Dehydration Process for Organic Solid Material

The present invention relates to a dehydrating process and more particularly to a process for dehydrating porous organic solid materials such as brown coal which can be dehydrated under an atmosphere in which water cannot be evaporated.

Hithertofore, dehydration of solid material such as brown coal has been carried out by an evaporation process such as a dry air process or an indirect heating process. These processes, however, have disadvantages in that the heat consumption is excessive, that the material has to be crushed in advance and that the dust is produced upon dehydration. Further, fire is likely to occur in the dehydrated material.

In order to eliminate the above disadvantages, a saturated steam dehydration process has been developed. The process utilizes the liquid form dehydration phenomenon in which physical and chemical changes are produced in a porous solid material such as brown coal when it is heated under a non-evaporating atmosphere with the result that water in liquid form is removed from pores in the solid material. In actual practice, the process is carried out by using a plurality of pressure vessels in which batch processes are proceeded with phase differences between the vessels. In one of typical processes, the brown coal to be dehydrated is sieved to remove fine powders and put into the vessels with time differences. Each of the vessels is then closed and supplied with saturated steam to be heated thereby so that water in the coal is removed in liquid form to produce hot water which is a mixture of the water removed from the coal and the condensed steam. The hot water is then introduced into a hot water reservoir. The vessel is thereafter depressurized so that the residual water in the brown coal is further evaporated.

The liquid form dehydrating process is characterized in that lumps of organic solid material of a substantial size can be dehydrated as they are, however, there are problems when it is applied to a dehydration of fine particles because water is constrained in spaces between particles under a capillary action and may be drawn into the material when the vessel is depressurized. Further, the fine particles of the material may be carried by the hot water out of the vessel and may sometimes cause clogging of pipe lines. Thus, in the liquid form dehydrating process, the material is at first sieved as described previously to remove fine particles, however, even after the sieving step, fine particles may be produced when the material is thrown into the vessel.

Further, in the conventional process, there is a further problem in that the heat in the waste water cannot be sufficiently recovered. The hot water as produced in the first vessel in the heating step is at once stored in a reservoir and then introduced into the second vessel which is in a preheating step to apply the heat in the water to the material in the vessel. The steam in the first vessel is also introduced into the third vessel to heat the material therein. The water is introduced into the second vessel flows downward through the layer of the brown coal and a heat exchange is performed between the water and the coal before the water is exhausted from the vessel. In this step, it will be noted that the heat exchange time is insufficient so that the waste water has a substantial residual heat. In the preheating step, however, since the transfer of the heating medium, that is, the hot water and the steam, is carried out under a pressure difference between the pressure vessels, it is impossible to transfer the waste water to a further vessel for recovering the heat therein.

It is therefore an object of the present invention to provide a process for dehydrating porous organic solid material in which heat can be effectively recovered from the waste water.

Another object of the present invention is to provide a dehydrating process in which the material can be protected from being pulverized during introduction into the pressure vessel.

According to the present invention, the above and other objects can be accomplished by a process for dehydrating water-containing organic solid material which comprises steps of introducing the material into a pressure vessel, applying saturated steam to said vessel to heat the material so that water in the material is removed and mixed with condensate of the steam to be exhausted from the vessel as a waste water, said waste water being transferred to a pressure vessel in which the material is not charged yet, the material being then charged into the vessel to carry out the same dehydrating steps. In another mode of the present invention, the material may be introduced into the vessel together with the waste water.

According to the features of the present invention, a sufficient amount of heat exchange is performed between the material and the waste water so as to recover the heat in the waste water. Further, the waste water functions as a cushion for the material which is being thrown into the vessel so that the material can be protected from being pulverized. In a preferable aspect of the present invention, the waste water is introduced into another vessel which is already charged with material and closed to thereby preheat the material therein. Through this preheating process, the temperature of the waste water is decreased to approximately 100.degree. C. so that there will be no substantial heat loss when the waste water is introduced into an open vessel.

The above and other objects and features of the present invention will become apparent from the following descriptions of a preferred embodiment taking reference to the accompanying drawing which fragmentarily shows an example of the system for carrying out the process of the present invention.

Referring to the drawing, the dehydrating system includes a belt conveyor 19 for transferring a brown coal to be dehydrated to a sieving device 18. The sieving device 18 includes a primary sieve 2, a crusher 3 and a secondary sieve 4 and functions to remove fine particles which are unsuitable for the liquid form dehydration. Beneath the sieving device 18, there is provided a belt conveyor 7 which extends above a series of pressure vessels 8a, 8b and 8c. The number of the pressure vessels may of course be determined as desired to properly carry out the dehydrating steps. The conveyor 7 is provided with a series of trippers 9 which distribute the material on the conveyor 7 to the respective ones of the vessels 8a, 8b and 8c at appropriate times. Hoppers 25a, 25b and 25c may be provided between the conveyor 7 and the vessels 8a, 8b and 8c, respectively, for receiving the material from the conveyor 7 and supplying it respectively to the vessels 8a, 8b and 8c at desired times.

