BE646129A - - Google Patents

Description

       

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  Sublimation at low pressure in a gaseous vehicle,
The present invention relates to the freeze-drying of cellular articles or porous material, for example food products or the like. Lyophilization involves, freezing the articles, placing the frozen articles in a base drying chamber. pressure, to apply heat;

     to sublimate the ice, to remove water vapor from the chamber to measure its formation, and to continue this process until the products are dried. If desired, the items can be frozen directly in the chamber drying before the sublimation operation.



   In this process, practically no liquid is released during the drying of the articles, the fibers or the matrix structures of the articles such as the products.

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 Foods are not wet enough at the drying temperatures * involved to reduce their market appeal as dry, quickly reconstituting items.



   On the other hand, the articles should not be subjected to temperatures high enough to cause changes or deterioration. When the dried articles are reconstituted by adding water thereto, their original characteristics, including their taste, are restored to a large extent. Processes of this kind to which the present invention relates are commonly referred to as lyophilization processes, and this term is used in the remainder of the description to denote the process of the invention, and the apparatus for the mine carrying out this process. .

   Although the elm of the invention which will be described is a lyophilization process in the broadest sense, the process can be used to dry products which are not frozen but which would be damaged by the application of. excessive heat. On the other hand, the process can be used to remove liquids other than water, from substances other than food products.



   Two major obstacles have prevented the diffusion of high-capacity freeze-drying processes for drying food products such as fruits, vegetables, meat, summer *
The first obstacle is that the drying time is considerable, which causes inefficient use of personnel and machines. The other obstacle is that the processing costs such as pumping costs, motive power costs and heat loss factors were high.

   As dry food products have to be sold in a very competitive market, an apparatus which may be suitable for the laboratory, for example for the preparation of relatively small quantities of high-priced biological products often becomes impossible to apply. from an economic point of view when working with quantities of the order of a ton, required in the food industry.

   

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   As will be discussed in detail below, the apparatus of the present invention utilizes a gasoline vehicle which circulates at low pressure in the drying chamber during the drying process and performs the dual function of providing the heat of sublimation (or digging). ) to liberate water vapor from the ice core of the drying material and to entrain the sublimated water vapor by sweeping over the surface of the drying material in the chamber.



   In a freeze-drying process, the ice on the surface of the product first sublimates, leaving a dry porous outer layer that surrounds a core of frozen product. In the present invention, the gaseous vehicle diffuses through the
A shell of dried material * which surrounds the ice core and enters the gates, "cells", or interstices of the material's salt. Therefore, heat is transmitted to the ice core from the gas vehicle @ = by conduction through the gas in the pores and by conduction through the shell of dried material that surrounds the ice core to the latter. .



   In the present invention, the gaseous vehicle and the entrained water vapor exit the drying chamber to pass into a refrigerated condenser where these vapors are condensed.



  The gaseous vehicle is condensed into a liquid and the water vapor is condensed into ice which is then melted * The liquefied gas vehicle is separated by gravity from the water, and the gaseous vehicle is reheated to return it to the state vapor or gas phase suitable for recirculation in the drying chamber
In operation of this apparatus, the pressure in the drying chamber is maintained at a value low enough that the ragged vehicle and the water vapor are not cooled to their respective dew points as they pass through the drying chamber. drying.

   The pile vehicle is vaporized before being introduced into the drying chamber and condensed at a lower temperature after exiting the chamber.

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 such. so that a pressure difference establishes you between the inlet and the outlet of the drying chamber and provides the driving force necessary for the gaseous vehicle to circulate in this chamber.

   The gaseous vehicle therefore supplies the heat of sublimation and sweeps the sublimated water vapor without the need to use a blower to circulate the gaseous vehicle in the drying chamber.
In addition, the food products in the drying chamber serve as a current restriction in the gas carrier circuit and as the hot gas carrier transfers its heat to the food.

   the temperature and the partial tension of the gaseous vehicle decrease and finally reach the partial tension present in the vapor condenser 'This pressure difference between the inlet and the outlet of the drying chamber is used for pumping or to ease the circulation of the vehicle gas and entrained water vapor.



   As mentioned above, the gaseous vehicle leaving the drying chamber at the indicated pressure entrains the sublimated water vapor in the vapor condenser. In this condenser, refrigerated coils absorb the heat required to condense (liquefy) the gas carrier and to freeze the water vapor entrained in ice crystals.



   An object of the invention is to provide a gaseous vehicle freeze-drying process which allows the drying cycle to be carried out in a relatively short period and at an economically acceptable cost. Processes in which all the factors are combined to accelerate drying are known, but known methods of this type are not commercially acceptable * For example, it has been proposed that the heat of sublimation be supplied to the ice core at an accelerated rate using radiant heat for the first stage of dried and completing the drying using microwave.

   However, the cost of installing such an apparatus on a large scale is excessive and uncompetitive. According to the present invention, the

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 radiant heating and by lI101O-on41. may be useful later in the drying cycle when sublimation is Pe.

     de, and a gaseous vehicle can then be used in the cycle when sublimation becomes more difficult
 EMI5.2
 Another object of the invention is to provide a cake-like freeze-drying apparatus which does not require mechanical pumping to achieve the desired circulation of the vehicle through the drying chamber. These stops are reached. U t41S are an irhiou3i. <Caseu oon 1en ... b1 ... d <i9 tuperatures and practical pretition, and preferably 1mmi.oib1, at the vice.



  Another object is to operate the device at the driving knees dwtw a gaont of pressures and d4. ttmptfraturts which makes it possible to approach the maximum breakage drying rate likely to be reached in theory in a gas vehicle appliance. The volume "rate of edouge dt tun" can be defined as the weight of water vapor dlwnde per hour per unit area and per unit thickness of the material.
 EMI5.3
 nechèe. The bulk drying rate can also be defined as the weight of water removed per hour divided by the initial weight of the product including water in the product.

   In the present
 EMI5.4
 Invention, the mass drying rate is made optimum by choosing an operating zone in which the pressure
 EMI5.5
 of the gas is high enough to provide excellent heat transfer to the ice core by conduction but not high enough to indeeirably inhibit the transport of water vapor released from the ice core and through the shell of dried material *.
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  Another goal is to provide a propb1: U.- apparatus. : nation which allows the use of continuous loading by carrier. This characteristic can be applied in the gas vehicle apparatus of the invention because the drying takes place fairly quickly and for a determined capacity or flow rate, it is not necessary for the drying chamber to contain a significant quantity of product. at any given time * trust

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 this report,

   the gaseous vehicle apparatus of the invention is superior to conventional vacuum drying apparatus where water vapor is simply extracted or condensed as it is formed and where the pressure may be less than 1 mm of mercury.



  In conventional ultra-low pressure forms, a large volume of material to be dried must be in the drying chamber to ensure adequate flow.
Yet another object of the invention * and to provide an apparatus of the type described in which virtually any loss of vehicle in the sago% or liquid state * is avoided.
Another object of the invention is to lyophilize with a minimum total energy input, either in the form of thermal energy or in the form of mechanical or electrical energy.

   A refrigeration unit is required to condense the vehicle into liquid and to freeze the water vapor into ice crystals. In the apparatus according to the invention, part of the energy required to operate the refrigeration compressor is recovered by passing the hot refrigeration gas through the ice melting apparatus, this refrigerant melting the ice. ice crystals. The water thus obtained and the liquefied vehicle are separated by gravity.

   The refrigerant preheats and evaporates the liquefied vehicle in a boiler to heat it and recirculate it in the drying chamber,
The manner in which these objects of the invention can be achieved will become apparent from the detailed description given below of a method and apparatus for carrying out the invention.



    In the drawings:
Fig. 1 is a schematic representation of an apparatus according to the invention; Fig 2 is a graph showing how the thermal conductivity of the occluded gas, that of the solid matter and the total thermal conductivity vary with pressure in the drying chamber for a typical food product, peaches by

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Fig. 3 is a graph showing how the capacity of the vapor 4'.au iL to flow through the shell of the material. stain varies with pressure in the drying chamber;

     Wire- 4 this a composite graphite of the total thermal conductivity of the mass transport coefficient of water vapor and the mass drying rate for different pressures in the drying chamber) Fig. 5 is a graph showing the pressure characteristics -temperature of water vapor and different vehicles.



   The most salient features of the apparatus for implementing the invention will first be described. Referring to Fig. 1, the food product is placed in a drying chamber 10 through which is passed a heated, non-toxic, water-insoluble gaseous vehicle (eg heptane). The gaseous vehicle provides the heat of sublimation and entrains the sublimated water vapor in a refrigerated condenser 20. In this condenser, both the water vapor and the gaseous vehicle are condensed. A suspension of ice crystals and liquid vehicle passes through an ice melter 30 where the water is separated by gravity.

   The liquid exiting the ice melter and the liquid vehicle coming directly from the lower part of the condenser 20 are directed to the boiler 36 and re-vaporized by hot refrigerants.



  The gaseous vehicle vaporized in the boiler is heated as it passes through a superheater above the boiler. The dry, hot gaseous vehicle re-enters the seohag chamber and the freeze-drying cycle continues. As the gaseous vehicle is condensed at a pressure lower than its inlet pressure, a positive pressure builds up in the drying chamber, eliminating the need for a blower or a circulating fan.



