Dehydration of natural gas using solid desiccants

Transcription

1 Energy 26 (2001) Dehydration of natural gas using solid desiccants P. Gandhidasan *, Abdulghani A. Al-Farayedhi, Ali A. Al-Mubarak Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 28 April 2000 Abstract Natural gas is an important source of primary energy that, under normal production conditions, is saturated with water vapor. Water vapor increases natural gases corrosivity, especially when acid gases are present. Several methods can be used to dry natural gas and, in this paper, a solid desiccant dehydrator using silica gel is considered due to its ability to provide extremely low dew points. The design analysis of a two-tower, silica gel dehydration unit to dry one million standard m 3 of natural gas per day is presented in this paper and the effects of various operating parameters on the design of the unit are discussed. The study also covers the analysis of energy requirements for the regeneration of the weak desiccant bed based on some simplified assumptions and it is found that the higher the regeneration temperature, the smaller are the required quantities of regeneration gas Elsevier Science Ltd. All rights reserved. 1. Introduction Saudi Arabia s natural gas reserves were estimated at 5.8 trillion m 3 in January Most known reserves are in the form of associated gas contained in the country s oilfields. There has been a substantial increase in natural gas production since The importance of natural gas to the petrochemical industry is revealed by the master gas system. The three existing gasprocessing plants have the capacity to process million m 3 ofrawgasperday.afourth gas processing plant is now being built and is scheduled to come on stream in mid-2001 [1]. Natural gas contains many contaminants, of which the most common undesirable impurity is water. Most natural gas will be near water saturation at the temperature and pressure of production. Dehydration of natural gas, which is a critical component of the natural gas conditioning process, is the removal of the water that is associated with natural gases in vapor form. Removal of water from the gas stream reduces the potential for corrosion, hydrate formation, * Corresponding author. Tel.: ; fax: address: pgandhi@kfupm.edu.sa (P. Gandhidasan) /01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 856 P. Gandhidasan et al. / Energy 26 (2001) Nomenclature a Additional vessel height for desiccant supports and distributor, m c Drying cycle, h C Specific heat, kj/kg K D Diameter, m E Joint efficiency, % F Loss factor for unsteady state heating, dimensionless G Regeneration gas volume, m 3 h Enthalpy of regeneration gas, kj/kg H Total heat required for regeneration, kj L Length, m m Mass, kg M Total water adsorbed, kg p Pressure, kpa P Pressure, MPa Q Gas flow rate, 10 6 std m 3 /day S Allowable stress, MPa t Temperature, C T Temperature, K u Useful design capacity of the desiccant, mass fraction v Superficial gas velocity, m/min V Volume, m 3 w Wall thickness, m W Water content of gas, kg/10 6 std m 3 Y Heat of water desorption, kj/kg Z Compressibility factor, dimensionless Greek symbols T Temperature difference between the bed and gas, K r Density, kg/m 3 Subscripts B Bed D Desiccant G Gas H Heater V Vessel 1 Inlet to the heater 2 Exit from the heater

