Natural gas separation and design

Mr. K. Sarkodie

outline

Receiving Gas at a Plant

• Liquid Separations.

• Design of equipment.

• Operating problems.

Introduction

• When a gas enters a plant, nearly always there is a large vessel through which the gas passes.

• The function of these vessels is to remove any free liquid from the gas stream before it is further processed. Since most gas enters the plant as a two phase fluid, these vessels are of prime importance.

Function of Inlet Separators

These vessels have two functions.

• These are:

1. Separate the fluid received into gas and liquid.

2. To handle liquid slugs and prevent the receiving of them at a plant from upsetting the rest of the plant.

Particle Size

• When separating liquid and solids from a gas, the size of particles is of considerable importance.

• Generally, we are concerned in the gas processing industry with the separation of liquid droplets - generally 10 microns in size (i.e, 1/100 of a millimeter or larger).

Description of Common Separators

• In the gas processing industry generally some form of an impingement separator is used. Some of the more common ones are:

• Vertical Separator

• Horizontal Separator

• Spherical Separator

• Cyclone Separator

Vertical separator • Most common type of separator • Used for separation of gas from relatively large

volumes of liquids. Advantages • Liquid level control is not critical since large

quantities of foreign materials such as sand without plugging up or otherwise affecting the operation of the vessel; and it is easy to clean

Disadvantages • expensive of the three types of separators; • it does not adapt readily to a skid-mounted unit; • has a lower capacity than any of the other kinds

of separators when compared on the basis of effective diameter

Mode of operation

• The well effluent enters the vertical separator at approximately the midpoint of the vessel.

• Separation of the gas from the liquid commences at this point. The gas travels upwards through the vessel, dropping out the larger diameter liquid particles in its journey towards the top of the vessel.

• Most separators are designed on a basis of a ten micron diameter particle being excluded from the gas at exit point of the separator.

• All liquid droplets which are larger than ten microns will be either dropped out in the main body of the separator or will impinge upon the mist extractor and be removed at that point.

Horizontal separator • The Horizontal separator is most commonly used for the

separation of large volumes of gas from small volumes of liquid.

• It is also used extensively for handling liquid slugs from gathering systems.

• This type of unit is used on gas wells, gas condensate wells, and, generally, as inlet separating devices at gas processing plants.

Advantages

• Cheaper then the vertical unit (1.5 times the gas capacity of a vertical unit of the same diameter).

Mode of operation

• The well effluent enters one end of the horizontal separator, hits a deflector plate to drop out liquid drops,

• then the gas continues to the gas exit of the separator, where it passes through a mist extractor and then out the gas outlet.

• The liquid is collected in the bottom half and if necessary, a • boot is put on at the outlet end to separate water from oil. • In the horizontal separator, the force of gravity acts on the

liquid droplets throughout the length of the vessel causing the particles to "arc" to the bottom of the unit and thus separate more easily than in the vertical separator.

Spherical separator

• The spherical separator is most commonly used for the separation of large volumes of gas from extremely small volumes of liquid.

• This type of unit is used mainly as a scrubber, i.e. removing small amounts of liquid.

Advantage

• More compact than either of the other units.

Disadvantage

• limited separation space and liquid surge capacity.

• the liquid level control is extremely critical.

Assignment

• Submit a page length on the mode of operation of cyclone separators and filter separators NEXT WEEK

• (HAND WRITTEN)

Separation Theory Momentum

• Fluid phases with different densities will have a different momentum.

• If a two phase stream changes direction sharply greater momentum will not allow the particles of the heavier phase to turn as rapidly as the lighter fluid, so separation occurs.

• Momentum is usually employed for bulk separation of the two phases in a stream.

Gravity settling

• Liquid droplets will settle out of a gas phase if the gravitational force acting on the droplet is greater than the drag force of the gas flowing around the droplet.

• These forces can be described mathematically using the terminal or free settling velocity.

