Article pubs.acs.org/EF
Hydrate Formation in Gas-Dominant Systems Using a Single-Pass Flowloop Mauricio Di Lorenzo,*,†,‡ Zachary M. Aman,† Gerardo Sanchez Soto,‡ Michael Johns,† Karen A. Kozielski,§ and Eric F. May† †
Centre for Energy, School of Mechanical and Chemical Engineering, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia ‡ CSIRO Earth Science and Resource Engineering, Kensington, Western Australia 6151, Australia § CSIRO Earth Science and Resource Engineering, Ian Wark Laboratory, Clayton, Victoria Australia ABSTRACT: A 130 ft single-pass, gas-dominant flowloop has been constructed to study hydrate formation in an annular flow regime by exposing warm process fluids to a cold pipe wall. Hydrate was formed in six experiments from a natural gas mixture, with 6−18 °F subcooling from hydrate equilibrium. At lower subcooling values a stenosis-type hydrate film growth model without adjustable parameters was used to estimate the resulting pressure drop and yielded an average deviation of 15.8 psi from the experimental value. The accuracy of this model decreases appreciably with increasing subcooling, suggesting the occurrence of a transition after which the pressure drop becomes dominated by additional hydrate phenomena such as particle deposition or wall sloughing. For experiments with 18 °F subcooling, the pressure drop signal contained periodic peak-and-trough behavior and the primary hydrate restriction was observed to migrate downstream at a rate of 3 ft/min over the course of the experiment. Average hydrate growth rates varied linearly with subcooling over a range of 0.2−1.2 L/min and were an order of magnitude larger than formation rates predicted using models developed for water-dominant systems. These results demonstrate the need for a new gas-dominant hydrate formation model, which incorporates stenosis-type growth, particle deposition from the liquid phase, and deposit sloughing from the wall.
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INTRODUCTION Gas hydrates are crystalline inclusion compounds, where molecular water cages trap light hydrocarbon species such as methane or ethane. Hydrates are stable at high pressure and low temperature, and represent a flow assurance challenge in deepwater−oil and gas flowlines.1 Hydrate crystals may agglomerate or adhere to the pipeline wall, resulting in complete occlusion of the pipe.2 As hydrate plug remediation represents a severe operational and safety hazard,2 focus has turned to enhancing the accuracy of predictive tools and models, which may enable engineers to effectively manage the risk of flowline blockage.3 Previous efforts have focused on understanding hydrate plug formation mechanisms in oil-dominated systems, which are typically characterized by a liquid hydrocarbon continuous phase in which the water and hydrate phases are dispersed. Turner et al.4 presented a four-step conceptual mechanism for plug formation in oil-dominated systems: (i) emulsification of water droplets in oil;5 (ii) hydrate nucleation at the gas−water interface;6 (iii) agglomeration of hydrate shells or particles leading to large increases in viscosity;7 and (iv) catastrophic plug formation with hydrate-wall interaction.8 New hydrate management techniques, such as the injection of antiagglomerant chemistries, have been devised to exploit the aggregation step within this mechanism9 and minimize viscosity increases within the hydrate-in-oil slurry. Gas-dominant systems may be characterized by a minority volume fraction of liquid water or hydrocarbon and are particularly susceptible to hydrate formation risks.2 Lingelem et al.10 proposed one conceptual mechanism for the development © 2014 American Chemical Society
of a hydrate blockage in gas-dominant systems (adapted in Figure 1), based on the formation and growth of a hydrate film at the pipeline wall. As the initial deposit thickens, enhanced shear stress from the flowing fluid may cause cohesive (hydrate−hydrate) or adhesive (hydrate-steel) failure (i.e., sloughing of deposits from the wall). In the latter stages of the plug formation, Lingelem et al.10 suggested a jamming-type failure scenario may be dominant.11 Only half of the four conceptual phenomena in Figure 1 have been explored in laboratory studies. Early analogues to hydrate film nucleation and growth have been drawn from ice crystallization,12 although the additional heat released during hydrate formation may result in unique growth patterns and surface area-to-volume characteristics such as the dendritic growth observed by Aman et al.13 Rao et al.14 measured methane hydrate film growth along a cold tube inside a highpressure visual cell, where heat transfer-limited growth resulted in three distinct regions of porosity: (i) greater than 90% during initial nucleation stage; (ii) 20−80% during typical growth; and (iii) less than 10% during the annealing stage, where additional outward growth was limited by the temperature of the bulk gas phase. No direct studies have yet investigated the interaction of flowing hydrate particles with hydrate film along the wall as a complementary growth mechanism. Preliminary micromechanical force measurements by Aspenes et al.15 concluded that cohesive forces were 3−4 Received: February 9, 2014 Revised: April 15, 2014 Published: April 17, 2014 3043
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Figure 1. Proposed conceptual mechanism for hydrate plug formation in gas-dominant systems, adapted from Lingelem et al.10
Figure 2. (above) Photograph of the Hytra flowloop. (below) Simplified layout of the Hytra flowloop, with six pressure and temperature gauges (denoted “P-T”) and four viewing windows (denoted “VW”) along the test section, and liquid and gas flow meters (“LFM” and “GFM”, respectively) prior to entering the test section.
