Drying and gas or vapor contact with solids – Process – With contacting of material treated with solid or liquid agent
Reexamination Certificate
1999-11-29
2001-06-26
Gravini, Stephen (Department: 2162)
Drying and gas or vapor contact with solids
Process
With contacting of material treated with solid or liquid agent
C034S348000, C034S357000, C034S363000, C034S586000, C034S588000, C034S181000, C034S187000
Reexamination Certificate
active
06249989
ABSTRACT:
The present invention relates to a method and an apparatus for transferring heat to or from a charge of material containing solids.
The present invention relates particularly to a method and an apparatus for transferring heat to or from a low thermal conductivity charge of material containing solids.
A number of industrial methods require that a charge of material containing solids be heated or cooled in order to initiate and sustain a chemical reaction or physical changes. Typically, it is necessary to heat the charge to an elevated temperature for the chemical reaction or physical change to occur. Unfortunately, many charges of solid materials have very low thermal conductivities and it is difficult to heat such charges using indirect heat transfer. Such charges are frequently heated by direct heat transfer, for example, by supplying hot gases to packed beds or fluid beds of the charges.
As used throughout this specification, “direct heat transfer” refers to heat transfer in which a heat transfer fluid comes into direct contact with the material to be heated or cooled. “Indirect heat transfer” refers to heat transfer in which the heat transfer fluid is separated from the material being heated or cooled by a physical barrier, such as the wall of a tube.
Some methods are not amenable or suitable for direct heat transfer. The ratio of heat capacitance between solids and gases is such that large volumes of gas or fluid are required to transfer the heat. For example, flow of the large volumes of gas required for heat transfer-through a packed bed is not possible unless the bed is very coarse or heating and cooling times are very long. In the case of methods involving coal and other materials which contain substances which may be volatile at elevated temperatures, direct heat transfer may result in volatile material being driven off with the heating gas which could cause difficulties in cleaning the offgas prior to emission of the offgas through the flue or stack. In other methods, direct heat transfer may lead to solids handling difficulties or maintenance problems caused by solids carry over in gas streams. In such methods, it is necessary to utilise indirect heat transfer to heat the charge.
One known indirect heat transfer method is the upgrading of coal, particularly low rank coal, by the simultaneous application of temperature and pressure described in U.S. Pat. No. 5,290,523 to Koppelman. In this method, heating a charge of coal under elevated pressure results in water being removed from the coal by a squeeze reaction caused by structural realignment of the coal and also by decarboxylation reactions. Furthermore, some soluble ash-forming components are also removed from the coal. This results in upgrading of coal by thermal dewatering and upgrading of the calorific value of the coal. By maintaining the pressure sufficiently high during the upgrading process, vaporization of the removed water can be substantially avoided which reduces the energy requirement of the method. Furthermore, the by-product water is produced mainly as a liquid rather than as steam or vapour.
The thermal processing of coal requires heat transfer to the coal (typically 300-600 Btu/lb), but the effective thermal conductivity of a packed bed of coal is approximately 0.1 W/mK, making the coal bed a good thermal insulator.
Options that might be considered to accelerate heating of coal to provide a process which achieves a reasonable heat-up time of a coal bed include:
Increase of thermal driving force by increasing the temperature of the heat transfer medium. This tends to lead to devolatilisation of coal which for low rank coal upgrading reduces the heating value of the product. Moreover, this also leads to condensation of tars and other volatilised materials in other parts of the vessel system.
Use of fluid beds. This leads to the need to circulate large volumes of (inert) gas which again accentuates the problem of devolatilisation of the coal. It also requires gas cooling and cleaning before recompression or the operation of a hot dirty compressor, both of which involve capital and maintenance. Further, fluid beds tend to separate fines.
Use of agitated beds such as a rotary kiln. The operation of such reactors at elevated pressures, with inert atmosphere, involves massive engineering difficulty and expense. Indirect heat transfer is preferred, but this further complicates the engineering difficulties and the volume occupancy of coal in the vessel can be low.
Use of flash drying of a ground feed. This requires subsequent agglomeration to produce a marketable product. It also requires an inert gas for heat transfer and the reactive volumes tend to be large because of the dispersed state of the solids.
Hydrothermal dewatering of coal in which the coal is ground to a small particle size and mixed with water to form a slurry and the slurry is subsequently heated to an elevated temperature at elevated pressure to maintain liquid conditions. This process requires grinding of coal which must then be either agglomerated or used directly in a process, such as at a power station. Furthermore, the mass of water heated to elevated temperature is large and this requires large heat exchangers for heat recovery.
With the simultaneous application of high pressure (greater than 10 barg), each of the above options become more difficult.
A packed bed combined with indirect heat transfer is preferred for heating or cooling coal because of the minimisation of volatile loss, the lower energy consumption, and the production of the majority of the by-product water as liquid.
A packed bed also allows a wider range of coal sizes, and coarser coal sizes than would be preferred for a fluid bed operation. A packed bed also gives the smallest volume to contain in a high pressure vessel, provided the heating rates are high. A small vessel volume leads to savings in pressurisation time and vessel cost.
The classical approach for enhancing indirect heat transfer is to provide sufficient surface area between the heating medium and the charge to be heated. This leads to shell and tube arrangements, with heating medium either on the inside or the outside of the bundles of tubes. Such tube bundles may be appropriate to transfer heat to liquids and gases (although they are prone to scaling and buildup, requiring maintenance) but they have some limitations when used in the heating of solids. This is particularly so in the case where the solids comprise coal that may have a particle size of up to 19 mm (0.75 inch), or even export size coal having particle sizes of up to 50 mm (2 inches), where problems of bridging and sticking are encountered. Any heat transfer system for such materials must be designed to allow free flow of solids, either at the start and end of a cycle in a batch process, or during a continuous process.
A further difficulty with the above-described prior art shell and tube arrangements arises from the fact that most prior art process vessels require a discharge cone to be positioned at the lower end of the tube bundle in the vessel in order to discharge the coal from the vessel. It is almost impossible to have the tube bundle extend into the discharge cone and accordingly the appreciable volume of coal that is contained in the discharge cone is not heated by the tube bundle. To overcome this difficulty, some methods incorporate water injection or steam injection into the coal bed. These are known as working fluids. Such working fluids may be vaporised (if liquid) and superheated in the upper sections of the bed and then flow to the outlet at the bottom of the discharge cone. Cold solids in the discharge cone are thereby heated by the working fluid (by convection and possibly by condensation of the working fluid). However, injection of a working fluid has serious consequences for the energy utilisation of the process.
One prior art method utilises a shell and tube type heat transfer apparatus in which coal is fed to the tube side and a heat transfer oil flows through the shell side. The tubes have a diameter of typically 7
Conochie David S.
Greenwood Mark J.
Gravini Stephen
Harness & Dickey & Pierce P.L.C.
KFx Inc.
LandOfFree
Method and apparatus for heat transfer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for heat transfer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for heat transfer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2499181