AMMONIA AND UREA

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AMMONIA & UREA INDUSTRY

In ammonia industry, natural gas is converted into hydrogen. The hydrogen is then combined with nitrogen to produce ammonia via the Haber-Bosch process.
In urea industry, urea is manufactured by reacting urea and carbon dioxide to form ammonium carbamate, which eventually is decomposed to urea.

Ammonia Process

A typical modern ammonia-producing plant first converts natural gas, liquified petroleum gas, or petroleum naphtha into gaseous hydrogen. The method for producing hydrogen from hydrocarbons is known as steam reforming. The hydrogen is then combined with nitrogen to produce ammonia via the Haber-Bosch process. The plant used a four-case centrifugal compressor to compress the syngas to a pressure of 152 bar, as presented on Figure 1.

Figure 1 – Typical Ammonia production plant

Starting with a natural gas (CH4) feedstock, the first step in the process is to remove sulfur compounds to avoid catalysts deactivation. Steam is then added, and the mixture goes to two reformer reactors where hydrogen plus carbon monoxide are formed. Atmosferic air or Air separation plant is the source of the nitrogen required for the ammonia reaction. Before this stream can be converted to ammonia, the carbon monoxide need to be converted in HT and LT Shift converters to carbon dioxide and more hydrogen.

Carbon dioxide needs to be removed from the process, since it will ruin the catalyst in the ammonia reactor. Amine absorber or pressure swing adsorption (PSA) units are often used to strips out the CO2 from the stream. Any remaining carbon dioxide and/or carbon monoxide are converted to methane in the methanator reactor. Finally, the stream then enters the final ammonia syntesis reactor, where desired end-product ammonia is produced by reaction of hydrogen and nitrogen. This step is known as the ammonia synthesis loop, also referred to as the Haber-Bosch process.

Urea Process

Urea is manufactured by reacting liquid ammonia and carbon dioxide over the compression section to an autoclave (inside of urea synthesis reactor) to form ammonium carbamate. The chemical reaction is an endothermic reaction. Therefore, ammonia is maintained in excess to shift the equilibrium towards urea formation. Urea production is based on two main reactions: formation of ammonium carbamate and dehydration of ammonium carbamate to produce molten urea.

In distillation tower and associated flash drum this high-pressure slurry is flashed to 1 atm pressure and distilled to remove excess ammonia. Decomposed ammonia carbamate salts are then removed and recycled. The solution is fed to vacuum evaporator for concentrating the slurry and eventually it is dried in a prilling tower and passed from the top of the tower into a bucket as granules. These urea granules are sent by conveyor to the bagging section, as presented on Figure 2.

Figure 2 – Typical Urea production plant

Primary Process Control Improvements for Ammonia & Urea Processes

Prior to any advanced process control (APC) project, even in ammonia and urea processes, base-level PID tuning and optimization is a critical pre-requisite step. Unless base-level PID control loops are well tuned, advanced process control (APC) cannot work well, since advanced process control (APC) will be manipulating the setpoints of the base-level PID control loops. Therefore, the first necessary step in the overall process control improvement procedure for ammonia and urea processes is PID tuning and optimization of primary or base-level PID controllers.

The benefits of PID tuning and optimization in ammonia and urea processes is the reduction of the oscillation amplitude or increase of the controller action by a factor of 2 or 3. This allows to enable smoother running of the ammonia and urea processes with increased stability in all control loops avoiding unnecessary ammonia and urea processes problems such as: damage and/or to fast wear and tear of the equipment, plant irregular shutdowns or off spec product properties and/or grades.

PiControl Solutions LLC has extensive experience in PID tuning and optimization for ammonia and urea controllers in the ammonia and urea industry. We understand and know how to tackle with typical PID control loop problems and have customized PID tuning and optimization software tools to help optimize all ammonia and urea unit controllers.

Our unique and novel closed-loop system identification technology makes it possible to tune and optimize base-level PID control loops quickly, efficiently and precisely. With our closed-loop technology we can perform system identification and PID tuning optimization of the following critical base-level PID controllers easily. Moreover, all process and data analysis and PID tuning and optimization work can be easily performed remotely by PiControl Solutions process control engineers.