3 P. Gandhidasan et al. / Energy 26 (2001) and freezing in the pipeline. It stops sluggish flow conditions that may be caused by condensation of water vapor in natural gas. Water is also removed to meet a water dew point requirement of a sales gas contract specification range from 32.8 to 117 kg/10 6 std m 3 [2]. Hence, the dehydration of natural gas is mandatory. The primary objective of this study is to establish a design procedure for a solid desiccant dehydrator suitable for natural gas using silica gel as the desiccant. There are different techniques employed for dehydrating natural gas, but only two types of dehydration equipment are in current use: they are absorption by liquid desiccants and adsorption by solid desiccants. The unit is called a liquid desiccant dehydrator and a solid desiccant dehydrator respectively. 2. Liquid desiccant dehydrator Water vapor may be removed from natural gas by bubbling the gas counter currently through certain liquids that have an affinity for water. This operation is called absorption. There are numbers of liquids that can be used to absorb water from natural gases such as calcium chloride, lithium chloride, and glycols. The glycols have proved to be the most effective liquid desiccants in current use since they have high hygroscopicity, low vapor pressure, high boiling points and low solubility in and of natural gas. The four types of glycol that have been successfully used to dehydrate natural gas are ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (T 4 EG) [3]. TEG has gained nearly universal acceptance as the most cost effective of the glycols due to superior dew point depression, operating cost, and operation reliability. However, there are several operating problems with glycol dehydrators. Suspended foreign matter, such as dirt, scale and iron oxide, may contaminate glycol solutions. Also, overheating of the solutions may produce both low and high boiling decomposition products. The resultant sludge may collect on heating surfaces, causing some loss in efficiency or, in severe cases, complete flow stoppage. Liquids (e.g. water, light hydrocarbons) in inlet gas may require installation of an efficient separator ahead of the absorber. Foaming of solution may occur with resultant carry-over of liquid [3]. Some leakage around packing glands of pumps may be permitted since excessive tightening of packing may result in scoring of rods. This leakage may be collected and periodically returned to system. Highly concentrated glycol solutions tend to become viscous at low temperatures and, therefore, are hard to pump. Glycol lines may solidify completely at low temperatures when the plant is not operating. Further, there are substantial environmental problems due to fugitive emissions, soil contamination, and fluid disposal problems. Efforts are currently underway to determine the extent of emissions associated with the process and develop ways of reducing these emissions [4]. Hence, solid desiccant dehydrator is considered in the present study. 3. Solid desiccant dehydrator A large number of solid materials take up water vapor from gases; some by actual chemical reaction, others through formation of loosely hydrated compounds, and a third group by adsorp-

4 858 P. Gandhidasan et al. / Energy 26 (2001) tion. Solid desiccant dehydration is an adsorption process, which is any process wherein molecules from the gas are held on the surface of a solid by surface forces. The adsorption process, as opposed to the absorption process, does not involve any chemical reaction. Adsorption is purely a surface phenomenon. The degree of adsorption is a function of operating temperature and pressure; adsorption increases with pressure increase and decreases with a temperature increase [5]. This type of dehydration has several advantages [6] over a liquid desiccant dehydration system. Lower dew point can be obtained over a wide range of operating conditions using solid desiccant dehydrators. Dehydration of very small quantities of natural gas at low cost can be achieved and the unit is insensitive to moderate changes in gas temperature, flow rate, and pressure. They are relatively free from problems of corrosion, foaming, etc Properties of solid desiccants The following are the general requirements of solid desiccants with particular reference to dehydration of natural gas [7]. The solid desiccants must have large surface area for high capacity and high mass transfer rate. They must possess a high bulk density and activity for the components to be removed. They must be easily and economically regenerated. The resistance to gas flow through the desiccant bed must be small to have low pressure drop since the unit cost is sensitive to pressure drop. They should have high mechanical strength to resist crushing and dust formation. They must be fairly cheap, non-corrosive, non-toxic, and chemically inert. There should be no appreciable change in volume during adsorption and desorption, and should retain strength when wet. Desiccants that operate by adsorption are of primary importance for commercial gas dehydration. The types most commonly used for this purpose are [6,8]: 1. Alumina-based adsorbents. These include impure, naturally occurring materials such as bauxite and purity activated aluminas derived from gels or crystalline minerals. 2. Molecular sieves. This category cover a large family of synthetic zeolites characterized by extremely uniform pore dimensions. 3. Silica-based adsorbents. This group includes pure activated silica gel and special formulations containing a small percentage of other components. Table 1 summarizes typical desiccant properties. The selection of a suitable desiccant is primarily an economic exercise. Alumina is the cheapest but requires larger towers for given water load, which increases capital cost and head load. Its two main disadvantages are the co-adsorption of hydrocarbons, which reduces its capacity for water and can lead to the loss of valuable hydrocarbon components to the fuel gas, and rehydration, which destroys its activity [6]. Molecular sieves exhibit higher adsorption design loadings, greater resistance to fouling and coking, and high removal of impurities from the process streams. However, molecular sieves are the most expensive and require high temperature for regeneration. The main advantages of silica gel are that it has a high capacity for water, can be regenerated at a low temperature, and are not catalytic for sulfur conversion reactions. Silica gel also has a high capacity for pentane and higher hydrocarbons and can be used for combined dehydration/hydrocarbon recovery process. A problem with silica gel is its tendency to shatter

