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Inhibition of gas hydrate formation by means of chemical additives—II. An evaluation of the screening method
Pergamon
Chemical Engmeering Science, Vol. 51, No. 13, pp. 3449-3458, 1996 Copyright 0 1996 Elsewer Science Ltd Printed m Great Britain. All rights reserved ooO9%2509/96 $15.00 + 0.00
0009-2509(95)00410-6
INHIBITION OF GAS HYDRATE FORMATION BY MEANS OF CHEMICAL ADDITIVES-II. AN EVALUATION OF THE SCREENING METHOD
SINTEF,
Applied
ARE LUND Chemistry, 7034 Trondheim,
Norway
and OLAV URDAHL* STATOIL, Research
and SIGRID S. KIRKHORN Centre, 7005 Trondheim, Norway
(First Received 12 June 1995; accepted in revised form 6 December 1995)
Abstract-A
given hydrocarbon gas-water-model oil system, used as a reference system for testing of new inhibitors, has been evaluated for experimental reproducibility of hydrate formation and behavior at flow conditions. The system was remixed five times in a 2” ID high-pressure flow loop and tested for the reproducibility both within one mixture (closed system) and between the remixtures. The results indicated good reproducibility both within the closed system and between the remixtures. Copyright 0 1996 Elsevier Science Ltd hydrate
INTRODUCTION
Hydrocarbon reserves in the North Sea include several marginal fields, which are either small or located at large water depths. Feasible economic exploitation of these reserves requires a shift towards lower cost options in subsea production, or the use of minimumprocessing platforms tied to centralized or land-based processing facilities. This will require the transportation of unprocessed or minimum-processed fluids over long distances. The fluids may consequently be exposed to low-temperature regions, where gas hydrate formation is expected. Gas hydrates, or clathrate hydrates, are one class of inclusion compounds (Makogon, 1981; Sloan, 1990). Common for all hydrates is that water molecules, due to hydrogen bonding, form a three-dimensional network, whose voids are occupied by non-polar molecules with low molecular weight and small rotational diameter. Whereas the lattice is usually not stable itself, the encaged gas molecules will contribute to stabilize it. At high pressure and/or at low temperatures, thermodynamically stable hydrates will form. If the pressure-temperature conditions in the subsea-pipeline favours the formation of these hydrates, extensive formation may take place and this can result in clogging of the pipeline due to deposits on the pipe wall or by formation of plugs in bends, valves, etc. (Hammerschmidt, 1934; Nyg&d, 1989; Austvik, 1992). Subsea transportation of unprocessed or minimumprocessed well fluids over long distances today re-
*Corresponding
author.
quires the use of large amounts of methanol or glycols for hydrate inhibition. The effect of these additives is to decrease the water activity to an extent that markedly reduces its ability to participate in hydrate formation, and thereby in a lowering of the hydrate formation temperature. The amount of inhibitor nccessary to obtain the desired lowering of the hydrate formation temperature is substantial, usually in the range of 20-40 wt% of the aqueous phase. This has prompted the search for new types of additives capable of inhibiting hydrate formation at far lower concentrations. In a previous paper (Urdahl et al., 1995) we have described an experimental set-up and a procedure for testing and evaluation of new gas hydrate inhibitors. The choice of the set-up, reference system and experimental procedure was based on earlier experiences. Different systems [trichlorofluoromethane (Rl l)-water, gas-water, gas-water-condensate and gas-water-oil systems] have been tested for hydrate properties in six different flow loops of variable inner diameters (1”-5”) and lengths (1.4430 m) (Austvik et al., 1990; Lund et al., 1991, 1992; Lund and Lysne, 1992). The main results from these experiments have been in agreement with later results published from other scientific groups (Lippmann et al., 1995; Reed et al.., 1994). However, most of the efforts in these experiments have been on evaluating different experimental conditions and not on the reproducibility of a single experiment (except for Rll-water systems which were interrupted by hysteresis effects). In this paper we have therefore looked at the reproducibility of our method with emphasis on nucleation time, subcooling and hydrate macrostructure build up.
3449
A.
3450 EXPERIMENTAL
LUNI )
SET-UP
The experiments were carried out in a high-pressure loop formed as a wheel at PETRECO A/S. The test wheel was filled with the desired fluid at a specified temperature and pressure, and then set under rotation. The rotation created a relative velocity between the pipe wall and the fluid and transport through a pipeline was thereby simulated. The high-pressure wheel is made from stainless steel with an inner tube diameter of 52.5 mm and a wheel diameter of 2.0 m. The volume is 13.4 1. The wheel includes two high-pressure windows for visual inspection, of which one is equipped with a Sony V800E Hi-8 video camera. The flow simulator is placed in a temperaturecontrolled chamber. The temperature is controlled using a programmable regulator, a heating fan and a refrigerator system. The temperature development in the chamber as a function of time is preset on the regulator. The wheel is attached to a motor/gearbox system enabling rotation between 0.3 and 5.0 m/s translation velocity. A torque sensor is installed as part of the rotational shaft enabling torque measurements to be performed during rotation. Pressure and temperature sensors on the wheel measures in the ranges of O-250 bara and - 10 to + lSO”C, respectively. All signals are transferred through cables and slip rings to a realtime PC-based data acquisition system. The accuracy of the measurements is estimated to be + 0.2 Nm for torque, f 0.5 bar for pressure, f O.l”C for fluid temperature in the wheel and * l.O”C for temperature in the chamber. The 2” ID flow simulator used is shown in Fig. 1. EXPERIMENTAL
WORK
The objective of the experiments was to study the reproducibility of hydrate formation and behaviour
Fig. 1. The PETRECO
et al. for a given hydrocarbon gas-water-model oil system. The system to be evaluated consisted of about 31.3 mol methane, 6.4 mol ethane, 3.7 mol propane, 1.0 1 (55.5 mol) distilled water, and 5.0 1 (31.1 mol) Exxsol D60 (Table 8). Experiments of two essentially different kinds were performed: (a) The wheel was rotated with a constant peripheral velocity (1 or 3 m/s) as the temperature was decreased from 70 to 4°C at a given cooling rate. These experiments are in the report referred to as continuous-flow experiments. The velocity of 1 m/s created a laminar flow regime in the liquid phases (prior to hydrate formation) while a velocity of 3 m/s gave a mixed turbulent flow regime. The experiments simulate flow in a long pipeline where the fluid gradually cools to sea bed temperature as it flows along the pipeline. (b) The system was first heated to 70°C while the wheel was rotating (peripheral velocity of 1 m/s). The wheel was then stopped and cooled down to a temperature of 4°C where it was kept constant for a period of a minimum of 12 h. The wheel was then restarted and rotated at a constant peripheral velocity of 1 m/s for the rest of the experiment. These experiments are referred to as stop/start experiments. This simulates a restart of a pipeline after a shut-in period. In order to minimize the number of adjustable parameters for the experiments, only one given cooling rate (temperature in chamber from 70 to 4°C in 1.5 h) was used both for the continuous-flow experiments and the stop/start experiments. The system to be evaluated was remixed five times in the test wheel. These mixtures are in this paper
2” ID fluid flow simulator.
Inhibition
of gas hydrate
numbered as systems REFl-REFS. For system REFl each experiment (1 and 3 m/s continuous-flow experiments and the stop/start experiment) was repeated five times in order to evaluate the reproducibility within a closed system. Before each experiment, the system was heated to 70°C for a minimum of 1 h in order to minimize any hysteresis effects from previously formed hydrate structures. Systems REF2-REFS were tested once with 1 and 3 m/s continuous-flow experiments. Before each experiment these systems were heated to 70°C. Compositional analysis of the gas phases used (before and after addition of liquid phases) were carried out for each mixture. All experiments were video, photo and data recorded.
formation-II
- 1, - 3, - 5, flow experiments - 8 and - 10, subnumbered -
Table 1. Gas analysis
- 7 and - 9, the 3 m/s continuousare subnumbered - 2, - 4, - 6, and the stop/start experiments are 11 to - 15.
