First Casale Lem™ Melamine Plant in Operation
As demonstrated by CASALE, melamine can be produced starting from fresh ammonia and carbon dioxide directly fed from the ammonia plant and processed together with the off-gases coming from melamine synthesis in a dedicated off-gas treatment unit that produces only the required amount of urea to feed the melamine plant.
This was the original request of the Client (GSFC) in order to avoid the revamp of their existing urea plants.
GSFC (Gujarat State Fertilizers and Chemicals) is a public limited company located in Vadodara (India) and a leading manufacturer of nitrogenous and phosphatic fertilizers as well as other petrochemical products.
Around 2013 GSFC decided to expand its business with the construction of a new melamine plant.
After the evaluation of the melamine market in India, GSFC decided for a melamine plant capacity of 40000 tons per year (corresponding to 120 metric tons per day). Urea section shall produce the required quantity of urea to feed the new melamine plant and a small additional urea export (50000 tons per year, corresponding to 150 metric tons per day) to feed the existing LP melamine plants.
In the bid phase GSFC invited the major technological competitors in the field of melamine and urea. The services requested by GSFC includes: the license, the delivery of the basic engineering package, the supply of the proprietary items, the review of the detailed engineering and the assistance during commissioning and start-up.
At the end of the bid phase GSFC selected Casale as licensor for several reasons:
- better capital investment;
- single licensor, responsible for the design of both the melamine and urea sections;
- very low energy consumption, leading to the lowest operating costs.
In the configuration proposed by Casale, the urea synthesis section is fed with fresh ammonia and CO2 coming from the existing ammonia plant, together with the off-gases recycled from the melamine synthesis. The 99.7% urea melt produced is partly fed to the melamine synthesis of the new plant designed by Casale and partly exported to the existing LP melamine units. In the melamine synthesis section urea is converted into melamine. The melamine melt from the synthesis is then treated in the melamine LP section for removal of byproducts, crystallization, filtration, drying, final transportation, storage and bagging of the melamine product. The off-gases from the melamine synthesis are recycled to the urea synthesis section for further production of urea.
Typical issues of the integration between melamine and urea plant are:
- the excess water sent to the urea reactor through the melamine off-gases
- the lack of fresh CO2 available to the urea reactor as heat source.
In the melamine plant, the off-gases recycled to the urea synthesis section are at high pressure and water free. This goal is achieved with the installation of a scrubber that washes the off-gases coming from the first and second reactor, removing the entrained melamine that could cause major problems in the urea section. The off-gas washing is done by means of urea melt.
With reference to the off-gas treatment section designed by Casale, its peculiarity is the fact that urea is produced from ammonia and CO2 contained in the off-gases coming from melamine section together with the fresh feed from the existing ammonia plant. This leads to the reduction of the fresh CO2 available to the urea reactor and consequent reduction of the heat source for the urea synthesis reaction.
This issue is solved by Casale with the application of the split flow loop concept.
The fresh CO2, after stripping of the ammonia contained in the urea solution coming from the reactor, is splitted into two streams. Only the necessary amount for the heat balance of the reactor is sent to the urea reactor, while the remaining portion is fed to the off-gas condensation together with the melamine off-gases and a small amount of carbamate to help their condensation. The heat recovery from the condensation of the hot off-gases generates steam, optimizing the energy consumption of the plant. The condensed ammonium carbamate is collected in a carbamate receiver and then pumped to the urea reactor.
In the melamine LP section, the use of sodium hydroxide in place of ammonia to maintain an alkaline environment is a further contribution to the minimization of the energy consumption:
- steam consumption for ammonia stripping in the LP section is minimized;
- vacuum adiabatic crystallization of melamine (at lower temperature) is carried out, thus maximizing melamine recovery from the solution.
- By-products solubility in the mother liquor is higher, thus allowing reduction of mother liquor purge to the waste water treatment section (minimizing melamine losses and reducing waste water treatment section energy consumption).
In 2019 the first melamine plant with integrated urea production, designed according to Casale low energy melamine technology for GSFC has been commissioned and started-up and is now on stream.
The energy consumption of the melamine section is about 2.7 Gcal/t of melamine produced. This figure includes the steam recovery obtained from the condensation of the melamine off-gases (approximately 1.0 Gcal/t of melamine produced).
The overall specific raw material consumption of the integrated melamine-OGT sections is practically stoichiometric (considering that 150 MTPD of urea melt are exported to existing melamine plants):
- 1522 kg of ammonia per ton of melamine
- 2017 kg of CO2 per ton of melamine
The overall energy consumption of the integrated melamine-OGT section is about 7.6 Gcal per ton of melamine. This includes steam, natural gas and electricity.
All the liquid effluents (in terms of NH3 concentration, TKN, TDS) and all the gas emissions (in terms of NH3, dust and NOx concentration) are in line with the expectations and well below the environmental limits.
In conclusion, the combination of the low energy melamine and split flow loop offered by Casale gives the opportunity to produce melamine starting from ammonia and CO2 with the best integrated melamine-urea plant, leading to the lowest overall energy consumption and capital investment cost.
