DRYING OF NATURAL GAS USING LOW TEMPERATURE SEPARATION

Semester Project Work

December 27, 2005

Mohammed Mamun Azad Student No.: 677698

Department of Petroleum Engineering and Petroleum Geosciences

Norwegian University of Science and Technology (NTNU)

Abstract An approach for removal of water from natural gas by using low temperature separation (LTS) method is presented here. Two types of LTS methods: Joule-Thomson valve and Turbo-expander were considered. Initial gas composition, temperature and pressure were used for doing a steady state simulation of dewatering process under process engineering program Hysys. Hysys made all the analysis by using the provided input data and some necessary assumptions and the simulation output were used to compare Joule-Thomson valve and Turbo-expander technically and thermodynamically. The composition of dry natural gas (in mole %) was obtained from Troll after processing at on-shore process plant Kollsnes. Hysys mixed this gas with water in a fictitious mixer in order to make it saturated with water which was considered to be the inlet stream for LTS units. Hysys helped to estimate the cooling and dewatering capacity, efficiency, advantages and disadvantages of Joule-Thomson valve and Turbo-expander. On the basis of the above simulation, an attempt was made to choose a suitable LTS type dehydration method from small to medium to large scale in a wide range of pressure and temperature condition, technically and thermodynamically efficient, easy to operate, at a reasonable installation, operating and maintenance cost.

Acknowledgements I wish to express my sincere appreciation to my supervisor Professor Jon Steinar Gudmundsson. I am very grateful to him for his support, guidance, assistance, patience and enthusiasm. I am especially grateful to Mr. Anwar Hossain Bhuian (PHD student) for his valuable remarks and comments during the entire project. I would like to thank Salako Abiodun for providing prompt support and technical knowledge in working with Hysys simulator. I appreciate my wife for her great support during the time of creating the project report, and much longer than that. Last but not least thanks to my friends and colleagues who helped me in making numerous improvements not only to its wording but to its technical content.

Nomenclature cp Specific heat of gas at constant pressure, cv Specific heat of gas at constant volume, H Enthalpy of gas, P, P1, P2 Pressure of gas, Q, q Heat flow, R Molar gas constant, s, s1, s2 Entropy of gas, T Temperature of gas, U Internal energy of gas, V Volume, w Work done by the gas, µJT Coefficient of Joule-Thomson’s effect.

List of contents 1 Introduction.................................................................................................................... 1

1.1 Objective ................................................................................................................... 1 1.2 Methodology ............................................................................................................. 1 1.3 Data Type.................................................................................................................. 1

2 Literature Review .......................................................................................................... 2 2.1 Absorption................................................................................................................. 2 2.2 Adsorption................................................................................................................. 3 2.3 Gas Permeation ......................................................................................................... 4 2.4 Low Temperature Separation.................................................................................... 5

2.4.1 Joule-Thomson Valve ........................................................................................ 5 2.4.2 Turbo expander: ................................................................................................. 7 2.4.3 Thermodynamic description of Joule-Thompson Process:................................ 8 2.4.4 Thermodynamic description of Turbo Expander (Isentropic) Process: ........... 10 2.4.5 Comparison between J-T Process and Turbo Expander Process ..................... 11

3 Data Analysis and Results........................................................................................... 12 3.1 Hysys Simulation Package...................................................................................... 12 3.2 Joule-Thomson Method .......................................................................................... 13 3.3 Turbo Expander Method ......................................................................................... 13

4 Discussion ..................................................................................................................... 27 4.1 Joule-Thomson Method .......................................................................................... 27

4.1.1 Advantages of Joule-Thomson Method ........................................................... 27 4.1.2 Limitations Joule-Thomson Method................................................................ 28

4.2 Turbo Expander Method ......................................................................................... 28 4.2.1 Advantages of Turbo Expander Method.......................................................... 28 4.2.2 Limitations of Turbo Expander Method .......................................................... 28

5 Conclusion .................................................................................................................... 29 6 References..................................................................................................................... 30

List of figures

Figure 2.1: Simplified flow diagrams for a glycol dehydration unit

------------------- 3

Figure 2.2: Dehydration by adsorption

------------------- 3

Figure 2.3 Gas Permeation Modules

------------------- 5

Figure 2.4 Typical LTS system without hydrate inhibitor

------------------- 6

Figure 2.5 Typical LTS system with hydrate inhibitor

------------------- 7

Figure 2.6 Typical Turbo Expander

------------------- 8

Figure 2.7 Effect of Joule-Thompson Coefficient

------------------- 9

Figure 2.8 Isenthalpic and isentropic cooling

------------------- 11

Figure 3.1 Process flow diagram of Joule-Thomson Method

------------------- 18

Figure 3.2 Process flow diagram of Turbo Expander Method

------------------- 18

Figure 3.3 Enthalpy-Entropy diagram of Joule-Thomson method

------------------- 19

Figure 3.4 Pressure-Entropy diagram of Joule-Thomson method

------------------- 19

Figure 3.5 Pressure-Temperature diagram of Joule-Thomson method

------------------- 20

Figure 3.6 Temperature-Enthalpy diagram of Joule-Thomson method

------------------- 20

Figure 3.7 Temperature-Entropy diagram of Joule-Thomson method

------------------- 21

Figure 3.8 Pressure-Enthalpy diagram of Joule-Thomson method

------------------- 21

Figure 3.9 Pressure-Entropy diagram of Turbo-Expander method

------------------- 22

Figure 3.10 Pressure-Temperature diagram of Turbo-Expander method

------------------- 22

Figure 3.11 Temperature-Enthalpy diagram of Turbo-Expander method

------------------- 23

Figure 3.12 Temperature-Entropy diagram of Turbo-Expander method

------------------- 23

Figure 3.13 Enthalpy-Entropy diagram of Turbo-Expander method ------------------- 24