5 P. Gandhidasan et al. / Energy 26 (2001) Table 1 Summary of typical desiccant properties Property Silica gel Alumina Molecular sieves (4Å to 5Å) Surface area (m 2 /g) Pore volume (cm 3 /g) Pore diameter (Å) Cavities 11.4 Å in dia. with circular openings 4.2 Å in dia. Bulk density (kg/m 3 ) App. specific gravity Specific heat (kj/kg C) when contacted with liquid water. If the adsorption flow is from top to bottom of the desiccant bed, then providing a layer of water-resistant desiccant on top of the bed can prevent this problem. The final choice of desiccant must be based on equipment costs, service life, and applicability to process needs, etc. In most adsorption plants proper design and operation is more critical than choice of adsorbents. Silica gel is chosen as the solid desiccant for the present study Equipment description Fig. 1 shows the flow diagram for a two-tower, solid-desiccant dehydration unit [9]. The solid lines represent the gas being dried, while the dashed lines represent the regeneration gas flow. One bed is on the adsorption cycle, while the other bed is being regenerated. The unit also consists of a heater, which may be a fired heater or a shell-and-tube heat exchanger using steam or hot oil, to supply the necessary heat for regeneration. Usually adsorption flow is from top to bottom to prevent lifting of the bed by extreme gas velocities and channeling. Regeneration flow is Fig. 1. Schematic of a two-tower solid desiccant dehydrator.

6 860 P. Gandhidasan et al. / Energy 26 (2001) normally countercurrent to adsorption flow to ensure complete regeneration of the bottom of the bed that is the last area contacted by the gas being dehydrated. For good dehydration, the bed should be switched to regeneration just before the water content of outlet gas reaches an unacceptable level. A bed must be properly regenerated and this depends on both the quantity and temperature of the regeneration gas. When dehydration becomes inadequate to meet downstream requirements, either the adsorption cycle must be shortened or the regeneration gas flow must be increased Mechanism of water adsorption As the wet inlet gas flows downward through the tower on the adsorption cycle, all of the adsorbable gas components are adsorbed at different rates [5]. The water vapor is directly adsorbed in top layers of the bed. Dry hydrocarbons gas components (propane, butane) passing on down through the bed are also adsorbed, with the heavier components displacing the lighter components as the cycle proceeds. The pore size of silica gel has the broadest range of the three primary solid desiccants as given in Table 1. Therefore, it can adsorb larger molecules in addition to water. After the process has proceeded for a very short period of time, a series of adsorption zones will form and move down through the desiccant bed. These zones represent the length of tower involved in the adsorption of any component. As the upper layers of desiccant become saturated with water, the lower layers begin to see wet gas and begin adsorbing the water vapor, displacing the previously adsorbed hydrocarbon components. When the bed is saturated with water vapor, the tower must be switched from adsorption cycle to regeneration cycle before the bed has become completely saturated with water. 4. Analysis of dehydration of natural gas The following assumptions are made to dry 1 MMscmd (million standard m 3 per day) of natural gas: 1. The vessel outlet temperature is 85% of the heater outlet temperature. 2. Average bed temperature is based on 75% of the bed at heater outlet temperature and 25% of the vessel outlet temperature. 3. The specific heats of steel and desiccant are and kj/kg K respectively. Heat of desorption is 3256 kj/kg H 2 O. 4. Flat heads are used on vessel ends. Total vessel weight is increased by 10% for support. 5. Heat losses to dryer during heating period is calculated at 5%. 6. The densities of steel and regeneration gas are assumed as 7860 and kg/m 3 respectively. 7. The wet gas entering the tower is assumed as saturated. 8. The range of operating pressure of the natural gas is 2 10 MPa and the temperature of the gas is 40 C. In order to design a dehydration system suitable for natural gas using silica gel as the solid desiccant, a number of charts and tables must be referred to and further, manual calculation is

