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The composition of natural gas varies depending on the field, formation, or reservoir from which it is produced. Since the composition of natural gas is never constant, there are standard test methods that can be used to determine the composition of natural gas and thus prepare it for use. The impurities contained in the gas are led to the formation of hydrates causing the blockage of the pipelines. Determining the optimal technological parameters of the drying plant and recommending a good choice of drying and glycol regeneration process will improve the field gas preparation system. Absorption using triethylene glycol (TEG) is one of the most popular methods for the dehydration of natural gas. However, the thermodynamic parameters of the gas as well as the parameters of the TEG always remain conditions to be studied to ensure an optimization of the process of regeneration of the glycol. The available experimental data were selected and correlated using a thermodynamic model based on the Peng-Robinson equation of state. The results obtained show that the model provided is a valuable tool for the design of the natural gas dehydration process.
Department of Petroleum Engineering, Tomsk Polytechnic University, Russia
*Address all correspondence to: fatimatatall29@gmail.com
1. Introduction
Absorption is known to be the most common drying method among the various gas drying processes.
During the absorption process, water vapor in the gas stream is absorbed in the liquid solvent stream. The liquid solvents used are glycols and the most commonly used are triethylene glycol and diethylene glycol.
The main technological parameters of the absorption process are temperature, pressure, the nature of the absorbent, the concentration of the regenerated absorbent, and the frequency of circulation of the absorbent.
However, according to several studies, TEG is the most used because of its ease of regeneration at concentrations from 98 to 99.9% as they approach properties that meet the criteria for commercial use [1, 2].
In general, the choice of the process mode and absorbent depends on the required depression of the gas dew point.
The depth of gas drying from moisture significantly depends on the concentration of glycol at the inlet to the absorber.
To deepen gas drying, vacuum desorption of moisture from glycol is used (at a pressure of 0.060–0.080 MPa and a temperature of about 205°C). The concentration of regenerated glycol, in this case, increases to 99.5%, and the dew point depression increases to 50–70°С [3].
The aim of this study is to dehydrate natural gas and evaluate the influence of the parameters involved in the absorption process in order to be able to optimize the absorption process using the most optimal parameters in accordance with the desired level of dehydration.
At present, di- and tri-ethylene glycol are used as absorbents for gas preparation [4].
The choice of solvent depends on the application. Typically, the solvent must meet the following criteria—high solubility of the solute to reduce the amount of solvent used; low volatility to reduce losses; does not cause corrosion; low viscosity for a high rate of mass transfer; safe; does not foam.
Glycols meet these criteria in the case of gas dehydration. The most popular glycol used for offshore gas processing is triethylene glycol (TEG).
The efficiency of natural gas dehydration by the absorption method depends on the nature of the absorbent, the concentration of the absorbent at the inlet to the absorber, the circulation rate of the absorbent, and the thermodynamic parameters of absorption [5].
This process is a “traditional” dehydration process based on triethylene glycol (TEG). The goal is to reduce the amount of water in natural gas using TEG, which is used as an extraction solvent. This process is necessary to avoid the formation of hydrates at low temperatures or corrosion problems due to the presence of carbon dioxide or hydrogen sulfide which are regularly found in natural gas.
The effectiveness of the gas dehydration method is measured by the water content in the dry product gas. Most gas sale contracts specify the maximum amount of water vapor allowed in the gas. The maximum amount of water vapor is estimated at 7 lbs./MMscf. Standard gas specifications also limit the water dew point to −10°C to ensure flow in export pipelines on the seabed [6]. Distribution specifications depend on the geographic region in which they are applied. For example, in Russia, the dew point temperature of the water during winter is −20°C, and during summer it is −14°C for natural gas at a pressure between 4 and 7 MPa [7].
The composition, temperature, and pressure of the Yamburg X-field gas were selected to simulate wet gas dehydration in Unisim Design R460.
Table 1 represents the composition of the gas of the X field and Table 2 represents the parameters of the gas and the triethylene glycol.
The moisture content of the gas is 600 mg/m3, this value exceeds the specification (100 mg/m3), so the gas must be recycled.
We accept the multiplicity of the TEG, equal to 1.500 m3/h, the mass percentage is 99.7%.
The mass flow rate of moisture in the gas is calculated by the formula:
Qm.=Wi×VE1
where V is the volumetric amount of hydrocarbon raw materials, m3/h.
Wi—initial moisture content.
In most installations, the rate of the TEG absorbent is 1–10 m3/h and depends on the degree of gas purification [8].
