Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011 ),
Edinburgh, Scotland, United Kingdom, July 17-21, 2011.
EVIDENCE OF A MEMORY EFFECT IN THE FORMATION AND
REFORMATION OF CYCLOPENTANE HYDRATE
Hami dreza Sefidroodi, Eirin Abrahamsen and Malcolm A. Kelland
Department of Mathematics and Natural Science
Faculty of Science and Technology
Uni versity of Stavanger
N-4036 Stavanger
NORWAY
ABSTRACT
While investigating the possibility of testing the performance of low dosage hydrate inhibitors
(LDHIs) using cyclopentane hydrates, we noticed that hydrate formation was kinetically very slow
with or without any additives even at 0.1 o C and with fast stirring in glass, jacketed beakers. The
addition of several solids were able to promote hydrate formation including chalk, barite, sand, clay,
ice and cyclopentane hydrate itself. However, to study LDHIs we preferred not to add solids to avoid
LDHI-solid interactions which might affect hydrate heteronucleation. Therefore we attempted to use
the superheated hydrate or “precursor test method”, pioneered by TOTAL and University of Pau with
gas hydrates, in which hydrates are first melted and then reformed. We found that provided the
superheating above the equilibrium temperature for cyclopentane hydrates (7.7 o C) was no more than 23 o C, we could obtain hydrates much faster during cooling to 0.0 o C than if hydrates were formed from
the liquids for the first time. This is evidence for a so-called memory effect for the formation and
reformation of cyclopentane hydrates. Further, it was found that transference of a small amount of
water from melted cyclopentane hydrates (made with no more than 2-3 o C superheating) to a much
larger amount of fresh water and cyclopentane, with no previous cyclopentane history, initiated
cyclopentane hydrate formation significantly faster than using the unspiked fresh water alone. A
memory effect has been reported previously for other clathrate hydrates but not Structure II
cyclopentane hydrates. This superheated hydrate method has allowed us to study more easily the
effect of LDHIs on fluids giving cyclopentane hydrate formation at atmospheric pressure.
Keywords: cyclopentane hydrates, memory effect, low dosage hydrate inhibitors
NOMENCLATURE
CP – cyclopentane
CPH – cycleopentane hydrate
 subcooling
KHI – kinetic hydrate inhibitor
To – detected onset temperature for hydrate
formation
t i – induction time
Mw - we ight average molecular weight

INTRODUCTION
The first screening of the performance of low
dosage hydrate inhibitors (LDHIs) for use in
oilfield applications is normally carried out in
small equipment at high pressures with gases that
make either Structure I or Structure II clathrate
hydrates.1-3 Rocker cells and autoclaves are the
most
common
types
of
equipment.
T etrahydrofuran (THF) hydrate, which makes
Corresponding author: Phone: +44 51831823 Fax +47 51831750 E-mail: [email protected]
Structure II hydrate, the most common structure
encountered in upstream oil and gas operations,
has also been investigated for screening LDHIs.
This avoids the use of high pressure equipment.
However, some researchers have questioned
whether the use of THF hydrate is appropriate
since THF is very water-soluble but oilfield gas
hydrates are formed by gas diffusion into the
aqueous phase. (THF hydrate has been useful to
measure the growth rate of SII hydrate crystals
with various additives). Therefore, we wondered if
LDHIs could be screened using cyclopentane
hydrates which also would avoid the use of high
pressures during testing. Cyclopentane has been
reported to form Structure II hydrates at
atmospheric pressure without the need for any help
gas at equilibrium (dissociation) temperatures
usually varying from 7.7-7.9 o C.3-5 Sakemoto and
co-workers
determined
the
equilibrium
temperature to be 7.0 o C using very small samples
and observations with a microscope.3 Even with
so-called help gases under pressure cyclopentane
still forms SII hydrates.6-9
Figure 1. The complete equipment for the
experiments on cyclopentane hydrate.
This paper describes attempts to make
cyclopentane hydrates by various methods, a
redetermination of its equilibrium temperature and
an investigation of the memory effect by melting
and reforming hydrates. Our studies on the use of
cyclopentane hydrates to screen kinetic hydrate
inhibitors (KHIs) will be reported in separately.
EXPERIMENTAL EQUIPMENT
The equipment in which most of the cyclopentane
hydrate studies was carried out wa s a parallel
series of four previously unused jacketed Pyrex
glass beakers coupled to a cooler bath (Figures 13). The beakers have an internal volume of 100ml.
