Thermal reactor with self-regulating transfer mechanism

A reaction product is formed and transferred from an autoclave to a receiving vessel at a laminar flow rate, using a self-adjusting transfer mechanism. A specific amount of water in the receiving vessel is heated and vaporized prior to the reaction product transfer to raise the pressure in the receiving vessel to saturation pressure. A flow passage between the autoclave and the receiving vessel is now opened, and a resulting pressure differential between the autoclave and the receiving vessel initiates the transfer process. A heat exchanger cools the reaction product flowing from the autoclave to the receiving vessel, where the amount of cooling is dependent upon the transfer rate of the reaction product. An increased transfer rate will cause the hotter reaction product entering the receiving vessel to increase the pressure inside the receiving vessel, thereby reducing, or self-adjusting, the transfer rate. The heat exchanger may also generate steam which is fed into the receiving vessel to adjust the pressure in the receiving vessel and thus adjust the transfer rate of the reaction product.

BACKGROUND OF THE INVENTION
 1. Technical Field
 This invention relates to the formation of reaction products such as
 ceramic, magnetic, electrolyte, electrode and other powders, to the use of
 high temperatures to disintegrate unwanted compounds, and to a structure
 employed for these purposes which may operate at near critical, critical
 or supercritical temperatures and the corresponding saturated pressures of
 the working fluids.
 2. Description of the Related Art
 In my earlier U.S. Pat. Nos. 4,238,240, 4,366,121, 4,545,970, 4,753,787,
 4,912,078, 4,983,374 and 5,026,527, I describe numerous structures and
 processes for forming reaction products. These patents are hereby
 incorporated by reference.
 In my '240 patent I disclose a method for forming a reaction product in
 which the reaction constituents are mixed in an autoclave. The mixed
 reaction constituents are then reacted for a selected time to form
 reaction products, and the reaction products are transferred, at the end
 of the reaction, from the autoclave to another vessel (sometimes called a
 "receiving vessel" and sometimes called an "antipressure vessel")
 connected to the autoclave by a flow passage. The pressure in the vessel
 is held in a controlled manner beneath the pressure in the autoclave
 during the transfer of the reaction products from the autoclave to the
 vessel. To maintain the pressure in the vessel in a controlled manner
 beneath the pressure in the autoclave during the transfer of the reaction
 products from the autoclave to the receiving vessel, I disclose an
 electronic control system which measures the pressures in the autoclave
 and the receiving vessel and which opens or closes a valve (shown as valve
 101 in FIG. 1 of the '240 patent) attached to the receiving vessel (vessel
 12 in the '240 patent) to maintain the pressure in the vessel a controlled
 amount beneath the pressure in the autoclave (shown as autoclave 10 in the
 '240 patent).
 I also disclose an alternative embodiment in the '240 patent wherein the
 electronic control system is replaced by a throttle valve or by a valve
 and a vent pipe. Before the start of the transfer operation, a suitable
 pressure difference is established between the autoclave and the receiving
 vessel. Then, to start the transfer of the reaction product from the
 autoclave to the antipressure vessel, a valve between the autoclave and
 the vessel is opened and simultaneously or subsequently, as desired, a
 pressure release valve on the top of the receiving vessel is opened and
 left open during the transfer process. As a result, the reaction product
 from the autoclave flows into the receiving vessel at an instantaneous
 rate determined by the instantaneous pressure difference between the
 autoclave and the receiving vessel. As I disclose in the '240 patent, this
 pressure difference is controlled by the sizes of the valve and vent pipe
 or the setting of the throttle valve. This embodiment avoids the use of a
 control circuit but has the potential disadvantage that the transfer is
 not as precisely controlled as with a control circuit.
 In my '787 patent, I provide a substantially simplified system for
 transferring the contents of the autoclave (shown as autoclave 10 in the
 '787 patent) to the antipressure vessel (shown as vessel 12 in the '787
 patent). The system of that invention incorporates a pressure release
 valve on the antipressure vessel, the setting of which is precisely
 controlled by a control signal from a flow meter used to measure the
 volumetric flow of the reaction product. In the preferred embodiment, the
 pressure release valve is controlled to maintain a constant flow of
 reaction product from the autoclave to the antipressure vessel. A novel
 method of initializing the pressure in the antipressure vessel is
 disclosed in the '787 patent whereby gas (typically steam) is released
 from the autoclave through a vent pipe into the antipressure vessel prior
 to the transfer of reaction product from the autoclave to the antipressure
 vessel. When the pressure in the antipressure vessel is equal to the
 pressure in the autoclave, the vent pipe is closed and the pressure in the
 antipressure vessel falls slightly beneath the pressure in the autoclave
 as a result of the natural cooling of the gas in the antipressure vessel
 due to heat transfer to the relatively cooler walls of the antipressure
 vessel. As the antipressure vessel comes to a relatively steady state
 temperature after several batches of reaction product have been passed to
 the vessel, the pressure difference between the autoclave and the vessel
 due to this natural cooling effect becomes less. And, when the gas is
 steam, relatively little steam condenses to create this pressure
 difference. This method and structure avoids the use of costly compressors
 as in the prior art to initialize the pressure in the antipressure vessel.
 When the gas is steam, the method requires a surprisingly small amount of
 steam from the autoclave to pressurize the antipressure vessel due to the
 fact that the steam in the autoclave is at a high pressure and temperature
 and, therefore, contains a low volume of water per cubic meter. However,
 this embodiment has the disadvantage of requiring a flowmeter and
 expensive monitoring equipment in order to trigger a pressure relief valve
 if, for example, the flow enters the turbulent regime.
 SUMMARY
 In accordance with the present invention, I provide a substantially
 streamlined system for transferring the contents of an autoclave to a
 receiving vessel, even with respect to the earlier system embodied in my
 '787 patent. This invention employs a self-siphoning method for effecting
 the transfer. In the preferred embodiment, this is accomplished by
 introducing a certain amount of water into a preheated receiving vessel,
 prior to the transfer of the autoclave contents, where the temperature of
 the receiving vessel is below the temperature of the autoclave contents.
 The quantity of water added to the receiving vessel is calculated, taking
 into account the temperature of the receiving vessel, to be sufficient to
 yield a saturated vapor pressure in the receiving vessel. Heating the
 receiving vessel may then be stopped.