Gas analysis
Gas analysis for each system (before and after addition of liquid phases) are given in Table 1, while compositions calculated from measured filling data are shown in Table 2. Hydrate
equilibrium
data
Figure 2 shows the hydrate equilibrium curve for system REFI. The equilibrium curve is calculated using the hydrate program SHP. Degree
EXPERIMENTAL RESULTS
The main results and measurements from the experiments with the systems REFl-REF5 are shown in Figs 3-10 and in Tables l-5. For system REFl, the 1 m/s continuous-flow experiments are subnumbered
3451
of subcooling
at hydrate
for systems
REFl-REF5 Vol. %
P (bar)
0;
N;
CH.,
CzHs
CJHs
only gas phase in the wheel Test 1 26.9 59.4 Test 2 27.0 59.3
-
0.21
70.13 73.42
17.87 16.41
12.00 9.96
Gas phase in equilibrium with water and ExxsolD60 Test 3 25.0 70.1 0.05 0.13 87.45 Test 4 25.0 69.4 0.06 0.25 87.55
9.52 9.36
2.85 2.78
71.76 72.62
16.47 16.56
11.77 10.82
Gas phase in equilibrium with water and ExxsolD60 Test 3 24.8 71.0 88.26
9.16
2.80
45.02 73.80
19.52 16.21
13.38 9.99
87.01
9.50
3.09
71.88 73.60
17.16 16.73
10.96 9.66
Gas phase in equilibrium with water and ExxsolD60 Test 3 27.3 70.8 87.94
9.39
2.67
67.87 65.89
18.33 15.98
13.48 11.37
87.76
9.35
2.89
Test System: REFl
System: REF2 Only gas phase in the wheel Test 1 26.9 59.4 Test 2 26.2 59.2
-
-
System: REF3 Only gas phase in the wheel Test 1 26.8 59.6 Test 2 27.0 59.3
4.52 -
17.56
Gas phase in equilibrium with water and ExxsolD60 Test 3 27.7 72.4 0.07 0.33 System: REF4 Only gas phase in the wheel Test 1 27.0 59.3 Test 2 27.1 59.1
-
-
System: REFS Only gas phase in the wheel Test 1 27.8 59.4 Test 2 27.9 59.3
0.06 1.35
0.26 5.40
Gas phase in equilibrium with water and ExxsolD60 Test 3 25.9 71.0 +Mixed with air in test bottle.
initiation
The degree of subcooling (“C) is the difference between experimental hydrate initiation temperature and the hydrate equilibrium (melting) temperature at the experimental pressure. Figure 3 and Tables 3 and
A. LUND et al.
3452 Table 2. Composition
(mole) for systems REFl-REFS data)
System
Methane
Ethane
REFl REF2 REF3 REM REFS
31.4 31.2 31.2 31.4 31.3
6.4 6.3 6.4 6.4 6.4
Propane 3.1 3.8 3.1 3.1 3.7
(calculated
from filling
Water
Exxsol D60
55.5 55.5 55.5 55.5 55.5
31.1 31.1 31.1 31.1 31.1
structures observed after start-up in the stop/start experiments are shown in Fig. 6 and Table 5.
Fig. 2. Calculated
hydrate
equilibrium REFl.
0
0
0
0
?? ?? ??
curve
for system
0
??
+
?? Q
?? +
0
0
0
0
(ml%
.
3mA
Fig. 3. Degree of subcooling obtained prior to hydrate equilibrium initiation for the systems REFl-REFS in the 1 and 3 m/s (peripheral velocity of the wheel) continuous-flow experiments. Tabulated values are given in Tables 3 and 4.
4 show
the degree of subcooling obtained prior to hydrate initiation in the continuous-flow experiments.
Hydrate macrostructures A description of the hydrate macrostructures observed (slurry, slush and powder-like hydrates) have previously been given by Austvik (1992). Figures 4 and 5 and Tables 3 and 4 give the time from hydrate initiation to the observed macrostructures in the continuous-flow experiments. The hydrate macro-
Changes in fluid flow properties during conversion of free water to hydrates Information on changes in fluid viscosity, deposits of hydrates on the pipe wall and clogging of the pipe by hydrate lumps were obtained from the recorded torque measurements on the rotational shaft of the wheel. The relative change in torque, or apparent viscosity, should be negligible during the conversion process of the free water to hydrates to obtain a transportable system in a pipeline. The deductions regarding the formation and macrostructure (slurry, slush, dry powder, etc.), based on the torque measurements, correlated well with the information obtained visually from the video recordings. Figure 7 and Tables 3 and 4 show the relative change in maximum torque (maximum average torque measured during the hydrate conversion process divided by average torque measured about 1 min before hydrate initiation) measured during hydrate formation in the continuous-flow experiments. For the stop/start experiments for system REFl shown in Fig. 8 and Table 5, the relative change in maximum torque for each experiment is defined as the maximum average torque measured during the hydrate conversion process before the pipe was clogged by hydrates divided by average torque measurement about 0.5 min after startup. Deposits of hydrates on the pipe wall If hydrates are going to be accepted in a pipeline, no hydrates can be allowed to stick to the pipe wall. Figures 9 [continuous-flow experiments (Tables 3 and 4)] and 10 [stop/start experiments system REFI (Table 5)] show a classification of the hydrate deposits observed on the pipe wall at the end of the experiments. For systems where the pipe was clogged by hydrate lumps, the results are based on a point of time immediately before the clogging occurred. Evaluations are based on flow resistance (torque) measurements supplied by visual observations and are classified as: - No deposits of hydrates on the pipe wall - Only traces or small amounts of hydrates on the pipe wall - A thin hydrate layer on the pipe wall giving some flow resistance
10.5 10.9 11.2 10.5 10.4 12.5 11.4 12.5 11.9
REFl-1 REFlL3 REFl-5 REFlG7 REFl-9 REF2 REF3 REM REFS
66.1 66.3 66.4 66.1 66.0 67.2 66.8 66.5 66.5
PO (bara)
6.4 6.0 5.7 6.4 6.5 4.5 5.6 4.5 5.1
AT (“C) 40.9 39.6 38.5 40.9 41.2 33.6 38.1 32.8 35.6
AP (bar) 11.4 12.9 10.9 10.7 9.2 8.9 9.0 12.7 11.8
r*lr**
12.2 11.6 11.8 10.9 11.6 11.4 11.5 11.3 11.1
67.0 66.7 66.8 66.3 66.7 66.7 66.9 65.9 66.1
PO (bara)
4.8 5.4 5.2 6.0 5.4 5.6 5.5 5.6 5.8
Note: The figures given in parentheses are subcooling and overpressurization at hydrate about 1 min before hydrate initiation. (-) No The pipeline was clogged by hydrate lumps
REFl-2 REFl-4 REFl-6 REFl-8 REFl-10 REF2 REF3 REF4 REF5
System (-repetition) 1.7 2.0 1.8 1.8 1.8 2.2 2.1 2.0 2.2
8.0 7.5 8.0 8.0 9.0 8.0 9.0 8.0 9.0
(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1)
34.0 52.5 38.5 35.0 35.0 37.5 40.0 38.0 34.0
(0.5) (0.6) (0.5) (0.6) (0.5) (0.4) (0.4) (0.5) (0.4)
Hydrate lumps starts to stick to pipe wall (min)
Hydrate lumps (slush-like) (min)
1.0 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.5
(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.0) (0.0) (0.1)
experiments
14.0 16.0 15.5 15.5 16.5 19.0 17.0 16.5 17.0
(0.3) (0.4) (0.4)’ (0.4)’ (0.4)r (0.4) (0.4) (0.4)” (0.4)
Hydrate lumps (slush-like) starts to stick to pipe wall (mm)
from continuous-flow
Slurry/slush hydrates (min)
and measurements
r*lr**
(0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0)
(u = 1 m/s) for systems
experiments
47.0 66.5 44.5 42.0 53.5 52.0 57.5 46.5 40.5
(1.0) (1.0) (0.8) (0.8) (0.9) (1.0) (1.0) (0.9) (0.8)
Clogging of the pipe (min)
REFlLREFS
Hydrate layer Thin hydrate layer Hydrate layer Hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Hydrate layer Hydrate layer
Deposits observed on the pipe wall at the end of the experiment
25.0 41.0 40.0 43.0 27.0 27.0 40.0 26.5
(1.5) (1.9) (2.0) (2.0) (1.5) (1.5) (2.1) (1.5)
Slush/ powder-like hydrates (min)
REFl-REFS
30.0 51.0 50.5 52.0 31.0 30.0 50.0 30.0
(2.1) (3.0) (3.0) (3.0) (2.1) (2.2) (3.0) (2.2)
Powder hydrates (min)
(u = 3 m/s) for systems
30.5 (1.1)
Clogging of the pipe (min)
Hydrate layer Hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer
Deposits observed on pipe wall at the end of the experiment
mole gas consumed by hydrate formation. To and PO are temperature and pressure at hydrate initiation. AT and BP are the degrees of is the maximum average torque measured during the hydrate conversion process divided by average torque measured initiation. r+/r,, observations. in ‘22.0 min, ‘21.5 min, ‘22.5 min and ’ 19.5 min.
