GB2122635A - Retorting process and apparatus - Google Patents

Abstract

In a process for retorting carbonaceous material, such as oil shale, by contact with hotter recycled heat-carrying bodies, such as ceramic balls, the critical weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material below which chipping and cracking of the hot recycled heat carrying bodies readily occurs is experimentally determined as a function of the temperature difference between the heat carrying bodies and the carbonaceous material and the weight ratio of the recycled heat-carrying bodies to the carbonaceous material is maintained during the process at a weight ratio which is slightly greater than the critical weight ratio corresponding to the temperature difference sensed during the process. <IMAGE>

Description

SPECIFICATION Retorting process and apparatus Field of the Invention This invention relates to a retorting process and apparatus in which carbonaceous material, such as oil shale, is retorted through the use of recycled hotter heat-carrying bodies, such as ceramic balls.

Background of the Invention It has previously been proposed to retort carbonaceous material, such as oil shale, in a rotating retort by the use of recycled hotter heatcarrying bodies, such as ceramic balls, which are fed to the rotating retort together with the carbonaceous material. As the very hot balls engage the cooler preheated oil shale, the oil shale temperature is raised to a point at which volatile liquids and combustible gases are driven off and coliected. Typical prior art patents which disclose processes of this type include U.S. patents Nos.

2,872,386; 3,020,227; 3,265,608; and 3,925,1 90.

With regard to the weight ratio of balls to oil shale which is employed, up to the present time this has been handled on a rather hit or miss basis (generally between 1:1 and 3 :1), without a thorough study of the optimum weight ratio of the balls or other heat-carrying bodies to the oil shale, at the particular temperatures which were employed.

With regard to the use of ceramic balls, a persistent operational problem has been the tendency for the balls to crack, chip and break under severe operating conditions. As a result, some operators preferred not to use a difference in temperature between the hot recycled balls and the oil shale feed which exceeded a certain preconceived temperature difference, on the basis that the thermal shock as each ball encountered the cold or cool oil shale would crack the ceramic balls or cause surface chips if too great a temperature difference were present, regardless of the heat transfer coefficient of the system.

Accordingly, it is the principal object of the present invention to provide such a retorting process and apparatus wherein chipping and cracking of the recycled heat-carrying bodies, such as ceramic balls, is minimized while achieving high efficiency operation.

Summary of the Invention The present invention provides a process for retorting carbonaceous material involving the addition of hotter recycled heat-carrying bodies to carbonaceous material and moving the carbonaceous material and hotter recycled heatcarrying bodies in heat-transferring proximity with one another in a retort to raise the temperature of the carbonaceous material to retorting heat ievels, wherein the hotter recycled heat-carrying bodies have a critical weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material below which chipping and cracking of the hotter recycled heat-carrying bodies may readily occur, which process is characterized by the steps of (a) experimentally determining the critical weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material, expressed as a continuously increasing function of temperature difference between the hotter recycled heatcarrying bodies and the carbonaceous material having a substantial positive slope with increasing temperature difference; (b) sensing the temperature difference between the hotter recycled heat-carrying bodies and the carbonaceous material during the process; and (c) providing an optimum weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material which is slightly greater than said critical weight ratio for the temperature difference sensed during the process.

The process of the present invention is also characterized by the further steps of (d) sensing the temperature difference between the hotter recycled heat-carrying bodies and the carbonaceous material as the temperature difference changes during the process; and (e) changing the weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material to a new, different optimum weight ratio as said temperature difference changes, said new, different optimum weight ratio being slightly greater than said critical weight ratio for the changed temperature difference.

In the process of the present invention the weight ratio can be changed by varying the rate of feeding the hotter recycled heat-carrying bodies while maintaining the rate of feeding the carbonaceous material relatively constant.

Alternatively, the weight ratio can be changed by varying the rate of feeding the carbonaceous material while maintaining the rate of feeding the hotter recycled heat-carrying bodies relatively constant.

The apparatus of the present invention for use in the retorting process is characterized by a retort; means for feeding carbonaceous material to the retort; means for feeding hotter recycled heat-carrying bodies to the retort; means for moving the carbonaceous material and the hotter recycled heat-carrying bodies in heat-transferring proximity with one another in the retort; means for controlling the amount of the hotter heat-carrying bodies and the amount of the carbonaceous material fed to the retort; means for sensing the temperature difference between the hotter heat-carrying bodies and the carbonaceous material fed to the retort; and means responsive to the sensed temperature difference between the hotter heat-carrying bodies and the carbonaceous material for controlling the weight ratio of the hotter heatcarrying bodies to the carbonaceous material at an optimum weight ratio which is slightly greater than the critical weight ratio for the sensed temperature difference.