   Before proceeding to a detailed description of one form of the invention, a summary of some of the

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 physical problems and considerations involved in freeze drying and particularly in the use of a. mangy vehicle. There are two basic problems which relate to performing a freeze-drying process in a relatively short time or cycle. These problems are i (I) The problem of heat transfer to the ice core in the Dobby.



   (II) The problem of removing water vapor which sublimates from the lace core.



   The rate of water vapor removal must be high enough to maintain the partial pressure of water vapor on the ice core low enough to prevent excessive temperature of the ice core and the resulting melting. annealed.



   Each of these problems brings into play several factors which are described later * In some cases, one factor can only be made optimum at the expense of another and one obtains apparently incompatible parameters. As will be seen, the apparatus of the invention operates within a range of pressures and temperatures which has been carefully chosen to take account of these conditions. According to the present invention the apparatus is set to operate according to the compromise the most efficient between apparently incompatible criteria so as to obtain the optimum mass drying rate.

   These factors' 1 and II indicated above will be discussed in detail to facilitate an understanding of the importance of the operating ranges which have been chosen in accordance with the present invention.



     (I) Heat transmission to the dacian core in the material (a) Effect of the temperature of the ice core
The heat transmission to the ice core is greater the lower the core temperature because the rate of heat transmission between two bodies is a function of their temperature difference Therefore when

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 EMI9.1
 the ice core is cooler e'a-ary the lower its temperature compared to the temporary of the vehicle axtst, the heat transmission from the mangy vehicle to the ice core is all the greater.

   This criterion would suggest bringing to the sa *
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 xlmua it temperature of the gaseous vehicle and At minimum the tmpdra. ture of the ice core, null in the apparatus of the invention the setting of other * so-called operating conditions slows down the temperature of the ice core Consequently, as will be seen, in 10 & ppartil of the invention it Do not attempt to adjust the temperature of the ice core directly.



  (b) Effect of the tea.pér # 1fftreil on the surface of the geouit of matibri The transmission of heat by conduction through the material itself towards the ice core is all the greater as the surface temperature of the shell of dried material * surrounding the core is higher.

   The temperature of the outer surface of the hull of dried material which surrounds the ice core is primarily a function of the temperature of the vehicle.
 EMI9.3
 pasty cule passing through the drying chamber, Amdo in the apparatus of the invention, the surface temperature of the
 EMI9.4
 dried material @ is controlled by regulating the temperature of the gaseous vessel * However, the temperature of the dried material shell * must be kept below a temperature which, as a bonus would be the dried parts of the product,

   so that the temperature of the gaseous vehicle mentioned above cannot be raised beyond extrinsic limitations.
 EMI9.5
 



  (c) Effect of heat wave activity on dried oopue and occluded vapor on heat transfer to nex at the crack.
In the apparatus of the invention, the heat of sublimation must be provided by transmitting heat from the gas vehicle to the ice core. This transmission of the gaseous vehicle current sweeping the exterior of the hull to the ice core
 EMI9.6
  'performs in two ways. One way is conduction through

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 the solid material of the composite shell of dried material @ Until
 EMI10.1
 core.

   The other is a transaisaion by conduotion of the ah * lw? through the gas mixture retained in the cells, pores, capillaries or matrix of the shell d .... t1r .. 'ob' ,. This occluded gaa mixture comprises the water vapor forgetting from the ice core and If, .lhiW1. gaseous Which d1tf \ & l 'in the hull. This heat transmission erfeotue you from the occluded case to a cell, a pore or there pMla'ua capluairel of this wall to the gas oc <Mu <d4ma 1 <port fti.tr1J, Xa opposite wall of the pores and so on. until a4, that the heat is transmitted to the ice core.

   The total thermal conductivity is the volume of these two effects *
In the case of cellular material such as fruits, vegetables, meat or the like, or porous materials such as coffee concentrates, most of the time has dried.
 EMI10.2
 of the volume of the shell of elder material which surrounds the ice core is occupied by this occluded gas mixture * The effect is the same whether it is pores or cells and the more general terms "porous" and " pores "are used for the whole thing.



  When the apparatus is operated in the range of pressures most suitable for the lyophilization of food products or the like using n-heptane as a gaseous vehicle according to the invention, about 54% of the net thermal conductivity of this composite shell. of material and occluded gas are responsible for the transmission of heat through the gas occluded in the pores of the material. The corresponding number for a subject
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 re ¯uoroch1que (FC-75) is' or, and other fluorooarbons give similar results.

   As mentioned, since the shell of seo-material is always in contact by one of
 EMI10.4
 its parts with the ice core, total thermal conductivity also includes the effect of the direct transmission of heat from the external vehicle gas stream through the solid matter itself and to the ice core.
 EMI10.5
 



  We will now refer to Flg. 2 which includes a curve representing the total thermal conductivity K of a

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 EMI11.1
 the CondUat4Vtt $ thetaiqse 40 occlui (heptane) at din4, rente # pressures and at the temperatures entering 4a game in the process. The Fi g. 2 also presents curves showing the individual contribution of gas and the individual contribution of solid matter to the total thermal conductivity curve.
 EMI11.2
 referring to the Udder curve.

   2 at lower presiio1! At 0.1 m of mercury the gas contribution is leaked but as the pressure increases beyond this point the gas contribution increases rapidly. This can be explained as follows * When the pressure of the occluded gases is very low, the number of gas molecules which occupy a given volume (e.g. a pore or capillary of the food material) is also reduced * Under these conditions , the path
 EMI11.3
 free mean of molecules between collisions intemoleculators is greater than the diameter of the pores, According to the kinetic theory of gases,

   the thermal conductivity is proportional to the product of the density of the gas and the mean free path * In this case, where the mean free path is kept constant by the wall of the pore or cell, the thermal conductivity of the gas is proportional to the density of gas which, in turn, is
 EMI11.4
 tional to the pressure * This is represented on the '"part' * of the gas" curve in Fig. 2 and corresponds to the essential part) *?
 EMI11.5
 right of the curve to the left and below point "x". [\:

   It will be noted that when the right part of the curve representing 'the contribution of the gas to the left of the point "tE" is added 1 tributton in constant value of the solid matter, the horseradish expressed by the straight line this lost on the graph d * 4j4 fatal thermal activity get! ' The kinetic theory of the gases shows that the thermal conduction of the gas is independent of pressure if the average free path of the molecules is reduced relative to the dimensions of the space surrounding them.

   In the present case, at the highest prelts (greater than approximately 30 mm of mercury), the number of molecules which occupy a pore of the food product * * is greatly increased so that the laws of collisions between loti

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   molecule * dominate as opposed to collisions between molecules and the pore walls. Under these circumstances, the average free path of the molecules is reduced relative to the pore diameter, and the thermal conductivity remains or becomes constant as the pressure increases further.

   This state is represented on the "part of gas" curve of Fig. 2 by the essentially constant value part of the curve to the right of the "y" point.



   Between point "x" and point "y" is a transition zone where the ratio of the contribution of the thermal conduction of the gas to the pressure changes from a direct proportionality ratio to a conductivity ratio. constant when the pressure changes.

   This gradual transition is the result of variations in the size of the pores in the dried material @, such that there is no single pressure at which the mean free path becomes equal to the pore diameter * Further, the phenomenon called " temperature jump "which manifests itself as a definite difference in temperature between the solid surface and the gas immediately adjacent to the surface becomes noticeable when changing from high pressures to lower pressure. The" temperature jump "causes a reduction apparent thermal conductivity when moving to internal pressures in the transition zone.



   The total thermal conductivity curve of Rod 2 is the algebraic sum of the curves of the contribution or share of gas and of the contribution or share of solid matter. The shape of the sinuous section of the curve which. a second positive derivative, ie the section of about 0.01 to 1.0 mm of mercury, is well below the operating range.

   The shape of this section comes simply from the algebraic addition of a straight horizontal line and a nearly straight line inclined on a logarithmic ordinate, and in this case the effect of direct conduction dominates., However, the another curved section of the curve, namely the section between 2.0 and 30 mm in mercury, has a physical significance in relation to the dimensions of the

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 cells or pores, and the free molecular path Costing medium has been explained,
 EMI13.1
 To sum up, the relationship between pressure and thermal conductivity can be understood by considering how the mean free path and the number of collisions of gas molecules vary with the pressure of the gas.

   At extremely high pressures
 EMI13.2
 weak, there are few collisions tntermo14cu1a1rl ', and the molecules of the gas mainly collide with the walls of the
 EMI13.3
 cell, pore or capillaries of the porous natlère, The average free path of the molecules is limited to the average ditacneion of the interstices or pores in which they are contained * however the number of molecules is so low that the thermal conductivity due to the occluded gas is negligible.



   As the pressure increases, a point is reached
 EMI13.4
 (adjacent point nxn, end. 2) where the number of molecules colliding with the solid matrix increases appreciably in direct propor- tion with the increase in pressure, when the near
 EMI13.5
 The pressure is raised to a value of several millimeters of mercury (in the transition zone, Fig. 2), a significant number of intermolecular collisions begin to occur.

   In this pressure region (the Ion where the device operates), the
 EMI13.6
 The behavior of the gas undergoes a gradual transition to the vaporization it exhibits at even higher pressures above the "y" point where the dominant type of collisions is inter-molecular as opposed to collisions with the solid matter which causes them. surrounded. As the pressure continues to rise
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 (above the area of point nys, Fil. z) 1nte01éoulairel collisions dominate and, in fact, at pressures which are not too far from this region, the thermal conductivity remains constant and no longer increases as the pressure increases.