7 P. Gandhidasan et al. / Energy 26 (2001) tedious. The purpose of this paper is to replace the use of tables, charts, and manual calculations to design a dehydration system for natural gas. By making use of the equations provided in this paper, a computer program can be easily written and a PC can perform the calculations faster System analysis Under normal production conditions, natural gas is saturated with water. Estimating the saturated water vapor content of the natural gas is fundamental to the design and operation of the solid desiccant dehydrator. Water content of the saturated gas entering the dehydrator at the given operating conditions is given by: W EXP( t G ) P ( ) (1) It is reasonable to assume that virtually all of the water is adsorbed on the desiccant bed. In determining the drying cycle the designer is influenced by the desiccant capacity, water load to dehydrator, allowable system pressure drop, regeneration gas facilities and the investment economics [10]. Drying cycles generally range from 4 to 24 h. A common value of 8 h has been selected to avoid the tedious mechanism of determining optimum cycle length. The amount of water adsorbed per cycle is given by: M (QcW) (2) 24 The capacity of a desiccant for water is expressed normally in mass of water adsorbed per unit mass of desiccant. There are three capacity terms used namely static capacity, dynamic capacity, and the useful design capacity. It is prefer to use design capacity based on field data since it recognizes loss of desiccant capacity with time as determined by experience and economic considerations. The useful design capacity is influenced by inlet gas temperature and water content, regeneration gas temperature and water content, desired dew point, unit pressure, adsorbent aging and contamination history and bed length. The useful design capacity for any given dehydration is specific and the rate of its decline depends on operating conditions and the type of gas being dehydrated. For normal service, 7 9 kg of water per 100 kg of desiccant is commonly specified as useful design capacity for silica gel. The desiccant volume using the useful design capacity is given by: V D M ur D (3) Any one of the following two methods can be used to determine the bed diameter. In the first method, allowable gas mass velocity can be calculated and divided with the total gas flow rate to find the desiccant bed area. In the second method, the allowable superficial gas velocity can be used as provided by company specifications. The superficial gas velocity generally varies from 9 to 18 m/min. The second approach is chosen in the present analysis. Combining the continuity equation and the real gas equation of state, the desiccant bed diameter can be determined. After simplification, the desiccant bed diameter is given by:

8 862 P. Gandhidasan et al. / Energy 26 (2001) D B 327QZT G pv (4) where Z is the compressibility factor and the value of 0.88 is assumed. The volume of the desiccant divided by the cross-sectional area of the bed gives the length of the bed. When the vessel is subjected to an internal pressure, three mutually perpendicular principal stresses namely hoop stress, longitudinal stress, and radial stresses are developed in the vessel material [11]. The vessel can be treated as a thin cylinder and the radial stress can be neglected since its magnitude is small. By assuming the ends of the vessel are closed, hoop stress is set up in resisting the bursting effect of the applied pressure. Hoop stress can be expressed in terms of the radius of the circle passing through the midpoint of the thickness. Hence, the vessel thickness can be expressed as: w V 0.5PD B (SE 0.6P) where the joint efficiency (E) is assumed as 100% or 1. (5) 5. Regeneration of weak desiccants Regeneration considerations are the key to successful and economical operation of the dehydration of natural gas system. A portion (approximately 10%) of the entering wet gas is utilized for regeneration purposes. This gas is sent through a fired heater or a shell-and-tube heat exchanger using steam or hot oil where it is heated from about 200 to 325 C [10] and piped to the tower being regenerated. Initially, the hot gas must heat up the tower and the desiccant. At about 120 C, water will being boiling or vaporizing and the bed continues to heat up but more slowly since water is being driven out of the desiccant. After all the water has been removed, heating is continued to drive off any heavier hydrocarbons and contaminants. With cycle times of 4 h or greater, the bed will be properly regenerated when the outlet gas temperature has reached C. Following the heating cycle, the bed will be cooled before it is switched to adsorption. The bed will usually be cooled by unheated regeneration gas that flows downward through the bed so that any water adsorbed from cooling gas will be at the top of the bed and will not be adsorbed into the gas during the dehydration step. The cooling cycle will normally be terminated when average bed temperature has dropped to about 50 C since further cooling might cause water to condense from the wet gas stream and to presaturate the bed before the next adsorption cycle begins. A bed must be properly regenerated, or its adsorptive capacity is reduced. Proper regeneration depends on both the quantity and temperature of the regeneration gas. The highest regeneration temperatures are the most efficient for desorption. However, heater cost, metallurgy, and the thermal stability of the desiccant and the gas must be considered [12]. For example, if the temperature is above 325 C for silica gel, then the desiccant will be ruined. When dehydration becomes inadequate to meet downstream requirements, an operator must either shorten the adsorption cycle or increase the regeneration gas flow or do both. These changes

