THE EFFECT OF AUTOCLAVE DESIGN AND TEST PROTOCOL ON HYDRATE TEST RESULTS

Kevin McNamee

Nalco7705 Hwy 90A, Sugar Land, TexasUNITED STATES OF AMERICA

Pete ConradNalco

7705 Hwy 90A, Sugar Land, TexasUNITED STATES OF AMERICA

ABSTRACTHigh-pressure autoclave tests are one of the industry accepted tools used to evaluate KHI performance. Experimentation has shown that the autoclave set-up, stir-bar type and stirring rate can have a dramatic effect on the reproducibility and quality of KHI performance test results.

Autoclave experiments to evaluate KHI performance were observed to have inconsistent results. The cause

of the inconsistencies was explored and determined to be a direct result of the autoclave design, specifically the inlet/outlet gas line set-up. Due to safety reasons, modifications to the existing manufacturer gas lines were implemented, resulting in increased dead-leg and accumulation points for unprotected condensed water. Visual inspection of tests with suspected premature failures, showed hydrate formation occurring outside of the reaction liquor, indicating the quick formation was from uninhibited condensed water, and not KHI performance related. The premature formation was only observed in autoclave tests with the extra

dead-leg and condensation accumulation points, and not after retrofitting inlet/outlet lines (with safety devices) to reduce/minimize dead-leg and condensation accumulation points, providing evidence that the design of the autoclave can have a dramatic effect on hydrate test results.

Additionally, the stirring method and rate of autoclave tests was explored to determine the best operational practice for autoclave testing of KHI performance. It was determined that changing the stirrer type and

speed can lead to a decrease in the dispersion of hydrate nucleation times. Optimizing the stirrer type and speed resulted in at least a 2-fold decrease in the relative standard deviation of the hydrate nucleation times.

This paper describes the effect of autoclave inlet/outlet gas line design and stirring type/rate on the resulting test data. Combining a proper autoclave design and stirring regime can result in a higher reproducibility of

test data and enhanced data quality.

Keywords: gas hydrates, kinetic inhibitors

Corresponding author: Phone: + 1 281 263 7627 Fax +1 281 263 7221 E-mail: kpmcnamee@nalco.com

NOMENCLATURE1 Autoclave design #1

2 Autoclave design #2A Cylindrical stirrer type

AC autoclaveB Wedge stirrer type

DSC Differential scanning calorimeterC Overhead stirrer

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

GHA Gas hydrate autoclaveGOR Gas to oil ratioHET Hydrate equilibrium temperature [oC]KHI(s) Kinetic hydrate inhibitor(s)LDHI(s) Low dose hydrate inhibitor(s)M Multiple data point method

MEG Mono ethylene glycolRPM Revolutions per minute%RSD Percent relative standard deviationS Sin gle data point methodsI Type I hydrate

sII Type II hydrateSC Sub-cooling [oC]t ave Average hold-time [h]th Hold-time [h]t i Induction time [h]tmin Minimum hold-time [h]

tmax Maximum hold-time [h]to Time to reach set-point temperature [h]THF TetrahydrofuranTHI Thermodynamic hydrate inhibitor(s)T sp Set-point temperature [oC]

standard deviation

INTRODUCTIONHydrate inhibition studies are a crucial part of flow assurance within the oil and gas industry [1-3]. Hydrate formation in oil and gas production lines poses significant safety risks as well as risk

for production loss and equipment and pipeline damage. A number of modeling programs have been developed that accurately predict hydrate risk within production pipelines, and are often used to help design mechanical solutions for hydrate management such as thermal insulation, choke

specifications to regulate pipeline pressure, electrical heating, or dead oil displacement[1]. Additionally, modeling can be used to predict treat rates of thermodynamic hydrate inhibitors (THI), such as methanol or MEG. Unfortunately, modeling software has not been developed to

accurately predict dose rates of low dose hydrate inhibitors (LDHIs). Instead, laboratory experiments designed to simulate pipeline conditions are one of the core methods for the design and application of LDHIs in the field. It is therefore crucial that the test design accurately reflect the true performance of the chemical

without artifacts that may produce false negative or false positive results.

