NATURAL GAS HYDRATE PREVENTION BY USING COMBINATION …

NATURAL GAS HYDRATE PREVENTION BY USING COMBINATION OF TWO
THERMODYNAMIC HYDRATE INHIBITORS {MEG+NACL} & {MEG+KCL}
M. RAMADAN1, M. KAOUD1, S. ALY2 M. E. AWAD2
1Khalda Petroleum Company, EGPC, Petroleum Ministry, Egypt
2Chemical Eng Department,Faculty of Petroleum And Mining Eng, Suez University
ABSTRACT
The gas hydrates cause serious operational and safety problems to oil and gas industry. Hydrates can plug gas
pipelines, wellbore and processing units leading to production loss, pipelines rupture and explosion. Many chemicals
injected to gas pipeline to decrease hydrate formation temperature and prevent hydrate formation. The most chemicals
used are thermodynamic hydrate inhibitors (THIs) like mono-ethylene glycol and methanol. Salt is one of
thermodynamic hydrate inhibitors and have the same effect as methanol and MEG. The aim of our work is to perform
a simulation study to detect the effect of mixed THIs like (MEG+NaCl) and (MEG+KCl) on gas hydrates at various
pressures and temperatures. We studied the hydrate inhibition effect for each inhibitor alone compared the results
with the mixed THI. The effect on hydrate formation temperature and the required amount of inhibitor for different
cases were studied. The simulation software used to perform this study was Aspen HYSYS. The data used in our
research are collected from a gas field located at north of Egypt. The results show that hydrate inhibition is improved
by using combination of two thermodynamic inhibitors.
KEYWORDS: Hydrate inhibition, Thermodynamic hydrate inhibitors, Mono-ethylene glycol, hybrid hydrate
inhibition, Salt, HYSYS simulation.
Received: Jun 09, 2020; Accepted: Jun 29, 2020; Published: Sep 21, 2020; Paper Id.: IJMPERDJUN20201359
1. INTRODUCTION
Gas hydrates are nonstoichiometric ice-like crystalline solid compounds and considered as a part of Catharses family. Gas
hydrates formed by combination of water (host) and gas molecules (guest) like CO2, ethane, methane and propane at the
right pressures and temperatures (1, 2). Hydrates structure stabilized through weak Vander Waals interaction between
the host water molecules and the guest gas molecules and no chemical reaction exist (3). There are three different
types of gas hydrates namely cubic structure I or type I (sI), cubic structure II or type II (sII) and hexagonal
structure H or type H (sH) (4). Hydrate formation increased at High pressure and low temperature. Serious
operational, economical and safety problems result from hydrate formation. Hydrate blockages will form in
pipeline, valves, wellbore and processing units result in shut down, production and financial loss. Many hazards
caused by hydrate plugs as they can move at speed of 300 Km/h (5). The removal and dissociation of hydrate is
a high cost and time-consuming process that may take several weeks. The cost of hydrate prevention is $1 billion
of annual production costs of oil and gas industry (6). The technologies used for gas hydrate inhibition include
water removal (dehydration), Thermal heating, reducing the system pressure and injecting chemical inhibitors (7). The
most common prevention method used is chemical compounds injection. Chemical inhibitors used are Low dosage hydrate
inhibitors (LDHIs) and thermodynamic hydrate inhibiters (THIs) as (alcohol, salt). Low dosage hydrate inhibitors (LDHIs)
O rig
in a
Engineering Research and Development (IJMPERD)
ISSN(P): 2249–6890; ISSN(E): 2249–8001
Vol. 10, Issue 3, Jun 2020, 14243–14254
© TJPRC Pvt. Ltd.
