STANDARD OPERATING PROCEDURE

FOR

DENSITY MEASUREMENT

WITH RUBOTHERM APPARATUS

Written by:

Armand Karimi

The University of Western Australia – Fluid Science Division – Prof. Eric F. May

October 2015 – Version 1.1

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EXPERIMENTAL OVERVIEW

Rubotherm is a magnetic suspension balance densimeter which makes high accuracy density measurement of single phase gas (mixtures) possible. The system is based on Archimedes principle of buoyancy: A sinker, or weight, of well-known volume is weighed in the present of the fluid of interest. The balance reading, W, corresponds to:

W m Vρ= − , (1)

where m is the sinker mass, ρ is the density of the fluid and V is the volume of the sinker. This equation can be simply rearranged to calculate the fluid density:

m WV

ρ −= . (2)

The centerpiece of this apparatus is a magnetic suspension micro-balance system that is used for direct mass measurement of sinkers. The micro-balance can read to 10 µg with a long-term reproducibility of 20 µg. To perform a density measurement on fluids at extreme conditions, such as a high pressure and low or high temperature, a magnetic coupling is used between the sinkers and the balance. As it is illustrated in figure 1, a permanent magnetic resides within the fluid inside the vessel containing the sinkers. On the other side of the coupling is an electromagnet at ambient conditions. The voltage supply to the electromagnet is controlled so that the position of the permanent magnetic remains constant. Our Rubotherm single sinker densimeter supports pressures up to 35 MPa (5000 psia) and it has an operational temperature range of 256 K to 422 K.

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Figure 1 - Measuring principle of magnetic suspension balance

REFERENCE READINGS

Source theory

W. Wagner, R. Kleinrahm, N. Pieperbeck, M. Jaeschke, A new apparatus for high-precision gas-density measurements to calibrate gas-density transducers. Proceedings of the 1989 International Gas Research Conference, Tokyo, November 6-9, 1989, Government Institutes Inc., Rockville, U.S.A., 1990, 462-471

EQUIPMENT

Equipment Description Gas Dosing System (GDS) Rubotherm’s control box ISCO Pump ISCO 260 D syringe pump Vacuum Pump Vacuubrand RZ 2.5 Balance Mettler Toledo Rubotherm’s Bath Julabo FP-50 (Circulator ME series) Cylinder Heating Bath Thermomix® 1442 D Pressure Transducer Paroscientific 46K-HHT-101 (6000 PSI) Temperature Probe Fluke 5606 Multimeter & Bridge IsoTech miliK CH4 Cylinder Pure methane 99.995% Gas Mixture Cylinder 12.2 L Faber Scuba cylinder Fittings and Valves HiP and Swagelok (Metric + Imperial) Manual Pressure Regulator Gascon systems

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SAFTEY

RISK & HAZARDS SUMMARY

The following table presents the general hazards associated with Rubotherm. It is highly recommended to review the formal risk assessment documents prepared for Rubotherm before starting to work with it.

Hazards Controls Flammable gases • No ignition source in the laboratory

• Check all the fittings for leakage regularly High pressure gas • Check the pressure limit of the system

• Check the valves, venting and exhaust • Burst disk • Read the manual for using ISCO pumps

Slips & trips • Keep the work area clean • Avoid touching with naked hand the inner surface of cell • Always were gloves

Silicon oil (Julabo bath)

• Always wear laboratory coat and gloves • Use proper silicon oil for each temperature range • Check all fittings in bath hoses for leakage

Manual handling • Make sure the cell is empty before moving or opening it. • Open the cell patiently and step by step • Change the sealing after opening each time • Ask for assistance for opening the cell

Magnetic field • Avoid placing magnetic materials near the equipment

EMERGENCY SHUTDOWN

There is an emergency safety button on the GDS unit for emergency shut-down of the system. By pushing this safety button the measuring unit will be disconnected from electrical power. Furthermore, by pushing this safety button the temperature control system and Julabo bath get switched off. In cases which the temperature in the cell is lower than the ambient temperature, disconnection of the bath from electrical power will result in an increase of the pressure inside the cell. In these cases, a proper burst disk will protect the system against overpressure in the lines.

Figure 2 - Safety off-button on the GDS unit.

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REPORTING AND DUTY OF CARE

• The system must be checked regularly for leakage in the fittings, valves and the sealing. • Perpendicularity of the balance to the table must be checked before each measurement. • The equipment environment must be cleaned regularly and the inner surfaces of the cell

must be protected from oil stains, dust or any other contamination. • Heat transfer in the Julabo bath must be chosen with respect to the measurement

temperature range. • It is your responsibility to fix and/or report any hazards that you notice in the

laboratory. • If an event or miss does occur you must report it to the lab supervisor ASAP. • Ensure you are aware of the nearest emergency stop, fire extinguishers and fire alarm is

to your work area.