Figure 3.14 Polytropic efficiency at different pressure in turbo expander

------------------- 24

Figure 3.15 Polytropic efficiency at different temperature in turbo expander

------------------- 25

Figure 3.16 Energy production at different pressure in turbo expander

------------------- 25

Figure 3.17 Energy production at different temperature in turbo expander

------------------- 26

Figure 3.18 Water removal in Joule-Thomson and turbo expander methods

------------------- 26

Figure 3.19 Water removals in Joule-Thomson and turbo expander methods

------------------- 27

List of tables Table 1.1 Table for gas composition

------------------------------- 1

Table 1.2 Table for normalized gas composition data

------------------------------- 2

Table 3.1 Thermodynamic properties for Joule-Thomson method (At different temperature)

------------------------------- 15

Table 3.2 Thermodynamic properties for Joule-Thomson method (At different pressure)

------------------------------- 15

Table 3.3 Outlet gas composition in Joule-Thomson method (At different temperature)

------------------------------- 16

Table 3.4 Outlet gas composition in Joule-Thomson method (At different pressure)

------------------------------- 16

Table 3.5 Thermodynamic properties for Turbo Expander method (At different temperature)

------------------------------- 17

Table 3.6 Thermodynamic properties for Turbo Expander method (At different pressure)

------------------------------- 17

Table 3.7 Outlet gas composition in Turbo Expander method (At different temperature)

------------------------------- 18

Table 3.8 Outlet gas composition in Turbo Expander method (At different temperature)

------------------------------- 18

1 Introduction Natural gas needs to be dried before pipeline transport, because the water molecules present in gas in both vapor and liquid state form hydrates which cause flow restrictions, pressure drops, lower the heating value of gas and corrode pipelines and other equipments. Several methods are used world-wide to dry gas, including absorption method, adsorption method and low temperature separation (LTS) method.

1.1 Objective Two methods are used in LTS: Joule-Thompson (J-T) expansion through valve or choke and expansion through turbine. Here my aim was to compare the two LTS methods with focus both technically and thermodynamically on their advantages and disadvantages in gas dehydration plants.

1.2 Methodology We need composition of gas to analyze any gas dehydration process because gas properties are highly influenced by the composition of gas. Gas composition (mole fraction) data of Troll (1), Norway were taken from the Home Page of Professor Jόn Steinar Gudmundsson in order to start the analysis of LTS methods. HYSYS simulator was the main tool of my analysis. Gas composition and some other relevant assumptions were the main inputs for HYSYS. Natural gas coming from subsurface reservoirs is saturated with water but here we started with dry gas. To avoid this problem, we introduced a fictitious mixer at the beginning of HYSYS which made the gas saturated with water. Then HYSYS analyzed that gas to give me the desired output data.

1.3 Data Type Mole fraction in percent of Troll (1) gas, Norway is given below: Table 1.1 Table for gas composition Component Troll (1) Methane 93.070 Ethane 3.720 Propane 0.582 Iso-Butane 0.346 N-Butane 0.083 C5+ 0.203 Nitrogen 1.657 Carbon-dioxide 0.319

Total 99.98 (1) After processing at Kollsnes (on-shore processing plant), average for Nov, 2000. Kollsnes is one of the largest systems in the world. Kollsnes receives the gas from Troll A, the largest gas field in Norway. Since total summation of mole fractions must be 100%, these data were normalized into following table: Table 1.2 Table for normalized gas composition data Component Mole % Methane 93,1024 Ethane 3,7041 Propane 0,5806 Iso-Butane 0,3504 N-Butane 0,0801 C5+ 0,2002 Nitrogen 1,6618 Carbon-dioxide 0,3204 Total 100 %

2 Literature Review The water present in natural gas may, depending on the temperature and pressure prevailing in an installation, condense and cause the formation of hydrates, solidify, or favor corrosion if the gas contains acid components. To avoid such situations, natural gas must be dehydrated (Rojey A. et. al.., 1997). Four types of processes are currently being used which are: a) Absorption, b) Adsorption, c) Gas Permeation and d) Low Temperature Separation.