Performance evaluations of hydrate inhibitors can be done using several different techniques, as there

are several industry-accepted methods for hydrate inhibition testing. In fact, each testing method has certain features and benefits that will cause slight variations in sub-cooling, GOR, and sheering of fluids that will ultimately result in inhibitor performance variations from one piece of

equipment to the next. Some examples include high-pressure rocking-cells, flow loops, wheel boxes, high-pressure autoclaves [4, 5], and more recently high-pressure differential scanning calorimeter (DSC) [6-11]. The instruments used for testing are designed to simulate the operating

conditions observed in the field, and are used to evaluate the performance of various hydrate inhibitors, such as kinetic hydrate inhibitors (KHIs).

Our experience shows the design and setup of

autoclaves, in addition to the test protocol, has a dramatic effect on data reproducibility and elimination of errors. Of chief concern is the elimination of “dead-legs”, effective fluid mixing, and the set-up of effective heat cycles for multi-cycle hydrate inhibitor tests. Herein, this paper

summarizes the best practices for high-pressure isocratic autoclave hydrate inhibition tests.

EXPERIMENTALHigh-Pressure Autoclave ApparatusA high-pressure gas hydrate autoclave was used to

measure hydrate induction times in the presence and absence of KHIs. The autoclave design used was a GHA 200 Gas Hydrate Autoclave system (PSL Systemtechnik, Figure 1). The 450 cc static chamber is equipped with a magnetically driven stirrer, a cooling jacket and chiller, and is fitted

with a head containing a thermocouple, boroscope, analog pressure gauge, and pressure transducer. Software provided by PSL was used to control stirrer rate, and temperature variables. The software also collects pressure, temperature and

stir rate as a function of time. A Nalco custom-built charging panel was used to deliver synthetic natural gas to set pressures (Figure 2). The autoclave temperature and pressure specifications are –10 oC to 60 oC (with an accuracy of 0.1 oC) and a maximum pressure of 200 bar. The

autoclave material is Stainless steel (AISI 316 Ti), with Viton® seals, and a sapphire-glass window (used for visual observations with a boroscope-camera).

Figure 1. PSL Systemtechnik GHA 200 gas hydrate autoclave set-up with magnetic stirring

Early equipment design is shown in Figure 3. The

pressure relief valves, outlet valves, and pressure transducer were mounted away from the autoclave and connected to the outlet port of the autoclave head by a 2’ section of flexible high-pressure tubing. An alternative design places the pressure relief valve, gas inlet and outlet valves, and

pressure transducer directly on the autoclave head (Figure 4).

Figure 2. Gas charging panel and autoclave set-up.

General Autoclave Test ProcedureAutoclaves equipped with a 2” by ¼” round cylindrical Teflon stir bar were loaded with 200 mL of total liquids (KHI, brine, and hydrocarbon phase) at room temperature. The autoclave was sealed and the appropriate gas (compositions shown in Table 1 and Table 2) was charged to the

set pressure at 20 oC. The pressure was allowed to equilibrate at 20 oC for at least 1 h to allow the gas to completely absorb into the fluids. The pressure was re-boosted to the set pressure if needed. The inlet valve was closed, and the temperature

ramped to the set temperature over 1 h, while stirring at 500 rpm. Temperature was held constant until the onset of hydrate nucleation, denoted by >0.2 bar drop in pressure. The hold-time (th) for each test was calculated as the time at which hydrates nucleated minus the time to reach set

point (to). For all autoclave test procedures reported hold-times are from 6-7 replicate runs.

Component Mole%

Nitrogen 0.39

Carbon Dioxide ---

Methane 87.26Ethane 7.57

Propane 3.10

iso-Butane 0.49

n-Butane 0.79iso-Pentane 0.20

Table 1. Green Canyon gas composition.