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SECTION 1. PRESENTATION OF GAS DOSING SYSTEM (GDS)

Before starting to learn density measurements with this system, it is necessary to learn how to operate the Gas Dosing System (GDS) provided by Rubotherm. GDS provides the ability to inject gas from three different inlets into the measuring cell with controlled pressures and flow rates. We can also use GDS to vent the gas, evacuate the cell and keep it under vacuum. These are the major modes of operation, which will be used extensively in SECTION 2 where the measurement procedure is described.

A. FILLING THE DENSITY MEASUREMENT CELL WITH GAS (MIXTURE) 1. Make sure that all of the valves of the Gas Dosing System (GDS) are closed (as shown

in the photo below).

2. Close the manual valves of ISCO pump (Valve N°1 and Valve N°2)1. 3. Expand the ISCO pump cylinder to its maximum volume i.e. 266.35 ml.

For expanding the ISCO pump cylinder to its maximum volume press <Refill> icon on the pump panel.

4. Make sure the gas cylinder pressure is larger than inside the ISCO pump, and the

open the gas cylinder’s valve. 5. Open valve N°1 and let the gas enter the ISCO pump.

1 The manual valves of the ISCO pump are labelled.

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6. When the pressure in the ISCO pump has reached the maximum cylinder pressure

(or the desired value), close valve N°1 and then close gas cylinder valve. 7. Set the ISCO pump to the desired pressure.

Follow the steps described below to set the pressure on the ISCO pump panel: >> Press STOP icon >> Press CONST PRESS >> Press ‘A’ (if the ISCO pump is connected to port A of the control panel) >> Enter the desired pressure value >> Press ENTER >> Press RUN Pump will start compressing the gas.

8. Once the desired pressure is reached in ISCO pump, open valve N°2 (to GDS inlet).

ATTENTION! Inlet pressure of GDS must always be 10 bar higher than the target pressure inside the cell.

9. On the GDS panel, press <GO> icon (green triangle). 10. A new wizard window appears to set the dosing control parameters step by step. 11. Enter the desired pressure in the measurement cell and press <next>.

12. Choose the gas injection channel and then press <next>.

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13. Select the gas that is going to be injected into the measurement cell and then press <next>.

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14. Set the desired gas injection flowrate and then press <next> (The maximum

acceptable flowrate for each gas is different and is predefined by Rubotherm software).

15. Check the summary of the dosing operation and then press <next>.

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16. Once the dosing wizard is completed press <Dosing>. Dosing operation will begin by opening of the valves of the selected channels.

17. After pressing <DOSING> a series of self-test windows appear. Answer to all

question (the answer must be “yes” to proceed) and run the dosing system.

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18. During the gas dosing process, the default panel of GDS shows necessary

information of the operation such as pressure, flow rate set-point and the state of each valve. Pressure inside the cell is shown by PI1 indicator (for low pressures) and by PI2 for high pressures. The value of the flowrate set-opint is shown in fornt of each inlet valve. On this panel, the green icons show the active parts of the system; red stands for inactive parts.

19. For detailed information of the dosing process, the user can click on the “viewer”

icon in the top right corner of the panel and switch to the advanced mode. In this panel a real-time view of the flow parameters is accessible. (Password: 3333)

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20. If the target pressure inside the cell cannot be achieved because the inlet pressure

generated with the ISCO pump is not high enough, press <STOP> icon on GDS panel and go to step 1. Repeat step 1 to step 18 until the desired pressure is reached.

B. CHANGING THE PRESSURE OF THE SYSTEM

For increasing or reducing the pressure inside the measuring cell, it is sufficient to press the <GO> icon on the GDS panel and modify the target pressure within the application wizard. The control system will automatically open the inlet valves or outlet valves (or both) for increasing or decreasing the pressure, respectively.

ATTENTION! Inlet pressure of GDS must always be 10 bar higher than the target pressure inside the cell.

C. EVACUATION & VACUUM OF THE CELL

There are two ways to evacuate the cell:

1. To release the pressure by pressing <Pressure Release> icon on the panel: In this case, the system automatically vents the gas to the exhaust through the ventilation exit and reduces the pressure to a pressure slightly higher than ambient pressure (1.2 bar). (This option may be used after before changing to another isotherm.)

2. To press <VAC.> icon on the screen panel: Here, the GDS system vents the gas to the exhaust through the ventilation exit of the GDS system. When the pressure is low enough the automatic valve of the ventilation system

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shuts off and the system gets connected to the vacuum pump. This method is usually used when a new gas/mixture is going to be introduced into the cell.

