Accepted Manuscript

Recovery of Monoethylene Glycol Combined with Kinetic Hydrate Inhibitor

Shurui Xu, Shuanshi Fan, Yanhong Wang, Xuemei Lang

PII: S0009-2509(17)30369-XDOI: http://dx.doi.org/10.1016/j.ces.2017.05.050Reference: CES 13637

To appear in: Chemical Engineering Science

Received Date: 19 January 2017Revised Date: 24 May 2017Accepted Date: 27 May 2017

Please cite this article as: S. Xu, S. Fan, Y. Wang, X. Lang, Recovery of Monoethylene Glycol Combined withKinetic Hydrate Inhibitor, Chemical Engineering Science (2017), doi: http://dx.doi.org/10.1016/j.ces.2017.05.050

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1

Recovery of Monoethylene Glycol Combined with Kinetic

Hydrate Inhibitor

Shurui Xu, Shuanshi Fan, Yanhong Wang*, Xuemei Lang

Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education,

School of Chemistry and Chemical Engineering, South China University of

Technology, Guangzhou 510640, China

Corresponding Author: Yanhong Wang *. Tel: + 86-20-22236581, Fax: +86-20-22236581,

E-mail: wyh@ scut.edu.cn

Authors

Shurui Xu: E-mail: xu.sr@mail.scut.cn

Shuanshi Fan: E-mail: ssfan@scut.edu.cn

Yanhong Wang: E-mail: wyh@ scut.edu.cn

Xuemei Lang: E-mail: cexmlang@scut.edu.cn

1

Abstract. Kinetic hydrate inhibitors (KHIs) combined with thermodynamic inhibitors (THIs)

such as monoethylene glycol (MEG) have been good additives for the prevention of hydrate

blockages in oil and gas industry operations. The regeneration and recycling of MEG are

conventional process steps used to reduce costs. However, the recovery of THIs in the presence of

KHIs or the recovery of the KHIs alone has rarely been investigated. In this paper, a series of

experiments was designed to study the recovery of both a KHI based poly (N-vinylcaprolactam)

and MEG. The results showed that the MEG recovery rate was closely related to the recovery

temperature, but was not influenced by the KHI. The MEG recovery rate from solutions consisting

of MEG and the KHI was as high as 94.52%, and the KHI was recovered along with the MEG.

The polymer structure of the KHI was rarely changed when the recovery temperature was close to

its polymerization temperature. The presence of the KHI had a negative impact on the

thermodynamic inhibition efficiency of the MEG. The KHI performance of the recovered solution

obtained at the KHI polymerization temperature could reach the level of the fresh combination

inhibitor, but the recovered solutions obtained at temperatures far above the KHI polymerization

temperature demonstrated worse inhibitory performance. The kinetic performance could be

restored by adding 5.0 wt% fresh MEG. MEG enabled a subcooling temperature decrease into the

range in which KHI which could play its role effectively, leading to the improved kinetic

performance of the recovered solution.

Keywords: Hydrate inhibitor recovery, monoethylene glycol, kinetic hydrate inhibitor, composite

inhibitor

2

1. Introduction

Natural gas hydrates are ice-like crystalline solids that contain small guest molecules in cages

formed by hydrogen-bonded water molecules[1, 2]

. Under appropriate conditions of temperature

(typically less than 30.0 °C) and pressure (typically greater than 2.0 MPa), gas hydrates can form

anywhere that water molecules coexist with potential guest molecules such as methane, carbon

dioxide, or nitrogen[3, 4]

. Consequently, natural gas hydrates have posed serious problems in oil

and gas production systems, because clogging of the gas and oil transmission pipelines by the

hydrates can lead to safety risks and financial loss[4, 5]

. To solve this problem, a variety of methods

such as thermal heating, depressurization, and mechanical elimination have been suggested. The

injection of hydrate inhibitors into the pipelines has become the most popular approach in the gas

and oil industries [1, 6]

.

Hydrate inhibitors can be generally classified as thermodynamic inhibitors (THIs) or

low-dosage hydrate inhibitors (LDHI). Historically, THIs such as methanol, monoethylene glycol

(MEG), and a variety of salts have been widely used in deep-water drilling operations[7]

. These

agents shift the thermodynamic conditions of hydrate formation outside the temperature and

pressure ranges used during operation, leading to the complete prevention of hydrate formation.

