Patent Publication Number: US-2011067438-A1

Title: System and process for producing frozen carbon dioxide pellets

Description:
The present invention relates to a system and a process for the production of frozen carbon dioxide granules (solid CO 2  pellets), e.g. of the type used for cryogenic sand blasting. 
     The processes currently known in the art for producing dry ice granules comprise the use of liquid CO 2  and expanding it to generate carbonic snow; this is then pressed and passed through an extruder to produce small cylinders (pellets) of dry ice. 
     The starting CO 2  in the liquid state is commercially available in cylinders kept at room temperature, i.e. at 20° C. and at a 57 bar pressure (MPT—Middle Pressure Tank), or stored in a thermally insulated tank at a temperature of −20° C. and at a 20 bar pressure (LPT—Low Pressure Tank). The range of use of CO 2  in the liquid state ranges from the critical point (31° C./74 bar) to the triple point (−56.6° C./5.18 bar). 
     According to known processes, the liquid CO 2  taken from MPT or LPT type tanks is made to expand in a chamber where pressure can vary from 0 to 2.5 bar, or at least below the triple point pressure of 5.18 bar to generate carbonic snow. 
     The conversion percentage of liquid CO 2  into carbonic snow is variable and depends on the initial temperature and pressure conditions: for example the percentage may vary between about 20% for high pressures and temperatures (MPT bottles) to about 40% for low pressures and temperatures (LPT tanks). In fact, following the phase diagram of carbon dioxide it is assumed that the lower the temperature and pressure, the greater the amount of solid CO 2  can be generated. 
     As previously mentioned, the carbonic snow is then pressed and passed through an extruder to produce small cylinders (pellets) of dry ice. 
     The remaining fraction not converted into carbonic snow consists of CO 2  in the gaseous state that is generally recovered to be used again in the process: this allows saving on raw material costs but inevitably leads to further system and process costs. Alternatively it is also possible to allow the gaseous CO 2  to disperse into the atmosphere, but this entails higher costs for raw materials and may prove to be unacceptable from an environmental point of view. 
     However, from the phase diagram of carbon dioxide one can derive that if liquid CO 2  is cooled below the sublimation point temperature at 1 bar, i.e. approximately −78.5° C., and at a pressure higher than the triple point, i.e. 5.18 bar, it is all converted into solid CO 2 : therefore, even at the atmospheric pressure (1 bar), CO 2  remains solid, without expanding and without entering a gaseous phase. This way theoretically achieves the full transformation of liquid CO 2  into solid CO 2 , i.e. with a theoretical percentage of conversion of 100%. 
     Based on these observations, EP-A-0663371 proposes a process and a system that allow, in theory, to completely transform the liquid carbon dioxide into carbon dioxide in the solid state, i.e. without any fractions to be recovered and/or be dispersed. Practically, this known document simply proposes to cool the liquid carbon dioxide by using liquid nitrogen to bring it directly from the liquid to the solid state, avoiding the intermediate step of CO 2  expansion for generating carbonic snow and the consequential loss of a substantial portion of carbon dioxide in the gaseous form. 
     Particularly, the liquid carbon dioxide is passed into a plurality of cylindrical ducts placed in an exchanger where liquid nitrogen is circulated, i.e. a cooling fluid that removes heat from liquid carbon dioxide causing its solidification. The pressure required to form and expel the pellets from the cylindrical ducts is ensured by the same carbon dioxide supplied in the liquid state. 
     However, the carbon dioxide within the cylindrical ducts tends to solidify immediately along the whole length of ducts, thus preventing the hydraulic pressure of carbon dioxide provided in the liquid state to push the solidified pellets to the outlet of the ducts. 
     Accordingly, one aim of the present invention is to propose a system and a process for the production of pellets or granules of carbon dioxide in the solid state that provide a theoretical yield of 100% in the conversion of carbon dioxide from the liquid state to the solid state. 
     Another aim of the present invention is to propose a system and a process of the above mentioned type enabling optimal use of the heat-removing ability of the cooling fluid. These aims are achieved by the present invention through a system according to claim  1  and to a process according to claim  9 . Additional peculiar characteristics of the present invention are set forth in the respective dependent claims. 