The optimum weight ratio is considered to be slightly greater than the critical weight ratio when the optimum weight ratio has a value within a few tenths higher than the value of the critical weight ratio at the sensed temperature difference, for example, 2.1 to 2.3 versus 2.0.

Brief Description of the Drawings Fig. 1 is a diagrammatic showing of a system illustrating the principles of the present invention; and Fig. 2 is a diagram showing various ball/shale weight ratios, including the critical and optimum weight ratios, plotted against the temperature difference between the recycled balls and the oil shale being retorted.

Detailed Description of the Invention Referring more particularly to the drawings, Fig.

1 is a block diagram of one illustrative system embodying the principles of the invention. A central portion of the invention is a retort 12 to which crushed carbonaceous material, such as oil shale, is fed by line 14, and to which hotter recycled heat-carrying bodies, such as small ceramic balls, are fed by line 1 6. The retort 12 can be rotated by a drive motor (not shown). Rotation of the retort 12 moves the carbonaceous material and the hotter recycled heat-carrying bodies in heat-transferring proximity with one another in the retort. The crushed raw oil shale input is indicated by arrow 18, and a conventional preheater for the oil shale is represented by block 20.

Concerning the retort 12, a trommel, or rotating cylindrical screen, 22 is employed at the output of the retort 12 to separate the oil shale residue from the recycled ceramic balls. In this connection, the balls might typically be in the order of zg inch in diameter and the oil shale would be crushed to a diameter less than this 21 inch diameter of the balls. Openings of the trommel or screen 22 would be in the order of 2 inch or slightly less, so that the residual oil shale particles, but not the balls, will drop through the screen 22 into an accumulator 24. Arrow 26 indicates the carrying away of the accumulated residual shale solids for further processing. Accumulator 28 receives the ceramic balls which roll through and out the open end of the trommel 22.The ceramic balls are routed via a line 30, elevator 32, and line 34 to a ball heating apparatus 36, which reheats the ceramic balls to an elevated temperature. In this connection, air and fuel are supplied over lines 38 and 40, respectively, to a combustion chamber 42, and the incoming balls from line 34 are exposed to the resultant heat.

The weight ratio of the recycled balls or other heat-carrying bodies to the oil shale being processed may be varied or determined in any desirable way. In the illustrative embodiment shown in Fig. 1, a feed control unit 44 is provided to vary the amount of balls which are permitted to pass through line 1 6 to the retort 12 in a given period of time. Another feed control unit 45 is provided to vary the amount of crushed shale being fed to the retort 12 through the line 14 from the preheater 20. Thus, the weight ratio of balls to shale may be easily varied.

The temperature of the preheated oil shale is sensed at point 46, and an electrical signal indicating the temperature at this point 46 is transmitted to a control and monitor circuit 48 over a control input lead 50. (Control leads are shown in dashed lines to distinguish them from the feed lines in this diagram.) Similarly, the temperature of the recycled ceramic balls is sensed at point 52 and a crorresponding electrical signal is transmitted to controil circuit 48 over a control input lead 53. If desired, instead of sensing at points 46 and 52, the temperature of the oil shale and the ceramic balls may be sensed at the point where the lines 14 and 1 6 feed the shale and ceramic balls directly to the rotating retort 12.

Appropriate electrical signals are transmitted from the control circuit 48 to the feed control units 44 and 45 over control output leads 56 and 57, respectively.

Preferably, the temperature of the retorted shale is sensed at point 47, or at another convenient location at the discharge of the retort 12, and a corresponding electrical signal is similarly transmitted to control circuit 48 over a control input lead 49. By providing the control circuit 48 with input and output temperatures and with appropriate circuitry, control of the retorting process may be achieved by maintaining an appropriate heat balance for the retort 12. As an example, if the air and fuel supplied to the ball heating apparatus 36 through lines 38 and 40, respectively, is held constant, the recycled ball feed control 44 may be utilized to maintain a constant reheated ball temperature at point 52.

Under equilibrium conditions, a constant flow rate of heat-carrying balls will be maintained.

Coincidentally, the feed rate of raw shale may be varied to maintain a constant, predetermined shale discharge temperature at point 47, In response to minor variations in the temperature of the raw shale feed, as sensed at point 46, control circuit 48, in one preferred embodiment, increases or decreases the raw shale flow rate to maintain the desired discharge temperature at point 47 which will correspond to the appropriate weight ratio of heat-carrying balls to shale.