2.1 Absorption The most common method for dehydration in the natural gas industry is the use of a liquid desiccant contactor-regeneration process. In this process, the wet gas is contacted with a lean solvent (containing only a small amount of water). The water in the gas is absorbed in the lean solvent, producing a rich solvent stream (one containing more water) and a dry gas. In case of absorption based natural gas dehydration processes the gas is dried by countercurrent scrubbing with a solvent that has a strong affinity for water. The solvent is usually a glycol, although other liquid desiccants are met which are calcium chloride, lithium chloride, zinc chloride, etc. The dehydrated gas leaves at the top of the column. The glycol leaving the bottom is regenerated by distillation and recycled. Before

undergoing the actual dehydration process any free liquids in the natural gas stream are removed. A separator should be included upstream of the contactor to separate any hydrocarbon liquids and free water. The separator could be a two-phase or three-phase separator depending on the amount of free water expected.

Figure 2.1: Simplified flow diagrams for a glycol dehydration unit (reprinted from unpublished diploma thesis of Artur Ryba, 2005)

2.2 Adsorption Separation processes by adsorption uses a solid phase with large surface area, which selectively retains the components to be separated. The adsorbents are generally characterized by a micro porous structure which affords a very large specific surface Adsorption processes are generally applied when a high purity is required for the processed gas. Adsorbents are naturally unsuitable for continuous circulation, owing to mechanical problems and also due to the risks of attrition (erosion of adsorbent particles due to friction and collisions during movement). This is why adsorbents are normally used in fixed beds with periodic sequencing. The flow scheme of a dehydration operation by adsorption in a fixed bed is shown in Figure 2.2.

Figure 2.2: Dehydration by adsorption (reprinted from Rojey A. et. al., 1997)

The process is conducted alternately and periodically, with each bed going through successive steps of adsorption and desorption. During the adsorption step, the gas to be processed is sent on the adsorbent bed which selectively retains the water. When the bed is saturated, hot natural gas is sent to regenerate the adsorbent. After regeneration and before the adsorption step, the bed must be cooled. This is achieved by passing through cold natural gas. After heating, the same gas can be used for regeneration. In these conditions, four beds are needed in practice, two beds operating simultaneously in adsorption, one bed in cooling and one bed in regeneration (Rojey A. et. al., 1997). The desorption step is carried by different methods, such as: a) Lowering the pressure, sometimes even under vacuum b) Sweeping by an inert natural gas to lower the partial pressure of the component to be desorbed c) Sweeping by a displacement agent, which, by being adsorbed, allows more effective desorption than with a simple evolution gas d) Heating, in which the temperature rises, facilitates desorption in a fixed-bed operation. The most widely used adsorbents are: Activated Alumina, Silica Gel and Molecular Sieves (Zeolites).

2.3 Gas Permeation In the process of dehydration by permeation, the dried natural gas is going through a membrane leaving particles of water and impurities on its surface. Industrial applications of dehydration by gas permeation are currently very limited. However, many investigations have demonstrated the potential value of such a process which, in comparison with a glycol dehydration unit, could prove to be more economical and more compact, which is extremely important for offshore production. These advantages only appear clearly in the case of single-stage operation without recycle or recompression of the permeate.

For the separation to be effective, the membrane must be very permeable with respect to the contaminant to be separated, which passes through the membrane driven by pressure difference, and it must be relatively impermeable to methane. The permeability of methane must be accepted to avoid an excessively large membrane area nevertheless means a significant loss of methane in the permeate. Membrane separation processes require large membrane areas, which are generally expressed in thousands of square meters. The membrane surface is dependent on the amount of gas permeating through it. Compact permeation modules with a high membrane area are therefore needed (Rojey A. et. al., 1997). The most widely used industrial modules belong to two types are (Figure 2.3): a) Modules with plane membranes wound spirally around a collector tube b) Modules with a bundle of hollow fibers

Figure 2.3 Gas Permeation Modules (Reprinted from Rojey A. et. al., 1997)

2.4 Low Temperature Separation In LTS type dehydration process, the pressure of incoming gas is reduced by using choke or expander in order to reduce temperature. This temperature drop makes the water in the gas condense and come out in liquid form from gas. Two methods are used in LTS:

(a) Joule-Thomson Valve (b) Turbo Expander

2.4.1 Joule-Thomson Valve The Joule-Thompson Expansion (Constant Enthalpy) systems use the refrigeration effect that results from a pressure drop taken on a high pressure well stream. This expansion occurs across a choke and the resulting refrigeration effect is dependent on the temperature of the upstream side of the choke, the pressure differential across the choke, and the amount of liquid formed. For obtaining the maximum removal of liquids from the gas stream for a given pressure differential and sales-gas pressure, the lowest possible temperature within reasonable limits should be attained in the separator. This in turn means the lowest possible temperature upstream of the choke. Two basic methods commonly used for dehydration purpose are:

(a) LTS without hydrate inhibitor, (b) LTS with hydrate inhibitor.

(a) LTS without hydrate inhibitor The basic unit for low-temperature separation without hydrate inhibitor includes essentially a choke, separator, and heat exchange coils. It is assumed that the inlet well stream contains a minimum amount of free water and is of sufficient temperature to prevent formation of hydrates upstream of the choke. The complete system is shown in figure 2.3. The well stream flows through the coil in the low temperature separator where it is slightly chilled, then to the inlet high pressure liquid separator where free liquids are separated from the gas. The gas then flows through the gas-gas heat exchanger, through the choke and into the low-temperature separator. The cold gas flows from the separator, through the gas-gas heat exchanger, and into the sales gas line. The liquids from the of the low temperature separator are dumped to some form of stabilization before going to storage.