Unisim’s simulation of a typical absorption gas dehydration process is shown in Figure 1 and combines gas absorption and glycol recovery.
Figure 1.
Gas dehydration process in Unisim.
The drying process illustrated in Figure 1 is explained below.
The wet gas is first directed to a valve that operates to reduce the gas pressure.
The wet gas enters the absorber column from below and the lean TEG enters from the top, falling into the absorber when it comes into contact with the wet gas flowing upward. Wet gas enters the T100 contactor at a pressure of 3862 kPa. This column absorbs part of the water contained in the gas thanks to the presence of triethylene glycol.
The dried gas is directed to the valve and then leaves it at a pressure of 3462 kPa.
The rich TEG leaves the bottom through the level control, passes through a valve where it is depressurized, and exits at a pressure of 3080 kPa.
The TEG containing water and hydrocarbons is sent to a stripping column (T101). The regenerator distillation column includes a reboiler operating at 205°C to prevent degradation of the TEG. The outgoing gases are condensed by the condenser, so the water vapor is directed to the outlet. The regenerated TEG sent to the bottom of the column (Regen Bottom), passing through the heat exchanger, will be sent to the mixer and mixed with a quantity of glycol to compensate for the losses of glycol.
2.1 Simulation results
The simulation results allow us to know a large number of process parameters, as well as the dew point temperature of the gas after absorption, which will allow us to know if the gas meets the conditions of the transport specification.
At a dry gas pressure of 3462 kPa, the gas dew point temperature is −26°С (Table 3).
2.1.1 Absorption column profile
We see that the temperature gradually increases, starting from the 2nd stage to the 9th stage, and then decreases to the 10th stage. This can be explained by the fact that as the gas goes up to meet the glycol, and heat transfer becomes more and more important; and this allows the glycol to absorb more water from the gas before saturation. At stage 10, when the glycol loses its ability to absorb water the temperature drops.
As shown, the pressure increases when moving up the stage, this is because the absorption phenomenon occurs at high pressure, in contrast to the regeneration phenomenon.
Temperature dew point
After drying, the gas comes out with a composition and pressure different from the original composition and initial pressure. The dew point temperature of this new gas composition can be determined by using the Unisim software (Tables 4 and 5).
Composition
Mass percentage
N2
0.0018
CO2
0.065
CH4
0.81
C2H6
0.054
C3H8
0.036
n-C4H10
0.0095
i-C4H10
0.019
Sum
1
Table 1.
Gas composition.
Parameters
Values
Wet gas
TEG 99.7%
Temperature
25°С
25°С
Pressure
4 MPa
3.800 MPa
Flow rate
330,000 m3/hr
(1700 kg/hr)
Initial water content
0.600 g/m3
Table 2.
Parameters of the initial wet gas and TEG.
Stages
Temperature (°С)
Pressure (kPa)
Pure liquid (kgmol/hr)
Pure vapor (kgmol/hr)
1
23.53
3600
13.24
2
23.54
3611.11
14.19
2.4128
3
23.55
3622.22
15.06
2.4129
4
23.56
3633.33
19.91
2.413
5
23.59
3644.44
16.78
2.4131
6
23.61
3655.55
17.72
2.4132
7
23.62
3666.66
18.8
2.4133
8
23.628
3677.77
20.14
2.4134
9
23.621
3688.88
21.94
2.4135
10
23.6
3700
2.4137
Table 3.
Profile of the absorption column.
Composition
Molar fraction
N2
0.001181
CO2
0.027063
CH4
0.915635
C2H6
0.032489
C3H8
0.014767
n-C4H10
0.002954
i-C4H10
0.005907
H2O
0.000025
TEG
0.000000
Table 4.
Composition of dry gas.
Water dew point temperature
−26°С
Moisture contents
0,015 г/м3
Critical temperature
−63°С
Critical pressure
6170 кПа
Critical compressibility
0.300
Table 5.
Composition of dry gas.
2.1.2 Regeneration column profile
Triethylene glycol is recovered in a distillation column containing a reboiler and a condenser. The reboiler temperature is 205°C to avoid decomposition of the glycol, and the condenser is at 102°C because the boiling point of water is 100°C. The reboiler is pressurized to below atmospheric pressure to increase the glycol purity. Under these conditions, glycol is recovered by 98.06% in molar percentage or 99.10% in mass percentage (Table 6).
Distillation type
Glycol concentration (%)
Glycol loss (kg/hr)
Atmospheric pressure (kPa)
100
93.420
0.665
Vacuum pressure (kPa)
80
94.423
1.242
60
96.512
2.313
35
98.061
5.701
Table 6.