34.7 37.1 36.3 39.6 37.1 38.1 37.8 37.6 38.6
(bar)
AP
Table 4. Observations
3.0 3.0 3.0 2.5 3.0 3.0 3.0 3.0 3.0
Slurry/slush hydrates (min)
from continuous-flow
by hydrate formation. To and PO are temperature and pressure at hydrate initiation. AT and AP are the is the maximum-average torque measured during the hydrate conversion process divided by average r,/~,,
and measurements
Note: The figures given in parentheses are mole gas consumed degrees of subcooling and overpressurization at hydrate initiation. torque measured about 1 min before hydrate initiation.
(2)
System (-repetition)
Table 3. Observations
Z
2 r=. g
Z E: B 0”
6 5 B c : s 09 c
A. LUND et al.
3454
I
18 -
3 2 z $9
5.; ?a kc!
$5 ;a
0 14-
12 -
10
0
-
u’n
*
??$
6--
22
0
16 -
0 0 +
g; P
,m 8
+++++++++ A
A
A
4
+
2-
+
+
??
?? + +
*
A
A
A
A
A
0
1
A
A
5
z
?
F
5
P
Fig. 6. Hydrate macrostructure
pheral velocity of the wheel) continuous-flow for systems REFl-REFS. Tabulated values Table 3.
experiments
:
?
G
!ti
I
z LG
observed in the stop/start
for system REFl. Tabulated Table 5.
All hydrates deposited a large flow resistance
~
t
I
Fig. 4. Hydrate macrostructure observed in the 1 m/s (periexperiments are given in
A
I
values are given in
on the pipe
wall giving
DISCUSSION
??
*
??
??
3 d 0’ ?? ??
c
?? ?? 0 0
a
?? ?? ? ? ??
* 0
? ? ??
Fig. 5. Hydrate macrostructure observed in the 3 m/s (peripheral velocity of the wheel) continuous-flow experiments for systems REFl-REFS. In some experiments the pipeline was clogged for some time by hydrate lumps. (1 22.0 min (REFl-6), 2 21.5 min (REFlL8) 3 22.5 min (REFlI10) and 4 19.5 min (REF4) before dissociation into slush/powderlike hydrates. Tabulated values are given in Table 4.
- A hydrate layer on the pipe wall giving flow resistance - Large deposits on the pipe wall giving a large flow resistance
All trends in the results are unambiguous regarding the reproducibility of the degree of subcooling at hydrate initiation, development of hydrate macrostructures, changes in fluid flow properties during conversion of free water to hydrates and deposits of hydrates on the pipe wall. The results are also in agreement with similar systems tested earlier (Lund et al., 1991, 1992; Lund and Lysne, 1992). This study has shown that it should be unnecessary to reproduce the experiments in order to confirm the results for a given hydrocarbon gas-water-model oil system, if deposits, agglomeration or plugging by hydrates are obtained. On the other hand, when no deposits, etc., by hydrates are obtained, all significant system parameters should be carefully examined. Reproducibility of the test mixture The test mixture was remixed five times in the test wheel. For each mixture, gas analysis was taken before (tests 1 and 2) and after (test 3) addition of the fluid phases to the wheel. Sampling of the gas was done through the inlet and outlet valves on the test wheel. The ports of these valves are parallel to the inside wall of the pipe and have no sealing of the high-pressure parts from the pipe wall. This results in accumulation of liquids (e.g. condensed propane and ethane) in the valves. The liquid film on the pipe wall is also drawn into the valves by bleeding of gas from the loop. These factors are reflected in the gas analysis given in Table 1 by the decreasing content of propane and ethane from test 1 to test 2 (taken immediately after
12.0 12.0 12.1 12.1 12.3
AT (“C) 53.4 53.3 53.5 53.5 53.6
AP (bar) 5.6 7.0 7.5 7.0 6.5
U,*
from stop/start
0.8 0.8 0.8 0.8 0.8
(0.3) (0.3) (0.3) (0.3) (0.2)
Slurry/slush hydrates (min)
and measurements
2.0 1.5 2.0 1.3 2.0
(0.4) (0.3) (0.3) (0.3) (0.3)
(0.4) (0.3) (0.4) (0.4) (0.4)
15.3 6.3 9.8 16.3 9.0
(1.0) (0.6) (0.8) (1.0) (0.7)
Clogging of the pipe (min)
Hyd. lumps starts to stick to pipe wall (min) 4.0 3.0 5.5 5.0 5.5
for system REFl
(u = 1 m/s after startup)
Hydrate lumps (slush-like) (min)
experiments
Thin hydrate layer Large deposits Hydrate layer Thin hydrate layer Hydrate layer
Deposits observed on the pipe wall at the end of the experiment
Relative change I” maximum torque due to hydrate formaton I” the stop/start experiments
??
??
??
??
??
??
??
??
w
Relative change on maxwnum torque due to hydrate formation
Note: The figures given in parentheses are mole gas consumed by hydrate formation. T0 and P, are temperature and pressure at hydrate initiation. AT and AP are the is the maximum average torque measured during the hydrate conversion process before clogging of degrees of subcooling and overpressurization at hydrate initiation. r*/t,, the pipe divided by average torque measured about 0.5 min after startup of the wheel.
64.1 64.0 64.0 64.0 63.8
REFl-11 REFl-12 REFl-13 REFl-14 REFI-15
4.7 4.7 4.6 4.6 4.4
PO (bara)
System (-repetition)
Table 5. Observations
A. LUND et al.
il
?? ??
test 1) for each gas mixture. More suitable test valves have to be installed on the test wheel in order to obtain more proper gas analysis. For use in calculation of the hydrate equilibrium data for the systems, the mole compositions of the remixtures (Table 2) were recalculated from measured filling data by the program HYSIM. The reliability of the given compositions is estimated to be about + 0.1 mol.
1 “v, 3rTd8
Hydrate equilibrium data The reliability of the calculated hydrate equilibrium data for system REFl given in Fig. 2 is estimated to be about + 0.5”C. For systems REF2-REFS, the calculated hydrate equilibrium data deviated +O.O5”C from the data for system REFl in the temperature region l-15°C.
Fig. 9. Classification of hydrate at the end of the experiment for ments for systems REFl-REFS. was clogged by hydrate lumps,
depositions on the pipe wall the continuous-flow experiFor systems where the pipe
the evaluations were based on a point of time immediately before the clogging occurred. Tabulated values are given in Tables 3 and 4.
??
*
*
Fig. 10. Classification of hydrate depositions on the pipe wall at the end of the experiment for the stop/start experiments for system REFl. The evaluations were based on a point of time immediately before clogging of the pipe. Tabulated values are given in Table 7.
Table 6. Groups Group
A B C D E
of experiments
Experiments 1 m/s 1 m/s 3 m/s 3 m/s
continuous continuous continuous continuous Stop/start
Degree of subcooling at hydrate initiation The grouping of the experiments used for the statistical analysis is given in Table 6. Average degree and standard deviations of subcooling at hydrate initiation for the continuous-flow experiments are given in Table 7. The results indicate that the subcooling is around 5.5”C for both the 1 and 3 m/s continuousflow experiments. As hydrate nucleation is a stochastic process on the interfacial areas between water and gas-oil phases, the time at subcooling for a given cooling rate seems to be the dominant effect when large interfacial areas are already available in the 1 m/s continuous-flow experiments. Similar results are also indicated for the loop (wheel shaped) used by Lippmann et al. (1995). The variance between the experiments indicate that the 1 m/s continuous-flow experiments have a larger variance between the remixtures (group B) than when reproduced within one mixture (system REFl, group A). For the 3 m/s continuous-flow experiments no significant difference was indicated between the experiments performed between remixtures (group D) or reproduced within one mixture (system REFl, group C). The reason for the apparent larger variance in the 1 m/s continuous-flow experiments may be due to the impact of impurities to the stochastic effect on the hydrate nucleation process. In the 3 m/s continuousflow experiments this factor is probably less dominating due to the increased available interfacial area. The results also indicate that heating of the systems to 70°C between each experiment is sufficient to eliminate hysteresis effects from previously formed hydrates.
used for statistical
analysis
Systems flow flow flow flow
REFlLl, -3, -5, REFlLl, REF2, REF3, REFl&2, -4, -6, REFl-2, REFZ, REF3, REFl-11, -12, -13,
-7, -9 REF4, REF5 -8, -10 REF4, REF5 -14, -15
Inhibition Table 7. Means
and standard
of gas hydrate deviation
formation-II
(in parenthesis)
3451
within each group
(Table 6)
Group A (1 m/s cont. flow exp.)
Variable
B (1 m/s cont. flow exp.)