     In the system according to the present invention at least one heat exchanger is provided for the formation of pellets having a container in which a bundle of tubes is housed each having an inlet opening, in which carbon dioxide enters in the liquid state, and each having an output opening from which the carbon dioxide comes out in the solid state in the form of pellets. The container includes at least one inlet connection and at least one outlet connection for the circulation of a cooling fluid within the device. The container is advantageously subdivided into at least three separate sections and connected in series to form, in said heat exchanger device for the formation of pellets, at least three zones at different temperatures. 
     In this way, a device is obtained for the formation of pellets in which a section receives carbon dioxide in the liquid state without solidifying it immediately, a subsequent section lowers the temperature of carbon dioxide to facilitate the liquid/solid phase transition and only in a further sequential section the temperature is lowered to values low enough to obtain exiting solid pellets. In this way, the tubes in which carbon dioxide is conducted are not obstructed by a solid fraction throughout their whole length, but only in the final output section of solidified pellets. 
     In the present invention, nitrogen as a cooling fluid in either a liquid or a gaseous state is preferably used. 
     In fact, the container of the device that forms pellets includes at least a first end section, in which the cooling fluid enters in the liquid state and at least a second end section from which the cooling fluid comes out in the gaseous state. 
     Temperature controlled means are preferably provided for regulating the flow of cooling fluid between the output of the cooling fluid from a section of the container and the input of the cooling fluid into the next section. 
     In the system according to the present invention is also preferably provided at least one intermediate heat exchanger placed between the source of carbon dioxide in the liquid state and the heat exchanger device for the formation of pellets. 
     In the intermediate heat exchanger at least a fraction of the cooling fluid is circulated in the gaseous state that comes out of the container of the device for the formation of pellets. In fact, the cooling fluid exiting from the container of the device for the formation of pellets is in the gaseous state at a temperature low enough to be conducted into the inlet of the intermediate heat exchanger. 
     In addition, the cooling fluid in the gaseous state is periodically circulated along a closed circuit within the intermediate heat exchanger. This allows to exploit as much as possible the heat removal capacity of the cooling fluid being used. 
     In this way, the liquid carbon dioxide is preventively cooled in the heat exchanger before being fed to the intermediate heat exchanger device for the formation of pellets, bringing the temperature near the triple point (−56.6° C.), and then gradually further cooled in the heat exchanger device for the formation of pellets. 
     The pellets can be formed in the device by way of hydraulic pressure provided for example, by a pump or, alternatively, by way of mechanical pressure exerted by at least one mobile element, such as a piston with mechanical, hydraulic or electric drive. According to another aspect of the present invention, a process is proposed for the production of frozen carbon dioxide pellets in the solid state, comprising the steps of:
         a) withdrawing carbon dioxide in the liquid state from a source at a pre-set temperature and pressure and feeding it to at least one heat exchanger device for the formation of pellets, the device including a container which houses a bundle of tubes, each having an inlet opening, in which carbon dioxide enters in the liquid state and each having an outlet opening from which the carbon dioxide comes out in the solid state in the form of pellets, the container including at least one inlet connection and at least one outlet connection for the circulation of cooling fluid within the device;   b) circulating a cooling fluid in the container that houses the tubes to keep them at a temperature lower than that of the carbon dioxide in the liquid state within its source;   characterized in that the container is divided into at least three separate sections and connected in series to form, within the heat exchanger device for the formation of pellets, at least three zones at different temperatures.       