Concentration of glycol after regeneration.
2.2 Analysis and optimization of absorption process
An analysis of the various parameters involved in the absorption process will allow us to better understand the effect of these variables on the process. The result of this analysis will allow us to select variable parameters to obtain the most optimal process. The most important process parameters are the solvent flow rate in the absorber, the gas contact temperature, the number of equilibrium stages in the absorber, and the stripping gas used (if a very concentrated TEG is required).
2.2.1 Influence of TEG flow rate on dry gas content
To determine the influence of the number of equilibrium stages in the absorber, as well as the TEG flow, the feed gas is taken into account and the TEG flow rate and the number of stages are varied. The water content of the gas will be calculated for each option. The TEG flow rate was varied between 1500 and 3000 kg/h (Figure 2).
Figure 2.
Influence of TEG flow rate on dry gas content.
The result of the analysis shows that the water content in the dry gas decreases with an increase in the circulation rate of the triethylene glycol.
It can be seen that starting from 1500 kg/h, the water content in the gas is 0.019 g/m3, and with an increase in the TEG consumption, the water content decreases.
With regard to the optimization of the TEG circulation, it can be seen that 1700 kg/h allows a water content of 0.015 g/m3 to be achieved, which corresponds to the specification for the water content in the gas. Therefore, it is preferable to reduce the amount from 2500 to 1700 kg/h and it will be more cost-effective from an economic point of view.
In addition, high circulation rates can overload the reboiler and prevent proper glycol regeneration. The heat required for the reboiler is directly proportional to the circulation rate. Thus, increasing the circulation rate can lower the reboiler temperature, reduce the lean glycol concentration, and actually reduce the amount of water that the glycol removes from the gas.
2.2.2 Influences of gas contact temperature on water dew point
To determine the effect of the gas contact temperature, the given feed gas is taken into account and the temperature is varied. The water dew point of the gas will be calculated for each option (Figure 3).
Figure 3.
Influence of gas contact temperature on water dew point.
The graph shows that as the gas contact temperature decreases, the amount of water contained in the gas after drying also decreases. This is due to the fact that low temperatures increase the capacity of absorbents, allowing them to absorb more water contained in the gas; therefore, the lower the gas temperature, the lower its equilibrium moisture capacity and then low water dew point. In addition, high temperatures increase TEG losses. The lower the process temperature, the lower the calculated concentration of glycol is used to obtain the target gas dew point. A decrease in temperature also leads to the destruction of the heat consumption for the operation of the regeneration unit, since the amount of water extracted from the gas decreases.
2.2.3 Effects of the stripping gas on TEG regeneration
After drying the gas, it is necessary to check the flows to determine at what temperature the gas will be hydrated.
The analysis of the conditions of hydrate formation of dry gas at a pressure of 3462 kPa and a temperature of 23°С conditions is given in Table 7.
Formation temperature
−22,18
Freezing temperature
−26,54
Type of hydrate
types I and II
Table 7.
Hydrate formation conditions.
At a pressure of 3462 kPa and a temperature of 23°C, dry gas can be transported without the formation of hydrates.
The formation of hydrates in process plants and pipelines can occur within a few minutes; therefore, it is necessary to know the range of hydrates formation.
Table 7 shows the range of temperatures and pressures at which hydrates are formed (Table 8).
Pressure (kPa)
Temperature (°C)
429
−38
908
−32
1838
−26
3431
−21
4362
−19
5469
−17
6058
−16,68
6419
−16,23
6616
−16,01
6703
−15,9
6722
−15,89
6694
−15,92
Table 8.
Thermodynamic range of hydrate formation.
The following figure allows better visualization of hydrate formation as a function of temperature and pressure (Figure 4).
Figure 4.
Diagram of the formation of hydrates.
The diagram shows the critical properties of the dry gas flow; critical temperature Tcr = −63.12°C, critical pressure Pcr = 6170 kPa.
The figure shows the temperatures and pressures indicated in Table 2 at which the hydrate is formed.
Thus, in order to ensure the transportation of gas without the formation of hydrates, it is necessary that the transportation be carried out outside these values of temperature and pressure.
2.2.4 Effects of the stripping gas on TEG regeneration
Stripping gas is an optional element that is used to achieve very high glycol concentrations that cannot be obtained with normal regeneration. This will ensure maximum dew point reduction and greater dehydration. It is also used to ensure intimate contact between hot gas and lean glycol after most of the water has been removed by distillation.