C (3 m/s cont. flow exp.)
D (3 m/s cont. flow exp.)
E (stop/start exp.)
10.70 (0.34)
11.76 (0.84)
11.62 (0.47)
11.50 (0.42)
4.60 (0.12)
(bara)
66.18 (0.16)
66.62 (0.41)
66.70 (0.26)
66.52 (0.49)
63.98 (0.11)
,4,T)
6.20 (0.34)
5.22 (0.80)
5.36 (0.43)
5.46 (0.38)
12.10 (0.12)
gr)
40.22 (1.14)
36.20 (3.33)
36.96 (1.77)
37.36 (1.53)
53.46 (0.11)
11.02 (1.33)
10.76 (1.72)
1.82 (0.11)
2.04 (0.2 1)
6.72 (0.72)
2.90 (0.22)
3.00 (0.00)
1.oo (0.00)
1.30 (0.27)
0.80 (0.00)
Hydrate lumps (slush-like) (min)
8.10 (0.55)
8.40 (0.55)
Hydrate lumps starts to stick to pipe wall (min)
39.00 (7.74)
36.70 (2.64)
PO
%I%* Slurry/slush (min)
hydrates
Slush/powder-like hydrates (min) Powder Clogging (min)
hydrates
(min)
of the pipe
Deposits (factor) observed on the pipe wall at end of the experiment
-
1.76 (0.34)
15.50 (0.94)
16.70 (1.79)
-
37.25 (8.26)
30.12 (6.59)
-
45.87 (10.60)
35.25 (9.84)
50.70 (9.81)
48.70 (6.39)
2.60 (0.55)
2.60 (0.55)
Hydrate macrostructures and changes in fluid flow properties during conversion of free water to hydrates In the development of hydrate macrostructures no significant difference was indicated for the 1 m/s continuous-flow experiments (Fig. 4 and Table 3) or the stop/start experiments (Fig. 6 and Table 5). The only appreciable difference between the experiments was the time period from when the hydrate lumps started to stick to the pipe wall to the clogging of the pipe. From evaluations of the data, this factor mainly seems to be dependent on the amount of hydrate deposits on the pipe wall observed visually and through the torque measurements. In some of the 3 m/s continuous-flow experiments (Fig. 5 and Table 4) the pipe was clogged by lumps of hydrates for some time. As more water in the lumps was converted to hydrates, the lumps started to move again before being converted to powder-like hydrates (except for experiment REFl-2 which stayed clogged). The process of clogging of the pipe seems to be influenced by the experimental equipment. When hydrate is formed there is a tendency for hydrates to gather together in the lower part of the wheel forming one (or more) plug(s) sliding along the pipe wall. This long plug will almost behave like a solid sliding on the pipe wall. As hydrate deposits on the pipe wall increase the friction forces, the entire hydrate plug starts
4.60 (1.08)
11.34 (4.29) 2.40 (0.55)
2.80 (0.45)
2.80 (0.84)
to oscillate. At a certain time the plug is carried over the top of the wheel and may develop into a plug permanently clogging the pipe. This seems to happen when slush-like hydrates, both in the plug and on the pipe wall, develop a more roughly fibrous texture. As there is no large pressure buildup on the upstream side of a plug, the clogging of the experimental equipment does not mean that a real system would be clogged at similar conditions. The ratio of pipe cross-sectional area to perimeter is larger (directly proportional) in a big pipe than in a small pipe. This means that for a given pressure difference the total shear strength or adhesion to the pipe walls will be less at higher diameters for a plug of a given length. This may be one of the reasons why earlier experiments by a similar system (Lund et al., 1992) did not clog 3 and 5 in ID wheels (similar wheel diameters) as easily as the 2 in ID wheel. The decreased tendency of plugging in the 3 m/s continuous-flow experiments seems for the given system mainly to be due to the increased effective gravity forces (gravity plus centrifugal forces) on the system. Deposits of hydrates on the pipe wall For the stop/start experiments the degree of deposits on the pipe wall was observed to be somewhat larger than for the continuous-flow experiments. With
A. LUND et al.
3458 Table 8. Composition
and physical
properties
of Exxsol D60
Composition, Exxsol 060 wt % Normal + isoparaffin hydrocarbons Naphthenic hydrocarbons Minimum
99.5% aliphatic
hydrocarbons.
Cg
Cl0
Cl,
C12
Cl3
Total C
0 1
5 13
23 18
23 8
6 3
57 43
Maximum
0.5% aromatic
hydrocarbons
Physical properties, Exxsol 060 Initial boiling point End point/dry point Flash pt. tag closed cup Relative volatility (n-butylacetate Colour (Saybolt) Density 15°C Viscosity 25°C Refractive index Bromine number Kauri Butanol value Aniline point Sulphur content Benzene content
= 1.00)
190°C 221°C > 61°C 0.05 + 30 0.79 kg/l 1.28 mPa 1.436 50 31 70 l mg/kg (l ppm) < 20 mg/kg
(-c 20 ppm)
161
Average molecular weight
a tendency of the given system to form hydrate deposits on the pipe wall, this was expected due to the higher degree of subcooling. In the experiments it was not possible to measure quantitatively the amount or thickness of a hydrate layer on the pipe wall by direct means. Classification of hydrate deposits on the pipe wall at the end of the experiments was mainly evaluated from torque measurements supplied by video recordings. As the visual part depends on the observer, all experiments were evaluated by at least two experienced scientists.
CONCLUSIONS
A given hydrocarbon gas-water-model oil system, used as a reference system for testing of new hydrate inhibitors, has been evaluated for experimental reproducibility of hydrate formation and behaviour at flow conditions. The system was remixed five times in a 2” ID high-pressure flow loop and tested for the reproducibility both within one mixture (closed system) and between the remixtures. The results indicated good reproducibility both within the closed system and between the remixtures. All trends in the results were unambiguous regarding the reproducibility of the degree of subcooling at hydrate initiation, development of hydrate macrostructures, changes in fluid flow properties during conversion of free water to hydrates and deposits of hydrates on the pipe wall. The results were also in agreement with similar systems tested earlier. This study has shown that it should be unnecessary to reproduce the experiments in order to confirm the results for a given hydrocarbon gas-water-model oil system, if deposits, agglomeration or plugging by hydrates are obtained. On the other hand, when no
deposits, etc., by hydrates are obtained, all significant system parameters should be carefully examined.
Acknowledgements-The Statoil Multiphase Flow Program is acknowledged for giving permission to publish the paper. Petreco A/S, Stjsrdal, Norway is acknowledged for performing the experimental work. REFERENCES
Austvik, T., 1992, Dr. Ing Thesis, The Norwegian Institute of Technology, Trondheim, Norway. Austvik, T., Lund, A., Lysne, D., Lindeberg, E. and Leken, K.-P., 1990, Report Nr: STF21 A90OO6, SINTEF Applied Chemistry, Norway. Hammerscmidt, E. H., 1934, Ind. Engng Chem. 26, 851. HYSIM, 1994, Process simulation program by Hydrotech Ltd, Calgary, Alberta, Canada. Lippmann, D., Kessel, D. and Rahimian, I., 1995, Gas hydrate nucleation and growth kinetics in multiphase transportation systems. In Proceedings of the 5th International Ofihore and Polar Engineering Conference, The Hague, The Netherlands. Lund, A. and Lysne, D., 1992, Report Nr: STF21 A92073, SINTEF Applied Chemistry, Norway. Lund, A., Lysne, D., Austvik, T. and Arde, B.A., 1991, Report Nr: STF21 A91040, SINTEF Applied Chemistry, Norway. Lund, A., Lysne, D., Grande, 0. and Austvik, T., 1992, Report Nr:. STF21 F92034, SINTEF Applied Chemistry, Norway. Makogon, Y. F., 1981, Hydrates of Natural Gas. Penn Well Books, Tulsa. Nygird, H. F., 1989, Transportability of hydrates in multiphase systems. In Proceedings of the 4th International Conference on Multiphase Flow, Nice, France. Reed, R. L., Kelley, L. R., Neumann, D. L., Oellke, R. H. and Young, W. D., 1994, Some preliminary results from a pilot-size hydrate flow loop. Ann. N. Y. Acad. Sci. 715,311. SHP, Statoil Hydrate Program, 1989, Statoil, Research Centre, Trondheim, Norway. Sloan, E. D. Jr, 1990, Chlathrate Hydrates of Natural Gas. Marcel Dekker, New York. Urdahl, O., Lund, A., Msrk, M. and Nilsen, T. N., 1995, Chem. Engng Sci. 50, 863.