Unless otherwise stated experiments were carried
out by adding 30ml of distilled water and 10ml of
>99% cyclopentane (from Sigma-Aldrich) to each
beaker. Each beaker contained a Teflon stirrer bar
which wa s stirred by a magnetic stirrer below the
beaker at calibrated speeds of 100-660rpm using
an oscilloscope. Thus, the only surfaces available
to promote cyclopentane hydrate formation from
the fluids were Pyrex glass and Teflon, both
smooth surfaces. Above each beaker was placed a
glass lid which gave a tight fit onto the flat ground
glass on the beakers. This lid was used to prevent
dust entering the beakers and to avoid evaporation
of volatile cyclopentane.
Figure 2. The four jacketed and stirred glass
beakers for cyclopentane hydrate studies.
Figure 3. View from above, as seen by the video
camera, of the jacketed beakers.
The cooling-heating unit was a Julabo F12 cooling
bath with temperature stability ±0.03°C. The
cooling fluid wa s a mixture of distilled water and
20% monoethyleneglycol (from VWR) to avoid
ice formation in the cooling unit. The distance via
the plastic tubing to each of the jacketed beakers
wa s the same for each beaker in order to keep the
temperature equal in all beakers at any one time.
having a thermometer actually placed in the
beaker. These values were double-checked by
carrying out additional experiments with
thermometers in the beakers. It was also found that
the temperature at any one time in each of the
beakers wa s the same within ±0.05°C irrespective
of the cooling bath temperature.
To measure the temperature in the fluids in the
beakers, and to check that the display for the
cooling bath temperature was accurate, we used a
digital stick thermometer with accuracy ±0.05°C.
The thermometer was removed during actual
memory effect experiments to avoid introducing
new surfaces and to keep the beaker lids on tight
to avoid loss of volatile cyclopentane. A second
thermometer with additional real time display was
used in the cooling bath to record the actual
temperature and to show this display on the video
recordings at all times. This gave good control of
any temperature in the bath and beakers when left
unattended because the video recording could
always be checked. The video recording was also
critical to judge when cyclopentane hydrate had
occurred when no one was present to observe this.
The video camera was placed about 1 meter above
the cells so that the bottom of all beakers could be
seen at the same time. Cyclopentane hydrate
formation could be observed to start when the
clear stirred solution of cyclopentane and water
became cloudy (Figures 4-7). The video camera
recording wa s displayed on a PC at all times and
the PC coupled to a remote observation system so
that experiments could be watched from other PCs
via the Internet.
INITIAL EXPERIMENTS TO DETERMINE
APPROPRIATE
CONDITIONS
FO R
CYCLOPENTANE HYDRATE FORMATION
It appears from the literature that making
cyclopentane hydrates by simply cooling stirred
mixtures of water and cyclopentane, without the
addition of a promoter, is not straightforward.
One research group added ice crystals to
cyclopentane and water at temperatures below 0 o C.
Thereafter, the mixture wa s warmed to a
temperature above 0 o C to give cyclopentane
hydrates.4 Another research group lowered the
temperature of cyclopentane-water mixtures to
below 0 o C for over 24 hours to obtain
cyclopentane hydrates.9 In both cases, it is not
clear whether hydrate formation was initiated by
ice. Other groups added a small amount of
preformed cyclopentane hydrate to a supercooled
mixture of cyclopentane and water, or an
emulsion,
to
promote
further
hydrate
formation.10,13 Another method used to make
cyclopentane hydrate has been to melt a mixture of
ice and cyclopentane above 0.0o C but below the
equilibrium
temperature
of
cyclopentane
hydrate.14-15
There was a difference in the actual temperature in
the beakers and the cooling bath when cooling
below room temperature. This difference varied
depending on the cooling bath temperature.
Therefore, to know the actual temperature in the
beakers, experiments were carried out to determine
this difference at all temperatures between -0.520.0 o C in the cooling bath. Measurements were
made of the beaker temperature, the cooling bath
temperature using the video thermometer against
the display of the cooling bath temperature.