14244 M. Ramadan, M. Kaoud, S. Aly & M. E. Awad
Impact Factor (JCC): 8.8746 SCOPUS Indexed Journal NAAS Rating: 3.11
are categorized to Kinetic hydrate inhibitors (KHIs) and Anti-Agglomerants(AA). Kinetic hydrate inhibitors (KHIs) are
used to postpone hydrate nucleation and growth. Anti-Agglomerants(AA) are used to maintain hydrate crystals dispersed
in oil phase. Thermodynamic hydrate inhibiters (THIs) like (methanol, mono-ethylene glycol and salts) are used to inhibit
hydrate formation by changing the equilibrium conditions that hydrate will form in lower temperatures and higher pressures
(8). Hydrate prevention with methanol and mono-ethylene glycol is costly because of high dosage required (10% to 60%
of the water phase).The high injection rate of methanol may cause pipeline corrosion and environmental prohibitive (9).
Methanol can mix with liquefied petroleum gas (LPG) producing azeotropes which are difficult to separate (10). Both
MEG and methanol have safety and environmental restrictions. This leads to searching for new techniques to reduce the
quantity of MEG and methanol used in gas hydrate prevention. Hydrate inhibition is enhanced by mixing two or more THIs
or THI and LDHI and the required inhibitor dosage is decreased. This combination is called Hybrid hydrate inhibition
(HHI) (11, 12).
2.1. Data gathering
In our research, the data used was gathered from a gas field located at north of Egypt. We modified the collected data to
test different simulation parameters to achieve the goal of our study.
2.1. 1. Gas Stream Composition
Table 1 shown the dry base composition of the natural gas stream included in this simulation study. We assumed most
sever operating conditions for hydrate calculation (13). To calculate the water amount we assumed that gas stream is
saturated gas at reservoir conditions (10). The water quantity was determined from McKetta–Wehe Chart.
Table 1: Components of the Natural Gas Produced from the Gas Field
Molar
Molar
H2S 0.000199 Benzene 0.00003 N-NONANE 0.000123
Methane 0.983844 Cyclohexane 0.00001 124-
MBenzene 0.00002
N-Pentane 0.00011 M - Xylene 0.00006
2.1.2. Gas Parameters
Gas stream parameters at reservoir, wellhead and slug catcher separator are shown in Table 2.
* Water flow rate was calculated at reservoir conditions from McKetta–Wehe Chart and found to be qw = 151.2 kg/h. We
made the simulation study at various Temperatures and pressures for gas stream at slug catcher. Hydrate inhibiter is mixed
with the wellhead fluid before choke valve to get better mixing. The chemical inhibitor injected with same pressure and
temperature of the fluid at wellhead (p =300 bar & T = 41.4 C). The MEG rate used in the field to prevent hydrate is 220
kg/h. We tested various injection rates of MEG for better simulation study. It is assumed that lean thermodynamic hydrate
inhibitor (THI) concentration is 100 %. It is not applicable to inject salt solution as separate stream in HYSYS but the effect
Natural Gas Hydrate Prevention by Using Combination of Two Thermodynamic 14245 Hydrate Inhibitors {Meg+Nacl} & {Meg+Kcl}
www.tjprc.org SCOPUS Indexed Journal [email protected]
of salt is studied from stream analysis option (hydrate study). Natural gas stream is transferred to onshore facility via a
seabed pipeline.
Table 2: Parameters of the Natural Gas Stream Produced from the Field
Stream
Flow Rate
2.2. Process Simulation
Several software packages have been developed for hydrate formation prediction such as CSMHYD, PVTSim, EQUI-
PHASE Hydrate (14). The most simulation software programs used for hydrate prediction and study are Promax, Hysys
and VMGSim. We used HYSYS simulation program to complete our simulation study (10). Aspen HYSYS software is a
process modeling program powered by aspen tech. HYSYS offer a lot of thermodynamic packages for process calculation
(15).
2.2.2. Simulation Model
Fluid package choice is the first step to use HYSYS. We selected Peng-Robinson package as it is recommended for oil
and gas systems and gives high amount of accuracy (16). The next step is to input the components required in the simulation.