Replicate Measurement ProcedureFor replicate runs with recycled fluids, the following additional steps were conducted after hydrate formation: the autoclave temperature was ramped to 35 oC over a 1 h period and maintained

Pressure Transducer

Pressure Relief Valve

Pressure Gauge

Gas Charging Panel

Boroscope Camera

Outlet Valve

Figure 3. Autoclave set-up with addition of pressure gauge and pressure relief valve (Autoclave Design #1)

for 8 h to eliminate any “hydrate-memory” effect. After the 8 h heat cycle the autoclave temperature was ramped to the desired set point temperature over 1 h, and held until hydrate formation occurred.

Component Mole%

Nitrogen 0.06Carbon Dioxide 0.21

Methane 99.22

Ethane 0.36

Propane 0.11iso-Butane 0.01

n-Butane 0.02

iso-Pentane 0

Table 2. Synthetic type I gas composition.

RESULTS AND DISCUSSIONEffects of Dead-Leg on Test ResultsIdentical KHI performance tests were run in two different autoclave designs (Design 1 shown in

Figure 3 and Design 2 shown in Figure 4). The difference between the two systems is the un-insulated tubing connecting the autoclave to the outlet valve and pressure transducer in Design 1. Design 2 contains the outlet valve and pressure transducer mounted directly on the autoclave head.

The excess tubing is effectively a dead-leg in the system, where there is no temperature control. The dead-leg also presents a location for condensed water to accumulate, which is essentially unprotected water that can easily be converted to

hydrates. The accumulation of the condensed water is an artifact of the test design and does not allow an accurate assessment of the KHI.

Figure 4. Modification of gas inlet and outlet design for dead-leg minimization and decreased condensate accumulation points (Autoclave Design #2)

Autoclave KHI performance tests were conducted

under two different sub-cooling, 13 oC and 10 oC, which was set by varying the autoclave

temperature (Table 3). Hold-times had a larger scatter in data under higher (13 oC) sub-cooling conditions. It is interesting to note that all 13 oC sub-cooling tests in Design 1 had failures < 2 h and the maximum hold-times were always less than the maximum hold-times observed in Design

2.

13 oC Sub-Cooling ResultsAC Design

a

(1 or 2)tave

(h)tmin

(h)tmax

(h)%RSD

1 31.2 0.2 53.8 79 %

2 57.4 20.8 88.1 38 %10 oC Sub-Cooling Results

AC Designa

(1 or 2)tave

(h)tmin

(h)tmax

(h)%RSD

1 56.8 36.5 69.1 20 %

2 51.6 35.5 65.5 24 %aWhere 1 denotes autoclave with 2’ long flexible tubing on outlet and 2 denotes redesigned shorter metal tubing outlet type

Table 3. Autoclave results vs. outlet design.

Previously it has been documented that hold-timevariability narrows with increased sub-cooling[1-3]. This decrease in variability could be a direct result of relative experimental error, where the data range scales proportionally to the hold-time

length. Analysis of hold-time vs. sub-cooling results from rocking-cell and autoclave tests corroborated the theory that hold-time variability

(standard deviation, ) is proportional to the hold-time values (i.e. larger hold-times had larger standard deviation). It was theorized that the large number of quick failures at higher sub-cooling

were false failures. To better evaluate the changes in hold-time dispersion, %RSD (equation 1) was

used to adjust for scaling of associated with changes in hold-times.

avetRSD

%100% (1)

Rocking-cell testing was used to corroborate the autoclave results (not shown). Comparisons between the results from 13 oC SC tests on the

autoclaves (Table 3) with analogous tests on the high pressure rocking-cells a t 13 oC SC, further illustrated the discrepancies in the high sub-cooling data obtained on the autoclaves. It was presumed that data discrepancies from the

autoclave tests at higher sub-cooling were either a result of the test protocol and/or the autoclave set-up. A boroscope-camera was used to investigate the location of hydrate formation in the autoclaves, for both the high sub-cooling (13 oC) and lower sub-cooling (10 oC) tests. Visual monitoring of

the hydrate formation illustrated a difference in the location of hydrate formation for the two sub-cooling (with the only difference in tests being the set-point temperature). For the low sub-cooling tests the formation of hydrates occurred along the

fluid-gas interface, generally at the wall of the autoclave, and then either broke free and grew in the reaction liquor or grew inward from the autoclave wall. The same phenomena occurred for the longer hold-times during the 13 oC sub-cooling tests in autoclave Design #1. In contrast, when

there was an early failure the hydrates formed on the top of the autoclave (outside the reaction media), followed by an inward growth, which eventually obscured visual observations. Hydrates formed in this location, led to catastrophic failures either before set-point was reached or very early

on in the test, where the replicate tests with longer hold-times had hydrates forming at the reaction liquor-gas interface only.

