were again based on GPSA (2004). The results were compared with the papers reported by Gandhidasan (2001) and Kumar (1987) [7]. The calculation procedure for GPSA and Gandhidasan arose from the summation of the particular heats, i.e. the heat for adsorbent warming, the heat for column warming and the heat for water desorption. Kumar’s calculation procedure run differently. The regeneration step was divided into four regions reproted in Ref. [7]. Afterwards we determined what individual phenomena proceeded in each region, what the border and average temperatures were, and how much energy was required to cover these phenomena. Finally, ◦
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the demands for each region were added. The basic parameters for all procedures were: temperature of the regeneration gas of 300 C, time of adsorption/regenerationof 12 h and two column designs. The condensation method was calculated on the basis of industrial data provided by TEBODIN s.r.o. and sup plementary calculations of the JT effect. The key parameter influencing energy demand was the pressure of NG from UGS. Because it was not feasible to apply this method under low pressures, and the provided data start at 10 MPa, the pressure range was adjusted. The total energy demand consisted of the air pre-cooling unit, the external cooling, the pumps for MEG injection and condensate off take, the heat for MEG regeneration, and flash heating.
Under high pressures of NG ( >16 MPa), the energy demand of the condensation method was at its lowest, and it remained nearly constant with an average value around 36 kW. The courses of the energy demand for the adsorption and absorption methods were quite similar: with increasing pressure of dehydrated NG, the energy demand slowly decreased. The absorption method was less demanding on the whole pressure scale, and began with consumption of 120 kW at 7 MPa. The adsorption method started with 234 kW at 7 MPa, but the energy demand decreased slightly more as the pressure of NG in UGS was risen. This led to a gradual decrease in the difference between these two methods, and the energy demand at the final pressure values of 20 MPa were equal to 54 kW for absorption and 103 kW for adsorption.
4. Results
5. Discussion
The results obtained by TEG absorption method were the same for each of the calculation procedures, and good agreement with industrial data was also obtained. However, the calculation procedures by the adsorption method led to different results. Hence, the average energy demand value was taken as the reference. The maximum deviation from it was below 20% for all the calculation procedures. The source of the deviation lies in the “loss factor and the non-steady state factor”. In the case of the condensation method, the calculated values for the JT effect were in good agreement with the industrial data, but the amount of data was limited, resulting in limited representation of the condensation method. The final energy consumption results for each dehydration method are summarized and shown in Figure 5.
By far the highest energy demand of the condensation method at low pressures of NG from UGS is due to the pressure being close to the distribution pressure, so that pressure cannot be used for the JT effect in flashes. Cooling is then compensated by the air pre-cooler and the external cooling device, which are unsuitable for large volume of processed NG. However, as the pressure difference between UGS and the distribution site increases, the space for expansion rises and the JT effect proceeds with increasing impact. This is projected into a linear decrease in the energy demand of the air precooler and the external cooling device. From the point where there is a pressure of NG >14 MPa, flash heating is gradually turned on to prevent any freezing caused by the strong JT effect. The energy demand of flash heating is reflected in the total energy consumption. Finally, at pressures of NG>16 MPa, total cooling and subsequent condensation are achieved by the JT effect. The total energy demand, which consists of flash heating and inhibitor injection and regeneration, remains constant. For adsorption and absorption dehydration methods, the similar falling courses of the energy demand with increasing pressure of NG can be explained by the fact that with increasing pressure within a UDG, the amount of H2 O present in the NG decreases. Generally, the absorption method consumes less energy, because the regeneration of TEG is less demanding than adsorbent regeneration. The composition of the total energy demand of the adsorption method can be divided into three parts. The heat for H2 O desorption, warming the adsorbent and the column is approximately 55%, 31% and 14%, respectively. It also has to be assumed that just part of the heat in the regeneration gas transfers to the adsorbent, the column and heat loss leave to the atmosphere, and the balance leaves with the hot gas. In brief, from the viewpoint of energy demand, the most appropriate dehydration method in cases of high pressures is the stored NG condensation method. This holds for NG from UGS with pressure >15 MPa and distribution pressure requirement of 7 MPa. Under lower pressures, the condensation method is used if the objective is to recover NGL and remove
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Figure 5. Final results of energy consumption for each dehydration method
Under low pressures (pressure of NG from UGS <13 MPa), the condensation method was the most demanding one. Its demand decreased linearly with pressure to a value of 145 kW for 13 MPa. At this point, the energy demand for the condensation method was roughly the same as that for the adsorption method. When NG pressure was further increased from 13 MPa to 16 MPa, the energy demand for the condensation method still decreased, but with a lowering tendency.
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water simultaneously. However, this is usually not the case when storing NG in a UGS. In cases when insufficient pressure difference is available, the absorption method is therefore favored over the adsorption method in terms of energy demand. TEG absorption is nearly twice less demanding. However, if a gas contaminated with sulfur or higher hydrocarbons is being processed, TEG in the reboiler foams and degrades with time. This occurs when a depleted oil field is used as a UGS. Adsorption is preferred in cases when very low T dew (H2 O concentration lower than 1 ppm) of NG is required, for example, when NG is liquefied. Abbreviations NG NGL UGS bcm T dew
TEG MEG T SA
LBTSA JT effect
natural gas natural gas liquid underground gas storage billion cubic meter dew point temperature triethylene glycol monoethylenglycol temperature swing adsorption layered bed temperature swing adsorption Joule-Thompson effect
BTEX
benzene, toluene, ethylbenzene and xylenes
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