D. INTERNAL ADJUSTMENT OF THE BALANCE

Internal adjustment of the balance is an important step before beginning any measurement. It is considered as an internal calibration of the balance, which should be done every day before starting measurements.

To initiate the internal balance adjustment, the magnetic suspension coupling must be switched to Zero-point position. Once the coupling is in a stable suspension position, the <Adjust Int.> icon on the Mettler Toledo balance screen panel must be pressed. Depending on the stability of the system, the internal adjustment may take a few minutes. At the end, the internal temperature of the balance at which the balance is adjusted is reported on the panel increased. This temperature should be documented in the measurement report (MS Excel file) and in the lab book. (This temperature is also a good scale of the efficiency of nitrogen flushing inside the balance at high and low temperatures).

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E. RUBOTHERM’S CONTROL SOFTWARE

Rubotherm’s control software provides the facility to switch the sinker positions and setting the pressure in the cell. It also gives real-time information of the mass flow rate of the dosing channels, the temperature of the electrical heater at the cell, the pressure and apparent sinker mass changes over time.

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SECTION 2. DENSITY MEASUREMENT PROCEDURE

Gas density measurements with magnetic suspension balance are carried at different isotherms. At a constant temperature, the measuring cell and the GDS lines must be filled with the gas at the highest desired pressure. Further, low pressures are measured by reducing the pressure of the cell.

For each measurement at each isotherm following procedures must be followed:

A. WEIGHING THE SINKER(S) INSIDE THE EVACUATED MEASURING CELL

Whenever a new gas (mixture) is going to be investigated, it is necessary to determine the mass of the sinker(s) inside the evacuated measuring cell for each isotherm of interest. For this purpose, the cell must be evacuated and should be kept under vacuum for at least 3 hours. When the system is at thermal equilibrium, the sinker(s) can be weighed. The resulting value needs to be documented for the subsequent density determinations. Please note: The vacuum inside the cell needs to be good enough to provide a sophisticated value for the sinker mass in vacuum.

B. VACUUM/PURGE PROCESS BEFORE BEGINNING WITH DENSITY MEASUREMENTS

It is necessary to run a series of vacuum-purge-cycles before introducing a new gas or a new mixture to the system. This process is descried below:

1. Evacuate the system and keep it under vacuum for 15 minutes. 2. Fill the cell with the gas (mixture) under study up to a pressure of about 5 bar. Cell can

be filled with the maximum flow rate, e.g., for methane 340 ml/min. 3. Stop GDS once 5 bar are achieved and leave the system for 3 minutes at this pressure. 4. Vent the gas and evacuate the measuring cell. Begin to wait for 3 minutes when the

pressure transducer shows 0 bar 5. Repeat steps 2 to 4 for 3 times and then fill the cell with the gas (mixture).

C. FILLING THE GAS (MIXTURE) FOR DENSITY MEASREMENT

For the filling procedure follow the steps described above in Part A of Section 1. Once the desired pressure is reached, the gas must be flushed through the measuring cell at constant conditions (T and p) - similar to measurement conditions - for 15 minutes. (Choose enough flow rate to exchange the gas volume inside the cell.) This is for reducing the systematic errors of adsorption and desorption of gas molecules on the internal surfaces of the measurement cell.

After 15 minutes, stop the dosing system by pushing the <STOP> icon on the GDS panel and leave the system to reach equilibrium, i.e. constant pressure and constant temperature. From the current experience this will probably take (3 to 4) hours.

D. STARTING THE MEASUREMENT

When the system is at the equilibrium, density measurements can be carried out:

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1. Before starting the measurement, the balance should conduct an ‘internal adjustment’

(see PART D in SECTION 1). Record the temperature given by the balance after the adjustment.

2. Open the proper excel file designed for reporting a density measurement and fill in the date and details of operation:

• Choose the type of measurement (pure fluid, binary mixture…). • Choose the type of sinker and the settle time of the balance. • Insert the temperature of the balance taken from internal adjustment. • Insert important comments at the beginning (or at the end of measurement)

describing special procedures and/or the goal of the measurements.

3. Switch the sinker position multiple times between ‘Zero-point’ and ‘MP1’ for 2 minutes before starting the measurements to align the magnetically suspended parts inside the cell.

4. Start the measurements by making the balance Zero at the zero point. At the same time run the timer and change the position to MP1. At each position record the sinker mass, cell temperature and pressure and change the sinker’s position. ATTENTION! Always read the sinker mass from the Mettler Toledo screen panel, which provides ±1 µg readability.