   As can be seen in Fig. 2, the total thermal conductivity of the dried food products has a value

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 relatively low substantially constant (0.009) at pressures less than 0.01 mm Hg, and a second substantially constant value (0.0194) which is 2.15 times greater than the first substantially constant value. This second practically constant value occurs at pressures greater than about 30 mm of mercury.

   According to the present invention, there is an important transition zone between these two pressures, namely the sound between 2 and 30 mm of mercury where the lower value of the total thermal conductivity X (using heptane as gaseous vehicle ) is still relatively high (greater than 0.15 units) and where K increases from this value almost goes up to the possible maximum of 0.0194 units.



  The lower operating range of 22 mm Hg is chosen because it represents the lower end of the transition zone. Below a pressure of about 2 mm of mercury, thermal conductivity decreases rapidly in a ratio expressed by a practically straight line,
When all these factors are made optimum, it is found that when considering thermal conductivity alone in the case of food products such as beef, vegetables, fruits etc., there is a total vapor pressure (which can be going down to 5 to 15 mm of mercury and sometimes going up to about 30 mm of mercury) for which the thermal conductivity is almost at its maximum. Above .



  At 30 mm of mercury, the rate of increase in thermal conductivity with pressure drops so quickly that no benefit is derived from these high pressures.



   On the other hand, according to the present invention, it is taken into account that as the water vapor pressure increases, the temperature of the ice core increases since the temperature of the ice core adapts to the pressure of. vapor to which it is subjected.



   The liquid in the food is usually a solution, and the euteotic temperature point of this liquid is

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 lower than the temperature corresponding to the triple point of water.



   It is generally not feasible from a commercial point of view to work below the sutectic temperature, of sort, * that the partial pressure of the water vapor will be maintained, between the temperature of the euteotic point and that of the triple point, at a point which reduces to an acceptable extent the amount of liquid solution remaining in the material during drying,
When the drying is completely complete, the material is completely dry and the very small amount of water in the solution which was present is evaporated.



   As emerges from the. Fig. 2, thermal conductivity
K can be regulated by acting on the pressure in the drying chamber and, in the range examined, the thermal conductivity increases as the pressure in the drying chamber increases.



   It may also be mentioned that the heat transfer capacity of the gaseous vehicle itself which sweeps the product varies directly with its density and therefore with its pressure. Thus, in addition to its low thermal conductivity at very low pressures, the gaseous vehicle itself has a low heat transport capacity per unit volume at these low pressures. In the working pressure zone used in the invention, the thermal conductivity of the gas vehicle is relatively high.



     II 'Elimination of water vapor from the material.



   (a) Effect of ice core temperature.



   The rate of water vapor removal increases as the temperature of the ice core increases. As the temperature of the ice core increases, the vapor pressure of the ice also increases which increases the force which pushes the vapor away from the ice core and therefore increases the rate of vapor removal from the ice core. water. Note that the effect of this phenomenon with respect to the advantages obtained is incompatible with paragraph 1 (a) or an increase in the temperature of the ice core lowers the heat transmission to this core.

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 and, therefore, is a disadvantage.



   However, as already mentioned, in the operation of the apparatus according to the invention, the temperature of the ice core is determined by the choice of other parameters. , the partial pressure of the water vapor is kept sufficiently low, that is to say between the eutectic point and the triple pointa so that the temperature of the ice core does not become too high and so that the ice does not melt * (b) Effect of the concentration of water vapor on the surface of the shell of dried material
The concentration of water vapor on the surface of the dried material shell,

   that is, material between the sur. face of the product and the lace core, slows down any further sublimation of water vapor from the ice core. The water molecules leave the ice core by sublimation according to statistical laws and, according to the same laws, the molecules of the water vapor which surrounds the nucleus can also re-enter it. This feedback effect is increased as the concentration of water vapor on the surface of the material increases.



   It will be remembered that the laws of physics indicate that the ratio between the intrinsic vapor pressure on ice at a given temperature and the actual water vapor pressure that may be present is a factor which determines the rate of sublimation. The water vapor molecules come out of the ice core and reenter it again and these molecular phase changes offset each other at equilibrium conditions, that is, when the two vapor pressures are equal * This sublimation action of the surface of the ice core is not affected by the coexisting partial pressure * of the molecules of the mange carrier.

   The preferred gaseous vehicle, n-heptane, is immiscible with water and does not dissolve in ice, so that the two gases (or vapors, if we prefer to use that term) react independently to the water. point of view of initiation of the sublimation process.

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  The concentration of water vapor at the surface of the
 EMI17.1
 shell of dried material * can be adjusted and, in fact, made to a low value in the present invention by acting on the temperature and the gas current. This last one is
 EMI17.2
 doubting more effective in lowering the concentration of water vapor on the surface of the material shell oh that it is drier $ e'et that is to say that its relative humidity is lower # The rate of sublimation from the ice core is increased * in proportion.
 EMI17.3
 



  (c) Effect of the transport coefficient of tasie of water vapor through dry matter ocapoeite fitjjRdirftfon.aai-P.ççJHis. , .. ,,,,,,.,,, ... M1 #. ,,.



   If all other factors are ignored the drying time is reduced as the corn transfer coefficient of water vapor is increased. The coefficient of trans-
 EMI17.4
 breakage rate includes the effects of the current or diffusion of water vapor molecules from the surface of the ice core and through the shell Intermediate of matter
 EMI17.5
 eechee and occluded gas vehicle,

   and the hydrodynamic current of molecules on the slaughterhouse # pathµ $ The extent to which each of these Individual effects contributes to the overall mass transfer coefficient cannot be expressed in a simple way. The net effect is proportional to the difference between the water vapor pressure in the ice core and the total pressure in the drying chamber, a pressure difference which establishes the motive force which pushes the water vapor away from the ice core. ice.

     However, the effect of a given driving force is itself related to the physical properties of the material subjected to drying and to the properties of the gaseous carrier gas, for example its molecular weight. As the pressure of the gas vehicle or vapor occluded in the pores of the material subjected to drying decreases,

   water vapor molecules that have been sublimated from the ice core can move around
 EMI17.6
 all the more easily through the pores of the dried material and through the porous walls to the surface of the material where they can be swept by the gas vehicle.

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   We will now refer to the graph of FIG. 3 which largely goes without saying. As can be seen in fig 3, there are two distinct phases in the curve of the mass transport coefficient * At pressures below about 0.2 tin of mercury, the mass transport coefficient D is constant at its maximum value .



   At pressures above about 2 mm Hg, the breakage transport coefficient decreases as the pros- sion increases ,, There is a transition zone between approximately the pressures of 0.2 and 2 mm Hg which connects the two aforementioned areas.



   As in the case of thermal conductivity, the mass transport coefficient can be adjusted by acting on the pressure in the drying chamber * This coefficient is all the higher and the speed of the stream of water vapor coming from of the ice core passing through the pores and joining the gaseous vehicle stream is greater the lower the pressure. As shown in Fig. 3, the operating zone according to the present invention is between the upper pressure limit of the transition zone of the mass transport coefficient (about 2 mm of mercury) and a pressure of about 30 mm of mercury.

   As shown in Fil-2, the lower of these pressures (2 m of mercury) corresponds to the lower pressure of the thermal conductivity transition zone.



  The highest pressure is the pressure at which the thermal conductivity K (Fil. 2) has become practically constant.



   The comparison of Figs. 2 and 3 reveal that in the working area of the present invention, as the total vapor pressure is reduced from the higher working pressure of 30 mm of mercury to the lower pressure of 2 mm of mercury, the coefficient mass transport increases by several hundred percent (about 800%), while the corresponding reduction in thermal conductivity is only a little over 20%.



  In the present invention, the mass transport coefficient

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 in the operating zone shown in Fig. 3 is sufficient to release the water vapor released as a result of the heat supplied. to the ice core following the principles discussed in connection with thermal conductivity.



     It is now possible to explain how the various operating parameters are chosen according to the indications of the present invention. By comparing the three factors listed all (I) heat transfer to the ice core, and (II) removing water vapor from the material, as mentioned, the factor (a) in each case, namely the effect of the temperature of the ice core, is not directly adjustable. when the process is started and therefore cannot be independently adjusted to reduce the drying time.

   This is because the temperature of the ice core during drying is only determined by the heat and mass transfer equations corresponding to the apparatus after factors (b) and (c) of items 1 and II above have been fixed.



   Items (b) of the two main headings (I) and (II) are easily adjusted and are not incompatible. These items are: the temperature of the dried material shell and the concentration of water vapor at the surface of the shell. These stations can be optimized by adjusting the temperature of the gas vehicle, its flow rate and its degree of dryness, so that in the present invention a choice of the characteristics of the gas vehicle ensuring advantages over one of the factors of positions (b) mentioned above also provides advantages over the other factor.



   Items (c) of item CI) transmission of heat to the ice core, and item (II) elimination of water vapor are generally incompatible. These items are: the thermal conductivity of the gas occluded in the shell of dried material and the mass transport coefficient of the water vapor sublimated through the composite shell and its large volume of gas occluded.

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   As described above, if the pressure in the drying chamber is increased by a small amount (for example
1 or 2 mm of mercury) at a higher value (eg 30 mm of mercury), the thermal conductivity of the dried intermediate composite part of the material and the occluded gas increases and then remains practically constant.



   Conversely, as the prose ± = in the drying chamber increases in the same range, the mass transport coefficient of water vapor from the ice core through the intermediate composite dried shell of occluded gas material and up to the surface of the material to be centered by the. gaseous vehicle decreases but remains sufficiently high.