Gas mixture component list mentioned before in table 1 plus H2O and MEG are added to HYSYS components. The next
step is to select the input data measuring units. We used SI units in this work. Composition and operating conditions for
gas mixture stream is needed for the simulation. We used the data gathered from the gas field. Simulation process for gas
transportation from the offshore well to the onshore facility is shown in Fig 1.
Figure 1: “Print Screen” Of HYSYS Simulation for the Gas Transportation Process.
2.2.3. Hydrate Calculation Model
Reference to thermodynamic, the hydrate formation process is occur in two steps. First step is transforming pure water to
an empty hydrate cage. This is a hypothetical step and necessary for calculations. The next step is filling the lattice of
hydrate. The process is like the following:
Pure water (α) empty hydrate crystal (β) filled hydrate crystal (H)
The chemical potential change for this process is illustrated in Eq. (1). (10, 18)
(1)
Where µH & µα & µβ are the chemical potential for the various
phases. The term (µH - µβ) represents the hydrate crystal stabilization. This term distinguishes the hydrate calculation
models as different models is used to estimate it. The term (µβ - µα) explains the water change from phase to phase and
(
(2)
Where R is universal gas constant, P is system pressure, T is system absolute temperature, v is molar volume, H is enthalpy,
the subscript O is a reference state, and Δ term is the phase transfer of water from its pure state to a hydrate state (type I or
type II).
These basic steps are included in most of hydrate calculation models such as Ng and Robinson hydrate model (18). Hydrate
calculations in HYSYS is based on Ng and Robinson model. This model is used to figure the hydrate formation in
equilibrium with hydrocarbon liquids. For calculating brine solution effect on Hydrate formation Hu Lee Sum correlation
was used in HYSYS as shown in Fig.2.
Fig 2: “Print Screen” of the Option Used in HYSYS to Study the Electrolyte Impact On Hydrate Formation.
2.2.4. Simulation Study Steps
Using HYSYS to study the effect of mixed thermodynamic hydrate inhibitors (MEG+NaCl) and (MEG+KCl) on hydrate
formation the following steps were used:
In the presence of pure water and without adding the inhibitors, hydrate formation temperature T0 and the required
inhibitor rate (MEG rate) to prevent hydrate formation are evaluated at various operating conditions.
In the presence of salt solution, hydrate depression temperature (salt ΔT) and the required inhibitor rate (MEG
rate) to prevent hydrate formation are calculated at various operating conditions.
In the presence of MEG, hydrate depression temperature (MEG ΔT) are evaluated at various operating conditions.
sum up MEG ΔT and Salt ΔT to get Total ΔT
Using a method of J.M. Campbell. (17), In the presence of the mixed inhibitor (MEG + salt), the hydrate formation
temperature is computed by deducting total ΔT from T0.
Using HYSYS program, In the presence of the mixed inhibitor (MEG + salt), the hydrate formation temperature
is evaluated.
We made comparison between required MEG rate in case of pure water and required MEG rate in case of salt
injection.
We made comparison between hydrate formation temperature at pure water T0 and the hydrate formation
temperature when (salt or MEG or (MEG+ salt)) is injected to the system.
* We studied two types of salts (NaCl and KCl), and the previous steps were done for both types.
3. RESULTS AND DISCUSSIONS
3.1. Effect of Mixed Inhibitor (MEG + salt) on Hydrate Formation Temperature of Gas Mixture
Table 3 shown the detail of simulation calculations of every inhibitor to the hydrate formation temperature for the gas
stream at various parameters (pressures, temperatures and combined inhibitor concentrations). Table.3 also presents a
comparison between the accuracy of HYSYS simulation Method with J.M. Campbell method (17). The last column in the
table represents the hydrate formation temperature calculated from HYSYS when mixed inhibitor is used. The column
before last one represents the hydrate formation temperature for mixed inhibitor calculated when method of J.M. Campbell
is used.
