Pressure measurements, hydraulic transport

The sensitivity of the pressure drop to the coefficient of solids-surface friction /j.f may well account for the wide scatter in the results shown earlier in Figure 5.10. Unfortunately this quantity has been measured by only very few investigators. It must be emphasised that in the design of any hydraulic transport system it is extremely important to have a knowledge of the coefficient of friction. 

Hydraulic transport can be achieved using a coal-water slurry, which can be efficiently pressurized by means of displacement pumps up to 70-100 bar with no inert gas detriment [1]. In addition to stability and viscosity, the main measure for slurry quality is the solids concentration. This determines the energy density and the amount of water, which hampers process efficiency. Figure 4.4 shows the influence of the slurry solids concentration on the cold gas efficiency for several solid fuels. Figures 4.4 is compiled using a GE gasifier model with radiant and... 

Saturated zones are common in porous material, and provide significant pathways for the subsurface transport of water and solutes. The term "ground water" commonly refers to continuously saturated zones of appreciable thickness. Saturated conditions also occur on a small scale or short term basis in association with the infiltration and drainage of precipitation or surface runoff. As indicated previously, the pressure and elevation components are the primary contributors to the total moisture potential in the saturated zone. These two are commonly combined into a "piezometric head", representing the addition of the water pressure head to the elevation at which the pressure head is measured. The hydraulic conductivity does not change significantly with... 

Under steady-state operation, local mechanical equilibrium prevails at all microscopic and macroscopic interfaces in the membrane. It fixes the stationary distribution of absorbed water. The condition of chemical equilibrium is, however, lifted to allow for the flux of water. Continuity of the net water flux in the PEM and across its interfaces with adjacent media adjusts the gradients in water activity or pressure in the system. Water fluxes occur by diffusion, hydraulic permeation, and electro-osmotic drag. At external interfaces, vaporization and condensation proceed at rates that match the net water flux. These mechanisms apply to PEM operation in a working cell, as well as to ex situ water flux measurements that are conducted in order to investigate the transport properties of PEMs. 

The work of Adachi et al. (2009) represented a first attempt to correlate and validate ex situ and in situ water permeation phenomena in PEMs. Water permeabilities of Nafion PEMs and water transport in operating PEFCs were investigated under comparable ex situ and in situ values of temperature and RH. The examined parameters included the type of driving forces (RH, pressure), the phases of water at PEM interfaces, PEM thickness, and the effect of catalyst layers at the membrane interfaces. Several experimental setups and schemes were designed and explored. Water permeability at 70°C was determined for Nafion membranes exposed to either liquid or vapor phases of water. Chemical potential gradients of water across the membrane are controlled through the use of differences in RH (38-100%), in the case of contact with water vapor, and hydraulic pressure (0-1.2 atm), in the case of contact with liquid water. Three types of water permeation experiments were performed, labeled as vapor-vapor permeation (VVP), liquid-vapor permeation (LVP), and liquid-liquid permeation (LLP). Ex situ measurements revealed that the flux of water is largest... 

In this Section, it is implicitly assumed that the mass transport resistance at the fluid-membrane interface on either side of the membrane is negligible. Also the following is information that is made available publicly by the membrane manufacturers, when not otherwise noted. As in technical processes, mass transport across semipermeable medical membranes is conveniently related to the concentration and pressme driving forces according to irreversible thermodynamics. Hence, for a two-component mixture the solute and solvent capacity to permeate a semipermeable membrane under an applied pressure and concentration gradient across the membrane can be expressed in terms of the following three parameters Lp, hydraulic permeability Pm, diffusive permeability and a, Staverman reflection coefficient (Kedem and Katchalski, 1958). All of them are more accurately measured experimentally because a limited knowledge of membrane stmcture means that theoretical models provide rather inaccurate predictions. 

---