Note: Descriptions are shown in the official language in which they were submitted.
:
This invention relates to a no~el carbon dioxide
product and to the method by which it is manufactured.
In accordance with the present invention, finely
divided, particulate, solid phase carbon dioxide is produced
. as a dispersion in liquid carbon dioxide to form a pumpable
:~: slurry. Although the solid particles in the slurry may settle
~` on standing to form a thicker slurry beneath a layer of clear
:. liquid, this thicker slurry flows easily and the finely divided
~,
;. solid particles do not agglomerate on standing.
,~ Heretofore carbon dioxide (C02) has been commercially
` available only in any one of its three phases; i.e. as compressedgas, as liquid at superatmospheric pressure, or in solid phase,
usually in block or pellet form and referred to colloquially
as "dry ice". While carbon dioxide in each o~ these forms has
.,
, ,~ ., ., . ;j ;;
.~67(~09
great commercl~l value and utility, no one of them is ideally
suited to derlve maximum advantage of carbon dioxide as a
refrlgerant. For example ~olid phase carbon dioxide contain~
about twice the refrigeratlon value of a like weight of liquid,
but solid phase material does not lend itself to economical and
convenient materi~l~ handling. It i3 much preferred to handle
fluids that can be ~ped and pumped from point to p~int rather
than to handle blocks of solid. Finely divided solid C02
agglomerates on standing and cannot be handled as a free flowing
dry powder. Accordingly carbon dioxide iB now widely used for
refrigeration by expanding liquid from ~uperatmospheric pressure
(typically 300 p.s.i.g.) to atmospheric through an expansion
noæzle (cnow horn) to form solid particles (usually referred to
as snow) and cold gas. Whlle some useful refrigeration is derived
~rom the cold gas, it is the snow that is desired as the effec-
tive refrigerant and by expansion of liquid only about 45% of the
total weight of C02 passed through the nozzle is converted to
solid .
The present invention therefore has as its primary
ob~ect the production of a novel carbon dioxide product compris-
ing a pumpable slurry of the solid and liquid pha~es of carbon
dioxide which retains most of the thermodynamic advantage of
solid C02 and at the same time adapts to convenient material
handling methods.
The slurry of this invention provides a mixture of
solid and liquid carbon dioxide with the solid being in the
form of finely divided dispersed particles 80 that the mixture
i8 fluid and pumpable, It i8 a two-phase mixture that can exist
at any pressure and temperature along the solid-liquid equili-
brium curve for carbon dloxide a~ shown in Figure 1, which is aphase diagram ~or carbon dioxide. m e lowest pressure for liquid
on this equilibrium cur~e is 60 p.6.i.g. (75 p.s.i.a.) and the
--2--
.lL06'700~
lowest temperature is -70F, which pressure and temperature
occurs at the triple point, the only point on the curve where
all three phases of carbon dioxide, i.e. gas, liquid and solid,
can exist simultaneously. The solld particles in the slurry
show little or no tendency to agglDmerate on standing.
It is desired to have as high a concentration of
æolids in the slurry as possible and stlll have a pumpable
slurry. The proportion of ~olid carbon dioxlde in the ~lurry
of this lnvention can vary over a wide range up to about 85~
by weight and still exist a$ a pumpable fluid. Generally, the
solids concentration i8 in excess of about 10% by weight. This
10% product is A watery substance resembling skim milk in appear-
ance and as the solids increase the appearance becomes that of
a thicker more viscous white fluid.
A mixture of liquid C02 and solid C02 cry8tal8 i8 known as
a tran~bDry lntermediate during the production of æolld C02
blocks by evaporation of liquid at the triple point. Typical
prior patents disclosing such a method of making solid C02
blocks are Small U.S. Patent 1,884,313 and Jones U.S. Patent
1,877,180. However, since the ob~ect of these prior art processes
was to make dense compacted blocks of solid C02 no steps were
taken to cause the solid particles to remain in the liquid as
dispersed discrete particles that do not agglomerate on standing,
and no ~uch result was achieved.
