Patent Publication Number: US-2012027627-A1

Title: Compressor system for a process gas plant having heat return, and the process gas plant for carbon dioxide gas separation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2010/054272, filed Mar. 31, 2010 and claims the benefit thereof. The International Application claims the benefits of German application. No. 10 2009 015 861.8 DE filed Apr. 1, 2009. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention refers to a compressor system for a process plant having heat return, and the process plant for carbon dioxide gas separation having the compressor system. 
     BACKGROUND OF INVENTION 
     During the combustion of fossil fuels, especially carbon dioxide gas results as flue gas and is to be seen as loading the environment if the carbon dioxide gas is discharged into the atmosphere. In particular, a fossil power plant emits considerable quantities of carbon dioxide gas, which it is necessary to reduce. To this end, a method is known in which the carbon dioxide gas emission of the power plant is separated from the flue gas, compressed and stored underground. For separating the carbon dioxide gas from the flue gas, a carbon dioxide gas separation plant, which has a compressor unit for compressing the carbon dioxide gas, is known. The compressor unit has a compressor having a multiplicity of compressor stages with which the carbon dioxide gas is compressed in stages. As a rule, the carbonaceous gas is moist since water is produced during the combustion of the fossil fuels. 
     The compression of the carbon dioxide gas takes place in the compressor polytropically and, in proportion to the pressure ratio of the compressor, leads to a temperature increase of the carbon dioxide gas. The compressor can be constructed from a multiplicity of compressor stages, wherein after the individual compressor stages the carbon dioxide gas is cooled by means of a cooler. As a result, the effort which is required for driving the compressor can be reduced. 
     In the cooler, heat from the carbon dioxide gas is yielded to a cooling medium. Cooling water, which flows through the cooler in a cooling water circuit, is conventionally used as the cooling medium, wherein heat is extracted from this for tempering the cooling water. For increasing the thermal efficiency of the carbon dioxide gas separation plant, it is advantageous to feed the heat, which is removed from the cooling water, to the process of the carbon dioxide gas separation plant at a suitable point. For example, two cooling water circuits with different temperature levels are provided in the carbon dioxide gas separation plant, wherein the cooling water circuit with the higher temperature level is provided for cooling the carbon dioxide gas directly after discharging from the compressor stage. For cooling the cooling water in this cooling water circuit, industrial water, for example, can be heated. The cooling water circuit with the lower temperature level is then used for further cooling of the carbon dioxide gas to a required temperature level which, for example, is suitable for entry of the carbon dioxide gas into the next compressor stage. 
     The cooler conventionally has a housing which is exposed to admission of the carbon dioxide gas and in which two cooler bundles are accommodated, wherein one of the cooler bundles is connected to the one cooling water circuit and the other cooler bundle is connected to the other cooling water circuit. The two cooler bundles are advantageously arranged next to each other in the housing, wherein the diameter of the housing is large. As a result, the cooler bundles, for construction-related reasons, have no common cross section so that an efficiency-optimized design of the cooler bundles is complicated. During the cooling of the carbonaceous gas, a falling short of the dew point usually occurs so that water precipitates in the cooler. Therefore, for reasons of corrosion resistance, the housing and the cooler bundles are constructed from stainless steel, as a result of which the production costs for the cooler are high. 
     Furthermore, the use of stainless steel in the cooler on heat transfer surfaces is disadvantageous since the thermal conductivity of stainless steel is sufficiently high only to a limited extent. 
     SUMMARY OF INVENTION 
     It is the object of the invention to create a compressor system for a process plant having heat return and a process plant for carbon dioxide gas separation having the compressor system, wherein the process plant has high thermal efficiency and the compressor system is cost-effective in production. 
     The compressor system according to the invention for a process plant having heat return has a compressor for compressing moist process gas, having at least one compressor stage, and a process gas cooler unit which, for cooling the process gas, is connected downstream to the compressor stage, and has at least one first and one second process gas cooler which is operated with a cooling medium, wherein the process gas coolers have in each case an individual process gas cooler jacket, which is exposed to admission of the process gas, with a process gas cooler bundle accommodated therein and exposed to admission of the cooling medium, are connected directly one after the other on the process gas side, and are designed and can be operated with the cooling medium in such a way that from the process gas cooler which is arranged upstream on the process gas side a predetermined heat flow can be removed from the process gas, as a result of which the thermodynamic state of the process gas between the process gas coolers is located in the region of the dew point front, and the process gas can be cooled to a predetermined temperature by means of the process gas cooler which is arranged downstream. The line in a pressure-enthalpy diagram for the process gas which marks the thermodynamic states of the process gas during which the moisture precipitates in the process gas, is to be understood by dew point front. 
     According to the invention, the process gas bundles are accommodated in separate process gas cooler jackets so that the process gas coolers are thermodynamically decoupled from each other. As a result, each process gas cooler can be advantageously individually designed with regard to its choice of material and its geometry, especially taking into account a diameter of the process gas cooler jackets which is as small as possible. A lower production cost and a reduced material consumption for the process gas coolers result from this. 