The ball feed control unit 44 and the shale feed control unit 45 may be of any desired form, and either or both could, for example, involve the use of a variable speed electric motor with an associated feedscrew, a flapper valve assembly as disclosed in U.S. patent No. 3,550,904 or any suitable valving structure which would accurately regulate the amount and the corresponding weight of the ceramic balls or carbonaceous material fed to the retort 1 2.

Of course, if the temperature of the oil shale and the ceramic balls is essentially constant, the feed rates of ceramic balls and oil shale may also be held constant. Step changes in the flow rate of ceramic balls or oil shale would be necessary if changes were introduced which would affect the heat in the system. Such changes might include, for example, a change in the heat-carrying bodies (i.e., dimensionally or with respect to material of construction), a change in the rotational speed of the retort, a change in the type of carbonaceous feed being processed, or a change in the configuration of the inlet section of the retort.

Each such change may require a change to effect a new optimum weight ratio of ceramic balls to carbonaceous material.

Reference will now be made to Fig. 2 in which the ball to shale weight ratio is plotted against the temperature difference in degrees Fahrenheit between the hot balls from the ball heater 36 and the preheated shale from the preheater 20.

Initially, attention is directed to experimental data indicated by the circles with dots in them and the circles with X's in them. The circles with X's in them represent experimental test runs in which the ceramic balls did not chip or crack to any great degree as a result of heat shock. However, the circles with dots in them represent experimental test runs in which the ceramic balls suffered high chip or crack formation as a result of heat shock.

The intermediate line 61, therefore, represents the experimentally determined critical weight ratio of the hotter balls to shale and has a substantially positive slope with increasing temperature difference. The shaded area 62, which lies slightly to the right of line 61, represents the optimum weight ratio where there is very little chip or crack formation and where there is the minimum amount of recycled heat-carrying bodies or balls for a given amount of shale. In the present system, by reducing the amount of balls which are required to retort a given amount of shale, the retorting process becomes more economical.

Viewed in another light, more shale could be retorted in a given apparatus, or the same amount of shale could be retorted in a smaller apparatus.

As mentioned above, it might logically be thought that below a certain preconceived temperature difference, there might be no, or very little, chip or crack formation, while above the certain preconceived temperature difference it might be expected that there would be high chip formation. That would, of course, conform to a horizontal line in the showing of Fig. 2, and that obviously is not the case, as indicated by the experimental data.

Concerning the data employed in the preparation of Fig. 2, the recycled heat-carrying bodies were in the form of balls - inch in diameter and made principally of alumina, or aluminum oxide. The temperature of the balls as fed to the rotating retort was in the range of 9000F to 1 2500F. In pilot plant operation the rotating retort is about 2 feet to 5 feet in diameter, while in commercial operations, a diameter in the order of 12 feet to 14 feet could be used. The speed of rotation was in the order of two to five revolutions per minute.

Now, it is considered appropriate at this point to discuss the control and monitor circuit 48 and its mode of operation. As mentioned above, the temperature may be sensed at points 52 and 46 for the reheated balls and the preheated shale, respectively, or immediately at the input to the retort 12; and the temperature at point 47 at the output of the retort 12 is also sensed. Of course, there will be a small temperature drop between points 46 and 52 and the input to the retort 12, and a correction factor may be introduced to correct for these differences.For one exemplary set of steady state conditions, the temperature of the reheated balls was approximately 12500 F, that of the preheated shale was approximately 5000F and the temperature at the output from the retort was approximately 9000F. The difference between the two input temperatures was about 7500F and the ball-to-shale weight ratio was approximately 1.7, as shown in Fig. 2. With these parameters, and considering the heat losses in the retort and the heat content of the volatile products which are obtained from the retort, the output temperature at point 47, as mentioned above, is approximately 9000F.

Now, the control and monitor circuit 48 may be operated in any of several modes to maintain the operating point of the process in the desired optimum shaded area 62, as indicated in Fig. 2.

Specifically, the temperatures from points 52, 46 and 47 are displayed, so that drastic departures from normal values may be readily detected, and suitable changes or adjustments made. Further, the control and monitor circuit 48 may be operated in a mode in which the reheated balls are fed at a relatively constant rate, and control is exercised by varying the rate of feed of the preheated shale by the feed control unit 45. This can be accomplished using a "forward" acting control system sensitive to the recycled ball and the shale temperatures sensed at points 52 and 46, to change the rate of feed for the shale (with a relatively constant rate of recycled bail feed) so as to provide a ball/shale weight ratio in the desired optimum shaded area 62 of Fig. 2. The monitored temperature at point 47 will verify that the process is operating within the desired optimum shaded area 62.