times larger than adhesive forces, except when the wall was wetted with a bulk water phase. Nicholas et al.16 observed that the nucleation of hydrate directly on steel increased adhesive forces by at least a factor of 100 when compared to the contact force between a preformed hydrate particle and steel. Together, these studies suggest that hydrate sloughing may be supported by an adhesive-type failure, except in cases where the hydrate has nucleated directly on the steel surface or when the steel has been wetted by water. Many flowloop studies to date have focused on liquiddominant systems, and their results are not extendable directly to gas-dominant systems. Nicholas et al.16 measured pressure drop in a single-pass flowloop, where hydrate was formed at a cold wall with a water-saturated condensate; this study
concluded that hydrate formation from water dissolved in liquid hydrocarbon resulted in a slow pressure drop increase (1 psi/h in cases when the condensate was cooled past the liquid water saturation curve (i.e., the point at which a free water phase appears). Hatton and Kruka17 studied hydrate formation in an industrial gas-condensate line, and presented three consecutive stages in the pressure drop profile’s evolution: (i) smooth pressure drop increase, on the order of 10 psi from the upstream measurement; (ii) continued pressure drop increase with moderate fluctuations, on the order of 50 3044
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psi; (iii) severe pressure drop fluctuations as the hydrate plug moved downstream, on the order of 100 psi. The present study introduces a new single-pass flowloop (Hytra), capable of achieving the annular flow regime and low liquid loading that may be encountered in gas-dominant industrial systems. The pressure drop profiles are, as a first approximation, interpreted as a combination of hydrate film growth initially, followed by an amalgamation of more complex behavior, such as deposit sloughing, particle deposition, and hydrodynamic effects.
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Operating Procedure. Before each experimental test, the flowloop was purged of any remaining water and pressurized to the desired set point. The test section was then cooled to the target temperature, under constant gas flow without any liquid injection, until a steady-state gas temperature profile was established along the test section. The values of the initial pressure and temperature were recorded with gas circulation halted, to temporarily eliminate frictional pressure drop contributions due to fluid flow. After resuming the gas circulation, tests commenced by initiating the water injection pump (hydrocarbon liquids were not used in this study). Hydrate formation was confirmed visually in each experiment, and the flowloop pressure drop was allowed to increase until the pressure drop approached the allowable limit of 250 psi, which usually required 20−40 min. Hydrates were fully dissociated after each experiment, and the resultant liquids were flushed into the separator. The system was fully emptied by discharging all liquids, which were collected at the bottom of the separator for disposal or recycling. The flowloop was finally depressurized and warmed to ambient temperature for at least 24 h before another experiment was performed. Experimental Conditions. Deionized water and natural gas were used to form structure II hydrate in the Hytra flowloop; the dry gas composition is reported in Table 1. The hydrate equilibrium curve is
EXPERIMENTAL METHODS
Gas-Dominant Flowloop. The Hytra flowloop was specifically designed to study gas-dominant industrial conditions, where high gas velocities and low liquid fractions support annular flow regimes. During operation of the Hytra loop, fresh liquid is injected continuously from an ambient-pressure storage tank, while the gas phase is recirculated. This procedure enables the gas space to be treated as a known volume, allowing direct calculation of hydrate formation based on the overall change in pressure. A single-pass design was used to target deposition-type hydrate phenomena in particular, which require prohibitive experimental times to observe in multipass flowloop configurations.18 The Hytra loop consists of a test section with a 0.8-in. internal diameter and 130 ft length (Figure 2). The test section is enclosed in a four-inch pipe-in-pipe insulated cooling jacket, which is connected to an external cooling bath with ±0.9 °F stability. Upon exiting the loop, the multiphase mixture is first released into a 145 gal gravimetric separation tank, where the gas outlet is redirected into a 21 gal cyclone separator to remove any liquid entrained in the gas stream before being reinjected into the loop through the gas