There are two ma~or advantages for refrigeration pur-
poses in supplying carbon dioxide as a slurry instead of the con-
ventional liquid carbon dloxide at 300 p.s.i.g. The higher den-
sity of slurry (11.04 lb./gal. for slurry containing 50~ sollds
at the triple point pressure of 60 p.s.i.~. V8. 8.48 lb./gal. for
300 p.~.i.g. llquid) allows a greater amount (weight) of C02 to
occupy a given volume (therefore more refrigeration per unit
volume). Also ince solid C02 contains more refrigeration than
~067(:~09
a like weight of liquid, its presence ln the slurry contributes
to a higher refrigeration content per pound of this novel carbon
dioxide product. me refrigeration content of slurry and 300
p.s.i.g. liquid can be compared by noting that each pound of
carbon dioxide slurry containing 50% weight solids at 60 p.s.i.g.
can be converted into 0.77 pounds of carbon dioxide "snow" at
-109F and 0 p.s.i.g. ~hile one pound of 300 p.s.i.g. liquid
yields o.46 pounds of such "snow". This relationship is of more
than academic interest because C02 when used as a refrigerant is
generally used as solid blocks, pellets or snow. Expressed
differently, a slurry containing 50% by weight of solid contains
about 156~ of the refrigeration value of a like weight of 300
p.s.i.g. liquid. Similarly a 10~ solids slurry contains about
131~ and an 85% solids slurry contains about 178~ of the refrig-
eration capacity of pure liquid.
A pumpable slurry of finely divided, discrete particles
of solid C02 in liquid C02, in a form and state suitable for use
or sale on a commercial scale9 was unknown prlor to the present
invention. However in the broadest sense it is old to provide a
slush or filurry of solid particles in liquid of the same or di~fer-
ent composition for refrigeration and other purposes. The provi-
sion of solid particles of ice in water is of course a notorious~
old method for maintaining the water at or near the freezing
temperature as the body absorbs heat. Until all the ice is melt-
ed the heat absorbed changes the solid to liquid without raising
the liquid temperature. Similarly, but in a totally different
frame of reference, it i5 now known to prevent premature evapora-
tion of liquid hydrogen used as a rocket fuel by storing the
hydrogen as a slushl i.e. a mixture of solid particles of hydrogen
dispersed in liquid hydrogen at cryogenic temperatures. As heat
leaks into such a system solid is converted to liquid and until
all of the solid is melted no evaporation takes place and it is
10679
not neces~ary to valve off any of the precious and hazardous
fuel. In both the water and hydrogen systems referred to above
the solid phase is dispersed in the liquid primarily to provide a
heat sink that would, in effect, provide self-refrigeration and
maintain the sub-ambient temperature of the body of liquid even
though heat is absorbed by the body as a whole. While a similar
benefit is obtained from the C02 slurry of the present invention
a primary purpose of the invention is to provide a high percentage
of solids at the end point of use, usually at atmospheric pres-
sure, where C02 cannot exist as liquid at any temperature. Inthe hydrogen and water examples the slurry provides cold liquid
at the point of end use. In the present invention the slurry pro-
vides a maximum amount of solid material at the point of end use.
Elsewhere in the prior art U.S. Patent No. 3,393,152 to
Smith and Townsend, assigned to the same assignee as the present
inventlon, discloses a composition consisting of finely divided
particles of solld C02 in a medium of liquid nitrogen. In that
patented invention too, refrigeration capacity of the solid C02
; enhances the re~rigeration capacity of the liquid nitrogen withcut
impairing its characteristic as a fluid that can be piped and
pumped. The liquid nitrogen can, of course, exist at the point of
end use as a boiling liquid at atmospheric pressure which vapor-
izes first leaving the solid C02 behind to sublime. The total re-
frigeration capacity of tl~is product is greater than that of liqu~d
nitrogen alone, but it delivers its refrigeration in two steps and
at two temperatures, and therefore is not ideally suited to many
applications. In any event it is in no way a system for increas-
lng the yield of solid C02 from liquid C02 at the point of end use.
