Nitric Acid Plants- The old, the new and the revamped
The design capacity of nitric acid plants continuously increased over the last decades. To stay competitive, operators of old nitric acid plants, which were designed for a lower capacity compared to today’s plants, are trying to run their plant as efficient as possible to be competitive. However since the equipment gets older and might require a refurbishment many operators of old nitric acid plants have to answer themselves the question to invest and improve or not to invest and maintain the current situation.
The answer to that question highly depends on the technical possibilities of the existing equipment:
- replacement of old equipment,
- re-vamping combined with capacity increase or
- installation of new and state-of-the-art equipment.
Replacement of old equipment reduces investment costs, but does not solve the problem with low production capacity. Re-vamp is an attractive option which balances increased production capacity and reasonable investment costs. Installation of new equipment increases the capacity at the level of modern state-of-art plants, but is related with higher investment costs.
These options get more important when considering a key component of the process – the nitric acid boiler. The decision for a single equipment of the process chain might easily be made, but a solution for the complete waste heat recovery system can only be achieved by serious assessments and substantial checks since all pieces of the process and apparatus chain have to work together as an ensemble.
Choosing the right option depends on the answers of the following questions:
- What are hurdles and bottlenecks?
- How to overcome them?
- What actions are time and / or cost consuming?
ARVOS GmbH | SCHMIDTSCHE SCHACK of Germany, previously known as ALSTOM Power Energy Recovery GmbH, is technology leader, designer and fabricator of relevant high duty heat exchanger components and systems for chemical processes. For fertilizer production processes process gas cooling systems are applied for ammonia processes and nitric acid generation. It attributes the impact of a nitric acid plant capacity increase up to 120% to the re-vamp of the waste heat recovery package. The major focus is on the identification and discussion of bottlenecks since each nitric acid plant is more or less unique and design solutions have to be tailor-made.
Use case capacity increase from 1500 MTPD to 1800 MTPD: applicability and necessary modifications of related hardware
A typical waste heat boiler system consists of a nitric acid boiler (also called process gas cooler), steam drum, flash tank, blow down tank, steam separator and corresponding piping. An example of such a system is shown in the process diagram in figure 1.
Step 1: A kind of inventory survey
It is not sufficient to assess the design and construction drawings of the waste heat recovery system in order to identify hurdles and bottlenecks. During the long operating time of the nitric acid plant a number of minor or major changes at the nitric acid boiler as well as at the package might be performed. Those changes may have an unexpected and undesired impact onto the whole modifications which are necessary to increase the capacity. For that reason a complete survey of the existing package is mandatory to document the present state of equipment and the System.
Step 2: Analysis of the gas side - heat exchanger and related hardware
Gas outlet temperature of waste heat boiler
The increase of production capacity by 20% firstly leads to an increase of volume flow rate to the boiler. Without any modification the temperature profile along process gas cooler increases since the heating surface is no longer sufficient. Depending on the original design conditions this will have a negative impact onto the mechanical integrity in case that the design temperature of the shell might be exceeded. The corresponding design temperatures are limiting the maximal possible capacity increase. To overcome this hurdle, the shell and/or existing heating surfaces have to be replaced. This measure is comparable with the installation of new equipment and is related with high capital cost, which might not be an option.
In case that the operating temperature after capacity increase is still lower than the original design temperature, this margin is reduced compared to the original design. This aspect gets more important since the existing equipment is aged and material properties might have changed over the operating period.
In addition to the increased process gas outlet temperature the higher process gas flow rate impacts the downstream process and changes the effect of this equipment (flue gas treatment (DeNOx system, i.e.), economiser, etc.).
The higher process gas cooler outlet temperature has also an impact onto the downstream economizer. Due to the higher temperature the boiler feed water is more pre-heated and in consequence more steam is exported by the steam drum to the steam super-heater module. The higher steam flow impacts the heat transfer in the super-heater.
To reduce or compensate those effects mentioned above, additional heating surfaces have to be installed in the nitric acid boiler. This measure enables effective recovery of extra heat due to the increased flow rate. This is the most efficient possibility with limited cost impact. The design approach of ARVOS GmbH | SCHMIDTSCHE SCHACK is to keep the new heating surface separated from the existing ones. This concept has the advantage to keep the original configuration.
A modification with an additional heating surface is shown in red colour in figure 2. The additional heating surface ensures flexibility as needed by the operator and process. It enables to compensate a potential continuous fouling of the process gas cooler and a related increase in process gas outlet temperature during operation. This gets more important if the further downstream process requires a constant process gas temperature over the complete operational range and time, like necessary for a tertiary abatement system.
Pressure drop on gas side of process gas cooler and economiser
Increasing the flow rate from 100% to 120% is directly related to the increase of pressure drop across the catalyst basket system and the heating surfaces by 44%pts. This higher pressure drop might be acceptable from a process point of view. However, this has also impact onto the mechanical structure of the process gas cooler. The total load of the support system consists of the weight of the hardware (catalyst basket, heating surfaces) and the forces introduced by the pressure drop. The increase in pressure drop is directly related to an increase in the pressure induced forces, whereas the weight of the hardware remains constant after modification. ARVOS GmbH | SCHMIDTSCHE SCHACK experienced that the support system has either design margins which can be used for the higher mechanical load or the support system can be reinforced. In case that none of the two options lead to a sufficient solution a secondary N2O abatement-catalyst located in the catalyst basket can be eliminated or replaced and incorporated into the downstream process, which reduces the weight and corresponding pressure drops caused by N2O-catalyst.