circulation compressor. The liquid phase and hydrates are collected in the separators for disposal. The liquid is injected from a 153 gal storage tank held at atmospheric pressure with temperature control and is injected into the flowloop using a high-pressure pump. The fluids are not mixed prior to injection in the flowloop, and the average residence time of liquids in the test section is estimated at 20−30 s. If hydrates are formed continuously in the flowloop, a typical experiment may run for 20−40 min prior to reaching the flowloop’s allowable pressure drop limit. The flowloop is constructed from 316 stainless steel and can operate in the temperature range 18−86 °F under pressure conditions up to 1750 psig, with a maximum pressure drop of approximately 250 psi. The liquid phase pump allows for liquid volumetric flow rates of 2.1−17.6 ft3/h (up to 1.3 ft/s), while the gas compressor allows for 294−589 SCF/min. (up to 27.9 ft/s). The gas compressor maintained a constant set-point pressure (either 1250 or 1500 psig, discussed below) throughout the experiment, until the flow was suspended for static pressure measurement. The superficial velocity of each phase, and the consequential liquid loading, depends on the operating loads of the liquid and gas phase pumps. Under typical operation, the current pump and compressor allow a maximum of 10 vol % liquid loading. A second 132 gal injection tank, operating up to 100 psi with temperature control, is available for the injection of hydrocarbon liquids. Temperature and pressure are measured at different points in the test section (P-T0 to P-T6 in Figure 2) using resistance temperature detectors and pressure transmitters with uncertainties of ±0.27 °F and ±3.9 psi, respectively; these instruments are mounted in measurement thermowells inside the flowloop. The gas and liquid flow rates are measured using a turbine gas flow meter and a positive displacement liquid flow meter with relative uncertainties of ±0.3% and ±1%, respectively. The pressure, temperature and flow rate readings are transmitted to a data acquisition system (DAQ) at one-second intervals. Fluid transport in the flowloop may be captured visually using high-speed digital video cameras outfitted with 10× objective lenses. Four visualization windows are available for inspection at different locations in the flowline (VW1 to VW4 in Figure 2).
Table 1. Natural Gas Composition components
composition (mol %)
CH4 C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 C6 C6+ CO2 N2
87.30 6.02 1.51 0.14 0.21 0.04 0.04 0.02 0.02 2.30 2.40
provided for reference in Figure 3a, obtained from Multiflash v4.1.19 The pressure−temperature operating space of the current experiments is highlighted for comparison in Figure 3a. Gas and water were injected into the test section at constant rates of 170 L/min (6 acfm) and 2.0 L/min, respectively. From the flow regime model presented by Barnea,20 these superficial phase velocities suggest that the flowloop was operating in the annular flow regime (Figure 3b), which was qualitatively confirmed through visual analysis of the flowloop windows (discussed below). Six experiments were performed, and the conditions are summarized in Table 2. In all experiments, approximately 6 vol % water injection was maintained at a superficial velocity of 0.3 ft/s, with a gas velocity of 26 ft/s. The final flowloop pressure was measured after liquid and gas pumps were halted. The final pressure in the loop increased during each experiment, because continuous liquid injection decreased the volume available for the gas phase. The total volume of hydrate formed by the end of the run Vhyd,final was calculated from a change in the static (nonflowing) pressure at known temperature conditions (Pinitial and Pfinal in Table 2), where the flowloop volume (1045 L) was reduced by the total amount of water injected (Vwater,inj). The hydrate volumes formed are listed in Table 2, which also reports the average fluid temperature (Tfluid,avg, from P-T0 through P-T6 in Figure 2), the average hydrate equilibrium temperature (Thyd,avg), the average subcooling from hydrate equilibrium (Tsub,avg = Thyd,avg − Tfluid,avg), the maximum pressure drop (ΔPmax) achieved in each experiment, and total run time of each test. The average hydrate equilibrium temperature, Thyd,avg, was calculated from the average flowloop pressure and the gas composition reported in Table 1 using the RKS-advanced model set implemented in Multiflash 4.1.19 Based on the initial pressures and temperatures provided in Table 2, the 3045
