Liquid settling rates

Basic design aspects of three-phase separation are identical to those discussed earlier for two-phase separation. The only additions are that more concern is placed on liquid-liquid settling rates and that some means of removing free water must be added. Liquid-liquid setding rates will be discussed later. Water removal is a function of control methods used to maintain separation and removal from the oil. Several control methods are applicable to three-phase separators and shape and diameter of the vessel will, to a degree, determine types of control used. 

Dissolved Solids Boiling Pt/Freezing Point For Liquids/Solid Mixtures Bulk Density Total Solids Content Solids Size Distribution Suspended Solids Content Suspended Solids Settling Rate Dissolved Solids Content Free Water Content Oil and Grease Content Viscosity For Gases Density... 

ASpGr = Difference in specific gravity of the particle and the surrounding fluid tyvg = Average residence time based on liquid flow rate and vessel volume, min tmjp = Minimum residence time to allow particles to settle based on Stokes Law, min u = Relative velocity between particle and main body of fluid, ft/sec... 

Thickeners, thickeners are primarily used for liquid-solid separation (see Section 10.4). When used for classification, the feed rate is such that the overflow rate is greater than the settling rate of the slurry, and the finer particles remain in the overflow stream. 

Vapor-Liquid Gravity Separator Design Fundamentals The critical factors in the performance of a horizontal separator are the vapor residence time and the settling rate of the liquid droplets. However, two other factors enter into the design—the vapor velocity must be limited to avoid liquid entrainment, and there must be sufficient freeboard within the vessel to allow for a feed distributor. For vertical separators, the design is based on a vapor velocity that must be less than the settling velocity of the smallest droplet that is to be collected, with due allowance for turbulence and maldistribution of the feed. The vapor residence time is a function of the vapor flow rate (mass), vapor density, and volume of vapor space in the separator, based on the following ... 

The majority of the process vessels you see in your plant are gravity, vapor-liquid separators. Their main purpose is to settle out droplets of entrained liquid from the upflowing gas. Factors that affect the settling rate of these droplets are... 

Density of liquid. The more dense the liquid droplets, the faster the droplet settling rate. 

All these ideas have been put into one equation, called Stokes law. Nothing against Sir Frederick Stokes, but vapor viscosities are almost always so small that they do not affect settling rates. Also, we never know the particle size distribution of the droplets. There is a more fruitful way to look at the settling tendency of droplets of liquid in an upflowing vapor stream, as shown in Fig. 26.1. The method states... 

But liquid viscosities are extremely variable. At a constant viscosity, the settling rate of water in another liquid, due to gravity is proportional to... 

The transport phenomenon for any spray material released In the air Is foremost a function of the particle size and size distribution of the released spray. The particle density plays a minor role, the settling rate from Stokes law for example varies as the square root of the density. Further, the density differences between liquids commonly used for pesticides Is very little, varying only slightly from water at density of 1 gm/ml. Other formulation physical factors of surface tension, viscosity and viscoelasticity play significant roles In the atomization process. These are altered by the addition of petroleum and vegetable oil as solvents and carriers as well as a host of adjuvants In varying... 

Rate of Settling—At the present time two methods are widely used for computing settling rates of liquid suspensions. Rollason, cited by Egolf and McCabe (1937), proposed the use of Stokes equation in the form... 

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