Hydrate formation in the unprotected condensed

water along the top of the autoclave and/or in the gas outlet line was presumed to be the cause of the premature failures. It was assumed that the condensed water was forming in other locations than only the top of the autoclave. This was due to the fact that the condensed water is able to fall

back into the autoclave when forming along the roof of the autoclave and condensed water did not lead to catastrophic failures outside of the reaction liquor at lower sub-cooling (higher autoclave temperatures). Since the false failure phenomena was only occurring at elevated sub-cooling (lower

autoclave temperatures) it was assumed that the increased cooling of the autoclave was leading to condensed water in the unprotected gas outletlines. Due to the design of the outlet lines, any condensed water formed will accumulate without

KHI protection. In contrast, condensed water formed on the top of the autoclave will be collected and returned to the mother liquor with sufficient mixing and splashing from the stir bar. The combination of higher formation of condensation in the outlet lines at lower

temperatures, plus the low probability outlet line condensation flowing back into the autoclave, and

the complete lack of protection of the condensed water in the outlet with KHI, all contribute to the probability of hydrate formation seeding in the outlet lines and growing down into the autoclaves. These factors are consistent with the premature formation of hydrates at lower autoclave

temperatures (sufficient to cool the outlet lines enough to cause condensation), and lack of these types of observed failures at warmer autoclave temperatures, when testing in autoclave Design #1.

In order to test if it was condensation forming in the gas outlet lines, which led to premature hydrate formation at elevated sub-cooling (achieved by lowered autoclave temperatures), autoclave design modifications were implemented. These modifications were done to target the formation of

hydrates due to condensed water formation in the outlet lines. The 2’ flexible tubing was replaced with metal fittings for two reasons. First, the design modification served to remove the dead-leg. Second, any condensation will be limited to the top of the autoclave, where sufficient mixing

wo uld re-dissolve the condensation back into the protected autoclave fluids. This tubing type and length was initially used to allow for the installation of pressure relief valves, which were fitted to the wall for ease of use and cleaning of

the autoclave heads. By redesigning the gas inlet and outlets of the autoclaves the pressure relief and outlet valves could be installed directly on top of the autoclave head. This allowed for the complete removal of the flexible tubing. Without the need for the long length of flexible tubing, the

extra dead-leg was decreased from 2’ down to a few inches.

Decreasing the dead-leg works to, both, lower the amount of unprotected space and decrease the area for and amount of unprotected condensate

formation. The second benefit of replacing the flexible tubing is the increased probability of any condensate that forms in the outlet line flowing back into the autoclave and becoming protected by the treated fluids. The decreased amount of

unprotected condensed water is a direct result of replacing the flexible tubing with a short section of vertical piping. The short metal tubing no longer has any low spots where condensation can accumulate unprotected. The new design for the autoclave inlet and outlets, using short metal

fittings to decrease dead-leg and condensate accumulation points, is shown in Figure 4. Figure

4 shows the decrease in relative error of the tests when switching from autoclave Design #1 to Design #2.

Effect of Stirring on ReproducibilityThe effect of stirring on autoclave test result

quality and reproducibility was investigated. Different stirrer types and stir rates were tested in an effort to completely mix the reaction liquor and maximize splashing of fluid throughout the entire autoclave. Splashing of the inhibited liquid onto

the upper portions of the autoclave will continuously coat the unprotected space with inhibited solution, in turn minimizing any unprotected space. By mixing the fluids throughout the autoclave and protecting the dead-leg space the resulting data should be more

consistent as the formation of hydrates in unprotected condensed water is drastically reduced. It was previously discovered that while using an overhead stirring system in the autoclaves that a stirring rate of > 600 rpm resulted in complete circulation of the reaction liquor

throughout the entire autoclave [12]. The data from the overhead stirred tests showed lower dispersion in hold-times from test to test when the stir rate was above the splashing threshold than tests below the 600 rpm rate Gaining the same

effect with the magnetic stirred autoclaves was desired as the installation of an overhead stirrer wo uld remove our visual capabilities. Testing was done to further align the results obtained with the overhead stirred autoclave with the magnetic stirred autoclaves.

