Liquid / Gas Separation Technology

Introduction

Removing liquids and solids from a gas stream is very important in refining and gas processing applications. Effective removal of these contaminants can prevent costly problems and downtime with downstream equipment like compressors, turbines, and burners. In addition, hydrocarbons and solid contaminants can induce foaming in an amine contactor tower and can contribute to premature catalyst changeouts in catalytic processes. In compressors that use oil to lubricate cylinders, the lube oil often gets into the discharge gas causing contamination downstream. A thin film of hydrocarbon deposited on heat exchangers will thicken and coke, decreasing heat transfer efficiency, increasing energy consumption and creating a risk of hot spots and leaks.

Types Of Removal Mechanisms To Remove Liquids And Solids From Gases

Before evaluating specific gas liquid separation techniques, it is important to understand the mechanisms used to remove liquids and solids from gases. These can be divided into four different categories.2

Gravity Settling

The first and easiest to understand is gravity settling, which occurs when the weight of the droplets or particles (ie. The gravitation force) exceeds drag created by the flowing gas.

Centrifugal Separation

A related and more efficient mechanism is centrifugal separation which occurs when the centrifugal force exceeds the drag created by the flowing gas. The centrifugal force can be several times greater than gravitational force.

Inertial Impaction

The third solid/ liquid gas separation mechanism is called inertial impaction which occurs when a gas passes through a network, such as fibers and impingement barriers. In this case, the gas stream follows a tortuous path around these obstacles while the solid or liquid droplets tend to go in straighter paths, impacting these obstacles. Once this occurs, the droplet or particle loses velocity and/or coalesces, and eventually falls to the bottom of the vessel or remains trapped in the fiber medium.

Liquid/Solid Gas Separation

And finally, a fourth mechanism of liquid/ solid gas separation occurs with very small aerosols (less than 0.1 µm). Called diffusional interception or Brownian Motion, this mechanism occurs when small aerosols collide with gas molecules. These collisions cause the aerosols to deviate from the fluid flow path around barriers increasing the likelihood of the aerosols striking a fiber surface and being removed.3

Throughout this paper, reference to droplet and particle sizes will be in the unit micron. One micron is 1/1000 of a millimeter or 39/1,000,000 of an inch.

Figure 1 shows the size of various material in microns.

Types of Liquid/Gas Separators And Technologies Used In Them

Several technologies are available for liquid gas and solid gas separation. This article will first provide selection criteria for the following gas/liquid separation technologies:

• gravity separators
• centrifugal separators
• filter vane separators
• mist eliminator pads
• liquid/gas coalescers

and then focus on the separation of fine aerosols from gases using liquid/gas coalescing technology.

Gravity Separators

In a gravity separator or knock-out drum, gravitational forces control separation. The lower the gas velocity and the larger the vessel size, the more efficient the liquid/gas separation. Because of the large vessel size required to achieve settling, gravity separators are rarely designed to remove droplets smaller than 300 microns⁴. A knock-out drum is typically used for bulk separation or as a first stage scrubber. A knock-out drum is also useful when vessel internals are required to be kept to a minimum as in a relief system or in fouling service⁵. Gravity separators are not recommended as the soul source of removal if high separation efficiency is required.

Centrifugal Separators

In centrifugal or cyclone separators, centrifugal forces can act on an aerosol at a force several times greater than gravity. Generally, cyclonic separators are used for removing aerosols greater than 100 µm in diameter and a properly sized cyclone can have a reasonable removal efficiency of aerosols as low as 10 µm. A cyclone’s removal efficiency is very low on mist particles smaller than 10µm⁶. Both cyclones and knock-out drums are recommended for waxy or coking materials.

Mist Eliminators

The solid/ liquid gas separation mechanism for mist eliminator pads is inertial impaction. Typically, mist eliminator pads, consisting of fibers or knitted meshes, can remove droplets down to 1-5 microns but the vessel containing them is relatively large because they must be operated at low velocities to prevent liquid reentrainment.

Filter Vane Separators

Vane separators are simply a series of baffles or plates within a vessel. The mechanism controlling separation again is inertial impaction. Vane separators are sensitive to mass velocity for removal efficiency, but generally can operate at higher velocities than mist eliminators, mainly because a more effective liquid drainage reduces liquid reentrainment. However, because of the relatively large paths between the plates constituting the tortuous network, vane separator can only remove relatively large droplet sizes (10 microns and above). Often, vane separators are used to retrofit mist eliminator pad vessels when gas velocity exceeds design velocity⁷. 

