The invention relates to a method of regenerative heat transfer from one flow to another based on the use of circulating fluidized bed reactors . The method can be employed to solve problems associated with conventional fixed-element, fixed-bed and fluidized-bed heat regenerators .

Heat transfer from one flow to another is one of the basic unit processes in process and energy technology. Currently, heat transfer is implemented chiefly using two heat exchanger types called recuperative or regenerative according to their operating principle.

In recuperative heat exchangers, heat transfer takes place via a nonper eable wall which isolates the flows from each other. In the basic type of a recuperative heat exchanger, thermal energy is conducted directly through the wall from one flow to another. A specific subgroup of recuperative heat exchangers is formed by the so-called intermediate circulation recuperators in which a heat transfer medium is recirculated between two recuperative heat exchangers. Such heat exchangers are employed in, e.g., nuclear power plants in which it is necessary to assure that the high-activity flow cannot mix with the secondary circulation in accident situations. Another exemplifying group of intermediate-circulation recupera¬ tors is formed by fluidized-bed boilers equipped with superheaters placed external to the combustion chamber; in these boilers the sand heated in the combustion chamber is cooled in a separate fluidized-bed super¬ heater. The chief limitations of recuperative heat exchangers are related to the erosion, corrosion and temperature of the heat exchanger vessel wall materials. Today, no practical wall materials are available for con¬ ditions exhibiting high mechanical or chemical stresses .

The highest allowable temperature in recuperators is often limited by the strength properties of the wall material. Moreover, recuperators are expensive and restricted in their control possibilities. Good control- lability can, however, be achieved in intermediate- circulation recuperators .

In regenerative heat exchangers, thermal energy is trans¬ ferred by way of allowing the heated heat-transferring medium to release energy into a colder flow and then reheating the cooled heat-transferring medium in a hotter flow. Regenerative heat exchangers are further divided into cyclically and continuously operating types on the basis of their operating principle.

In cyclically operating regenerators the hotter and the cooler flow are cyclically routed via a single solid structure which thus alternatingly stores and releases thermal energy. The batch-heated rock stove of a sauna is without doubt the oldest application of the cyclically operated regenerator.

In continuously operating regenerators the heat-storing medium is continually recirculated from one flow to another. The best-known type of continuously operating regenerator is the Ljungstrom regenerator in which a rot¬ ating, cylindrically shaped heat exchanger disc transfers thermal energy from one material flow to another. This regenerator type has been modified for different applica- tions such as, for example, the air-conditioning regen¬ erator which additionally provides moisture transfer on surfaces coated with lithium chloride paste.

Besides the regenerator types of the above-described kinds with a fixed-shape, contiguous heat-transferring element, regenerators based on granular heat transfer media are known in the art.

Several different types of regenerators are known having the granular heat transfer medium in the fixed-bed state and the heat transfer medium is then mechanically recirculated between the layers of the bed.

German patent DE 3,225,838 /A/ employs a granulated heat transfer medium (e.g., porcelain pellets) for heat trans¬ fer between the gas flows. The granular bed material is fluidized, whereby the pellets remain clean and clogging of the heat exchanger is avoided. US patent 4,307,773 discloses another type of process and apparatus in which a regenerator system based on bubbling fluidized bed layers is employed for heat recovery from contaminating gases. Besides the above-described patents, different types of regenerators are known based on alternate heating/cooling of granular material in separate, parallel, bubbling fluidized bed layers. UK patent 2,118,702 A discloses a regenerator based on downward dribbling fixed bed layers .

A central issue of regenerators based on a fixed heat transfer element and fixed layered zones of granular material is how to keep them clean. Also the prevention of flows from mixing with each other causes sealing prob- lems in these regenerators. Furthermore, the temperature differentials formed into the heat transfer material impose mechanical stresses which limit the life of the heat transfer element or material. A drawback of the layered fixed bed regenerator is the channelling of flows in the fixed bed layers. Moreover, the fixed bed layers obviously develop inevitable temperature gradients in the direction of the flow and the temperature of a layer is difficult to control.

In fluidized bed reactors the flow velocities must be adapted according to the physical properties of the heat transfer material employed, and the control range of the

regenerator is limited between the minimum fluidization velocity and the pneumatic transportation velocity. In practice this means that the heat transfer medium of the regenerator must have a coarse granular size, or alter- natively, the flow velocities employed must be kept low. Furthermore, the recirculation of the heat transfer medium between the fluidized bed layers in a manner avoiding excessive mixing of the layers is problematic. This problem is accentuated at high pressure differentials between the heat-transferring flows. Herein, it is generally necessary to use mechanical valves whose wear and temperature limitations eliminate an essential portion of the benefits of this regenerator type. Prior-art fluidized-bed and fixed-bed regenerators require the use of a mechanical or pneumatic conveyor for recycling the heat transfer medium from the lower unit to the upper unit. In terms of equipment and process technology, such conveyors are almost impossible to implement. In fact the recycling system of the granular heat transfer medium is a characterizing property of the present regenerator invention.

The above-described drawbacks are overcome in a crucial manner by way of the method according to the invention which is characterized in that the regenerator according to the invention employs circulating fluidized bed reactors rather than fixed-bed or fluidized bed reactors . The method is characterized by what is stated in the characterizing part of claim 1.