Chemical Engmeering Science, Vol. 51, No. 13, pp. 3449-3458, 1996 Copyright 0 1996 Elsewer Science Ltd Printed m Great Britain. All rights reserved ooO9%2509/96 $15.00 + 0.00
0009-2509(95)00410-6
INHIBITION OF GAS HYDRATE FORMATION BY MEANS OF CHEMICAL ADDITIVES-II. AN EVALUATION OF THE SCREENING METHOD
SINTEF,
Applied
ARE LUND Chemistry, 7034 Trondheim,
Norway
and OLAV URDAHL* STATOIL, Research
and SIGRID S. KIRKHORN Centre, 7005 Trondheim, Norway
(First Received 12 June 1995; accepted in revised form 6 December 1995)
Abstract-A
given hydrocarbon gas-water-model oil system, used as a reference system for testing of new inhibitors, has been evaluated for experimental reproducibility of hydrate formation and behavior at flow conditions. The system was remixed five times in a 2” ID high-pressure flow loop and tested for the reproducibility both within one mixture (closed system) and between the remixtures. The results indicated good reproducibility both within the closed system and between the remixtures. Copyright 0 1996 Elsevier Science Ltd hydrate
INTRODUCTION
Hydrocarbon reserves in the North Sea include several marginal fields, which are either small or located at large water depths. Feasible economic exploitation of these reserves requires a shift towards lower cost options in subsea production, or the use of minimumprocessing platforms tied to centralized or land-based processing facilities. This will require the transportation of unprocessed or minimum-processed fluids over long distances. The fluids may consequently be exposed to low-temperature regions, where gas hydrate formation is expected. Gas hydrates, or clathrate hydrates, are one class of inclusion compounds (Makogon, 1981; Sloan, 1990). Common for all hydrates is that water molecules, due to hydrogen bonding, form a three-dimensional network, whose voids are occupied by non-polar molecules with low molecular weight and small rotational diameter. Whereas the lattice is usually not stable itself, the encaged gas molecules will contribute to stabilize it. At high pressure and/or at low temperatures, thermodynamically stable hydrates will form. If the pressure-temperature conditions in the subsea-pipeline favours the formation of these hydrates, extensive formation may take place and this can result in clogging of the pipeline due to deposits on the pipe wall or by formation of plugs in bends, valves, etc. (Hammerschmidt, 1934; Nyg&d, 1989; Austvik, 1992). Subsea transportation of unprocessed or minimumprocessed well fluids over long distances today re-
*Corresponding
author.
quires the use of large amounts of methanol or glycols for hydrate inhibition. The effect of these additives is to decrease the water activity to an extent that markedly reduces its ability to participate in hydrate formation, and thereby in a lowering of the hydrate formation temperature. The amount of inhibitor nccessary to obtain the desired lowering of the hydrate formation temperature is substantial, usually in the range of 20-40 wt% of the aqueous phase. This has prompted the search for new types of additives capable of inhibiting hydrate formation at far lower concentrations. In a previous paper (Urdahl et al., 1995) we have described an experimental set-up and a procedure for testing and evaluation of new gas hydrate inhibitors. The choice of the set-up, reference system and experimental procedure was based on earlier experiences. Different systems [trichlorofluoromethane (Rl l)-water, gas-water, gas-water-condensate and gas-water-oil systems] have been tested for hydrate properties in six different flow loops of variable inner diameters (1”-5”) and lengths (1.4430 m) (Austvik et al., 1990; Lund et al., 1991, 1992; Lund and Lysne, 1992). The main results from these experiments have been in agreement with later results published from other scientific groups (Lippmann et al., 1995; Reed et al.., 1994). However, most of the efforts in these experiments have been on evaluating different experimental conditions and not on the reproducibility of a single experiment (except for Rll-water systems which were interrupted by hysteresis effects). In this paper we have therefore looked at the reproducibility of our method with emphasis on nucleation time, subcooling and hydrate macrostructure build up.
3449
A.
3450 EXPERIMENTAL
LUNI )
SET-UP
The experiments were carried out in a high-pressure loop formed as a wheel at PETRECO A/S. The test wheel was filled with the desired fluid at a specified temperature and pressure, and then set under rotation. The rotation created a relative velocity between the pipe wall and the fluid and transport through a pipeline was thereby simulated. The high-pressure wheel is made from stainless steel with an inner tube diameter of 52.5 mm and a wheel diameter of 2.0 m. The volume is 13.4 1. The wheel includes two high-pressure windows for visual inspection, of which one is equipped with a Sony V800E Hi-8 video camera. The flow simulator is placed in a temperaturecontrolled chamber. The temperature is controlled using a programmable regulator, a heating fan and a refrigerator system. The temperature development in the chamber as a function of time is preset on the regulator. The wheel is attached to a motor/gearbox system enabling rotation between 0.3 and 5.0 m/s translation velocity. A torque sensor is installed as part of the rotational shaft enabling torque measurements to be performed during rotation. Pressure and temperature sensors on the wheel measures in the ranges of O-250 bara and - 10 to + lSO”C, respectively. All signals are transferred through cables and slip rings to a realtime PC-based data acquisition system. The accuracy of the measurements is estimated to be + 0.2 Nm for torque, f 0.5 bar for pressure, f O.l”C for fluid temperature in the wheel and * l.O”C for temperature in the chamber. The 2” ID flow simulator used is shown in Fig. 1. EXPERIMENTAL
WORK
The objective of the experiments was to study the reproducibility of hydrate formation and behaviour
Fig. 1. The PETRECO
et al. for a given hydrocarbon gas-water-model oil system. The system to be evaluated consisted of about 31.3 mol methane, 6.4 mol ethane, 3.7 mol propane, 1.0 1 (55.5 mol) distilled water, and 5.0 1 (31.1 mol) Exxsol D60 (Table 8). Experiments of two essentially different kinds were performed: (a) The wheel was rotated with a constant peripheral velocity (1 or 3 m/s) as the temperature was decreased from 70 to 4°C at a given cooling rate. These experiments are in the report referred to as continuous-flow experiments. The velocity of 1 m/s created a laminar flow regime in the liquid phases (prior to hydrate formation) while a velocity of 3 m/s gave a mixed turbulent flow regime. The experiments simulate flow in a long pipeline where the fluid gradually cools to sea bed temperature as it flows along the pipeline. (b) The system was first heated to 70°C while the wheel was rotating (peripheral velocity of 1 m/s). The wheel was then stopped and cooled down to a temperature of 4°C where it was kept constant for a period of a minimum of 12 h. The wheel was then restarted and rotated at a constant peripheral velocity of 1 m/s for the rest of the experiment. These experiments are referred to as stop/start experiments. This simulates a restart of a pipeline after a shut-in period. In order to minimize the number of adjustable parameters for the experiments, only one given cooling rate (temperature in chamber from 70 to 4°C in 1.5 h) was used both for the continuous-flow experiments and the stop/start experiments. The system to be evaluated was remixed five times in the test wheel. These mixtures are in this paper
2” ID fluid flow simulator.
Inhibition
of gas hydrate
numbered as systems REFl-REFS. For system REFl each experiment (1 and 3 m/s continuous-flow experiments and the stop/start experiment) was repeated five times in order to evaluate the reproducibility within a closed system. Before each experiment, the system was heated to 70°C for a minimum of 1 h in order to minimize any hysteresis effects from previously formed hydrate structures. Systems REF2-REFS were tested once with 1 and 3 m/s continuous-flow experiments. Before each experiment these systems were heated to 70°C. Compositional analysis of the gas phases used (before and after addition of liquid phases) were carried out for each mixture. All experiments were video, photo and data recorded.
formation-II
- 1, - 3, - 5, flow experiments - 8 and - 10, subnumbered -
Table 1. Gas analysis
- 7 and - 9, the 3 m/s continuousare subnumbered - 2, - 4, - 6, and the stop/start experiments are 11 to - 15.