The present invention is a new and useful carbon
dioxide product consisting of a body of liquid carbon dioxide
at a pressure equal to or greater than the triple polnt pressure
containing small particles of solid carbon dioxide as discrete
--5--
OQ~3
free flowing unagglomerated particles, the whole composition
being capable of being handled as a fluid, such as by pipes and
pumps. ~he total refrigeration capacity of the composition is
the sum of that contained in the liquid and solid components and
therefore has the potential to deliver a greater amount of solid
when expanded to atmospheric pressure than does a like weight of
liquid alone. Since a slurry cannot readily be expanded through
a nozzle with a small orifice without either clogging the oriflce
or straining the solids from the liquid, the slurry i~ preferably
dispensed by a posltive displacement device such as a gear or
vane type pump operated as an expander rather than a pump.
Essentially the method employed to make the slurry,
according to the present invention, comprises evapo~ ting C02
from a body of liquid C02 at the triple point temperature and pre~
sure by venting the reaction vessel to permit further vaporizat~n
of liquid. The heat absorbed from the liquid body causes the
formation of solid C02 in the liquid in an amount related to the
amount of vapor formed. As the solid is formed, the bath i8 con-
tinuously and vigorously agitated. By controlling the rate of
formation of the solid carbon dioxide (by controlling the rate
at which gas is vented from the system) and the agitation, a
slurry is produced containing up to about 85% by weight of fine-
ly divided, particulate carbon dioxide dispersed in liquid car-
bon dioxide. The particulate material so formed does not show
any tendency to agglomerate on standing. Carbon dioxide slurry
prepared by the method of this invention flows freely and can be
handled as a &lurry at solid concentrations up to about 85~ on a
weight basis. The solid particles are relatively fine and do not
agglomerate on storage. Analysis of settling rates i~dicated
that the smalle~t particles are of the order of 4 mlcrons in max-
imum cross sectional dimension. It is believed that still finer
particle sizes can be produced according to the invention, for
1067009
example by increasing agitator speeds. The solid appears to be
crystalline in form. For most pumping systems particles
larger than about 1/4 inch (approximately 6 mm) in maximum
cross sectional dimension are undesirable as lumps of this
size, and larger, can more readily plug pipelines and pumps.
The larger particles are considered undesirable, not only
because they tend to plug tne pipelines and pumps, but also
because they are more difficult to retain in dispersion.
The preferred range of particle size is considered to be from
about 4 microns minimum to about 2 mm maximum.
The novel carbon dioxide product may be made by either
a batch process or continuously. The concentration of solid
in the slurry made by the batch process depends essentially
on the amount of gas vented from the reactor and the heat
leak into the reactor. The amount of gas vented is, of
course, the product of the rate at which gas is vented from
the reactor and the length of time the reaction is permitted
to proceed. In the continuous process the concentration of
solid depends on the liquid feed rate and the rate at which
gas is vented from the reactor as will be more fully explained
hereinafter. By either method the ultimate solids
concentration can be increased by concentrating the solids in
a vessel other than the reactor and recycling excess liquid.
In accordance with one broad aspect, the invention
relates to a process for the preparation of a pumpable carbon
dioxide slurry which comprises confining a body of liquid CO2
at a pressure above the critical pressure, venting CO2 vapor
above said body of liquid CO2 until the critical pressure is
reached, further venting carbon dioxide vapor from above said
body of liquid carbon dioxide at the triple-point pressure and
temperature in a reactor to cause discrete non-agglomerated
~7-
1067009
particles of solid carbon dioxide to be formed at an
evaporating surface of the liquid, agitating said liquid
carbon dioxide while venting said carbon dioxide vapor to
prevent agglomeration of said discrete particles as a crust
and to remove said solid carbon dioxide from said surface as
non-agglomerated fine discrete particles which are submerged
by said agitation in the body of said liquid thereby dispersing
said discrete solid particles in said liquid to form a solid
` liquid slurry, and removing the solid-liquid slurry so formed
from the reactor.