Regression analysis was used and graphs produced
so that at all times and all bath temperatures it was
possible to know the actual temperature in the
beakers by readin g off from the graphs without
We also found it difficult to make cyclopentane
hydrates without the use of a promoter. For
example, by stirring various ratios of cyclopentane
and water (or 3.5 wt.% NaCl solution) at 200400rpm at 0.0 o C or 0.1o C (and no lower, to avoid
ice formation) for 24 hours gave cyclopentane
hydrates in only two tests out of 20 experiments
after 190 and 500 minutes. Some of these
experiments were continued for 72 hours with still
no cyclopentane hydrate formation. This was done
in two ways, either by adding the fluids to the
beakers at room temperature and then cooling, or
by cooling the empty beakers and then adding the
fluids. However, if the stirrer bar did not rotate
smoothly but rattled around the beaker, which
occurred on a few occasions, cyclopentane hydrate
wa s often initiated within 24 hours but only after
reaching the minimum cooling temperature of
0.0 o C, which took approximately one hour. We
also conducted 12 tests at 0.0 o C with only distilled
water in the beakers to see if ice formation would
occur over 72 hours but no ice was ever formed.
THF has been used as a promoter for Structure II
gas hydrate formation.16 THF was used because of
its high solubility in water. THF has also been
used as a promoter for Structure I methane hydrate
formation.17 However, in our jacketed beakers,
addition of a small amount (0.1 or 1.0 wt.%) of
THF to the aqueous phase did not promote any
cyclopentane hydrate formation either in 24 hours
at 0.1 o C and 200rpm stirring. Accelerated
formation of cyclopentane hydrates has been
shown to occur using CO2 under pressure mixed
with cyclopentane and water.18 To avoid high
pressures, which wa s the whole point of the
exercise, we tried saturating distilled water with
CO2 gas at atmospheric pressure or used a
purchased bottle of carbonated mineral water
(Farris) as the aqueous phase. Neither method
gave cyclopentane hydrates at the same subcooling
and stirrer speed as used with THF spiking.
We wondered if increasing the interfacial area
between the aqueous phase and the upper
cyclopentane phase would help promote hydrate
formation. Therefore, we emulsified the
cyclopentane and water with both a high HLB
non-ionic surfactant mixture (a local washing up
liquid or polyoxyethylene (20) sorbitan
monolaurate (Tween 20) and a low HLB non-ionic
surfactant (sorbitan monolaurate (Span 20)) in
order to form cyclopentane-in-water and water-incyclopentane
emulsions
respectively.
A
concentration of 0.1 wt.% or 1.0 wt.% surfactant
wa s used based on the aqueous phase. Neither
emulsion gave hydrate formation in 20 hours when
stirred at 0.1 o C at 200-400rpm, even when 1 wt.%
THF was added to the aqueous phase. The same
result was obtained with 1 wt.% of the ionic
surfactants, sodium dodecyl sulphate (SDS) or
tetradecyltrimethylammonium chloride.
However, as with other groups’ studies discussed
earlier, we found we could promote cyclopentane
hydrates by adding a small quantity of ice to the
water phase at 0.1o C. The ice slowly melted but
gave cyclopentane hydrate formation in less than 5
minutes with stirring at 200rpm. In this and all
subsequent experiments reported herein we used
10ml of cyclopentane and 30ml of distilled water
in each jacketed glass beaker. In our 100ml
beakers and with stirring at 200rpm the
cyclopentane is partially dispersed as large, clear
drops in the aqueous phase (Figure 4). When
hydrate formation starts the droplets become
cloudy and cyclopentane hydrates grow around the
edges of the droplets (Figure 5-7). After several
minutes the droplets become more and more white
with hydrates and the walls become coated with
hydrates due to the centrifugal force from the fast
stirring.
Figure 4. Clear, stirred fluids before the start of
cyclopentane hydrate formation seen from the top
of a glass cell.
Cyclopentane hydrate formation in our jacketed
glass beakers is clearly much slower than
experiments we have conducted with natural gas
hydrates in 23ml sapphire cells at 40-150 bar
pressure at equivalent stirring rates (200-400rpm)
and subcoolings.19-24 With natural gases (either
methane to form SI or a C1-C4 gas mixture
forming SII hydrates), nucleation of gas hydrates
occurs in a maximum of a few minutes at 7.67.7 o C subcooling in our e quipment. Once gas
hydrate formation is initiated further hydrate
formation is very rapid, leading to a plug of
hydrate and stopping of the stirrer within a few
minutes. In contrast, cyclopentane hydrates are
much harder to nucleate without a promoter in
either our jacketed, glass beakers or the sapphire
cells despite the cells having additional steel
surfaces. In addition, once cyclopentane hydrate
formation has been initiated in either equipment,
further hydrate formation is relatively slow,
compared to gas hydrate formation, taking up to
20-30 minutes to form sufficient hydrates to stop
the rotation of the stirrer. We speculate that the
slower nucleation and growth rates of
cyclopentane hydrates is due to cyclopentane
being less soluble in water at atmospheric pressure
than natural gases at high pressure.
