Synthesis loop, example

Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

As an example, consider ammonia synthesis. In an ammonia synthesis loop, hydrogen and nitrogen are reacted to ammonia. The reactor effluent is partially condensed to separate ammonia as a liquid. Unreacted gaseous hydrogen and nitrogen are recycled to the reactor. A purge on the... 

Figure 99. Example of a synthesis loop for 1500 t/d NH (Krupp - Uhde)...

Figure 99 is an example of modern ammonia synthesis loop (Krupp-Uhde) with a two-vessel converter system and three indirectly cooled catalyst beds producing 1500 t/d NH3 at 188 bar. 

The main advantage of the indirect routes via syngas is the very high carbon efficiency. As an example, a modem methanol synthesis loop based on natural gas may operate with more than 50% conversion per... 

After a water-gas shift reaction at low temperatures (LTS) and the following CO2-removal, the synthesis gas can be purified in a so-called methanation to remove smaller traces of CO and CO2. Due to the exothermal reversed steam reforming reaction (5.1) smaller quantities of methane are formed which can be accepted as inert component, for example, in the ammonia synthesis, however, only at the expense of a so-called purge gas flow from the synthesis loop [5.18]. Usually, this last purification step is not suitable for the generation of high-purity hydrogen. 

Figure 8.17 shows that with increasing particle size, the ammonia concentration at the outlet decreased. For example, when the particle size increases from 0.6-0.9 mm to 4.0-6.7mm, the ammonia concentration at the outlet at 15.0 MPa and 400°C decreases from 23.42% to 19.43%, representing a 17.0% reduction in ammonia concentration. At a low pressure of 7.0 MPa, the net value of ammonia is not so high, and the percentage of decrease in ammonia concentration is especially considerable. Taking the bed resistance and other factors into consideration, low pressme ammonia synthesis loop should as far as possible use smaller catalyst particles. 

The N2-H2 ratio is stoichiometric at all points in the traditional synthesis loop. But, at low N2-H2 ratio loop, N2-H2 mixture with the mole ratio of H2 to N2 at inlet is 2 (1-0.671) (1 1 will be better) is sent into a converter. During the reaction process, it keeps stoichiometric ratio, but because the raw material with H2/N2 of 1 1 is sent into the converter, more H2 is consumed than N2, so H2 is needed to be complementary to the system in order to maintain the proportion needed. For example, someone suggested that H2 could be injected to every point of the system respectively. ... 

The recovery of reaction heat at synthesis loop includes both the quantity and the quality of the heat. The former is determined by Eq. (9.3), i.e., the net value of ammonia and the temperature difference loss of the recovery system. For example, in Fig. 9.25, inlet temperature of converter is 141°C, the outlet temperature is 284°C, so the adiabatic temperature rise of the converter is 143°C, the temperature difference lost at the cool side of the preheater front-converter is 19°C, and the rest heat can be applied to the feed water of the boiler. The recovery ratio of the reaction heat reaches 86.7%. 

The selectivity of catalyst is not involved in synthesis loop of ammonia. Therefore, the influence of catalyst selectivity upon the process of ammonia production mainly takes place in the working procedure such as steam reforming etc. In the catalytic conversion of organic compounds, some raw materials transform more or less to by-products. The yield of purpose product is influenced by the catalyst selectivity to a great degree. For example, the decisive factor of the economy of oxidation process is just the catalyst selectivity. As a result, for this kind of chemical process, the catalyst selectivity has great economic significance than its activity. 

The final energy input is power for the three gas-compression duties synthesis gas compression, synthesis loop circulation, and ammonia refrigeration. The power required for these duties depends, for example, on synthesis pressure and on converter design. Refrigeration power is strongly dependent on ambient temperature. However, typical figures for a modern 1000 ton per day low-pressure... 

Synthesis loop heat exchangers, and especially the waste heat boilers, are critical items in ammonia plants. The ammonia content and the high partial pressure of hydrogen in the reactor effluent makes proper boiler design and selection of construction materials essential. Several designs are available. Reviews and examples may be found in [558, 559, 608-615, 919]. 

M. W. Kellogg has also suggested a process (the KAAP process) based on a new catalyst [928, 949]. This technology has been used in a revamp project where a new reactor was installed downstream of the existing reactor in an ammonia synthesis loop (see Sect. 6.4.3.4). In addition, more radically new process schemes deviating from the traditional route have been described. An example is a scheme based on so-called parallel reforming [30,32,120,121] and a low pressure loop with ammonia recovery by water absorption [769]. None of these new developments have been implemented in practice. 

The Exxon Chemical process is a good example of processes which are designed for a specific case in cooperation between the plant owner (Exxon Chemical), the contractor (Bechtel), and process licensors (Haldor Topsoe for the synthesis loop and converter). The process and experience from operation of the plant is described in [776, 777]. 

Multiple feedback loops can provide additional fine control. For example, as shown in Figure 9—5, the presence of excess product B decteases the tequitement for substrate 3. Howevet, Sj is also tequited fot synthesis of A, C, and D. Excess B should thetefote also curtail synthesis of all font end products. To circumvent this potential difficulty, each end product typically only partially inhibits catalytic activity. The effect of an excess of two or more end products may be strictly additive or, alternatively, may be greater than their individual effect (cooperative feedback inhibition). 

Like direct synthesis, xc is the closed-loop time constant and our only tuning parameter. The first order function raised to an integer power of r is used to ensure that the controller is physically realizable. 2 Again, we would violate this intention in our simple example just so that we can obtain results that resemble an ideal PID controller. 

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