Nowadays, instead of the typical methanol inhibitor, MEG has become a popular choice for its

environmental friendliness[8]

. However, this inhibitor is expensive, because large amounts of MEG

must be injected into the pipeline (20 to 60 wt% of the water produced)[9]

. To reduce operating

costs, the MEG is usually recovered and recycled. The most common regeneration system for

monoethylene glycol is a MEG recovery unit (MRU)[10]

. Although the MRU affords an

economical method for MEG recovery, the system is bounded by an upper limit in terms of how

much MEG can be recovered. Consequently, the operator must limit production by shutting down

high water-producing wells or adding fresh MEG. In an industrial production process, either

option will severely impact the cost profile. Hence, the more desirable solutions to reducing the

costs of high MEG injection dosages are the LDHIs.

Kinetic hydrate inhibitors (KHIs), a type of LDHI, are water-soluble polymers which delay

hydrate nucleation and growth for a certain amount of time under particular subcooling

conditions[11]

. Typical KHIs are polymers based on 5- and 7-membered-ring N-vinyl lactams, such

3

as poly(N-vinylpyrrolidone) (PVP) and poly(N-vinylcaprolactam) (PVCap)[12-16]

. One of the main

advantages of KHIs is that the effective dosage is 0.5 ~ 2.0 mass% of the water produced, which is

one percent of THIs. However, KHIs are not the best solution for hydrate prevention because the

current commercial inhibitor requires effective subcooling below 12.0 °C in field applications

[17-20]. This could be an issue, for example, the subcooling required in offshore deep-water or

ultra-deep-water systems as high as 20.0 °C [21-25]

. In such systems, hydrate prevention cannot be

optimally controlled using KHIs alone.

Fortunately, the use of combined KHI/THI treatments has provided a solution for overcoming

these problems, by improving the subcooling requirements for the KHIs and reducing the THI

dosages. Thus, the THIs can modulate the subcooling temperature into the range appropriate for

KHI application, while the KHIs can significantly reduce the THI injection amounts. Thus,

composite inhibitors consisting of KHIs and THIs have become active areas of research [10, 12, 26–29]

.

In laboratory-based testing, the good performance of a composite inhibitor that included a KHI

and THI has been confirmed. Kim et al.[26]

indicated that replacing a high dose of MEG with 0.2

wt% KHI was a good option, reducing the injection of MEG by 20.0 wt%. Mozaffar et al.[27]

proved that MEG has a very positive effect on PVCap as hydrate inhibitor and the combination of

MEG +PVCap or MeOH + PVCap offers far better inhibition by mass/volume inhibitor than MEG

or MeOH alone. In field applications, it is rare to use inhibitor combinations consisting of KHIs

and THIs; successful cases have only injected a KHI or THI. Table 1 summarizes the applications

of KHIs replacing THIs in the oil and gas fields.

Table 1. Parameters for KHI application replacing THI in typical oil and gas fields

Type

of KHI

Dose of

KHI (L/day)

Type

of THI

Dose of THI

(L/day)

Subcooling

(°C)

Location of oil and gas

field

Hytreat 84.5 MEG 1817.3 10.0 Southern North Sea[22]

Ice-Chek 50.9 MeOH 200.0 14.0 Alberta Gas Field[23]

KHI/CI (0.2 wt%) - - 5.6 Karan Field[24]

EC 6423A 1.89 MeOH 140.0 8.0 West Pembina Field[28]

PVP family 8.5 MeOH 596.2 8.3 Gulf of Mexico[29]

PVCap family 5.0 MeOH 300.0 3.3 Gulf of Mexico[30]

GHI-7182 5.6 MeOH 64.3 15.0 North Soldado Field[31]

4

In oil and gas industry applications, THIs are usually recovered to a large extent due to their

mature recovery processes, whereas the low doses of KHIs are directly discharged. However, this

lack of KHI recovery may result in fouling of the produced water. To reduce the pollution caused

by KHI discharge, Anderson et al.[32]

introduced a method to remove them from the produced

water, achieving a KHI removal rate of 90.0% by solvent extraction. In an actual field application,

special techniques such as centrifugal separation or filtration must be included in water treatment

processes, which may incur extra operational expenses and even alter the process. Compared with

the KHI removal method, other approaches such as the recycling of both the KHI and THI would

be convenient and less costly, if the recycling measures were feasible. The recovery of THIs in the

presence of KHIs or the recovery of KHIs alone has been rarely investigated. It is not clear

whether both kinds of inhibitors can be recovered or the inhibitory performance of the recovered

inhibitors would be good. These problems must be addressed to advance the industrial application

of combined KHI/THI inhibitors to prevent hydrate formation.