Table 3: Hydrate Formation Temperature of gas Mixture when Inhibitor or Mixed Inhibitor is used at Various
Operating Conditions.
Fig.3 (a) shown the effect of NaCl at 15wt% concentration and effect of MEG at 20.5wt% concentration on hydrate
formation temperature of the gas mixture. The hydrate formation temperature at pure water and the hydrate inhibition effect
of mixing 15wt% NaCl with 20.5 wt% MEG are shown in the same Figure. We can see the enhancement in hydrate
prevention and the decrease on hydrate formation temperature in case of the mixed inhibitor is used. Fig.3 (b) shown the
hydrate prevention effect of injecting 12wt% NaCl, 30wt%MEG and (12.2wt% NaCl + 30wt% MEG) to the system. We
replaced NaCl salt with KCl and investigated its effect. In Fig.3 (c) KCl at 10wt% and MEG at 23wt% is used to study the
change in hydrate formation temperature. The mixed inhibitor (MEG+KCl) can be used to decrease hydrate formation
temperature and show good results as the mixed inhibitor (MEG+NaCl). Fig.3 (d) shown the hydrate formation temperature
with 8wt% KCl, 35wt%MEG and the mixed (8wt%KCl+35wt%MEG).
Figure 3: a. Contribution of 15wt% NaCl and 20.5wt% MEG to the hydrate formation temperature of gas
mixture. b. Contribution of 12wt% NaCl and 30wt% MEG to the hydrate formation temperature of gas mixture.
C. Contribution of 10wt% KCl and 23wt% MEG to the hydrate formation temperature of gas mixture. d.
Contribution of 8wt% KCl and 35wt% MEG to the hydrate formation temperature of gas mixture.
3.2. Comparison between the Effect of (MEG+NaCl) and (MEG+KCl) on Hydrate Formation Temperature
Comparison between the hydrate inhibiting effect of (MEG+NaCl) against (MEG+KCl) at same concentrations and
operating conditions is shown in Table.4.The mixed (NaCl+ MEG) have more decrease in hydrate formation temperature
than (KCl+MEG) with the same concentration as shown in Fig.4.
Table 4: Comparison between Hydrate Formation Temperature of NaCl /KCl at Same Concentration 15wt % and
MEG Concentration 20.5wt%
NaCl KCl NaCl KCl NaCl KCl NaCl KCl NaCl KCl
82 -10 13.97 5.69 7.7 7.25 8.28 6.27 6.72 15 12.99 -1.03 0.98 0.237 2.09
90 -6.7 14.81 6.49 8.5 8.06 8.32 6.31 6.75 15.07 13.06 -0.26 1.75 1 2.87
115 2.18 16.96 8.51 10.56 10.1 8.45 6.4 6.86 15.31 13.26 1.65 3.7 2.94 4.84
140 9.24 18.59 10.06 12.12 11.64 8.53 6.47 6.95 15.48 13.42 3.11 5.17 4.42 6.34
165 14.92 19.9 11.3 13.3 12.8 8.6 6.6 7.1 15.7 13.7 4.2 6.2 5.61 7.5
Figure 4: Contribution of (15wt% KCl + 20.5wt% MEG) and (15wt% NaCl + 20.5wt% MEG) to the Hydrate
Formation Temperature of Gas Mixture.
3.3. Effect of Salt Solution on Required MEG rate to Prevent Hydrate Formation
Table.5 illustrates the required MEG rate to prevent Hydrate formation at various operating conditions in case of pure water
(no salt added).Table.5 also shown Comparison between required MEG rate at pure water and required MEG rate in case
of adding salt (NaCl or KCl) at different concentrations. Results show that required MEG rate to prevent hydrate formation
is decreased by salt solution injection. From results we found that in some cases presence of salt alone was enough to
prevent hydrate formation and there was no need to inject MEG as shown in Fig.5 (a-b).