Figure 2.4 Typical LTS system without hydrate inhibitor (Reprinted from Petroleum Engineering Handbook, Third Printing)

(b) LTS with hydrate inhibitor The formation of hydrates can be prevented by changing the character of the water in such way that it will not form hydrates with natural gas. This is accomplished through the use of a substance known as ‘Hydrate Inhibitor’. The most commonly used inhibitors are glycols and alcohols. A typical system is shown in figure 2.5. The inhibitor is injected between the inlet high pressure separator and the regenerative heat exchanger. The inhibitor mixes with free water for cooling and prevents hydrate formation.

Figure 2.5 Typical LTS system with hydrate inhibitor (Reprinted from Petroleum Engineering Handbook, Third Printing)

The low temperature separation system with hydrate inhibitor eliminates the formation of hydrates and allows the gas to be cooled below the hydrate temperature before expansion. This results in an increase in the amount of condensate removed from the well stream. The operating costs are higher than the system without inhibitor, but the increased recovery will normally be more than offset this.

2.4.2 Turbo expander: The turbine expansion low temperature dehydration system differs from choke expansion is that the turbine turns a shaft from which a work is extracted. A typical turbo-expander process is shown in figure 2.6.

Figure 2.6 Typical Turbo Expander (Reprinted from Petroleum Engineering

Handbook, Third Printing) The gas enters through an inlet separator with any liquid separated at this point being introduced to a low point in the stabilizer tower. The gas then goes through heat exchange with the cold gas leaving the stabilizer. Another separator is installed if sufficient liquid is formed in the gas-gas heat exchanger with the liquid being introduced at an intermediate point in the stabilizer. The cold gas then flows to the expander where the pressure is reduced and low temperature is achieved. The gas and liquid mixture leaves the expander and flows to the separator that normally is on the top of the stabilizer column. Sales gas flows back through the exchanger and may be compressed in the direct connected centrifugal compressor before being put into the sales gas line. Since extremely low temperatures are achieved in a typical turbo expander plant, dehydration is normally the first step though some plants do use alcohol injection. The gas frequently is expanded below sales gas pressure and then recompressed to make use of the work that must be extracted from the shaft of the turbine (Bloch H. P. and Soares C., 2001). A fairly recent development in gas processing, the turbo expander process, is one of the simplest and easy of operable. The favourable operating characteristics allow the plant to run unattended through long periods and its simplicity and relatively low investment cost make it an attractive option (I. Ross and T. Robinson, 1981).

2.4.3 Thermodynamic description of Joule-Thompson Process: Consider a sample of gas initially at P1, V1 and T1 was forced into a system at constant pressure P1. The gas came out of the system at P2, V2 and T2. The system is insulated such that Δq=0. The work has two terms, work done on the system to force the gas through the

system and the work done by the system on the surroundings as it came out the other side of the plug. The total work is:

( ) ( )2211

2211 00VPVP

VPVPw−=

−−−−=

Since Δq =0, the change in internal energy of the gas is, ΔU= Δq+w = 0+P1V1-P2V2

= P1V1-P2V2 The enthalpy is then given by, ΔH= ΔU+Δ(PV)= P1V1-P2V2 +P2V2- P1V1= 0 So Joule-Thompson’s effect (Throttling process) is a constant enthalpy process. Co-efficient of Joule-Thompson’s effect, µJT can be defined as,

(www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html, 12.12.2005)

Figure 2.7 Effect of Joule-Thompson Coefficient (Reprinted from Equipment Modules,

Volume-2) The coefficient of J-T effect is important in the cooling operation of gas for the purpose of liquefaction or dehydration. It tells whether a gas cools or heats on expansion. It turns out that this coefficient is a decreasing function of temperature and it passes through zero at the Joule-Thompson inversion temperature. In an expansion dp<0, whether dT is

P

TJT

P

T

H

HJT

cPH

THPH

PT

PT

⎟⎠⎞

⎜⎝⎛∂∂

−=⇒

⎟⎠⎞

⎜⎝⎛∂∂

⎟⎠⎞

⎜⎝⎛∂∂

−=⎟⎠⎞

⎜⎝⎛∂∂

⇒

⎟⎠⎞

⎜⎝⎛∂∂

=

μ

μ

positive or negative depends on the sign of µJT. It can be seen that if µJT is positive then dT negative upon expansion so that the gas cools. On the other hand, if µJT is negative then dT is positive so that the gas warms upon expansion (Campbell J. M., 1994).