In this work, the recovery efficiency of MEG and a KHI was studied under different

operating conditions. The thermodynamic performance of the recovered MEG was also evaluated

in a high-pressure autoclave. Additionally, the kinetic inhibitory performance of recovered

solutions consisting of MEG and the KHI was investigated systematically. Finally, the

thermodynamic and kinetic inhibitory performance properties were studied with a multicomponent

gas having the same composition of the natural gas distributed in the South China Sea. These

studies will lead to a completely new understanding of the recovery and reuse of combined

MEG/KHI hydrate inhibitors.

2. Experimental section

2.1. Materials. A proprietary KHI (HI-121; number-average molecular weight, Mn = 1120)

was synthesized by the free-radical polymerization of N-vinylcaprolactam in diethylene glycol

monobutyl ether[33]

. MEG with a minimum purity of 99.9 mol% was obtained from Sinopharm

Chemical Reagent Co., Ltd. (Shanghai, China). Acetone and methanol were obtained from

Aladdin Co., Ltd. (Shanghai, China). The composition of the synthetic gas which was supplied by

the Guangzhou Zhuozheng Gas Industry Co. Ltd. is shown in Table 2. The

simulated formation water used in all experiments was weighed on an electronic balance with an

5

accuracy of ± 0.1 mg, its composition is shown in Table 3.

Table 2. Composition of multicomponent gas used in this work

Component Mole fraction (%) Component Mole fraction (%)

CH4 91.12 iso-pentane 0.16

C2H6 5.03 n-hexane 0.41

C3H8 1.49 n-heptane 0.033

n-butane 0.37 CO2 0.48

iso-butane 0.32 N2 0.46

n-pentane 0.12 Total 99.99

Table 3. Formation water ingredients and suppliers used in this work

Chemical Supplier Purity/

mol%

Mass fraction

(%)

KCl Sinopharm Chemical Reagent Co., Ltd. 99.5 0.207

NaCl Tianjin Kermel Chemical Reagent Co., Ltd. 99.5 1.769

CaCl2 Sinopharm Chemical Reagent Co., Ltd. 98.0 0.511

2.2. Apparatus.

A rotary evaporator was used to recover the MEG and HI-121. Qualitative gas

chromatography-mass spectrometry (GC-MS) analysis was carried out on a GCMS-QP2010

system, consisting of a GC2010 gas chromatograph and a QP2010 Plus quadrupole mass

spectrometer (Shimadzu). Detailed parameters for the analyses are provided in the Supporting

Information (SI). Gas chromatography was performed to confirm the amount of MEG in the

recovered liquid, using an Agilent 6820 instrument equipped with a flame-ionization detector.

Detailed information is also provided in the attached SI file. Liquid chromatography–mass

spectrometry (LC-MS) analysis was conducted with Agilent 1290 and Bruker maXis impact

instruments.

A high-pressure autoclave, equipped with a magnetic stirrer, was used to measure the

phase-equilibrium conditions of fresh and recovered MEG. The apparatus was described in our

previous work[34, 35]

. The effective volume of the cell is 300.0 mL and the allowable operational

6

temperature and pressure ranges for the vessel are −20.0 °C ~ 50.0 °C and 0.0 MPa ~ 20.0 MPa

respectively.

The inhibitory performance of the recovered solutions comprising MEG and HI-121 was

evaluated in an apparatus[18, 33]

consisting of six identical magnetically stirred steel cells.

High-pressure nitrogen and a vacuum pump were used to convey the solutions. The temperatures

of the gas and liquid phases in the cell were measured by two internal platinum resistance

thermometers (Westzh WZ-PT100) with an accuracy of ± 0.1 °C. The internal pressures of the

vessels were measured by a pressure transducer (Senex DG-1300) with an accuracy of ±0.01 MPa.

2.3. Methods

Recovery of MEG/KHI. Typical atmospheric distillation was used to recover MEG and

HI-121 from a solution of 20.0 wt% MEG and 3.0 wt% HI-121 (60 g) in water at 140 °C. Vacuum

distillations were also conducted at 100 °C and 60 °C, under pressures of 0.020 MPa and 0.085

MPa, respectively. For comparison, the recovery of pure MEG solution was also examined at

60 °C and 0.016 MPa; this sample was designated as VR0-60. The notations VR-60, VR-100, and

AR-140 correspond to recovered solutions (RSs) consisting of MEG and HI-121 collected at

60 °C, 100 °C, and 140 °C. The recovery of pure HI-121 solution designated as AR1-140 was also

examined at 140.0 °C in order to study the change of HI-121 in experimental process.