Magnetic stir-bars of variable size and shape were tested at stirring rates of (400 to 1000) rpm, to attempt to achieve a stirring regime similar to overhead stirred autoclaves. This was in an effort to achieve optimum stirring without a loss of

visual capabilities. Of the various stir-bar geometries tested, only the wedge type design was observed to be capable of obtaining mixing close to the overhead stirrer. The optimum stirring rate for the wedge design was found to be 650 rpm.

Autoclave testing was conducted to compare the reproducibility of hold-times with the standard cylindrical design and the wedge stir-bar. The wedge shaped stir-bars, at 650 rpm, provided more consistent hold-times than the smooth cylindrical

or octagonal cylinder shaped stir-bars (at their maximum stable stir rate of 500 rpm) as is shown

in Table 5. These results were also compared to analogous tests using the overhead stirred autoclave at a stirring rate of 650 rpm (Table 5). When using the overhead stirrer or wedge shaped magnetic stir-bars at 650 rpm, the hold-times are more consistent (lower %RSD) than using the

cylinder stir-bar at 500 rpm. The %RSD for autoclave testing with the cylindrical stir-bars of 34 % to 41 %, was much higher than the %RSD on the overhead stirred autoclaves (9%) or when

using the wedge stir-bars (8-12%). This illustrates the benefits of splashing the inhibited media up into the unprotected headspace of the autoclaves, on hold-time reproducibility.

HydrateTypea

(sI,sII)

Sub-Cooling

(oC)

StirrerTypeb

(A,B,C)

tave

(h)%RSD

sI 6 A 49.3 39 %sI 6 B 56.2 8 %sI 6 C 53.0 9 %sII 10 A 51.6 34 %sII 10 B 52.7 8 %sII 13 A 50.1 41 %sII 13 B 47.0 12 %

Note: All tests conducted with autoclave Design #2aWhere sI is a type I hydrate test (Table 2 gas) and sII is a type II hydrate test (Table 1 gas)bWhere A denotes smooth cylindrical stir-bar at 500 rpm, B denotes wedge stir-bar at 650 rpm and Cdenotes stirring with an overhead stirrer at 650 rpm

Table 4. Stirrer design vs. hold-time reproducibility.

Replicate Measurement MethodPrevious studies have shown that after forming hydrates, heating up to dissociate hydrates and

cooling back down to form hydrates again, without changing the reaction liquor, will often lead to shorter observed hold-times. This phenomenon has been referred to as the “hydrate-memory” effect [4, 5, 13]. The memory effect has been

reported as being caused by residual hydrate or ordered water structures, which were not sufficiently dissociated, during the hydrate melting cycle, leading to hydrate seeding. It has also been reported as potentially being caused by higher than equilibrium levels of gas dissolved in the water

upon hydrate formation and dissociation just above the equilibrium temperature. In the experience of the authors, the latter appears to be a more suitable explanation. This had become

apparent when attempting to erase the “hydrate-memory” effect with gases containing high CO2

concentrations, as the CO2 exhibits a reverse temperature-solubility relationship. Further evidence for “hydrate-memory” effect being a result of higher than equilibrium concentrations of

gas can be seen by the lack of a memory effect when using pure THF hydrates. Duchateau et al.[4, 5] determined that dissociation of hydrates at too low of a temperature and/or too short of a heat-up cycle will result in the occurrence of the

“hydrate-memory” effect; however, heating the reaction solution to ≥ 12 oC above the hydrate equilibrium temperature, (HET) for > 2 h will eliminate the memory effect [4, 5]. Their approach was to heat the solution of formed

hydrates at a temperature and duration sufficient to melt the hydrates, without completely erasing the memory effect. In doing this they observed a narrower spread in hold-times. However, these hold-times will be much shorter than what would

be observed in the field, where the solutions are fresh and free of any residual hydrate or ordered water structures.

