Methane Hydrate Crystallization on Sessile Water Droplets

Summary

We describe a method to form gas hydrate on sessile water droplets to study the effects of various inhibitors, promoters, and substrates on the hydrate crystal morphology.

Abstract

This paper describes a method to form methane hydrate shells on water droplets. In addition, it provides blueprints for a pressure cell rated to 10 MPa working pressure, containing a stage for sessile droplets, a sapphire window for visualization, and temperature and pressure transducers. A pressure pump connected to a methane gas cylinder is used to pressurize the cell to 5 MPa. The cooling system is a 10 gallon (37.85 L) tank containing a 50% ethanol solution cooled via ethylene glycol through copper coils. This setup enables the observation of the temperature change associated with hydrate formation and dissociation during cooling and depressurization, respectively, as well as visualization and photography of the morphologic changes of the droplet. With this method, rapid hydrate shell formation was observed at ~-6 °C to -9 °C. During depressurization, a 0.2 °C to 0.5 °C temperature drop was observed at the pressure/temperature (P/T) stability curve due to exothermic hydrate dissociation, confirmed by visual observation of melting at the start of the temperature drop. The "memory effect" was observed after repressurizing to 5 MPa from 2 MPa. This experimental design allows the monitoring of pressure, temperature, and morphology of the droplet over time, making this a suitable method for testing various additives and substrates on hydrate morphology.

Introduction

Gas hydrates are cages of hydrogen-bonded water molecules that trap guest gas molecules via van der Waals interactions. Methane hydrates form under high-pressure and low-temperature conditions, which occur in nature in the subsurface sediment along continental margins, under Arctic permafrost, and on other planetary bodies in the solar system¹. Gas hydrates store several thousand gigatons of carbon, with important implications for climate and energy². Gas hydrates can also be hazardous in the natural gas industry because conditions favorable for hydrates occur in gas pipelines, which can clog the pipes lead....

Protocol

1. Design, validate, and machine the pressure cell.

1. Design the cell to allow direct visualization of hydrate formation from a water droplet. Ensure that the cell has a main chamber with a see-through sapphire window and four ports for fluid/gas inlet, outlet, light, and wires (Figure 1). Create the final design in engineering design software (Supplemental Figure S1).
2. To check that the pressure cell is safe under working high pressure, conduct a finite element analysis using simulation software.
   1. Input the full-size pressure cell model from the engineering design software into the simulatio....

Results

With this method, a gas hydrate shell on a droplet can be monitored visually through a sapphire window of the pressure cell and via temperature and pressure transducers. To nucleate the hydrate shell after pressurizing to 5 MPa, dry ice can be added to the top of the pressure cell to induce a thermal shock to trigger rapid hydrate crystallization. There is a clear morphologic difference upon dry ice-forced hydrate shell formation. The water droplet transitioned from a smooth, reflective surface (Figu.......