GC-MS was used to qualitatively analyze the substances in the RSs, and GC was used to

confirm the MEG recovery rates. The GC-MS and GC test samples were prepared by diluting the

recovered liquids in acetone in a 1:4 volume ratio. LC-MS was used to monitor the changes in the

HI-121 during the recovery process. The LC-MS test samples were prepared by diluting the RSs

with methanol in a 1:4 volume ratio. To distinguish between materials at different recovery stages,

recovered MEG and fresh MEG are referred to as RMEG and FMEG, respectively. Similarly, the

terms RHI-121 and FHI-121 represent recovered HI-121 and fresh HI-121. A detailed diagram for

the recovery and detection processes is shown in Figure 1.

7

vacuum distillation

（60 ºC)

vacuum distillation

（100 ºC)

atmospheric

distillation（140 ºC)

MEG recovery rate

VR-60

VR-100

AR-140

LC-MS analysis

20.0wt%MEG+3.0wt%

HI-121 solution

GC analysis

VR-60 VR-100 AR-140

GC -MS analysis

KHI recovery feasibility

20.0wt%MEG

solution

vacuum distillation

（60 ºC)acetone dilution

VR0-60 VR0-60

VR-60 VR-100 AR-140

GC analysis

3.0wt% HI-121 solution

atmospheric

distillation（140 ºC)

AR1-140

methanol dilution

AR1-140

LC-MS analysis

Figure 1. Schematic diagram for the recovery and analysis of the composite inhibitors.

Thermodynamic performance tests. The phase-equilibrium conditions for the FMEG and

RMEG with or without the KHI were measured using the classical method [34, 36]

. The key steps of

the pressure-search method are (i) cooling to deduce hydrate formation and (ii) heating to

dissociate the hydrate. In general, the cooling rate was 3 °C/h and the heating rate was 0.025 °C/h

until a little hydrate was left in the cell. The intersection of the hydrate formation line and the

hydrate dissociation line is considered as the phase-equilibrium point, i.e., the moment at which all

hydrate has been dissociated completely. A typical P–T curve is shown in Figure 2.

Figure 2. Typical P–T curve for a system containing 15.0 mass% FMEG obtained using the

isochoric pressure-search method.

Kinetic performance test. The standard induction time (ti) method was used to evaluate the

inhibitory performance of the hydrate inhibitors by isothermal cooling [11, 37, 38]

. The term ti is

defined as the time from the start of cooling to the first detected hydrate formation, as shown in

Figure 3. In the test, the temperature was adjusted to 32.0 °C at the desired pressure (15.0 MPa in

this work). Then, the temperature of the vessel was cooled to a set temperature (2.0 °C) which was

below the equilibrium point by more than 16 °C. When the liquid-phase temperature reached

2.0 °C, the conditions were held for 48.0 h, with stirring at 750 rpm. It should be noted that if the

system with the inhibitor solution did not form a hydrate, ti was recorded as 48.0 h. In addition,

8

the hydrate crystal growth rate could be measured by the rate of gas consumption after the onset of

nucleation.

Figure 3. Typical curve obtained in the induction time (ti) test for 10.0 wt% VR-100 plus 1.0 wt%

aqueous FHI-121.

3. Results and discussion

3.1 Recovery of MEG/KHI

The qualitative GC-MS analysis results (Figure S1) show that MEG and diethylene glycol

monobutyl ether were existed in all three RS samples. For both the FMEG and VR0-60 samples,

the first chromatographic peak is acetone (solvent peak) and the second is MEG. The two peaks in

the chromatogram of recovered solution (AR1-140) elute in the order acetamide and diethylene

glycol monobutyl ether, which are the decomposition products of polymers in HI-121 and its

solvent respectively. As expected, the peaks in the VR-60 and VR-100 chromatograms are

acetamide, MEG, and diethylene glycol monobutyl ether. The harsher recovery conditions used

for AR-140 also altered its product composition. The substances observed in the different RS

samples are shown in detail in Table 4.

Table 4. Qualitative identification of substances in recovered solutions by GC-MS

Solution Substance

FMGE Acetone, MEG

VR0-60 Acetone, MEG

VR-60 Acetamide, MEG, Diethylene glycol monobutyl ether

VR-100 Acetamide , MEG, Diethylene glycol monobutyl ether

AR-140 L-Alaninol , MEG, Diethylene glycol monobutyl ether

AR1-140 Acetamide, Diethylene glycol monobutyl ether

9

The rates of MEG recovery are confirmed by GC analysis, which showed that the presence of

HI-121 had little impact. Thus, the rate of MEG recovery in a solution consisting of MEG and

HI-121 is close to that obtained for a solution containing only MEG. However, the MEG recovery

rate is seriously affected by the recovery temperature, and declined with increasing operating

temperature. As shown in Figure 4, the MEG contents are qualified by the external standard

method. The weight percents of MEG in the four diluted RSs are 22.89%, 22.93%, 21.56%, and