   In the case of the mass transport coefficient, the increase in nearness instead of being favorable as in the case of the thermal conductivity factor becomes a disadvantage * In fact, at atmospheric pressure, the mass transport effect quoted has such a low value that, although the gaseous vehicle could sweep all the water vapor from the surface of the hull and thus maintain zero water vapor pressure at the surface of the dried material, the mass transport of the sublimated vapor from the ice core to the surface of the dried material would be so slow that the drying time would become unreasonably long.



   It should also be noted that the transmission capacity of heat per unit volume of gas increases with pressure and that at very low pressures, large volumes of gaseous vehicle must circulate to provide the heat of sublimation.



   As mentioned and according to the invention, these apparently incompatible factors are made optimum by choosing a working pressure in the drying chamber in a pressure area where the thermal conductivity curve lies in a range of. transition, namely a pressure sound where the average free path of the molecules of the gaseous vehicle are of the same general order as the average interior dimensions of the pores,

   

 <Desc / Clms Page number 21>

 cells or capillaries of the matrix of the material to be dried * Any further increase in the pressure of the gas vehicle does not promote the transmission of heat from the outer surface of the shell of the dried material to the ice core $
Reference is made to the composite graph of FIG. 4.

   This graph superimposes the thermal conductivity and mass transport curves discussed above and adds a curve representing the resulting mass drying rate W for a typical food product .. such as peaches. Note * that the already quoted overall working range area * includes the bump of the mass drying speed curve * However,

     according to the present invention a pressure range! typical working speed is much smaller and 5.67 to 11 mm Hg in the example cited * This range practically coincides with the are of the optimum mass drying rate on the curve * In this pressure range,

     the effects of the transport of ses are still sufficiently great although rapidly reducing as the pressure increases that the water vapor sublimated from the ice core reaches the surface of the dried material to be entrained by the gaseous vehicle at a speed ensuring reduced drying time.

     As already indicated, if a sufficient quantity of heat for forgetting has reached the ice core, as soon as the water vapor reaches the surface of the matter, there is no longer a problem because the gaseous vehicle can be easily adjusted to sweep this water vapor out of the drying chamber * The partial pressure of the water vapor never exceeds approximately 1.3 mm of mercury which is well below the pressure of the triple point so that sublimation occurs.



   In the processes already proposed using a gaseous vehicle, this gas circulates in the apparatus at a relatively high total pressure, for example at atmospheric pressure. Under these conditions, it can be seen in Figs.

   3 and 4 that characterized them

 <Desc / Clms Page number 22>

 The mass transport rates of water vapor through the gas occluded in the intermediate dried material are so low that even when the surface of the dried material is freed of water vapor by the gaseous vehicle, the amount of vapor of water removed from this region of the ice core is not large enough and additional amounts of water vapor cannot be sublimated from the ice core through the intermediate shell of dried material to make the process. convenient.



  These devices require unreasonably long drying times.



   On the other hand, if the known devices of the non-condensable gaseous vehicle type were used at total pressures low enough to achieve an acceptable mass transport of water vapor through the intermediate shell of dried material, the conditions the volume and speed required for the pumping device of the gaseous vehicle would be very severe, which again would make the cost of the device economically uninteresting.



   The optimum working area, in terms of water vapor removal rate, is the region in which the pressure is high enough to achieve relatively high thermal conductivity and high heat transfer to the ice core, but this pressure cannot be raised to the point; to reduce the mass transport sufficiently to inhibit excessively the passage of water vapor from the ice core through the shell of dried material and the process is carried out without increasing the temperature of the ice core sufficiently to clean a commercially unacceptable amount of unfrozen concentrated solution of solids and water during the drying process.

   As mentioned, the very small amount of solution which forms is also evaporated during the drying cycle.



   As indicated, the operating zone in terms of pressure range can be defined as the transition zone from the thermal conductivity curve to temperature.

 <Desc / Clms Page number 23>

 rature chooses * This can also be defined as the range of pressures in which the average free path of gas molecules and the average dimensions of pores, cells, or capillaries are practically equal or of the same order.



   The working pressure zone * can be further described as' being the sound system in which the thermal conductivity is as high as possible but where, yet, the pressure is not so high as to prevent the passage of vapor from it. water out of the matrix to such a degree that the maximum allowable temperature for the ice core would be exceeded *
The basic criteria and parameter. discovered as determining the operation according to the present invention having been explained, reference will now be made to FIG. 1 which is a diagrammatic representation of the essential elements of a freeze-drying apparatus of the discontinuous type for the implementation of the invention.



  GENERAL DESCRIPTION OF THE APPLIANCE.



   Referring to Fig. 1, the product to be dried is placed in the drying chamber 10, the details of which do not form part of the present invention. In the case of vegetables, mushrooms or the like, the product is cut to size or otherwise processed into fairly small pieces. In the case of meats, the product is diced or relatively thinly sliced in the usual way. In the discontinuous apparatus shown, the usual sealed door 12 is provided for loading and unloading the product.

   Before being placed in the drying chamber, the product is distributed over mesh trays 14 which are arranged horizontally and which allow the passage of gas through, the product and the bottoms of the trays. The product to be dried, generally indicated at 16, has been previously frozen in conventional industrial apparatus, the details of which do not form part of the invention.



   An outlet passage 18 connects the drying chamber 10 to the condenser 20, this condenser being provided for condensing and releasing.

 <Desc / Clms Page number 24>

   to defy the mangy vehicle HV and freeze the input water vapor WV to ice crystals X. As already mentioned, the mangy vehicle HV useful in the form of the invention which will be described is steam. of heptane in the gaseous state. Associated with the main steam condenser 20, a refrigeration apparatus is indicated by R.

   This device includes the usual compressor
22 wherein the refrigerant gas is compressed and conducted through a hot refrigerant pipe 24 to a coil 28 in an ice melter 30. The ice melter 30 serves as a first stage condenser of the refrigerant and provides the heat required to melt the ice crystals X formed during the drying process.



   An outlet pipe 34 passes the hot refrigerant exiting the ice melting coil 28 into a boiler 36 where the liquefied heptane HL is evaporated, to form the vapor or gas phase HV. This evaporation is carried out by means of an evaporation coil 38 which is connected to the pipe 34 and which is immersed in the liquid vehicle HL in the boiler 36 * A refrigerant outlet pipe 40 connects the evaporator coil. - heptane controller 38 to an expansion valve 42 and the cooled and condensed refrigerant leaving the expansion valve 42 expands in the refrigeration coils 44 mounted in the upper part of the main steam condenser 20,

   The refrigerant, which is now gaseous, is returned to compressor 22 from the condenser coils
44 by a suction line 46.



   To rid the refrigeration coils 44 of accumulated ice crystals, a coil sprinkler pump 50 is provided with an inlet line. 51 which receives the liquid heptane HL from the lower part of the condenser. The output of the pump is connected to a distributor 52 which is located above the refrigeration coil 44.

   This distributor is provided with small nozzles or orifices so that the liquefied vehicle HL extracted from the lower part of the condenser pump 50 is sprayed on the refrigeration coil 44 to loosen and entrain the ice crystals X which are deposited there. At the lower part of the condenser, an agitator 54 is provided, driven by a motor.

 <Desc / Clms Page number 25>

 
 EMI25.1
 reducer 55, This agitator 54 not only acts * $ 1 Mttà of ice and liquid heptane but also serves as a pump to make
 EMI25.2
 pass this saga * through a line 56 into the melting apparatus 4th the ice 30, If necessary, a pump (not shown) can also be installed in the line 56 to transport more POU- t,

  rrea0.errt the magma to the ice melting device. Water II, formed in the apparatus for melting ice by heat tower. *
 EMI25.3
 ni * from the condensing coil the refrigerant is pumped to atmospheric pressure and discharged * by a water charIf pump 58 whose inlet is lt1iê. to the fusion apparatus 30.
 EMI25.4
 



  A primary line for liquid heptanw 60 is fitted to the upper part of the main steam tank casing 20 and terminates in a liquid vehicle secondary line 62 coming from the top of the steam generator. fusion
 EMI25.5
 ice z0, The pipes 60 and 62 ae join to form a common pipe 64 which leads to the boiler 36 in a are located below the coil 38 of the boiler.
 EMI25.6
 The pressure "b" between the level of the liquid in the vapor condenser 20 and the liquid in the boiler b is made equivalent to the difference between the pressure in the boiler and that in the condenser. As can be seen, this pressure
 EMI25.7
 equivalent to .8 mm Hg, which corresponds to 116 aat 4 'hep tan * liquid.



   In the lace termination apparatus 30, the interface between liquid heptane and water must be maintained at a level sufficiently low to prevent water from reaching the conduit 62 leading to the boiler 36, and yet sufficiently high.
 EMI25.8
 so that the heptane can reach the pump µ $ On Useful a level control device and an electromagnetic valve 65. in line 60,
The level adjuster is shown schematically. It uses a 66 sensor with a * probe or electrode
 EMI25.9
 high and low level and a control device 67.

   The ap

 <Desc / Clms Page number 26>

   such a control comprises a part which operates the valve 65 and a part which starts or stops the motor of the pump 58. When the heptane-water interface falls to the level of the low level electrode the pump 58 is stopped and the valve 65 opened to increase the passage of heptane in the line 62. This allows the entry of a greater quantity of magma into the ice melter and the interface rises.



