Water/Dew Point

When a customer agrees to purchase gas, product quality is specified in terms of the calorific value of the gas, measured by the Wobbe index (calorific value divided by density), the hydrocarbon dew point and the water dew point, and the fraction of other gases such as Nj, COj, HjS. The Wobbe index specification ensures that the gas the customer receives has a predictable calorific value and hence predictable burning characteristics. If the gas becomes lean, less energy is released, and if the gas becomes too rich there is a risk that the gas burners flame out . Water and hydrocarbon dew points (the pressure and temperature at which liquids start to drop out of the gas) are specified to ensure that over the range of temperature and pressure at which the gas is handled by the customer, no liquids will drop out (these could cause possible corrosion and/or hydrate formation). 

Another nonregenerative drying appHcation for molecular sieves is their use as an adsorbent for water and solvent in dual-pane insulated glass windows. The molecular sieve is loaded into the spacer frame used to separate the panes. Once the window has been sealed, low hydrocarbon and water dew points are maintained within the enclosed space for the lifetime of the unit. Consequently, no condensation or fogging occurs within this space to cloud the window. 

Fouling Industrial streams may contain condensable or reactive components which may coat, solvate, fill the free volume, or react with the membrane. Gases compressed by an oil-lubricated compressor may contain oil, or may be at the water dew point. Materials that will coat or harm the membrane must be removed before the gas is treated. Most membranes require removal of compressor oil. The extremely permeable poly(trimethylsilylpropyne) may not become a practical membrane because it loses its permeability rapidly. Part of the problem is pore collapse, but it seems extremely sensitive to contamination even by diffusion pump oil and gaskets [Robeson, op. cit., (1994)]. 

Natural gas must meet certain specifications before it qualifies for fuel. It should also meet certain dew point characteristics before entering a transmission pipeline. Water dew point is controlled and maintained by stationary equipment such as molecular sieves or dehydration by glycol. Hydrocarbon dew point, on the other hand, can... 

One option for water dew point eontrol is to alter the freezing point by glyeol eontaet before eooling. The seeond option is to dry the gas by moleeular sieve prior to refrigeration and subsequent separation of heavy eomponents for hydroearbon dew point eonditioning. 

The two major conditions that promote hydrate formation are (1) the gas being at the appropriate temperature and pressure, and (2) the gas being at or below its water dew point with free water present. For any particular composition of gas at a given pressure there is a temperature below which hydrates will form and above which hydrates will not form. As the pressure increases, the hydrate formation temperature also increases. If there is no... 

The contactor works in the same manner as a condensate stabilizer tower described in Chapter 6. As the glycol falls from tray to tray it becomes richer and richer in water. As the gas rises it becomes leaner and leaner in water vapor. Glycol contactors will typically have between 6 and 12 trays, depending upon the water dew point required. To obtain a 7 Ib/MMscf specification, 6 to 8 trays are common. 

Figure 8-9. Equilibrium water dew points at different temperatures for gases.

The hydrocarbon dew point is reduced to such a level that retrograde condensation (i.e., condensation resulting from pressure drop) cannot occur under the worst conditions likely to be experienced in the gas transmission system. Similarly, the water dew point is reduced to a level sufficient to preclude formation of Cl to C4 hydrates in the system. 

Water Dew Point For flue gas, the water dew point is that temperature at which the actual water vapor pressure equals the water saturation vapor pressure. Cooling the flue gas below this temperature will result in the formation of liquid water [or ice, below 273 K (0°C)]. For example, burning natural gas with 3 percent excess oxygen (15 percent excess air), the flue gas water dew point would be (Fig. 24-57) 330K(56.7°C). 

For gas-side materials, one available technology employs Teflon-covered metal tubing and Teflon tube sheets. This technology is often operated across both the acid and water dew points, and can accept inlet gas temperatures to 533 K (260°C). Typical applications may achieve a cold-end AT below 45 C (80 F), improve the boiler thermal efficiency by 10 percent (LHV basis), and have a simple payback of 2 to 3 years, based on fuel avoidance (Figs. 24-58 and 24-59). 

FIG. 24-59 Characteristic curves for boiler thermal efficiency as a function of flue gas effluent temperature and flue gas water dew points. Based on the LHV of a fuel, and stoichiometric reaction, 100 percent efficiency would be achieved if sufficient combustion heat were recovered and removed, so that the temperature of the effluent flue gas was reduced to 25°C. For a flue gas with a 55°C dew point, recovering additional heat via condensation by cooling from 175 to 35°C (as shown) would increase the overall efficiency by more than 13 percent. Courtesy Combustion b- Energy Systems, Ltd. www.condexenergy.com.)... 

What is the water dew point of a COz-water mixture containing 1 mol% water at 50°C ... 

This mixture has two dew points - a normal dew point and a retrograde dew point. The figure below shows the amount of aqueous phase present as a function of the pressure. At pressure below the first dew point, there is no aqueous phase present. At 3.4 MPa, the water dew point, the first infinitesimal amount of water appears. At first as the pressure increases so does the amount of water. From the figure we can see that the amount of water present reaches a maximum at about 7 MPa. At higher pressures the amount of water starts to decrease. 

To recover the liquid product from the reactor product gas stream. Biocarbons Corporation uses a venturi scrubber with a closed water recirculating loop that (for commercial operation) is kept above the water dew point of the gas to avoid net water production (An electrostatic precipitator would be used downstream of the scrubber... 

The treated gas leaving the M D E A unit will require downstream dehydration since the pipeline gas specifications require a minimum water dew point. This is best achieved with a conventional TEG glycol dehydration unit, as the gas is very lean without condensable hydrocarbons. 

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