Using the “memory effect” method for determining KHI performance, the hold-times

observed by will not accurately represent the field conditions, in which there were no previously formed hydrates, as reported in the findings of Duchateau [4, 5]. Therefore, in the work done in our lab, the reverse approach was utilized, in which the autoclave is heated to a high enough

temperature and long enough duration to erase the memory effect of previously formed hydrates, by allowing for dissolved gas levels to return to equilibrium concentrations. This procedure resulted in a reaction liquor with the same fluid

characteristics of a fresh test. Using this approach the fluids can be reused for another test, without the need to remove and replace the fluids an d gas, and the resulting hold-times will be more representative of what can be expected in the field. The ability to reuse the same fluids allows for

testing the hold-times under the exact conditions as the first test (i.e. fluid amounts and pressure), which will eliminate any discrepancies associated with slight changes in fluid concentrations or test pressures from one test to another. Reusing the fluids also allows for quickly changing the set

point temperatures to test at variable sub-cooling, as well as saving on cost and cleaning/loading times.

Experimentation was done to develop a method for repeatable hold-time determination on the autoclaves without the need to replace the fluids after each test. Heating the autoclaves above 35oC (≥ 15 oC above the HET) for ≅ 8 h was

sufficient for erasing the” hydrate-memory” effect. Although shorter times at higher temperatures will erase the memory effect, the 35 oC temperature was chosen to assure the KHI remained in

solution, as many KHIs become insoluble in water at elevated temperatures. The hold-times of replicate tests performed after the 8 h heating step,without replacing the fluids, were observed to be consistent as well as having similar %RSD as replicate tests performed with fresh fluids for each

test (Table 6). By assuring the return to equilibrium levels of dissolved gases, replicate testing can be achieved without the necessity to replace the fluids. In turn, this will lower the cost of testing and greatly decrease the chances of any

discrepancies from slight differences in fluid concentrations and test pressures, which can occur when replacing the fluids for each test. As a result hydrate performance curves can be obtained with the knowledge that each data point has exactly the same test parameters (aside from changes in T sp).

Methoda

(S,M)tave

(h)%RSD

S 54.3 8%M 56.2 7%

Note: All tests conducted with autoclave Design #2aWhere S denotes single data point per run, M denotes

multiple data points with recycled fluids.

Table 6. Comparison of single data point vs. multiple data point autoclave methods.

CONCLUSIONSAutoclave design/set-up has an integral role in the accuracy of data obtained from hydrate testing. Experimental evidence shows that the architecture of the gas inlet and outlet sections of the autoclave can lead to drastic differences in test results. When the autoclave outlet was designed with a

long (2’) section of flexible tubing (Design #1), premature failures were observed. The frequency of the premature failures was sho wn to occur at a higher rate when testing at lower autoclave temperatures. It was shown by the work outlined in this paper that redesigning the autoclave inlet

and outlet lines, to minimize dead-leg and

condensed water accumulation spots (Design #2), greatly lowered the probability of forming hydrates in unprotected condensed water. The result was more accurate data, free of premature hydrate formation, allowing for more reliable determination of KHI hold-times. It was also

shown that stirring rate and stirrer type can play an integral role in the variability of hold-times. When the autoclave fluids are sufficiently stirred, allowing for splashing and complete circulation of protected fluids throughout the autoclave, the

range of observed hold-times was decreased (decreased %RSD), resulting in more precise results.

It was also shown that heating the reaction liquor for a long enough duration and high enough

temperature to remove any “hydrate-memory” effect, allowed for the recycling of the test media, without the necessity of replacing the fluids for each test (excluding gases with ≥ 1 mol% CO2). Recycling of the fluids will allow for the complete removal of any minor discrepancies in KHI hold-

times associated with slight differences in fluid concentrations and gas pressures for each individually loaded test. This protocol to eliminate the memory effect can then be compounded with the improvements in autoclave

design/set-up and experimental procedure, allowing for more accurate and precise hold-time measurements.

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