     If, for example, the process gas is carbon dioxide, then as the moist process gas it is chemically aggressive, as a result of which the materials for the process gas cooler bundles and for the process gas cooler jackets are to be selected as being corrosion-resistant. In particular, stainless steel could come into consideration as corrosion-resistant material. However, a construction of the process gas coolers with stainless steel leads to increased production costs so that the separation of the process gas coolers according to the invention is especially advantageous. Furthermore, different materials can be used for the individual process gas jackets and the individual process gas coolers and are optimally selected with regard to corrosion-resistance, strength, thermal conductivity and costs. 
     In addition, the process gas coolers can be individually designed in such a way that an optimized flow distribution can be established in the process gas cooler bundles, wherein the narrowest cross section in the process gas cooler jackets is large. As a result, pressure losses in the process gas coolers are advantageously reduced. 
     The process gas cooler which is arranged downstream is preferably equipped for removing the heat of condensation of the water which precipitates from the process gas and for separating out this water. In addition, it is preferred that the thermodynamic state of the process gas between the process gas coolers is located just ahead of the dew point front. The process gas coolers, by their process gas cooler jackets, are preferably interconnected by two transfer pipes for the parallel conducting of process gas from the process gas cooler which is arranged upstream on the process gas side to the process gas cooler which is arranged downstream. In this case, at least one of the transfer pipes is preferably equipped with a compensator. 
     At least one of the process gas cooler bundles is preferably arranged eccentrically in its process gas cooler jacket. In addition, it is preferred that at least one of the process gas cooler bundles is of a square-shaped construction and the process gas cooler jacket is of a hollow-cylindrical construction, and that the process gas cooler bundle is arranged in a tilted manner around the longitudinal axis of the process gas cooler jacket for the process gas inflow and/or for the process gas outflow in the process gas cooler jacket. As a result, an enlargement of the inlet cross section and of the outlet cross section of the process gas cooler in question is advantageously achieved, as a result of which pressure loss on the process side in the process gas cooler is reduced. 
     The process gas plant according to the invention for carbon dioxide gas separation having the compressor system, has a first cooling medium circuit which is equipped for operating the process gas cooler which is arranged upstream on the process gas side, and a second cooling medium circuit which is equipped for operating the process gas cooler which is arranged downstream, wherein the process gas is moist carbon dioxide and the first cooling-medium circuit can be used for re-feeding heat into the process gas plant. 
     The process gas, which discharges from the compressor stage, is cooled by the process gas cooler which is arranged upstream on the process gas side. Due to the fact that this process gas has achieved its maximum temperature directly after discharging from the compressor stage, the first cooling-medium circuit can advantageously be operated at a high temperature level. As a result, the re-feeding of heat can also take place at a high temperature level, as a result of which the re-feeding of heat is efficient. The re-feeding of heat can be used for heating a consumer water circuit, for example. 
     Cooling water is preferably the cooling medium. In this case, it is preferred that in the first cooling water circuit, in the inflow to the process gas cooler which is arranged upstream on the process gas side, the temperature of the cooling water is 40° C., and in the outflow from the process gas cooler which is arranged upstream on the process gas side, the temperature of the cooling water is from 120° C. to 160° C., wherein the temperature of the process gas at the process gas inlet of the process gas cooler which is arranged upstream on the process gas side is between 140° C. and 175° C. In addition, in the second cooling water circuit, in the inflow to the process gas cooler which is arranged downstream on the process gas side, the temperature of the cooling water is 24° C. and in the outflow from the process gas cooler which is arranged downstream on the process gas side the temperature of the cooling water is 32° C., wherein the temperature of the process gas at the process gas outlet of the process gas cooler which is arranged downstream on the process gas side is 34° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following text, a preferred embodiment of a compressor system according to the invention and a preferred embodiment of a process gas cooler unit according to the invention are explained with reference to the attached schematic drawings. In the drawings: 
         FIG. 1  shows a schematic view of the embodiment of the compressor system, 
         FIG. 2  shows a perspective view of the embodiment of the process gas cooler and 
         FIG. 3  shows a cross-sectional view of the embodiment of the process gas cooler from  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     As is evident from  FIG. 1 , a compressor system  1  has a compressor  2  which is provided for compressing process gas in a process gas plant, wherein the process gas is moist carbon dioxide. The process gas enters the compressor  2  via a compressor inlet  3 , is subjected to compression and, in a compressed state, discharges from the compressor  2  at a compressor exit  4 . 