example, at 0.1 o C and 200rpm with 10ml
cyclopentane and 30ml distilled water in the
jacketed beakers, addition of 20mg of the
following solids initiated cyclopentane hydrate
formation within 2-20 minutes in all cases after
first stirring without the addition of solids for 20
hours:
 Rust from corroded iron
 Rust-free iron metal powder
 Calcium carbonate powder (outcrop chalk
sourced from Denmark)
 Barium sulphate powder (from a deposited
scale sample)
 Powdered sandstone
 Powdered Bentonite clay (aluminosilicatebased)
However, we did not want to use the addition of
solids for studying the relative performance of
KHIs in case the primary hydrate inhibition
mechanism became absorption of the KHIs onto
the solid surface thereby inhibiting initiation of
hydrate formation. This included cyclopentane
hydrates itself since one would then be studying
primarily crystal growth inhibition rather than
nucleation inhibition of hydrates.
Figures 5-7. Views from above glass cells showing
chronological stages of CPH formation.
Besides ice and cyclopentane hydrate, we found
that addition of various other solids, that could be
found in actively producing oilfields, could also
initiate
cyclopentane
hydrate
formation
significantly faster than with no solids present. For
Von Solms and co-workers have used silica
particles to initiate gas hydrate formation in
studies of some KHIs and have found that hydrate
formation is less stochastic giving more
reproducible induction times than if no silica was
added.25 A second method this research group used
to improve the reproducibility of the induction
time to gas hydrate formation was the superheated
hydrate or so-called “precursor KHI test method”
first introduced by TOTAL oil company and the
University of Pau.26-27 In this method, gas hydrates
are formed, melted and reformed, all in the
presence of a KHI. The second formation of
hydrates is more reproducible because, as the
authors propose, the aqueous phase contains some
residual structure or precursors (polyhedral
clustering or embryos) from the melted hydrates
which promotes hydrate reformation when cooled
again.28 Both constant cooling experiments and
isothermal experiments were carried out.
Residual structure or precursors in the aqueous
phase has been proposed as the mechanism for the
so-called “memory effect”. Thus a system that has
formed hydrates and then been melted will reform
hydrates at a higher and more reproducible
temperature when re-cooled than a system with no
hydrate history. The reformed hydrates will still
melt at the same dissociation temperature. This
memory effect for clathrate hydrates is a type of
hysteresis.29 However, rather than there being
partial clusters of hydrates (precursors), molecular
modeling work has shown that there may be a
persistent higher-than-equilibrium concentration of
methane in the water phase after melting methane
hydrate that could cause the memory effect.30-31
Several groups have observed a memory effect
from experiments with gas hydrates at
temperatures well above the normal melting point
of ice.3,32-38 The first observation of memory effect
in a semi-clathrate hydrate system has been
reported for tetragonal tetra-n-butyl ammonium
bromide
semi-clathrate
hydrate.39
Recrystallization was shown to occur in the
vicinity of the place where the last piece of initial
crystal was dissociated. The authors state that this
implies that a small amount of residual structures
remain in the dissociated water, but they could not
confirm this with Raman micro-spectroscopy.
One group has reported results on THF hydrate
stating that no memory effect exists.40-41 However,
in this study THF hydrates were melted at a
minimum of 6.5 o C above the equilibrium
temperature which may have destroyed any
significant residual hydrate structures and thus the
memory effect may have been lost. Cooling rates
were fast and nucleation observed to occur on the
wa lls of the vessel. Others have reported that if
hydrates are melted we ll above the equilibrium
temperature and/or for very long periods the
memory effect is lost.3,26,37,41 There was observed
no residual structure from a neutron diffraction
study on melted methane hydrates, i.e. there was
no difference in the water structure before hydrate
formation and after melting the hydrates.42 The
researchers speculate that it is possible that other
studies, where it was implied that there is a
memory effect, may have not reached equilibrium
as melting was only 2 hrs usually.17,43 Thus, there
may have been microscopic hydrate crystals still
present.
Very recently, a molecular simulation study on a
hydrate former “M”, akin to CH4 or CO2 , has been
reported.44 On warming the hydrate above the
equilibrium temperature the hydrate was observed
to melt from the periphery outwards leaving a
hydrate core. The researchers speculate that the
whole hydrate including the core takes time to
fully melt away, and thus the core remnants can be
sites for reforming hydrates on recooling.