 An initial pressure difference now exists between the receiving vessel and
 the autoclave. The reaction product slurry in the autoclave is then
 transferred via a pipe and heat exchanger(s) to the receiving vessel due
 to this initial pressure difference. An increased rate of transfer through
 the heat exchanger(s) causes the temperature of the reaction product
 slurry entering the receiving vessel to rise. The slurry entering the hot
 receiving vessel generates more steam and pressure at a higher slurry
 temperature than at a lower slurry temperature. Since an increased
 transfer rate causes the pressure within the receiving vessel to rise,
 which in turn lowers the transfer rate, the present system maintains a
 self-adjusted pressure difference between the two vessels to control the
 transfer rate of the slurry.
 Additionally, a two-stage heat exchanger is preferably used, where a first
 heat exchanger acts as a steam generator by converting an externally
 supplied flow of water into steam as the water cools the hot slurry from
 the autoclave. This steam is supplied to the receiving vessel to
 additionally control the pressure differential between the two vessels.
 The flow rate of cooling water through the steam generator is used to vary
 the temperature of the steam generated (a higher flow rate of cooling
 water gives a lower steam temperature) and the temperature of the slurry,
 where a lower steam temperature reduces the pressure in the receiving
 vessel so as to increase the slurry flow rate. A higher steam temperature
 will raise the pressure in the receiving vessel and slow down the slurry
 flow rate.
 The self-adjusting and self-siphoning reaction system described allows the
 autoclave to discharge the slurry at a desired flow rate without the prior
 art requirement of continuously monitoring and controlling the flow rate
 of reaction product and the relative pressures of the autoclave and
 receiving vessel.
 This invention will be more fully understood in conjunction with the
 following detailed description taken together with the attached drawings.

DETAILED DESCRIPTION
 The following detailed description is intended to be illustrative only of
 selected embodiments of the invention and not to limit the invention.
 As will be apparent from a comparison of the present FIG. 1 with FIG. 1 of
 the '787 patent, the system of this invention for the formation of a
 reaction product is substantially changed from that disclosed in the '787
 patent.
 In the present FIG. 1, an autoclave 10 is used which, in the preferred
 embodiment, is made of a fine grain carbon steel, such as WB36 or 15 Ni Cu
 Mo Nb 5 from Thyssen Stahl AG, of Duisburg, Germany, and cladded with a
 layer of nickel or stainless steel, such as AISI 316 Ti.
 The minimum layer thickness of the stainless steel cladding should be about
 3 mm. For particular applications, such as the manufacture of precisely
 grown crystals, the cladding layer is preferably nickel. This nickel layer
 can be applied by electrodeless plating or electrode deposit techniques.
 The thickness of this nickel layer may be in the range of 125 to 150
 microns. Such a thin layer of nickel reduces the heat-transfer losses to
 virtually zero.
 The limitations of chemical nickel deposits in a corrosive atmosphere are
 those inherent in any thin protective film over a substrate where the film
 is susceptible to attack. Specifically, almost all inorganic acids and
 short-chain carboxylic acids exhibit penetration rates which are too large
 for practical use of electrodeless coatings. This is equally true for
 materials forming soluble nickel complexes such as cyanides, ammonias,
 short-chain organic amines, many mono- and polyhydroxycarboxylic anions,
 etc., and for some neutral compounds yielding highly corrosive hydrolysis
 products. The same limitations apply, of course, to an electroplated
 nickel coating, except that as the thickness of the nickel coating is
 increased, the nickel coating may be more economically applied by
 electrodeposition.
 Plate made by electrodeless deposition, which is relatively brittle, is not
 recommended in applications where flexing or resistance to violent shocks
 is required.
 Chemical nickel deposition should not be used to plate parts which are to
 be welded. Welds made on plated areas may be embrittled by phosphorous
 from the plating entering the weld. Also, the plating usually develops a
 crack pattern which destroys the protective value of the nickel-alloy
 film.
 Catalytic reduction plating, having a low hothardness, should not be used
 in cases where both wear and heat are involved.
 Ordinarily, chemical nickel coating is more expensive than
 electrodeposition primarily because the reducing agent, namely, sodium
 hypophosphite, is more costly than electric power. In some specific
 applications, for instance, the lining of large tanks, the opposite may be
 true, especially for thinner coatings.
 The nickel coating is able to sustain high alkali environments, and the
 coating can be used at up to 700.degree. C. provided that heating up and
 cooling down of the autoclave 10 are controlled in order to avoid the
 formation of micro-cracks due to thermal shock.
 The preferred carbon steel construction of autoclave 10 offsets the low
 heat transfer coefficient of stainless steel alone. Thus, the time for
 heating up and cooling down can be reduced advantageously. Furthermore, a
 fine grain carbon steel, such as WB36, has a tensile strength which
 diminishes only 12% between 100.degree. C. and 350.degree. C., and a
 maximum of 28% between 100.degree. C. and 450.degree. C., a regime wherein
 stainless steel's tensile strength drops dramatically, necessitating thick
 stainless steel autoclave walls and exacerbating the heating/cooling time
 problem. Table I provides data on the tensile strength of WB36 or
 15NiCuMoNb5 at various thicknesses and temperatures.
 TABLE I
 Data of Tensile Strain at Elevated Temperatures
 Shell R.sub.p 0.2 at temperature .degree. C.*
 Thickness Sample 100 150 200 250 300 350 400
 450
 mm Direction Newtons/mm.sup.2
 .ltoreq.50 transverse 422 412 402 392 382 373 343
 304
 &gt;50-.ltoreq.100 402 392 382 373 363 353 333
 304
 &gt;100-.ltoreq.125 392 382 373 363 353 343 324
 294
 &gt;125-.ltoreq.150 382 373 363 353 343 333 314
 284
 &gt;150-.ltoreq.300 373 363 353 343 333 323 304
 274
 *R.sub.p 0.2 = 0.2% elasticity boundary condition of fine grain carbon
 steel WB 36 or 15 NiCuMoNb 5.
 Carbon steel alone not only suffers an incrustation problem but is also
 unable to withstand the expected acid pH range (0&lt;pH&lt;7) or alkali pH range
 (7&lt;pH&lt;14), whereby such acidic or alkali media would attack the carbon
 steel. This is effectively remedied by the cladding with nickel or
 stainless steel.
