MECHANISM OF SMALL PARTICLE SEPARATION
Centrifugal Force Separation
5.1 GRAVITY SEPARATION
Gravity settling is by far the most widely used mechanism of separation. This results primarily from the simplicity of the equipment necessary and the readily available force of gravity.
Gravity separation depends upon the following conditions.
1. The relative density of the liquid particle compared to that of the gas under vessel operating conditions. If a mixture of oil and water is allowed to settle then the water particles which are heavier will settle down and oil particles will move up and we get these two components separated. If the differnce in densities between oil and water is more then the rate of settling will be faster.
2. The relative size of the particle.
3. The vertical velocity of the gaseous stream, in spite of which the particle tends to separate downward. If the velocity of the gas moving up is high then it will reduce the settling rate of liquid particles from the gas.
4. The turbulence which exists in the vapour space of the separation vessel.
The function of a gravity settler is basically to reduce the velocity of the incoming well stream from one which is turbulent and permits entrainment of particles to a velocity which is less turbulent, allowing the suspended particles to collect. Any knock-out pot, accumulator, or expanded section in a flow line will act as a gravity settler. The flow line itself will allow gravity settling if the flow rate is sufficiently low.
Since the basic requirement for gravity settling is to minimize turbulence, it is important to decrease the gas velocity immediately upon entering the separator and in some manner cause the flow of gas to be uniform throughout the separation section. When this is accomplished the laws of settling uses to determine a maximum gas velocity for the separation of liquid particles of certain diameters.
If the gas flow in the separation section is vertical and upward the particles to be separated must settle against the flow of gas. The calculated settling velocity for a certain particle diameter indicates a maximum gas velocity for the separation of that diameter particle.
5.2 CENTRIFUGAL FORCE SEPARATION SECTION
In centrifugal force separation the direction of the inlet stream is changed many times. When a gas stream changes direction, the liquid drops, having a density, offer more resistance to the change in direction and tend to continue in a straight line.
This results in a collision of the larger liquid particles with the confining wall and a separation from the less dense gas. Since most oilfield separation problems involve the suspension of relatively heavy liquid particles in a less dense gas, the particles can be separated by centrifugal force.
The use of centrifugal force by itself for particle removal is usually limited to the initial separation of large volumes and large drops of liquid. This principle, however, may also be used for mist extraction. The lower practical limit of particle size which can be removed in centrifugal force (cyclone type) mist extractors is thought to be about 5 to 10 microns in diameter. Separation of very smaller particles by this methods results in pressure reduction.
5.3 IMPINGEMENT SEPARATION
Impingement is probably the most widely used principle for tiny particles collection in liquid and gas separation. This type separation depends upon entrained particles striking an obstruction rather than the containing walls. The obstructions act as collecting surfaces.
There are two general types of impingement mist extractors which are presently in wide usage, the difference in the two types being the intensity of the centrifugal force utilized.
1. Vane type
2. Knitted wire mesh type
As gas approaches an obstruction it tends to flow around it. As in the case of centrifugal force separations, the heavier drops tend to continue in a straight line and strike the collecting surface.
When the diameter of the obstructions are large compared to the size of the liquid particles to be removed and the change in directions of the gas in drastic, a large amount of centrifugal force is generated. This type impingement separator, usually referred to as a vane type.
Refering to the figure, its shows the approaches the operation of centrifugal force separators and are characterized by moderately high pressure drops. Liquid that is collected within the vanes must be returned to the liquid accumulating section. Most variations in vane type mist extractors result largely from different methods used to protect and drain the collected liquid.
KNITTED WIRE MESH EXTRACTORS
A second type of impingement separation device is a knitted wire mesh pad. The primary mechanism of separation in the knitted wire mesh is impingement. It also utilizes centrifugal and gravitational force in the collection of small liquid Particles.
As the gas and entrained particles approach the wire obstructions as indicated in figure the gas turns to miss the wire while the particle tends to continue in a straight line. A portion of the particles in line with the obstructions impinge upon them and are collected.
After particles strike the collecting surface in a knitted wire mesh, they form into a thin liquid film, wetting the wire surface. If the gas flow is upward, the liquid film slowly moves to the lowest point on the wire. Here the film grows until a droplet forms large enough to break away from the collecting surface and drop back against the flow of gas.
This droplet size is many times larger in diameter than the average particle collected. For instance, a mist extractor may be removing many 10 micron diameter particles but the diameter of the drops draining from the mist extractor will be in the order of 1000 to 5000 microns in diameter.
If the gas passes horizontally through a knitted wire mesh pad the large droplets formed will flow through the pad in the direction of the gas velocity, but they will quickly fall by gravity from the flowing stream because of their greatly increased size.
The presence of a liquid film on the wire and the large droplets formed cause some scrubbing action which further aids the removal of tiny particles.
Knitted wire mesh mist extractors usually used in oil and gas separators are designed to produce high separation efficiencies for particles 10 microns and larger in size.
Coalescence of small particles into those large enough to settle by gravity is provided by two mechanism
The surface of the element is usually wet and small particles striking same are absorbed.
A common example of coalescing occurs when water drops form on the wind-shield of a car as it is driven in fog. As the tiny water drops which make up the fog strike the windshield, they combine with other drops and eventually form a drop large enough to run down the glass.
Several of the internal devices of a separator such as deflector plates, straightening vanes, baffles, etc. and even the vessel walls are forms of coalescers. In each device, liquid drops adhere to the surface of the device and combine with other drops until a large drop forms which will fall out. The effectiveness of separation will also depend upon the total amount of coalescing surface area that is present.
In the mist extraction section specially designed coalescing devices may be installed.