In accordance with another aspect, the invention relates
to a method for producing a refrigerant composition comprising a
a pumpable slurry consisting essentially of finely divided
particles of solid phase carbon dioxide dispersed in liquid
phase carbon dioxide comprising, withdrawing vapor phase
carbon dioxide at a controlled rate from a vessel containing
liquid phase and vapor phase carbon dioxide at the carbon
dioxide triple-point pressure to cause discrete non-
agglomerated particles of solid phase carbon dioxide to be
formed at an evaporating surface of the liquid, while
continuously agitating the liquid to prevent agglomeration
: of said particles as a crust, said vapor withdrawal rate being
controlled to produce a fluid and pumpable slurry of from 10%
to 85% by weight of finely divided particles of solid carbon
dioxide dispersed in the liquid, and said agitation being
sufficient to remove said particles of solid from said
evaporating surface to a position under the surface as said
particles are formed while preventing agglomeration of the
solid particles within the body of liquid, and continuously
recycling a portion of the removed mixture from said vessel
to said reactor to provide for further evaporation of liquid
in said reactor and concentration of the solids content in
~ -7a-
H
~067009
said vessel, with the remaining portion of said removed
mixture constituting the product slurry.
In accordance with a further aspect, the invention
relates to a continuous method for producing a refrigerant
composition comprising a pumpable slurry consisting essentially
of finely divided, small particles of solid carbon dioxide
dispersed in liquid carbon dioxide, comprising withdrawing
carbon dioxide vapor at a controlled rate from a vessel
containing liquid carbon dioxide at the triple-point pressure
of carbon dioxide to cause discrete non-agglomerated particles
of solid carbon dioxide to be formed at an evaporating surface
of the liquid while continuously agitating the liquid to
prevent agglomeration of said particles as a crust, said rate
of vapor withdrawal being controlled to produce a fluid and
pumpable slurry of from 10~ to 85~ by weight of small, finely
divided particles of solid carbon dioxide dispersed in liquid
carbon dioxide, said agitation being sufficient to remove said
particles from said evaporating surface as said particles are
formed while preventing agglomeration of the solid particles
within the body of liquid, and continuously withdrawing slurry
from the vessel and continuously introducing liquid carbon
dioxide into the vessel under triple-point conditions of
pressure and temperature at a rate corresponding to the rates
at which solid and liquid are withdrawn as product slurry and
vapor is withdrawn as by-product, and continuously recycling
a portion of the removed mixture from said vessel to said
reactor to provide for further evaporation of liquid in said
reactor and concentration of the solids content in said vessel,
with the remaining portion of said removed mixture constituting
the product slurry.
For a more complete description of the novel product and
~ -7b-
H
~067009
its methods of manufacture reference should be made to the
accompanying drawings in which:
Figure 1 is a phase diagram for carbon dioxide;
Figure 2 is a diagrammatic showing of a batch method
for the manufacture o CO2 slurry of the desired novel
characteristics;
Figure 3 is a diagrammatic showing of a continuous
method for the manufacture of CO2 slurry of the desired
-7c-
~6~0as
novel characteristics;
Figure 3A is a sectional plan vlew of the slurry gen-
erator shown in Fig. 3;
Figure 4 is a graph showing the relationship between
average density o~ the slurry and the percent solids contained
therein; and
Figure 5 i8 a graph showing the maximum percent solids
that can be formed for différent liquid evaporation rates~
A simple apparatus to carry out the novel process and
produce the novel product of the present invention is illustrated
' in Fig. 2.