 As previously stated, it is recommended that the inside of the fine grain
 carbon steel vessel be cladded with stainless steel to a thickness of 3
 mm. On the other hand, there are certain reactions that are carried out
 under extreme alkali conditions leading to leaching of the stainless steel
 cladding where those impurities are consequently leading to an undesired,
 contaminated end product, as is the case with, for example, Nickel Zinc
 Ferrite, which is used as a magnetic ceramic for application in the high
 frequency telecommunications and computer data storage. This can be
 overcome by coating the inside of the autoclave 10 wall with a layer of
 nickel through well known electrodeless and electrode deposit techniques.
 Referring back to FIG. 1, autoclave 10 has a pressure outlet port 10a
 controlled by pressure release valve 10b and an inlet port 10c controlled
 by valve 10d, both valves being of a type well known in the art. An
 agitator 10e has a plurality of paddles 100a, 100b to 100i where i is an
 integer equal to the maximum number of paddles used with agitator 10e. The
 blades on the paddles are preferably of the Interprop.RTM. type supplied
 by Ekato Corporation of Germany. The Interprop.RTM. mixing blades achieve
 a greater heat transfer from the wall of autoclave 10 into the suspension
 at a lower energy consumption. In FIG. 1, three paddles are shown on
 agitator 10e; however, a different number of paddles can be used if
 desired based upon experimental results. Agitator 10e is, in accordance
 with one embodiment of this invention, a variable speed agitator with a
 speed which varies from 30 rpm to 240 rpm. Of course, these speeds can
 also be changed if desired to achieve appropriate results depending upon
 the reaction product desired.
 Autoclave 10 is heated by the use of a thermal oil of well known
 constituents. The thermal oil is first heated in a thermal oil boiler (not
 shown but well known in the arts) and is then pumped through hollow
 semicircular coils wound in a plurality of banks on the outer surface of
 autoclave 10. FIG. 1 shows eight cross-sections 15a through 15h of one
 bank of such semicircular coils. Typically, four banks of coils are used,
 and one bank contains eight (8) spirals of heating coils which pass the
 thermal oil in one direction. The adjacent bank also contains eight (8)
 spirals of heating coils but passes thermal oil in the other direction.
 The use of the plurality of banks of coils minimizes the temperature drop
 of the heating oil in any one bank to ensure that the surface of autoclave
 10 is fairly uniformly heated in the steady state. In one embodiment, the
 temperature drop of the heating oil from the inlet to the outlet of the
 bank is kept to less than twenty degrees celsius. This small temperature
 drop coupled with the use of the agitators allows the temperature of the
 reaction product in autoclave 10 to be kept substantially uniform within
 about .+-.5.degree. C.
 Agitator 10e within autoclave 10 is controlled to mix the reaction product
 within autoclave 10 to ensure substantially uniform temperature throughout
 the reaction product. As a result, the crystal growth of the reaction
 product within autoclave 10 is also controlled to be substantially
 uniform.
 Autoclave 10 in FIG. 1 also includes an outlet valve 10f connected to an
 outlet line or pipe 14 (composed of sections 14a, 14b, and 14c). Outlet
 line 14 passes the reaction product from autoclave 10 through two heat
 exchangers 16 and 18. Heat exchanger 16 contains an inlet 16a and an
 outlet 16b for the passage of a fluid, such as water, into a secondary
 portion of heat exchanger 16 to withdraw heat from the reaction product
 flowing through line 14a. In the preferred embodiment, heat exchanger 16
 is used to generate steam, to be explained later, and will hereinafter be
 referred to as steam generator 16.
 The reaction product flowing through line 14a passes into steam generator
 16 at inlet 16c and out from steam generator 16 through outlet 16d.
 FIG. 2 illustrates one embodiment of steam generator 16. Steam generator 16
 cools the reaction product flowing into the primary portion of steam
 generator 16 such that the reaction product exiting steam generator 16 is,
 for example, 50.degree. C. cooler than the reaction product entering steam
 generator 16. To cool the reaction product as it flows through steam
 generator 16, a low temperature liquid, such as water, at a temperature
 of, for example, 15.degree. C. enters into a top portion of steam
 generator 16 via inlet 16a. This cool water entering steam generator 16
 flows around a plurality of internal pipes or tubes 16e formed of, for
 example, stainless steel, through which the reaction product flows.
 Steam generator 16 is designed so that the cool water entering inlet 16a
 accepts enough heat from the reaction product to exit as steam having a
 temperature of, for example, 200.degree. C.-400.degree. C. via outlet 16b.
 To maintain a laminar flow through lines 14a and 14b, the cross-sectional
 areas of lines 14a and 14b should be approximately the same as the sum of
 the cross-sectional areas of tubes 16e so that steam generator 16 will not
 present a flow resistance to the reaction products. Of course,
 consideration must also be given to other factors, well known to those
 skilled in the art, such as friction between the reaction product and
 tubes 16e, to maintain a laminar flow.
 As will be described later, the steam exiting from outlet 16b is supplied
 to a receiving vessel 12 so as to make use of the energy in the steam and
 to control the pressure in receiving vessel 12.
 Preferably, steam generator 16 uses a counter-flow of cooling fluid,
 whereby the cool fluid enters near the cooled reaction product exit
 portion 16d of steam generator 16, and the steam exits near the hotter
 reaction product entrance portion 16c of steam generator 16 so as to
 minimize mechanical stresses within steam generator 16 due to temperature
 differences.
 The calculations which one may use to design steam generator 16 are given
 in the example below. For the sake of simplicity, it is considered that
 the slurry contains 100% water, when actually the slurry has about a 10%
 solids content. Accurate calculations, however, require that the energy of
 the solids be taken into consideration, but the deviation in the final
 results is not very significant.
 Assume autoclave 10 has a volume of 6000 liters, and a reaction product
 fills 80% of this total volume, equalling 4800 liters or kilograms of
 reaction product.
 Assume this amount has to be transferred to receiving vessel 12 within 20
 minutes or 1200 seconds. Then the flow rate of reaction product must be:
EQU 4800 kgs/1200 sec=4 kg.multidot.sec.sup.-1 (eq. 1)
 Steam generator 16 is a counterflow type and the following coolant/reaction
 product input/output temperatures apply:
EQU T.sub.slurry in 250.degree. C.=524 K
 T.sub.slurry out 200.degree. C.=474 K
EQU T.sub.cooling in 15.degree. C.=289 K
EQU T.sub.steam out 200.degree. C.=474 K
EQU U=0.825 kW.multidot.m.sup.-2.multidot.K.sup.-1, (eq. 2)
 where U is the heat transfer coefficient of steam generator 16, as
 determined by its stainless steel construction, where m is meters, and K
 is Kelvin.
