Fluidos

FUNDAMENTALS OF GAS SOLIDS/LIQUIDS SEPARATION

Fundamentals of Gas Solids/Liquids Separation

Many process operations require the removal of entrained non-gas particles from multi-phase gas streams. The removal of these non-gas particles is the process in itself (capture of a valuable product) or the process of cleaning a gas stream in order to protect either stationary or rotating equipmentfrom the harmful effects due to non-gas particles entering those devices. The removal of entrained non-gas particles from a multi-phase gas stream is a separation process involving the removal of: Liquid particles from a gas or vapor stream; Solid particles from a gas or vapor stream; Combination of both liquid and solid particles from a gas or vapor stream. In order to understand mechanicalseparation concepts, a basic understanding of particle formation, physics and motion is required.

Particle Formation
Non gas particles are generated and entrained in the gas stream (spray regime) or are converted to a sheet flow (annular regime) and carried on the fluid stream boundary. These liquid non-gas particles can be generated from a pure gas due to a gas-liquid phase change occurringwithin a state change of the gas. This change of state is commonly known as condensation. The following graphic is the pressure-volume chart for a typical vapor.
130

120 110 100 90 Critical Isotherm 80 TEMP. INCREASE

It is only in the gas liquid multi-phase zone that liquid non gas particles can exist.
GAS REGION

(Also called Superheat Region)

(Constant Temperature)

ISOTHERMSPRESSURE

LIQUID

C

70

X

CRITICAL POINT

The critical point "C" (See PressureVolume Chart Graphic) is the state point at which the liquid and gas phases are identical. At higher pressures along the critical isotherm, a slight increase or decrease in temperature will instantaneously produce a complete phase change. At a temperature higher than the critical isotherm, no liquid phase will beencountered. At temperatures below the critical isotherm gas can exist as both a gas or liquid. The amount of liquid per unit volume of mixture can be estimated using the known physical properties of the mixture. Liquid particles can be formed in mid stream by condensing initially at the molecular size level. These particles then can be carried by the fluid stream. Condensation on cool surfaceswill produce a liquid film which is swept off the surface by drag force due to the gas stream velocity being greater than the surface tension on the liquid. Liquids injected into the stream may be borne along in the stream if in small

60 50 BUBBLE POINT

DEW POINT GAS-LIQUID MULTI-PHASE ZONE

40 30

B

0

A

100

200

300

SPECIFIC VOLUME

A to C Liquid Line B to C SaturatedGas Line Figure 1 Pressure - Volume Relation For Typical Real Gas

particle form. The liquid stream injected will be disbursed initially due to that force overcoming the surface tension forces. The liquid will become part of the mixture and has the potential to also change state. Small liquid particles of like substances will join with one another upon collision in what is called coalescence.The rate at which the particles with no net movement will join together is dependent on gas viscosity and temperature and the number of particles present. Higher temperatures, lower viscosities, and flow stream agitation will provide higher coalescence rate. Liquid present within the gas stream will flow in one of several flow regimes.

Flow Regimes
GAS (a) Bubble
GAS GAS

(e) Slug GAS (f)Semi-annular GAS (g) Annular

(b) Plug GAS (c) Stratified GAS

Figure 2 depicts the various flow regimes that (h) Spray (d) Wavy occur within multi-phase gas streams. The two flow regimes that provide the lowest liquid to gas weight ratio are annular and spray flow. Annular Figure 2 flow is characterized by finite liquid amounts carried in suspension with significant amounts along the pipe...