Temperature gradient in retort for pyrolysis of carbon containing solids

A temperature gradient in staged turbulent bed retort established by adding heat transfer material at different levels along vertical length of retort to prevent thermal cracking of product vapors.

BACKGROUND OF THE INVENTION 
Certain naturally-occurring materials such as oil shale, coal, tar sands, 
and diatomaceous earth contain a carbonaceous fraction. During retorting, 
these carbon-containing solids release an oil useful in petroleum 
processing. Oil shale is a shale-like rock containing an organic 
component, usually referred to as kerogen, which upon heating releases 
volatile hydrocarbons that may be recovered as shale oil. A residual 
carbonaceous material typically remains with the inorganic component left 
following pyrolysis. 
The pyrolysis or retorting process may be carried out in a retorting vessel 
of various designs. In one method the raw oil shale is crushed into 
particulate material containing a particle size distribution not exceeding 
about 2-1/2 mesh (Tyler Standard Sieve) which is fed into the upper 
portion of the retorting vessel. The raw oil shale is mixed with hot solid 
heat carrier particles which, along with the raw shale, moves downward as 
a continuous bed of material through the retorting vessel. An upward flow 
of a non-oxidizing stripping gas counter-current to the downward moving 
shale carries the volatile hydrocarbons away from the bed. See U.S. Pat. 
No. 4,199,432. In order to prevent gross vertical backmixing and slugging 
of solids passing downward with the bed, the retorting vessel is equipped 
with a plurality of dispersing elements in the form of perforated plates, 
bars, screens, packing, or other suitable internals. The dispersing 
elements also serve to limit the size of gas bubbles passing up the 
retorting vessel. 
In operating the retorting vessel it is important to remove the vapor 
products without thermal cracking if high liquid yields are to be 
obtained. One way of minimizing thermal cracking is to operate the 
retorting vessel at a relatively low temperature. However, lower 
temperatures require longer retorting times, which means increased 
residence time in the retort. A longer residence time requires a larger 
retort or a decrease in the amount of raw shale that can be processed. The 
present invention is directed to a method for efficiently pyrolyizing the 
raw shale with a minimum of thermal cracking. 
BRIEF SUMMARY OF THE INVENTION 
In its broadest aspect, the invention concerns a process for retorting a 
particulate carbon-containing solid wherein said carbon-containing solid 
is passed into the upper portion of a vertically-elongated retorting 
vessel equipped with a plurality of dispersing elements so constructed and 
arranged as to substantially limit backmixing and slugging of solids 
passing downward therethrough, heating the carbon-containing solids to 
retorting temperature principally by means of hot solid heat carrier 
particles to drive off volatile hydrocarbons, passing a non-oxidizing 
stripping gas upward through the retorting vessel, removing volatile 
hydrocarbons and stripping gas from the upper portion of the retorting 
vessel, and withdrawing the resulting retorted solids from the lower 
portion of the retort, the improvement comprising establishing an 
ascending temperature profile in the retorting vessel by introducing the 
hot solid heat carrier particles into the retorting vessel at two or more 
different levels along the vertical length of the retorting vessel, 
whereby a temperature gradient is created along the vertical axis of the 
retorting vessel-the highest temperature being at the bottom of the vessel 
with the temperature declining near the top. 
More specifically, the invention concerns a process for retorting oil shale 
wherein particulate raw oil shale is retorted by passing said raw oil 
shale into the upper portion of a vertically-elongated retorting vessel 
equipped with a plurality of dispersing elements so constructed and 
arranged as to substantially limit backmixing and slugging of solids 
passing downward therethrough, heating the raw oil shale to retorting 
temperature principally by means of hot solid heat carrier particles to 
drive off volatile hydrocarbons, passing a non-oxidizing stripping gas 
upward through the retorting vessel, removing the volatile hydrocarbons 
and stripping gas from the upper portion of the retorting vessel, and 
withdrawing the resulting retorted oil shale from the lower portion of the 
retort, the improvement comprising establishing an ascending temperature 
profile in the retorting vessel by introducing the hot solid heat carrier 
particles into the retorting vessel at two or more different levels along 
the vertical length of the retorting vessel, whereby a temperature 
gradient is created along the vertical axis of the retorting vessel-the 
highest temperature being at the bottom of the vessel with the temperature 
declining near the top. 