 Given the above assumptions, steam generator 16 is required to cool the 4
 kg.multidot.sec.sup.-1 slurry from 524 K to 474 K by means of X
 kg.multidot.sec-1 cooling water entering inlet 16a at 289 K and leaving
 outlet 16b as steam at 474 K. The heat transfer constant of steam
 generator 16 is 0.825 kW.multidot.m.sup.-2.multidot.K.sup.-1.
 The constant heat load of the slurry is:
EQU Heat load Q=4 kg.multidot.sec.sup.-1.multidot.4.18
 kW.multidot.sec.multidot.kg.sup.-1.multidot.K.sup.-1 (524 K-474 K)=836 kW.
 (eq. 3)
 The flow (X) of cooling water entering inlet 16a is calculated as follows:
EQU 836 kW=X kg.multidot.sec.sup.-1.multidot.4.18
 kW.multidot.sec.multidot.kg.sup.-1.multidot.K.sup.-1 ( 474 K-289 K) (eq.
 4)
 Therefore,
 ##EQU1##
 Therefore, in 20 minutes (1200 secs), 1320 kgs (1200 sec.multidot.1.1
 kg.multidot.sec.sup.-1) of cooling water are required. The 1320 kgs of
 water leave steam generator 16 as steam at a temperature of 474 K and a
 saturated steam pressure of 1.55 Megapascals (MPa) (15.5 bars). The above
 simplified calculations ignore the nonlinearity of water temperature
 versus heat absorbed when changing phase from liquid to steam.
 According to the standard tables of properties of water and steam in
 SI-units, the volume of 1 kg steam at 1.55 MPa equals 0.1275 m.sup.3.
 Therefore, 1320 kgs of steam has a volume of 1320.times.0.1275=168.3
 m.sup.3.
 As previously discussed, the steam exiting steam generator 16 from outlet
 16b is fed into receiving vessel 12, which is preheated to approximately
 the temperature of the steam leaving steam generator 16. If receiving
 vessel 12 has a volume of 8 m.sup.3 (8000 liters) and it was filled with
 air to 2.0 MPa (20 bars) pressure, then receiving vessel 12 would require
 8.times.20 bars"160 m.sup.3 of air. This is virtually the same as the
 total volume of steam generated by steam generator 16.
 In order to initiate a flow of reaction product from autoclave 10 to
 receiving vessel 12, a pressure difference (.DELTA.P) is required. The
 following equation applies:
EQU P.sub.r -.DELTA.P=P.sub.s, (eq. 6)
 where
 P.sub.r =reaction pressure in autoclave 10 in MPa
 .DELTA.P=pressure difference in MPa
 P.sub.s =saturated steam pressure in receiving vessel 12 in MPa
 Transfer always should take place under laminar conditions (Reynolds
 Number.ltoreq.2000), usually not exceeding 0.4 MPa (4 bars) pressure
 difference between autoclave 10 and receiving vessel 12, however, the
 pressure difference could be as high as 1.0 MPa (10 bars) in the case the
 viscosity of the slurry is high. The latter could be the case in the event
 the solids content in autoclave 10 is high or reaction times have exceeded
 a certain time, leading to an increase of viscosity, thus requiring a high
 pressure difference to enable a proper discharge from autoclave 10,
 through steam generator 16, through heat exchanger 18, and into receiving
 vessel 12.
 Nevertheless, assuming .DELTA.P=0.4 MPa (4 bars), this would mean that the
 pressure in receiving vessel 12 should be about 4 bars below the saturated
 steam pressure in autoclave 10. Since at 250.degree. C. autoclave 10 will
 have an internal pressure of about 4 Mpa (40 bars), a pressure of 3.6 MPa
 (36 bars) in receiving vessel 12 is a minimum requirement.
 The logarithmic mean (.theta..sub.m) temperature difference between the
 slurry and the cooling water is calculated to be:
 ##EQU2##
 The total surface area (A) of the high pressure steam generator 16 required
 in order to lower the temperature of the slurry from 524 K to 474 K while
 raising the temperature of the coolant water from 289 K to 474 K is:
 ##EQU3##
 where Q is heat load in kW (previously calculated);
 A is the surface area (m.sup.2) of steam generator 16; and
 U is the heat transfer coefficient (W.multidot.m.sup.-2.multidot.K.sup.-1)
 of the steam generator 16 material (stainless steel).
 Assume the inner diameter of the high pressure line 14a connecting the
 bottom of autoclave 10 with the intake port 16c of steam generator 16 is
 0.148 m. The cross-sectional area of line 14a thus equals .pi.r.sup.2 or
EQU .pi..multidot.0.074.sup.2 =1.72.multidot.10.sup.-2.multidot.m.sup.2. (eq.
 9)
 Assume the cross-sectional area of one tube 16e in steam generator 16 is
 3.14.multidot.10.sup.-4.multidot.m.sup.2 (i.e., 0.02 m inner diameter).
 This means one would need
 ##EQU4##
 tubes 16e to get the same total cross-sectional area of tubes 16e as the
 cross-sectional area of line 14a. The total surface area (A) was
 calculated to be 9.8 m.sup.2 and the outside circumference of each tube
 16e is assumed to be 7.86.multidot.10.sup.-2 m (i.e., 0.025 m outer
 diameter). From that data the total length of tubes 16e can be calculated:
 ##EQU5##
 Since the minimum number of tubes 16e is 55, each individual tube 16e
 length is:
EQU 125/55=2.27 meters (eq. 12)
 Thus, given the above assumptions and calculations, team generator 16 of
 FIG. 2 will be approximately 3.27 including two semispherical heads)
 meters in length and contain 55 tubes 16e.
 From outlet 16d of steam generator 16, the reaction products flow through
 pipe 14b and into a second heat exchanger 18 (FIG. 3). Heat exchanger 18
 includes a coolant inlet 18a and a hot water outlet 18b.