Gas analysis
Gas analysis for each system (before and after addition of liquid phases) are given in Table 1, while compositions calculated from measured filling data are shown in Table 2. Hydrate
equilibrium
data
Figure 2 shows the hydrate equilibrium curve for system REFI. The equilibrium curve is calculated using the hydrate program SHP. Degree
EXPERIMENTAL RESULTS
The main results and measurements from the experiments with the systems REFl-REF5 are shown in Figs 3-10 and in Tables l-5. For system REFl, the 1 m/s continuous-flow experiments are subnumbered
3451
of subcooling
at hydrate
for systems
REFl-REF5 Vol. %
P (bar)
0;
N;
CH.,
CzHs
CJHs
only gas phase in the wheel Test 1 26.9 59.4 Test 2 27.0 59.3
-
0.21
70.13 73.42
17.87 16.41
12.00 9.96
Gas phase in equilibrium with water and ExxsolD60 Test 3 25.0 70.1 0.05 0.13 87.45 Test 4 25.0 69.4 0.06 0.25 87.55
9.52 9.36
2.85 2.78
71.76 72.62
16.47 16.56
11.77 10.82
Gas phase in equilibrium with water and ExxsolD60 Test 3 24.8 71.0 88.26
9.16
2.80
45.02 73.80
19.52 16.21
13.38 9.99
87.01
9.50
3.09
71.88 73.60
17.16 16.73
10.96 9.66
Gas phase in equilibrium with water and ExxsolD60 Test 3 27.3 70.8 87.94
9.39
2.67
67.87 65.89
18.33 15.98
13.48 11.37
87.76
9.35
2.89
Test System: REFl
System: REF2 Only gas phase in the wheel Test 1 26.9 59.4 Test 2 26.2 59.2
-
-
System: REF3 Only gas phase in the wheel Test 1 26.8 59.6 Test 2 27.0 59.3
4.52 -
17.56
Gas phase in equilibrium with water and ExxsolD60 Test 3 27.7 72.4 0.07 0.33 System: REF4 Only gas phase in the wheel Test 1 27.0 59.3 Test 2 27.1 59.1
-
-
System: REFS Only gas phase in the wheel Test 1 27.8 59.4 Test 2 27.9 59.3
0.06 1.35
0.26 5.40
Gas phase in equilibrium with water and ExxsolD60 Test 3 25.9 71.0 +Mixed with air in test bottle.
initiation
The degree of subcooling (“C) is the difference between experimental hydrate initiation temperature and the hydrate equilibrium (melting) temperature at the experimental pressure. Figure 3 and Tables 3 and
A. LUND et al.
3452 Table 2. Composition
(mole) for systems REFl-REFS data)
System
Methane
Ethane
REFl REF2 REF3 REM REFS
31.4 31.2 31.2 31.4 31.3
6.4 6.3 6.4 6.4 6.4
Propane 3.1 3.8 3.1 3.1 3.7
(calculated
from filling
Water
Exxsol D60
55.5 55.5 55.5 55.5 55.5
31.1 31.1 31.1 31.1 31.1
structures observed after start-up in the stop/start experiments are shown in Fig. 6 and Table 5.
Fig. 2. Calculated
hydrate
equilibrium REFl.
0
0
0
0
?? ?? ??
curve
for system
0
??
+
?? Q
?? +
0
0
0
0
(ml%
.
3mA
Fig. 3. Degree of subcooling obtained prior to hydrate equilibrium initiation for the systems REFl-REFS in the 1 and 3 m/s (peripheral velocity of the wheel) continuous-flow experiments. Tabulated values are given in Tables 3 and 4.
4 show
the degree of subcooling obtained prior to hydrate initiation in the continuous-flow experiments.
Hydrate macrostructures A description of the hydrate macrostructures observed (slurry, slush and powder-like hydrates) have previously been given by Austvik (1992). Figures 4 and 5 and Tables 3 and 4 give the time from hydrate initiation to the observed macrostructures in the continuous-flow experiments. The hydrate macro-
Changes in fluid flow properties during conversion of free water to hydrates Information on changes in fluid viscosity, deposits of hydrates on the pipe wall and clogging of the pipe by hydrate lumps were obtained from the recorded torque measurements on the rotational shaft of the wheel. The relative change in torque, or apparent viscosity, should be negligible during the conversion process of the free water to hydrates to obtain a transportable system in a pipeline. The deductions regarding the formation and macrostructure (slurry, slush, dry powder, etc.), based on the torque measurements, correlated well with the information obtained visually from the video recordings. Figure 7 and Tables 3 and 4 show the relative change in maximum torque (maximum average torque measured during the hydrate conversion process divided by average torque measured about 1 min before hydrate initiation) measured during hydrate formation in the continuous-flow experiments. For the stop/start experiments for system REFl shown in Fig. 8 and Table 5, the relative change in maximum torque for each experiment is defined as the maximum average torque measured during the hydrate conversion process before the pipe was clogged by hydrates divided by average torque measurement about 0.5 min after startup. Deposits of hydrates on the pipe wall If hydrates are going to be accepted in a pipeline, no hydrates can be allowed to stick to the pipe wall. Figures 9 [continuous-flow experiments (Tables 3 and 4)] and 10 [stop/start experiments system REFI (Table 5)] show a classification of the hydrate deposits observed on the pipe wall at the end of the experiments. For systems where the pipe was clogged by hydrate lumps, the results are based on a point of time immediately before the clogging occurred. Evaluations are based on flow resistance (torque) measurements supplied by visual observations and are classified as: - No deposits of hydrates on the pipe wall - Only traces or small amounts of hydrates on the pipe wall - A thin hydrate layer on the pipe wall giving some flow resistance
10.5 10.9 11.2 10.5 10.4 12.5 11.4 12.5 11.9
REFl-1 REFlL3 REFl-5 REFlG7 REFl-9 REF2 REF3 REM REFS
66.1 66.3 66.4 66.1 66.0 67.2 66.8 66.5 66.5
PO (bara)
6.4 6.0 5.7 6.4 6.5 4.5 5.6 4.5 5.1
AT (“C) 40.9 39.6 38.5 40.9 41.2 33.6 38.1 32.8 35.6
AP (bar) 11.4 12.9 10.9 10.7 9.2 8.9 9.0 12.7 11.8
r*lr**
12.2 11.6 11.8 10.9 11.6 11.4 11.5 11.3 11.1
67.0 66.7 66.8 66.3 66.7 66.7 66.9 65.9 66.1
PO (bara)
4.8 5.4 5.2 6.0 5.4 5.6 5.5 5.6 5.8
Note: The figures given in parentheses are subcooling and overpressurization at hydrate about 1 min before hydrate initiation. (-) No The pipeline was clogged by hydrate lumps
REFl-2 REFl-4 REFl-6 REFl-8 REFl-10 REF2 REF3 REF4 REF5
System (-repetition) 1.7 2.0 1.8 1.8 1.8 2.2 2.1 2.0 2.2
8.0 7.5 8.0 8.0 9.0 8.0 9.0 8.0 9.0
(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.1)
34.0 52.5 38.5 35.0 35.0 37.5 40.0 38.0 34.0
(0.5) (0.6) (0.5) (0.6) (0.5) (0.4) (0.4) (0.5) (0.4)
Hydrate lumps starts to stick to pipe wall (min)
Hydrate lumps (slush-like) (min)
1.0 1.0 1.0 1.0 1.0 1.0 1.5 1.5 1.5
(0.1) (0.1) (0.1) (0.1) (0.1) (0.1) (0.0) (0.0) (0.1)
experiments
14.0 16.0 15.5 15.5 16.5 19.0 17.0 16.5 17.0
(0.3) (0.4) (0.4)’ (0.4)’ (0.4)r (0.4) (0.4) (0.4)” (0.4)
Hydrate lumps (slush-like) starts to stick to pipe wall (mm)
from continuous-flow
Slurry/slush hydrates (min)
and measurements
r*lr**
(0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0)
(u = 1 m/s) for systems
experiments
47.0 66.5 44.5 42.0 53.5 52.0 57.5 46.5 40.5
(1.0) (1.0) (0.8) (0.8) (0.9) (1.0) (1.0) (0.9) (0.8)
Clogging of the pipe (min)
REFlLREFS
Hydrate layer Thin hydrate layer Hydrate layer Hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Hydrate layer Hydrate layer
Deposits observed on the pipe wall at the end of the experiment
25.0 41.0 40.0 43.0 27.0 27.0 40.0 26.5
(1.5) (1.9) (2.0) (2.0) (1.5) (1.5) (2.1) (1.5)
Slush/ powder-like hydrates (min)
REFl-REFS
30.0 51.0 50.5 52.0 31.0 30.0 50.0 30.0
(2.1) (3.0) (3.0) (3.0) (2.1) (2.2) (3.0) (2.2)
Powder hydrates (min)
(u = 3 m/s) for systems
30.5 (1.1)
Clogging of the pipe (min)
Hydrate layer Hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer Thin hydrate layer
Deposits observed on pipe wall at the end of the experiment
mole gas consumed by hydrate formation. To and PO are temperature and pressure at hydrate initiation. AT and BP are the degrees of is the maximum average torque measured during the hydrate conversion process divided by average torque measured initiation. r+/r,, observations. in ‘22.0 min, ‘21.5 min, ‘22.5 min and ’ 19.5 min.
