Mitsui Toatsu Chemicals, Incorporated

Toyo Engineering Corporation

Ricoh Company, Ltd.

Shigeru Inoue

Hiroshi Ono

Akito Fukui

Hidetsugu Fujii

Haruyuki Morikawa

Keizo Konoki

Tetsuo Kimura

Michio Nobue

Eiji Otsuka

Yasuhiro Tani

Toshiyuki Enomoto

Kazumichi Kanai

Toshiki Kato

Tadao Shirasu

Katsumi Kagechika

Hisashi Miyagawa

Suguru Watanabe

Toshinori Takae

Morio Tsuji

Shinji Yoshimura

A processing tool is used to carry out a fixed-abrasive grinding process on a surface of a silicon work-piece. The processing tool includes abrasive grains made up silica grains. A primary average grain size of the silica grains is desirably 0.8 nm to 10 &mgr;m.

Processing tool, method of producing tool, processing method and processing apparatus

Disclosed herein is a process of synthesizing urea including reacting ammonia and carbon dioxide at a urea synthesis pressure and temperature in a urea synthesis zone, separating excess ammonia and unreacted ammonium carbamate from the thus-obtained urea synthesis melt as a gaseous mixture containing ammonia and carbon dioxide, recirculating the gaseous mixture to the urea synthesis zone, and, on the other hand, obtaining urea from an aqueous urea solution which has been obtained by separating the excess ammonia and unreacted ammonium carbamate. The above process features ingeniously combined conditions of various process steps. It produces urea using less high-pressure steam and recovers less low-pressure steam. A stripping operation making use of carbon dioxide can be effectively incorporated in the above process. The above process permits to cut the construction cost of a urea synthesis plant.

Process for synthesizing urea

The specification describes a process for synthesizing urea. The process comprises reacting ammonia and carbon dioxide in a molar ratio of 3:1-5:1 and at a pressure of 150-250 Kg/cm.sup.2 G, subjecting the resultant reaction mixture to a stripping step using gaseous carbon dioxide at a pressure substantially equal to the urea synthesis pressure and a temperature of 195.degree.-210.degree. C. to remove unreacted ammonia and unreacted carbon dioxide contained in the reaction mixture so that the content of unreacted ammonia is lowered to 10-15% by weight. This invention ensures a high conversion ratio from carbon dioxide to urea, considerably little formation of biuret during the stripping step, and reduced consumption of high pressure steam.

Synthesis of urea

In a synthesis of urea using ammonia in a highly excessive molar ratio, unreacted materials are decomposed and separated by subjecting the urea synthesis effluent to a stripping step using carbon dioxide at a pressure equal to the urea synthesis pressure. The thus-separated gaseous mixture of ammonia and carbon dioxide is condensed through an indirect heat exchange with an effluent stream discharged from the stripping step and lowered to a predetermined pressure level. Resulting condensation heat is used for the decomposition and separation of unreacted materials still remaining in said effluent stream. By choosing suitable operation conditions for each step of the present invention, it is possible to reduce the amount of high pressure steam to be required and to minimize the amount of low pressure steam to be recovered.

An inert gas containing carbon dioxide and ammonia and separated from a urea synthesis effluent under urea synthesis pressures is brought into contact with an absorbent along with unreacted carbon dioxide and ammonia separated in high pressure distillation at a pressure of from 10 to 25 kg/cm.sup.2 G to absorb substantially all of the carbon dioxide and the major portion of the ammonia, and is cooled to be separated from the resulting liquid ammonia and then is discharged. The inert gas can be treated with little or no danger of explosion by the above method.

A process for synthesizing urea in which a urea synthesis effluent obtained by reacting carbon dioxide and ammonia at urea synthesis pressures and temperatures is subjected to stripping treatment with carbon dioxide under pressures substantially equal to urea synthesis pressures to separate the unreacted carbon dioxide and ammonia contained in the urea synthesis effluent as a gaseous mixture, and a sufficient amount of said gaseous mixture to maintain the urea synthesis temperatures at a predetermined level is recycled to the urea synthesis in the gaseous state, the balance being subjected to condensation to be recycled in the liquid state to the urea synthesis.

In a urea synthesis process, the reaction product from the reaction vessel is flowed in series through high pressure, medium pressure and low pressure decomposing and stripping devices to decompose ammonium carbamate to NH.sub.3 gas, CO.sub.2 gas and water vapor, and to remove those gases and unreacted starting materials from the aqueous urea solution. CO.sub.2 gas is used as the stripping gas in the decomposing and stripping devices.