     The compressor  2  is constructed as a multistage compressor and has a first up to a sixth compressor stage  5  to  10 . For intercooling, a first process gas cooler unit  11  is provided between the second compressor stage  6  and the third compressor stage  7 , a second process gas cooler unit  12  is provided between the fourth compressor stage  8  and the fifth compressor stage  9 , and a third process gas cooler unit  13  is provided downstream of the sixth compressor stage  10  and upstream of the compressor exit  4 . Therefore, the corresponding process gas cooler unit  11  and  12  and  13  is provided after two compressor stages  5 ,  6  and  7 ,  8  and  9 ,  10  respectively. 
     The process gas cooler units  11 ,  12 ,  13  are formed in each case from two process gas coolers  14  to  19  which are exposed to throughflow by the process gas one after the other. The process gas coolers  14  to  19  have in each case an individual process gas cooler jacket  34 , which is exposed to admission of the process gas, and a process gas cooler bundle  35  which is accommodated therein and exposed to admission of cooling water. The process gas cooler bundles  35  of the process gas coolers  14 ,  16 ,  18  which are arranged upstream on the process gas side are integrated in a first cooling water circuit  28  and the process gas cooler bundles  35  of the process gas coolers  15 ,  17 ,  19  which are arranged downstream on the process gas side are integrated in a second cooling water circuit  31 . The first cooling water circuit  28  is formed from an outflow line  29 , by which cooling water from the process gas cooler bundles  35  of the process gas coolers  14 ,  16 ,  18  is discharged, and an inflow line  30 , with which cooling water is directed to the process gas cooler bundles  35  of the process gas coolers  14 ,  16 ,  18 . The second cooling water circuit  31  is formed from an outflow line  32 , from which cooling water from the process gas cooler bundles  35  of the process gas coolers  15 ,  17 ,  19  is discharged, and an inflow line  33 , with which cooling water is directed to the process gas cooler bundles  35  of the process gas coolers  15 ,  17 ,  19 . The temperature level of the cooling water in the first cooling water circuit  28  is higher than the temperature level of the cooling water in the second cooling water circuit  31 , wherein the temperature of the cooling water in the inflow line  30  of the first cooling water circuit  29  is 40° C. and the temperature of the cooling water in the inflow line  33  of the second cooling water circuit  31  is 24° C. 
     During operation of the compressor system  1 , the process gas at the exit of the second compressor stage  6  and therefore at the inlet  20  of the first process gas cooler  14  of the first process gas cooler unit  11  has a temperature of 175° C., at the exit of the fourth compressor stage  8  and therefore at the inlet  23  of the first process gas cooler  16  of the second process gas cooler unit  12  has a temperature of 149° C., and at the exit of the sixth compressor stage  10  and therefore at the inlet  26  of the first process gas cooler  18  of the third process gas cooler unit  13  has a temperature of 140° C. The first process gas cooler  14  of the first process gas cooler unit  11 , just as the first process gas cooler  16  of the second process gas cooler unit  12  and the first process gas cooler  18  of the third process gas cooler unit  13 , are designed in such a way that a heat flow is removed from the process gas, as a result of which the thermodynamic state of the process gas between  21  the process gas coolers  14 ,  15 , just as between  24  the process gas coolers  16 ,  17  and between  27  the process gas coolers  18 ,  19 , is located in the region of the dew point front. The process gas cooler  15  of the first process gas cooler unit  11 , just as the second process gas cooler  17  of the second process gas cooler unit  12  and the second process gas cooler  19  of the third process gas cooler unit  13 , cools the process gas to 34° C. In this case, the cooling water in the first cooling water circuit  28 , in the outflow line  29 , is heated to 120° C. to 140° C. and in the second cooling water circuit  31 , in the outflow line  32 , is heated to 32° C. 
     In  FIGS. 2 and 3 , the process gas cooler unit  12  is shown representatively for the process gas cooler units  12 ,  13  and  14 . 
     The process gas cooler bundle  35  is of a square-shaped design and arranged in the hollow-cylindrical process gas cooler jacket  34 . The longitudinal center axis of the process gas cooler jacket  34  is arranged in a parallel offset manner from the longitudinal center axis of the process gas cooler bundle  35  so that the process gas cooler bundle  35  is arranged eccentrically in the process gas cooler jacket  34 . In  FIGS. 2 and 3 , the process gas cooler jacket  34  is arranged in a horizontally disposed manner, wherein the process gas cooler bundle  35  is arranged in a tilted manner around the longitudinal axis of the process gas cooler jacket  34  and is therefore tilted in relation to the horizontal. The process gas cooler jacket  34  of the first process gas cooler  14  and the process gas cooler jacket  34  of the second process gas cooler  15  are formed with two transfer pipes  36  which foam the process gas-side transfer point  21  from the first process gas cooler  14  to the second process gas cooler  15 . The transfer pipe  36  is arranged horizontally and the inlet  23  and the outlet  25  are arranged in a vertically extending manner. As a result, the process gas cooler bundles  35  are arranged in a tilted manner around the longitudinal axis of the process gas cooler jacket  34  for the process gas flow through the inlet  23 , the transfer pipe  36  and the outlet  25 .