Patent Publication Number: US-2016231025-A1

Title: Controlled heating method of a process fluid through concentrating solar thermal plant and heat carrier system and apparatus thereof

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of concentrating solar thermal plants and their application in the traditional refining and petrochemical sector for heating service purpose. Heating services in refining and petrochemical sectors, are those normally realized by means of fired heaters with fuel consumption and combustion products released to the atmosphere. Refinery fluids like petroleum, or its fractions or derivatives, or petrochemical process fluids that need to be heated up to a controlled process temperature may be heated up by means of solar heat absorbed and stored through a concentrating solar thermal plant (preferably the type based on parabolic trough concentrators and receivers and molten salts circulation and storage) with a considerable fuel saving and emission reduction (CO2, SOx, NOx, dusts and unburned compounds). Main advantage of solar plants application is to reduce the overall use of fossil fuels which should be, a part from a mere economic saving, one target of the sustainable development of industrial countries, now suffering from a too heavy environmental impact of industrial activities and trying to transform their economy toward a larger use of renewable energy. Introducing solar energy application in conventional industrial sectors could be strategic for reaching an economy of scale and therefore allowing a wider and faster spread of thermal solar system. 
     The use of solar energy in place of fossil fuels has a double benefit on global warming, first deriving from direct reduction of fossil fuels consumption and related greenhouse gases emissions, second by reduction of the solar energy reflected by earth in the infrared, being this last one of the main contribute to greenhouse effect. 
     BACKGROUND OF THE INVENTION 
     Concentrating thermal solar plant utilizes a “radiation concentrator collector” which concentrates the solar radiation by focusing it into a smaller area, using mirrored surfaces. In this system, a reflector, which is typically parabolic, receives and reflects (focuses) incoming solar radiation into a radiation absorber, which is formed as a tube. The tube radiation absorber is conventionally surrounded by a treated glass enclosure tube to limit the loss of heat and the space between the tube radiation absorber and the glass enclosure tube is under vacuum. 
     The tube radiation absorber is made of metal with a coating having a high solar radiation absorption coefficient to maximize the energy transfer from the solar radiation reflecting off the reflector. A thermal fluid constituting a heat transport medium (either a diathermic oil or molten salts; molten salts mixture is preferred in this case due to the higher temperature that can be reached by the thermal fluid in the solar field) flows within the tube radiation absorber and can reach temperatures up to 580° C. with conventional molten salts mixture based on Na and K Nitrates. 
     The thermal energy is then transported by the thermal fluid toward end users (most commonly steam generators and super-heaters for electric power generation) and is in part stored in a storage tanks system in order to extend the operation time. The storage system consists of a minimum of two storage tanks, a cold fluid storage and a hot one. The solar heat absorbed and stored in the CSP system can be than carried in different ways, have different use, and can be eventually integrated with a firing contribute to compensate the solar system variations before distribution to selected users. 
     Main applications of concentrating solar system already realized are in the field of electric power generation by using hot oil which in turn is used for generating high pressure steam generation and superheating which is then expanded in a steam turbine. More recently the use of molten salts was introduced as a way to raise the Rankine cycle efficiency (cit. Patent Application No. EP12167509.4 filed on May 10, 2012) and reduce the cost of thermal energy storage (TES). 
     Recent pilot projects&#39; applications funded by EC include the Mats project where power, sea water and cold are produced and Comethy project where hydrogen is produced by a low temperature steam reforming process heated by molten salts and integrated with membrane separation (cit. Patent Application No. EP12159998.9 filed on Mar. 16, 2012). 
     Concentrating solar system have at present a high investment cost but can significantly reduce the operating cost in those application where heat required is generated by firing a fuel stream, as it is in conventional heating services of refineries and petrochemical operations. 
     As a background to present invention there are also known applications of Heat Recovery Units (HRU) for heating refinery fluids, based on use of Gas 
     Turbines exhaust streams. In this case the exhaust stream is fully discharged to stack after heat recovery, at a temperature which depends on the heating service temperature level, usually in the range of 150° C.-300° C., with very large heat dispersion. 
     With the present invention this is avoided, because a large part of the heat carrier stream downstream heating service is recycled back to the heat carrier circuit, while only a part of the total stream, corresponding to the inlet fuel for post-firing and fresh combustion air, is sent to stack. 
     SUMMARY OF THE INVENTION 
     The present invention refers to one specific application of concentrating solar thermal plant, based on molten salt circulation and storage, where the solar heat absorbed by collectors and receivers and stored in the storage system is transferred to an air stream used as a heat carrier, it is eventually integrated/substituted with post-firing under continuous duty control function, and it is finally conveyed to a heat exchanger where it is transferred to a process fluid. Any heating service for refinery fluids, petrochemical process fluids, demineralized/boiler feed water for steam generation and steam superheating actually realized through fired heaters is suitable for the application of present invention. 
     One general problem of concentrating thermal solar plant is that of having a variable content of heat available, due to variability of sun radiation along with day time, weather condition, season and geography. If solar heat is to be used for a heating service under temperature controlled conditions, it has to be integrated with a variable heat producing system that is capable to provide the supplementary heat required in a controlled and viable manner. 