Alternatively, with a predetermined temperature and rate of flow of recycled balls, the system may be operated as a servo or feed back system with a relatively long time constant (greater than the transit time through retort 12), and the shale flow through control unit 45 varied to produce the predetermined temperature at point 47 at the output of retort 1 2. If the temperature at point 47 increases, the rate of flow through control unit 45 will be increased, and vice versa. Further, through monitoring, verification of the correct operating point within shaded area 62 of Fig. 2 may be confirmed. Also, for example, if the temperature of the preheated shale should decrease slightly, this change would have the initial effect of reducing the output temperature at point 47; the result would be to slow the feed of shale through control unit 45, thereby increasing the ball/shale weight ratio, as called for by the increased temperature difference, in accordance with Fig. 2. Similarly, of course, both the forward and feedback modes of operation can be readily impiemented with the shale feed rate being held relatively constant, and varying the recycled ball feed through control unit 44.

It is to be understood the+ the foregoing parameters merely represent one practical system for the retorting of oil shale, and that variations are to be expected for varying conditions and materials. For example, the feed control units 44 and 45 may be combined with the ball and shale heating units 36 and 20, respectively. Also, different diameter ceramic balls, for example, of 4 inch and 1 inch diameters, have been successfully used, with the crushed carbonaceous material being in each case of smaller size to facilitate separation. Carbonaceous materials which have been successfully retorted include rubber and coal, in addition to oil shale. In the case of retorting rubber without preheating, a much higher optimum weight ratio of balls to the rubber was required in the order of 8:1 to 10:1 in part in view of the higher temperature difference between the reheated balls and the feed stock and the different heat transfer coefficient at the inlet of the retort.

Accordingly, the present invention is not limited to the precise conditions plotted and analyzed in detail in Fig. 2, for example.

Claims (8)

1. A process for retorting carbonaceous material involving the addition of hotter recycled heat-carrying bodies to carbonaceous material and moving the carbonaceous material and hotter recycled heat-carrying bodies in heat-transferring proximity with one another in a retort to raise the temperature of the carbonaceous material to retorting heat levels, wherein the hotter recycled heat-carrying bodies have a critical weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material below which chipping and cracking of the hotter recycled heat-carrying bodies may readily occur, characterized by the steps of (a) experimentally determining the critical weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material, expressed as a continuously increasing function of temperature difference between the hotter recycled heatcarrying bodies and the carbonaceous material having a substantial positive slope with increasing temperature difference; (b) sensing the temperature difference between the hotter recycled heat-carrying bodies and the carbonaceous material during the process; and (c) providing an optimum weight ratio of the hotter recycled heat-carrying bodies to the carbonaceous material which is slightly greater than said critical weight ratio for the temperature difference sensed during the process.

2. The process defined by claim 1, characterized by the further steps of (d) sensing the temperature difference between the hotter recycled heat-carrying bodies and the carbonaceous material as the temperature difference changes during the process; and (e) changing the weight ratio of +Lle hotter recycled heat-carrying bodies to the carbonaceous material to a new, different optimum weight ratio as said temperature difference changes, said new, difference optimum weight ratio being slightly greater than said critical weight ratio for the changed temperature difference.

3. The process defined by claim 2, characterized by the weight ratio is changed by varying the rate of feeding the hotter recycled heat-carrying bodies while maintaining the rate of feeding the carbonaceous material relatively constant.

4. The process defined by claim 2, characterized by the weight ratio is changed by varying the rate of feeding the carbonaceous material while maintaining the rate of feeding the hotter recycled heat-carrying bodies relatively constant.

5. The process defined by any of claims 1 to 4, characterized by the carbonaceous material is oil shale and the hotter recycled heat-carrying bodies are ceramic balls.

6. An apparatus for use in the process defined by any of claims 1 to 5, characterized by a retort; means for feeding carbonaceous material to the retort; means for feeding hotter recycled heat-carrying bodies to the retort; means for moving the carbonaceous material and the hotter recycled heat-carrying bodies in heat transferring proximity with one another in the retort; means for controlling the amount of the hotter heat-carrying bodies and the amount of the carbonaceous material fed to the retort; means for sensing the temperature difference between the hotter heat-carrying bodies and the carbonaceous material fed to the retort; and means responsive to the sensed temperature difference between the hotter heat-carrying bodies and the carbonaceous material for controlling the weight ratio of the hotter heatcarrying bodies to the carbonaceous material at an optimum weight ratio which is slightly greater than the critical weight ratio for the sensed temperature difference.

7. A process for retorting carbonaceous material substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

8. Apparatus for retorting carbonaceous material substantially as hereinbefore described with reference to and as shown in Fig. 1 of the accompanying drawings.