Ammonia synthesis separator temperature

The simplest way to recover the product from an ammonia plant is by condensation followed by separation of liquid ammonia from the circulating gas. The optimum temperature for the separator is a function of the synthesis pressure. At high pressure, such as 300-350 bar, simple water cooling can be utilized with a separator temperature just above ambient. At lower pressures, however, the dewpoint of the converter effluent is reduced significantly and refrigeration must be used to condense the product ammonia. Typical separator temperatures in modem plants are in the range --10°C to -25 C. 

Table 6-5 shows the eonditions for whieh NH3 produetion is possible. Both low temperatures or very high pressures aehieve favorable equilibrium. At 25°C, the equilibrium eonstant is very high, while at higher temperatures, both and deerease rapidly. Generally, ammonia synthesis reaetors operate at about 350°C and 200 atm with an equilibrium eonversion of about 70% in eaeh pass. The NH3 is separated from unreaeted Hj and N2, whieh are reeyeled baek to the reaetor. For the overall proeess involving tlie tubular reaetor, separation and reeyele produee about 100% ammonia eonversion. 

In most processes the reaction takes place on an iron catalyst. The reaction pressure is normally in the range of 150 to 250 bar, and temperatures are in the range of 350°C to 550°C. At the usual commercial converter operating conditions, the conversion achieved per pass is only 20% to 30%53. In most commercial ammonia plants, the Haber recycle loop process is still used to give substantially complete conversion of the synthesis gas. In the Haber process the ammonia is separated from the recycle gas by cooling and condensation. Next the unconverted synthesis gas is supplemented with fresh makeup gas, and returned as feed to the ammonia synthesis converter74. 

Liquid ammonia is separated by condensation from the synthesis loop and is either subcooled and routed to storage, or conveyed at moderate temperature to subsequent consumers. 

Different reactor types are used in the individual ammonia-synthesis plants. They have in common, that the catalyst mass is to be found in a separate container inside the reactor chamber. Between the catalyst holder and the reactor wall there is a gap through which the cold synthesis gas can be fed in, such that the reactor wall is not heated to the same temperature as the catalyst holder. 

In contrast to sulfur species, there are no differences in principle between natural and anthropogenic processes in the formation and release of reactive nitrogen species. Industrial nitrogen fixation (in separated steps N2 NH3, N2 NOx, NOx NO3) proceeds via the same oxidation levels as biotic fixation and nitrification, either on purpose in chemical industries (ammonia synthesis, nitric acid production) or unintentionally in all high-temperature processes, namely combustion, as a byproduct due to N2 + O2 2 NO. 

The separator temperature, together with operating pressure and location of make-up gas addition point, determines the ammonia concentration at the converter inlet. A low temperature means a low ammonia concentration at the converter inlet, which again means either a low catalyst volume or high conversion. But low separator temperature can only be obtained by increased power consumption and cost of the refrigeration units. In the end the selection of loop pressure as well as the choice of separator temperature is a compromise between energy consumption and capital costs in various parts of the synthesis unit. 

---