In general the equipment consists of a 50 gal. reactor
10, equipped wlth a propeller type agitator 11, drlven by a vari-
able speed motor 12. Liquid C02 is admitted to the reactor from
a sultable supply which is typically a storage tank maintained at
approximately 300 p.s.i.g. and 0F. The liquid supply is admi~d
to the reactor under pressure through the supply line 13 and con-
trol valve 14. A suitably controlled nitrogen supply line 15
communicates with the head space of the reactor 10. A vapor line
16 leads from the head space of the reactor through a heat ex-
changer 17 and a vapor control valve 18 to a meter 19 and a vent
to atmosphere. Valve 20 and discharge line 21 provide for dis-
charge of the contents of the reactor through a sight flow in-
dicator 22 into a receiver 23 via line 24, or alternatively
direct discharge from the reactor to a point of use through valve
25 and discharge llne 26.
In operation a suitable quantity of liquid C02, for
example 35 gallOns~ is admitted to the reactor at 76 p.s.l.g.
~rom the 300 p.s.i.g. source of supply by closing valve 20 and
opening valve 14 and the vent valve 18. During the filli~¢ oper-
ation it i8 necessary to vent the reactor to remove displaced
air, the gas generated as a result of the pressure reduction and
~ Q67009
the gas generated as a result of boil off caused by heat absorb-
ed from the warm reactor. When the liquid level in the reactor
is stabilized at the desired level, valve 14 is closed. To make
a slurry according to the invention the agitator 11 is activated
and valve 18 is again opened to valve off vapor and reduce the
reactor pressure. When the pressure fallæ to the triple point
pressure of 60 p.s.i.g. further venting of gas will result in
the formation of solld C02 at the surface of the liquid masR and
not in further pressure reduction. Any crystals of solid C02 80
formed are instantly sucked below the surface and into the body
of the liquid by the action o~ the agitator. Since the amount of
solid formation is directly related to the amount of vapor gener-
ated, the volume of gas vented is carefully measured by a suit-
able gas meter 19 in the vent line. To get a meaningful reading
from the meter 19 it is necessary to admit the ga~ to the meter
at a suitable and known temperature. For this purpose the gas
vented from the reactor is warmed in heat exchanger 17 before
passing through the gas meter 19.
When the vented gas volume indicates sufficient solids
have ~ormed the valve 18 is closed and the venting stopped. The
ag1tator 11 is then slowed down and the reactor i~ pressurized
with nitrogen from the supply line 15 to 100 p.s.i.g. When valve
20 is opened, the slurry flows into the receiver 23 and its pass-
age can be visually monitored through the window of the sight
flow indicator 22.
In the batch method of C02 slurry preparation, such as
described above, the percent solids in the solid-liquid mixture
is determined in general by the amount of liquid that is vapor-
ized and vented. However, as a practical matter, the rate at
which the vapor is vented limits the maximum solid content that
can be produced in a form that achieves the object of the inven-
tion, i.e. to produce a pumpable fluid. Figure 5 relates the
:1 067~1)9
evaporation rate~ and the related solids formation rate, to the
percent solids that will circulate freely in the reactor as re-
quired to form a pumpable fluid. m e evaporation rate specified
in Fig. 5 is only the evaporation ~rom the triple point condition
and does not include evaporation necessary to reduce the pressure
of the liquid feed to the triple point. Accordingly, while more
solid can be produced in a given period of time the higher the
evaporation rate, the solids concentration of the slurry that can
be made will decrease as the evaporation rate increases. One
reason for this phenomenon is that with lncreasing rate of forma-
tion of solid particles, fluffier and more porous particles are
formed so that, in essence, one is able to get less solid into
the slurry and have it remain in condition to circulate freely in
the reactor and permit continued reaction. The bulk density in
the reactor of a slurry produced with a high rate of solid ~orma-
tion, for example, is lower than the bulk density o~ a slurry
produced with a low rate of solid formation.
m e rate of evaporation or withdrawal of gaseous carbon
dioxide from the vessel, because of its ph~sical tie-in to the
rate of solid formation provides a convenient method to define
the desired rate of solid formation. In general, it is desired
to maintain the evaporation rate of the carbon dioxide per unit
of liquid surface area between about 10 and 250, preferably 75 to
17~ pounds per hour per square foot of area. In addition to rate
of evaporation the residence time of solids in the liquid in the
vessel (aging) can affect the size and density of the solid par-
tieles and thus the solid concentration attainable in the slurry.
