Marc Scott Hodes

Salt precipitation and scale control in supercritical water oxidation-Part A: fundamentals and research

Abstract Supercritical water oxidation (SCWO) is an effective technology for treatment of organics and organic components of aqueous wastes. Commercialization of SCWO processes has been hindered by concerns about corrosion and scale buildup/fouling which, when present, must be accommodated by system design and/or operational procedures. Salts are formed during SCWO when acidic solutions are neutralized to reduce corrosion and may also be present in the waste stream itself. Because salts have low solubility in supercritical water (SCW), they precipitate. Precipitated salts often form agglomerates and coat internal surfaces, thereby inhibiting heat transfer from/to exterior surfaces. When scale buildup is left uncontrolled, plugging of transport lines and/or the reactor can occur. The required cleaning can result in substantial and costly downtime in the SCWO process. General principles and research relevant to SCWO have been reviewed elsewhere. A review of the many technologies available to control scale during SCWO is given in the companion paper by Marrone et al. [J. Supercrit. Fluids (in press)]. Presented here is a review of fundamental principles and research pertinent to the precipitation of salts and scale control at the elevated temperatures and pressures found in an SCWO reactor. First, SCWO is introduced and the physics leading to scale buildup during SCWO is discussed. Next, the phase diagrams of model salt–water systems at relevant conditions are presented. Then, the many phenomena which complicate modeling of heat transfer in SCW (buoyancy, rapidly varying thermophysical properties, etc.) are reviewed and a set of correlations to calculate heat transfer coefficients is provided. Finally, the limited number of controlled experimental studies on scale buildup during SCWO are reviewed.

Salt precipitation and scale control in supercritical water oxidation—part B: commercial/full-scale applications

Despite the potential of supercritical water oxidation (SCWO) as a viable technology for organic waste destruction, its commercial development has been hindered by the problems of corrosion and salt precipitation/solids buildup. The extremely low solubility of polar inorganic salts in the supercritical water environment causes salts present in the feed, or formed during reaction, to precipitate inside the reactor. If left unchecked, these salts can rapidly accumulate on reactor walls or process surfaces and form plugs, causing expensive and frequent downtime of the SCWO system. Other solids such as oxides exhibit low solubility in water over the range from ambient to supercritical conditions and, although they have much less tendency to adhere to process surfaces, may still hinder operations if not accommodated. Many wastes will have a combination of salt-type and oxide-type solids, and may have an intermediate tendency to stick to process surfaces. Many of the companies that have attempted to commercialize the SCWO technology over the past two decades have developed innovative approaches to dealing with the corrosion and salt precipitation/solids buildup problems. These are often the distinguishing features of each companys SCWO process. This paper objectively reviews several commercial approaches that have been developed and/or used to control salt precipitation and solids buildup in SCWO systems. The approaches reviewed consist of specific reactor designs and operating techniques, and include the following: reverse flow tank reactor with brine pool, transpiring wall reactor, adsorption/reaction on a fluidized solid phase, reverse flow tubular reactor, centrifuge reactor, high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence/homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation. Recent commercial SCWO applications utilizing these approaches are also discussed. A companion paper by Hodes et al. (J. Supercrit. Fluid., see this volume) reviews fundamental principles and research pertinent to scale control in SCWO processes.

On the Potential for Homogeneous Nucleation of Salt From Aqueous Solution in a Natural Convection Boundary Layer

Recent studies have examined the rate of salt deposition by natural convection on a cylinder heated above the solubility temperature corresponding to the concentration of salt in the surrounding solution at conditions typical of the Supercritical Water Oxidation (SCWO) process (Hodes et al. [1,2], Hodes [3]). The total deposition rate of salt on the cylinder is the sum of the rate of deposition at the salt layer-solution interface (SLSI) formed on the cylinder and that within the porous salt layer. The rate of deposition at the SLSI cannot be computed without determining whether or not salt nucleates homogeneously in the adjacent (natural convection) boundary layer. A methodology to determine whether or not homogeneous nucleation in the boundary layer is possible is presented here. Temperature and concentration profiles in the boundary layer are computed under the assumption that homogeneous nucleation does not occur. If, under this assumption, supersaturation does not occur, homogeneous nucleation is impossible. If supersaturation is present, homogeneous nucleation may or may not occur depending on the amount of metastability the solution can tolerate. It is shown that the Lewis number is the critical solution property in determining whether or not homogeneous nucleation is possible and a simple formula is developed to predict the Lewis number below which homogeneous nucleation is impossible for a given solubility boundary and set of operating conditions. Finally, the theory is shown to be consistent with experimental observations for which homogeneous nucleation is absent or present.

A Natural Convection Model for the Rate of Salt Deposition From Near-Supercritical, Aqueous Solutions

A model is developed for the rate of salt deposition by natural convection from aqueous salt solutions onto a horizontal cylinder heated beyond the solubility temperature for the dissolved salt. The model accounts for the deposition rate at the salt layer-solution interface (SLSI) formed on the cylinder, but it does not account for deposition which may occur inside the porous salt layer (PSL). Dissolved salt is transported to the SLSI by molecular diffusion (with advection) and subsequently nucleates heterogeneously there. The model is applied to the experimental deposition rate data acquired by Hodes et al. (1998, 2002) at conditions pertinent to Supercritical Water Oxidation (SWCO). The ratio of the predicted deposition rate to the measured one ranges from roughly 0.5 to 2 indicating that deposition inside the PSL can be considerable.

