Design of an integrated melamine-urea process - ippe.com

4. NEW INTEGRATED UREA-MELAMINE PROCESSThe main goal of the development of an integrated urea-melamine process is the reductionof the energy and capital costs of the process. The main improvements compared with theconventional gas-phase melamine process are:o heat recovery from the byproducts ammonia and carbon dioxideo export of water-free byproduct gases to the urea plant without a concentration unito synthesis of melamine melt instead of melamine vaporo integrated purification of the melamine melt without a recrystallization unito water-free quench process producing directly dry powdero low cost ammonia recovery due to separation of byproduct gas before the quenchA block scheme of the integrated urea-melamine process is shown in figure 5. The process isoperated at high pressure (150 – 250 bar) in order to produce melamine melt instead ofmelamine vapor. Urea melt coming from the urea plant is preheated in the direct contactor by thehot (360 – 440 ° C) byproduct gases ammonia and carbon dioxide, which were produced in thereactor. This off-gas is simultaneously cooled to temperatures below 250 ° C and steam isproduced in the contactor. In this way heat from the byproduct gases is recovered and can beused in the process for preheating process streams or equipment. In the reactor the preheatedurea is converted into a melamine melt and ammonia and carbon dioxide gas using HeatTransfer Salt (HTS) as a heating medium. In the separator the melamine melt and the gases areseparated creating theNH 3 + CO 2 toUrea Steam urea plantContactorAmmoniaRecoveryReactor Separator Quench Melamineurea NH 3 +CO 2NH 3HTS NH 3Figure 5. Block diagram of the integrated high-pressure melamine process. The bold lines showthe heat flows; the thin lines show the process flows.capability to recover heat from the byproduct gas in the contactor. The melamine melt is purifiedin the separator by addition of ammonia gas. The hot ammonia gas used for the purification stepis also sent to the contactor for heat recovery. The purification occurs direclty in the melt and noseparate recrystallization step is required. Then the melt is sprayed and quenched with theevaporating liquid ammonia in the quench section. The amount of liquid ammonia is just enoughto cool the melamine to the required temperature and all ammonia is evaporated. Dry melaminepowder of high quality is obtained directly without a drying step. The evaporated ammonia fromthe quench is condensed in the ammonia recovery section and is re-used. The ammonia recoveryis much simpler than in the gas-phase process, since the byproduct gas has been separatedbefore the quench.2

An extra advantage of the new melamine process is that the off-gas coming from thecontactor is water-free and can be recycled directly to the urea plant without a concentration unitand without a negative impact on the urea conversion. If the melamine process is operated at apressure higher than the pressure of the urea plant a direct coupling (Meessen et al. [9] and vanWijck [10]) with the high-pressure section in a urea plant without condensation of the gases isfeasible. This results in a fully integrated urea-melamine process.Utilities (per metric ton of melamine):Natural gas:7 GJ (reactor)Steam:< 1 tonElectricity:< 0.4 MWhCooling water: < 400 tonTable 3. Utilities consumption of New Integrated Melamine processSafety precautions are an important issue during design and operation because of the highpressure and temperature. During start-up and shut-down the process is operated at reducedpressure to minimize risks in case of a process failure. The direct coupling requires asophisticated automatic pressure control system.Although the high-pressure section is expensive the total capital cost of the new process islower compared to the conventional process. The integrated high-pressure melamine processrequires only 40% of the energy consumption of a conventional gas-phase melamine processand has no negative impact on the urea conversion and stripping in the urea process. Moredetailed information on the utility consumption of the new integrated melamine process in givenin Table 3.5. SUMMARYThe main improvements of newly developed integrated urea-melamine process comparedwith the conventional gas-phase melamine process are:o heat recovery from the byproducts ammonia and carbon dioxideo export of water-free byproduct gases to the urea plant without a concentration unito synthesis of melamine melt instead of melamine vaporo integrated purification of the melamine melt without a recrystallization unito water-free quench process producing directly dry powdero low cost ammonia recovery due to separation of byproduct gas before the quenchAs a result of this the integrated high-pressure melamine process has lower investment costsand requires only 40% of the energy consumption of a conventional gas-phase melamineprocess. In addition the new melamine process produces high quality melamine.3