Net value of ammonia

ICI 74-1 catalyst which contains cobalt has been successfully developed, and applied in the low pressure ICI-AMV process by ICI. The catalyst used in this process is ICI 74-1. The diameter of the converter is 2.9m, with the height of 24m. The volume of the catalysts is 96 m (250 tons in weight) in total, which is separated into three catalyst beds. The operation conditions are Pressure of 10 MPa, temperature of 450°C, space velocity of 5,00h net value of ammonia (10%-11%) and pressure drop of 0.4 MPa. The content of inert gases such as methane is limited to about 7%. The reduction temperature at which water is produced is 370°C. The highest reduction temperature is about 480°C. In Hainan Fudao Fertilizer Plant of China, the volume of the catalyst of the converter is increased to 122.4 m in Ude-ICI-AMV process. 

The design of industrial converter is commonly calculated by volume of catalysts. From (6.24), with the same ammonia concentration, the reaction rate of volume is increased by 3.6 times because space velocity is increased by about 3.6 times. But nh3 must meet the requirements of industrial ammonia net value because with increase of space velocity, the outlet concentration or net value of ammonia decreases. 

The above equations show that the production of ammonia depends on the inlet gas flow rate, the concentration of ammonia at the outlet and the net values of ammonia at the inlet and outlet of the converter. The production increases with increasing Vj and and from (8.1). The concentration (y>) of ammonia at the outlet is related to the pressme (p), temperature (T), space velocity (S v), H2/N2 ratio, content of inert gases ( i), type (M), volume (Vk), particle size (dp) and reduction R) of catalyst, gas pm-ity (AT), and the concentration of ammonia at the reactor inlet (y>o)- These factors can be expressed by the function below. 

Figures 8.6 and 8.7 respectively show the operating temperature profile in a continuous heat-exchange reactor and in an adiabatic fixed-bed reactor. The operating line in Fig. 8.6 is based on the heat balance, t2 = to + (14—15)Ay x 100%, where to is the entrance temperature, <2 is the outlet temperature. Ay is the net value of ammonia.

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 performances of the converter. It includes the net value of ammonia, the pressure difference between inlet and outlet of the converter, the rate of utilizing the high-pressure space, distribution of temperature, the degree of the difficulty in reduction, the complex degree of the structure, operation flexibility and so on. 

These technical factors and the criteria interact. For example, degassing amount per ton of ammonia is related to the pressure level, the content of the inert gas in fresh S3mgas and the net value of ammonia (concentration difference of ammonia in outlet and inlet of reactor). The freezing separation series of ammonia are related to the pressure level and the net value of ammonia. The reclaim way of the waste heat is related to the energy equilibrium of whole plant and the net value of ammonia. They are all related close to the performance of catalyst because of the operating conditions of whole plant which is determined according to performance of catalysts. Therefore, the performance of catalysts has a great influence on the economic benefits of whole catal dic process. 

According to the estimation from some relative experts, it is economic when the net value of ammonia is more than 8.4% in low-pressure ammonia synthesis. When the A301 catalyst is in the conditions of 7.0 MPa and space velocity of 4000-8000 h , and the net value of ammonia is 10%-12%, it can meet the needs of economy to net value. Table 9.5 shows the amoimt of inlet syngas (S in) and catalyst volume (V at) needed at isotonic pressure ammonia plant with capacity of 1000 t/d at 7.0-7.5MPa. 

The process mentioned is about the cooling purification process. For most of modern ammonia plants, it is more popular to use the thermal purification method. In the thermal purification process, the final purification is methanation, and the content of inert gas (CIH-I-Ar) is usually higher. The increase of the inert gas content will reduce the concentration and the net value of the outlet ammonia. The research results show that, while the inert gas content increases by 1%, the net value will reduce by about 0.3%-0.4%. In the condition No. 2 of Table 9.5, if the inert gas content increases to 6%, the net value will decrease from 10.69% to 9.49%. Then the amount of inlet gas of converter and the catalyst volume are 64.45 x 10 m h and 107.4 m , respectively. If the inlet concentration of ammonia of the converter is reduced to 2% (condensing temperature is about —30°C), the net value of ammonia will reach 11.15%, and the amount of inlet gas of converter and the catalyst volume are 54.85 x 10 m h and 91.4 m , respectively. It will be seen from this that, for the thermal purification process, it is necessary to limit the inert gas content below 6%-8% and to decrease the condensing temperature of ammonia to about —30°C. 

In the micro-pressurized process at 9-10 MPa, the first section can be omitted. The syngas pressure would be increased to 8.5-10 MPa only through the circulating section. It functions as both compressor and circulator. This is the so-called micro-pressurized process. Table 9.6 shows the outlet concentration and the net value of ammonia for A301 at 8.5 MPa and 10.0 MPa. 

Table 9.6 shows that in the micro-pressurized process, the outlet concentration and net value of ammonia is 16.66% (425°C) and 13% at 8.5 MPa and 6000h /. It is 17.20% (425°C) and 13.57% at lOMPa and 8,000h respectively. Table 9.7 shows the amount of inlet syngas (5jn) and catalyst volume (Foat) needed at micro-pressurized ammonia synthesis plant with capacity of 10001 d at 8.5-10 MPa. 

It can be seen from Eq. (9.30 that the main way of increasing the recovery rate of the reaction heat is to raise the net value of ammonia and decrease the temperature difference loss of the recovery system. 

The main measures of raising the net value of ammonia are to adopt high activity catalyst, converter with high net value and operate in low space velocity. 

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 converter technology is up-to-date, and net value of ammonia for the new designed converter increases, usually to 12%-16%, and adiabatic temperature rises of the reaction to 174-232°C. When KAAP technology with Ru catalyst is adopted. 

The catal3dic activity will gradually decrease with the prolongation of catalyst service time. At this time, it can be known from Eq. (9.4) that in order to keep the liquid ammonia output, it should increase the reaction pressure, reduce the content of inert gas and inlet ammonia of the converter, to retain A

out) in the equation unchanged or the flow of recycle gas (Vjn) must be increased which is the easiest way. The flow of recycle gas (Vjn) is related to catalytic activity (outlet concentration of ammonia). Here the recycle ratio is related to catalytic activity (net value of ammonia). The so-called recycle ratio is the ratio of recycle gas to fresh syngas (Fig. 9.29). 

When the decrease of catalytic activity leads to the decrease of net value of ammonia, it is necessary to increase the flow of recycle gas (inlet gas of converter) in order to keep liquid ammonia output (see Eq. 9.4). However, the power consumption of circulating compressor or ammonia compressor is proportional to the volume of inhalational gas i.e. the flow of recycle gas. When other operating parameters of circulating compressor or ammonia compressor are not changed, the power consumption will increase AN/Nq-... 

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