21.29%, respectively. Hence, the weight percents of MEG in the four RSs are 87.89%, 88.83%,

82.79%, and 81.74%, respectively. The corresponding MEG contents in the VR0-60, VR-60,

VR-100, and AR-140 RS are 11.42 g, 11.39 g, 11.01 g, and 10.95 g, respectively. Fresh MEG

contents in four initial solutions are 12.03 g, 12.05 g, 11.95 g and 11.98 g, respectively. Thus, the

rate of MEG recovery in VR0-60 is 94.93% and the MEG recovery rates (RR) for the three RSs

consisting of MEG and HI-121 are 94.52%, 92.13%, and 91.40%, respectively. That the MEG

recovery rate is closely related to the recovery temperature may be due to the ineffective return of

the MEG to the liquid phase, leading to on-stream loss at high temperature. The RR of MEG was

determined as shown in equation 1.The RRs of MEG in four RSs are listed in Table 5.

12 14 16 18 20 22 24 26 28 30

8.0x105

1.0x106

1.2x106

1.4x106

1.6x106

RMEG in diluted AR-140

RMEG in diluted VR-100

RMEG in diluted VR-60

RMEG in diluted VR0-60

MEG concentration（ wt%）

Inte

nsi

ty

MEG standard sample

Figure 4. External standard curve for MEG and the MEG contents (wt%) in the diluted solutions.

Table 5. MEG contents in the recovered solutions (RSs) and MEG recovery rates (RR)

Sample Weight of RS (g) RMEG in RS (g) FMEG in the

initial solution (g) MEG RR

VR0-60 13.0 11.42 12.03 94.93

VR-60 12.9 11.39 12.05 94.52

VR-100 13.3 11.01 11.95 92.13

AR-140 13.4 10.95 11.98 91.40

10

The LC-MS analysis results confirm that the HI-121 inhibitor is also recovered during the

MEG recovery process. At recovery temperatures near its polymerization temperature, the

structure of HI-121 was hardly changed; However, at higher temperatures, the additive tended to

undergo loss of monomer units. Changes in the HI-121 during the recovery process are closely

related to the recovery temperature. As the recovery temperature increases, the polymer chain of

HI-121 becomes shorter and the degree of polymerization also decreases. The molecular weight

(Mn) of RHI-121 at a recovery temperature of 60 °C is mainly distributed around 980.0 Da, which

is one less monomer unit than contained in FHI-121. When the recovery temperature exceeds the

polymerization temperature (70 °C), the HI-121 polymer chains are easily cleaved into smaller

fragments. At recovery temperatures of 100°C and 140 °C, RHI-121 contains three and five fewer

monomer units than FHI-121. The detailed LC-MS test results are shown in Figure S2, and the Mn

of the RHI-121 samples recovered under different conditions are summarized in Table 6.

Table 6. Molecular weight values for HI-121 recovered under different conditions

Sample Molecular weight (Da) Number of lost monomer unit(s)

FHI-121 1120 0

RHI-121 in VR-60 980 1

RHI-121 in VR-100 700 3

RHI-121 in AR-140 420 5

3.2 Thermodynamic performance of RMEG

The phase-equilibrium temperatures of 15.0 wt% and 40.0 wt% solutions of FMEG and the

corresponding sets of three RSs were evaluated. As shown in Figure 5, at the same pressure (9.8

MPa) for the 15.0 wt% samples, the phase-equilibrium temperatures for VR-60, VR-100, and

AR-140 are shifted higher by 1.2°C, 1.6°C, and 1.9 °C, compared with the 15.0 wt% FMEG,

respectively. According to the Hammerschmidt equation[39]

, as shown in equation 2, the theoretical

concentrations of RMEG in VR-60, VR-100, and AR-140 should be 12.45 wt%, 11.64 wt%, and

10.95 wt% at these phase-equilibrium temperatures respectively. The calculation formula of the

theoretical concentration was also shown in equation 3. The actual RMEG concentrations in

VR-60, VR-100, and AR-140 are 14.17 wt%, 13.82 wt%, and 13.71 wt%, respectively.

Consequently, the thermodynamic inhibitory efficiency (TIE) of the RMEG, as defined in equation

4, under the three recovery conditions (VR-60, VR-100, AR-140) are 87.86%, 84.22%, and

79.86%, respectively.

11

Where, k is a constant which equals to 1297 for MeOH but 2222 for MEG, W is the mass fraction

of the THI in the solution (it is the theoretical concentration of THI), M is the molecular weight of

the THI, and ΔT is the hydrate suppression.