In aging~ the pores in each particle become filled with solid so
that the material is less fluffy and the slurry is pumpable at
high solids concentration. Residence times of 2 to 125 minutes,
preferably 8 to 16 minutes, are suitable for this invention.
In addition to controlling the rate of solid formation,
-10--
~(~6700~
a substantial agitation is maintained in the production ve6sel.
If no agitation is used, solid tends to form a crust that covers
the liquid surface. Any method that instantly removes sollds
~rom the surface as they are formed and immerses them in the
body of the liquid may be used. The problem is preferably over-
come by use of mechanical agitation sufficient to prevent forma-
tion of a crust and to move the solid~, as they are formed, away
from the liquid surface. Agitation also prevents agglomeration
of the particulate solid carbon dioxide in the initial stages
of the formation. Additionally, it is desired to prevent gas
entrapment in the solids. In the absence of agitation, or at
a slow rate of agitation, the carbon dioxide solids immediately
begin to agglomerate as they are formed and undesirable large
lumps are formed. Under certain conditions, it is even possible
that the liquid carbon dioxide will convert to a solid block of
carbon dioxide. At optimum rates of agltation, a fine slurry is
made with a fast production of solids. The type of agitator
used is not critical, but it is necessary to use an agitator
designed to move the solids from the surface and to provide ade-
quate shear in the slurry mass to control particle size, suchas an impeller type agitator. The type of agitation and the
power input of agitators will vary depending upon the size of
the vessel. For a reactor 23 inches in diameter a single 8 . 9
inch diameter propeller driven at 420 rpm by a 1/3 HP motor has
proved highly successful. For a 50 gallon reactor above about
0.003, preferably above 0.005~andparticularly agitator power
input about 0. oo8 to 0.025 HP per gallon of slurry in the re-
actor is suitable for this inventlon. Power inputs above 0.05
HP per gallon will generally be unnecessary. In all cases
agitation sufficlent to prevent formatlon of a cru~t on the
surface of the liquid, to remove the particles from the surface
as they are formed and to prevent agglomeration of the solids
-11-
within the body of the liquid is necess~ry for the production
of the slurry of this invention.
Table I contains data from five representative runs
with the equipment and in the manner described above.
TABLE I
Evap,
Rate
% lbs/
R _ Solids hr/ft2 RPM Comments
A 59 22 420 Shows capability for high solids
production
B 57 41 420 Transfer made after 19 hours with-
out agitation
C 14 134 420 Evap. rate to produce maximum
solids; Transfer after 2 hours
settling
96 420 Representative of good reactor
operation (normal operations 11ne)
E 51 32 420 Representative of good reactor
operation (normal operations line)
The solid content in the slurry of the present inven-
tion is such that the slurry is pumpable. m e present invention
contemplates slurries containing from about 10% to about 85
(by weight) solids. The higher percent solids slurries are
preferably produced by concentration outside the reactor. With-
out agitation the solids in the carbon dioxide slurry of this
invention will settle leaving a layer of clear liquid above
thicker slurry with a diætinct slurry-liquid interface. This
clear liquid can be removed after the initial settling, which
takes less than 1 minute. The bulk density of the slurry in-
crea~es at a slower rate. Agltation o~ the settled slurry
quickly redispersee the solid C02 particles throughout the liq-
uid C02 to form a substantially homogeneous slurry. Thesettlement phenomena can be used to produce concentrated
slurries in short periods of time. The settled ~lurry will
seldom, if ever, exceed about 85~ weight solids concentration.
m e final solids concentration of the final slurry product of
this invention can, using this phenomena, become independent
of the evaporation rate that was used in making the initial
slurry.
A problem encountered in the operation of the reactor
illustrated in Fig. 2 was the build up of solid C02 on the
agitator shaft above the liquid (slurry) surface and on the
reactor walls in the vicinity of the liquid-vapor interface.
