Heat Exchangers Hydrogen

Gas exposure times to furnace temperatures are kept short by a direct spray of ethylene dichloride at ambient temperatures onto the exit gases and by an indirect heat exchanger. Hydrogen chloride may be recovered by scrubbing with water, or by some form of fractionation if it is to be fed in anhydrous form to an oxychlorination unit. Many vinyl chloride producers operate an oxychlorination unit solely to convert the by-product hydrogen chloride from the ethylene dichloride cracker to produce additional ethylene dichloride feedstock. Vinyl chloride (b.p. —13.4°C) is almost entirely consumed for the production of poly(vinyl chloride) (PVC). 

Advantages of the foam include a choice of metals to 1000°C, low weight, compaction and the abihty to be formed in complex shapes. Some potential applications include gas turbine tail gas heat exchangers, hydrogen liquefaction plant and partial oxidation of hydrocarbons to hydrogen. The foam can be catalysed, of relevance to HEX-reactors. Within the area of heat exchangers, the work at ETH was based on aluminium foam of the type shown in Figure 4.11. 

A flow diagram for the system is shown in Figure 5. Feed gas is dried, and ammonia and sulfur compounds are removed to prevent the irreversible buildup of insoluble salts in the system. Water and soHds formed by trace ammonia and sulfur compounds are removed in the solvent maintenance section (96). The pretreated carbon monoxide feed gas enters the absorber where it is selectively absorbed by a countercurrent flow of solvent to form a carbon monoxide complex with the active copper salt. The carbon monoxide-rich solution flows from the bottom of the absorber to a flash vessel where physically absorbed gas species such as hydrogen, nitrogen, and methane are removed. The solution is then sent to the stripper where the carbon monoxide is released from the complex by heating and pressure reduction to about 0.15 MPa (1.5 atm). The solvent is stripped of residual carbon monoxide, heat-exchanged with the stripper feed, and pumped to the top of the absorber to complete the cycle. 

Compact brazed aluminum plate-fin heat exchangers can be used in most cryogenic hydrogen purification apphcations. The use of these relatively low cost heat exchangers, combined with low separation energy requirements, results in a highly economical process for hydrogen purification. 

As temperatures decrease, closer temperature approaches are needed in the heat exchangers to achieve low energy requirements. Consequendy, temperature pinches in Hquid hydrogen plants range from 1 degree K at 20 K to 6 degrees K at 300 K. 

Fig. 6. Hydrogen hquefaction process. A represents adsorber C, catalyst bed HX, heat exchanger E, expander.

Problems sometimes occur in trying to minimize the temperature difference at the cold end of the heat exchanger, particularly if the specific heat of the warm fluid decreases with decreasing temperature as is the case with gaseous hydrogen. 

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