Heat storage systems for use in an industrial batch process

Transcription

1 ECN-RX Heat storage systems for use in an industrial batch process (Results of) A case study R. de Boer S.F. Smeding P.W. Bach Contribution to The Tenth International Conference on Thermal Energy Storage, ECOSTOCK, May 31 - June 2, 2006, New Jersey, USA A B 130 June 2006, draft version 17 July~2l~[lal version Revisions _ ~.~_ k~- ~ / ~./~1 ~ c_~,_, R. de Boer?~t "/ ~ P.W. Bach C hecke~by~ Issued by: ~~ ECN Energy Efficiency in Industry Industrial Heat Technology JUNE 2006

2 Acknowledgement The Dutch agency SenterNovem (sustainable development and innovation) is acknowledged for financially supporting the present study. Ing. Mark Bramer, manager operations of the company Dr. W. Kolb Nederland BV is acknowledged for providing process data and for his kind cooperation in the study. Abstract Thermal energy storage can be an attractive technology to enable re-use of waste-heat, especially for batch processes. A case study was carried out to evaluate the technical and economical feasibility of an industrial heat storage system. The study focused on integration of a heat storage system within an existing production facility of organic surfactants. Three different thermal storage systems with operating temperatures between 110 C to 160 C were designed to store the heat released during the exothermic reaction phase and re-use the heat for preheating of the reactants in the following batch. The first system uses a Phase Change Material (PCM) contained in metal balls with an assumed phase change temperature at 140 C. The second system uses a concrete volume as a sensible heat storage material and the third system is also based on concrete, but with a doubling of the storage capacity. A dynamic simulation was performed of a reference cycle of a batch reactor coupled with a thermal storage system to calculate energy savings for preheating of the reactants. It was required to oversize the storage capacity of the PCM system, in order to obtain a heat transfer rate that matches with the conditions of the actual process. The calculated energy savings for heating of the batch reactors is 50 to 70%, resulting in financial savings of 26 to 38 k on an annual basis. The total capital investment of the storage systems is estimated at 440 to 540 k. Simple pay out time is higher than 10 years, with the best result for the concrete heat storage. The bare cost of the thermal buffer is 15% to 30% of the total capital investment. Because the cost of integration of the storage system in an existing facility are a large part of the total cost, it is recommended to evaluate the use of thermal storage systems for grass-roots situations. 2 ECN-RX

3 Contents List of tables 4 List of figures 4 1. Background 5 2. Design of a thermal storage system with a batch process 6 3. Calculation of the energy savings 8 4. Techno economic evaluation Discussion and conclusions Nomenclature 13 References 14 ECN-RX

4 List of tables Table 2.1 General process sequence for the alkoxylation reaction 6 Table 3.1 General description of three storage systems designs 9 Table 3.2 Results of the calculations of energy input and output of a single batch production with different thermal storage options 9 Table 4.1 Overview of calculated annual natural gas usage and financial savings for thermal storage options 10 List of figures Figure 1.1 Energy balance of the Dutch industry, from energy carrier to end use, [Spoelstra, 2005] 5 Figure 2.1 Birds-eye view of the production facility of Dr. W. Kolb Nederland BV 6 Figure 2.2 Process scheme for a thermal storage system connected to the two existing batch reactors 7 Figure 3.1 Basic representation of the existing batch reactor and external heat exchanger configuration 8 Figure 3.2 Modified process scheme for integration of a thermal storage system with the batch reactor 8 4 ECN-RX

5 1. Background More than 80% of the total energy use in the Dutch industry involves the need for heat, either in fired furnaces or in the form of steam at different pressure levels, see Figure 1.1. Most of this heat is eventually released to the ambient atmosphere through cooling water, cooling towers, flue gasses, and other heat losses. We call this heat loss Industrial waste heat. Large energy savings are possible, if this waste heat could be reused. The total industrial waste heat in the Netherlands is estimated at more than 250 PJ per year. Heat integration (pinch technology) forms the first solution to reduce waste heat. This has already been applied to a great extent in the Dutch industry. Thermal energy storage can also be an attractive technology to enable re-use of waste-heat, especially for batch processes. Several studies have estimated the energy savings potential in batch processes in the Dutch industry [Herder et al.1999, Kiesewetter 2001, Steen en Kee 1997, Beer et al.1994, Ashton 1993]. The resulting energy savings potential varies from 2 to 5 PJ/year. The technologies available for heat storage are mainly based on sensible storage. Phase Change Materials for latent heat storage are commercially available up to 117 C (MgCl 2 6H 2 O). Materials for latent heat storage at higher temperatures are still in development. Technologies for storage of heat from concentrated solar power plants could also offer interesting opportunities for application in an industrial environment. Several large scale and high temperature (>250 C) heat storage systems are under development or are already applied in combination with a solar power plant. The storage of heat in solar systems strongly increases the effective use of the solar collecting system by enabling a time shift between harvesting of solar thermal power and production of electric power. For industrial application of thermal energy storage, the benefits should come from a significant reduction of primary energy input for heat production and thus reduction of energy costs. A case study was carried out to evaluate the technical and economical feasibility of an industrial heat storage system for temperatures above 100 C. Electricity 120 PJ Coal 83 PJ Conversion loss 98 PJ Electricity 124 PJ Oil 524 PJ Industrial energy use 1227 PJ Heat 531 PJ < 100ºC 63 PJ ºC 91 PJ ºC 111 PJ ºC 69 PJ ºC 92 PJ > 1000ºC 105 PJ Final energetic energy us 655 PJ Natural gas 432 PJ Feedstock 474 PJ Miscellaneous 69 PJ Figure 1.1 Energy balance of the Dutch industry, from energy carrier to end use, [Spoelstra, 2005] ECN-RX

