High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

Summary

The high pressure sapphire cell apparatus is a unique tool to study, without sampling, phase behavior under a wide range of pressures. Using a cathetometer, very precise volume measurements can be recorded to measure liquid expansion and phase composition. Thus, this synthetic method enables the study of (1) phase equilibria of multi-component mixtures and (2) the partition behavior of catalyst or model compounds as a function of pressure.

Abstract

The high pressure sapphire cell apparatus was constructed to visually determine the composition of multiphase systems without physical sampling. Specifically, the sapphire cell enables visual data collection from multiple loadings to solve a set of material balances to precisely determine phase composition. Ternary phase diagrams can then be established to determine the proportion of each component in each phase at a given condition. In principle, any ternary system can be studied although ternary systems (gas-liquid-liquid) are the specific examples discussed herein. For instance, the ternary THF-Water-CO₂ system was studied at 25 and 40 °C and is described herein. Of key importance, this technique does not require sampling. Circumventing the possible disturbance of the system equilibrium upon sampling, inherent measurement errors, and technical difficulties of physically sampling under pressure is a significant benefit of this technique. Perhaps as important, the sapphire cell also enables the direct visual observation of the phase behavior. In fact, as the CO₂ pressure is increased, the homogeneous THF-Water solution phase splits at about 2 MPa. With this technique, it was possible to easily and clearly observe the cloud point and determine the composition of the newly formed phases as a function of pressure.

The data acquired with the sapphire cell technique can be used for many applications. In our case, we measured swelling and composition for tunable solvents, like gas-expanded liquids, gas-expanded ionic liquids and Organic Aqueous Tunable Systems (OATS)^1-4. For the latest system, OATS, the high-pressure sapphire cell enabled the study of (1) phase behavior as a function of pressure and temperature, (2) composition of each phase (gas-liquid-liquid) as a function of pressure and temperature and (3) catalyst partitioning in the two liquid phases as a function of pressure and composition. Finally, the sapphire cell is an especially effective tool to gather accurate and reproducible measurements in a timely fashion.

Introduction

When reactions are being conducted with a hydrophilic catalyst and a hydrophobic substrate to form a hydrophobic product, it is quite common to employ mixed solvents in order to provide a homogeneous reaction system. For example, THF-water and acetonitrile-water are commonly mixed solvent vehicles for these homogenous reaction processes. Ideally, it would be advantageous to develop a process in which the reaction is performed under homogeneous conditions followed by an induced phase split to separate the aqueous and organic solvent components. The hydrophilic catalyst would then be located in the aqueous phase and the hydrophobic product in the organic phase. The over....

Protocol

1. Assembly of the Sapphire Cell

1. Place a 116 size backing ring and 210 size O-ring onto the piston. Verify that O-ring material is compatible with chemicals used during the experiment prior to assembly.
   1. Some backing rings have a flat and a curved edge. If this is the case, place the flat edge down and the curved edge against the O-ring.
2. Thread rod into the bottom of the piston using a rod with a threaded tip (Figure 3).
3. Wrap the rod with a layer of lab towel (or other nonabrasive lab wipes) to prevent scratching while inserting the piston into the cell.
4. Inser....

Results

The schematic of the high-pressure sapphire cell is shown in Figure 2, along with a picture of the cell. The sample is in the top cell and in the bottom cell is water with blue dye for demonstration purposes. The liquid components are fed via a syringe and valve, while the CO₂ (gas component) is pumped through a high pressure syringe pump. The pressure can be controlled through the piston (the water is also fed via high pressure syringe pump in our setup). The liquid and gas phases can be clea.......

Discussion

The sapphire cell apparatus is a unique tool for measuring phase behavior without sampling, and thus equilibrium is not disturbed. To ensure accurate repeatable data, there are critical steps in the protocol (Protocol 4 entitled “Operation of the Sapphire Cell Apparatus”) that must be followed. For any system in which phase composition is measured, it is critical to reach equilibrium prior to measurement. The sapphire cell is placed on a rotating shaft that facilitates mixing to more rapidly achieve equilibri.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Hollow sapphire cylinder			50.8 mm O.D. x 25.4±0.0001 mm I.D. x 203.2 mm L
Pressurizing fluid	Water
Syringe pumps	Teledyne Isco Model 500D
Digital temperature controller	Omega CN76000
Digital readouts	HH-22 Omega
Thermocouples	Omega Type K
Pressure transducer & readout	Druck, DPI 260, PDCR 910
CO2			SCF grade
Cathetometer	Gaertner Scientific Corporation or any scientific lab suppliers
Relief valve	Spring loaded relieve valve (Swagelok)
Mounting bracket	Unistrut bracket
Hollow spacers			3/4 in
4 stainless steel bolts, 4 nuts, 2 washers			3/4 in
3 O-rings	Kalrez, 210 size
3 backing rings			116 size for piston; 2 8210 size for end caps
1 multiport fitting	HiP
High pressure tubing			Stainless steel, 1/16 in