The chief benefits of the present invention over fixed- bed and fluidized-bed regenerators are listed as follows: 1. The flow and heat-transfer properties of the circulat¬ ing fluidized bed technique are decisively better than in any prior-art regenerator type. With the help of an ef¬ fective separator, a fine-grained heat transfer medium can be employed in circulating fluidized bed regenera-

tors, simultaneously permitting the dimensioning of the equipment for high flow velocities.

2. Owing to the high flow velocity, regenerators based on the circulating fluidized bed technology are relatively small and have lower investment costs with respect to prior-art regenerators .

3. Regenerators based on the circulating fluidized bed technology operate over a significantly wider part-load range than conventional regenerators based on fixed-bed or fluidized-bed technology.

4. Circulating fluidized bed regenerators can employ media with heterogeneous physical properties, thus facilitating, for instance, simultaneous use of two or more heat transfer materials of different physical and/or chemical properties.

5. The medium mixing tendency in parallel reactors can be prevented in regenerators based on the circulating fluidized bed technology by way of a simple arrangement of the recycle channel of the circulating medium that additionally permits a rapid and accurate control of the regenerator.

6. Owing to the efficient mixing which is characteristic of the circulating fluidized bed technology, excellent temperature control of the reactors is attained, which has a decisive importance in the handling of contaminat¬ ing flows.

7. The implementation of circulating fluidized bed reac¬ tors in the regenerator configuration avoids the use of separate mechanical or pneumatic conveyors for transfer- ring the circulating medium from one regenerator unit to the other. By contrast, regenerators based on bubbling fluidized beds or dribbling fixed bed layers require high volume rates of the pneumatic carrier gas conveyors in relation to the process gas volume rates, thus making the use of such conveyors impossible in most cases.

8. The suspension density in the rising chambers of circulating fluidized bed regenerators can be easily

controlled by complementing the external recycling loops of the circulating fluidized bed regenerators with a conventional internal recirculation of the solids.

In the following the invention is examined in greater detail with reference to the annexed drawing of Fig. 1.

Fig. 1 shows the basic construction of a regenerator according to the invention comprised of two circulating fluidized bed regenerators A and B, each comprised of a chamber 1, a cyclone 2 and a return channel 3, while the regenerators are connected as illustrated in Fig. 1. The flows participating in the heat transfer enter the regenerator via nozzles 9 and mix with the particles recirculated in the circulating fluidized bed reactor in the lower part of chambers 1. Here, the temperature dif¬ ferentials between the flow and the recirculated par¬ ticles is rapidly equalized and the flow in practice as¬ sumes the temperature of the isothermic chamber. The flow carries the particles upward through the chambers 1 into inlet channels Ϊ0 of cyclones 2, through which the sus¬ pended medium enters the cyclone chamber 2 tangentially. The particles are separated from the gas flow which next exits via a central pipe 11. The separated particles land on the bottom of the cyclone chamber 2, where the upper surfaces 12 are kept at a desired level . The levels of the surfaces 12 are determined so that the surface in one cyclone is adjusted to an accurately controlled level, while the surface level in the other cyclone varies within predetermined limits dictated by the amount of recirculated material and the running conditions . From the cyclones the recirculated material is routed into channels 3 and 4 as shown in Fig. 1. A portion of the recirculated material is directed via the channels 3 and a pneumatic valve 6 back to chamber 1. The internal recirculations of the reactors A and B can be used for controlling the amount of particles contained in the

chambers 1, whereby in comparison to a situation without internal recirculations, a lower chamber can be employed, for instance. The internal recirculations can also be utilized to assure the cleanliness of the regenerator chambers at low volume rates of the recirculating material between the chambers. From the bottom part of the channel 4, a portion of the recirculating particles of reactor A are transferred via a pneumatic valve 8 to reactor B. The operation of the pneumatic valves 6 and 8 is based on the control of the supporting reactive force at the lower part of the fixed bed layers with the help of a gas flow of very low volume rate. When the support¬ ing reactive force becomes smaller than the equilibrium limit force, the fixed bed commences gravitational fall downward. The benefits of the above-described operating principle over mechanical and pneumatic conveyors are, among others, a low wear rate and very low volume rate of required control flow. A column of recirculating par¬ ticles in fixed-bed state is maintained in the channels 3 and 4, and the column is kept gravitationally moving downward. The fixed bed layers 4 prevent the mixing of the flows A and B. Correspondingly, the fixed bed layers 3 prevent the gas flow from entering the cyclone through an incorrect path, though operating conditions in the channels 3 can also be kept acceptable for the regenera¬ tor function without the benefit of the fixed-bed state. The situation and operation of reactor B is fully analogous. The heat transfer between the flows A and B is controlled by adjusting the particle flow rate between the reactors .

The above description elucidates only the basic configur¬ ation of the regenerator according to the invention, and for those versed in the art it is evident that the dif- ferent embodiments of the invention can be varied within the scope and spirit of the described invention. For instance, the channel 4 can be advantageously tilted in

some cases, thus permitting horizontal transfer of the particles, yet preventing in the above-described manner the flows from mixing with each other. If a pressure differential between the units of the regenerator config- uration is very high, in some cases the channel 4 can be divided into several series-connected sections to avoid the regenerator height from becoming excessive. The con¬ figuration of two parallel connected reactors can be replaced by multiple parallel-connected reactors operat- ing at different temperature levels, whereby their mutual gas and solids flows can be connected in varied ways . Obviously, such reactor units can also be operated in a series connection. Using chemically active heat transfer processes, desired kinds of chemical processes can be implemented. The described regenerator can also be employed for implementing a chemical or physical process, particularly when an accurate temperature control is crucial for running the process .