Heat may be added at these points to prevent the problem. This
may be by heat exchange with warm liquid or by electric or other
direct heating.
To overcome the foregoing problem and to demonstrate
the feasibility of continuous slurry manufacture the apparatus
of Fig. 2 was modlfied to provide for the continuous introduction
of make-up liquid C02 against the wall of the reactor at the liq~
uid vapor interface~ to melt the solid that tended to accumulate
at that point. As a further modification a pump was placed in
l~ne 21 between the reactor and the receiver, and a flow-
controlled recycle line provided from the downstream (high pres-
sure) side of the pump to return product to the reactor at a
point below the liquid surface. A screen was placed at the out-
let of the reactor to prevent any inadvertent large solid par-
ticles from entering the outlet line and avoid the possibility
of clogging the pump. With these modifications the apparatu6 of
. 2 was operated with the continuous introduction of liquid
feed stock at a rate to m~n~n the ~u~ level in the reactor while
vapor was vented as before, and product slurry was continuously
transferred to the receiver or other point of end use. The res-
idence time, and thus the solids concentration, was controIled bythe proportion of product withdrawn ~rom the reactor th~twas recycled.
Table II contains data from two representative runs
-13-
1067009
with the equipment modified and operated as described above.
TABLE II
Evap.
Rate
% lbs/
R Solids hr/ft2 RPM Comments
AA 10 126 420 30 minute continuous run
BB 3 126 420 85 minute continuous run
A care~ul analysis of forty-one batch runs and fifteen
continuous runs with the apparatus of Figure 2 revealed the
following cignificant parameters con~idered essential to the
production of a high solid concentration, unagglomerated, pump-
able slurry. First it i8 necessary to provide a high degree of
agitation throughout the entire body of liquid in the reactor
so as to produce only fine solid particles. It is necessary
to apply heat to the reactor wall at the level o~ the liquid
vapor interface to prevent the formation of large solld particles
thereon, which could then fPll into the slurry and which would
not necessarlly be reduced to small particles thereafter. Since
it is always possible that some larger particles of ~olid may be
formed in spite of precautions to prevent this, it i8 necessary
for continuous reliable operation to provide a screen at the out-
let of the reactor to prevent inadvertently formed large parti-
cles from clogging the outlet and any downstream pump or orifice.
A fine particle slurry, made in accordance with the present in-
vention, i6 a pumpable fluid. However, it appears that the slurry
pump suction line Ehould be kept large enough in diameter to
keep the velocity in the line below about 1 foot per second, at
which rate the pres~ure drop in the line is insufficient to
cause significant vaporization which, if present, tends to block
the pump and c~use some agglomeration of the solid particles.
A preferred embodiment of the invention, suitable for
-14-
~ ~067~.)09
large scale continuous production of the novel carbon dioxide
product i8 illustrated in Fig. 3. C02 in liquid phase at 300
p.s.i.g. and 0F is provided as starting material for the process.
This can be conveniently supplied from any conventional storage
facility such as tank 51 which receives liquid from a liquefier
52 to which compressed gas is supplied from a compressor 53.
The reactor in which the liquid i8 converted to a
solid-llquid slurry is identified generally by the reference
number 54 and consists of a pressure vess~l 56 having inwardly
extending ba~fles 57 attached to the wall as shown in Fig. 3A,
which is a sectional view along line A-A of Fig. 3. The reactor
is equipped with a bottom entering agitator 58 driven by agitator
motor 59 which provides su~ficient agitation to generate a ~ine
particle slurry. Location o~ the agitator shaft wholly within
the liquid in the reactor eliminates the problem of solid C02
forming on the shaft in the vapor filled head space of the re-
actor. The preferred toroidal pattern of circulation in the
reactor i8 indicated. m e liquld feedstock from ~torage tank 51
is first passed through an adsorber 60 to remove any moisture
2~ present and is then passed in heat exchange relation with the
walls of the reactor by passing it through a suit~ble coll 61
affixed to the outside of the reactor vessel 56 at the level of
the liquid-vapor interface. As the liquid feedstock gives up
its heat from 0F to -50F it melts any solid deposit forming on
the reactor walls in the head space. The entrance to the product
withdraw~l pipe 62 at the bottom of the reactor is protected by
a screen 63 to prevent an occasional larger solid particle from
entering the pipe 62 and the slUrry pump 64 through which the
slurry is discharged. Provision may be made uch as by liquid
line 65 and control valve 65', to admit clear llquid to the re-
actor to reverse ~lush the protective screen should it become
clogged.