     Object of present invention is the architecture and method of supplying solar heat to a refinery or process fluid by means of an intermediate heat carrier circuit having the capacity of integrate the variable heat required by process heating service through post-firing. To do this, an air stream, used as a heat carrier between molten salts and final user, is heated up by hot molten salts circulating from solar plant up to a temperature in the range between 480° C. and 550° C., typically 500° C. during the ‘active phase’ (phase 1) when direct sun radiation is heating molten salts and during the ‘storage phase’ (phase 2) when the hot molten salts are taken from the hot storage tank and then discharged in the cold storage tank. During the ‘inactive phase’ (phase 3) of the solar field the heat carrier air stream is instead heated up through post-firing, realized by means of duct burners, firing a suitable fuel (refinery gas, natural gas, bio-fuel or any mixture of them). 
     Post-firing is active along all three phases, although only a minimum firing contribute, with the aim of controlling the process fluid temperature, shall be used during phase 1 and 2, while during phase 3 it will provide the full heat duty required by process heating service. The necessary post-firing contribute in any phase shall be determined by an automatic control system the main function of which is to control the fluid service temperature and therefore to adapt the firing contribute during the different operating phases. 
     The hot air/flue gas stream is then sent to heat exchange with the process fluid and it is recycled back in the circuit to heat exchange with molten salts and controlled post-firing. 
     Together with firing contribute, the automatic control system determines also the fresh air stream required by fuel combustion and the purge stream, equal in mass to inlet fuel and fresh air stream, to be sent to stack to avoid the build-up of combustion products. During the inactive phase of the solar field the control system will also allow deviation, through an appropriate control valve, of a secondary hot air stream to be sent to heat exchange with cold molten salt, to eventually avoid molten salts cooling below a safe temperature (safely above condensation temperature). 
     The here described architecture and method for heating a process fluid up to a required temperature is suitable for substituting any fluid heating service up to a temperature of 550° C. or slightly above. 
     The advantage of the proposed innovation is a meaningful reduction of the fuel consumption (in the order of 65%) and greenhouse gas emission compared to conventional heating systems. 
     These and various other characteristics and advantages of the present invention will be readly apparent to these skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of the controlled heating system of a process fluid through concentrating solar thermal plant based on molten salts circulation and storage according to the present invention. Main Items:  10  Concentrating solar absorption field  11  Hot molten salts storage tank  12  Cold molten salts storage tank  20  heat exchanger (molten salts to heat carrier and vice-versa)  30  post-firing duct burners  40  heat exchanger (heat carrier to process fluid)  50  heat carrier circuit fan  60  fresh air inlet fan  70  stack  80  automatic control system  81  inlet fluid temperature measure  82  fluid flow-rate measure  83  outlet fluid temperature measure  84  fuel flow-rate control loop  85  fresh air flow-rate control loop  86  purge stream flow-rate control loop  87  cold molten salts temperature  88  heat carrier secondary stream valve Fluids:  100 ÷ 105  molten salts  200  fresh air inlet  201 ÷ 210  heat carrier fluid (air/flue gas)  300  fuel to post-firing  400 ÷ 401  process fluid to be heated up 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated schematically in  FIG. 1  main components of the full architecture of present invention consist of a concentrating solar thermal plant  10 , a heat exchanger  20  between molten salts and heat carrier and a heat exchanger  40  between heat carrier and process fluid. 
     The concentrating solar field  10  and storage system  11  and  12  based on molten salt circulation and storage is designed for absorbing and storing the heat necessary for the active and storage phase (about ⅔ of the continuous operation time of the required process heating). 
     The solar collection system  10  comprises one or more tube radiation absorbers and a plurality of through collectors, such a single axis parabolic reflectors. 
     Alternatively, any suitable means for concentrating solar radiation, such as Fresnel collectors, may be provided. The thermal fluid is heated within the receiver tubes upon exposure to direct and concentrated solar radiation. 
     The hot thermal fluid  100  coming from solar field is stored in the hot storage tank  11  and then circulated by means of suitable pumps (not shown) to provide a heat source to the intermediate heat carrier stream  201  through a heat exchanger  20 . The heat exchanger  20  transfers heat from molten salts to the heat carrier fluid during phase 1 and 2 (active and storage) while it is suitable also for transferring heat back to molten salts during phase 3 (inactive solar) to avoid molten salts cooling down below safe temperature (safely above condensation temperature). 
     The post-firing  30 , realized by means of duct burners located inside the air duct, is capable of firing any capacity from 0% to 100%. 
     Heat release is calculated in continuous by control system  80  on the base of heat required by the process fluid  400  to reach its final controlled temperature  83 . The firing contribute LHV is delivered as a flue gas stream  300  under flow control  84  in the heat carrier stream  202  for the subsequent delivery of heat carrier stream  203  to heat exchanger  40  with process fluid  400 . 
     The heat exchanger  40  is capable to transfer heat from the heat carrier  203  to the process fluid  400  to reach the final temperature through temperature control  83 . 
     The heat carrier circuit fan  50  is capable to provide the pressure head necessary to the heat carrier fluid necessary to win the circuit pressure drops. 
     The fresh air fan  60  is capable to admit the required fresh air flow  200  under automatic control by means of flow control  85 . 
     The flow control loop  86  is capable to control the purge gas stream to be sent to stack  70 . The control valve  88  permits to derive a heat carrier by-pass stream to allow temperature control of the cold molten salts circulating during CSP un-active phase.