Advanced Process Control (APC) Improvements for Ammonia & Urea Processes

PiControl Solutions LLC has extensive experience in advanced process control optimization for ammonia and urea processes. We understand the economics factors that drive the profit margin and have customized multivariable closed-loop system identification and advanced process control (APC) design and optimization tools to help optimize and improve the ammonia and urea processes.

We focus to analyze the process and provide the right economic advanced control solution for each ammonia and urea process. The main opportunity for achieving APC (advanced process control) benefits is based on the fact that the ammonia and urea production process is not very interactive, it is rather serial process. Poor control, oscillations and deviations in one control loop can cause problems only in the downstream unit, but not upstream of the process since the process is very sequential.

Our DCS-based APC (advanced process control) methodology has proven particularly successful in this ammonia and urea area. Our DCS-based APC (advanced process control) design will result in the following ammonia and urea benefits:

There are a couple of key measurements that must be made to keep the ammonia plant operating efficiently:

The most critical measurement is the hydrogen to nytrogen ratio between the synthesis reactor and the compressor in the recycle stream. The ratio must be keep at the proper levels to get the maximum formation of ammonia at each pass.
Another important ratio to maintain is steam to hydrocarbon ratio. The ratio must be keep at the proper levels to get the maximum formation of hydrogen needful for ammonia production. Many times this is tracked by methane conversion on the exit stream of the secondary reformer.
To maximize operating efficiency of ammonia production unit it is also good to keep track of CO and CO2 production and separation.
The control of process inert gases such as argon, methane and others is important in order to maintain constant converter feed ratios, since much of the converter feed make up is a result of recycled converter gases after ammonia production.
Waste water is often high in ammonia production, therefore even in wastewater treatment unit (as a part of ammonia production) certain variables needs to be tightly controlled.
Improved operation of feed compressor
Improved operation of feed furnace
Improved heating operation of furnaces related to Primary/Secondary Reformers
Improved High/Low Temperature Shift Convertors (HTSC/LTSC)
Improved operation of Synthesis Compressor
Minimization of Ammonia flaring
Improved stability and efficiency of Steam production unit

There are a couple of key measurements that must be made to keep the urea plant operating efficiently:

Improved operation of Stripper, Carbamate Condenser and Urea reactor
Improved temperature control of a Flush Drum
Improvements of Evaporator unit operation considering % of biuret and moisture
Improved operation of a Prilling section
Improved operation of Vacuum section
Improvement of Scrubber operation
Improvement of Adsorber section operation
Improvement of Desorption and Hydrolysis units operation
Other potential benefits could be also done in Ad-Blue and NP sections

Maintaining tight control of these variables results in significant economic benefits to ammonia and urea production, as follows:

lower fuel costs
lower steam production
higher yields
overall optimized efficiency

After DCS work on advanced process control (APC) schemes is complete and all advanced process control (APC) parameters are calculated and optimized, PiControl Solutions LLC will over factory acceptance test (FAT) make sure that the advanced process control (APC) design is complete, correct and operable. After completion of process control (APC) control project, PiControl Solutions LLC will conduct dedicated process control training for ammonia company.

For ammonia and urea processes we typically ensure 2-4% increase in profits, smoother plant operation, automated loading and unloading and less work for the operators.

PiControl is an innovative process control software and services company that offers state of-the art, modern solutions at a lower cost, implemented in shorter time compared to competitors.

PiControl Solutions is the only process control and automation company in the world which can perform any advanced process control (APC) project completely remotely. Nowadays, every country uses high speed and reliable internet connection and with a help of medium to high resolution cheap web-cameras or even over widely used remote meeting and screen sharing applications it is possible to do complete design, tuning and optimization, FAT test and start-up of any advanced process control project. Over this low cost online/remote approach huge travel and accommodation costs can be saved and human health and safety can be kept on the high level.

For more information and details, please send us an email: [email protected] or call the Tel: (832) 495 6436.