High pressure autoclave experiments carried out
by Dicharry’s group showed that the repeatability
of KHI performance tests diminished rapidly when
the hydrates were melted at a temperature more
than 2-4 o C above the equilibrium temperature for
2 hours.26 It was speculated that the residual
hydrate structures are destroyed under these
conditions (i.e. loss of memory effect) and
therefore the process reverts to being strongly
stochastic again, as if hydrates had never been preformed at all with these fluids. We have also
carried out a comprehensive study of the
superheated hydrate KHI test method with natural
gas hydrates in high pressure sapphire and steel
cells. We also obtained better reproducibility of
hold times using this test method compared to
more common non-superheated hydrate methods.45
The superheated hydrate test method seemed a
suitable method to test KHIs with cyclopentane
hydrate systems. The reason is that the hydrates
can be formed and melted at a given temperature
at atmospheric pressure, and then the KHI can be
added. Using this method, the amount of residual
hydrate structure in the aqueous phase (i.e. the
intensity of the memory effect) is kept constant for
all KHIs. This is important as it is known that
KHIs can affect the dissociation temperature of
gas hydrate to different degrees.1-3,46-50 For
example, 5000ppm of poly(N-vinylcaprolactam),
originally based on the water phase, can raise the
dissociation temperature of Structure II gas
hydrates by several degrees Celsius at a range of
pressures. Poly(N-vinylpyrrolidone), a poorer
KHI, has a much weaker effect on the dissociation
temperature. Therefore, if you dose the KHI to the
water phase first, make gas hydrates and melt them
for a short time period at 1-4o C above the true
equilibrium temperature, the amount of residual
hydrate structure will vary depending on the
structure of the KHI. In some cases, such as with
poly(N-vinylcaprolactam), you may not even get
complete macroscopic melting of the gas hydrates.
Therefore, as the University of Pau suggested in a
later research paper, it may be best to measure the
dissociation temperature for each system with its
added KHI and superheat to the same degree
above these measured dissociation temperatures
for each KHI separately. This can be a fairly timeconsuming experimental procedure.
With cyclopentane hydrates one can avoid the
problem of measuring individual dissociation
temperatures for each KHI. This is because the
KHI can be easily added to the aqueous phase after
melting the hydrates because the experiment is
conducted at atmospheric pressure. In our case,
this involves simply lifting a glass lid off the
beaker and injecting the required amount of a
concentrated solution of the KHI in water. Adding
the KHI to a system under high pressure is not so
easy and would require some kind of pressurized
injection equipment.
Work carried out by the University of Pau with
natural gas hydrates suggests that gas hydrates
melted about 4 o C above the equilibrium
temperature for a few hours lose their memory
effect. Therefore, in order to screen KHIs with
cyclopentane hydrates we wanted to be sure that
there was indeed a memory effect operating and at
which temperature above the equilibrium
temperature the memory effect began to disappear.
This led us to conduct a thorough investigation in
which cyclopentane hydrates were melted at
various temperatures and then reformed on cooling
again. It is this work that is discussed in the rest of
this paper.
MEMORY
EFFEC T
EXPERIMENTAL
METHOD
The preparation of the cyclopentane hydrate in
each beaker in all memory effect experiments was
carried out as follows. The beakers were cooled to
0.0 o C before the addition of any fluids. Then 10ml
cyclopentane and 30ml distilled water was added.
After stirring at 200rpm for 10 minutes, to bring
the fluid temperature to 0.0 o C, a small crystal of
cyclopentane hydrate, made earlier and kept in a
refrigerator at 3-4 o C, was added to each beaker.
The time to the observation of the formation of
further cyclopentane hydrate was less than 2
minutes in all experiments. Then, before the actual
memory tests were started, the temperature in the
beakers wa s adjusted to 9.2 o C 30 or 60 minutes
after nucleation of cyclopentane hydrates began.
After complete melting of the hydrates to give a
clear mixture of fluids the beakers were cooled
again to 0.0 o C at a constant rate in all experiments
(this requires setting the cooling bath to a
temperature of 0.4 o C). Hydrate formation occurred
almost at the same time in each beaker in each
experiment. The start of the hydrate formation
time was determined as the time when hydrate
formation occurred in the last beaker. After a
designated
hydrate
formation time,
the
temperature in the beakers was increased to 0.3 o C
below the hydrate equilibrium temperature, i.e.
