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Coalescence of electrolytically-generated, 50-to 600-pm-diameter gas bubbles was observed using an optical technique that employs a linear photodiode array to detect inteface movement with a resolution of 10-6s. when two bubbles coalesce, the suface energy that is released causes interface velocities of 2 to 4 m/s; these are followed by large-amplitude damped oblateprolate oscillations. Within the viscosity range studied, the oscillation period is insensitive to the viscosity and damping is insensitive to surface tension, in agreement with a scaling analysis based on a damped harmonic oscillator. Surface waves are superimposed on the motion. Finite-element solutions of the freesugace, nonlinear Navier-Stokes equations are consistent with the experiments.

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Batch operation of reverse osmosis (RO) has been proposed as a method to reduce seawater RO (SWRO) energy consumption and fouling propensity. In this paper, we use a transient numerical model of the RO process to investigate the impact of several practical loss mechanisms on the overall energetic performance of batch SWRO compared to a conventional continuous system. A critical variable that controls the energetic advantage of batch RO is the reset time between cycles. A large reset time necessitates higher operating flux and therefore results in increased energy consumption. On the other hand, ensuring a low cycle reset time requires higher energy for the refilling process. A batch SWRO design with an atmospheric pressure feed tank and pressure exchangers for energy recovery does not show promise for energy savings. Batch SWRO must be designed with a large number of short pressure vessels (with fewer membranes each) and lower energy recovery losses (e.g., by using pressurized feed storage) in order to reduce energy consumption by up to 8%. These modifications are more complex and hence capital expenses would determine overall feasibility of such designs to improve seawater desalination.

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A B S T R A C TLegacy seawater reverse osmosis (SWRO) desalination plants used turbine-type energy recovery devices (ERDs) connected with a shaft to the high-pressure pump. These ERDs, commonly known as Pelton wheels or energy recovery turbines, were default equipment in SWRO plants until quite recently.Today, however, over 80% of new SWRO plants are being designed and built to utilize isobaric-chamber ERDs. Isobaric ERDs such as Energy Recovery, Inc. (ERI's) PX Pressure Exchanger (PX ) device are positive displacement devices that operate with energy transfer efficiencies as high as 98%. High SWRO plant operating efficiency can be obtained over a wide range of membrane water recovery rates, typically between 35% and 50%. Recovery rates can be adjusted in response to changes in seawater temperature or salinity or as the membrane elements age. Flexible recovery and low-recovery operation are tremendous advantages for lowcost SWRO operation provided by isobaric ERD technology.Removing legacy ERDs and installing modern isobaric ERDs makes it possible to reduce the power consumption of existing systems by as much as 60%. Such retrofits can also significantly increase the capacity of existing systems while adding little or no additional power requirements. These benefits can be realized at a fraction of the cost of constructing new plants. For these reasons, many owners of legacy desalination plants worldwide are upgrading their processes by incorporating isobaric ERD technology.The authors recognize that each energy recovery technology comes with its own unique advantages and disadvantages, which should be compared and studied for each individual system. This paper, therefore, provides detailed analyses comparing SWRO energy consumption with various ERDs. It presents many examples of retrofits, including replacements of Pelton turbines and turbocharger devices. It also estimates the potential energy savings and capacity increase benefits for retrofitting some of the largest SWRO projects in the world including facilities in Fujairah, UAE and Trinidad.