     Preferably, the carbon dioxide in the liquid state is cooled to a temperature between the temperature of the liquid carbon dioxide of its source and the temperature of the triple point of the carbon dioxide, the cooling being achieved by way of an intermediate heat exchanger in which the cooling fluid is put into circulation. 
    
    
     
       Additional features and advantages of the present invention will become apparent from the following description, made with reference to the attached drawings by way of illustration and not limitation, in which: 
         FIG. 1  is a diagram of a system for the production of carbon dioxide pellets in the solid state according to a possible embodiment of the present invention; 
         FIG. 2  is a schematic cross-section view of a heat exchanger device for the formation of pellets; 
         FIG. 3  is a longitudinal section view illustrating the heat exchanger device for the formation of pellets and the means to exert pressure according to an embodiment of the present invention; 
         FIG. 4  schematically illustrates the formation of pellets in one of the tubes of the heat exchanger device for the formation of pellets; and 
         FIG. 5  is a longitudinal section view illustrating a heat exchanger device for the formation of pellets and its mechanical means to exert pressure according to another embodiment of the present invention, and 
         FIG. 6  is a diagram illustrating the process according to the present invention on the phase diagram of carbon dioxide. 
     
    
    
     In the system of  FIG. 1  a thermally insulated tank  10 , preferably of the LPT type (Low Pressure Tank) is shown, which is the source of carbon dioxide in the liquid state. The carbon dioxide in the tank  10  is maintained at a temperature of −20° C. and at a pressure of 20 bar. 
     Further represented herein is a thermally insulated tank  20  which is the source of a cooling fluid, such as nitrogen, which is kept in the liquid state at a temperature of −196° C. and at a pressure of 5 bar. 
     The liquid carbon dioxide is drawn from the tank  10  and fed to an intermediate heat exchanger  30  to further reduce the temperature to a predetermined value, for example, to about −50° C. Along the connecting duct  13  between tank  10  and heat exchanger  30  is provided a pressure regulating valve  12  to reduce the pressure of liquid carbon dioxide at a value of about 7 bar. 
     The heat exchanger  30  is supplied with a cooling fluid, in particular nitrogen in the gaseous state, which is drawn from the top of the tank  20  and fed to the exchanger  30  by way of a duct  23 . Along the duct  23  a pressure regulating valve  42  and a modulating pressure regulator  22  are arranged in series. A further modulating pressure regulator  32  is disposed downstream of the heat exchanger  30 . 
     The nitrogen in the gaseous state can be periodically circulated in a closed circuit within the heat exchanger  30  by way of a fan  35 . When the gaseous nitrogen is no longer capable of removing heat from carbon dioxide to lower the temperature to the prefixed value, the gaseous nitrogen is expelled from the heat exchanger  30  and discharged into the atmosphere through the modulating pressure regulator  32 . The discharge of gaseous nitrogen into the atmosphere may be directed, in whole or in part, towards the inside of the container  100  (dashed line  37 ) in order to maintain a cooled internal atmosphere substantially devoid of moisture or, in other words to obtain the “thermal inertization” of container  100  for the pellets. 
     The liquid carbon dioxide exiting from the heat exchanger  30  is therefore at a temperature of about −50° C., or at least at a temperature close to that of the triple point (−56.6° C.), and is fed to the heat exchanger device  40  for the formation of pellets, adjusting the flow through a solenoid valve  41 . A pump  87  can be provided to exert pressure on the device  40  as will be explained in detail below referring to the embodiment of  FIG. 3 . 
     The heat exchanger device  40  for the formation of pellets comprises a container  50  ( FIGS. 2 ,  3  and  5 ) in which nitrogen is circulated, according to the modality that will be described below, which is drawn in the liquid state from tank  20  at a temperature of about −196° C. and sent in the container  50  through a duct  21 . A pressure regulating valve  24  is arranged along the duct  21  to maintain a liquid nitrogen supply pressure at around 5 bar. 
     Subsequent sections of the device  40  for the formation of pellets are interconnected in series by way of temperature controlled means  60  to regulate the flow of cooling fluid between the outlet of a section and the inlet of the next section. 
     The gaseous nitrogen exiting from the device  40  is directed to the heat exchanger  30  through a duct  43 , directly mixing it for example, to gaseous nitrogen drawn from the top of tank  20 . A modulating pressure regulator  44  placed along the duct  43  allows compensation for any variation in pressure between the circuit of liquid nitrogen and the circuit of gaseous nitrogen. 
     The carbon dioxide fed into the device  40  is maintained at a value of pressure higher than the triple point (5.18 bar), for example at a pressure of about 7 bar, so as to cause solidification of liquid carbon dioxide in the device  40  that is exposed to compression to form and expel the pellets  90 . The latter can then be collected within a thermally insulated container  100  where a temperature below −78.5° C. is maintained, i.e. a temperature below the sublimation temperature of carbon dioxide in atmospheric pressure conditions (1 bar). 
     A device  80  assembles together the plant control systems and in the case of the embodiment to be described with reference to  FIG. 5 , comprises for example, also the mechanical means acting on a mobile element of compression, as will be further explained in more detail. 
       FIG. 2  shows a cross section of a device  40  for the formation of pellets in accordance with the present invention. The device  40  essentially comprises a container  50  which houses a bundle of tubes  62 . 