 This second heat-exchanger 18 is similar in its general construction to
 steam generator 16 of FIG. 2; however, it requires a different total area
 as calculated below. Assume the following conditions:
EQU T.sub.slurry in 200.degree. C.=474 K
EQU T.sub.slurry out 100.degree. C.=374 K
EQU T.sub.cooling in 15.degree. C.=289 K
EQU T.sub.cooling out 90.degree. C.=364 K
EQU U=0.825 kW.multidot.m.sup.-2.multidot.K.sup.-1
 Heat-exchanger 18 is required to cool 4 kg.multidot.sec.sup.-1 slurry from
 200.degree. C. (474 K) to 100.degree. C. (374 K) by means of X
 kg.multidot.sec.sup.-1 cooling water entering at 15.degree. C. (289 K) and
 leaving as hot water at 90.degree. C. (364 K). The heat load Q is
 calculated to be:
EQU Heat load Q=4 kg.multidot.sec.sup.-1 4.18
 kW.multidot.sec.multidot.kg.sup.-1 K.sup.-1 (474 K-374 K) (eq. 13)
 or
EQU Q=1672 kW.
 The amount of cooling water is calculated as follows:
 ##EQU6##
 In 20 minutes (1200 sec),
EQU 1200.times.5.3=6,360 kgs (eq. 15)
 The logarithmic mean temperature difference .theta.m is:
 ##EQU7##
 The total surface area of heat exchanger 18 is therefore:
 ##EQU8##
 Using the same type tubes as in steam generator 16, the total length of the
 required tubes would be:
 ##EQU9##
 Since the same number (55) of tubes in steam generator 16 is needed in heat
 exchanger 18, the total length of each tube in heat exchanger 18 is:
 ##EQU10##
 Thus, given the above assumptions and calculations, heat exchanger 18 of
 FIG. 3 will be approximately 6.1 meters in length (including two
 semi-spherical heads) contain 55 tubes.
 The cooled slurry exiting heat exchanger 18 flows into line 14c. Line 14c
 is connected to receiving vessel 12 via valve 12c.
 Vessel 12, like autoclave 10, contains an agitator 12a containing a
 plurality of paddles 120a, 120b . . . through 120i, where i is an integer
 representing the number of paddles on agitator 12a. The blades of paddles
 120 are also preferably the Interprop.RTM. type from Ekato Corporation of
 Germany. One embodiment of this invention uses four such paddles 120
 although, again, the number of paddles used can be determined empirically
 depending upon the quality desired for the resulting product.
 Vessel 12 is heated by thermal oil with FIG. 1 showing ten cross-sections
 13a through 13j of the semicircular coils through which the thermal oil is
 pumped. In one embodiment, this thermal oil is pumped at a rate
 substantially higher than for the autoclave 10. The walls of vessel 12 are
 sized to have a relatively large latent heat capacity to prevent vessel 12
 from cooling down too quickly.
 Vessel 12 has an outlet 12e with a valve 12f for controlling the removal of
 material from vessel 12. In addition, a standard valve 12d is provided to
 relieve the remainder of the pressure in vessel 12 after the total
 discharge of vessel 12 has taken place. Valve 12d may be controlled by
 control mechanism 12g.
 The self-siphoning mechanism in the preferred embodiment is established by
 first introducing, via inlet valve 12b, a fixed amount of water necessary
 to achieve a saturated steam pressure in vessel 12 after vessel 12 is
 heated. Upon heating by use of the heating coils 13a-13j, the water and
 vessel 12 will both be at the desired same temperature, with the entire
 vessel 12 acting as a heat reservoir. Thus, any cooling of vessel 12 will
 take place slowly and evenly. An initial pressure is now established
 within vessel 12 which should be less than the pressure in autoclave 10.
 In one embodiment, vessel 12 is initially heated to 474 K (200.degree. C.)
 while the reaction in autoclave 10 is taking place at 524 K (250.degree.
 C.). The slurry exiting heat exchanger 18 may be approximately 100.degree.
 C.
 Thus, when the cooled reactant leaves heat exchanger 18 and enters into the
 hot receiving vessel 12, it will immediately start to evaporate, creating
 a certain amount of pressure within receiving vessel 12 which controls the
 self-siphoning due to the pressure being less than in autoclave 10. In the
 preferred embodiment, the pressure so produced is such that the reaction
 product is transferred through line 14 under laminar flow conditions
 thereby preventing the crystal structure of the reaction product from
 degrading.
 For safety's sake, pressure transducers placed on the top of autoclave 10
 and vessel 12 are also connected to safety control circuits to prevent the
 inadvertent opening by individuals operating the system of any valves
 during the reaction process. In addition, safety valves are placed on the
 top of autoclave 10 and receiving vessel 12 to relieve pressures within
 these vessels should these pressures exceed safety limits.
 A more detailed description of the operation of steam generator 16 and its
 function in controlling the pressure in receiving vessel 12 will now be
 described. Assuming the initial temperature of the slurry within autoclave
 10 is 524 K, steam generator 16, in one embodiment, may be designed to
 cool the slurry down to 474 K and produce steam exiting at outlet 16b of
 steam generator 16 at a temperature of 474 K. A pressure transducer (not
 shown) connected at the output 16b of steam generator 16 is set at the
 required pressure to only allow steam to exit. The cool water pumped into
 the secondary side of steam generator 16 always has a higher pressure than
 the steam pressure outputted at output 16b of steam generator 16. Thus,
 once the pressure transducer allows steam to exit from output 16b, water
 is continually pumped into steam generator 16 so as to provide this
 continuous generation of steam.
 The steam outputted at output 16b is not required to be injected into
 receiving vessel 12 in order for the self-siphoning effect to take place;
 however, in the preferred embodiment, this steam is provided to receiving
 vessel 12 via line 19 and valve 19a. This not only conserves energy and
 improves the efficiency of the system, but the steam generated by steam
 generator 16 may be used to keep receiving vessel 12 at a certain pressure
 just equal to or slightly below the pressure in autoclave 10 to control
 the flow rate of the reaction product.
 For example, it is primarily the temperature of the slurry exiting heat
 exchanger 18 that sets the requirements for the self-siphoning effect
 (i.e., the pressure within receiving vessel 12).
