This invention relates to a method and an apparatus for handling high pressure material, particularly to a method for reducing pressure of a high-pressure solids-gas mixture and for separating the solids from the solids-gas mixture in a vessel. The vessel is in communication with a reactor that is operated under a pressure of at least about 2 bars, and serves to maintain desired reactions, such as the combustion or gasification of fuel. The vessel serves to remove ash or other particulate material from the reactor.
The term "solids", as used in the specification and claims, refers to all residues known to a person skilled in the art that may be obtained from reactions taking place in pressurized reactors, such as circulating fluidized bed reactors.
A known method for reducing the pressure of gas-solids mixtures is disclosed in Knowlton, T. M.; Findlay, J. G.; and Chan, I., "Continuous Depressurization of Solids Using a Restricted Pipe Discharge System", Presented at the 1989 AIChe Annual Meeting, San Francisco, Calif., Nov. 5-10, 1989. In this reference, a method for de-pressurizing a gas-solids mixture by using a Restricted Pipe Discharge System ("RPDS") is disclosed. In the RPDS, solids are discharged in moving packed-bed flow from higher pressure to lower pressure through a pipe restricted at its outlet, while the gas is forced to flow through the packed bed of solids, thus causing de-pressurizing. In U.S. Pat. Nos. 2,684,868, 2,684,870, 2,684,872, and 2,684,873 methods for conveying or transporting of granular solids, which can be used in the RPDS, are disclosed in more detail.
Typically, the main set of components of the RPDS are depressurizing piping ("transporting pipe"), a de-entrainment vessel wherein a packed bed of solid material is maintained for acting as a solids flow restriction at the end of the transporting pipe, and means for controlling the flow of the solids from the de-entrainment vessel.
An RPDS may be successfully utilized, for example, in connection with Pressurized Fluidized Bed reactors such as Pressurized Fluidized Bed Combustors or Gasifiers and processes executed therein. For example, in a system where a Pressurized Fluidized Bed Gasifier, which provides a partial gasification of fuel, is connected to a Fluidized Bed Combustor for oxidizing the residue char from the gasification, and in a transportation system for conveying the solids resulting from a partial gasification, such as char, to the final combustion, the RPDS may be successfully employed. Any corresponding processes can be equipped with an RPDS as well.
In such systems, the reactions take place at elevated pressure, e.g., 2-20 bar, and at elevated temperature, preferably about 1200.degree. C., and residues are produced which should be subjected to further processing at lower pressure. The solids discharged from a Pressurized Fluidized Bed reactor may be introduced to the RPDS either with or without first cooling the solids. If cooling is utilized, the temperature of the solids may be lowered even to about 300.degree. C. temperature if the temperature of the solids-gas stream prior to introduction into the de-entrainment vessel is about 300.degree.-1200.degree. C. (e.g., about 300.degree.-799.degree. C.); otherwise, the temperature is still reduced to below that of the reactor, e.g., to about 800.degree.-1200.degree. C.
In the prior art, a necessary feature of the RPDS system, in order to achieve continuous movement of the solids with a simultaneous pressure drop of the gas, is to cause the gas to flow faster than the solids in the piping. Under these circumstances, the RPDS usually smoothly and reliably de-pressurizes the solids. It also has some advantages over a lockhopper as a depressurization device (e.g., the flow of the solids is continuous, and valve maintenance time and costs are decreased). However, if, e.g., the gas flow rate is increased to too great an extent, the whole de-pressurizing process is handicapped due to the fluidization of the solids in the de-entrainment vessel. The fluidization of the solids is detrimental to the function of the device since restriction to solids flow is destroyed, and the flow of the solids becomes uncontrollable. Therefore, it is very important to guard against fluidization of solids, to guarantee the smooth running of the entire process.