Liquid/Gas Coalescers

Liquid/gas coalescer cartridges combine features of both mist eliminator pads and vane separators, but are usually not specified for removing bulk liquids. In bulk liquid systems, a high efficiency coalescer is generally placed downstream of a knock-out drum or impingement separator. Gas flows through a very fine pack of bound fibrous material with a wrap on the outer surface to promote liquid drainage (See Figure 2 below). A coalescer cartridge can trap droplets down to 0.1 micron. When properly designed and sized, drainage of the coalesced droplets from the fibrous pack allows gas velocities much higher than in the case of mist eliminator pads and vane separators with no liquid reentrainment or increase in pressure drop across the assembly.

Table 1 summarizes each of these technologies and provides guidelines for proper selection. As you can see, for systems containing very fine aerosols, under 5 µm, a coalescer should be selected. Removing very fine aerosols from gases results in major economic, reliability, and maintenance benefits in compressor systems.

Formation of Fine Aerosols

There are several different ways that very fine liquid aerosols can get into a gas stream.

1. Condensation from a saturated vapor,
2. Atomization (spray effect through a flow restriction) and, 
3. Liquid reentrainment.8

Recent studies on aerosol size distribution in a natural gas stream have identified that significant quantities of droplets below 5 microns are the norm whenever choke valves and other restrictions are present⁹ or when vapors are at their dew points¹⁰ . The measurements shown in Figure 3 were performed to determine concentration ofliquid aerosols in natural gas stream sampled downstream of vane separators (combination of gravity separator and horizontal filter barrier and equivalent to a mist eliminator pad). Results show that in many cases, large quantities of aerosols can go through this type of separator because the droplets are too small to be trapped by these separation devices. As a result, a liquid/gas coalescer should be the technology of choice whenever high recovery rates are required to protect downstream equipment or to recover valuable liquid products.

Traditional means of coalescer performance validation is the DOP (dioctylphthalate) test¹¹. In this test, a monodispersed aerosol of 0.3 µm diameter is continuously generated by a condensation of DOP vapor under controlled conditions. When aerosol generation is stabilized (constant particle size and aerosol concentration), the concentration of DOP is measured upstream and downstream of the coalescer by a light scattering photometer. Results are expressed as a percent of DOP penetration at the flow rate used.

Some major drawbacks of the DOP test include¹²

1. The test is performed on a dry or unsaturated cartridge. A dry cartridge, in essence, acts like a sponge, absorbing any liquid which goes through it. What the DOP test does not measure is the coalescer’s ability to retain liquids when liquids saturate the coalescer medium and could be re-entrained downstream.

2. This leads to a second drawback; the pressure drop measured across the assembly is underestimated when compared with actual pressure drops across a saturated element. The saturated DP is approximately 2-4 times greater than the clean DP.

3. The test is performed under a partial vacuum where gas properties (density and viscosity) are very different from those prevailing at actual operating pressure. DOP test conditions tend to overstate the efficiency of the coalescer element.

In order to avoid shortcomings of the DOP test, Pall has developed the Liquid Aerosol Separation Efficiency (LASE) test. This test was developed solely for the purpose of measuring coalescer performance in a compressed gas stream under conditions more similar to those found in a refinery or a gas processing plant. The system used for this test is schematically represented in Figure 5. 

The LASE test differs from the DOP test in the following ways:

1.It gives a more accurate and meaningful measure of efficiency. The DOP efficiency essentially tells you what percent of 0.3 µm dioctylphthalate droplets will be removed by a dry coalescer; the LASE test tells you what ppmw of contaminants will be in the gas downstream of the coalescer. In other words, what the LASE test tells you is how much contaminant your downstream equipment will be exposed to.

2. The DOP uses monodispersed (ie. same sized) droplets of DOP, a liquid not commonly encountered in a gas processing or refinery gas streams; the LASE test uses a lube oil which has droplet sizes that range from 0.1-0.9 µm.

3. The LASE test more closely simulates process conditions, by being run on a saturated cartridge and being performed under positive pressure. 

Table 2 shows a comparison of the DOP and LASE test.