Stamicarbon was the original pioneer of the stripping concept, in which one of the input materials - carbon dioxide - is used to strip free ammonia from the urea synthesis reactor effluent, promoting decomposition of its residual ammonium carbamate content and allowing it to be reconstituted at the full system pressure. This had a profoundly b eneficial effect on the economics of urea production by simplifying and reducing the cost of the plant and raising both energy and material conversion efficiencies. Today Today all the most modern urea processes p rocesses embody the stripping principle. Because of the high ammonia and a nd carbon dioxide conversions in the synthesis section of a Stamicarbon carbon dioxide stripping plant, there is no need for a medium-pressure recirculation stage downstream of the high-pressure stripper. In the high-pressure stripper, which is essentially a shell-and-tube heat exchanger, the incoming inco ming carbon dioxide feed flows counter-current to the urea solution leaving the reactor. On the shell side, the high-pressure stripper is heated he ated with steam. The off-gas of the high-pressure stripper, containing the feed carbon dioxide along with additional carbon dioxide and ammonia from the dissociated carbamate, is then fed into the carbamate condenser. Thans to the low ammonia and carbon dioxide concentrations in the stripped urea solution, the Stamicarbon !O" stripping process is the only process that re#uires $ust a single low-pressure recirculation stage. https%&&www.stamicarbon.com&co"-stripping-process
Conventional CO2 stripping process This conventional process re#uires the same e#uipment as the pool condenser process except the pool condenser. The The conventional high pressure carbamate condenser is a vertical shell and tube heat exchanger. Since it does not provide sufficient volume for taing over a part of the final reaction step building urea, the reactor volume needs to be bigger than for the pool condenser process. Thus, the conventional process is applicable for small and medium si'e si'e plant capacities only. !lic to enlarge
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Urea Process
CO2 stripper process is unmatchable in its efciency because it uses carbon dioxide as stripping-agent. As shown in the diagram, the process is allowing excess unconverted ammonia rom the synthesis sector to be recycled as carbamate in ust one silgle stage. !n this recirculation section the unconverted "#$ and CO2 are removed rom the main product stream, condensed into carbamate again and recycled to the synthesis sector using a high pressure carbamate pump. %he eedstodt consumption &gures are almost e'ual to the stoichiometric values or ammoni and carbon dioxide, leaving no room or urther reduction. %he e(uent and emission values are extremely low and meet the environmental re'uirements o most countries.
Synthesis: 3 )ool condenser. 3 )ool reactor 3 !O" stripping process with falling-film type carbamate condenser. The pool condenser is basicalty a hori'ontal reactor with a submerged 4-tube bundle. It combines the function of falling-film type carbamate condenser in the conventional !O" stripping process with part ollie reactor function. / 5 - 65 7 reduction in the reactor volume is achieved by shifting the reaction voume to the pool condenser. This is of particular advantage for high-capacity single-train plants since the reactor is one of the heaviest items of e#uipment. The pool condenser represents the state-of8the-art concept in modern urea plants. The advantages of new technology with pool condenser are% 3 Investment cost savings due to the si'e reduction of high pressure items. 3 Operational advantages drawn from recent ex perience with a pool condenser within a synthesis loop, which include
- more stable level&pressure control - less sensitivity to changes in the load or the 9&! ratio. 3 / reduced construction height, resulting in reduced construction costs. In all !O" stripping processes, ammonia and carbon dioxide are fed directly to the synthesis section. Optimum process conditions of approx. 065 bar and 0:5;! are maintained. !arbon dioxide, to which a small #uantity of air is added to prevent corrosion, is compressed to s ynthesis pressure in a multistage compressor, while the ammonia pressure is raised by a high-pressure pump. The high-pressure sections of recently-built urea plants are made of Safurex<, a duplex steel specially developed for the Stamicarbon urea process by Sandvi, Sweden. This steel offers excellent corrosion resistance and high tenacity and permits a reduction in the amount of added passivation air. =ydrogen is removed from the fresh carbon dioxide feed stream in the =" removal reactor located between the compression stages. The exothermic condensation to ammonia carbamate as well as the endothermic dehydration of the carbamate to urea and water taes place in the synthesis section. The reaction described results in a chemical e#uilibrium> part of the ammonia and carbon dioxide is not converted to urea and water. ?or this reason, the reaction mixture is sub$ected to a stripping process, using carbon dioxide to strip off the unreacted ammonia. This design feature is highly effective because of its low energy re#uirement and retention of unconverted reactants in the synthesis section. The stripper offgases are introduced into the high-pressure pool condenser together with the carbamate solution from the high-pressure scrubber and fresh ammonia. The heat released b y the formation of carbamate in the high-pressure pool condenser is recovered to generate low-pressure steam. Subse#uently, the mixture of gas and li#uid flows into the urea reactor in which the main urea formation taes place. The li#uid reaction mixture which leaves the reactor via an overflow is introduced into the stripper top. The exhaust gases @inert gases, 9=, !O" and ="OA, which are separated from the li#uid at the reactor top, are scrubbed in the high-pressure scrubber with carbamate solution from the low-pressure recirculation section. Thus, most of the gases are recovered and returned to the pool condenser via the highpressure e$ector. The non-condensables withdrawn from the high pressure scrubber are scrubbed in a low-pressure absorber, thereby minimising ammonia emissions. Recirculation / Evaporation:
9ormally one recirculation stage is re#uired due to the low ammonia and carbon dioxide concentrations in the stripped urea solution. The ammonia and carbon dioxide still contained in the urea solution discharged by the stripper are recovered in this low-pressure stage. The ideal ratio of ammonia and carbon dioxide in the recovered gases means that dilution by the resulting ammonium carbamate solution is minimised despite the low pressure of around bar. /s a result, the carbamate pump re#uires a much lower capacity and less undesirable water is recycled to the synthesis section. The urea solution leaving the recirculation section is further concentrated in the evaporation section to meet the re#uirements of the granulation process. acuum evaporation
is chosen to minimise biuret formation. / urea solution tan is provided to collect the solution during the periodical cleaning period of the granulator. The entire process condensate is collected in the process condensate tan, from where it is sent to the desorption section. Desorption / Hydrolysis:
In the first desorption column, ammonia and carbon dioxide are expelled from the process condensate, which is then pumped to the hydrolysis column where any urea still present therein is dissociated. The liberated ammonia and carbon dioxide are desorbed in the second desorption column with the aid of steam. The gas leaving the first desorption column at the top is fed to the reflux condenser, where the 9=, !O" and ="O v apours are condensed, and the ma$ority is then pumped to the C) carbamate condenser. The rest is returned to the first desorption column. The process water leaving the second desorber meets even the most stringent statutory environmental re#uirements, allowing this waste water to be used as mae-up for the cooling water system or even as boiler feedwater. !onse#uently, there is no waste water effluent stream from this urea process. http%&&rotunds.com&4reaDen.html