   When the interface reaches the high level electrode of the sensor 66, the valve 65 is closed and the pump 58 is started, which lowers the level of the interface * Level regulators of the type described are manufactured by Fielden Electronics, Inc., Huntington, New York and sold as "Tektor". The details of the level adjustment do not form part of the invention and are not described.



   The pressure h1 between the liquid level in the vapor condenser 20 and the top of the liquid heptane in the ice melter 30 should only be sufficient to prevent vaporization of the heptane in the liquid. ice melting apparatus. At 40 F (4.4 C) this pressure is approximately 20 mm Hg. Since the total vapor pressure in the vapor condenser 20 is 5.67 mm Hg, the indicated pressure hl should be 11.1 inches (282 mm) for a liquid vehicle having the density of heptane.



   In order to remove air and other non-condensing gases which might be entrained in the apparatus or which might enter the apparatus through leaks during drying, a refrigerated trap 70 is provided in the condenser. 20. This trap 70 is formed by a closed vessel comprising an inlet pipe 72 coming from the upper wall of the condenser 20. A vacuum pump 74 is connected to the vessel of the trap to remove non-condensable gaa such as air. .

   In order to recover the vapor from the gaseous vehicle, possibly not condensed, which would be sucked by the pump 74, the trap is cooled by a coil 76 which is connected to a refrigeration device (not shown), of which

 <Desc / Clms Page number 27>

 the details are not part of the invention. The condensed gaseous vehicle and any condensed water vapor are returned from the trap to the condenser 20 through a return pipe 78.



   Returning to the boiler 36, directly above this boiler is a gas vehicle superheater 80 in which the vaporized gas vehicle HV is superheated by steam pipes 82 disposed in the superheat chamber. Steam is supplied at the temperature necessary to give the desired superheat to the vaporized gas HV, and is controlled by a steam valve 83. The superheated dry gaseous vehicle is conducted from the superheater 80 to the inlet port. from a drying chamber 10 by a passage 84. The passage 84 and the other parts of the apparatus are isolated, for example by the caulking 86 shown in part on the passage 84. This caulk and any other insulation that may be necessary have been removed from the Wire diagram. 1 for clarity.



   In order to save the gaseous vehicle remaining in the drying chamber and in passages 18 and 84 during the loading and unloading operations, isolation valves 90 and 92 are used. A refrigerated trap 94 is connected to the drain passage. return 84 via a pipe 96 and a valve 98 * A vacuum pump 100 removes air and non-condensable gases. A refrigerated coil 102 liquefies the condensable gases and these liquids exit from the trap 94 through a return line 104. A valve 106 directs the condensed liquid to the condenser 20 through a line 108 or discharged through a line 110.



  GENERAL OPERATION OF THE APPLIANCE.



   The general operation of the apparatus according to the invention will be described below.



   The criteria envisaged for the choice of a gaseous or liquid vehicle for operating the freeze-drying apparatus when processing food products will be examined below. It will suffice here to specify that the vehicle used in the form of the the invention described is heptane, the characteristics of which

 <Desc / Clms Page number 28>

   as 8 physical are shown above * In the form described the heptane used is n-heptane (normal heptane).



    PRELIMINARY PROCESS. PmJM. Ui.Em.



   The valves 90 and 92 have been made at the end of the previous drying cycle and after the product has been introduced into the drying chamber and the door has been closed, the first operation of the process is to remove the air of the drying camber and the pipes leading to it * For this purpose, the bleeder valve 98 is opened and the vacuum pump 100 is easily started and operated until virtually all the air trapped between the valves 90 and 92 has been sucked. This results in the removal of a certain amount of water vapor which has formed during the evacuation, but if desired, the water vapor can be condensed in trap 94.

   When the evacuation is complete, the valve 98 of the trap is closed and the vacuum pump 100 is stopped.



   The valves 90 and 92 of the gaseous vehicle passages can then be opened. To start a new drying cycle, the refrigeration appliance R is switched on and when the valve 83, controlling the supply of steam to the superheater, is opened for the admission of steam, the appliance is ready to use. function.



   Gaseous vehicle cycle.



   The vehicle cycle will be described below starting at station H of passage 94 at the entrance to the drying chamber. The conditions in this section of passage 84 are substantially the same as those of the entire passage. The states and conditions of the vehicle and the water at the various stations of the apparatus are shown in Table I which is based on a water removal of 100 lbs / hour 45 kg / hour. Table I can be taken to represent the conditions at the start of the cycle of a batch process or the average conditions in a continuous process.

 <Desc / Clms Page number 29>

 
 EMI29.1
 



  TABLE 1 - PHOCEDS CONDITIONS ftMï'l al 'Js2'W'1'rélt, t1QQ t1'e 41 100 liyp <t / hmiy.
 EMI29.2
 
<tb>



  Posts <SEP> Teap. <SEP> Pressure <SEP> Pressure <SEP> Flow
<tb>
 
 EMI29.3
 #F ma de as total pounds / liat 922tane the% jHL & J: bourg agewe JGfiU A 6.0 4e36 1.31 3 # 67 1980 Vapeur Vapor B 5 # 0 no .1n1t1- 1780 Liquida
 EMI29.4
 
<tb> cative
<tb>
<tb> C <SEP> 5.0 <SEP> "<SEP> 200 <SEP> Liquid <SEP> Solid
<tb>
 
 EMI29.5
 D 32.0 100 # # Liquid.



  32.0 100 Liquid P 6.8 If 80. Liquid 0 32.3 11.5 - 1105 1880 vapor
 EMI29.6
 
<tb> H <SEP> 161.3 <SEP> 11.0 <SEP> - <SEP> 11.0 <SEP> 1680 <SEP> steam
<tb>
 
 EMI29.7
 At the H station, the hot, dry optic steam has a temperature of: 1. '(72 "C) and a pressure of 11% of Hg.



  After stable water conditions * leaving the gttwux 00 vihaule are established, the conditions which can be reached very early in the cycle, the steam leaving the drying chamber of the state at is a mixture of cold heptane vapor. UV and Wu water vapor. In this station the temperature of the vapor mixture is 6 * F (-14 ° C) and as can be seen in Table 1 the total prison is 5, p67 May of Hg.

   These conditions establish a pressure difference between the drying chamber and the stations.
 EMI29.8
 H and A of more than 5 mm of Hg which creates the pumping force necessary to circulate gas and water vapor in the drying chamber. The average pressure in the drying chamber
 EMI29.9
 is about 8 am Hgt Passing through the drying chamber, the gaseous vehicle supplies the heat of sublimation to the dry matter and the ice core and the ice core is sublime by thermal conduction and vapor diffusion water out of the ice core,

   as described previously * The gaseous vehicle entrains and sweeps the water vapor which has been sublimated out of the ice core and which has diffused up to the surface of the dried material shell.

 <Desc / Clms Page number 30>

 
The food product in the upper part of the drying chamber is the first exposed to the hot gaseous vehicle
 EMI30.1
 entering the room and is the pr8ld.tr t..Iob, r.

   As the drying and continuing process it forms a layer of dried material at the top of the drying chamber which has been heated to the same temperature as the gaseous vehicle, p6huS ... in the chamber below the drying chamber. layer of dried material is the drying xono where the heat is transmitted from the gas vehicle to the ice core of the frozen product while the
 EMI30.2
 sublimated water vapor is swept away by the beggar vehicle.

   Vara the bottom of the drying zone, the vehicle is cooled considerably and is mixed with a large proportion of water vapor In this region, the rate of mass drying of the product particles is slower because the difference in temperature between the surface and the ice core is lower so that the heat transfer to the core is slower. The diffusion of water vapor from the core to the surface is also slower because there is less difference in the water vapor pressures.
 EMI30.3
 between the ice core and the surface. At the lower part of the drying zone, the vehicle and the water vapor have been cooled to the temperature of the frozen products and the partial pressure of the water vapor is equal to the vapor pressure of the ice in the product. frozen.

   Below the drying zone, no drying takes place and the product remains frozen until the
 EMI30.4
 drying area go down to it. During the drying cycle,; the drying zone gradually descends from the upper part to the lower part of the drying chamber and finally the drying process is completed. The food or other product subjected to desiccation constitutes an obstacle to the flow of the gas vehicle so that the pressure difference of
 EMI30.5
 more than 5 WI1 of ES can be maintained during the drying cycle.



   As the cold and wet mixture of gaseous vehicle and water vapor enters the condenser 20, it flows

 <Desc / Clms Page number 31>

 on the cooling coil 44 which bends @ the liquid heptane vapor HL and the water vapor is frozen to form ice crystals X. The liquid vehicle falls to the bottom of the condenser 20 to form a mass of liquid heptane HL.

   The ice crystals X are detached from the coils by spraying liquid heptane from distributor 52 and since they are heavier than liquid heptane HL; the ice crystals are deposited by gravity at the bottom of the condenser. Stirrer 54 driven by 5µ reducer maintains ice crystals in a fluid state so that ice crystal magma and liquid carrier can flow into ice melter 30.

   '
As mentioned above, in order to remove air and other non-condensable gases which leak into the apparatus or which have been trapped the vacuum pump 74 and the refrigerated trap 70 are maintained in operation during the drying cycle. The gaseous vehicle and the water vapor which would possibly be entrained in the trap are condensed there by the refrigeration coils 76 and returned to the condenser via the line 78.



