By Michael Goff
A large number of ammonia plants currently in operation are more than 50 years old. Given the technological advances over the last couple of decades, plant operators now have the opportunity to implement new technologies to increase capacity and/or increase energy efficiency.
In most cases, when revamping a plant to increase production rate, the energy efficiency of the plant is also improved due to more efficient processes. Even if a full revamp isn’t in the immediate plans, performing a revamp study can prove valuable, particularly when it comes to identifying plant bottlenecks. The study can also help inform a roadmap towards planning future replacement of equipment that may be nearing end of life.
As plant production has increased over the years, the primary reformer is fired more than the initial design. This results in more flue gas, at higher temperature, going to the convection section. With the existing convection section tubes, the stack temperature goes up and overall plant efficiency decreases. There are multiple methods of modifying the existing convection section to extract more heat from the flue gas and bring the stack gas temperature down, significantly decreasing energy consumption. Possibilities for revamp include heating combustion air, feed gas saturator, modifying tube metallurgy, tube spacing and adding more tube rows.
A pre-reformer can be placed upstream of the primary reformer to reform any components heavier than methane and perform about 10 percent of the duty. Adding a pre-reformer can result in significant energy savings because it allows a lower steam to carbon ratio to be used in the primary reformer, reducing the concern of carbon formation on the primary reformer catalyst.
Medium Pressure Condensate Stripper
The process condensate in many older plants is still stripped at low pressure, which results in the steam either going into atmosphere or being condensed at a low pressure to collect the water. This results in energy loss and increased load on the cooling tower. In a medium-pressure condensate stripper (MPCS), medium pressure process steam is used to strip the process condensate, with the steam from the top of the stripper containing small amounts of CO2, NH3 and methanol going to the reformer to be converted back to syngas. The improved energy efficiency results in lower energy consumption and reduced emissions since less VOCs are emitted into the atmosphere. The typical energy efficiency improvement by the addition of MPCS is about 0.3-0.5 MMBTU/ton. An alternative to a medium-pressure condensate stripper is the addition of a feed gas saturator that accomplishes the same reduction in emissions.
Energy required in the CO2 removal section of the plant is significant in terms of heat required to regenerate the solvent and energy required to drive the circulation pumps. The two main processes used for CO2 removal are hot potassium carbonate and amine-based solutions. The regeneration heat between the two are similar, but amine-based systems require a lower solvent circulation rate and have less CO2 in the absorber overheads.
Some of the common methods of reducing the energy consumption in the CO2 section of the plant include converting from single stage to two stage regeneration for better heat integration, changing from single stage flash drum with multi-stage flash drum with steam ejectors, adding a low pressure stripper, adding a hydraulic turbine on the high pressure solution going to the stripper, and changing packing in the absorber and stripper to improve performance and reduce pressure drop.
Pressure Drop/Syngas Compressor Capacity
As production capacity increases, the pressure drop in the front end of the ammonia plant increases as well. There are several opportunities to reduce this pressure drop to reduce the load on the syngas compressor and increase capacity – some of the more common methods include larger radiant tubes, parallel low temperature shift, new internals in the shift reactors, adding an ammonia wash instead of mole sieve drying, and adding chillers on the suction of some of the syngas compressor stages. There are many more options available for reducing the load on the syngas compressor, and these options will become more apparent once a good pressure survey of the ammonia plant is performed.
Original ammonia converters had a relatively low concentration of ammonia out of the converter, resulting in large recycle rates and higher refrigeration duties. Increasing the concentration out of the converter reduces the recycle rate and load on the syngas compressor while also shifting more of the ammonia condensing duty to cooling water instead of refrigeration. The higher ammonia concentration is the reason why the original refrigeration compressor is usually still capable of meeting the refrigeration requirements even though the production capacity has increased substantially. Most of the additional ammonia is condensed with cooling water and the increased conversion per pass results in a recycle rate similar to the original design.
There are three methods commonly employed to increase the conversion out of the ammonia converter: 1) replacing the original basket design; 2) adding another converter vessel downstream of the existing; and 3) replacement of the original converter with new internals. Almost all plants have updated the converter technology at some point, but there are usually still opportunities to increase performance with the existing vessel.
Many plants are considering replacing their original converters due to concerns over the integrity of the metallurgy over many years and upgrading to materials more resistant to high temperature hydrogen attack. A new larger converter may allow for a lower loop operating pressure that reduces energy consumption and/or allows the current syngas compressor to meet future operating demands.
The overall plant steam balance offers opportunities for improvement, as operating parameters and production capacity of the ammonia plant has evolved over the years along with changing product slates associated with urea, nitric acid and other products. There could be opportunities for significant energy savings by converting a motor drive to a steam turbine drive, or vice versa. Other times it may be advantageous to replace a letdown steam turbine with a condensing turbine. Upgrading existing steam turbines, specifically the syngas turbine, can result in better efficiency and increased power output. In all revamps, it is important to get a good base case and revamp steam balance that considers all operating cases.
Plant operators have many options and parameters that need to be evaluated to meet the objectives of the revamp. To define the best revamp options, a study should be performed to develop several schemes that can be compared against one other on capital cost, operating costs, implementation times, constructability and operability. The process of evaluating the revamp options should be done in close collaboration with plant personnel to achieve the best outcome.
Are you ready to make the investment but struggling with the many options and parameters that need to be evaluated to meet the objectives of the revamp? Let the professionals at Black & Veatch help to define the best revamp options and simply the decision-making process. Black & Veatch can perform a study to develop several schemes that can be compared against one other on capital cost, operating costs, implementation times, constructability and operability. The process of evaluating the revamp options should be done in close collaboration with plant personnel to achieve the best outcome.
Visit bv.com/oil-gas to learn more or email one of our experts today to discuss project development needs.