Figure 5. Phase-equilibrium points for a synthetic gas with different concentrations of FMEG and

recovery solutions. (Hollow symbols = 15.0 wt%; solid symbols = 40.0 wt%).

The TIE increases when the concentration of MEG increases to 40.0 wt%. At 11.3 MPa and

40.0 wt% RS, the increments in the equilibrium temperature for VR-60, VR-100, and AR-140 are

2.40 °C, 3.54 °C, and 4.5 °C, compared with FMEG. Using similar calculations, the TIE values of

the RMEG are 99.16%, 98.25%, and 96.14% (Table 7), respectively. The TIE of RMEG is less

than 100% because of the presence of HI-121-based N-vinylcaprolactam (VCap) monomers. A

previous study reported that the presence of KHI-based VCap monomers in the inhibited system

would induce non-classical crystalline structures in the gas hydrate, leading to a higher

equilibrium temperature (thermodynamic effect)[38, 40]

. On the other hand, various small molecules

such as the dimer of N-vinylcaprolactam that could arise from HI-121 chain cleavage may also

result in a decreased TIE. Those molecules may act as nucleation centers and promote hydrate

formation, leading to a higher equilibrium temperature.

12

Table 7. TIE values for RMEG as a function of concentration and recovery conditions

(Teq/°C, Peq/MPa) Actual RMEG

concentration in RS

Theoretical MEG

concentration TIE of RMEG in RS

(16.60, 9.91) 14.17 wt% 12.45 wt% 87.86 (VR-60)

(17.01, 9.82) 13.82 wt% 11.64 wt% 84.22 (VR-100)

(17.31, 10.01) 13.71 wt% 10.95 wt% 79.86 (AR-140)

(8.74, 11.38) 37.80 wt% 37.48 wt% 99.16 (VR-60)

(10.40, 11.25) 36.85 wt% 36.21 wt% 98.25 (VR-100)

(10.91, 11.30) 36.56 wt% 35.15 wt% 96.14 (AR-140)

3.3 Kinetic performance of the RMEG/RHI-121 combination

It is known that hydrate formation proceeds through two stages, (i) nucleation and (ii) growth.

With respect to these two processes, induction time and gas consumption have been used as

reliable parameters for evaluating the performance of hydrate inhibitors[11, 41]

. The induction time

reflects the effectiveness of the hydrate inhibitor in delaying hydrate nucleation, whereas the gas

consumption reveals the effect of the hydrate inhibitor on hydrate growth after hydrate crystals

have formed.

Reuse of the recovered solutions. The inhibitory performance of RS samples containing

different concentrations of RMEG and RHI-121 was investigated. Inhibition by the RS obtained at

60 °C (VR-60) is the best among the three RS samples, and nearly equal to that of the freshly

combined inhibitors. The kinetic inhibitory performance of the RSs significantly decreases with

increasing recovery temperature. As shown by the results in Figure 6, the induction time for

VR-60 (47.6 h) is reduced by only 0.83% compared with the time for the fresh combination

consisting of 1.0 wt% FHI-121 and 10.0 wt% FMEG. The induction times for VR-100 and

AR-140 are respectively reduced by 77.01% and 91.66% when compared with the fresh sample.

Figure 6. Induction times for different 11.5 wt% RSs. The blank sample contained 1.0 wt%

FHI-121 and 10.0 wt% FMEG. Error bars represent the standard deviation ti, the up arrows

13

represent the ti exceed 48.0 h.

The gas consumption data also reveals the superiority of the kinetic inhibitory performance

for the RS obtained at 60 °C in suppressing hydrate crystal growth. As shown in Figure 7, the 11.5

wt% VR-60 sample performs as effectively as the fresh combination and inhibits hydrate growth

for 48.0h. In contrast, the VR-100 and AR-140 samples do not effectively prevent hydrate growth.

The gas consumption in the systems containing VR-100 or AR-140 increases rapidly once hydrate

crystals are formed. The system with 11.5 wt% VR-100 forms hydrates at a rate of 0.25%/h

initially. The hydrates grow very slowly after 18 h, leading to a 4.6%/h rate of water conversion to

hydrate crystals. Even poorer results were obtained for the system with 11.5 wt% AR-140, in

which hydrate crystals grow at an average speed of 0.13%/h for as long as 46.0 h. Consequently,

5.98% of the water is consumed through conversion to hydrate crystals. The close relationship

between the kinetic inhibitory performance of the RSs and their recovery conditions can be

ascribed to the remarkable changes in the HI-121 polymer structure with recovery temperature.