     As mentioned, the stream coming from the steam condenser 20 is divided into two streams. Liquid heptane flows directly from an upper portion of the vapor condenser housing 20 through line 60 and joins the heptane exiting the ice melter in line 62 as described above. 60 and 62 meet to form the pipe 64 which directs the liquid heptane to the boiler 36,
In the ice melting apparatus,

   the refrigeration coils 28 melt the ice crystals X in water W and the water is pumped at atmospheric pressure and discharged by the pump 58. The level regulator maintains the heptane- interface. water in the ice melter between the electrodes of sensor 66 as described.



   The mass of liquid heptane HL in the boiler is

 <Desc / Clms Page number 32>

 vaporized by the hot refrigerant in coil 38 lice to form heptane vapor HV. The conditions of this steam at the. station 0 are given in Table 1 and it can be seen that the heptane vapor is at a temperature of 32.3 F (0.17 C) and has a pressure of 11.0 mm Hg. The heptane vapor HV is superheated by the superheat coils 82 as described above to return it to the H-station state where the steam is at a temperature of 161.3 F (72 C) and a pressure of 11 mm Hg.



   The above-described circulation of a condensable gaseous vehicle followed by its condensation and re-evaporation communicates heat to the ice core in the product and sweeps away the sublimated water vapor, all without the need for use. . to be a wind tunnel. The efficiency of this process is such that the drying time is greatly reduced compared to that of conventional drying operations by high vacuum freeze drying. For example, in a plant according to the invention, a 1 inch (25 mm) thick layer of food product such as peaches or peas can be dried in less than 1 hour, whereas conventional high vacuum freeze drying of same raw material takes 6 to 9 hours.



   The refrigeration cycle will be briefly summarized again. The refrigerant expands at the expansion valve 42 in the coil 44 of the vapor condenser 20. The refrigerant absorbs the heat of vaporization of the heptane vapor and the heat of sublimation of the water vapor in the coil 44. The expanded refrigerant enters. into the compressor through line 46 and hot compressed refrigerant passes through line 24 to the ice melter 30. As mentioned, in the coil 28 of the ice melter the refrigerant provides the heat necessary to shear ice crystals X in a body of water W.

   The refrigerant enters the boiler 36 through line 34 and by means of a coil 38 the refrigerant supplies the heat of vaporization of the liquid heptane HL which is reconverted to vapor heptane HV. This cooled liquid refrigerant comes out of the

 <Desc / Clms Page number 33>

 boiler through line 40 and passes through the detest valve 42 and the refrigerant expands back into the condenser coil
44 to freeze the water vapor and form ice crystals and condense the vehicle in the liquid state HL as described above,

   
When the drying cycle has been completed * the refrigeration appliance is stopped, the supply of steam to the superheater is Interrupted and valves 90 and 92 are closed * Using valves 98 and 106 and the refrigeration trap 94 it is possible to purge the passages 18, 84 and the drying chamber of the residual gas vehicle and return the condensed vehicle to the condensate,

     The nested product can then be removed from the drying chamber after the vacuum * has been broken.



   If a continuous process is used, with the usual air lock and bleeders at the product inlet and at the outlet of the dryer. $ Valves 90 and 92 are not required * GAS VESICLE - Essential characteristics for direct phase separation.



   1. When food products are dried, if any residue of the vehicle is taken up by the product that residue should be non-toxic in the amounts present.



   2. The vehicle cannot react (must be inert) with the material to be dried, nor with water for direct phase separation.



   3. The vehicle must have an intrinsic vapor pressure at the temperature of the ice core in the material that is higher than its partial vapor pressure in the drying chamber during operation at the temperature of the ice core. ice. This prevents condensation of the piggy vehicle in the drying chamber.



   The vehicle which is used in the apparatus which has been described is normal heptane. Its vapor pressure is indicated -T; as follows in the Handbook of Chemistry and Physics (Chemical Rubber Publishing Co., Cleveland, Ohio):

 <Desc / Clms Page number 34>

 Pressure mm of Hg, 1 10 40 100 400 760 Temperature (approximate) F -30 28 72 107 172 209 The specific gravity of liquid n-heptane is 0.684 at 68 F (20 C) compared to water at 39.2 F (4 C).



  GAS VEHICLE - Desirable features.



   1. It must be non-flammable,
2. It must have at the temperature of the ice core in the material an intrinsic vapor pressure which approaches as closely as possible its actual working pressure or partial pressure at the temperature of the ice core. However, as mentioned above, these pressures can be so close that the gaseous vehicle drops to its intrinsic vapor pressure at the temperature of the ice core due to a deviation in working conditions which could occur. due to small irregularities in operation * If the temperature of the gas vehicle drops too low, this vehicle will condense in the drying chamber.



   Fig. 5 is a graph comparing the pressure-temperature vapor characteristics of heptane with those of water. We see that the heptane curve follows quite closely the water curve but is above the water curve on the temperature scale. This displacement represents a safety factor during operation, which prevents the vehicle from condensing in the drying chamber.



   This characteristic, that is to say the fact that the pressure-temperature curve of vapor of the support is shifted towards the naut relative to the corresponding curve * -of water, the is not too far removed, improves the efficiency of the condenser. It allows the operation of the steam condenser without requiring extremely low temperatures. This in turn reduces the temperature difference between the vehicle liquefied in the condenser and the temperature to which the vehicle is to be heated in the vehicle boiler. The result is a reduction in the amount of work done by the refrigerator compressor in order to condense and re-evaporate the gaseous vehicle.

 <Desc / Clms Page number 35>

 



   3, The vehicle must have a relatively high ratio between specific heat (vapor phase) and latent heat of vaporization. As the specific heat of the mangy vehicle in the vapor phase increases, the mass flow rate required to provide the latent heat of sublimation decreases.



   It is desirable that the vehicle have as high a specific heat as possible but not at the expense of an excessively high latent heat of vaporization. The amount of heat easily circulated by the refrigeration compressor between the refrigeration evaporator (condenser). heptane and water vapor) and the heptane boiler is larger the higher the heat of vaporization. An increase in this amount of heat requires more work on the part of the refrigeration unit.



   Normal heptane in its gaseous state has a heat capacity of 0.38 BTU / pound F at 30 F (0.38 kcal / C kg at -1.11 C) and a latent heat of vaporization of 160 BTU / pound ( 88.9 kcal / kg) at a pressure of 12 mm Hg and a temperature of 32 F (0 * C) which practically corresponds to the state of the gas in the boiler before being superheated. The ratio between the specific heat and the heat of vaporization cited (times 100) is 0.237. The specific ratio of superheated heptane is somewhat higher.



   4. The molecular weight of the vehicle should be as low as possible. By taking into account a certain speed of mass passage through the drying chamber, an increase in the molecular weight of the vehicle reduces the weight fraction of the water vapor that the vehicle can entrain for a given ice temperature and pressure. steaming ice.



   As a result, the most desirable vehicle optimizes the effects of specific heat ratio and those of molecular weight. For example, n-heptane has a molecular weight of 100.2. Tests were carried out with a fluorocarbon vehicle supplied by Minnesota Mining & nd Manufacturing Co., St. Paul

 <Desc / Clms Page number 36>

 
 EMI36.1
 Minnesota, called #Fluoroeheaieal PC-'15 ". The PGrY is pnno1pa.



  Preparation of perfluoro-2-butyitetrahydrofuran of formula, '16. ' This material has a ratio (liver 100) between the specific heat Cal./g% C 110.C) and the latent heat of vaporization (boiling point calluses) of W- (100) 0.6 'compared to suppressant ratio (times 100) of 0 ± 'M (100) - 0.231 for n-heptane. This material is a colorless and odorless liquid, however the molecular weight of fluorearbur. "pou15" is four times that of n-heptane and despite the more favorable specific heat ratio, this increase in the molecular weight of the fluorocarbon reduces the vehicle's ability to sweep sublimated water vapor by approximately 30% relative to the capacity of heptane.



  However, this fluorooarbon possessed other essential and desirable characteristics of a vehicle and therefore may be considered for the process at the expense of a somewhat longer drying time. The temperature and vapor pressure characteristics of FC-75 are given in Fig. 5 with those of hexane and ethyl alcohol which can also be used.



   Another fluorocarbon suitable as a vehicle is
 EMI36.2
 manufactured by 3.1. Du Pont de Nemours de C *, 'W1lm1ncton, Delavare, under the name "Freon 2UN * This fluorinated hydrocarbon is 1,1,1,3-tetrachlorotetrafluoropropane of the formula CC1, CF2cra en.



   Its toxicity is low and it boils at 114.5 C, freezes
 EMI36.3
 at 9yd C, has a specific gravity at 25-C of 1.6927 and a specific heat of z20 oa1 / BI-C. 2560,
5. The vehicle and water (or ice) are preferably mutually insoluble so that the gas = liquefied vehicle can be separated directly from the water and ice without resorting to fractionation.



   6. For gravity separation, the vehicle must have a specific gravity at or below freezing water temperature that differs from that of water or ice at these temperatures. This difference must be sufficient
 EMI36.4
 health to ensure separation gr "Y1JIétnque'd \ 1 gaseous vehicle

 <Desc / Clms Page number 37>

 
 EMI37.1
 liquid at X * times the glac4; Normal lopheptans useful in ,, "'apP8..IU described m ditail is a saturated parattimic hydrocarbon and therefore is Inert # inactive and tr <<ttbl *.