9 P. Gandhidasan et al. / Energy 26 (2001) should allow continued operation while the cause of the deficiency is being determined. For the present study, the pressure and temperature of the inlet gas and the heater outlet temperature are fixed. The average temperature of desiccant bed based on 75% of the bed at heater outlet temperature and 25% at the vessel outlet temperature can be calculated as: t B 0.75t H ( t H ) (6) Sufficient heat is required to provide the latent heat of vaporization of the adsorbed water and to raise the temperature of the bed and associated equipment to the final regeneration temperature. In order to size the heater, one must calculate the total energy for regeneration which includes heating the (steel) vessel, heating the desiccant, heat to remove water from desiccant and the heat losses. Total heat required for regeneration is given by: H 1.05[(m VC V T) (m DC D T) (M Y)] (7) where T is the temperature difference between the bed and the gas. m V r V pd B w V(L B a 0.5D B ) 1.10 (8) where the additional vessel height is assumed as 0.6 m, and m D r D V D (9) There will be an added energy requirement to cover the losses as the regeneration gas temperature increases on the outlet side of the bed. Since the desiccant bed must be heated to regeneration temperature in a short time, the regeneration gas rate should be increased by some factor that will correct for unsteady state heat-up and this loss factor is given by [13]: F ln t H t G 0.15t H (10) Heat (energy) added to the regeneration gas to regenerate the desiccant bed is the product of the total heat required for regeneration and the loss factor. A portion of the inlet gas will be utilized for regeneration and therefore, it is to be heated from its inlet temperature to its exit temperature. The volume of gas required for regeneration is given by: H F G (11) (h 2 h 1 )r G 6. Results and discussion A computer program is written to carry out the numerical calculations to dry one million standard m 3 of natural gas per day based on the assumptions stated earlier. For this calculation, it is assumed that the useful design capacity for silica gel as 0.08 and the superficial gas velocity as

10 864 P. Gandhidasan et al. / Energy 26 (2001) Fig. 2. Effect of operating pressures and temperatures on the desiccant mass. 9 m/min. The results of the design of the natural gas solid desiccant dehydrator are shown in Figs The results based on the equations developed in this paper are compared with data available in the literature in Fig. 5. The amount of desiccant mass required dehydrating the natural gas for various operating pressures and temperatures are shown in Fig. 2. As the pressure increases, the required desiccant mass for dehydration decreases and also as the temperature decreases, the required desiccant mass decreases. From the economic considerations, pressurized, cool natural gas reduces the mass of the desiccant required for dehydration. From the figure it can be seen that the sudden change in desiccant mass at about the pressure of 6 MPa and further increase in pressure has negligible effect on the mass of the desiccant. The effect of pressure on the dimensions of the silica gel bed is shown in Fig. 3 for the Fig. 3. Effect of pressure on the bed dimensions.

11 P. Gandhidasan et al. / Energy 26 (2001) Fig. 4. Effect of operating pressures and temperatures on the tower shell thickness. superficial gas velocity of 9 m/min. As the pressure increases, the desiccant bed diameter decreases whereas the bed length increases. It is to be noted that the changes in desiccant bed diameter is insignificant compared to the changes in the bed length. Superficial gas velocities up to 18 m/min have been used in some installation [7]. The velocity affects both diameter and length of the bed. As the superficial gas velocity increases, diameter decreases and length increases. The effect of operating pressure and temperature on the dryer shell thickness is shown in Fig. 4. As the pressure increases, the shell thickness also increases to withstand for high pressures. It is interesting to note that the effect of natural gas temperature has virtually no bearing on the thickness of the dryer shell. In order to give confidence in the equations that have been developed in this paper, the desiccant volume required to dehydrate 1 million std m 3 of natural gas per day for the given operating conditions is compared with those estimated by Wunder [10] in Fig. 5. The present study over- Fig. 5. Comparison of present study with Wunder [10] results.