     In the embodiment of  FIG. 3 , the container  40  is separated into three sections by separating walls  65  crossed by tubes  62 . The liquid carbon dioxide is fed to the device  40  by a pump  87 , guaranteeing the maintenance of the necessary pressure to facilitate the ejection of pellets. 
     Taking as a reference the directional flow of carbon dioxide, the end section  40   a , in which carbon dioxide in the liquid state is fed, is kept at a temperature of about −50° C., the next section  40   b  is kept at a temperature of approximately −70° C. and the section  40   c  from which the pellets are ejected is kept at a temperature of about −90° C. The conditions that occur within a single tube  62  of the device  40  are schematically illustrated in  FIG. 4 . 
     The zones at different temperatures thus provided are obtained by feeding liquid nitrogen at a temperature of −196° C. through an inlet connection  51  to maintain in the section  40   c  of the container  50  a temperature of about −90° C. This also facilitates the starting step of production, quickly generating a “cap” of frozen carbon dioxide that allows the proper functioning of the subsequent steps of production in a continuous cycle. However, if necessary, the openings of the tubes  62  from which the pellets emerge can be sealed off at the beginning of the production to ensure the achievement of the desired temperature and pressure conditions. During the production cycle, the seal is ensured instead by the fraction of carbon dioxide in the solid state that is maintained in the section  40   c.    
     The nitrogen exiting from section  40   c  is fed to the section  40   b  by way of a means  60  which allows flow regulation of nitrogen to maintain the desired temperature for this section (about −70° C.). The same occurs to the nitrogen exiting from the section  40   b , which is drawn in the inlet of section  40   a  through a device  60  regulating the flow of nitrogen to maintain the desired temperature (approximately −50° C.) in the section  40   a . The tubes  62  inside the container  50  open at the level of a cutting device  95  which cyclically allows the cutting of the carbon dioxide pellets at a desired length during their ejection in the solid form. 
     In the embodiment of  FIG. 5 , the principle of cooling the sections at different temperatures is the same as the one shown in  FIG. 4 . 
     In this embodiment, the liquid carbon dioxide is fed into a chamber  47  in which a piston  81  slides activated by a hydraulic cylinder  82  with double effect to cyclically ensure the action of compression necessary for advancing carbon dioxide in the tubes  62  and ejecting the pellets  90  at the exit of the device  40 . 
     Although not explicitly stated, it is understood that all pipes, equipment, tanks and various system devices subject to temperatures lower than the environment are properly thermo-insulated. 
     The various steps of the procedure in accordance with the present invention are briefly summarized with reference to the phase diagram of carbon dioxide represented in  FIG. 6 : temperatures are expressed in degrees Celsius along the x-axis according to a linear scale, while the pressures are expressed in bars along the y-axis according to a logarithmic scale. In the diagram of  FIG. 6  the main reference points are also underlined for carbon dioxide, i.e. the critical point CP (31° C., 74 bar), the triple point TP (−56.6° C. 5.18 bar) and the sublimation point SP at atmospheric pressure (−78.5° C., 1 bar). 
     The point D 1  represents the conditions of liquid carbon dioxide in tank  10 , for example referred to the case of a tank type LPT (Low Pressure Tank), in which carbon dioxide is maintained at a temperature of −20° C. and a pressure of 20 bar. 
     The valve  12  reduces the pressure of carbon dioxide drawn from the tank  10  to a value of about 7 bar and the intermediate heat exchanger  30  reduces the temperature to a value of about −50° C. Practically, carbon dioxide exiting from the heat exchanger  30  will be found for instance under the given conditions close to the point D 2 , i.e. temperature and pressure conditions close to those of the triple point TP. 
     Starting from point D 2 , in the conditions of temperature and pressure under which carbon dioxide is fed in the section  40   a  of the device  40 , it is possible to subject the liquid carbon dioxide at subsequent stages of cooling to overcome the curve of liquefaction and achieve solid state conditions. 
     For example, considering for the sake of simplicity an isobaric condition, the cooling in sections  40   b  and  40   c  of the device  40  is pushed beyond the sublimation point temperature SP, i.e. beyond the point D 3  at −78.5° C. and 7 bar, to prevent that the pellets just produced, once exposed to atmospheric pressure, may be subject to sublimation. For example, the temperature of carbon dioxide in the section  40   c  of device  40  can be lowered to about −90° C. (point D 4 ), regardless of pressure variations that may occur in the feeding of carbon dioxide to the device  40 . 
     During the production of pellets, it is necessary to maintain the pressure in the pipes  62  of the device  40  to values higher than those of the triple point, i.e. greater than 5.18 bar to prevent the phenomena of liquefaction/sublimation of carbon dioxide already solidified. This can be achieved for example, by keeping the supply of liquid carbon dioxide always at pressures greater than the triple point, for example at 7 bar, or feeding the device  40  also with a fraction of gaseous carbon dioxide at a suitable temperature and at a pressure higher than that of the triple point TP. 
     Various changes may be made to the embodiments represented here, without going beyond the scope of the invention. For example, the source of carbon dioxide could also be formed theoretically by cylinders of the MPT type (Middle Pressure Tank), although this could result in a lower yield process. Similarly, other cooling fluids can also be used that are able to provide a component in the liquid state and a component in the gaseous state, such as argon, helium or the like, although currently, these types of fluids are less easily available and have higher costs than nitrogen.