Heat Exchanger in Ammonia Production

Heat Exchanger in Ammonia Production

High pressure heat exchange systems 
in Ammonia production processes

Part I

It is obvious, that the design of Ammonia plants is driven by as low as possible capital cost and a short payback period. This cost pressure is passed on to every process designer and equipment manufacturer. Apparently low hanging fruits exist to reduce the individual costs of equipment or control and safety systems. However, in some cases those cost reduction measures shift their impact into other sub systems and suppliers or also into the operation phase of an Ammonia plant, in which maintenance cost and reliability/availability gets more important.

The typical waste heat boiler package downstream the secondary reformer in Ammonia production consists of a horizontal process gas cooler (PGC) with a steam drum on top followed by a steam super heater (SSH) to improve the overall process efficiency. The PGC package operates in natural water / steam circulation.

In addition to the increasing plant capacity to benefit from the effect of economy of scale, the requirements of an increasing operation range from part-load to overload for the waste heat boiler package can also be observed, for which the equipment manufacturers have to guarantee the appropriate process temperatures. 

ARVOS | SCHMIDTSCHE SCHACK (previously known as ALSTOM Power Energy Recovery GmbH, founded 1910 in Kassel, Germany) as designer and fabricator of relevant high duty heat exchanger components and packages for the chemical, petrochemical, refining and metallurgical industry shares the impact and dependencies of special cost reduction measures on two focal Points: 

  1.  Temperature control of the waste heat boiler package
  2.  Process gas cooler design


Boiler package temperature control

Figure 1: P&ID of PGC package with simple temperature control (concept 1)

Due to downstream hardware requirements, the process gas outlet temperature of the SSH should not vary over the complete operation range. In addition, the super-heated steam outlet temperature shall be achieved to ensure the overall process efficiency.

In general, there are two different control concepts, which differ in hardware layout and operational behaviour.

The simple control philosophy is equipped with a gas outlet temperature control downstream the SSH as shown in figure 1. 

Figure 2: Temperature and by-pass mass flow over the operation range for simple temperature control (concept 1)

This temperature control activates the by-pass of the PGC to achieve the required process gas outlet temperature over the complete operation range. The SSH itself has no by-pass, which has a positive effect onto the capital and operational cost as well as on total availability of the package. Both heat exchangers are designed for the maximum mass flow and corresponding maximum heat load (and potentially required over design by customer). In case that the secondary reformer is operating in part-load with reduced gas mass flow, the by-pass of the PGC will be adjusted to the actual operating conditions, but for the SSH with no by-pass this leads to varying process temperatures depending on load conditions. Since the downstream process gas temperature has to be constant over the operation range, in consequence the gas inlet temperature to the SSH has to increase with decreasing process gas mass flow as shown in figure 2. This requires that the PGC by-pass flow increases in low part load range, which results in a lower steam production of the PGC and therefore a lower steam mass flow to be super-heated by the SSH. In consequence, the SSH steam outlet temperature increases with decreasing load.

From an operational point, the decreasing steam mass flow might be acceptable depending on the requirement of the steam consumers. But this control philosophy changes completely the mechanical design of the SSH, since now the part load temperatures at process and steam side are the leading factors for the design temperatures of the apparatus. As shown in figure 2, the temperatures increase by 15% at gas side and 25% at steam side compared to the nominal operation point. This leads to higher wall thickness and/or material changes for the gas and steam shell as well as for the heating surface with negative impact onto thermal stresses and apparatus weight. In addition, either the downstream steam system has to be designed according the highest steam temperature or the high steam temperature has to be cooled down again by a continuously operating spray water injection, which must use condensed steam without impureness to protect the downstream steam system. A further drawback is a large by-pass for the PGC, which directly impacts the PGC size and weight.

The benefits of this system are a simple control system and a low maintenance effort due to only one by-pass in the system. However, the disadvantages described above clearly compensate or even overcompensate this.

Figure 3: P&ID of PGC package with two temperature controls (concept 2)
Figure 4: Temperature and by-pass mass flow over the operation range for two temperature controls (concept 2)

The second concept, which is successfully installed by ARVOS | SCHMIDTSCHE SCHACK at several Ammonia plants since decades, comprises a temperature control of the PGC gas outlet, which activates the PGC by-pass. In addition, the SSH is equipped with a by-pass, which is activated by a steam outlet temperature control as shown in figure 3.

It can be seen clearly in figure 4, that with this concept all temperatures of process gas and steam can be kept constant over the complete operation range. In addition, the steam production of the PGC is nearly linear with the process gas mass flow and load, respectively.

The mechanical design temperature of the SSH at gas and steam side are lower compared to the concept mentioned above, which has beneficial impact onto shell wall thickness, material and thermal stresses. In addition, the downstream steam system requires also lower design temperatures and the installation of a spray water injection downstream the SSH can be performed as originally intended, namely as safe guard. The “price” to pay is only a second by-pass flap with impact onto maintenance effort and a second temperature control loop, but this is overbalanced clearly by the benefits described above. Furthermore this type of control philosophy can react much faster in case of process disturbances since there are no large volumes/masses between the temperature sensors and the by-pass actuators and both control loops are almost independent of each other.

For Part II follow us next week