7.4 o C and held for 5 minutes before further
increasing the temperature more slowly to a
designated melting temperature. This was done to
avoid overheating the contents of the beakers if the
melting temperature only had been programmed
into the heating unit. After holding the beakers at
the melting temperature for a designated time
period the beakers were again cooled to 0.0 o C at a
constant rate in all experiments. The onset
temperature (To) for cyclopentane hydrate
formation was recorded based on visual
observations. If hydrate formation occurred in any
beaker after the beaker temperature reached 0.0 o C
the induction time to formation of hydrate
formation at 0.0 o C (t i) was recorded. If no hydrate
wa s observed to form within 20 hours the
experimental result was recorded as “no hydrate”
(NH). T h is memory test procedure avoids the
possibility that the added cyclopentane hydrate
crystal might be the last hydrate particle to melt
and was the main initiator of hydrate formation in
the second cycle. It also avoided any suspicion that
ice may be nucleating hydrate formation. Any data
from experiments in which the stirrer bar did not
rotate smoothly in the centre of the beaker but
rattled around the beaker were ignored. As
discussed earlier, this behavior occurred on a few
occasions and accelerated cyclopentane hydrate
formation.
After each memory test, hot water (50-60 o C) was
added to the beakers to melt the hydrates. The
fluids were removed and the same hot water
wa shing procedure repeated at least three times.
The beakers were then cleaned with paper towels,
wa shed again with hot water and then rinsed
thoroughly with distilled water. The magnetic
stirrer bars were also washed well in hot tap water
and rinsed in distilled water. The same stirrer bar
wa s kept in the same beaker throughout the whole
study. The glass lids were also washed the same
way after each experiment and kept to the same
beaker. Blank tests were carried intermittently to
check that no hydrates formed from cyclopentane
and water at 0.0 o C within at least 20 hours using
fresh water with no previous cyclopentane hydrate
history. This made sure that our washing
procedure for the cells was adequate.
CYCLOPENTANE
HYDRATE
EQ UILIBRIUM
TES T
METHOD AND
RESULTS
To
determine the
cyclopentane
hydrate
equilibrium temperature, the same preparation of
the fluids in the cells (beakers) was carried out as
done for the memory effect experiments. This
made sure that the conditions in the four cells wa s
very similar before further experimental work.
After cyclopentane hydrate formation at 0.0 o C, the
beakers were warmed over to 7.3 o C over
approximately 5 minutes, then heated stepwise
0.1 o C at a time holding at each step for at least 30
minutes. The temperature at which the fluids
became totally clear and no longer cloudy with no
visible sign of hydrate particles wa s taken as the
dissociation (equilibrium) temperature. This wa s
found to be 7.7 o C in all experiments, which other
groups have also observed.10 It was sometimes
necessary to hold the beaker at this temperature for
much longer than 30 minutes to see complete
dissociation of hydrates (i.e. back to a clear
solution), sometimes for up to 18 hours.
RESULTS AND DISCUSSION OF MEMORY
EFFEC T EXPERIMENTS
Three sets of experiments were carried out which
are summarized in Figures 8-11 and Table 1. In
the first set of experiments, cyclopentane hydrates
were melted at various set temperatures for 20
minutes. In the second set of experiments the
melting period was 2 hours and in the third set it
wa s 24 hours. Figures 6-8 show data for the onset
temperatures (To) of cyclopentane hydrate
formation at melting temperatures that always
gave cyclopentane hydrate formation before
reaching the minimum temperature of 0 o C. This
graphical data is supplemented by Table 1 which
contains data for experiments in which
cyclopentane hydrate formation did not always
occur before reaching the minimum temperature of
0 o C. This tabulated data is either the onset
temperature before reaching 0.0 o C or the time after
reaching 0.0 o C at which hydrate formation did
occur. An experimental result of “NH” is given in
each case when no hydrates formed in 24 hours at
the minimum temperature.
Figure 8. Onset temperature results of memory
effect experiments with a melting time of 20
minutes at melting temperatures that always gave
cyclopentane hydrate formation before reaching
the minimum temperature of 0 o C.
Figure 9. Onset temperature results of memory
effect experiments with a melting time of 2 hours
at melting temperatures that always gave
cyclopentane hydrate formation before reaching
the minimum temperature of 0 o C.
superheated to higher temperatures or kept for
20hrs at 9 o C or more the solution was clear (Figure
13).
Table 1. Summary of the data for melting
temperatures that did not always give
cyclopentane hydrate formation before cooling to
the minimum temperature of 0.0o C. (NH means no
hydrates observed in 20 hours).
Figure 10. Onset temperature results of memory
effect experiments with a melting time of 20 hours
at melting temperatures that always gave
cyclopentane hydrate formation before reaching
the minimum temperature of 0 o C.