The concentration of the regeneration glycol depends on the rate of circulation of the stripping gas. We will analyze the effect of the boil-off gas flow rate adopted for dry gas on the glycol purity. The speed varies from 50 to 300 kg/hr (Figures 5 and 6).
Figure 5.
TEG concentration versus stripping gas velocity.
Figure 6.
TEG losses versus stripping gas velocity.
By analyzing the effect of the stripping gas flow rate, it can be seen that at a reboiler pressure of 100 kPa, the more the stripping gas flow rate increases, the more the glycol concentration increases, but on the other hand, there is little glycol loss. The amount of stripping gas produces a cleaner glycol but causes an increase in glycol loss.
To optimize the operation and from an economic point of view, it is recommended to work on a plant where the reboiler pressure is set at 100 kPa with a boil-off gas flow rate of 200 kg/h, which will allow us to recover glycol up to 99.62% with a loss of 7.5 g/1000 m3.
2.2.5 Regeneration of glycol with an azeotropic agent
The agent is used to form a ternary azeotrope with the mixture.
The DRIZO process provides glycol enrichment by using its own internal stripping medium, a mixture of paraffinic and aromatic hydrocarbons with a boiling range of C5 +.
The patent works with an isooctane solvent, but the typical composition of the stripping medium is aromatic hydrocarbons, naphthene, and paraffin.
In our study case, hexane is used to separate binary water and triethylene glycol.
Based on the DRIZO process principle, a 100% hexane hetero-azeotrope is used as our gas does not contain C5+ hydrocarbons (Figure 7).
Figure 7.
The process of regeneration using an azeotropic agent.
Azeotropic distillation was carried out at various reboiler pressure and the results allowed us to see the concentration of regenerated glycol and the loss of glycol according to the parameters (Table 9).
Reboiler temperature (°C)
Glycol concentration (%)
TEG glycol (kg/hr)
200
99.981
2.432
180
99.914
2.315
140
99.792
2.193
Table 9.
Results of the regeneration process.
Analyzing the results, we see that at atmospheric pressure and a reboiler temperature of 200°C, we can achieve a glycol concentration of 99.981% with a loss of 2.432 kg/h.
Lowering the temperature of the reboiler does not significantly reduce the TEG concentration; therefore, to reduce the heat consumption, the temperature of the reboiler can be set to 140°C.
The modeling of the absorption process was carried out using the Unisim program. The study of the various parameters allowed to identify their influence on the whole process in particular on the gas dew point and also on the glycol concentration after regeneration.
The simulation results showed that the parameters that have the greatest influence on the efficiency of the process are the gas contact temperature, the rate of circulation of triethylene glycol, as well as the stripping gas if regeneration of glycol with a high concentration is required.
A decrease of the gas contact temperature to 25°C led to a decrease in the amount of water in the dried gas.
A high TEG circulation rate results in dry gas with less water but can overload the reboiler and reduce good glycol recovery.
To increase the degree of TEG regeneration, a stripping gas and an azeotropic agent were used at atmospheric pressure.
With the stripping gas, the TEG was regenerated up to 99.71% by mass at a reboiler temperature of 205°C, while with the azeotropic agent the regeneration is 99.914% by mass for a temperature of 180°C and for a temperature of 200°C it is 99.981% by mass.
The particular conclusion of this study is that increasing the temperature of the reboiler does not allow a great increase in the purity of the regenerated triethylene glycol; therefore, to reduce the heat consumption, the temperature of the reboiler can be reduced.
References
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2.Gironi F, Maschietti M, Piemonte V. Natural Gas Dehydration: A Triethylene Glycol-Water System Analysis. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects. 2010;32(20):1861-1868
3.Eirini GP, Cristina C, Georgia DP, Giuseppe C, Epaminondas CV. Sensitivity analysis and process optimization of a natural gas dehydration unit using triethylene glycol. Journal of Natural Gas Science and Engineering. 2019;71:1875-5100
4.Nikolaev VV, Busygina NV, Busygin IG. The main processes of physical and physicochemical gas processing. Moscow: Nedra Publishing House; 1998. p. 184
6.Demirbas A. Methane Gas Hydrate. London: Green Energy and Technology; 2010. p. 192
7.STO Gazprom 089-2010. Combustible natural gas supplied and transported through main gas pipelines. Technical conditions. Moscow: JSC “Gazprom”; 2010. p. 19 (In Russian)
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Written By
Fatimata Tall
Submitted: 03 September 2021Reviewed: 11 September 2021Published: 28 June 2022
Open access peer-reviewed chapter