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Figure 4. Average subcooling in the flowloop as a function of time for experiments 1 and 4; dashed curves indicate combined error bounds from temperature and pressure measurements. to PT-3), section 3 (PT-3 to PT-4), and section 4 (PT-4 to PT-6). Accounting for the increased pressure drop due to the thermowells and viewing windows,21 the equivalent lengths of each section were 20.2, 24.5, 27.6, and 48.6 ft, respectively. The pressure drops for each section were calculated directly from the difference between the absolute pressures measured at the beginning and end of each section. At 1250 psi and a temperature of 66.1 °F (above the hydrate equilibrium temperature), the average flowloop pressure drop was 24.9 ± 1.6 psi (at one standard deviation). The pressure drop time series (dark line) plotted in Figure 5 was obtained from a ten-point running Figure 3. (a) Hydrate equilibrium boundary for the sII natural gas mixture listed in Table 1 with the experimental region highlighted by a gray boundary. (b) Flow regime map for the Hytra flowloop based on the model of Barnea,20 with the operating point highlighted. amount of hydrate formed in each experiment accounts for a static (nonflowing) pressure decrease of about 30−60 psi. The values of the Tsub,avg listed in Table 2 represent averages taken over the course of the experiment and the length of the flowloop. During an experiment, the subcooling (and hence the driving force for hydrate formation) varied because of two competing effects: (i) pressure increases of 8−77 psi at the injection point over the course of the experiment, which increased the subcooling; and (ii) temperature increases throughout the flowline, caused by reduced heat-transfer to the environment from thermally insulating hydrate deposits. The net results of these competing effects for experiments 1 and 4 are shown in Figure 4 in which the spatially averaged subcooling in the flowloop is plotted as a function of time. The subcooling in both cases can be seen to be relatively constant varying approximately by only 2−5 °F over the course of the experiments. This indicates that considering an average subcooling when interpreting the results from a given flowloop experiment is likely to be a reasonable first approximation. The use of the timedependent temperatures and pressures measured at multiple locations in the flowloop may be deployed in the future within a more sophisticated model for hydrate film growth and deposition, to account for local variations in driving force. Modeling and Data Interpretation. The flowloop was divided into four sections based on the location of pressure and temperature measurements in Figure 2: section 1 (PT-1 to PT-2), section 2 (PT-2
Figure 5. Pressure drop across the entire flowloop at 1250 psi and 66.2 °F (which is above the hydrate equilibrium temperature). The black line represents a ten-point running average of experimental data, and the straight brown line represents a prediction made using the Brill and Mukherjee22 pressure drop model for annular flow. average of the experimental data (symbols), which allows better visualization of the pressure drop trends with time by filtering noise in the pressure signals at frequencies greater than about 0.1 Hz: the average is calculated using the five data points preceding and five points following the desired time. Strong vibrations produced by the
Table 2. Summary of Hytra Flowloop Experiments expt.
Pinitial (psi)
Pfinal (psi)
Vwater,inj (L)
Vhyd,final (L)
Tfluid,avg (°F)
Thyd,avg (°F)
Tsub,avg (°F)
ΔPmax (psi)
run time (min)
1 2 3 4 5 6
1277 1270 1219 1270 1529 1541
1354 1339 1227 1290 1548 1549
88.9 87.0 47.0 71.5 45.2 42.4
20.5 22.3 27.8 27.2 27.4 3.9
56.4 56.2 48.0 51.2 48.1 61.6
66.9 66.9 66.6 66.8 68.3 68.5
10.5 10.6 18.6 15.6 20.2 6.9
195 191 292 177 266 189
48.2 45.2 22.6 39.5 22.7 20.7
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compressor, partially attenuated by pulsation dampeners fitted at the discharge, are the main cause for the data scatter. The average flowloop pressure drop was compared with predictions made using Brill and Mukherjee22 model; no adjustable parameters were used in this calculation. The Brill and Mukherjee22 model consists of two components: a flow regime selection algorithm and frictional pressure drop models for each flow regime. The flow regime algorithm is presented thoroughly in literature23 and predicts an annular flow for all experiments listed in Table 2, in agreement with visual observation and the flow regime model by Barnea.20 The model for pressure drop (ΔP) per unit length (ΔL) in annular flow is shown in eq 1:
⎛ fρ v 2 ⎞ ΔP 1 avg avg = ⎜⎜ + ρavg g sin θpipe⎟⎟ ΔL 4 R E pipe ⎝ ⎠ k