 In the case where the temperature of the slurry entering receiving vessel
 12 becomes too high (for example, above 100.degree. C.), less vapor will
 condense on the walls of receiving vessel 12, thus maintaining too high a
 pressure within receiving vessel 12 and thus lowering the transfer speed
 of reaction product from autoclave 10 to receiving vessel 12. By
 controlling the flow of water entering steam generator 16 via inlet 16a,
 the temperature of the slurry may be controlled as well as the temperature
 of the steam exiting outlet 16b. For example, a greater flow of coolant
 into inlet 16a will lower the temperature of the slurry exiting steam
 generator 16 as well as lowering the steam temperature at outlet 16b. This
 will cause a lowering of the steam temperature and pressure within
 receiving vessel 12 so as to increase the pressure differential between
 autoclave 10 and receiving vessel 12 to thus increase the transfer speed
 of the reaction product from autoclave 10 to receiving vessel 12.
 In the event that the flow rate of the reaction product is too great, less
 water will be pumped into steam generator 16 causing the steam outputted
 from steam generator 16 to be of a greater temperature and at a higher
 pressure so as to increase the pressure within receiving vessel 12 and
 thus slow down the transfer speed of the reaction product.
 Heat exchanger 18 is used to lower the temperature of the slurry to a
 selected temperature so that the resulting reaction product may be held in
 receiving vessel 12 without the reaction product undergoing any further
 reaction. Thus, the resulting reaction product may be transferred from
 receiving vessel 12 via outlet 12e at any later time. Frequently, it may
 be desired to conduct a secondary reaction process in receiving vessel 12,
 and thus heat exchanger 18 will be operated accordingly to reduce the
 temperature of the reaction product to the desired temperature for this
 secondary reaction process.
 Since the temperature of the slurry through heat exchanger 18 is fairly low
 compared to the temperature of the slurry through steam generator 16, the
 water exits from the output 18b of heat exchanger 18 as hot water as
 opposed to steam. This hot water may then be stored for use in a next
 reaction process within autoclave 10.
 When the reaction product has been completely transferred from autoclave 10
 to receiving vessel 12, there will still be a residual pressure in
 autoclave 10 which may exceed 1 MPa or 10 bars. To lower this pressure
 safely, the flow of water into the secondary portion of steam generator 16
 may be stopped while continuing to flow water into the secondary portion
 of heat exchanger 18 via inlet 18a. By doing so, the vapor pressure in
 autoclave 10 is reduced to virtually atmospheric pressure, while hot water
 is being generated by heat exchanger 18 for subsequent batches. A hot
 water storage vessel (not shown) may store the hot water outputted from
 output 18b.
 The residual hot water which has been outputted by heat exchanger 18 and
 stored may also be used to provide the initial pressurization of receiving
 vessel 12 so that less energy is required to be externally supplied to
 receiving vessel 12 to heat the water to the temperature needed to
 generate the required initial pressure in receiving vessel 12.
 Generally, the maximum temperature drop between the slurry entering and
 leaving steam generator 16 should not exceed approximately 50.degree. C.
 By using a separate heat exchanger to generate steam, as opposed to a
 single heat exchanger which would merely generate hot water, steam
 generator 16 may be made fairly small so as to withstand very high
 pressures and temperatures and produce an energy-valuable steam resource
 which may be used as a control tool in the transfer of reaction products.
 The specialized use of steam generator 16 enables one to form, using
 standard materials, a steam generator which may operate at temperatures
 near critical, at critical, or supercritical temperatures of water. The
 commercial applications of conducting reactions at such temperatures are
 known to those skilled in the art. Some applications regarding the
 destruction of toxic waste are described in the article "Supercritical
 Water, a Medium for Chemistry," by R. Shaw et al., C&EN, Dec. 23, 1991,
 incorporated herein by reference. At such high temperatures, a vast amount
 of energy would be needed to maintain a suitable pressure in reaction
 vessel 12. However, with steam being generated by steam generator 16
 around these temperatures, much less energy is needed to maintain an
 adequate pressure in reaction vessel 12.
 Although water has been specifically mentioned as the coolant for steam
 generator 16 and heat exchanger 18, and as an ingredient in the reaction
 process itself, other fluids may be used as would be obvious to those
 skilled in the art after reading this disclosure.
 By increasing the temperature and the corresponding saturated steam
 pressures in a reactor system to near critical, at critical, or above
 critical temperatures, the required dimensioning and ruggedness of
 autoclaves and heat exchangers become extraordinary, and investment costs
 cannot be balanced against the economic justification of producing fine
 ceramic crystals resulting from such reaction processes. The two-stage
 transfer system involving steam generator 16 and heat exchanger 18 avoids
 the shortcomings ascribed to these reactors in that smaller size heat
 exchangers are required which are simple to manufacture and are at the
 same time able to sustain extremely high temperatures and pressures. As a
 consequence of this, they are far less expensive to manufacture to meet
 the demands of such a reactor. By introducing the two-stage transfer
 system, high calorie-rich slurry can be cooled to generate high pressure
 steam. This steam, in conjunction with preheating the receiving vessel 12
 and by partly evaporating the relatively cool slurry leaving heat
 exchanger 18, leads to a very energy efficient and cost efficient
 self-siphoning discharge system for the reactor.
 One type of reaction product which has been shown to have better
 performance when formed under supercritical conditions is Nickel Zinc
 Ferrite for use in, for example, high frequency telecommunications, and
 computer data storage.
 An alternative embodiment of the reactor of FIG. 1 is shown in FIG. 4
 where, instead of a receiving vessel which is closed, receiving vessel 20
 is an open vessel where its internal pressure is maintained at
 approximately one atmosphere (1 bar). In FIG. 4, vessel 20 is depicted as
 having an opening 22 being open to the atmosphere. The elements in FIG. 4
 which are designated with the same designation as those elements in FIG. 1
 have similar structures and functions. However, in FIG. 4, there is no
 separate steam generator, and heat exchanger 18 must be designed and
 operated to lower the temperature of the slurry entering line 14a to the
 desired final temperature of the slurry entering open vessel 20.
 Once a reaction has been completed in autoclave 10, valve 10f is opened and
 the reaction product flows through line 14a and into heat exchanger 18 due
 to the pressure differential between autoclave 10 and open vessel 20. Due
 to the large pressure differential, the flow of reaction products through
 line 14 will be relatively swift, which may make this transfer system
 unsuitable for those reaction products whose crystal structures would
 degrade when subjected to rapid changes in temperature and pressure. The
 system of FIG. 4 may be suitable for processes involving the
 transformation of a raw material's mineral structure into another mineral
 structure.
