Definitions of Words and Phrases Used in Separation Equipment

Coalescing: The process or mechanism of merging small droplets or aerosols and creating larger droplets that can easily be removed by gravity. It also refers to the joining of liquid droplets dispersed in another immiscible liquid e.g. water drops in oil.

Gas coalescing filter: A separator containing changeable elements that is capable of the removal of sub-micron aerosols and solids. This coalescing and filtering occurs as the gas flows from the inside of the filter/coalescing element to the outside of this element in the vertical filter-coalescer. Properly designed, this coalescing stage will remove solids and fine aerosols down to 0.3 micron and larger. 

Electrostatic coalescer: A device used to remove dispersed water from oil by using a high voltage field to polarize and/or charge dispersed water droplets.

Emulsion: A stable dispersion of one immiscible liquid in another liquid. 

Entrainment: Fluid in the form of a mist, fog, droplets or bubbles carried along with the continuous phase.

Filter: A device used to separate solids from liquid or gas flow. Most filters utilize removable elements. 

Filter separators: A device to remove solids and entrained liquids from a gas stream. It usually has two compartments. The first compartment contains filter coalescing elements. As the gas flows through the elements, the liquid particles coalesce into larger droplets and when the droplets reach sufficient size, the gas flow causes them to flow out of the filter elements into the center core. The particles are then carried into the second compartment of the vessel where the larger droplets are removed. A lower barrel or boot may be used for surge or storage of the removed liquid.

Flash drum: A vessel used to separate liquids, generated due to pressure reduction and/or increase in temperature of a liquid stream, from the gas phase or two phase fluid. 

Gas-oil ratio (GOR): The ratio of gas to hydrocarbon at a defined condition, typically expressed as Sm³/m³. 

Heater-treater: A device used to process hydrocarbon, by warming and coalescence, in order to remove small quantities of residual water so as to meet transportation or product specifications. 

Line drop: A boot or underground vessel, used on a pipeline, to provide a place for free liquids to separate and accumulate. It is used in pipelines with very high gas-to-liquid ratios to remove only free liquid from a gas stream. It will remove bulk liquid, but not necessarily all the liquid. 

Knock out drum: Generic term used to describe vessels for gas-liquid separation. Separation can be either for high, or low, gas-to-liquid ratio streams.

Liquid coalescer: A vessel internal used for increasing the droplet size of immiscible liquids, so that they can be removed by gravity separation. Typical coalescing elements are stacked plates, vanes, wire or plastic mesh, or cartridge type elements. 

Liquid-liquid separators: A vessel where two liquid phases are separated. 

Mist eliminator: A fixed device used to enhance removal of smaller liquid droplets from a gas above which is not normally possible by gravity separation. Typical mist eliminator designs include knitted wire mesh, vane type, and cyclonic.

Production separator: A vessel typically used as the first separation device that the fluid encounters in the wellhead to processing plant production network (sometimes is called Wellhead Separator, when physically located at the well site). 

Retention time: For gas-liquid separation, the average time a flowing fluid remains within the liquid section of a separator at the design feed rate. For three phase separation, the retention time can be the time the total fluid remains in the separation section at the design feed rate, or if defined as phase retention time, the time the phase remains in the separation section. 

Scrubber: A category of separator used for high gas-to-liquid ratios. Scrubbers are used as the primary separator in systems where small amounts of liquid are produced, to ‘polish’ an already-separated gas stream by removing residual contaminants more completely, or as a backup in case of an operational upset upstream.

Separator: A generic term for a device which separates gas-liquid, gas-liquid-liquid, gas–solids, liquid-solids or gas-liquid- solids. 

Slug catcher: A particular separator design which is able to absorb sustained in-flow of large liquid volumes at irregular intervals. Usually found on gas gathering systems or other two-phase pipeline systems at the terminus of the pipeline. A slug catcher may be a single large vessel or a manifolded system of pipes. 

Surge drum: A vessel used to provide appropriate time for flow control and dampening during process variations and upsets. The capacity of the surge drum provides the ability to accept liquids from the upstream process, or provide liquids to down stream equipment without upsets. 

Surge time: The time it takes to fill a specified fraction of a vessel, defined as the volume between a specified level range in a vessel divided by the design feed flow rate. 

Test separator: A separator vessel used near the wellhead, which separates the phases for well test metering. 

Three phase separator: A vessel used to separate gas and two liquids of different densities (e.g. gas, water, and oil) into three distinct streams.

Distillation column: How to fix temperature and pressure

A distillation column can be illustrated as follows:

Pressure Profile

In a distillation column there exists a pressure gradient. The pressure at the bottom is higher and lowers towards the top of the column. This pressure gradient occurs due to the trickling liquid that restricts the upward flow of vapour thus creating a pressure loss on the flow. 

In steady-state distillation processes, the column pressure is kept constant and the temperature is varied to control the composition of the product streams.
In normal situations, the vapor pressure of the liquid on the top tray fixes the pressure at that location before the vapor enters the overhead condenser. This parameter fixes the column pressure. The pressure in the other sites in the column depends on the ability of the vapors and liquids to distribute themselves up and down the column with minimum pressure drops. 