2.4.4 Thermodynamic description of Turbo Expander (Isentropic) Process: An isentropic process is a constant entropy process. If a control mass undergoes a process which is both reversible and adiabatic, then the second law specifies the entropy changes to be zero. A steady state reversible flow through an adiabatic controlled volume also has no entropy change from inlet to outlet. Both are isentropic processes. Although an isentropic process might be an idealization of an actual process, it serves as a limiting case, for particular applications. The entropy change for an ideal gas can be presented as:

∫

∫

−=−

+=−

2

1

2

1

1

212

1

212

ln

ln

T

TP

T

Tv

PP

RTdTcss

vv

RTdTcss

For such isentropic process, s2-s1=0

∫

∫

−=

+=

2

1

2

1

1

2

1

2

ln0

ln0

T

TP

T

Tv

PPR

TdTc

vvR

TdTc

The final form used depends on the approximation made for the temperature dependence of the specific heats. Assuming that the specific heats are accurately approximated by constant values eliminates the integrals in the above equations. The constant specific equations are:

P

PP

cR

sP

cR

s

cR

ssv

PP

TT

PPR

TTc

vv

vv

TT

vvR

TTc

⎟⎟⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛⇒−=

⎟⎟⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛⇒+= −

1

2

1

2

1

2

1

2

2

1

1

2

1

2

1

2

1

2

lnln0

lnln0

Where, the subscript s indicates that the process occurs at constant entropy. The power on each expression is rewritten in terms of k = cP/cv by noting that cP-cv = R, so R/cP = (k-1)/k. Thus we get,

kk

s

k

ss

PP

TT

vv

TT

1

1

2

1

2

1

2

1

1

2

−

−

⎟⎟⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛

These equations are specified relations that are used for an ideal gas undergoing ideal process if the specific heats are considered to be constant. If the specific heat can not be assumed as constants then the temperature dependence of the specific heats must be included. The variable specific heat solution for an ideal gas undergoing an isentropic

process is obtained as: 1

2102012 ln)()(0

PP

RTsTsss −−==− . (Holman J.P., 1988)

2.4.5 Comparison between J-T Process and Turbo Expander Process The turbo expander is a mechanical device that produces work by expanding the feed gas stream from its initial high pressure to a lower pressure level. In the ideal case the expansion is isentropic. As mechanical work is produced the enthalpy of the gas is decreased. In reality, the expansion can not completely approach the isentropic case but produce a high percentage of the ideally possible work. The expansion and reduction in enthalpy lowers the temperature of the gas which results in partial liquefaction (I. Ross and T. Robinson, 1981.). By contrast, expansion across a valve is isenthalpic producing no work. Resulting temperatures are not as low as those achieved by the expander and less liquefaction takes place. In a work producing expansion, the temperature of the process fluid is always reduced; hence cooling does not depend on being below the inversion temperature prior to expansion. Additionally, the work producing results in a larger amount of cooling than in an isenthalpic expansion over the same pressure difference. This is illustrated diagrammatically in figure 2.8, where TA-TB is the isentropic cooling and TA-TC is the isenthalpic cooling for adiabatic expansion between the same pressure limits (Perry R. H., 1984).

Figure 2.8 Isenthalpic and isentropic cooling (Reprinted from Chemical Engineers’

Handbook, Perry)

3 Data Analysis and Results I started my simulation with Troll gas composition data, considering 1000 Kgmole/hr gas and 20 Kgmole/hr water being mixed in a fictitious mixer. The thermodynamic properties of gas were analyzed at different temperature and pressure levels by using HYSYS simulator. These properties were the key to make comparison between the low temperature separation processes: Joule-Thomson and Turbo Expander process.

3.1 Hysys Simulation Package Aspen Hysys 3.2 is a process modeling tool for steady state simulation, design, performance monitoring, optimization and business planning for oil and gas production, gas processing and petroleum refining industries. The program is built upon proven technologies, with more than 25 years experience supplying process simulation tools to the oil, gas and refining industries. It proves an interactive process modeling solution that enables engineers to create steady state models of plant design, performance monitoring, troubleshooting, operational improvement, business planning and asset management. Hysys helps process industries improve productivity and profitability throughout the plant lifecycle. The powerful simulation and analysis tools, real-time applications and the integrated approach to the engineering solutions enable the user to improve designs, optimize production and enhance decision-making (Aspen Tech, 2004). Hysys offers a high degree of flexibility because there are multiple ways to accomplish specific tasks. This flexibility combined with consistent and logical approach to how these capabilities are delivered makes Hysys a versatile process simulation tool (Aspen Tech, 2004). Another Hysys feature is that modular operations are combined with non-sequential solution algorithm, so not only is information processed as it is supplied, but the results of any calculation are automatically produced throughout the flow sheet, both forwards and backwards. The modular structure of the operation means they can be calculated in either direction, using information in an outlet stream to calculate inlet conditions (Aspen Tech, 2004). In Hysys, all necessary information pertaining to pure component flash and physical property calculations is contained within the Fluid Package, therefore choosing the right Fluid Package for given compounds is substantial. For the given composition of natural gas flowing through LTS unit, different Fluid Packages were checked, but finally the Peng-Robinson equation of state was chosen, as an ideal model for process calculations. Material streams are used to simulate the material traveling in and out of the simulation boundaries and passing between unit operations. For the material stream the user has to define their main properties and composition so Hysys can solve the stream. The parameters necessary are the temperature, pressure, flow based for example on molar flow, and composition (Aspen Tech, 2003). Energy streams are used to simulate the energy traveling in and out of the simulation boundaries and passing between unit