   This Ln41c \ & 1490 other hydrocarbon nozxument liquids in the aethaat arte may be suitable and o # is the case in characteristically oh1IÙqU ... lfouttto1 ', IPhydrocarbon tu4c11atfaent 1l1t'nw, in 14rit, namely loboxanop is not as desirable as 1'btptfAnt from the point of view of tonoUo's oQnora11 & U1 .. 'of 2.'app- p., e11.

   In the case of I # hexme # the trfhicuit in #te aonduemb at 6 ma Hg # <M6is <a condonteur temperature of -300? (4 * 0) <L''n.ra1. necessary so that an apparition of r4trigdrat4on could function at this low teapï '* tuï' <t is much greater than for a device working at 5 * F (** X5 * C) toulmento Cool ne .1nl1'1. not that 1 Vexant can not be useful but It is also important that the h <ptMt because the expenditure 4'8 \ ': & 1' was higher *, As we can 1-t see at Pis # 5, the Ethyl alcohol can also be useful when a y4hiCNJ .. * it is then necessary a * dîstill4ktion to repair 1'.10001.



  In the 0 &. of 1 $ hydrocarbon Immediately Wp4ri1U1 ', namely octane, the temperature at the boiling point t'Mffi'- sa = have raised # so that the gas is at a temperature more 4Xtt4t than heptane when it is introduced In the siobageo chamber, by confiscating, if the ice was used as a vehicle, the temperature of the ice core and the corresponding low pressure required in the drying chamber to ensure the sublimation so much, would represent such conditions. lJ21iU1 t .. such that the octane gas would condense in the drying oil and therefore, could entrain the Steam 4 '.. u.



  Although the form of the invention which has been described in
 EMI37.2
 detail corresponds to a batch process since the pressures
 EMI37.3
 Very low levels are not required in the drying chamber as with conventional high vacuum lyophilization processes, the intention and the continuous function. Air stockings are necessary, An airlock system suitable for Xa pre-

 <Desc / Clms Page number 38>

 The invention is described in U.S. Patent No. 2,515,098.



   On the other hand, concentrated liquids, such as coffee concentrates, can be dry in a continuous apparatus operating in accordance with the present invention. A continuous conveyor capable of receiving concentrates is described in US Pat. No. 2,602,573.



   Although a separate passage 18 is shown to connect the drying chamber 10 with the main steam condenser 20, it essentially acts as a single chamber and these two parts of the apparatus can be considered as a single chamber. *
In a batch process) the refrigeration coils 44 in the main condenser need not be defrosted during drying and can be defrosted at the end of the drying cycle.



   If heated trays 14 are used, some or all of the premise vehicle superheater 80 can be omitted and the gaseous vehicle can be relied upon to entrain the heat from the trays and conduct it to the ice core of the material.



   CLAIMS.

** ATTENTION ** end of DESC field can contain start of CLMS **.


    

Claims (1)