Discussion

We have developed a method to form methane hydrate shells on sessile water droplets safely and share this method to machine and assemble a pressure cell rated to 10 MPa working pressure, as well as the pressurizing and cooling systems. The pressure cell is outfitted with a stage for the droplet containing embedded thermocouples, a sapphire window for visualizing the droplet, and a pressure transducer fixed to the top of the cell. The cooling system includes chilled ethylene glycol circulating through copper coils in a ta.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CAMERA AND LAPTOP
Camera Body	Nikon	D7200	Name in Protocol: camera
Camera Control Pro 2 Software	Nikon		Name in Protocol: camera software
Laptop	HP Pavilion	hp-pavilion-laptop-14-ce0068st	Needs to be PC with plenty of storage (~ 1 Tb) Name in Protocol: laptop
Macrophotography Lens	Nikon	AF-S MICRO 105mm f/2.8G IF-ED Lens	Name in Protocol: lens
CONSUMABLES
Deionized water			Name in Protocol: DI water
Dry Ice	VWR or grocery store		Buy just before nucleation Name in Protocol: dry ice
Ethanol			Name in Protocol: ethanol
Ethylene Glycol			Name in Protocol: ethylene glycol
COOLING SYSTEM
1/2 in. O.D. x 3/8 in. I.D. x 25 ft. Polyethylene Tubing	Everbilt	Model # 301844	For circulating coolant from chiller to copper coils in aquarium Name in Protocol: 3/8” (inner diameter) plastic tubing
Circulating chiller	Polyscience		Name in Protocol: chiller
Economical Flexible Polyethylene Foam Pipe Insulation	McMaster-Carr	4530K162	3/4" thick wall; 1/2" inner diameter; R Value 3; 6' long Name in Protocol: foam pipe insulation
Plastic tubing			use any tubing that fits the airline connection in the lab and long enough to travel from the airline connection to the front of the aquarium
DATALOGGER
Armature Multiplexer Module for 34970A/ 34972A, 20-Channel	Keysight Technologies	34901A	Name in Protocol: datalogger multichannel
Benchvue or Benchlink software	Benchvue or Benchlink		Name in Protocol: temperature transducer software
Data Acquisition/Switch Unit. GPIB, RS232	Keysight Technologies	34970A	Name in Protocol: datalogger
USB/GPIB interface	Keysight Technologies	82357B	Name in Protocol: datalogger USB
datalogger multichannel
Schott Fostec -Llc 20510 Ace Fiber Optic Light Source	Schott Fostec	A20500	3115PS-12W-B20 115 V ~AC 50/60Hz 5/4.5 W Name in Protocol: light source unit
Schott Fostec light source guide - single bundle	Schott Fostec	A08031.40	Name in Protocol: fiber optic light source cable
METHANE GAS AND REGULATOR
1/4 OD in. x 20 ft. Copper Soft Refrigeration Coil	Everbilt	Model # D 04020PS	For pressurizing ISCO pressure pump. An additional pack is needed for coolant circulation, as listed below. Name in Protocol: high pressure-rated 1/4” copper pipe
Methane cylinder regulator	Airgas	Y11N114G350-AG	Name in Protocol: methane cylinder regulator
Methane gas cylinder	Airgas	ME UHP300	Name in Protocol: methane gas cylinder
PRESSURE PUMP
1/4 in.  flexible tubing, ~ 3 ft.			Connect to pump inlet for leak test Name in Protocol: 1/4"  flexible tubing
260D Syringe Pump W/Controller	Teledyne Instruments Inc.	67-1240-520	Name in Protocol: pressure pump
Controller − Ethernet/USB	Teledyne Instruments Inc.	62-1240-114	Purchase if you would like to install Labview onto computer and control pressure pump remotely. We did not do this.
Smooth-Bore Seamless 316 Stainless Steel Tubing, 1/4" OD, 0.035" Wall Thickness, 1 Foot Long (x5)	McMaster-Carr	89785K824	Name in Protocol: 1/4" pipe
Smooth-Bore Seamless 316 Stainless Steel Tubing, 1/8" OD, 0.02" Wall Thickness, 1 Foot Long (x4)	McMaster-Carr	89785K811	Name in Protocol: 1/8" pipe
Stainless Steel Swagelok Tube Fitting, Reducing Union, 1/4 in. x 1/8 in. Tube OD (x4)	Swagelok	SS-400-6-2	Name in Protocol: 1/8” to 1/4” adapter
PRESSURE CELL
316 Stainless Steel Nut and Ferrule Set (1 Nut/1 Front Ferrule/1 Back Ferrule) for 1/4 in. Tube Fitting (20)	Swagelok	SS-400-NFSET	Used for fitting connections where necessary Name in Protocol: ferrule set
316L Stainless Steel Convoluted (FM) Hose, 1/4 in., 316L Stainless Steel Braid, 1/4 in. Tube Adapters, 60 in. (1.5 m) Length	Swagelok	SS-FM4TA4TA4-60	Connects pressure pump to pressure cell Name in Protocol: 1/4" braided stainless steel flexible pressure-rated hose
ABAQUS	ABAQUS FEA		Name in Protocol: simulation software
Abrasion-Resistant Cushioning Washer for 7/8" Screw Size, 0.875" ID, 2.25" OD, packs of 10 (x1)	McMaster-Carr	90131A107	Name in Protocol: 2.25" rubber washer
Abrasion-Resistant Sealing Washer, Aramid Fabric/Buna-N Rubber, 3/8" Screw Size, 0.625" OD, packs of 10 (x1)	McMaster-Carr	93303A105	Used for illumination port