operations. The energy stream property view contains of fields allowing user to define stream parameters, view objects to which the stream is attached and specify dynamic information. The main parameter for energy streams is heat flow (Aspen Tech, 2003). Separator is a unit with one or multiple feeds, one vapor and one liquid product stream. The separator divides the vessel contents into its constituent vapor and liquid phases. Every separator may be provided with some common features like for example the geometry of the vessel and heat loss model which accounts for the convective and conductive heat transfer that occurs across the vessel wall. The user can choose between various heater types, which determine the way in which heat is transferred to the vessel operation (Aspen Tech, 2003). In both Joule-Thomson and Turbo Expander processes, the gas is allowed to expand and consequently a certain temperature drop is obtained which is very useful to separate water from gas. Hysys simulator can be used to analyze the processes if sufficient inputs are provided. In my analysis, the inputs were gas composition, gas flow rate, temperature, pressure and few assumptions. There is a provision of using hypothetical component with proper input of some of its properties say molecular weight, liquid density or boiling point. Aspen tech recommended hypothetical component is minimum C7+. But in my analysis, C5+ was hypothetical component and its boiling point was assumed 96oC. Initial gas flow rate was assumed 1000 Kgmole/hr. Then Hysys analyzed the inputs and gave a series of output data such as outlet temperature, pressure, composition, molar enthalpy, molar entropy, heat flow etc.

3.2 Joule-Thomson Method 1000 Kgmoles of gas and 20 Kgmoles of water were mixed together in a mixer in order to make the gas saturated with water. Then saturated gas was allowed to pass through the Joule-Thomson valve. The pressure was dropped down to 65 bar which is assumed to be slightly higher than the pipeline pressure. Consequently the gas became cold due to Joule-Thomson effect. A separator was installed immediately after the downstream of the valve. Water was removed from the gas at the separator. The top product from the separator is the dry gas ready to deliver into the pipeline (Figure 3.1). Hysys analyzed the entire process at varied conditions of inlet temperature and pressure to get the desired outputs such as outlet temperature, molar enthalpy and molar entropy. All the inputs and outputs were given in following tables and some figures were plotted by using those data.

3.3 Turbo Expander Method Here the saturated gas passed through an expander, which allowed the pressure to drop down to 65 bar (Figure 3.2). The temperature drop achieved was more than that in Joule-Thomson method. Similar to Joule-Thomson method, Hysys was used to analyze the

entire process at varied conditions of inlet temperature and pressure to get the desired outputs such as outlet temperature, molar enthalpy, molar entropy and apart from Joule-Thomson method certain amount of energy was recovered. This energy recovery was calculated in terms of heat. All the inputs and outputs are given in following tables and some figures were plotted by using those data.

Table 3.1 Thermodynamic properties for Joule-Thomson method (At different temperature)

Table 3.2 Thermodynamic properties for Joule-Thomson method (At different pressure)

Table 3.3 Outlet gas composition in Joule-Thomson method (At different temperature)

Table 3.4 Outlet gas composition in Joule-Thomson method (At different pressure)

Table 3.5 Thermodynamic properties for Turbo Expander method (At different temperature)

Table 3.6 Thermodynamic properties for Turbo Expander method (At different pressure)

Table 3.7 Outlet gas composition in Turbo Expander method (At different temperature)

Table 3.8 Outlet gas composition in Turbo Expander method (At different temperature)

Figure 3.1 Process flow diagram of Joule-Thomson Method

Figure 3.2 Process flow diagram of Turbo Expander Method

Enthalpy-Entropy Diagram

1,84

1,85

1,86

1,87

1,88

1,89

1,9

1,91

1,92

149 150 151 152 153 154 155 156 157 158

Molar Entropy, S, KJ/KgmoleoC

Enth

alpy

, H, K

cal/K

gmol

e, (*

104 )

HS Curve

Figure 3.3 Enthalpy-Entropy diagram of Joule-Thomson method

Pressure-Entropy Diagram

0

20

40

60

80

100

120

140

160

180

200

152 152,5 153 153,5 154 154,5 155

Molar Entropy, S, KJ/KgmoleoC

Pres

sure

, P, b

ar

PS Curve

Figure 3.4 Pressure-Entropy diagram of Joule-Thomson method

Pressure-Temperature Diagram

90

100

110

120

130

140

150

160

170

180

190

50 55 60 65 70 75 80

Temperature, T, oC

Pres

sure

, P, b

ar

PT Chart

Figure 3.5 Pressure-Temperature diagram of Joule-Thomson method

Temperature-Enthalpy Diagram

0

20

40

60

80

100

120

1,84 1,85 1,86 1,87 1,88 1,89 1,9 1,91 1,92

Molar Enthalpy, H, Kcal/Kgmole, (*104)