Process for preparation of urea

A process for treating water vapor generated in concentrating an aqueous urea solution wherein a urea synthesis effluent containing urea, unreacted ammonium carbamate and water from a urea synthesis zone is subjected to a plurality of decomposition stages, the pressures of which stages are stepwise reduced to decompose and separate substantially all of the unreacted ammonium carbamate from the aqueous urea solution. The aqueous urea solution which still contains small amounts of ammonia and carbon dioxide is concentrated to obtain crystalline urea or molten urea substantially free of water. The water vapor generated in concentrating said aqueous urea solution which contains small amounts of ammonia and carbon dioxide is cooled for condensation thereby forming a dilute aqueous ammonium carbamate solution which is subjected to rectification under a gauge pressure below 25 kg/cm.sup.2 to distil off a gaseous mixture of water vapor, ammonia and carbon dioxide, introducing the gaseous mixture into the unreacted ammonium carbamate decomposition stage using rectification under substantially the same pressure as the pressure of the rectification for the dilute aqueous ammonium carbamate solution to remove the water vapor from the gaseous mixture, and recovering the resultant ammonia and carbon dioxide together with a gaseous mixture of ammonia and carbon dioxide which is generated in the unreacted ammonium carbamate decomposition stage.

Treatment of water vapor generated in concentrating an aqueous urea

A reaction apparatus for corrosive material includes a heat exchanger that is coupled to a header. A first inlet is provided for fluid raw material and a second inlet is provided for introducing a non-corrosive material. There is also an outlet for reaction products. A tube baffle is positioned across the interior of the heat exchanger in order to define an inner space therein and an inner member which substantially conforms to the configuration of the inner space is positioned therein in closely spaced relationship with the inside surface of the header. A first plurality of tubes are attached to opening formed in the baffle and a second plurality of tubes are attached to the inner member and are positioned correspondingly to the first tubes in spaced relation thereto and in fluid communication therewith.

Chemical apparatus for corrosive materials

A process is described for preparing guanidine by separating guanidine from a process stream obtained in a urea synthesis process, wherein ammonia and carbon dioxide are reacted at a high temperature and a high pressure. The process stream includes a urea synthesis effluent from a urea synthesis zone, a solution obtained by separating unreacted materials from the effluent, a concentrate of the solution or a mother liquor derived from a urea crystallization zone.

Process for preparing guanidine

A urea synthesis process for converting ammonia and carbon dioxide to urea is improved by providing an easy means of maintaining the urea synthesis zone in the process at a constant temperature. In the process the starting CO.sub.2 and up to and including 100 percent of the starting NH.sub.3 are reacted in a heat-recovery zone maintained at a urea synthesis pressure. Some of the heat of reaction is removed. The molar ratio of NH.sub.3 to CO.sub.2 which is fed into the heat-recovery zone is less than 4. The reaction mixture and the rest of the starting NH.sub.3 are fed into a urea synthesis zone maintained at urea synthesis pressure to produce urea. The improvement involves adjusting the amount of starting ammonia which is fed into the urea synthesis zone in response to any change in the temperature in the urea synthesis zone so that the urea synthesis zone is maintained at a substantially fixed temperature.

Process for controlling the temperature during urea synthesis

A urea synthesis effluent is subjected to at least one high pressure decomposition stage and then a low pressure decomposition stage to decompose and recover unreacted materials contained in said effluent for recycle to the urea synthesis. The urea synthesis effluent discharged from a high pressure decomposition stage is cooled to 105.degree. - 170.degree. C. by indirect heat exchange with the urea synthesis effluent in the rectification zone of the low pressure decomposition stage and then is flashed into the top of said rectification zone, whereby the bottom of said rectification zone is heated to 100.degree. - 140.degree. C. and the temperature at the top of said rectification zone is maintained at 60.degree. - 120.degree. C. to minimize the water content in the distillate from the rectification zone.

Process for separating and recovering unreacted materials in urea

Carbon dioxide is reacted with a stoichiometric excess of ammonia at urea synthesis temperatures and pressures in a urea synthesis zone with the mol ratio of ammonia to carbon dioxide being in the range of from 5:1 to 12:1. The urea synthesis effluent from the urea synthesis zone is pressurized to a pressure higher than the urea synthesis pressure, and heated to a temperature higher than the urea synthesis temperature in a separation zone, wherein unreacted ammonium carbamate and excess ammonia contained in said urea synthesis effluent are separated from urea synthesis effluent in the form of a gaseous mixture of ammonia and carbon dioxide. The thus separated gaseous mixture of ammonia and carbon dioxide is recycled to said urea synthesis zone by means of the pressure difference.

Urea synthesis effluent obtained by reacting carbon dioxide with ammonia is subjected to a plurality of decomposition stages, e.g., three stages, to decompose and recover the unreacted ammonium carbamate with stepwise reduction of pressure, and the off-gas is absorbed in an absorbent in each stage and is recovered. The aqueous urea solution from the final decomposition stage is concentrated, water vapor generated upon said concentration is condensed, and the condensate containing small amounts of ammonia and carbon dioxide is stripped with steam recovered from the high pressure absorption zone. The steam discharged from the stripping step is introduced into the final decomposition stage to directly heat the urea synthesis effluent.

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Kamakura, Japan

Shigeru Inoue