The polymer chain of the RHI-121 recovered at 60 °C mainly consists of seven monomer units

(comparable to FHI-121). Accordingly, the reuse performance of VR-60 equals that of the fresh

combination. Unfortunately, during the recovery process at 100 °C, the HI-121 polymer suffers

partial cleavage to generate dimers, trimers, and some tetramers. These small fragments maybe

become nucleation agents that induce water to form clusters, thus accelerating hydrate formation

and leading to a shorter induction time. Furthermore, once the hydrate crystals form, the HI-121

polymer with a shorter chain length has no ability to hinder crystal growth because the shorter

hydrophobic end groups cannot effectively disrupt the water molecules. These reasons are further

corroborated by the results for the reuse of AR-140.

Figure 7. Gas consumption data for different RSs (11.5 wt%). The blank sample comprised 1.0 wt%

FHI-121 and10.0 wt% FMEG.

14

Adding FHI-121 to the RSs. To rescue the inhibitory performance of the RSs, the effects of

adding fresh HI-121 were explored. Adding 1.0 wt% FHI-121 could improve the induction time of

the RSs. Because the inhibitory effects of VR-60 are as outstanding as the fresh combination, the

addition of FHI-121 has little influence on its performance (Fig. 8). However, unlike the VR-60

sample, adding FHI-121 has an obvious effect on the performance of the VR-100 and AR-140

samples. The induction times for VR-100 and AR-140 respectively increase by factors of 2.5 and

3.2 after introducing 1.0 wt% FHI-121. Unfortunately, however, the induction times for VR-100

and AR-140 remain far below that of the fresh combination, even with the inclusion of 1.0 wt%

FHI-121. Thus, the addition of fresh HI-121 improves the inhibitory performance of the RSs, but

does not restore it to the level of the freshly combined inhibitors.

Figure 8. Induction times of 11.5 wt% RS and 10.0 wt% RS plus 1.0 wt% fresh HI-121 versus the

freshly combined inhibitors (10.0 wt% FMEG + 1.0 wt% FH1-121).

The gas consumption profiles in Figure 9 reveal that fresh HI-121 cannot successfully limit

hydrate crystal growth once nucleation has occurred. Introducing FHI-121 to VR-60 does not

successfully prevent the growth of hydrate nuclei. In the system containing VR-60 plus 1.0 wt%

FHI-121, gas is suddenly consumed after 46.0 h, leading to the conversion of 1.03% water to

hydrate crystals. The same case is also observed for VR-100 mixed with fresh HI-121. The

hydrate grows in a catastrophic way once the crystals form, and the maximum growth rate reaches

4.01%/h when fresh HI-121 is introduced, resulting in the conversion of 5.4% water to hydrate.

After 48 h, the VR-100 + FHI-121 system contains 0.9% more hydrate than the VR-100 system.

The performance of the aqueous AR-140 is also improved when adding fresh HI-121; shortly after

addition, the growth rate is fast (1.8%/h), but is retarded significantly after 2 h. Consequently, the

amount of water converted to hydrate crystal is 1.48% lower than the value in the system

15

containing only AR-140.

Figure 9. Gas consumption vs. time profiles for systems with 11.5 wt% RS and 10.0 wt% RS plus

1.0 wt% fresh HI-121.

Adding 1.0 wt% fresh HI-121 to the three RSs not only improves the concentration of the

HI-121 polymer molecules but also increases their polydispersity, which makes the HI-121

polymer-based VCap monomer exceed its optimum working conditions[18, 42, 43]

. The optimum

concentration and molecular weight of PVCap is 0.5 wt% and 900 Da. The kinetic inhibitory

efficiency of PVCap is decreased at lower and higher concentrations. Thus, introducing fresh

HI-121 would increase the concentration of HI-121 present in the RS beyond its optimum level.

Furthermore, adding HI-121 would increase the range of polymer dispersity, deviating from its

optimum molecular weight. Hence, HI-121 with different molecular chain lengths cannot assist in

reducing hydrate formation.

Adding FMEG to RS. Other measures to restore the inhibitory performance of the RSs to the

level of the freshly combined solution were also examined. Although adding fresh MEG had no

obvious effect on the induction time of VR-60, adding fresh MEG could sharply delay hydrate

nucleation for VR-100 and AR-140. As the data in Figure 10 shows, the induction times for

VR-100 increase more than 3.5- and 4.4-fold when adding 2.5 and 5.0 wt% FMEG, respectively.

Furthermore, the induction times for AR-140 improve by 8- and 12-fold after introducing 2.5 and

5.0 wt% fresh MEG. Most strikingly, the 5.0 wt% fresh MEG had the ability to completely restore

the inhibitory performance of the RSs. RS systems with 5.0 wt% additional MEG successfully

hindered hydrate formation throughout the experiment, reaching the inhibitory level of the fresh

combination.