Tem

pera

ture

, T,

o C

TH Curve

Figure 3.6 Temperature-Enthalpy diagram of Joule-Thomson method

Temperature-Entropy Diagram

0

20

40

60

80

100

120

149 150 151 152 153 154 155 156 157 158

Molar Entropy, KJ/KgmoleoC

Tem

pera

ture

, T,

o C

TS Curve

Figure 3.7 Temperature-Entropy diagram of Joule-Thomson method

Pressure-Entropy Diagram

0

20

40

60

80

100

120

140

160

180

200

147 148 149 150 151 152 153 154

Molar Entropy, S, KJ/kgmoleoC

Pres

sure

, P, b

ar

PS Curve

Figure 3.8 Pressure-Enthalpy diagram of Joule-Thomson method

Pressure-Entropy Diagram

0

20

40

60

80

100

120

140

160

180

200

147 148 149 150 151 152 153 154

Molar Entropy, S, KJ/kgmoleoC

Pres

sure

, P, b

ar

PS Curve

Figure 3.9 Pressure-Entropy diagram of Turbo-Expander method

Pressure-Temperature Diagram

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70

Temperature, T, oC

Pres

sure

, P, b

ar

PT Curve

Figure 3.10 Pressure-Temperature diagram of Turbo-Expander method

TH Diagram

0

20

40

60

80

100

120

1,856 1,857 1,858 1,859 1,86 1,861 1,862 1,863 1,864 1,865

Molar Enthalpy, H, Kcal/Kgmole, (*104)

Inle

t Tem

pera

ture

, T, o C

TH Curve

Figure 3.11 Temperature-Enthalpy diagram of Turbo-Expander method

Temperature-Entropy Diagram

0

20

40

60

80

100

120

146 147 148 149 150 151 152 153 154

Molar Entropy, S, KJ/KgmoleoC

Inle

t Tem

pera

ture

, o C

TS Curve

Figure 3.12 Temperature-Entropy diagram of Turbo-Expander method

Enthalpy-Entropy Diagram

1,856

1,857

1,858

1,859

1,86

1,861

1,862

1,863

1,864

1,865

146 147 148 149 150 151 152 153 154

Molar Entropy, S, KJ/KgmoleoC

Mol

ar E

ntha

lpy,

H, K

cal/K

gmol

e, (*

104 )

HS Curve

Figure 3.13 Enthalpy-Entropy diagram of Turbo-Expander method

Polytropic Efficiency at different pressure

72,4

72,6

72,8

73

73,2

73,4

73,6

73,8

74

74,2

0 20 40 60 80 100 120 140 160 180 200

Pressure, P, bar

Poly

trop

ic e

ffic

ienc

y,%

efficiency-P curve

Figure 3.14 Polytropic efficiency at different pressure in turbo expander

Polytropic efficiency at different temperature

72,9

72,95

73

73,05

73,1

73,15

73,2

73,25

73,3

73,35

0 20 40 60 80 100 120 140

Temperature, T, oC

Poly

trop

ic E

ffic

ienc

y,%

efficiency-T curve

Figure 3.15 Polytropic efficiency at different temperature in turbo expander

Energy production at different pressure

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160 180 200

Pressure, P, bar

Ener

gy p

rodu

ctio

n, Q

, KW

Q-P curve

Figure 3.16 Energy production at different pressure in turbo expander

Energy production at different temperature

0

50

100

150

200

250

300

350

400

450

500

0 20 40 60 80 100 120 140

Temperature, T, oC

Ener

gy p

rodu

ctio

n, Q

, KW

T-Q curve

Figure 3.17 Energy production at different temperature in turbo expander

Comparison of water removal

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140 160 180 200

Pressure, P, bar

Wat

er re

mov

al ra

te, K

gmol

e/hr

.

J-T MethodTurbo Expander Method

Figure 3.18 Water removal in Joule-Thomson and turbo expander methods (At different pressure)

Comparison of Water Removal

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120 140

Temperature, T, oC

Wat

er R

emov

al ra

te, K

gmol

e/hr

J-T Method

Turbo Expander Method

Figure 3.19 Water removal in Joule-Thomson and turbo expander methods (At different temperature)

4 Discussion Joule-Thomson and Turbo Expander methods were analyzed to compare their dehydration performance, operating efficiency and thermodynamic properties. These could finally give a conclusion on selecting a proper method for gas dehydration.

4.1 Joule-Thomson Method All the calculations were based upon 1000 Kgmole of gas and 20 Kgmole of water initially. At first, the whole process was analyzed with initial pressure of 150 bar and final pressure 65 bar. Initial temperatures were varied from 70oC to 120oC. In each case, outlet properties e.g. temperature, enthalpy, entropy and gas composition were observed. Then the same observation was made by changing pressure from 100 to 175 bar keeping the temperature constant at 90oC.

4.1.1 Advantages of Joule-Thomson Method This method is very simple and operation is very easy. It can operate efficiently. Isenthalpic operation is possible what was obtained from Hysys. It is shown in Table 3.1 and 3.2. It can remove more water at lower temperatures and higher pressures as shown in figure 3.18 and 3.19.

4.1.2 Limitations Joule-Thomson Method The major limitation is that, it can not cool the gas as low as Turbo expander can do. Since it operates isenthalpically, it does not produce any energy. It is not a good choice when high level of dehydration is required. Controlling of the valve opening may be a problem; here opening was fixed at 50%. A significant amount of energy may be required for the plant operation.

4.2 Turbo Expander Method Similar to Joule-Thomson method, all the calculations were based upon 1000 Kgmole of gas and 20 Kgmole of water initially. At first, the whole process was analyzed with initial pressure of 150 bar and final pressure 65 bar. Initial temperatures were varied from 70oC to 120oC. In each case, outlet properties e.g. temperature, enthalpy, entropy, energy production and gas composition were observed. Then the same observation was made by changing pressures from 100 to 175 bar keeping the temperature constant at 90oC. Hysys fixed its adiabatic efficiency at 75% and polytropic efficiency decreased with the increase of temperature and pressure.