 Since the initial rush of reaction product into an empty line 14 may be
 more rapid than once the reaction product has completely filled line 14,
 heat exchanger 18 will not adequately cool the reaction product during
 this initial flow. To slow down the reaction product when it is initially
 being passed through line 14, so as to sufficiently cool the reaction
 product by heat exchanger 18, valve 12c is initially closed to form an air
 block in line 14. When valve 10f is then opened, the back air pressure in
 line 14 will generate a pressure buildup in line 14 and slow down the flow
 of reaction product in line 14 so that heat exchanger 18 may adequately
 cool the reaction product to the final temperature. Once the flow of
 reaction product is sufficiently low to be adequately cooled by heat
 exchanger 18, valve 12c is opened so that the flow of reaction product
 reaches a steady state through line 14 and is cooled by heat exchanger 18
 to the desired temperature.
 As an example of how to design heat exchanger 18, the below analysis is
 provided.
 Assume the reaction in autoclave 10 will take place at 250.degree. C. and
 heat exchanger 18 is required to cool the slurry down to 90.degree. C.
 with a coolant being applied to inlet 18a at 15.degree. C.
 Also assume 4800 kgs of slurry are to be discharged in 1200 seconds. The
 rate is therefore 4 kg.multidot.sec.sup.-1. The initial temperature
 conditions are:
EQU T.sub.i (reaction) 250.degree. C..tbd.524 K
EQU T.sub.o (final temp.) 90.degree. C..tbd.364 K
EQU T.sub.i (coolant) 15.degree. C..tbd.289 K
EQU T.sub.o (hot water) 90.degree. C..tbd.364 K
 The heat load Q is calculated to be:
EQU Heat load Q=4 kg.multidot.sec.sup.-1.multidot.4.18
 kW.multidot.sec.multidot.kg.sup.-1.multidot.K.sup.-1 (524 K-364 K)=2675
 kW. (eq. 20)
 The rate of cooling water required is:
EQU 2675 kW=X.multidot.4.18
 kw.multidot.sec.multidot.kg.sup.-1.multidot.K.sup.-1.multidot.(364 K-289
 K) (eq. 21)
 ##EQU11##
 The mean logarithmic temperature (.theta..sub.m) is
 ##EQU12##
 The surface area (A) of heat exchanger 18 is calculated to be:
 ##EQU13##
 Using the same tubes 16e (FIG. 2) as previously described, the total length
 of tubes 16e must be:
 ##EQU14##
 The number of tubes 16e will depend on the cross-sectional area of line 14a
 in FIG. 4.
 Assuming line 14a in FIG. 4 is identical with line 14a in FIG. 1, then the
 individual pipe length is:
EQU 367.6/5.5=6.68 meters
 The process and structure described above is multi-purpose in the sense
 that the process and structure can be used to provide a number of
 different reaction products. Using a hydro/solvo thermal reaction with
 this invention to form ceramic powders or other substances saves a
 substantial amount of energy over standard methods for the formation of
 such ceramic powders. Moreover, the hydro/solvo thermal reaction provides
 ceramic materials or other substances of substantially uniform crystal
 size in a powder like form.
 The reaction product is formed by controlling the temperature of the
 reaction constituents within autoclave 10 to within a selected value for a
 selected period of time at a desired pressure.
 As disclosed in a report published by Lawrence Berkeley Laboratory,
 University of California (LBL-14722), entitled "A Database for Nuclear
 Waste Disposal for Temperatures up to 300.degree. C.," by Sidney L.
 Phillips and Leonard F. Silvester, September 1982, the amount of inorganic
 susbstance in solution can be calculated according to equation 15 set
 forth in that paper. That equation states that log S (where S is the
 solubility in water in gram moles per liter) is a function of temperature.
 Using that equation, and other equations set forth below, one can calculate
 the solubility in gmol per liter of reaction constituents dissolved in
 water at the preferred temperature. As the temperature goes up, the amount
 of material dissolved also goes up. Accordingly, there is a substantial
 advantage, not only with respect to solubility but also with respect to
 controlled crystal growth, to operating autoclave 10 at a higher
 temperature and pressure than previously considered advisable.
 To aid in selecting a process temperature for a specific reaction, the
 following calculations are provided. The symbols used are defined in
 Appendix A.
 Equilibrium Constants
 The database centers on values of equilibrium constants, log K.degree. at
 25.degree. C. and zero ionic strength. The chemical equilibria are mostly
 hydrolysis, complexation and ionization reactions. These intrinsic data
 are calculated from the equations:
EQU .DELTA..sub.r G.degree.=66.sub.r H.degree.-T.DELTA.S.degree. (eq. 26)
 ##EQU15##
 Values of .DELTA..sub.r H.degree. and .DELTA.S.degree. for each chemical
 reaction are calculated from the sum of the .DELTA..sub.f H.degree. and
 .DELTA.S.degree. of the products, minus this sum for the reactants.
 Temperature Effects
 Equilibrium quotients are computed from 25-300.degree. C. based on the
 following equation:
 ##EQU16##
 Equation 28 assumes heat capacity change is constant over the temperature
 range of interest; this assumption is certainly not valid above perhaps
 100.degree. C. for the majority of reactions. However, if chemical
 reactions are written such that both sides of a chemical equation have an
 equal number of like charges, then constancy of .DELTA.C.sub.p.degree. is
 a more tenable assumption. This "balanced like charges" approach and eq.
 28 are used in this database to the extent possible. C.sub.p.degree.=0
 will be assigned for uncharged aqueous substances such as U(OH).sub.4
 (aq). In absence of other data, this assumption is used for this database.
 Linearity is improved by writing eq. 28 in the following form, when
 plotting values of log Q as a function of 1000/T(K):
 ##EQU17##
 Because of the improved linearity, extrapolation can be made to higher
 temperatures with more confidence with eq. 29. This equation should be
 used whenever data are available on heat capacity of chemical reactions at
 25.degree. C.