formation19 from the flowloop’s measured change in static (nonflowing) pressure and volume at known temperature over the course of the experiment. As a first approximation, our model assumes that stenosis-type annular hydrate growth dominates the pressure drop signal. Per the discussion above, we have estimated the total gas consumed to form hydrate for each experiment in Table 2; the total volume of hydrate was estimated by assuming a constant gas-to-water ratio in the hydrate phase that was taken from Sloan and Koh.1 The frictional pressure drop increase attributable to stenosis-type hydrate film growth may be estimated by decreasing Rpipe in eq 1 by an amount corresponding to the uniform distribution of hydrate around the loop. Using the total volume of hydrate estimated from the experimental data in Table 2, we calculated the time-averaged hydrate growth rate throughout the experiment. The assumptions that the calculated growth rate is constant throughout the experiment and the hydrate deposit is radially homogeneous are, of course, first approximations. As previously indicated, temporal and spatial deviations from the average subcooling will give rise to nonuniform growth rates both along the flowloop’s length and over the experiment’s duration. The calculations further assume that the deposit is nonporous. Experimental data by Rao et al.14 suggest that porosity decreases with time (from 80 to 0% over approximately 60 h), for a heat transfer-limited system where hydrate was formed from water-saturated gas. Dynamic porosity and spatial cooling calculations may be incorporated into future studies but must be validated for both the geometry and liquid loading of the Hytra flowloop. To our knowledge of the published literature, no comprehensive model is available to describe the pressure drop caused by hydrate formation in gas-dominant multiphase systems. While the simplifying assumptions made here are significant, the model is useful because it provides a framework by which to interpret the observed pressure drop and, for example, identify when transitions in the dominant hydrate plugging mechanism have occurred. This is likely to assist in the challenging task of developing more sophisticated models for hydrate-induced pressure drops in gas-dominant multiphase flowlines by partially deconvolving the multiple effects that contribute to the observed signals.
(1)
where ρavg is the average mass density of the fluid stream, g is gravitational acceleration, vavg is the average fluid velocity, Rpipe is the hydrodynamic radius, and θpipe represents pipeline inclination above a horizontal plane (set to 0° for this flowloop). The friction factor ( f) is a function of both the average Reynolds number (Re) for the fluids and a loss coefficient FR, defined by Brill and Mukherjee22 as an empirical, fourth-order polynomial function of holdup. Finally, Ek is a phenomenological balance term for fluid momenta on pressure and is described thoroughly in the literature.22 The predicted pressure drop for the experiment shown in Figure 5 was 23.3 psi, which deviates from the measured average by an amount equal to the standard deviation of the measured data. Hydrate formation was confirmed visually in each experiment (Figure 6), and occurred within a few minutes of initiating the liquid injection pump (when the flowloop temperature was at the target subcooling).
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RESULTS AND DISCUSSION Pressure Drop Increase with Hydrate Formation. Experiments 1 and 2 were performed under near-identical conditions at around 1250 psi (initial pressure) and a temperature of 56 °F (subcooling of 10.5 °F) in part to establish the repeatability of the experiments. Figure 7a shows pressure drop as a function of time after water injection was initiated for experiment 1. The pressure drop across sections 1−3 increased monotonically with time, while local maxima were observed through time in section 4. As warm fluids enter the flowloop and cool due to contact with the wall, the subcooling and kinetic growth rate24 are likely to be the highest in section 4. In experiment 1, the time-averaged subcoolings across sections 1−4 were 6.8, 8.3, 9.4, and 11.9 °F, respectively; equivalent values (±0.2 °F) were observed in experiments 1 and 2. Figure 7b shows the deviation (in psi) between experimental measurements and the stenosis-type growth model (i.e., the pressure drop increase predicted by eq 1 for the calculated hydrate growth rate); the model had no adjustable parameters, and the average growth rate was calculated from the total pressure consumed throughout the experiment. For experiment 1, the calculated hydrate growth rate was 0.4 L/min, resulting in average deviations between model and experiment of 6.5, 16.0, and 15.8 psi for section 1, section 4, and the overall flowloop, respectively. These deviations correspond to 18.1, 12.1, and 8.3% of the maximum pressure drop values recorded in section 1, section 4, and the total flowloop, respectively.