Fig.5 (a) shown the required MEG quantity to prevent hydrates at various parameters in case no salt is added to
the system. The effect of NaCl with (15wt % and 12wt %) concentration on the required MEG rate to prevent hydrates at
different pressures and temperatures is shown in same figure. As salt concentration increased, the required MEG rate
decreased. The depression on MEG quantity when salt injected to the system is due to the inhibition effect for the salt.
These is due to the fact that the resulting ions in the aqueous solution of the salt will reduce the chemical potential of
liquid water and for sufficiently high concentration of ions the water will be more stable as liquid water rather than
hydrate water. In Fig.5 (b) the effect of (10wt%and 8wt %) KCl on the required MEG to prevent hydrate formation is
shown. The required MEG quantity in case of pure water is shown in same figure. From Fig5.b we found that the more the
quantity of salt is injected to the system the less quantity of MEG is needed for hydrates inhibition.
Table 5: MEG Required Flow Rate to Prevent Hydrate Formation at Different Salts Types and Concentrations
Salt
Salt
Flow
Rate
kg/hr
NaCl 21
KCl 17
KCl 13.2
82 -10 13.97 10.88 191.16 148.08
90 -6.7 14.81 11.71 164.5 129.36
115 2.18 16.96 13.81 99.43 81.19
140 9.24 18.59 15.41 55.55 44.15
165 14.9 19.9 16.7 26.19 14.17
Figure 5.a: Effect of NaCl Salt on Required MEG Rate to Prevent Hydrate Formation. b. Effect of KCl salt on
Required MEG Rate to Prevent Hydrate Formation.
3.4. Comparison between the Effect of NaCl and KCl on Required MEG rate to Prevent Hydrate Formation
Table 6 shown the required MEG rate at same concentration (15wt %) of NaCl and KCl.in case of injecting NaCl to the
system The required MEG rate is smaller than that in case of injecting KCl.
In Fig.6 We can find effect of 15wt% NaCl and 15wt% KCl on required MEG rate.
Table 6: Comparison between NaCl /KCl Effect on Required MEG flow Rate at Same Concentration 15wt %
OP
after Adding Salt
82 -10 13.97 5.69 7.7 191.16 121.22 132.14
90 -6.7 14.81 6.49 8.5 164.5 102.02 113.09
115 2.18 16.96 8.51 10.56 99.43 51.44 63.52
140 9.24 18.59 10.06 12.12 55.55 10.85 24.54
165 14.92 19.9 11.3 13.3 26.19 0 0
Figure 6: Effect of 15wt% NaCl and 15wt% KCl on Required MEG Rate.
CONCLUSION
Hydrate prevention with mono-ethylene glycol and methanol is expensive (regarding operational cost) because of the high
dosage required with high concentration. New techniques were developed to reduce or replace thermodynamic hydrate
inhibitors (MEG and methanol). One of the new techniques used is combination between MEG and salt. The results of this
study showed that hydrate-inhibition is improved by mixing two THIs (MEG+NaCl) and (MEG+KCl). The results showed
that Combination of (MEG+NaCl) is preferred over (MEG+ KCl) combination with the same concentration and at the same
operating conditions. The HYSYS simulation Results were compared with results measured by J.M. Campbell method and
indicated a good agreement. Using mixed inhibitors will reduce hazards of MEG and reduce the potential of environment
pollution resulting from MEG disposal as the used quantity of MEG is reduced. The result showed that injecting 12.2wt%
NaCl to the gas stream reduces the required MEG quantity to prevent hydrate by 30.8%. When 15.15wt% NaCl was used
the MEG quantity was reduced by 36.7%. The effect of salt on pipelines and processing units’ corrosion, scale formation
and other related problems need further study in more details before applying the mixed inhibitor technique in the field.
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1. INTRODUCTION
2. METHODOLOGY

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NATURAL GAS HYDRATE PREVENTION BY USING COMBINATION …