• The drag coefficient has been found to be a function of the shape of the particle and the Reynolds number of the flowing gas. For the purpose of this equation particle shape is considered to be a solid, rigid sphere.

• Reynolds number is defined as:

• In this form, a trial and error solution is required since both particle size, Dp, and terminal velocity, Vt, are involved.

• To avoid trial and error, values of the drag coefficient are presented in Figure 15.7

• as a function of the product of drag coefficient, C', times the Reynolds number squared; this eliminates velocity from the expression:

• The abscissa of Figure 15.7 is given by:

Gravity settling

Gravity Settling - Limiting Conditions

• As with other fluid flow phenomena, the drag coefficient reaches a limiting value at high Reynolds numbers.

Separator Design and Construction

• Separators are usually characterized as vertical, horizontal, or spherical. Horizontal separators can be single or double barrel and can be equipped with sumps or boots.

• Parts of a separator

• Consists majorly of four sections

Parts of separator-- for a gas – liquid

separator

Separation section, A, is used to separate the main portion of free liquid in the inlet stream.

• It contains the inlet nozzle which may be tangential, or a diverter baffle to take advantage of the inertial effects of centrifugal force or an abrupt change of direction to separate the major portion of the liquid from the gas stream.

• The secondary or gravity section, B, is designed to utilize the force of gravity to enhance separation

• of entrained droplets. In some designs, straightening vanes are used to reduce turbulence.

• The vanes also act as droplet collectors, and reduce the distance a droplet must fall to be removed from the gas stream.

• The coalescing section, C, utilizes a coalescer or mist extractor which can consist of a series of vanes, a knitted wire mesh pad, or cyclonic passages.

• This section removes the very small droplets of liquid from the gas by impingement on a surface where they coalesce.

• A typical liquid carryover from the mist extractor is less than 0.1 gallon per MMSCF.

• The sump or liquid collection section, D, acts as receiver for all liquid removed from the gas in the primary, secondary, and coalescing section.

• Depending on requirements, the liquid section should have a certain amount of surge volume, for degassing or slug catching, over a minimum liquid level necessary for controls to function properly.

• Degassing may require a horizontal separator with a shallow liquid while emulsion separation may also require higher temperature, higher liquid level, and/or the addition of a surfactant

SEPARATOR DESIGN

• The following discussion on oil –gas separation has been adapted from Sivall’s excellent treatment of the subject.

• Sivall’s tables, graphs and procedures are accepted as the standard of the industry

Design of a separator

• There are two design criteria.

• The allowable gas and liquid capacities

• Separator sizing (internal diameter and height)

Gas capacity

• This is done by means of a form of the Stokes’ Law which can be expressed by the following equation:

where: u = allowable velocity, feet per second or (m/s). ρg = density of the gas, lb/cu ft. (kg/m3). ρL = density of the liquid, lb/cu ft. (kg/m3).

K value

• By use of the previous equation and the actual flow, the area of the vessel can be determined from:

• Empirically,

LIQUID CAPACITY • The liquid capacity is of a separator is

dependent on the retention time of the liquid in the vessel.

• Good separation requires sufficient time to obtain an equilibrium condition between the liquid and gas phase at the temperature and pressure of separation.

• The liquid capacity of a separator is based can be obtained from the following equation;

Stage separation

• Stage separation is a process in which gaseous and liquid hydrocarbons are separated into vapour and liquid phases by two or more equilibrium flashes at consecutively lower pressures.

• As illustrated in Figure below, two-stage separation requires two separators and a storage tank; and so on.

• The tank is always counted as the final stage of vapour-liquid separation because the final equilibrium flash occurs in the tank.

• The purpose of stage separation is to reduce the pressure on the reservoir liquids a little at a time, in steps or stages, so that a more stable stock-tank liquid will result.

• Differential liberation can be closely approached by using three or more series-connected stages of separation, in each of which flash vaporization takes place.