Figure 6. Images from the high-speed camera positioned at VW-1 during experiment 6, illustrating the growth of a hydrate film. Throughout the experiment, the pressure drop increased across the flowloop with time. This increase during f low may be attributed to gas velocity increases and momentum dissipation caused by hydrate formation/deposition phenomena (confirmed visually), that result in (i) a smaller annulus for flow, (ii) decreased gas density due to hydrate formation, and (iii) dissipative effects related to, for example, increased effective viscosity and/or the movement of solid hydrate plugs through the loop. The overall decrease in gas density was estimated using Multiflash 4.1 and the RKS-advanced model set for hydrate 3047
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Figure 8. Total pressure drop as a function of time after liquid injection for experiments 1 and 2: repeat trials at initial pressures of 1270 and 1277 psi, with a subcooling from the hydrate equilibrium curve of 10.5 °F.
Figure 7. (a) Pressure drop increase after liquid injection begins at 1250 psi and temperature of 56 °F for experiment 1 and (b) deviation of experimental pressure drop from the stenosis-type growth model for experiment 1. Solid curves represent 10-point moving averages to guide the eye. Figure 9. Pressure drop in experiment 1 (ordinate) as a function of pressure drop in experiment 2 (abscissa) at equal time after liquid injection; the solid line represents 1:1, while the dashed lines represent ±20 psi boundaries.
The largest deviation between model and experiment (approximately 60 psi) occurs in section 4, where the fluid temperature has decreased by approximately 5 °F due to contact with the cold wall. It is likely that in this section of the loop, the hydrate growth rate was greater than the average value used for the simple stenosis model because of the increased local subcooling. This is consistent with the fact that the deviations of section 4 from the model’s predictions shown in Figure 7b are more positive and have a larger slope and curvature than section 1, suggesting that the rate of stenosis in section 4 was larger. However, the deviations for section 4 also exhibit more structure and sharp transient features, particularly in the latter stages of the experiment. As we discuss in later sections, such observations suggest that the onset of more complex hydrate phenomena (e.g., particle deposition or sloughing) may be associated with higher subcooling values. A comparison of experiments 1 and 2 is shown in Figure 8, which demonstrates the repeatability under these conditions of the full loop pressure drop signal during hydrate formation. The experiments were performed 4 days apart; the water phase was stored at ambient temperature between both experiments, while fresh gas charges were used in each experiment. To more closely examine the repeatability of the experiments, the overall pressure drop in experiment 2 was plotted as a function of the overall pressure drop in experiment 1 for equivalent experimental times. Figure 9 shows that the pressure drop signals generally fall within ±20 psi (or ±11% of the maximum pressure drop in experiment 2). This degree of variance in the total pressure drop between these two repeat
experiments is comparable to the scatter in the pressure drop signal observed in the absence of hydrates (Figure 5), which might be considered unexpected given the additional phenomena involved in the formation and transport of gas hydrates. Deviations to the right of the lower boundary correspond to pronounced late-term local maxima observed in experiment 2, which were not observed in experiment 1. These can be associated with sloughing events, which occur in both experiments but with different severities and at different times: 31 and 38 min in experiment 1; 33 and 42 min in experiment 2. Sloughing of hydrate from the wall occurs when the hydrate deposit is fractured by the fluid’s shear force. The force required to slough a hydrate deposit is qualitatively expected to decrease as the hydrate plug becomes more porous.14 To our knowledge of the literature, there are no predictive models available that describe either (i) the porosity of hydrate wall deposits with residence time or subcooling or (ii) the frequency or magnitude of hydrate sloughing events. Additional experiments and analyses are needed to systematically probe this behavior. Subcooling from Hydrate Equilibrium. Four additional experiments were performed to investigate the effect of subcooling on pressure drop increase: experiments 3 and 4 at initial pressures of 1219 and 1270 psi with temperatures of 48.0 and 51.2 °F (Tsub,avg = 18.6 and 15.6 °F), respectively; and 3048
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for experiments 3 and 4, respectively. When these growth rates were used in eq 1, the average relative deviations between the pressure drop predicted by the model and observed during experiments 3 and 4 were 55 and 41%, respectively. Similarly, the hydrate growth rates for experiments 5 and 6 were 1.2 ± 0.1 and 0.20 ± 0.03 L/min, respectively, which gave rise to average relative deviations in the total pressure drop between model and experiment of 71 and 60%, respectively. This comparison further supports the hypothesis that stenosis caused by a hydrate film dominates the pressure drop signal at low subcooling (