 A typical prior art process for forming ceramic powders involves melting
 ingredients at a very high temperature (2800.degree. C.-3000.degree. C.),
 allowing the melted ingredients to cool in a large block to ambient
 temperatures, crushing the block into smaller parts, coarsely grinding the
 smaller parts to yield rough crystals and then finely grinding the rough
 crystals to yield fine powders. By using my invention, this energy
 intensive process is totally avoided. My hydro/solvo thermal process will
 directly produce fine crystal powders. In one preferred method, the
 hydrothermal reaction takes place at a temperature at, near, equal to, or
 above supercritical temperatures rather than at several thousand degrees
 celsius. By controlling the time of reaction, the size of the ceramic
 crystals can be fairly accurately controlled to the desired dimension.
 Thus the process described above yields a substantial improvement in the
 formation of uniform crystals of reaction products over the prior art both
 in terms of energy consumed and the uniformity of the resulting structure.
 In addition, the prior art grinding procedure yields crystals of nonuniform
 and differing sizes even though the resulting materials are substantially
 fine. This creates certain problems in using these crystals to form
 finished products. In particular, ceramic materials are known to be
 brittle despite their other desirable characteristics. Because of this
 shortcoming, ceramic materials find fewer applications in advanced
 technology than justified by their potential benefits. Thus research is
 being done to increase the lifetime and prolong the fatigue limits of
 ceramic materials such that ceramic materials can be used in new
 applications to replace a variety of metal composites. However,
 nonuniformity of ceramic crystal size yields a nonuniform bonding force
 which in itself relates to discrepancies in the atomic structure of the
 ceramic crystal making up the ceramic materials. Scanning electron
 microscope (SEM) exposures of ceramic materials show that fatigue starts
 at those places where there are substantial differences in uniformity of
 the ceramic crystals. Apparently the bonding energy between nonuniform
 crystals is unable to find a so-called harmonic neighbor thus leading to
 spontaneous fatigue because of the differences in the bonding energy
 between different size crystals within the material. At this stage of the
 technological development of materials from ceramic crystals, several
 companies have acquired improved crystal size uniformity obtained using a
 grinding process but still the uniformity is not sufficient to allow the
 proven material to be used in high technology applications such as, for
 example, blades for jet engines. Thus considering these factors, the
 process of my invention makes possible the fabrication of uniform powders.
 The present invention also offers the following advantages over that
 disclosed in my previous '787 patent. There is no flowmeter for the
 reaction product required, since the flow of reaction product to the
 vessel 12 is self-adjusted. Nor is it necessary to install expensive
 electronic equipment to monitor any flowmeter signal in order to trigger a
 pressure relief valve on the receiving vessel. There is only a standard
 relief valve to vent the remaining pressure after total autoclave
 discharge has taken place. There is no requirement of gas transfer, via a
 by-pass line, from the autoclave to the receiving vessel prior to the
 reaction product transfer. The cooled reactant leaving the heat exchanger
 generates sufficient pressure upon entering the heated receiving vessel
 that the transfer takes place at a rate such that the flow is
 near-laminar, given an appropriate matching of the dimensions of the
 autoclave and receiving vessel. The initial thin film of water on the
 walls of receiving vessel 12 ensures that vessel 12 will not cool down too
 rapidly, with its potentially deleterious consequences on the quality of
 the reaction products. Also, the heating requirements on receiving vessel
 12, so as to evaporate the water and generate sufficient vapor pressure,
 is far less than in my previous invention. Thus, this invention offers
 tremendous scope for a batch procedure requiring less energy, with a far
 lower overall investment cost, and simplified procedure ensuring near
 continuous operation.
 My preferred process and system include the following additional structural
 and functional characteristics:
 1. Temperatures of the reaction process are controlled within .+-.5.degree.
 C.;
 2. Pressure transducers used are preferably sputtered film transducers of a
 type made available by CEC Corporation in Pasadena, Calif.;
 3. Variable speed stirring equipment uses INTERPROP.RTM. blades of a type
 provided by Ekato Corporation of Germany;
 4. Reaction products are reproducible as a function of reaction time and
 temperature;
 5. Less energy is used than prior art processes;
 6. Many different reaction products are capable of being made with the same
 system;
 7. Higher precalculable solubility for reaction constituents are attained
 to allow accurate characterization of the process;
 8. Autoclave wall size is reduced by the use of WB 36 or 15 Ni Cu Mo Nb 5
 from Thyssen Stahl AG, of Duisburg, Germany, cladded with nickel or
 stainless steel AISI 316 Ti permitting faster heat-up and cool-down, while
 marginally decreasing the autoclave tensile strength;
 9. The system is capable of being economically designed and fabricated so
 that it may operate near, at, or above critical temperatures.
 In view of the above, other embodiments of this invention will be obvious
 to those skilled in the art.
 APPENDIX A
 Symbols, Units and Conversion Factors
 .DELTA..sub.f G.degree.=Gibbs energy of formation, 25.degree. C., I=0; kJ
 mol.sup.-1
 .DELTA..sub.f H.degree.=Enthalpy of formation, 25.degree. C., I=0; kJ
 mol.sup.-1
 S.degree.=Entropy, 25.degree. C., I=0; J mol.sup.-1 K.sup.-1
 C.sub.p.degree.=Heat capacity, 25.degree. C.; J mol.sup.-1 K.sup.-1
 .DELTA..sub.r G.degree.=Gibbs energy of reaction, 25.degree. C., I=0; J
 mol.sup.-1
 .DELTA..sub.r H.degree.=Enthalpy of reaction, 25.degree. C., I=0; J
 mol.sup.-1
 .DELTA.S.degree.=Entropy change for reaction, 25.degree. C., I=0; J
 mol.sup.-1 K.sup.-1
 .DELTA.C.sub.p.degree.=Heat capacity change for reaction, 25.degree. C.,
 I=0; J mol.sup.-1 K.sup.-1
 J=joules; cal=calories
 kJ=kilojoules=1000 joules
 I=ionic strength, mol kg.sup.-1
 K=273.15+.degree. C., C.=celsius
 mol=gram molecular weight
 Q=equilibrium quotient, or product
 K.degree.=intrinsic equilibrium constant, 25.degree. C., I=0
 R=gas constant: 8.3143 J mol.sup.-1 K.sup.-1 ; 1.987 cal mol.sup.-1
 g=gaseous form
 s=solid form
 am=amorphous form
 aq=aqueous form
 l=liquid form
 ##EQU18##