io67no~
The liquid feed after having passed in heat exchange
with the reactor wall i~ admitted to the reactor below the
liquid surface through a flow control valve 66 actuated by a flow
controller 67. m is controller keeps the feed rate constant.
The reactor i~ malntained at the triple point pressure of 60
p.s.i.g. A constant percentage of solid~ is formed in the
reactor by evaporating liquid at a constant rate. This rate of
evaporation is regulated by valve 68 in the vapor discharge line
69 which communicates with head space of the reactor. The valve
68 is controlled by the vapor flow controller 71. This cold vapor
is returned to the high pressure StQge of compre6sor 53 where
it ls compressed to the liquefier pressure together with the
gaseous c~rbon dioxide feed. If desired the cold vapor can be
heated before entering the compressor by passage through a suit-
able heat exchanger to recover the available refrigeration.
It is preferred to control the liquid evaporation rate
to continuously produce in the reactor a slurry containing about
30~ solids. As the percent solid~ are controlled by maintaining
all flow~ constant, the flow through pump 64 is kept constant by
ad~usting its speed in response to the level of liquid in the
reactor by a reactor level controller 72.
Thus as slurry is formed it i8 continuously removed
from the reactor by pump 64 which delivers it to a concentrating
receiver 73. The bottom of the concentrating receiver 73 is
provided with a slow moving agitator 74 to assure dispersion of
the solids in the liquid for delivery of the concentrated slurry
to a point of storage or use. m e agitator 74 does not however
produce enough agitation in concentrating receiver 73 (which is
preferably a pressure vessel having a relatively large height-
to-depth ratlo) to prevent concentration by gravitational separa-
tion of the more dense solid particles and the less dense liquid.
Thus if 30% solid slurry i~ delivered to the concentrating
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1067009
receiver the solids will quickly settle out to produce a slurry having a
solids concentration of the order of 60% in the bottorn of the vessel and
clear liquid at the top. The clear liquid present at the top of the con-
centrating receiver 73 is returned to the reactor through return line 76
Residence in the concentxating receiver further enhances the densit~ of
the slurry by reason of the aging of the solid particles which takes placeO
It has been determined that it is more economical to continuously produce
30% solids in the reactor and concentrate them thereafter to whatever
higher percentage is desired than to initially produce the higher percent
1 0 solids.
To utilize fully the refrigeration capacity of the slurry of
the present invention it is necessary to be able to dispense the material
from i~s storage pressure, which in no event can be less than the triple
point pressure of 60 pO sOiogO, to atmospheric pressureO The solid
component of the slurry should be able to pass through the dispenser
substantially without phase modification and without separation from the
liquidO The liquid component, which will flash to solid and gas upon
pressure reduction to atmospheric, must be able to be dispensed without
the solid formation impeding the operation of the equipmentO The con-
20 ventional snow horn used for dispensing liquid CO2 is generally notsuitable for slurry dispensing. It has been found that certain positive
displacement pumps, such for example as gear pumpq and vane pumps,
can be used as dispensers for the dispensing of the slurry productO Such
a pump, to be used as a dispenser, must be operated in "reverse", i. eO
as an expander delivering slurry from a high pressure to a low pressureO
In so doing work is produced, thereby enhancing the refrigeration capacity
of the dispensed product~ This method of dispensing is being separately
claimed in our copending Canadian Patent application NoO 171, 519, filed
May 16, 19730