In a process of the type described above, the particles of raw oil shale 
and hot solid heat carrier particles normally do not exceed a maximum size 
of about 0.8 cm (2-1/2 mesh-Tyler Standard Sieve). Generally, the velocity 
of the flow of stripping gas in the retorting vessel falls within the 
range of from about 1 foot per second to about 5 feet per second. The hot 
solid heat carrier particles may be a ceramic composition, sand, alumina, 
burned shale (oil shale from which both the volatile hydrocarbons and 
residual carbonaceous material are removed), or the like. Most preferably, 
the heat carrier particles are burned oil shale used alone or mixed with a 
supplemental heat carrier material.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, fresh particulate oil shale enters the retorting 
vessel 2 by way of feed conduit 4. The oil shale forms a bed of material 
in the lower portion of the retorting vessel 6 and passes downward through 
a series of dispersing elements 8a, 8b, 8c and 8d which prevent slugging 
of the solids and limit gross vertical backmixing. Hot solid heat carrier 
particles enter the retorting vessel via conduits 10 and 12, the flow 
being controlled by valves 16 and 18, respectively. The preferred heat 
carrier particles are principally burned shale, i.e., retorted oil shale 
which has been burned to remove the carbonaceous residue. By controlling 
the rate at which the hot solid heat carrier particles enter the retorting 
vessel, a vertical temperature gradient is established with the lowest 
temperature being near the top of the bed of mixed material and the 
highest temperature being located near the bottom. This is illustrated in 
FIG. 2 wherein the reference positions in the retorting vessel are 
designated A and B which correspond to the levels at which conduits 10 and 
12, respectively, enter the vessel. 
Referring back to FIG. 1, a stripping gas is introduced into the retorting 
vessel via conduit 22 and passes upward in generally countercurrent flow 
to the downward moving shale. Product vapors and entrained fine material 
having a terminal velocity equal to or below the velocity of the exiting 
gas are carried upward and exit the retorting vessel via off gas conduit 
24. The upper portion 26 of the retorting vessel is enlarged and serves as 
a solids disengaging area to prevent particles larger than a preselected 
size from being carried away with the off gas. The product vapors and 
stripping gas are separated from the fines in a cyclone 28. 
Again referring to the retorting vessel 2, retorted oil shale mixed with 
hot solid heat carrier particles leaves the bottom of the vessel by means 
of conduit 30 and is carried to the combustor 32 where the carbonaceous 
residue on the retorted oil shale is burned. Typically, as shown, the 
combustor is a lift pipe with the retorted oil shale entering near the 
bottom. An air flow is introduced into the combustor via conduit 34 and 
entrains the shale particles carrying them upward. The residual 
carbonaceous material is combusted during the passage of the shale from 
the bottom to the top of the combustor. The burned shale leaves the 
combustor via solids conduit 36 and is passed to holding bin 38 where it 
is temporarily stored prior to recycling into the retorting vessel. Excess 
shale leaves the system via oulet 40. Flue gases from the combustor leave 
via flue gas outlet 42. 
In carrying out the process that is the subject of this invention, it is 
essential that the retorting vessel be of a design that prevents 
substantial gross backmixing of solids within the bed of downward moving 
shale particles and that the design assures localized rapid mixing of the 
hot solid heat carrier particles with the oil shale after introduction. 
Retorting vessels, having internal dispersing elements are described in 
U.S. Pat. Nos. 4,199,432 and 4,183,800, meet these criteria. In a 
retorting vessel such as described above, approximately 90% of the 
volatile hydrocarbons will be retorted away in the cooler portion of the 
bed, i.e., that portion at or below about 480.degree. C. The remaining 
volatiles will be recovered in the lower hotter zone of the bed. Using 
this process, thermal cracking of the evolved vapors would be minimized in 
the solids disengaging zone of the retort and in the bed itself. Higher 
oil product yields would be obtained instead of undesired coke or gas. 
In process schemes using burned oil shale as a hot solid heat carrier, it 
is usually necessary to provide some means for separating the finer 
grained material from the recycle material. Generally, burned shale 
particles smaller than about 75 to 100 microns are unsatisfactory for use 
as heat carrier material. This separation may be accomplished in a 
cyclone, a sifting means, or in the solids disengaging area of the 
combustor lift pipe.