1.- A method of drying a wetting agent contained in a material placed in a drying chamber, characterized in that a dry gaseous vehicle is introduced into the drying chamber at a pressure inside atmospheric pressure, on. supplies heat of vaporization to the gaseous vehicle and a pump pressure differential is established across the drying chamber to cause the mangy vehicle to sweep the chamber, providing heat of vaporization to the agent wetting and entraining the wetting agent vaporized by condensation of the mixture of the mangy vehicle and entrained wetting agent vapor, which leaves the material in the drying chamber at a total vapor pressure lower than the pressure of the gaseous vehicle entering the drying chamber <Desc / Clms Page number 39> 2. A method according to claim 1, characterized in that one further isolates the vehicle from the condensed wetting agent, the isolated condensed gaseous vehicle is re-vaporized and the re-vaporized vehicle is reintroduced into the drying chamber. 3. A method according to claim 2, characterized in that the heat for vaporization of the wetting agent is supplied to the vaporized gaseous vehicle before it enters the drying chamber. 4.- Closed-cycle process for drying frozen porous material placed in a drying chamber, characterized in that a dry gaseous vehicle is introduced heated into the drying chamber at a pressure below atmospheric pressure, the heat of which is supplied. sublimation to the gas vehicle to establish a pumping pressure difference from one side of the. drying chamber by condensing the mixture of gaseous vehicle and sublimated water vapor exiting the material in the drying chamber to establish a total vapor pressure below the exit of the drying chamber, the condensed vehicle is isolated and re-vaporizes the vehicle to reintroduce it into the drying chamber, the gaseous vehicle being inert.,. stable and having a pressure curve at vapor temperatures which roughly follows that of water but lies higher than water on the temperature scale to a sufficient extent to prevent condensation of the gaseous vehicle in the drying chamber due to small variations in operating conditions * 5.- A method according to claim 4, characterized in that the heat of sublimation is applied to the gas vehicle before the gas is reintroduced into the drying chamber. 6 ,, - Closed cycle method of drying frozen cellular material placed in a drying chamber, characterized in that Introducing a dry gaseous vehicle heated into the drying chamber at a pressure below atmospheric pressure, it is establishes a pumping pressure difference from one side of the drying chamber to the other by condensing the mixture of carrier gas and water vapor exiting the drying chamber <Desc / Clms Page number 40> to establish a lower total vapor pressure at the outlet of the drying chamber, the condensed vehicle is isolated and reheated to its initial state to reintroduce it into the drying chamber, the range between the pressures being maintained in the transition zone of the thermal conductivity-pressure curve of the gas vehicle in the dried porous layer. 7.- A method of drying frozen porous material placed in a drying chamber, characterized in that a dry gaseous vehicle heated into the drying chamber at a pressure inside the atmospheric pressure is introduced and a difference of pumping pressure from one side of the drying chamber to the other so that the gaseous vehicle sweeps the chamber, heat of sublimation is supplied to the ice in the frozen material and the liberated water vapor is swept away by condensing the mixture of gaseous vehicle and water vapor exiting the drying chamber to establish a lower total vapor pressure at the exit of the chamber. 8. A method according to claim 7, characterized in that further isolates the condensed vehicle from the condensed water vapor, the condensed vehicle is re-vaporized and the re-vaporized vehicle is reintroduced into the drying chamber. 9. A method according to claim 8, characterized in that the further superheats the revaporized vehicle to provide it with the heat of sublimation of the ice before reintroducing the revaporized vehicle in the drying chamber. 10.- Closed-cycle process for drying frozen porous material, characterized in that a hot, dry gaseous vehicle is introduced at a pressure below atmospheric pressure into a drying chamber containing the frozen material, and swept. matter by this gaseous vehicle by passing through the drying chamber to provide the heat of sublimation and entrain the water vapor from the surface of the matter, vapor which comes from the sublimation of the ice core of the matter, we maintain a sufficient vehicle gas flow rate to maintain <Desc / Clms Page number 41> below the triple point, the Vapor Pressure of the water on the ice core in the material, the gaseous vehicle mixture of water vapor which exits the drying chamber is condensed, thus reducing the total vapor pressure of the spoiled vehicle and of the water vapor exiting the drying chamber to a value lower than that at which the gaseous vehicle is Introduced into the licking chamber, the water is separated from the liquefied pasty vehicle, the liquefied gas vehicle is re-vaporized, the heat of ice sublimation is added to the gas vehicle, and the hot and hot vehicle gas is recirculated into the drying chamber. It * - Cycle process formed of drying poreu- frozen material, characterized in co that a dry and hot pasty vehicle is introduced at a pressure below atmospheric pressure in a drying chamber containing the frozen material, it is made sweep the material through this gaseous vehicle passing through the drying chamber, to provide the heat of sublimation and to entrain the water vapor given off from the surface of the material which has sublimated from the ice core of the material, a sufficient flow of gaseous vehicle is maintained to maintain below the triple point the water vapor pressure on the ice core in, the material, the latent heat of vaporization of the gas vehicle and the latent heat of sublimation of the water vapor exiting the drying chamber are subtracted to form a mixture of liquefied gas vehicle and ice crystals while simultaneously reducing the pressure of total vapor of the gaseous vehicle and the vapor pressure of water leaving the drying chamber to a value lower than that at which the gaseous vehicle was introduced into the drying chamber, the ice crystals are separated from the liquefied gas vehicle, the liquefied gas vehicle is re-vaporized, the heat of sublimation of the ice is added to the gas vehicle and the hot and dry vehicle gas is recirculated into the drying chamber, '12. - Closed cycle process for drying porous material. <Desc / Clms Page number 42> is frozen, characterized in and that a Sage = dry and hot vehicle is introduced at a pressure below atmospheric pressure into a drying chamber containing the frozen material, the material is swept through this gaseous vehicle and it is passed through the drying chamber to provide the heat of sublimation and to entrain from the surface of the material the water vapor which has been sublimated from the ice core of the material, a sufficient flow of gaseous vehicle is maintained to maintain the water vapor pressure on the ice core in the material below the triple pointa one condenses the mixture of gaseous vehicle and water vapor which leaves the chamber while simultaneously reducing the total vapor pressure of the gaseous vehicle and of the water vapor leaving the drying chamber, to a second pressure lower than that at which the gaseous vehicle was introduced into the drying chamber, it is maintains the difference between the first and the second pressure in the transition zone of the thermal conductivity-pressure curve of the gas vehicle, the water is separated from the liquefied gaseous vehicle, the liquefied gaseous vehicle is upgraded, the heat of sublimation of the ice is added to the gas vehicle and the dry and hot gas vehicle is recirculated into the drying chamber. 13.- A closed-cycle process for drying frozen porous material, characterized in that hot, dry heptane vapor is introduced at a pressure below atmospheric pressure, into a drying chamber containing frozen matter, this heptane vapor is swept over the matter to provide the heat of sublimation and entrain from the surface of the matter the water vapor which has been sublimated from the ice core of the matter, we maintains a flow of heptane vapor of water sufficient to maintain below the triple point the pressure of water vapor on the ice core of the material, the mixture of water vapor and heptane vapor which leaves the drying chamber is condensed, thus reducing the total vapor pressure of the heptane and of the water vapor leaving the chan- <Desc / Clms Page number 43> After drying to a value lower than that at which the heptane vapor was introduced into the drying chamber, the water is separated from the liquefied heptane, the liquefied heptane is re-vaporized, the heat of sublimation of vaporized heptane ice, and the hot, dry heptane was recirculated to the drying chamber. 14.- Dry cycle method of drying porous, frozen material, characterized in that dry and hot heptane gageate is introduced at a pressure below atmospheric pressure in a drying chamber containing the material to be consoled. sweeps the material by the heptane gas which passes through the drying chamber to provide the heat of sublimation and carry away from the surface of the material water vapor which has been sublimated from the ice core of the material , one maintains a flow of mangy heptane sufficient to maintain below the triple point the water vapor pressure on the gas core in the material, the mixture of gaseous heptane and water vapor which leaves the drying chamber is condensed while simultaneously reducing the total vapor pressure of the gaseous heptane and of the water vapor exiting the drying chamber at a second pressure lower than that at which gaseous heptane was introduced into the drying chamber, the difference between the first and the second pressure is maintained in the transition zone of the thermal conductivity-pressure curve of heptane gaseous, the water is separated from the liquefied heptane gas, the liquefied heptane is re-vaporized and the hot, dry heptane gas is recirculated in the drying chamber. 15.- The method of claim 14, characterized in that the vaporized heptane is superheated before recirculating it. 16.- A closed-cycle process for drying frozen porous material, characterized in that a gaseous vehicle consisting of a dry and hot fluorocarbon is introduced at a pressure below atmospheric pressure into a drying chamber containing the frozen material, it is swept by this vehicle <Desc / Clms Page number 44> gas the matter by passing it through the so-called licking chamber to provide the heat of sublimation and drag it from the face of the matter, the vapor which has been sublimated from the ice core of the matter, one maintains a current of pasty vehicle sufficiently to maintain below the triple point the pressure of water vapor on the core of ice in the matter, one condenses, the mixture of gaseous heptane and water vapor which leaves the drying chamber, while simultaneously reducing the total vapor pressure of the gaseous vehicle and the water vapor leaving the drying chamber to a second pressure lower than that at which the gaseous vehicle was introduced into the drying chamber, we maintain the difference between the first and the second! pressure in the transition zone of the thermal conductivity-pressure curve of the gas vehicle, the water is separated from the liquefied gas vehicle, the gaseous vehicle is re-vaporized and the hot, dry mangy vehicle is recirculated in the drying chamber. 17. - Closed-cycle process for drying frozen porous material, characterized in that a dry and hot mangy vehicle is introduced, at a pressure below atmospheric pressure, in a drying chamber containing the frozen material is made sweeping through this gaseous vehicle the material by passing it into the drying chamber to provide the heat of sublimation and entrain from the surface of the material the water vapor which has been sublimated from the ice core of the material, we maintains a sufficient flow of gaseous vehicle to maintain below the triple point the water vapor pressure on the ice core in the material, the mixture of gaseous vehicle and water vapor which leaves the drying chamber is condensed while simultaneously reducing the total vapor pressure of the gaseous vehicle and of the water vapor exiting the drying chamber to one second pressure lower than that at which the gaseous vehicle was introduced into the drying chamber, the range between the first and the second pressure is maintained in the transition zone of the thermal conductivity-pressure curve of the gas vehicle, one separates the water of the liquefied gaseous vehicle, <Desc / Clms Page number 45> EMI45.1 rswraporizes the liquefied gaseous vehicle, we add the heat of sublimation of the ice you convey gaxsux and we recirculate the dry and hot mangy vehicle in the drying chamber, this gaseous vehicle being inert and stable and having a relatively high ratio between the heat of corn * specific in vapor phase and latent heat of vaporization and a pressure- curve EMI45.2 vapor teapMture which generally follows that of water but is higher than water on the temperature scale to a sufficient extent to prevent condensation of the vehicle gaseous in the drying chamber by moth of small deviations of operating conditions. 18.- A method of drying frozen porous articles in a drying chamber, characterized in that it transmits the chorus of sublimation of the ice from a gaseous vehicle at a pressure below atmospheric pressure to the ice core in the article, thus transmitting the heat of vaporization of the gaseous vehicle and the heat of sublimation of the vapor to a EMI45.3 pressure Lower than the atmospheric pressure lower than that at the outlet of the drying chamber to a cold refrigerant to liquefy the gaseous vehicle and freeze the water vapor, the ice is separated from the gaseous vehicle, the refrigerant is compressed thus adding the energy which will later provide the heat of vaporization of the vehicle transmitting the heat of vaporization of the vehicle liquefies the refrigerant to the liquefied vehicle to re-vaporize the liquid vehicle, we add the heat of sublima * EMI45.4 The ice is removed from the revaportid vehicle and the hot, dry, revaporized vehicle is directed again to the drying chamber for a further sublimation and a new flushing of the water vapor. EMI45.5 19. "A method according to claim 18, characterized in that the vehicle is heptane * 20.- A method according to claim 18, characterized in, EMI45.6 what the vehicle is a fluorooarbide. 21. A method according to claim M, characterized in! that the vehicle is mainly composed of perfluoro-S-buiyl- 'tetrahydrofuran (Ca 2 0). <Desc / Clms Page number 46> 22. Method according to claim 18, characterized in that the vehicle is essentially made up of 1,1,1,3-tetrachlorotetrafluoroporpane (CCl3CF2Cl). 23.- Apparatus for separating a vaporizable constituent by sublimation from a material containing a vaporizable constituent in the Bolide state, characterized in that it comprises a drying chamber with an inlet passage of a gauze vehicle = ; , and an outlet passage for the mangy vehicle and the combined sublimated steam, a device for supporting the material to be dried in the drying chamber, a steam condensing chamber connected to the outlet passage of the drying chamber, an apparatus for refuse yours with a device for condensing the vapor inside the condensation chamber, a device for removing air or other inert gas from the condensation chamber, a boiler for the liquefied vehicle mounted to receive the vehicle the housing of the condensation chamber and a fluid vehicle in the device, this fluid vehicle being in the vapor phase in the passages, the drying chamber and the upper part of the condensing chamber of the vapor and liquid phase in the housing of the condenser in the boiler. 24.- Apparatus according to claim 23, characterized in that a superheater for the vaporized vehicle is mounted between the boiler and the drying chamber. 25. Apparatus according to claim 23, characterized in that the vehicle is a saturated paraffinic hydrocarbon from the methane group, consisting of hexane and heptane. 26.- Apparatus according to claim 23, characterized in that the vehicle is heptane. 27.- Apparatus according to claim 23, characterized in that the vehicle is a fluorocarbon. 28.- Apparatus according to claim 23, characterized in that the vehicle is mainly composed of perfluoro-2-butyltetrahydrofuran (C8F16O). 29.- Apparatus according to claim 23, characterized <Desc / Clms Page number 47> EMI47.1 and that the vehicle is mainly eoopoié of, '7,, 13tdtra chlorotetratluoropropane (Col 30? 201)' 30.- Apparatus according to claim 23, characterized in that the refrigeration apparatus comprises a coil for vaporizing the liquid vehicle and condensing the refrigerant in the boiler. EMI47.2 31. Apparatus according to claim 23 .. characterized in that the vapor condenser of the refrigeration apparatus freezes the condensed water vapor, a pump drives the liquid vehicle from the housing of the vapor condenser to directed nozzles. to the vapor condenser to detach the ice crystals therefrom and a combined liquid vehicle and water melting and separator apparatus is interposed between the vapor condenser housing and the boiler. 32. Apparatus according to claim 31, characterized in that the ice in the ice melting apparatus is melted by a condensing coil of the refrigerant of the refrigeration apparatus. 33.- Freeze-drying apparatus characterized in that it comprises a drying chamber with an inlet passage for the gas vehicle and an outlet passage for the gas vehicle of the combined water vapor, a device for supporting the material to be dried in the vacuum chamber, a condensing chamber connected to the outlet passage of the drying chamber for condensing the vehicle and freezing the vapor, an ice melting apparatus connected to the condenser housing to receive a liquid vehicle magna and ice crystals, a boiler connected to the casing of the condensation chamber and to the lower part of the condenser casing, a device connecting an upper part of the ice-melting apparatus to the boiler, a refrigeration apparatus comprising a coil for evaporating the refrigerant in the condenser and cooling coils for the hot refrigerant in the ice melting apparatus and the boiler, a superheater for the vehicle vaporized in the boiler, the overheater being connected to the inlet passage of the drying chamber <Desc / Clms Page number 48> EMI48.1 and a fluid vibicue in the apparatus 11, and #foicult fluid <at vapor phase in the * passages $ the discharge chamber of the partit, up'r1wJ. of the oon4tn.eur and batch mrchautteur and in phase liquidated in the housing of the oondtn.8Ur .. the ice melting apparatus and the boiler.