34.7 37.1 36.3 39.6 37.1 38.1 37.8 37.6 38.6
(bar)
AP
Table 4. Observations
3.0 3.0 3.0 2.5 3.0 3.0 3.0 3.0 3.0
Slurry/slush hydrates (min)
from continuous-flow
by hydrate formation. To and PO are temperature and pressure at hydrate initiation. AT and AP are the is the maximum-average torque measured during the hydrate conversion process divided by average r,/~,,
and measurements
Note: The figures given in parentheses are mole gas consumed degrees of subcooling and overpressurization at hydrate initiation. torque measured about 1 min before hydrate initiation.
(2)
System (-repetition)
Table 3. Observations
Z
2 r=. g
Z E: B 0”
6 5 B c : s 09 c
A. LUND et al.
3454
I
18 -
3 2 z $9
5.; ?a kc!
$5 ;a
0 14-
12 -
10
0
-
u’n
*
??$
6--
22
0
16 -
0 0 +
g; P
,m 8
+++++++++ A
A
A
4
+
2-
+
+
??
?? + +
*
A
A
A
A
A
0
1
A
A
5
z
?
F
5
P
Fig. 6. Hydrate macrostructure
pheral velocity of the wheel) continuous-flow for systems REFl-REFS. Tabulated values Table 3.
experiments
:
?
G
!ti
I
z LG
observed in the stop/start
for system REFl. Tabulated Table 5.
All hydrates deposited a large flow resistance
~
t
I
Fig. 4. Hydrate macrostructure observed in the 1 m/s (periexperiments are given in
A
I
values are given in
on the pipe
wall giving
DISCUSSION
??
*
??
??
3 d 0’ ?? ??
c
?? ?? 0 0
a
?? ?? ? ? ??
* 0
? ? ??
Fig. 5. Hydrate macrostructure observed in the 3 m/s (peripheral velocity of the wheel) continuous-flow experiments for systems REFl-REFS. In some experiments the pipeline was clogged for some time by hydrate lumps. (1 22.0 min (REFl-6), 2 21.5 min (REFlL8) 3 22.5 min (REFlI10) and 4 19.5 min (REF4) before dissociation into slush/powderlike hydrates. Tabulated values are given in Table 4.
- A hydrate layer on the pipe wall giving flow resistance - Large deposits on the pipe wall giving a large flow resistance
All trends in the results are unambiguous regarding the reproducibility of the degree of subcooling at hydrate initiation, development of hydrate macrostructures, changes in fluid flow properties during conversion of free water to hydrates and deposits of hydrates on the pipe wall. The results are also in agreement with similar systems tested earlier (Lund et al., 1991, 1992; Lund and Lysne, 1992). This study has shown that it should be unnecessary to reproduce the experiments in order to confirm the results for a given hydrocarbon gas-water-model oil system, if deposits, agglomeration or plugging by hydrates are obtained. On the other hand, when no deposits, etc., by hydrates are obtained, all significant system parameters should be carefully examined. Reproducibility of the test mixture The test mixture was remixed five times in the test wheel. For each mixture, gas analysis was taken before (tests 1 and 2) and after (test 3) addition of the fluid phases to the wheel. Sampling of the gas was done through the inlet and outlet valves on the test wheel. The ports of these valves are parallel to the inside wall of the pipe and have no sealing of the high-pressure parts from the pipe wall. This results in accumulation of liquids (e.g. condensed propane and ethane) in the valves. The liquid film on the pipe wall is also drawn into the valves by bleeding of gas from the loop. These factors are reflected in the gas analysis given in Table 1 by the decreasing content of propane and ethane from test 1 to test 2 (taken immediately after
12.0 12.0 12.1 12.1 12.3
AT (“C) 53.4 53.3 53.5 53.5 53.6
AP (bar) 5.6 7.0 7.5 7.0 6.5
U,*
from stop/start
0.8 0.8 0.8 0.8 0.8
(0.3) (0.3) (0.3) (0.3) (0.2)
Slurry/slush hydrates (min)
and measurements
2.0 1.5 2.0 1.3 2.0
(0.4) (0.3) (0.3) (0.3) (0.3)
(0.4) (0.3) (0.4) (0.4) (0.4)
15.3 6.3 9.8 16.3 9.0
(1.0) (0.6) (0.8) (1.0) (0.7)
Clogging of the pipe (min)
Hyd. lumps starts to stick to pipe wall (min) 4.0 3.0 5.5 5.0 5.5
for system REFl
(u = 1 m/s after startup)
Hydrate lumps (slush-like) (min)
experiments
Thin hydrate layer Large deposits Hydrate layer Thin hydrate layer Hydrate layer
Deposits observed on the pipe wall at the end of the experiment
Relative change I” maximum torque due to hydrate formaton I” the stop/start experiments
??
??
??
??
??
??
??
??
w
Relative change on maxwnum torque due to hydrate formation
Note: The figures given in parentheses are mole gas consumed by hydrate formation. T0 and P, are temperature and pressure at hydrate initiation. AT and AP are the is the maximum average torque measured during the hydrate conversion process before clogging of degrees of subcooling and overpressurization at hydrate initiation. r*/t,, the pipe divided by average torque measured about 0.5 min after startup of the wheel.
64.1 64.0 64.0 64.0 63.8
REFl-11 REFl-12 REFl-13 REFl-14 REFI-15
4.7 4.7 4.6 4.6 4.4
PO (bara)
System (-repetition)
Table 5. Observations
A. LUND et al.
il
?? ??
test 1) for each gas mixture. More suitable test valves have to be installed on the test wheel in order to obtain more proper gas analysis. For use in calculation of the hydrate equilibrium data for the systems, the mole compositions of the remixtures (Table 2) were recalculated from measured filling data by the program HYSIM. The reliability of the given compositions is estimated to be about + 0.1 mol.
1 “v, 3rTd8
Hydrate equilibrium data The reliability of the calculated hydrate equilibrium data for system REFl given in Fig. 2 is estimated to be about + 0.5”C. For systems REF2-REFS, the calculated hydrate equilibrium data deviated +O.O5”C from the data for system REFl in the temperature region l-15°C.
Fig. 9. Classification of hydrate at the end of the experiment for ments for systems REFl-REFS. was clogged by hydrate lumps,
depositions on the pipe wall the continuous-flow experiFor systems where the pipe
the evaluations were based on a point of time immediately before the clogging occurred. Tabulated values are given in Tables 3 and 4.
??
*
*
Fig. 10. Classification of hydrate depositions on the pipe wall at the end of the experiment for the stop/start experiments for system REFl. The evaluations were based on a point of time immediately before clogging of the pipe. Tabulated values are given in Table 7.
Table 6. Groups Group
A B C D E
of experiments
Experiments 1 m/s 1 m/s 3 m/s 3 m/s
continuous continuous continuous continuous Stop/start
Degree of subcooling at hydrate initiation The grouping of the experiments used for the statistical analysis is given in Table 6. Average degree and standard deviations of subcooling at hydrate initiation for the continuous-flow experiments are given in Table 7. The results indicate that the subcooling is around 5.5”C for both the 1 and 3 m/s continuousflow experiments. As hydrate nucleation is a stochastic process on the interfacial areas between water and gas-oil phases, the time at subcooling for a given cooling rate seems to be the dominant effect when large interfacial areas are already available in the 1 m/s continuous-flow experiments. Similar results are also indicated for the loop (wheel shaped) used by Lippmann et al. (1995). The variance between the experiments indicate that the 1 m/s continuous-flow experiments have a larger variance between the remixtures (group B) than when reproduced within one mixture (system REFl, group A). For the 3 m/s continuous-flow experiments no significant difference was indicated between the experiments performed between remixtures (group D) or reproduced within one mixture (system REFl, group C). The reason for the apparent larger variance in the 1 m/s continuous-flow experiments may be due to the impact of impurities to the stochastic effect on the hydrate nucleation process. In the 3 m/s continuousflow experiments this factor is probably less dominating due to the increased available interfacial area. The results also indicate that heating of the systems to 70°C between each experiment is sufficient to eliminate hysteresis effects from previously formed hydrates.
used for statistical
analysis
Systems flow flow flow flow
REFlLl, -3, -5, REFlLl, REF2, REF3, REFl&2, -4, -6, REFl-2, REFZ, REF3, REFl-11, -12, -13,
-7, -9 REF4, REF5 -8, -10 REF4, REF5 -14, -15
Inhibition Table 7. Means
and standard
of gas hydrate deviation
formation-II
(in parenthesis)
3451
within each group
(Table 6)
Group A (1 m/s cont. flow exp.)
