Synthetic ammonia heat and cold exchanger
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The old plants were equipped with two heat exchangers: the gas after water cooling – first heat exchanger – ammonia cooling – second heat exchanger – ammonia separation – secondary ammonia cooling – secondary ammonia separation; then the first and second heat exchangers connected to the circulation pump. In contrast, the newly designed 15MPa ammonia synthesis plant with an annual production capacity of 200,000 tons only has one heat exchanger: gas after water cooling – heat exchanger – ammonia cooling – ammonia separation – secondary ammonia cooling – secondary ammonia separation – heat exchanger – circulation pump. What are the advantages of this design? How do other manufacturers set up their systems? I would appreciate your insights. This post was last edited by tmny1 on 2008-1-15 08:22]
The use of a single heat exchange unit simplifies the process and facilitates its layout. Employing two heat exchange stages is advantageous for equipment manufacturing, as the temperature difference decreases, reducing the load on each individual device; this makes it easier to determine the size and materials required for the equipment
In the new design process, more heat is recovered compared to the first process, which reduces the load on water and ammonia cooling systems and thus leads to better energy savings. Additionally, there is one less heat exchanger, so the system resistance is lower than in the first design; this results in an optimized design.
The dual heat and cold exchanger configuration enables hierarchical heat and cold recovery; with the same total area, better recovery should be possible
Let’s discuss this with everyone: 1. If the evaporation pressure of gaseous ammonia is the same under one-ammonia cooling and two-ammonia cooling, then the temperatures of the hot and cold fluids at the second cooling stage will be essentially the same, meaning there is no need for a second cooling stage； 2. If the evaporation pressures of the two are not very different, and the condensation temperatures differ by less than 10°C, then the heat transfer temperature difference at the second cooling stage is too small, resulting in limited effectiveness in recovering cold energy ； 3. Adding another high-pressure low-temperature unit increases investment, raises system resistance, and leads to greater loss of cooling capacity ； It mainly depends on the condensation temperatures of single-ammonia cooling and double-ammonia cooling to determine whether double-cooling is worthwhile. Right? Please give me some advice.
1. The main function of cold exchange is to recover the cooling capacity of the system and reduce the cooling energy consumption per ton of ammonia； At the same time, the heat from the hot gas at the outlet of the cold gas recovery water cooler is transferred to the synthesis tower, thereby supplying heat to the catalyst bed and increasing the amount of by-product steam! 2. The size of the cold exchange area should be set reasonably; excessive recovery of cold energy results in a higher temperature of the cold air exiting the last stage of cold exchange, which increases the power consumption of the circulation machine and also affects its air delivery capacity ； With less cold energy recovery, the consumption of cold energy per unit increases! During engineering design, the safety factor for the heat exchange area of the cold exchanger is generally not more than 1.2 times (though this is not always the case; it depends on the empirical formulas used by the designer). Therefore, it is essential to design the cold exchanger properly in ammonia synthesis systems ； 3. The location and process settings within the system are also very important: a. In the first type of process, the technological route is long, the resistance is relatively high, and the engineering investment is also higher ； The second process route has a shorter length, lower resistance, and relatively lower engineering investment. Currently, for the design of low-pressure ammonia synthesis systems, the system resistance (at the inlet and outlet of the circulator) is generally required to be less than 0.8 MPa; therefore, option two for the process design is more appropriate ； b. In terms of project investment: Option 2 is also a more reasonable choice ； c. If the temperatures of the secondary ammonia cooling stages are both 0°C, the efficiency of ammonia separation and the ammonia content in the gas after separation are the same, as is the energy consumption per unit of cooling capacity for both cases! It’s just that the ammonia pressure in the primary ammonia cooler of Option 1 is relatively high (with an outlet temperature of around 8–10°C) ； In the second option, the pressure of gaseous ammonia in the primary ammonia cooler is low (with the outlet temperature controlled at 5°C to 8°C). If the gas temperature entering the first and second stages of ammonia separation is the same, the energy consumption per unit of cooling capacity is identical for both processes ； In terms of heat consumption, it’s similar in principle! d. As for the process to be adopted, the design is primarily determined by the ammonia pressure level that the plant can provide. 4. However, with the development of ammonia synthesis technology at home and abroad in recent years, for achieving the same energy-saving results, low investment, a simple process, and easy operation are the preferred options! Therefore, I support the second process technical solution!
The level of cooling capacity consumption depends primarily on the inlet and outlet temperatures of the hot and cold gases at the first heat exchanger. Through calculations, it can be seen that regardless of which of the two process options is used, as long as the gas temperatures at the inlet and outlet of the first heat exchanger are the same, and the temperature of the ammonia in the final stage is the same, the cooling capacity consumption will remain identical.
Regarding the design for cold energy recovery, there are now ammonia coolers and heat exchangers that offer the highest rate of cold energy recovery. In such systems, the primary and secondary ammonia coolers are connected in series, with a horizontal separator installed at the outlet of the secondary ammonia cooler. This reduces the number of connection pipes required. Meanwhile, the heat exchanger is placed inside both ammonia coolers, in the form of coiled heat exchange tubes: the inner tube contains the gas after ammonia separation, the annular space holds the syngas, and the outer side is filled with liquid ammonia as the refrigerant. In this way, the syngas is cooled by two layers of cooling effects, resulting in **reduced cold energy loss. However, manufacturing this equipment is difficult, and the equipment itself is quite long.
Ours is a heat exchanger – water cooling – heat exchanger – first ammonia cooling – second ammonia cooling – ammonia separation
A heat exchanger has a limited heat exchange capacity. In older systems, it may have been that the design did not take into account the need for heat exchangers with high heat exchange capacities; as a result, two heat exchangers were used in the same circuit. When the new design approach was introduced, heat exchanger production technology must have improved; one unit was sufficient, which allowed for **cost and space savings.
Reply to floor 11: Heat exchanger -- water cooling -- heat exchanger -- first ammonia cooling -- second ammonia cooling -- ammonia separation. This is the old process route; is there a new synthesis route available now? Is it a bit expensive to install 2 ammonia coolers as well?
There is also a water cooler in front of the cold exchanger in our factory; is it the same as what was mentioned upstairs?
The first process increases equipment investment and system resistance, with no significant difference in terms of energy savings.