Dry gas recovery

Figure 16.4 Dry gas recovery as a function of dew point for various sweep fractions sweep fractions soiid - 0 short dash - 0.1 iong dash -0.2...

The changes in performance for a lower water/nitrogen selectivity of 100 are illustrated in Figures 16.5 and 16.6. Relative to a selectivity of 1000, dry gas recovery decreases slightly but dry gas flow rates are significttntly lower. A dectease in recovery is expected with a decrease in selectivity since more dry air will permeate along with the water. For a dew point of 0°F and sweep fraction of 0.2, the recovery decreases from 0.79 to 0.75. 

Variability in the sweep flow around individual fibres has little effect on module performance. Only slight decreases in dry gas recovery and flow rate occur as the variability in sweep flow increases. The dry gas flow rate drops by less than 10% over the range of dew points considered as the variability in sweep flow rate increases to 20%. The dry gas recovery changes by less than 1%. 

Figures 16.9 and 16.10 compare the effects of inner diameter and sweep variation on module performance. Like sweep variability, inner diameter variability is detrimental to performance - dry gas recovery and flow rate deaease as the variability increases. However, the effect of inner diameter variability is significantly larger. The dry gas flow rate decreases by a factor of two with 20% inner diameter variation for the lower dry gas dew points. Changes in dry gas recovery are smaller and do not exceed 5% at the lowest dew points considered.

Figure 16,13 Effect of sweep configuration on dry gas recovery as a function of dew point diamond - internal, circle - shell, triangle - offset. The solid line corresponds to uniform sweep distribution. Note the diamond and circle symbols overlap...

In all cases, for a given product dew point, the dry gas flow rate increases as the fraction of the product used as permeate sweep increases. However, the dry gas recovery (the fraction of the wet feed recovered as product) simultaneously decreases. The optimal sweep fraction will depend on the trade-off between increased productivity and fractional loss of the dry, high pressure product. 

Methods of Purification. Although carbon dioxide produced and recovered by the methods outlined above has a high purity, it may contain traces of hydrogen sulfide and sulfur dioxide, which cause a slight odor or taste. The fermentation gas recovery processes include a purification stage, but carbon dioxide recovered by other methods must be further purified before it is acceptable for beverage, dry ice, or other uses. The most commonly used purification methods are treatments with potassium permanganate, potassium dichromate, or active carbon. 

Based on dryer cost alone, indirect-heat dryers are more expensive to build and install than direct-heat dryers designed for the same duty. As environmental concerns and resulting restrictions on process emissions increase, however, indirect-heat dryers are more attractive because they employ purge gas only to remove vapor and not to transport heat as well. Dust and vapor recovery systems for indirect-heat dryers are smaller and less cosdy to supply heat for drying, gas throughput in direct-heat dryers is 3—10 kg/kg of water evaporated indirect-heat dryers require only 1—1.5 kg/kg of vapor removed. System costs vary directly with size, so whereas more money may be spent for the dryer, much more is saved in recovery costs. Wet scmbbers ate employed for dust recovery on indirect-heat dryers because dryer exit gas usually is close to saturation. Where dry systems are employed, all external surfaces must be insulated and traced to prevent vapor condensation inside. 

Carbon dioxide is a gas at room temperature. Below -78°C it is a solid and is commonly referred to as dry ice. At that temperature it sublimes and changes directly from a solid to a vapor. Because of this unique property, as well as its non-combustible nature, it is a common refrigerant and inert blanket. Table 3.4 shows the uses of carbon dioxide in all its forms liquid, solid, and gas. Refrigeration using dry ice is especially important in the food industry. Beverage carbonation for soft drinks is a very big application. In oil and gas recovery carbon dioxide competes with nitrogen as an inert atmosphere for oil wells. 

The H2S formed exits the FCCU in the dry gas and is removed downstream in the sulfur recovery unit. The increase in H2S production, 5-20%, can typically be managed within a refinery s operations (Figure 16.2). A different mechanism of SO uptake has been presented by Magnabosco [5],... 

Because large amounts of gas are required to supply all the heat for drying, dust-recovery equipment may be very large and expensive when drying very small particles. 

In the inflammable gas recovery system, the gas is cleaned first by collecting coarse dust with a dry dust collector, then... 

Oil and gas reach the surface as a frothy mixture which is sent to a separation tank. The oil then is pumped into storage tanks and the gas is treated in natural gasoline plants for the removal of any remaining liquids. The dry gas is used in several ways. Some is used to supply power in the oil field or pumped back to maintain pressure in the reservoir. The balance is sold or burned off. Natural gas recovery has increased importance since it is used extensively in the manufacture of chemicals and as a source of gas in the home. 

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