Acrylic Sheet | White 2447 / WRT31 Extruded Paper-Masked (Translucent 55% (0.118 x 12 x 12)	Interstate Plastics	ACRW7EPSH	Machine a circle of acrylic to fit in the inner chamber of the pressure cell to serve as the background for imaging Name in Protocol: acrylic disc
AutoCAD	AutoCAD		Name in Protocol: engineering design software
Conax fitting	Conax Technologies	311401-011	TG(PTM2/)-24-A6-T, OPTIONAL 1/4" NPT Name in Protocol: pressure seal connector
High Accuracy Oil Filled Pressure Transducers/Transmitters for General industrial applications (x2)	Omega Engineering, Inc.	PX409-3.5KGUSBH	Buy two so there is a backup. Name in Protocol: pressure transducer
HIGH PRESSURE CHAMBER  PARTS	Wither Tool, Die and Manufacturing Company		Machining for pressure cell parts as listed in CAD drawings (Figure S1) Name in Protocol: Part B = stainless steel washer
High-Strength 316 Stainless Steel Socket Head Screw, M5 x 0.80 mm Thread, 14 mm Long (x20)	McMaster-Carr	90037A119	Used for illumination port
High-Strength 316 Stainless Steel Socket Head Screw, M8 x 1.25 mm Thread, 25 mm Long (x20)	McMaster-Carr	90037A133	Name in Protocol: M8 stainless steel screws
Oil-Resistant Hard Buna-N O-Ring, 3/32 Fractional Width, Dash Number 120, packs of 50 (x1)	McMaster-Carr	5308T178	Name in Protocol: 1" o-ring
Oil-Resistant Hard Buna-N O-Ring, 3/32 Fractional Width, Dash Number 128, packs of 50 (x1)	McMaster-Carr	5308T186	Name in Protocol: 1.5" o-ring
Omega Inc. pressure transducer software	Omega Engineering, Inc.		Name in Protocol: pressure transducer software
Polycarbonate Disc	McMaster-Carr	8571K31	Listed in CAD drawings for illumination port, Fig. S1 Part E
Sapphire windows (x3)	Guild Optical Associates, Inc.	Optical Grade Sapphire Window, C-Plane Diameter: 1.811” ±.005” Thickness: .590” ±.005” Surface Quality: 60/40 Edges ground and safety chamfered	Buy three so there are two backups. Name in Protocol: sapphire window
Solid Thermocouple Wire FEP Insulation and Jacket, Type K, 24 Gauge, 50 ft. Length (x1)	McMaster-Carr	3870K32	Name in Protocol: thermocouples
Stainless Steel Integral Bonnet Needle Valve, 0.37 Cv, 1/4 in. Swagelok Tube Fitting, Regulating Stem (x4)	Swagelok	SS-1RS4	Two will be used for the pressure pump as well. Name in Protocol: 1/4" needle valves
Stainless Steel Pipe Fitting, Hex Nipple, 1/4 in. Male NPT (x2)	Swagelok	SS-4-HN	Used for illumination port
Stainless Steel Swagelok Tube Fitting, Female Branch Tee, 1/4 in. Tube OD x 1/4 in. Tube OD x 1/4 in. Female NPT (x2)	Swagelok	SS-400-3-4TTF	Used with pressure transducer Name in Protocol: branch tee fitting
Stainless Steel Swagelok Tube Fitting, Male Connector, 1/4 in. Tube OD x 1/4 in. Male NPT (x4)	Swagelok	SS-400-1-4	Used on top port and side port leading to needle valves Name in Protocol: NPT screws
Stainless Steel Swagelok Tube Fitting, Port Connector, 1/4 in. Tube OD (x8)	Swagelok	SS-401-PC	Use as tube connections between NTP and valve connections Name in Protocol: port connector fitting
TANK
1/4 OD in. x 20 ft. Copper Soft Refrigeration Coil	Everbilt	Model # D 04020PS	For circulating coolant Name in Protocol: 1/4" copper pipe
10 gallon aquarium	Tetra		Name in Protocol: 10 gallon tank
2 oz. Waterweld	J-B Weld	Model # 8277	Name in Protocol: underwater sealant
3 in. x 25 ft. Foil Backed Fiberglass Pipe Wrap Insulation	Frost King	Model # SP42X/16	For wrapping around aquarium Name in Protocol: foil-lined fiberglass
3/8 7/8 in. Stainless Steel Hose Clamp (10 pack)	Everbilt	Model # 670655E	Name in Protocol: worm drive hose clamps
Styrofoam			Name in Protocol: insulating material
TOOLS
1-1/8 in. Ratcheting Tube Cutter	Husky	Model # 86-036-0111
1/2 in. to 1 in. Pipe Cutter	Apollo	Model # 69PTKC001
Adjustable wrench (x2)	Steel Core	Model # 31899	Need two wrenches with jaw at least 1"
Allen wrench set	Home Depot
Duct tape			Name in Protocol: duct tape
Flexible tubing, like an IV line, to fit on the end of grainger probe (canula)			Name in Protocol: IV tube
Grainger 18 gauge probe	Grainger		For inserting droplet Name in Protocol: cannula
High Vacuum Grease	Dow corning		Apply to o-rings before inserting sapphire window Name in Protocol: vacuum grease
Klein Tools Professional 90 Degree 4-in-1 Tube Bender	Klein Tools	Model # 89030	Name in Protocol: tube bender
Snoop liquid leak detector	Swagelok	MS-SNOOP-8OZ	To detect leaks when pressurized when methane Name in Protocol: liquid leak detector
Suction cup	Home Depot		For removing tight fitting sapphire window Name in Protocol: suction cup
Teflon Tape			Name in Protocol: plumber's tape
Temflex 3/4 in. x 60 ft. 1700 Electrical Tape Black	3M	Model # 1700-1PK-BB40	Name in Protocol: electrical tape