6 2. Design of a thermal storage system with a batch process The Dutch chemical company Dr. W. Kolb BV was willing to make their process conditions available for the evaluation of a high temperature storage unit connected to their batch process. This company produces non-ionic tensides by alkoxylation of fatty alcohols and acids, for use in detergents and cosmetics. The production facility was constructed in 1992 and put into operation one year later, see Figure 2.1. It produces tons of products using a batch-wise process. A gas-fired steam boiler provides the site with the required steam to operate the batch reactors and to keep storage of the raw materials at the right temperature. The annual use of natural gas is about 1 million m 3. The alkoxylation reaction is strongly exothermic. The reacting mixture is kept at a maximum temperature of about 180 C. A cooling water circuit and cooling towers remove the major part of this heat to the environment. Figure 2.1 Birds-eye view of the production facility of Dr. W. Kolb Nederland BV The production facility has two batch reactors, which operate independent of each other. The production facility produces many different non-ionic tensides and each product requires a specific duration of the reaction phase. Variations in the amount of reactants and products per batch are also very common. An average process condition was selected, based on information on the number of batches produced and the total weight of products during a year for the evaluation of a thermal storage system. The general process sequence of a batch is described in Table 2.1. The duration of this process sequence can vary from three to twelve hours. Table 2.1 Step General process sequence for the alkoxylation reaction Utility 1 Fill batch reactor with long chain hydrocarbon reactants 2 Preheating of reactants to 140 C Steam 8 bar 3 Addition of first alkoxy reactant and further heating to 160 C (exothermic) 4 Further addition of alkoxy reactant and reaction at max.180 C Cooling water 65 C 5 Post reaction phase (no further addition of alkoxy) Cooling water 65 C 6 Cool down to 120 C Cooling water 30 C 7 Pump down of products The batch reactors consist of a pressure vessel with an external loop to pump the reactant and products through a heat exchanger in order to add or remove heat. Steam is used for heating the reactants, and cooling water takes away the heat. Cooling water is present in two reservoirs, one at 65 C to control the temperature during the reaction phase and one at 30 C to cool the 6 ECN-RX

7 products at the end of the process. Reactants (long chain hydrocarbons) are preheated using steam at 8 bar from the steam boiler. The energy required for preheating of the reactants of an average batch is about 25 % of the energy released from the batch during the exothermic reaction, 2.6 GJ and 11 GJ, respectively. In addition, the temperature during reaction (180 C) is sufficient for preheating purposes at maximum 140 C. The purpose of the thermal storage system is thus to collect the exothermic heat of the reaction of one batch and to provide this heat again for preheating of the reactants in the next batch. In order to achieve this, a new process scheme was developed to integrate a thermal storage system with both batch reactors. This scheme is shown in Figure 2.2. Heat exchangers TSA1 and TSA2 are integrated in the existing piping to collect heat from or to add heat to the reactants and products. A pressurized water circuit (10 barg) is considered for heat transfer between reactors and the thermal storage buffer. Piping, pumps and valves are arranged such that the storage can be charged from 'top to bottom' and discharged from bottom to top. TSA3 and TSA4 are optional components to enable heat transfer between the storage system and the existing steam circuit. With this scheme the following options for heat management are intended: Storing heat from reactor 1 to buffer Storing heat from reactor 2 to buffer Extracting heat from buffer to reactor 1 Extracting heat from buffer to reactor 2 Heat transfer directly from reactor 1 to reactor 2 and vice versa (bypassing the buffer) The existing system for heating with steam and cooling with water of either 65 C or 30 C is maintained in the new scheme for additional heating and cooling supply for those situations where the storage system cannot provide sufficient thermal power for heating or cooling. The chemical company specified to maintain the heat-up and cool-down times as short as possible, at least as short as given in the general process sequence. With the suggested process scheme this can always be fulfilled. Steam 180 C Water 30/65 C Reactor vessel 1 Reactor vessel 2 Storage Figure 2.2 Process scheme for a thermal storage system connected to the two existing batch reactors ECN-RX

