METHY LAM IN E-HYDROGEN EXCHANGE PROCESSES

The deuterium exchange reaction between liquid methylamine and gaseous hydrogen, CH3NH2(0 + HDC^CHjNHDCO + H2(g)

is catalyzed by potassium methylamide, CH3NHK. This reaction proceeds with sufficient speed at —50°C to permit operation of a cold tower at this temperature, where the equilibrium constant, 7.9, is the highest known for any practical, deuterium exchange reaction. The optimum hot-tower temperature for a dual-temperature process using this reaction is +40°C, a limit set by thermal decomposition of potassium methylamide at higher temperatures. At +40°C the deuterium exchange equilibrium constant is 3.6. The ratio of these two separation factors, 7.9/3.6 = 2.19, is also higher than the ratio for any other practical system (Tables

12.17 and 13.18). For this reason, Atomic Energy of Canada, Limited (AECL), has undertaken a development program for a dual-temperature process using methylamine and hydrogen from a synthetic ammonia plant with a flow sheet similar to Fig. 13.37.

Sulzer Brothers Canada, Ltd., working with AECL, has given a partial description [W6] of a dual-temperature flow sheet modified from Fig. 13.37 proposed for use in recovering deuterium from ammonia synthesis gas made from Alberta natural gas containing 135 ppm deuterium. Figure 13.40 is a material flow sheet for the synthesis-gas generation section and first deuterium-enrichment stage of such a heavy-water plant. Deuterium contents have been given as [xN], where x is the ratio of the deuterium content to that of Alberta water containing 135 ppm deuterium. The deuterium contents of methane, water, and hydrogen are those given by Wynn [W6]. The deuterium contents of methylamine streams have been assumed to give a plausible number of stages in the various towers. Total flow quantities

Atom fraction D in liquid

Figure 13.39 McCabe-Thiele diagram for Fig. 13.38.

Transfer 0 from MA liquid to MA vgpor «*-D ;

Natural gos 1270.8 CH4 _[0.9N]_____ ^

" д/Г"*

1440 N2 381.6 02

Transfer D from MA vapor] a=| to water D —*■

assumed for this flow sheet are those for a plant permitting production of 1150 MT ammonia/day, after allowing for losses in the ammonia plant. The net deuterium extraction of

0. 422 kg-mol/h would produce 66.8 MT D2 О in 330 operating days per year.

The novel feature of this flow sheet is the production of synthesis gas enriched threefold relative to natural water to provide enriched feed for the exchange plant and thus reduce its size. In the synthesis-gas generator (A), natural gas whose deuterium content is 0.97V is re-formed with air and enriched steam, 5.7N, to produce threefold-enriched synthesis gas and unreacted enriched water, whose enriched deuterium content is recycled. Enriched synthesis gas is the vapor feed to the hot tower (C) of the first dual-temperature exchange stage. Here the deuterium content of synthesis gas is raised from 3N to 207V, while that of counterflowing methylamine and catalyst is reduced from 9151N to 10.916.V. In the cold tower (D), the deuterium content of synthesis gas is decreased from 20N to 0.2Л’ while that of methylamine is increased from 1.061V to 9SN. A portion of the 95-fold-enriched methylamine is fed to the second enriching stage, and an equal amount of partially depleted methylamine is returned; the resultant net flow of 0.422 kg-mol D/h, after further enrichment in higher stages, provides the plant’s heavy-water product.

Enriched steam for the synthesis-gas generator (A) is produced in the series of sieve-plate contactors (E), (F), and (G). In (E) deuterium is transferred from methylamine liquid to methylamine vapor, reducing the deuterium content of the liquid from 10.9167V to 1.061V while increasing that of the vapor from 1.02TV to 107V. In (F) deuterium is transferred from methylamine vapor to water, increasing the deuterium content of the latter from IN to 81V. This two-step transfer of deuterium from liquid methylamine leaving (C) to water leaving (F) is necessary to prevent chemical reaction between water and the catalyst dissolved in liquid methylamine.

Deuterium in enriched water (81V) leaving (F) and unreacted enriched water (5.5N) leaving (B) is transferred to steam in step G, producing enriched steam (5.7N) for the synthesis-gas generator from natural steam. This transfer step is used instead of simply recycling the water leaving (B) and (F) to avoid returning nonvolatile impurities to the synthesis-gas generator.

Because of the reduced rate of the deuterium exchange reaction at —50°C, the stages of the cold tower (D) are to be of the type developed by Sulzer [LI ] for the ammonia-hydrogen exchange process and used in the Mazingarbe plant, Sec. 9.1. For the methylamine-hydrogen system at —50°C, a stage efficiency of 70 percent has been obtained [W6].

At the temperature of the hot tower, 40°C, potassium methylamide slowly decomposes into potassium dimethyl formamidide:

2CH3NHj + CH3NHK -*■ CH3(NKXCH)NCH3 + 2H2 + NH3

This reaction is suppressed by addition of an equimolal amount of lithium methylamide, which has little catalytic activity but inhibits decomposition of the potassium compound.

The great advantage of this methylamine-hydrogen exchange process compared with the dual-temperature ammonia-hydrogen system is the much smaller number of stages needed with methylamine. Intratower flow rates relative to product D20 with methylamine are also smaller than with ammonia. Table 13.27 compares the two processes. The lower internal flow rates with methylamine also lead to lower utility requirements. A disadvantage of this methylamine — hydrogen flow sheet is the need to operate the synthesis-gas-generating section of the ammonia plant with enriched water. This necessitates recycle and strict control of losses of unreacted deuterium-enriched steam and water.

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