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5. At the end, the excel file provides the average and the standard deviation of mass

readings, pressure and temperature of the system. 6. Save the excel file with an appropriate file name and exit. 7. Do a replicate measurement after a few minutes to confirm the primary results.

ATTENTION! If the system is at equilibrium, and there is no vibration, mechanical or electromagnetic effect, the standard deviation of balance readings should be less than 20 µg. Otherwise, all the destabilizing effects must be reduced and measurements must be repeated.

E. REDUCING THE MEASURING CELL PRESSURE FOR THE NEXT SINLGE POINT (LOWER PRESSURE)

At each single point measurement the measuring cell must be flushed with fresh gas for about 15 minutes at the desired measuring pressure and temperature. For this purpose follow PART B of SECTION 1. After reaching equilibrium in the cell, go back to PART D of SECTION 2 and run the measurements.

F. CHANGING TEMPERARTURE OF CELL AND COUPLING HOUSING

A target temperature can be set via the bath-thermostat panel. (Please note: The Rubotherm software could be used as well, however, this option is currently disabled as the software sometimes changes set-point values randomly.) The temperatures of the measuring cell and the coupling housing should always be the same, otherwise convection of gas inside cell and coupling housing will lead to unstable balance readings.

As the sinker positions of the magnetic suspension coupling are adjusted when the system is “open”, which means at ambient conditions, the coupling positions need to be changed regularly when changing temperatures. This is important for the control unit to adapt the control parameters. If not doing so, the coupling might not work properly at the new set-point temperature. Changing sinker positions during temperature adjustment might be carried out automatically by using the density measurement option in the Rubotherm software (Please note: You can only do this when both sinkers are installed!).

When measuring at temperatures below the dew-point temperature of the ambient air, flush the balance hood (and therewith the surrounding of the electromagnet) to prevent condensation or freezing of water from the air. If not doing so, no stable balance readings can be achieved!

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SECTION 3. Ternary mixture preparation loop

A newly prepared sampling loop allows making of ternary mixtures inside an Isco-pump and then filling the isochoric cells with the gas mixture while keeping the total pressure above cricondenbar of the mixture. This sampling system is illustrated in figure 6.

Figure 3 - Ternary mixture preparation system

In this system a binary mixture is prepared in advance as a base for making a fixed composition ternary mixture. This binary mixture is prepared and kept in Gas Cylinder No.1. Knowing the composition of the binary mixture and the volume of Isco-pump, precise mass of heptane (C7) are injected into Isco pump no.1. This syringe pump has a maximum volume of 500 ml and is equipped with a mixer. The pressure of this pump is set above the cricondenbar of the mixture in order to keep the ternary mixture always in a single phase region.

The composition of the ternary system is then analyzed by GC using the sampling loop no.2. On the other hand the pressure cell is filled with pure methane. By opening the outlet valve of isco pump no.1 the mixture pushes back the pure methane in pure methane cylinder (cylinder no.2). The valves of the pressure cell are closed and the system is ready for experiment.

The system is under development and more detailed procedure will be provided after complete installation of the system.

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SECTION 4. TROUBLESHOOTING

1. Unstable sinker positions

Problem: MP1 (or ZP) is not stable and the balance position switches automatically. Balance is out of control.

Solution: This problem usually occurs at extreme temperatures (T>100°C or T<10°C) when either the sinker or the sinker hook are touching the deposit screws or the interior walls of the deposit cage. At high temperatures, it is recommended to open the cell while the housing coupling is kept at the measurement temperature i.e. T>100°C and then screw in/out the deposit screw to find the stable position for the deposit screw.

For low temperatures, however, the deposit position cannot be set while the cell is open because of the risk of condensate formation in the interior surfaces of the coupling. Therefore, each time the deposit screw must be tightened at ambient temperature and then after closing the cell, the stability must be verified at low temperature. This method is based on try & error and is time consuming.

As a general rule, the distance between the two magnets of the coupling becomes smaller when the weighing force increases. This means that inside the evacuated cell the distance between the two magnets is the smallest, and at low gas densities (i.e., at moderate pressures) it is pretty much the same. However, at higher pressures the density is larger and, thus, due to buoyancy the weighing force becomes smaller. This means the distance between both magnets becomes larger, and this is the point where the sinker might touch the sinker rest/deposit screw. This is also the case in low temperatures (and high pressures) where the distance between two sinkers increase and the sinker positions become instable.

2. Unstable mass readings

Problem: MP1 and ZP positions are individually stable but the mass readings are unstable (balance cannot be calibrated).

Solution: This problem usually occurs when the balance is not horizontally adjusted. In this situation, using a bubble-level the balance level must be adjusted.

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