0 5 10 15 20 25 30 35 40 45 50

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Gas

co

nsu

mp

tio

n/

mo

l

Time/h

10.0 wt% FMEG + 1.0wt% FHI-121

11.5wt% VR- 60

11.5wt% VR-100

11.5wt% AR-140

10.0wt% VR-60 + 1.0wt% FHI-121

10.0wt% VR-100 + 1.0wt% FHI-121

10.0wt% AR-140 + 1.0wt% FHI-121

16

Figure 10. Changes in induction times for RS systems in the presence of various concentrations of

fresh MEG.

The gas consumption results shown in Figure 11 also prove that the performance of the RSs

could be completely restored by adding fresh MEG. The most exciting result is that the kinetic

performance of the RSs returned to the original perfect level with the addition of 5.0 wt% fresh

MEG. None of the systems exhibit hydrate formation over 48.0 h after adding 5.0 wt% fresh MEG

to the RS. The addition of 2.5 wt% fresh MEG to the RSs also remarkably hinders hydrate crystal

growth and decreases the amount of hydrate. For VR-100, the amount of hydrate decreases from

4.6% to 3.0% after introducing 2.5 wt% fresh MEG. Similarly, the amount of hydrate formed is

reduced by 0.9% when adding 2.5 wt% fresh MEG to AR-100.

The addition of fresh MEG restores the inhibitory performance of the RSs primarily because

of the thermodynamic effects of MEG. Our previous work proved that the suppression value of the

equilibrium temperature was 1.25 °C with a 5 wt% increase in the MEG concentration in the same

gas/water system[44]

. Adding MEG enables the subcooling temperature to be lowered into the

range in which HI-121 can be effective. Hence, the kinetic effect of the RS could be restored once

the system entered the region in which HI-121 could play its role effectively.

Figure 11. Gas consumption versus time profiles for RS systems in the presence of various

concentrations of fresh MEG.

0 5 10 15 20 25 30 35 40 45 50

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

AR-140

AR-140 + 2.5wt% FMEG

AR-140 + 5.0wt% FMEG

VR-100

VR-100 + 2.5wt% FMEG

VR-100 + 5.0wt% FMEG

10.0wt% FMEG + 1.0wt% FHI-121

VR-60

VR- 60 + 2.5wt% FMEG

VR-60 + 5.0wt% FMEG

Ga

s co

nsu

mp

tio

n/

mo

l

Time/h

17

Conclusions

In this work, a series of experiments about MEG recovery in the presence of HI-121 at three

different conditions were conducted to reveal the change of combination inhibitors in the recovery

process. At the same time, the thermodynamic and kinetic inhibitory performance properties of the

recovered solutions were studied. The results showed that the HI-121 could simultaneously

recover with the MEG and the presence of HI-121 was found to have no negative effect on the

MEG recovery rate. The recovery properties of MEG and HI-121 were closely related to the

processing temperature. The best recovery condition of both inhibitors was 60 °C under vacuum

(0.085 MPa). At the best recovery condition, the MEG recovery rate was the highest (94.52%) and

the structure of HI-121 was hardly changed. Hence, the recovered solution obtained at the

polymerization temperature suppressed hydrate formation for 48.0 h and reached the level of the

fresh combination inhibitor. At recovery temperatures of 100°C and 140 °C, the HI-121 tended to

undergo loss of monomer units. This contributed to the negative impact of HI-121 on the

thermodynamic inhibition performance of the recovered MEG. Adding 5.0 wt% fresh MEG to the

recovered solutions completely restored their kinetic performance.

In the future, we need to consider the influence of salts on the recovery process and whether

the THI and KHI could not change after a desalination unit, which is a necessary unit in the

industrial recovery process.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (51576069),

National Natural Science Foundation of China (2016YFC0304006), Guangdong Natural Science

Foundation (2016A030313488) and Fundamental Research Funds for the Central University

(2015ZZ076 and 2015ZM121).

18

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21

TOC:

MEG + KHI ）solution

Recovered MEG and KHI

Feed

Gas stroge tank

MEG + KHI solution

60 ºC

WaterMEG solution

No hydrate

formation

22

Highlights

1. The recovery rate of MEG was affected by the temperature rather than HI-121.

2. HI-121 could be recovered along with MEG under proper recovery conditions.

3. HI-121 with various polymer chain lengths reduced the TIE of the recovered

MEG.

4. The solution recovered under milder conditions retained good inhibitory

performance.

5. Adding 5 wt% MEG restored the recovered solution performance to original

levels.