Synthesis loop design

Since the first commercial application of the Haber process for ammonia production in 1913, a wide variety of synthesis loop designs have been developed. A history of the early developments is given in Chapter 1, and has also been reviewed elsewhere. However, by the 1950s and early 1960s, a broad consensus about the optimum design conditions for an ammonia synthesis loop had been reached. A typical design from this period will be described. This will be used to demonstrate how the elements of the synthesis loop are applied to produce a practical design, and will also serve as a base case for the discussion of modern developments. A flowsheet for this type of synthesis loop is shown in Fig. 7.4. 

The first application in 1992 used a two-bed, hot-wall KAAP reactor that featured a low pressure drop and radial flow. Because of the KAAP catalyst s high activity, thin beds are necessary to keep operating temperatures within the desired range203. In 2002 the KAAP reactor had evolved to a four-bed design. A magnetite catalyst is used in the first bed of the synthesis loop when the ammonia concentration is below 2% of the feed. Then the ruthenium catalyst is used in the next three beds to bring the ammonia level up to 18% or more215. 

Uhde s solution to the challenge of building larger plants is a dual-pressure process that uses Synetix catalysts. A capacity of 3,300 tonnes/day can be designed with all proven high-pressure equipment for the synthesis loop and compressor. It is also possible to reach capacities of 4,000 tonnes/day if the casing of the synthesis gas compressor is increased to the next size, which is available218. 

Assuming the feedstock is methane, which is the major component of natural gas, the theoretical feed requirement would be equivalent to one-fourth of the potential hydrogen production or 16,713 SCF CH /ST NH3(15.2 MM BTU/ST). However, the actual process consumes on the order of 22,420 SCF CHi+/ST NH3 or about 20.4 MM BTU/ST NH3 (LHV). The required quantity of feed depends on the process design criteria chosen for the methane conversion in the reforming section, the efficiency of CO conversion, degree of CO2 removal and the inerts (CHi+ + Ar) level maintained in the ammonia synthesis loop. Thus, the potential hydrogen conversion efficiency of the feedstock in the steam reforming process is about 75%. Table 3 shows where the balance of the feed is consumed or lost from the process. 

After final cooling, the synthesis gas is compressed (7) and sent to the synthesis loop. The loop can operate at pressures between 70 to 100 bar. The converter design does impact the loop pressure, with radial-flow designs enabling low loop pressure even at the largest plant size. Low loop pressure reduces the total energy requirements for the process. 

A) Synthesis loop for pure and dry makeup gas B) Product recovery after recycle compression C) Product recovery before recycle compression (four-nozzle compressor design) D) Two stages of product condensation a) Ammonia converter with heat exchangers b) Ammonia recovery by chilling and condensation c) Ammonia recovery by condensation at ambient temperature d) Synthesis gas compressor e) Recycle compressor... 

Kellogg has developed for its ruthenium catalyst based KAAP ammonia process [404], [478] a special converter design. Four radial flow beds are accommodated in a single pressure shell with intermediate heat exchangers after the first, second and third bed. The first bed is loaded with conventional iron catalyst, the following ones with the new ruthenium catalyst. Figure 95 is a simplified sketch of the converter and the synthesis loop of the KAAP for a new plant. For revamps Kellogg has also proposed a two-bed version completely loaded with ruthenium catalyst to be placed downstream of a conventional converter [398]. 

The synthesis loop boiler on the exit of the converter is also a very important piece of equipment. In some modern plants not equipped with an auxiliary boiler it supplies nearly half of the total steam generation. It may generate as much as 1.5 t of steam per tonne of ammonia, equivalent to about 90% of the reaction heat. Fire-tube versions have been also used, including Babcock-Borsig s thin-tubesheet design. But compared to the secondary reformer service, where the gas pressure is lower than the steam pressure, the conditions and stress patterns are different. In the synthesis loop boiler the opposite is the case, with the result that the tubes are subjected to longitudinal compression instead of being under tension. Several failures in this application have been reported [993], and there was some discussion of whether this type of boiler is the best solution for the synthesis loop waste-heat duty. 

Exxon Chemical Process. The Exxon Chemical process [1092], [1093] was specifically designed for the company s own site in Canada and so far not built for third parties. It uses a proprietary bottom-fired primary reformer furnace and a proprietary hot potash carbon dioxide removal system with a sterically hindered amine activator. Synthesis loop and converter are licensed by Haldor Topsoe A/S. Synthesis is carried out at 140 bar in a Topsoe S-200 converter and total energy consumption is reported to be 29 GJ/t NH3. 

Casale advanced waste-heat boiler design in the synthesis loop. 

Ammonia Casale SA Ammonia Natural gas (NG) Process produces anhydrous ammonia from natural gas by applying Casale s high-efficiency secondary reformer design, axial-radial technology for shift conversion, ejector ammonia wash system, axial-radical technology for ammonia converter and advanced waste-heat boiler design in the synthesis loop. 5 2010... 

In the early days the synthesis gas was produced at atmospheric pressure, and the synthesis gas was compressed in reciprocating compressors to pressures as high as 100 MPa in some cases. Capacities were limited to around 300 - 400 MTPD due to limitations in reciprocating compressors. However, with the development of steam reformer based front-ends and the introduction of centrifugal compressors, the ammonia plant capacities suddenly increased to 1000 MTPD with ammonia synthesis loop pressures typically around 15 MPa. Since the 1960 s new developments have been in the ammonia converter designs, such as introduction of radial flow converters and introduction of converters with multiple catalyst beds to increase ammonia conversion. 

The operating conditions in the ammonia synthesis loop are described by a number of parameters, which in some cases may be independent variables, and in other cases a function of other parameters. The relationship between these parameters (and other parameters such as space velocity, inert level, concentrations and temperatures at various points in the synthesis loop, etc.) can be described in mathematical models that are used for design, simulation, and optimisation. 

A methanol synthesis loop with a stoichiometric feed of CO and H2 is to be designed for a 95% overall conversion of CO. All the methanol formed leaves in the product stream. Not more than 2% of the CO and 0-5% of the H2 emerging from the reactor is to leave in the product stream—the remainder is recycled. Calculate the single pass conversion, the recycle ratio and the composition of the product. 

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