It is the liquid composition on the top tray that defines the expected column operating pressure. 
The external reflux ratio (L/D) has a bearing on fixing that composition – as the various L/V’s (internal reflux ratios) that are generated down the column have on the various trays’ compositions. 

The bottom pressure will be determined by the pressure drop along the column. This depends on the relevant selected technology and the load of vapor and liquid inside the column. The corresponding bubble point of bottom pressure will specify the bottom temperature. 

The purpose of the reflux is to provide down-flowing liquid throughout the rectification section to contact with the up-flowing vapor in order to achieve stage-by-stage equilibrium heat and mass transfer thus purifying the top product. When sub-cooled reflux is introduced to the top tray, it must be heated up to its bubble point before the lighter components will vaporize.

Temperature Profile

The temperature distribution in a distillation column is warmer at the bottom and cooler at the top. For a binary feed mixture, the temperature at the bottom is just lower than the boiling point of the heavier component while the temperature at the top is just above the boiling point of the lighter component. 

At the bottom of the column, it is desired that the heavy component remains as a liquid and the lighter component remains as a gas, thus the temperature at the bottom should match this requirement. The temperature of the bottom is controlled by a reboiler. The heat added at the bottom is easy to control through steam or hot oil flow rates. 

It is the opposite at the top of the column, i.e., the light component is required to remain a gas while the heavier component is condensed and trickles down the column. The temperature at the top is set above the boiling point of the lighter component. The top product is usually needed in liquid form for easy storage hence the gaseous products need to be condensed. This liquid stream is then split into two where one stream is returned to the column and the other is sent to storage. 

The temperature at the top is controlled by adjusting the reflux rate. The Reflux Rate is the flow rate of the liquid sent back to the top of the column. A higher reflux rate results in more cooler liquid falling down the column against the rising warmer gas, and the top temperature is lower.

Overall heat is added at the bottom of the column awhile heat is extracted at the top of the column. Inside the column the temperature balance is created between the hot gas rising up the column and the cooler liquid falling down the column.

Inside the column, the temperature is set by the relative volatility or the partial pressure of the feed according to Raoult’s law. The volatility of components in the column is different. If P is the total pressure in the column, this is equal to the sum of P1 + P2 + P3 + ---- partial pressures at a different height. 

While total pressure is constant in the column, the partial pressures of the feed components are different along with the height. 

Partial pressure = Mole fraction x Vapor pressure of pure component at a height 

Since volatility of hydrocarbon increases as you go up, the vapor pressure increases, and consequently, saturation temperature gets lower. Therefore, the column gets cooler as we go up. In a steady-state, the partial pressures do not change much.

Remember: 

Raoult's law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present

What is L/D?

It is the external reflux ratio: It is the ratio of the liquid returned to the column divided by the liquid removed as product, i.e., R = L/D.  

As the external reflux cools the top of the tower, vapors consisting of heavier fraction condense and flow down the tower and it's referred to as internal reflux. The liquid/vapor flow ratio inside the upper section of the column.is referred to as the internal reflux ratio i.e., L/V.

SI and CGS Units summary table

Introduction to Fluid Mechanics

Mechanics studies the motion and deformation of material bodies under applied loads like forces and moments.

It involves loads, energy, motion, deformation and material properties. 

When the material is either in the liquid or gas phase it is called fluid mechanics. With fluid referring to both liquids and gases. 

The most basic differences between solids, liquids and gases are presented below:

Mechanics deals with both stationary and moving bodies under the influence of loads. 

• Statics  deals with bodies at rest. 

• Dynamics deals with bodies in motion.

Fluid mechanics deals with how fluids at rest (fluid statics) and in motion (fluid dynamics) behave.

• Gas dynamics studies compressible flow of gases with high density changes. 

• Aerodynamics is similar to gas dynamics, but also covers low speed flows. It focuses on air flow.

• Hydrodynamics studies liquids (incompressible flow) in motion. 

The Concept of Continuum

At the microscopic level fluids consist of molecules. However, in engineering we deal with the fluid on a macroscopic level and ignore the behavior of each individual molecule i.e. we treat the fluid as a whole, a continuum.

In Continuum it is assumed that the fluid and flow properties like density, velocity, pressure, temperature, etc. vary continuously throughout the fluid. 
In continuum, the smallest element of a fluid is NOT a fluid molecule, but rather a fluid particle, which contains enough number of molecules to make meaningful statistical averages.
The Knudsen Number can determine whether the Continuum Assumption is valid for any case.

Continuum is valid when Kn < 0.01

Dimensions and Units

In the MLT system, the basic dimensions are: Mass [M], Length [L], Time [T]. All other quantities are expressed in terms of these three.
For some problems, Temperature [𝜃] also serves as a basic dimension.
Equations should be dimensionally homogeneous, i.e. dimensions of the left and right sides should be the same.

Fundamental Flow and Fluid Properties

It is common to use pressure (p) and temperature (T) to fix the thermodynamic state. Then other properties can be expressed as a function of these two:

                                       𝜌=𝜌(𝑝,𝑇),       ℎ=ℎ(𝑝,𝑇),       𝜇=𝜇(𝑝,𝑇)