12 866 P. Gandhidasan et al. / Energy 26 (2001) Fig. 6. Effect of gas flow rate on the volume of the desiccant. predicts the desiccant volume as shown in the figure and the difference may be due to the estimation of the water content of the saturated natural gas entering the dehydrator. In designing a natural gas dehydration system, the engineer must often estimate the water content of the gas to be dried. In order to calculate this parameter, Eq. (1) is used in the present study whereas Wunder used the tedious manual calculation in addition to the use of steam tables. However, the error is small at low pressures and it is within 10%. The effects of volumetric flow rate of natural gas for dehydration are shown in Figs. 6 and 7. As the gas flow rate increases, the required desiccant volume for dehydration also increases to achieve the same level of dehydration as shown in Fig. 6. As the pressure increases, the required desiccant volume decreases and the variation is linear. The effect of volumetric flow rate on the ratio of the length to the diameter of the desiccant bed is shown in Fig. 7. As the gas flow rate Fig. 7. Effect of gas flow rate on the ratio of length to diameter of the bed.

13 P. Gandhidasan et al. / Energy 26 (2001) Fig. 8. Energy requirements for various exit temperatures of the heater. increases the ratio of length of the bed to the diameter of the bed decreases and, as the pressure increases the ratio also increases. The amount of energy (heat) needed for the regeneration of desiccant bed has been calculated for various operating pressures with different heater outlet temperatures and the results are shown in Fig. 8. The heat added to the regeneration gas increases with increase in the heater exit temperature. For a particular heater exit temperature, as the pressure increases the heat added to the regeneration gas decreases. Fig. 9 shows the effect of heater exit temperature on the volume of the regeneration gas required. As the heater exit temperature increases, the regeneration gas volume decreases for various operating pressures. For a particular heater outlet temperature, as the pressure increases Fig. 9. Regeneration gas volume for various heater exit temperatures.

14 868 P. Gandhidasan et al. / Energy 26 (2001) the regeneration gas volume decreases and this effect is significant at lower operating pressures. The higher the regeneration temperature, the smaller the required quantities of regeneration gas. 7. Conclusions Calculations have been made for the design of a two-tower, solid desiccant dehydrator using silica gel as one of the most versatile solid desiccants for the dehydration of natural gas. It is found that the operating temperature of the gas has negligible effect on the dryer shell thickness but it has considerable impact on the desiccant mass required for dehydration. The operating pressure of the gas increases, the desiccant mass required for dehydration decreases but the rate of decrease is insignificant for pressures greater than 6 MPa. From the analysis of the regeneration of weak desiccants, it is found that the higher the regeneration temperature, the smaller are the required quantities of regeneration gas. Acknowledgements The authors are grateful for the financial support and facilities provided by the King Fahd University of Petroleum and Minerals for this research. References [1] Arab oil and gas directory. Paris: Arab Petroleum Research Center, [2] Grosso S, Fowler AE, Pearce RL. Dehydration of natural and industrial gas streams with liquid desiccants. In: Mujumdar AS, editor. Drying 80, Proceedings of Second International Symposium. Washington (DC): Hemisphere Publishing Corp, 1980: [3] Ikoku CU. Natural gas engineering a systems approach. Houston: PennWell Publishing Company, [4] Rueter CO, Murff MC, Beitler CM. Glycol dehydration operations, environmental regulations, and waste stream survey. Topical report GRI-96/0049. Chicago: Gas Research Institute, [5] Petroleum Extension Service. Field handling of natural gas. 3rd ed. Austin: The University of Texas, [6] Kohl A, Nielsen R. Gas purification. 5th ed. Houston: Gulf Publishing Company, [7] Campbell JM. Gas conditioning and monitoring. 6th ed. Norman: Campbell Petroleum Series, [8] Aitani AM. Sour natural gas drying. Hydrocarb Process 1993;72(4): [9] Petroleum Extension Service. Plant processing of natural gasy. Austin: University of Texas, [10] Wunder JWJ. How to design a natural-gas drier. Oil Gas J 1962;60(32): [11] Shigley JE, Mischke CR. Standard handbook of machine design. New York: McGraw-Hill, [12] Lee AL, Erekson EJ. Point-of-use gas dehydration. Part II. Fundamental and technical information. Topical report GRI-93/ Chicago: Gas Research Institute, [13] Coker AK. Program sizes solid desiccant dryer for natural gas. Oil Gas J 1994;92(7):74 8.