Figure 11. Onset temperature results of memory
effect experiments with all three melting times at
melting
temperatures
that
always
gave
cyclopentane hydrate formation before reaching
the minimum temperature of 0o C. Lines of best fit
are also shown.
The turbidity of the fluids when cyclopentane
hydrates had been melted wa s dependent on the
superheating and the length of time the fluids were
kept at this temperature. If cyclopentane hydrates
were melted a t 8 o r 9o C for 20mins or 2hrs the
fluid was still cloudy but no particles could be
observed visibly in the cells (Figure 12). If
Melt
Temp
o
( C).
Melt
Time
(hours)
Expts that
o
gave To ( C)
Expts that
o
gave ti ( C) or
NH after
o
reaching 0 C
13.1
0.33
0.4, 0.4, 0.0
15, 105, NH,
25, 33, 961
14.1
0.33
0.3, 1.3, 2.0
NH, NH, 43,
7, NH, 7, NH,
NH
15.1
0.33
0.2, 1.7
NH, NH, NH,
NH, NH
16.1
0.33
11.1
2
2.0, 0.9, 0.6,
2.8, 1.7, 0.5,
0.3, 1.5
12, 53, 3, NH
12.1
2
0,3, 0.0, 1.6,
0.7, 0.4, 0.4,
1163, 1, >1
(final time not
recorded),
1034, NH,
NH, 12, 3, 22
13.1
2
0.7
161, 383, NH,
NH, NH
14.1
2
10.2
20
1.4
146, 12, 9, 5
11.1
20
1.4
56, 26, 79, 6,
16
12.1
20
609, 79, NH,
305, 14, NH
13.1
20
NH, NH, NH,
NH, NH, NH
NH, NH, 11,
36, NH, NH,
NH, NH
NH, NH, NH,
NH, NH, NH
over 1 hour. It should also be pointed out that no
beaker gave constantly worse results than any
another beaker, i.e. there was no trend that any one
beaker was different from another.
Even with the limited number of experiments
carried out in this study, these data are clear,
statistically significant evidence of some kind of
memory effect in the beakers which promotes
reformation of cyclopentane hydrates at an earlier
time and/or higher onset temperature in the
cooling cycle.
Figure 12. Cloudy solution
cyclopentane hydrates.
after
melting
Figure 13. Clear solution after melting
cyclopentane hydrates for a sufficiently long
period or with sufficient superheating.
From the figures and tables it is clear that
cyclopentane hydrate formation can occur much
earlier when the fluids have a history of previously
forming hydrates and the superheating and melting
time period are low. As discussed earlier, for fresh
fluids with no previous cyclopentane hydrate
history, we only observed cyclopentane hydrate
formation in 2 out of 20 experiments after at least
24 hours at 0.0 o C. In these 2 experiments hydrates
formed after 190 and 500 minutes after reaching
the minimum temperature of 0.0 o C. Using fluids
with previous cyclopentane history we could
obtain hydrates well before reaching the minimum
temperature and up to onset temperatures of about
4 o C with the cooling rate used. For example, to
cool from 9.2 o C to 0.0 o C took consistently just
In addition, the data in Figures 8-10 show a trend
by which longer melting temperatures cause a loss
of the memory effect. This trend is graphically
displayed in Figure 11 where lines of best fit
through the three sets of data at 0.33, 2 and 20
hours melting times have been added. For
example, using a melting temperature of 12.1 o C
for 0.33 hours gave onset temperatures from
reformation of cyclopentane hydrates of in the
range 0.5-2.4 o C whereas with 24 hours melting
time at the same temperature hydrates always
formed after reaching the minimum temperature of
0 o C or not all within the 24 hour experimental
period. However, when the melting period gets
longer and/or the melting temperature increased
the results become more random and the
probability of getting no hydrate formation in 24
hours increases. For example, with 20 hours
melting time, hydrates always formed during the
cooling period before reaching 0 o C when the first
hydrates were melted at 8.3 o C, whereas when
melted at 13.1 o C for the same time period no
hydrates were reformed after 24 hours in all the 6
experiments carried out. The number of
experiments at any one particular melting
temperature or melting time period is not very
large but enough to see a trend, and certainly with
all the data as a whole the trend is even more
evident and statistically significant.
A further 8 experiments (i.e. 32 beakers), were
also carried out with 95 minutes melting time at
various melting temperatures and these also fit
with the observed trends discussed herein. For
example at a melting temperature of 9.2 o C all
beakers gave cyclopentane hydrate formation at an
onset temperature of 2.8o C. At a melting
temperature of 12.1o C hydrates formed at 0.11.4 o C in 5 beakers and at 32-386 minutes after
reaching 0.0 o C in 3 other beakers.