A very important contributor to maximum production of nitric acid and optimal operation of the process gas cooler is the uniform inflow of ammonia/air mixture onto the platinum rhodium catalyst grid. Large deviations in the flow pattern from the uniform distribution lead to reduced nitric acid production, and moreover to an inefficient usage of the expensive platinum rhodium catalyst grid. The increase of flow capacity has an impact onto the flow field. The geometry of process gas inlet nozzle to the burner hood diameter is diverging channel in which the flow will be decelerated. If the flow will be decelerated too much, it tends to flow separations which cause vortices, turbulence and inhomogeneous flow distribution. This effect will get worse with increased flow rate.
To assess the impact of higher flow rate onto the flow distribution to the catalyst grid, ARVOS GmbH | SCHMIDTSCHE SCHACK performs three dimensional Computational Fluid Dynamics simulations. The numerical model considers all relevant installations which might contribute to a disturbed flow field. Based on the results of this assessment the current geometry is feasible for the increased flow rate or requires modifications of the internals. The analysis has to be performed in close cooperation with the process licensor since ARVOS GmbH | SCHMIDTSCHE SCHACK is not in the role to influence the process. This effort in the design phase is justified because the requirement for potential modifications to achieve a homogenous flow field can be assessed in an early project phase and not during commissioning of the process gas cooler.
In Figures 3 an example is presented of the flow velocity field at the catalyst grid before (left side) and after (right side) optimization of internals. It can be seen clearly that the original geometry leads to a non-uniform velocity distribution with high values in the outer region and low velocities in the center. This leads to an inefficient use of the catalyst grid. After small modifications of internals the flow field for the increased capacity is homogeneous.
Step 3: Analysis of the water side - heat exchanger related hardware
The higher flow rate on the gas side has also impact onto the water/steam side. Due to the flow rate increase by 20%pts. the transferred heat to the water/steam system rises by 15%pts to 20%pts depending on the upgrading of the heating surfaces. In consequence the steam production and the steam fraction increase which have also impact onto the flow rates of the related water circuit.
The heat increase in the water/steam system and the related density change directly impacts the circulating water flow rate. For that reason the circulation ratio, pressure drop, and stability of the water circuit between steam drum and process gas cooler have to be recalculated. The design target here is to maintain the water circulation rate on the same level as before the upgrade.
A reduced flow rate has a negative effect onto the cooling of the process tubes and the tube wall temperatures. In case that the existing pump has not sufficient margin, a new impeller for the circulation pump might be required to increase the flow rate.
The required Net Positive Suction Head of the circulation pump increases since the circulation flow rate rises. Thus, the driving power of the circulation pumps will be higher. To adjust the water flow into the different heating elements, new La Mont nozzles or / and upstream orifices may be necessary to optimize the hydraulic loop of the water circulation.
Boiler Feed Water pumps
Due to the higher steam production and the need for higher boiler feed water flow, the following critical factors have to be assessed:
- liquid phase condition at the outlet of the economiser
- available flowrate of boiler feed water and
- pressure at the inlet of the steam drum
In cooperation with the customer it has to be ensured that the boiler feed water inlet conditions into the steam drum are sufficient for the upgraded conditions.
The water velocity at the inlet of each evaporator water path is an indicator for tube wall temperatures and has to be greater than a minimum velocity to ensure sufficient cooling.
The steam/water mixture velocity at each evaporator outlet flow path has to be smaller than a maximal value to avoid erosion by steam bubbles. The same condition is valid for the velocity of the steam/water mixture of each line back into the steam drum.
Unbounded increase of those velocities leads to a rise in pressure drop and hence to a reduction of flow rate and a poor cooling effect. The latter means higher risk of dry-out and destruction of magnetite layer. A critical assessment has to be performed to avoid this Situation.
The higher steam production also impacts directly the steam space load of the drum. In case that the steam space load will be too high the steam quality decreases. The only possibility to decrease the steam space load to an acceptable level is to provide more space for the steam bubbles in the existing steam drum, which means to reduce the water level in the drum.
The reduced water level impacts the residence time or hold-up time. For the current upgrade case of 120% flow rate the hold-up time between normal level and shutdown level will be reduced by 10% (30 seconds). However, this measure is required to ensure the steam quality without any modifications at the steam drum.
Step 4: Analysis of instrumentation
In cooperation with the customer safety and code related requirements will be assessed. Examples are
- steam drum pressure safety valve set-points in combination with maximum steam production as well as the capacity and pressure of the valves
- measurement range / capacity of flow measurement in the water/steam circuit and the boiler feed water flow
- measurement range / capacity of pressure drop measurement Equipment
For a renewal of an old process gas cooler in combination with a plant capacity increase a cost effective option is to re-vamp the existing nitric acid boiler. However the preceding discussion clearly shows that not only the process gas cooler has to be assessed but also all interactions at gas and water side. The integration of additional heating coils is sufficient to operate the unit up to 120 % load and ensures full flexibility. In addition, the recommended actions have to be performed to overcome the identified bottlenecks. The above discussed use case also shows that this assessment will be performed in cooperation with the customer. Based on major design and construction experience of ARVOS GmbH | SCHMIDTSCHE SCHACK since 50 years, the company fleet of nitric acid process gas coolers comprises in total more than 1 million operating hours. This experience is basis for re-vamp applications at low risk. Several nitric acid plants have successfully been optimized and retrofitted on the basis of ARVOS | SCHMIDTSCHE SCHACK’s engineering studies.