Variable
B (1 m/s cont. flow exp.)
C (3 m/s cont. flow exp.)
D (3 m/s cont. flow exp.)
E (stop/start exp.)
10.70 (0.34)
11.76 (0.84)
11.62 (0.47)
11.50 (0.42)
4.60 (0.12)
(bara)
66.18 (0.16)
66.62 (0.41)
66.70 (0.26)
66.52 (0.49)
63.98 (0.11)
,4,T)
6.20 (0.34)
5.22 (0.80)
5.36 (0.43)
5.46 (0.38)
12.10 (0.12)
gr)
40.22 (1.14)
36.20 (3.33)
36.96 (1.77)
37.36 (1.53)
53.46 (0.11)
11.02 (1.33)
10.76 (1.72)
1.82 (0.11)
2.04 (0.2 1)
6.72 (0.72)
2.90 (0.22)
3.00 (0.00)
1.oo (0.00)
1.30 (0.27)
0.80 (0.00)
Hydrate lumps (slush-like) (min)
8.10 (0.55)
8.40 (0.55)
Hydrate lumps starts to stick to pipe wall (min)
39.00 (7.74)
36.70 (2.64)
PO
%I%* Slurry/slush (min)
hydrates
Slush/powder-like hydrates (min) Powder Clogging (min)
hydrates
(min)
of the pipe
Deposits (factor) observed on the pipe wall at end of the experiment
-
1.76 (0.34)
15.50 (0.94)
16.70 (1.79)
-
37.25 (8.26)
30.12 (6.59)
-
45.87 (10.60)
35.25 (9.84)
50.70 (9.81)
48.70 (6.39)
2.60 (0.55)
2.60 (0.55)
Hydrate macrostructures and changes in fluid flow properties during conversion of free water to hydrates In the development of hydrate macrostructures no significant difference was indicated for the 1 m/s continuous-flow experiments (Fig. 4 and Table 3) or the stop/start experiments (Fig. 6 and Table 5). The only appreciable difference between the experiments was the time period from when the hydrate lumps started to stick to the pipe wall to the clogging of the pipe. From evaluations of the data, this factor mainly seems to be dependent on the amount of hydrate deposits on the pipe wall observed visually and through the torque measurements. In some of the 3 m/s continuous-flow experiments (Fig. 5 and Table 4) the pipe was clogged by lumps of hydrates for some time. As more water in the lumps was converted to hydrates, the lumps started to move again before being converted to powder-like hydrates (except for experiment REFl-2 which stayed clogged). The process of clogging of the pipe seems to be influenced by the experimental equipment. When hydrate is formed there is a tendency for hydrates to gather together in the lower part of the wheel forming one (or more) plug(s) sliding along the pipe wall. This long plug will almost behave like a solid sliding on the pipe wall. As hydrate deposits on the pipe wall increase the friction forces, the entire hydrate plug starts
4.60 (1.08)
11.34 (4.29) 2.40 (0.55)
2.80 (0.45)
2.80 (0.84)
to oscillate. At a certain time the plug is carried over the top of the wheel and may develop into a plug permanently clogging the pipe. This seems to happen when slush-like hydrates, both in the plug and on the pipe wall, develop a more roughly fibrous texture. As there is no large pressure buildup on the upstream side of a plug, the clogging of the experimental equipment does not mean that a real system would be clogged at similar conditions. The ratio of pipe cross-sectional area to perimeter is larger (directly proportional) in a big pipe than in a small pipe. This means that for a given pressure difference the total shear strength or adhesion to the pipe walls will be less at higher diameters for a plug of a given length. This may be one of the reasons why earlier experiments by a similar system (Lund et al., 1992) did not clog 3 and 5 in ID wheels (similar wheel diameters) as easily as the 2 in ID wheel. The decreased tendency of plugging in the 3 m/s continuous-flow experiments seems for the given system mainly to be due to the increased effective gravity forces (gravity plus centrifugal forces) on the system. Deposits of hydrates on the pipe wall For the stop/start experiments the degree of deposits on the pipe wall was observed to be somewhat larger than for the continuous-flow experiments. With
A. LUND et al.
3458 Table 8. Composition
and physical
properties
of Exxsol D60
Composition, Exxsol 060 wt % Normal + isoparaffin hydrocarbons Naphthenic hydrocarbons Minimum
99.5% aliphatic
hydrocarbons.
Cg
Cl0
Cl,
C12
Cl3
Total C
0 1
5 13
23 18
23 8
6 3
57 43
Maximum
0.5% aromatic
hydrocarbons
Physical properties, Exxsol 060 Initial boiling point End point/dry point Flash pt. tag closed cup Relative volatility (n-butylacetate Colour (Saybolt) Density 15°C Viscosity 25°C Refractive index Bromine number Kauri Butanol value Aniline point Sulphur content Benzene content
= 1.00)
190°C 221°C > 61°C 0.05 + 30 0.79 kg/l 1.28 mPa 1.436 50 31 70 l mg/kg (l ppm) < 20 mg/kg
(-c 20 ppm)
161
Average molecular weight
a tendency of the given system to form hydrate deposits on the pipe wall, this was expected due to the higher degree of subcooling. In the experiments it was not possible to measure quantitatively the amount or thickness of a hydrate layer on the pipe wall by direct means. Classification of hydrate deposits on the pipe wall at the end of the experiments was mainly evaluated from torque measurements supplied by video recordings. As the visual part depends on the observer, all experiments were evaluated by at least two experienced scientists.
CONCLUSIONS
A given hydrocarbon gas-water-model oil system, used as a reference system for testing of new hydrate inhibitors, has been evaluated for experimental reproducibility of hydrate formation and behaviour at flow conditions. The system was remixed five times in a 2” ID high-pressure flow loop and tested for the reproducibility both within one mixture (closed system) and between the remixtures. The results indicated good reproducibility both within the closed system and between the remixtures. All trends in the results were unambiguous regarding the reproducibility of the degree of subcooling at hydrate initiation, development of hydrate macrostructures, changes in fluid flow properties during conversion of free water to hydrates and deposits of hydrates on the pipe wall. The results were also in agreement with similar systems tested earlier. This study has shown that it should be unnecessary to reproduce the experiments in order to confirm the results for a given hydrocarbon gas-water-model oil system, if deposits, agglomeration or plugging by hydrates are obtained. On the other hand, when no
deposits, etc., by hydrates are obtained, all significant system parameters should be carefully examined.
Acknowledgements-The Statoil Multiphase Flow Program is acknowledged for giving permission to publish the paper. Petreco A/S, Stjsrdal, Norway is acknowledged for performing the experimental work. REFERENCES
Austvik, T., 1992, Dr. Ing Thesis, The Norwegian Institute of Technology, Trondheim, Norway. Austvik, T., Lund, A., Lysne, D., Lindeberg, E. and Leken, K.-P., 1990, Report Nr: STF21 A90OO6, SINTEF Applied Chemistry, Norway. Hammerscmidt, E. H., 1934, Ind. Engng Chem. 26, 851. HYSIM, 1994, Process simulation program by Hydrotech Ltd, Calgary, Alberta, Canada. Lippmann, D., Kessel, D. and Rahimian, I., 1995, Gas hydrate nucleation and growth kinetics in multiphase transportation systems. In Proceedings of the 5th International Ofihore and Polar Engineering Conference, The Hague, The Netherlands. Lund, A. and Lysne, D., 1992, Report Nr: STF21 A92073, SINTEF Applied Chemistry, Norway. Lund, A., Lysne, D., Austvik, T. and Arde, B.A., 1991, Report Nr: STF21 A91040, SINTEF Applied Chemistry, Norway. Lund, A., Lysne, D., Grande, 0. and Austvik, T., 1992, Report Nr:. STF21 F92034, SINTEF Applied Chemistry, Norway. Makogon, Y. F., 1981, Hydrates of Natural Gas. Penn Well Books, Tulsa. Nygird, H. F., 1989, Transportability of hydrates in multiphase systems. In Proceedings of the 4th International Conference on Multiphase Flow, Nice, France. Reed, R. L., Kelley, L. R., Neumann, D. L., Oellke, R. H. and Young, W. D., 1994, Some preliminary results from a pilot-size hydrate flow loop. Ann. N. Y. Acad. Sci. 715,311. SHP, Statoil Hydrate Program, 1989, Statoil, Research Centre, Trondheim, Norway. Sloan, E. D. Jr, 1990, Chlathrate Hydrates of Natural Gas. Marcel Dekker, New York. Urdahl, O., Lund, A., Msrk, M. and Nilsen, T. N., 1995, Chem. Engng Sci. 50, 863.