Another point to mention from the results in
Figures 8-11 is that the reproducibility in the onset
temperatures is good (i.e. scattering of To in the
range of about 1 o C), only with 2 hours or less
melting time and superheating of less than about
2 o C. In comparison, Dicharry and co-workers
found that good reproducibility in onset
temperatures (or induction times at constant
temperature) was lost if the superheating was more
than 3-4o C for 2 hours using natural gas hydrate
systems.26
Another interesting set of tests that we carried out
wa s to see if cyclopentane hydrate formation could
be accelerated by transferring some of the melted
water from cyclopentane hydrates to a fresh
mixture of water and cyclopentane with no
cyclopentane hydrate history in another beaker
that was the cooled. The procedure was as follows:
10ml cyclopentane and 30ml distilled water were
added to one of the 4 beakers (the other 3 were
empty), cooled to 0.0 o C and a crystal of
cyclopentane hydrate from the refrigerator added.
After 30 minutes of hydrate formation the
temperature in the beakers was raised to 9.3 o C for
1 hour to fully melt the hydrates and then cooled
again to 0.0 o C for 30 minutes. Hydrates formed
again and the beaker warmed to 7.7 o C for 20 hours
to give a totally clear mixture of water and
cyclopentane. 30ml distilled water with no hydrate
history and 10ml cyclopentane was then added to
the other 3 empty beakers and left for 1 hour to
cool the fluids to 7.7 o C. Using a pipette pre-cooled
in a refrigerator, 3ml of the melted cyclopentane
hydrate water in the first beaker was transferred
quickly to each of the other three beakers. All 4
beakers were then cooled to 0.0 o C which took
about 50 minutes. Cyclopentane hydrate formation
occurred at 1.0, 1.6 and 3.6 o C in the three beakers
with the spiked fresh solutions and at 1.9 o C in the
original beaker with previous hydrate history. A
repeat experiment also gave the same result, i.e.
hydrate formation in all four beakers before
reaching 0.0 o C. In other experiments, one or two
of the beakers was not spiked with melted
cyclopentane hydrates and only these beakers did
not give hydrate formation when cooled to 0.0 o C
for 24 hours. In another experiment 1ml of melted
hydrate water from 3 beakers wa s added to the
fourth beaker with fresh water and cyclopentane
and cooled to 0.0 o C. Cyclopentane hydrate
formation occurred at 3.4 o C in the fourth beaker
and at 1.7, 3.4 and 3.4o C in the other three beakers.
These experiments indicate that the memory effect
is located in the aqueous phase and that it can be
transferred from one solution to a fresh aqueous
solution with no previous hydrate history.
CONCLUSION
Our experiments show that it can be difficult to
form cyclopentane hydrates at 0.0o C in stirrer glass
beakers even at 7.7 o C subcooling without the aid
of a promoter. Examples of promoters include
cyclopentane hydrate or ice crystals as we ll as
powdered iron metal, rust, chalk, barite or
Bentonite clay. The final cyclopentane hydrate
promoter investigated wa s residual hydrate
structure, or so-called precursors, in the water
phase from melting cyclopentane hydrates a
maximum of a few degrees Celsius (preferably <23 o C) for short time periods above the equilibrium
temperature. The equilibrium temperature wa s
determined to be approximately 7.7 o C. Thus, we
found that provided the superheating above the
equilibrium temperature was no more than 3-4 o C
for short time periods, we could obtain hydrates
much faster during cooling to 0.0 o C than if
hydrates were formed from the liquids for the first
time. Even after 20 hours superheating at 9.2 o C or
less. T he residual structure appeared to persist
since hydrate formation was still greatly
accelerated compared to fluids with no hydrate
history. These results are evidence for a so-called
memory effect for the formation and reformation
of cyclopentane hydrates. Further, it was found
that transference of a small amount of water from
melted cyclopentane hydrates (made with no more
than 2-3o C superheating) to a much larger amount
of fresh water and cyclopentane, with no previous
cyclopentane history, initiated cyclopentane
hydrate formation significantly faster than using
the fresh water alone. A memory effect has been
reported previously for other clathrate hydrates but
not Structure II cyclopentane hydrates. This
superheated hydrate test method has allowed us to
study the effect of LDHIs on fluids giving
cyclopentane hydrate formation at atmospheric
pressure.51
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Erik
Gisle
Dirdal,
Chandrakala
Arulanantham, Hamidreza
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