Referring specifically to the process carried out in the vessel 8a, the vessel which is in advance filled with hot water is at first charged with material to be dehydrated, such as brown coal and then closed and supplied with saturated steam which is passed from an external steam source 11 through a pipe 31a to the vessel 8a. The vessels 8a, 8b and 8c are connected at the top portions with the steam source 11 through pipes 31a, 31b and 31c. The material is thus heated and water in the material is removed in liquid form. The consumed steam is condensed in this process and mixed with the water from the material. The vessel 8a is provided with a water reservoir 10a which is connected with the vessel 8a through a pipe 21a. The hot water in the vessel 8a is therefore introduced into the reservoir 10a. The vessels 8b and 8c are also provided with water reservoirs 10b and 10c which are respectively connected with the vessels 8b and 8c through pipes 21b and 21c. After the dehydrating process, the vessel 8a is depressurized so that the residual water in the coal is evaporated. Thereafter, the dehydrated coal is taken out of the vessel 8a.

The pressure vessels 8a, 8b and 8c are connected with a pipe 13 respectively through pipes 32a, 32b and 32c which may be used when the dehydration in the vessel 8a is finished to transfer the steam in the vessel 8a to another vessel 8c for preheating the material in the vessel 8c. The reservoir 10a is connected through a pipe 12b with the next vessel 8b. Similarly, the reservoirs 10b and 10c are connected through a pipe 12c and a pipe (not shown) with the vessel 8c and a next vessel (not shown). The hot water in the reservoir 10a is therefore transferred through the pipe 12b to the vessel 8b by opening the valve in the pipe 12b when the liquid form dehydration in the vessel 8a is finished to thereby preheat the material in the vessel 8b. The water flows down the vessel 8b and is passed through the pipe 21b to the reservoir 10b. The reservoirs 10a, 10b and 10c are provided with discharge pipes 14a, 14b and 14c, respectively, which are connected with a pipe 15 opening to a waste water tank 16. The water introduced into the reservoir 10b as described above is therefore discharged through the pipes 14b and 15 into the tank 16.

The waste water in the tank 16 is drawn by a pump 17 to a head tank 29 which is connected with a pipe 33 which is in turn connected through pipes 20a, 20b and 20c with the vessels 8a, 8b and 8c. The waste water in the head tank 29 is introduced for example into the vessel 8c and then the material is thrown into the vessel 8c so that the heat in the water is given to the material. The water in the vessel 8c functions as a cushion for the material so that material is protected from being pulverized. Thereafter, the water is taken out of the vessel 8c into the reservoir 10c from which the water is discharged through the pipes 14c and 15 and then through a discharge pipe 18. Parallely with the process in the vessel 8c, dehydration process is being progressed in the vessel 8b in a similar manner as explained with reference to the vessel 8a. Thus, hot water is stored in the reservoir 10b. The hot water in the reservoir 10b is then transferred through the pipe 12c to the vessel 8c to preheat the material therein. The steam in the vessel 8b is also transferred to the vessel in the next stage. Similarly the hydration process is carried out in the vessel 8c.

It will thus be understood that according to the process as described above the heat in the waste water can be effectively recovered. Further, the water in the vessel protects the material from being pulverized when it is thrown into the vessel.

The line 33 from the head tank 29 may be connected through pipes 26a, 26b and 26c with the hoppers 25a, 25b and 25c, respectively, and through pipes 27a, 27b 27c with discharge chutes of the hoppers 25a, 25b and 25c so that the material may be thrown into the vessels 8a, 8b and 8c together with the waste water. With this process, it is also possible to protect the material from being pulverized.

As an example, brown coal from Australia is dehydrated in accordance with the process of the present invention. The net brown coal of 1 kg includes 2 kg of water and the liquid form dehydration process under 250.degree. C. and 40 atms of pressure causes a decrease in the water content to 0.6 kg. After depressurization, the water content in 1 kg of net brown coal is 0.25 kg. The steam consumption in a conventional process is 1.05 kg for the net brown coal of 1 kg. Thus, the calculated heat supply is 700 kcal for 1 kg of net brown coal. The waste water before recirculation is 2.8 kg for 1 kg of net brown coal if the amount of dissipated steam is neglected. The heat in the waste water of 100.degree. C. is 280 kcal for 1 kg of brown coal which is approximately 40% of the total heat supply. If the heat in the waste water is recovered so that the temperature is decreased to 50.degree. C., it is possible to attain an improvement thermal efficiency by about 20% so that the steam consumption is decreased to 0.875 kg for 1 kg of net brown coal.

It should of course be noted that the process of the present invention is not limited to an application to a dehydration of brown coal but it may be applied to any porous organic solid material. Thus, it should be understood that the invention is not limited to the details of the examples as described but changes and modifications may be made without departing from the scope of the appended claims.