Permeation dryers

Flue gas conditioning for moisture removal is commonly performed for criteria pollutant measurements, especially for extractive CEM systems at combustion sources. An implicit assumption is that conditioning systems specifically remove moisture without affecting pollutant and diluent concentrations. Gas conditioning is generally performed by passing the flue gas through a cold trap (Peltier or refrigerant dryer) to remove moisture by condensation, which is subsequently extracted by a peristaltic pump. Many air pollutants are water-soluble and potentially susceptible to removal in a condensation dryer from gas interaction with liquid water.

An alternative technology for gas conditioning is the permeation dryer, where the flue gas passes over a selectively permeable membrane for moisture removal. In this case, water is transferred through the membrane while other pollutants are excluded, and the gas does not come into contact with condensate.

Laboratory experiments were performed to measure the relative removal of a water-soluble pollutant (sulphur dioxide, SO₂) using the two conditioning techniques. A wet gas generator was used to create hot, wet gas streams of known composition (15 % and 30 % moisture, balance nitrogen) and flow rate. Pre-heated SO₂ was injected into the wet stream using mass flow valves to achieve concentrations of 20, 50, and 100 ppm. The mixed gas was directed through a heated sample line to either a peltier or a permeation conditioning system. Two different types of gas analyzer were used to measure the SO₂ concentration after sample conditioning. Both analytic methods demonstrated that SO₂ is removed to a significantly greater extent by the peltier dryer. These results have important implications for monitoring of SO₂ and other soluble gases.

Theory:

The selective and continuous removal of water vapour is performed by leading the flue-gas through a tube. Water vapour is absorbed through the tubing walls and moves through it. Dry purge gas is flowing at the outside of the drying-tube in the opposite direction and carries the water vapour away. Virtually all elements of the flue-gas sample remain unchanged, only the water vapour is removed. The drying effect is relative to the difference in vapour pressure between the gas outside the tube and the wet gas inside.

Gas permeation is the term used to describe a membrane separation process using a non-porous semi-permeable membrane. In this type of process, a gaseous stream is separated into permeating and non-permeating streams. The non-permeating stream is generally called the non-permeate in gas separations terminology. Transport occurs by a solution diffusion mechanism, and membrane selectivity is based on the relative permeation rates of the components through the membrane. Each gaseous component transporting through the membrane has a characteristic permeation rate that is a function of the ability to diffuse through the membrane material. The mechanism for transport is based on solubility and diffusion. The two relationships upon which the equations are based are Fick's law for diffusion and Henry's law for solubility.

Diffusive flux through the membrane can be expressed by Fick's Law related to the membrane system as given by:

(I)

Where: J_i : flux of component i (mole/m^₂/s)

D_i : diffusivity of component i (m²/s)

L : thickness of the membrane (m)

C_im1: concentration of component i inside membrane on feed side (mole/m3)

C_im2: concentration of component i inside membrane on permeate side (mole/m3 )

From Henry's Law:

C_im = S_i * p_i (II)

Where, S_i : solubility constant for component i in the membrane (mol/m³.Pa)

p_i : partial pressure of component i in the gas phase (Pa)

Permeation through the membrane is a function of solubility and diffusivity:

P_i = D_i * S_i (III)

Separation efficiency aij is based on the different rates of permeation of the gas components, data that is not commonly available:

(IV)

An experimental separation factor a*ij is frequently used to quantify the separation of a binary system of components i (oxygen) and j (nitrogen), where Cp and Cr represent molar concentrations in the permeate and retentate (non-permeate) streams, respectively. The separation factor can also be defined in terms of Cp and Cf, i.e., concentrations in the permeate and feed streams. These relationships can be written in terms of mole fractions xp, xr and xf, which is more convenient.

(Va)

(Vb)

Recovery is defined by the equations below, where Qp, Qr and Qf represent the volumetric flow rates of permeate (or non-permeate) and feed streams, respectively (m³/s).

(VIa)

(VIb)

Stage cut defines the ratio of permeate flow rate to total feed flow rate. This assumes that both concentrations and volumetric flow rates are measured at atmospheric pressure in the permeate and the non-permeate streams.

(VII)

The total flux of a component, J_i may be calculated from the expression below:

(VIII)

Where, Q_ip : volumetric flow rate of species i in the permeate (m³/sec).

ρ : density of permeate (mole/m³).

A : area of membrane (m²)

n : number of modules used.

The permeation dryer is one of the most effective and compact means available to remove water from a system, particularly if feedback of the dried gas is used to improve drying effect.