4.2.1 Advantages of Turbo Expander Method This is a modern method for gas dehydration. Its mechanical part can produce considerable amount of energy in the form of heat that can minimize energy cost for the plant operation. So it can offer higher temperature drop than Joule-Thomson method and eventually higher water recovery is possible. This method is a good choice when high water recovery is desired. At higher pressures and temperatures it creates higher temperature drop and consequently higher energy production and water recovery are achieved. It offers higher water recovery at high pressure up to 160 bar.

4.2.2 Limitations of Turbo Expander Method In ideal case, it is an isentropic process. But in reality, the expansion can not completely approach the isentropic case but produce a high percentage of the ideally possible work. Table 3.3 and 3.4 showed that entropy was decreasing with the increase of inlet pressure and it was increasing with increase of temperature. Polytropic efficiency increases with the increase of temperature and decrease of pressure. It can remove more water at high temperatures but then water composition at the gas stream of the separator outlet becomes high. Since this type of plant is expensive, it is not wise to choose when less water recovery is desired.

5 Conclusion Gas composition was the key data to start this simulation. Here Troll normalized gas composition (in mole fraction) data were used. Some input data such as inlet molar flow, pressure and temperature were assumed to specify the systems. Each system needed some assumption before they were simulated by Hysys. Hysys can calculate varieties of chemical, physical and thermodynamic properties by using some input data. Here Hysys generated outputs were enthalpy, entropy, outlet gas composition, energy recovery, efficiency etc. which were essential for describing any process for gas dehydration. Among many available dehydration processes, Joule-Thomson and Turbo Expander type processes were compared from both technical and thermodynamic point of view. Both Joule-Thomson and Turbo Expander methods are LTS type processes. Natural gas is expanded in these processes and consequently a rapid temperature drop is achieved. Due to this cooling, some water condenses out from water saturated gas. In Joule-Thomson method, gas passed through a choke type valve where its pressure dropped rapidly. A temperature drop was achieved immediately due to isenthalpic Joule-Thomson effect. The turbo expander is a mechanical device that produces work by expanding the feed gas stream from its initial high pressure to a lower pressure level. In the ideal case the expansion is isentropic. As mechanical work is produced the enthalpy of the gas is decreased. In reality the expansion can not completely approach the isentropic case but produces a high percentage of the ideally possible work. The expansion and reduction in enthalpy lowers the temperature of the gas which results in partial condensation of water. By contrast expansion across a valve is isenthalpic producing no work. Resulting temperatures are not as low as those achieved by the expander and less condensation of water takes place. A Joule-Thomson valve could be installed in parallel with the expander. This could be used during start-up and times of maintenance on the turbo expander. They might also be operated in parallel if there is too much gas for the expander. The performance of the combined Joule-Thomson and Turbo expander type processes can be studied in future because it may offer better water removal with a safe and uninterrupted operation.

6 References

(1) Ryba A., “Reduction in emissions and energy use at maćkowice natural gas dehydration facility”, Unpublished Diploma Thesis, 2005.

(2) Bloch Heinz P., Soares C., 2001, “Turboexpanders and process applications”, Gulf Professional Publishing, PP 3-6, 19-21.

(3) Bradley H. B., “Petroleum Engineering Handbook”, Third edition, Society of petroleum engineers, P 14-1, P 14-3, P 14-5, P 14-6, P 14-7, P 14-8.

(4) Campbell J. M., Lilly L. L., Maddox R. N., “Gas conditioning and processing Volume-2: The equipment modules”, Seventh edition, Campbell Petroleum Series, PP 252-258.

(5) Dorsett L. R., 1989, “LTX: Reapplication of Proven Technology”, SPE 19080, PP 1-3.

(6) Holman J.P., 1988, “Thermodynamics”, Fourth edition, McGraw-Hill Book Company, PP 160-162, 195.

(7) General Information About Hysys, Aspen Technology Inc, 2004, www.aspentech.com,

(8) Perry R.H., 1984, “Perry’s Chemical Engineers’ Handbook”, Sixth edition, McGraw-Hill Book Company, P 12-49, P 12-50, P 12-51.

(9) Rojey A., Jaffret C., Cornot-Gandolphe S., Durand B., Jullian S. and Valais M., 1997, “ Natural Gas Production processing and Transport”, Editions Technip, PP 252-276.

(10) Rose I., Robinson T., 1981, “Offshore gas conservation utilizing a turbo-expander based refrigeration extraction cycle”, OEB1 SPE 10391.1, PP 3-7.

(11) Maddox R. N., Bretz E., 1976, “Turbo-Expander Applications in Natural Gas Processing”, Journal of Petroleum Technology, PP 611-613.

(12) http://www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html (12.10.2005).

(13) http://www.ipt.ntnu.no/~jsg/undervisning/naturgass/GasCompositionExamples.pdf (18.09.2005).

(14) http://www.rwe-dea.com/en/172.htm (10.09.2005).