Optimized Thermoelectric Module-Heat Sink Assemblies for Precision Temperature Control

Robust precision temperature control of heat-dissipating photonics components is achieved by mounting them on thermoelectric modules (TEMs), which are in turn mounted on heat sinks. However, the power consumption of such TEMs is high. Indeed, it may exceed that of the component. This problem is exacerbated when the ambient temperature and/or component heat load vary as is normally the case. In the usual packaging configuration, a TEM is mounted on an air-cooled heat sink of specified thermal resistance. However, heat sinks of negligible thermal resistance minimize TEM power for sufficiently high ambient temperatures and/or heat loads. Conversely, a relatively high thermal resistance heat sink minimizes TEM power for sufficiently low ambient temperatures and heat loads. In the problem considered, total footprint of thermoelectric material in a TEM, thermoelectric material properties, component operating temperature, relevant component-side thermal resistances, and ambient temperature range are prescribed. Moreover, the minimum and maximum rates of heat dissipation by the component are zero and a prescribed value, respectively. Provided is an algorithm to compute the combination of the height of the pellets in a TEM and the thermal resistance of the heat sink attached to it, which minimizes the maximum sum of the component and TEM powers for permissible operating conditions. It is further shown that the maximum value of this sum asymptotically decreases as the total footprint of thermoelectric material in a TEM increases. Implementation of the algorithm maximizes the fraction of the power budget in an optoelectronics circuit pack available for other uses. Use of the algorithm is demonstrated through an example for a typical set of conditions.

Effects of Interfacial Position on Drag Reduction in a Superhydrophobic Microchannel

The use of superhydrophobic surfaces in confined flows is of particular interest as these surfaces have been shown to exhibit a drag reduction effect that is orders of magnitude larger than those due to molecular slip. In this paper we present experimental results of the pressure-driven flow of water in a parallel-plate microchannel having a no-slip upper wall and a superhydrophobic lower wall. Pressure-drop versus flow-rate measurements characterize the apparent slip behavior of the superhydrophobic surfaces with varying pillar-to-pillar pitch spacing and pillar diameter. The superhydrophobic surface consists of a square array of cylindrical pillars that are fabricated by deep reactive ion etching on silicon and coated with a hydrophobic fluoropolymer. A major challenge, in correlating our experimental results with existing theoretical predictions, is uncertainty in the location of the gas/liquid interface and the associated gas/liquid/solid contact line within the pillar features comprising the superhydrophobic surface. We present experimental results, from laser-scanning confocal microscopy, that measure the location of the gas-liquid interface and associated contact line for fluid flowing through a parallel-plate microchannel. Knowledge of the contact line location is then used to correlate experimental pressure-drop versus flow-rate data with a theoretical model based on porous-flow theory that takes into account partial penetration of liquid into a superhydrophobic surface.Copyright

Mechanical Gap Fillers: Concepts and Thermal Resistance Measurements

This paper discusses spring-loaded mechanical structures that can be used to make thermal connections between an object to be cooled, such as an integrated circuit, and a dissipative structure, like a cooling plate or heat sink. These metal structures are flexible and resilient, adapting to variations in position and orientation of the two objects to be coupled. Precision experiments demonstrate that they have much lower thermal resistance than elastomeric ldquogap-fillerrdquo pads that are usually used to perform this function.

Thermal-Resistance Measurements on Mechanical Gap Fillers

This paper discusses spring-loaded mechanical structures that can be used to make thermal connections between an object to be cooled, such as an integrated circuit, and a dissipative structure, like a cooling plate or heat sink. These metal structures are flexible and resilient, adapting to variations in orientation of the two objects to be coupled. Precision experiments and computations demonstrate that they have much lower thermal resistance than elastomeric “gap-filler” pads that are usually used to perform this function.Copyright

Sizing of Pellets in Thermoelectric Modules (TEMs)

Sizing the height and cross sectional area of the pellets within thermoelectric modules (TEMs) used to cool, heat and generate power is necessary to optimize their efficiency and/or performance. Here the heat flux that a TEM can accommodate, its coefficient of performance, and its operating current and voltage in refrigeration mode are provided as a function of pellet geometry. This enables designers to, for example, size pellets to refrigerate a load such that the total power consumption of a TEM and a power supply (that converts available voltage to that required by the TEM) is minimized. In generation mode, power output, conversion efficiency and operating voltage and current are provided as a function of pellet geometry and the electrical resistance of a load connected to a TEM. Finally, the effects of electrical contact resistance at the pellet interconnects on the aforementioned parameters are addressed.© 2005 ASME

Electrically Tunable Superhydrophobic Nanostructured Surfaces

In this work, we discuss dynamic electrical control of the wetting behavior of liquids on nanostructured surfaces spanning the entire possible range from superhydrophobic behavior to nearly complete wetting. It is demonstrated that a droplet of liquid can be reversibly switched between the superhydrophobic “rolling ball” state and the hydrophilic immobile droplet state by the application of electrical voltage and current. The nature of the transition mechanism is studied both experimentally and theoretically. The reported results provide novel methods of manipulating liquids at microscale.Copyright