Enhanced heat transfer system

Details Enhanced heat transfer system Enhanced heat transfer system

2000-02-08

2001-02-13

Gravini, Stephen (Department: 3749)

Drying and gas or vapor contact with solids

Process

With contacting of material treated with solid or liquid agent

C034S348000, C034S357000, C034S363000, C034S562000, C034S586000, C034S588000, C034S181000, C034S187000

Reexamination Certificate

active

06185841

ABSTRACT:

The present invention relates to processing a charge of a solid material to heat or cool the solid material.
The present invention relates particularly, although not exclusively, to processing a charge of a solid material, the charge having low thermal conductivity, under conditions including high temperature and pressure.
The present invention relates more particularly to:
(i) upgrading carbonaceous materials, typically coal, under conditions including high temperature and pressure to increase the BTU value of the carbonaceous materials by removing water from the carbonaceous materials; and
(ii) cooling the heated carbonaceous materials.
U.S. Pat. No. 5,290,523 to Koppelman discloses a process for upgrading coal by the simultaneous application of temperature and pressure.
Koppelman discloses thermal dewatering of coal by heating coal under conditions including elevated temperature and pressure to cause physical changes in the coal that results in water being removed from the coal by a “squeeze” reaction.
Koppelman also discloses maintaining the pressure sufficiently high during the upgrading process so that the by-product water is produced mainly as a liquid rather than steam.
Koppelman also discloses a range of different apparatus options for carrying out the upgrading process. In general terms, the options are based on the use of a pressure vessel which includes an inverted conical inlet, a cylindrical body, a conical outlet, and an assembly of vertically or horizontally disposed heat exchange tubes positioned in the body.
In one proposal to use a Koppelman-type apparatus, the vertically disposed tubes and the outlet end are packed with coal, and nitrogen is injected to pressurise the tubes and the outlet end. The coal is heated by indirect heat exchange with a heat exchange fluid supplied to the cylindrical body externally of the tubes. Further heat transfer is promoted by supplying water to the tubes, which subsequently forms steam that acts as a heat transfer fluid. The combination of elevated pressure and temperature conditions evaporates some of the water from the coal and thereafter condenses some of the water as a liquid. A portion of the steam generated following the addition of water also condenses as a liquid due to the elevated pressure. Steam which is not condensed, and which is in excess of the requirements for optimum pressurisation of the packed bed, must be vented. In addition, non-condensable gases (eg CO, CO
2
) are evolved and need to be vented. Periodically, liquid is drained from the outlet end. Finally, after a prescribed residence tine, the vessel is depressurised and the upgraded coal is discharged via the outlet end and subsequently cooled.
International applications PCT/AU98/00005 entitled “A Reactor”, PCT/AU98/00142 entitled “Process Vessel and Method of Treating a Charge of Material”, and PCT/AU98/00204 entitled “Liquid/Gas/Solid Separation” in the name of the applicant disclose inter alia an improved process for upgrading coal by the simultaneous application of temperature and pressure to that described by Koppelman.
International application PCT/AU98/00142 is particularly relevant in the context of the present invention. The International application discloses that the applicant found that enhanced heat transfer could be achieved in heating or cooling a charge of coal or other solid material having a low thermal conductivity in a pressure vessel by utilising a working fluid that is forced to flow through the vessel from an inlet end to an outlet end by virtue of an applied pressure and is recirculated to the inlet end. The preferred embodiment shown in FIG. 7 of the International application is based on the use of a centrifugal fan located externally of the vessel as the means of applying the required pressure to create flow of the working fluid.
An object of the present invention is to provide an improved process and apparatus for upgrading coal by the simultaneous application of temperature and pressure to that described by Koppelman and in the above International applications.
According to the present invention there is provided a method of heating or cooling a solid material in a process vessel, which method comprises:
(a) supplying a charge of the solid material to the vessel to form a packed bed;
(b) supplying a working fluid to the vessel;
(c) heating or cooling the solid material by heat exchange with a heat exchange fluid via internal heat transfer surfaces in the packed bed, whereby indirect heat exchange occurs between the heat transfer fluid and the charge and between the heat transfer fluid and the working fluid, and whereby direct heat exchange occurs between the working fluid and the charge; and
(d) enhancing heat exchange during heating or cooling step (c) by reversing flow of the working fluid by:
(i) causing the working fluid to flow in a first direction for a first period of time;
(ii) causing the working fluid to flow in a second direction for a second period of time; and
(iii) repeating steps (i) and (ii).
The above described heat exchange enhancing step (d) is hereinafter referred to as “reversing flow” of the working fluid.
It is preferred that the second direction be opposite to the first direction.
The present invention is based on the realisation that reversing flow of the working fluid can significantly enhance indirect heat exchange between the heat exchange fluid and the solid material and that the energy requirements for reversing flow of the working fluid are relatively low.
It is preferred that the method further comprises pressurising the packed bed prior to or during heating or cooling step (c) with externally supplied gas, internally generated steam, or both.
It is preferred particularly that the method further comprises pressurising the packed bed prior to or during heating or cooling step (C) to an operational pressure up to 800 psig.
It is preferred that the working fluid be a gas.
In situations where the working fluid is a gas, because the working fluid is compressible and the packed bed has resistance to flow, some of the flow will be stored as compressed gas in the vessel (and any associated pipework). The extent of this capacitance effect is dependent on a range of factors, such as particle size in the packed bed, operating pressure, mass flow, frequency, and compressible volume. It is preferred that the system be designed so that the capacitance effect accounts for less than 10% of mass flow of the working fluid.
It is preferred that the working gas does not undergo a phase change in the operating conditions of the method. It is noted that in some instances there may be a benefit in using a working gas that contains a condensable component.
Gases that may be used as the working gas include oxygen, nitrogen, steam, SO
2
, CO
2
, hydrocarbons, noble gases, refrigerants, and mixtures thereof.
It is preferred that the working fluid be unreactive with the bed.
It is preferred that the frequency of reversing flow be less than 10 HZ and, more preferably, less than 3 HZ. It is preferred particularly that the frequency of reversing flow be less than 2 HZ.
The duration of the first and second time periods of reversing flow may be the same so that there is no net flow of the working fluid through the vessel. Alternatively, the duration of the first and second periods of time may be different so that there is a net flow of the working fluid through the vessel which produces a net circulating flow of the working fluid in the vessel.
The reversing flow of the working fluid may be a series of successive steps with the flow in the second direction immediately following the flow in the first direction and these steps being repeated immediately thereafter. The reversing flow of the working fluid may also be any suitable variation. For example, there may be a pause between the reversing of the flow between the first and second directions. By way of further example, there may be a pause after the flow in one direction and thereafter further flow in the same direction before reversing the flow to the opposite direction. B

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