The dual-temperature principle for providing reflux for the ammonia-hydrogen deuterium exchange process was proposed by the British firm Constructors John Brown [C12], has been tested in pilot-plant experiments conducted by Friedrich Uhde Gmbh at the plant of Farbwerke Hoechst in Germany [W2], and is to be used in a commercial plant at Talcher, India (item 19, Table 132), being constructed by Uhde.

Figure 13.37 is a material flow sheet for a dual-temperature ammonia-hydrogen exchange plant using the same amount of synthesis-gas feed and producing the same amount of enriched ammonia product as the monothermal ammonia-hydrogen exchange plant of Fig. 13.23. Comparison of these figures shows that the hot exchange column of Fig. 13.37 performs both the function of the ammonia dissociation step D of Fig. 13.23 in providing enriched

synthesis-gas recycle vapor for the enriching cold column of Fig. 13.37 and the function of the ammonia synthesis step A of Fig. 13.23 in providing depleted liquid ammonia reflux for the stripping cold column of Fig. 13.37. Comparison of these figures shows the advantages of the dual-temperature system to be as follows:

1. Elimination of the ammonia dissociation step D

2. Elimination of the work of recompressing synthesis gas from 55 to 350 atm

3. Elimination of the net heat input needed to dissociate ammonia at 740°C

4. Elimination of the need to synthesize ammonia for reflux and the costs associated with this step

5. Elimination of the catalyst deuterium stripper, F, Fig. 13.23

The dual-temperature system, however, is not without its disadvantages. Because the hot exchange column of Fig. 13.37 returns synthesis gas with a much lower deuterium content than the ammonia dissociation step of Fig. 13.23 and returns liquid ammonia reflux with a much higher deuterium content than the ammonia synthesis step of Fig. 13.23, it is necessary to operate the cold columns of Fig. 13.37 with higher liquid and vapor flow rates than those of Fig. 13.23 and to run them closer to minimum reflux conditions. Consequently, a much larger number of theoretical stages is needed in the cold columns of Fig. 13.37 than in the corresponding columns of Fig. 13.23. In addition, the dual-temperature system requires a large hot exchange column. Table 13.26 compares the liquid and vapor flow rates and number of theoretical stages in the two systems.

Even though flow conditions for the dual-temperature system, Fig. 13.37, were chosen to give a minimum number of stages, the increase from 5.7 stages for the monothermal system to

72.9 for the dual-temperature system must be viewed as a serious disadvantage of the latter. Another disadvantage of the dual-temperature flow sheet would be the complicated heat exchange system needed for heat recovery and humidification between the hot and cold towers, which is not shown in Fig. 13.37.

Nitschke [N2] has given a partial description of the flow sheet used by Uhde for the dual-temperature ammonia-hydrogen heavy-water plant that company is building at Talcher, India (item 19, Table 13.2). Figure 13.38 is a qualitative material flow sheet for the first-stage exchange columns of that plant.

Feed for this heavy-water plant consists of synthesis gas for the ammonia plant of the Indian Department of Atomic Energy, at 190 to 200 atm. The heavy-water plant, however, operates at 300 atm. To avoid the need for compressing synthesis gas, and to isolate gas flow in the ammonia plant from gas flow in the heavy-water plant, deuterium in feed synthesis gas is transferred to a solution of potassium amide in ammonia in the transfer column A, and synthesis gas 85 percent stripped of deuterium is returned to the ammonia plant.

Ammonia for the heavy-water plant is pumped to 300 atm, cooled to —25°C, and introduced as feed between the stripping (B) and enriching (C) sections of the first-stage cold exchange tower, where it joins ammonia circulating at the rate L In C the deuterium content of the ammonia is raised to first-stage product level xp by exchange against synthesis gas flowing at rate Gi + G2 whose deuterium content is reduced from yp to yp. A portion of the ammonia is sent to the first of two higher stages for further enrichment, and an equal flow of partially depleted ammonia is returned, reducing the deuterium content of ammonia entering the hot enriching section D to x’p. Here, because of the lower separation factor, the deuterium content of the ammonia is reduced to x’p, somewhat below that of feed, while the deuterium content of synthesis gas is raised from yF to yp. The deuterium content of ammonia is further reduced to the tails level xw in the hot stripping section E, where the gas flow rate has been reduced to Gi because of the recycle at rate Gi to the enriching sections C and D. The gas in E is enriched from yw to yp. A portion F of the tails is reenriched to feed level xF in the transfer column A, and the remainder, L, is fed to the cold stripping column В to be reenriched to feed level while stripping synthesis gas flowing at rate G from yF toy^.

The function of the four exchange-column sections can be better understood by reference

F+ L, Xyy

to the qualitative McCabe-Thiele diagram Fig. 13.39, whose nomenclature is keyed to Fig. 13.38. The slopes of the four operating lines are

Cold stripping, L/Gi

Cold enriching, CF + L)l(Gl + G2)

Hot enriching, (F + L)/(Gi + G2 )

Hot stripping, (F + L)/G1

By providing enough stripping plates, xw and у у/ could be made as close to zero as desired. By providing enough enriching plates, Xp and yp could be made as close to unity as desired.