8 3. Calculation of the energy savings A thermal model was set-up to simulate the process conditions during a batch reaction and to derive the required utilities (steam, hot water and cooling water), in order to achieve the general process conditions. The scheme of Figure 3.1 is used for the calculation of the base case scenario. The heat exchanger can receive steam, and cooling water at 65 or 30 C. batch reactor heat exchanger WW E101 Figure 3.1 Basic representation of the existing batch reactor and external heat exchanger configuration For the cases with a thermal storage system the basic scheme is modified with another heat exchanger (of equal performance) and a thermal storage tank, see Figure 3.2. For the thermal storage tank three alternatives were considered. The first one consists of a tank filled with a high temperature PCM. The properties of this PCM are considered to be the same as that of a PCM with a melting temperature of 164 C, which is being developed (and sold) by the company EPS ltd. located in the UK. The PCM is contained in sealed stainless steel balls of 10 cm diameter. Complete melting and solidification of the PCM in these balls takes 170 and 130 minutes respectively at a temperature difference of 10 C.For the present case however the melting temperature was assumed to be 140 C, for a better match with the process temperatures. The number of balls, and thus the storage capacity, was chosen such that the total heat transfer matched reasonably well with the process requirements. The second buffer consists of a 20 ft transport container filled with concrete and a bundle of 1800 tubes inside the concrete mass to transfer heat. The third alternative is a 45 ft transport container also filled with concrete and 1800 tubes for heat transfer. Details of the storage systems are given below in Table 3.1. batch reactor HEX E101 HEX E201 Thermal storage Figure 3.2 Modified process scheme for integration of a thermal storage system with the batch reactor 8 ECN-RX

9 Table 3.1 General description of three storage systems designs Storage system unit 1 Phase Change Material (140 C) 2 Concrete mass 3 Concrete mass Buffer Volume (net) m Heat transfer fluid m Storage medium m Storage capacity MJ 4600 ( T=20 K) 611 ( T=20 K) 1440 ( T=20 K) Heat transfer rate kw/k The model calculations were done assuming one reactor connected to the storage system. An average of three and a half hour for the process sequence is used in the calculations. The results of these single reactor calculations are extrapolated to two reactors, both connected to the same thermal storage. It means that in this idealized situation the exothermic heat from the reactor 1 is used for preheating reactor 2, and that the exothermic heat from reactor 2 is again used for preheating reactor 1. In practice this would be very difficult to achieve because the durations of the batches vary strongly. On the other hand, situations where exothermic heat from one of the reactors could directly be used to preheat the other are also not considered in the calculation of the energy savings. Dynamic calculations were done with a fixed time step, during which the temperature of the storage system, the reactor content, and the resulting thermal power transferred from or to the storage system are determined. Whenever the thermal power of the storage is insufficient to maintain the required heating and cooling rates, a switch to the existing steam or the cooling water circuit was made. The results of the calculations of the utility use and storage use for a single batch production with the different storage options are summarized in Table 4.1. Table 3.2 Results of the calculations of energy input and output of a single batch production with different thermal storage options GJ/batch Base case 1 PCM storage 2 Concrete storage 3 Concrete storage Steam input Cooling water 65 C Cooling water 30 C Thermal storage: heat input heat output Savings: steam cooling water 65 C The large concrete storage, option 3, gives the highest energy savings in the present case. The storage capacity and heat transfer of option 3 reasonably match with the process requirements, resulting in he highest savings on steam. The smaller concrete storage, option 2, gives lower savings due to the limited storage capacity. For the PCM storage the limiting factor is the heat transfer rate. The process requires much faster heat transfer rates than the melting and solidification of the PCM can deliver, being contained in 10 cm steel balls. Less than 40% of the PCM material is effectively used as latent storage, the rest is just used for sensible storage. However, a reduction of the PCM storage capacity would also reduce the overall heat transfer rate of the PCM system, and therefore not lead to a more effective use of the PCM. To achieve a better match of the PCM storage with the batch process of the present case, different configurations of the PCM are required that have smaller heat transfer distances, e.g. smaller steel balls that contain the PCM. ECN-RX

10 4. Techno economic evaluation The total capital cost for the realization of the storage systems were calculated. The resulting estimated cost for the equipment and installation on site are: option 1: PCM storage 535 k option 2: 20 ft concrete storage 440 k option 3: 45 ft concrete storage 480 k The optional heat exchangers TSA3 and TSA4 (Figure 2.2) are not included in this cost calculation. The annual cost savings for steam supply (natural gas) are calculated from the savings per batch and shown in Table 4.1. Although energy cost savings as high as 70% are achieved with the large concrete storage system, the simple pay out time in the present case ranges from just a little less than 13 years up to 18 years for the PCM storage. Table 4.1 Overview of calculated annual natural gas usage and financial savings for thermal storage options Annual basis Base case PCM storage 20 ft container concrete 45 ft container concrete Steam consumption [TJ] Gas consumption for reactor [m 3 ] 213,200 93, ,100 62,400 Cost of gas for reactor [ ] 1 53,300 23,300 26,800 15,600 Savings of gas cost for reactor [ ] 30,000 26,500 37,700 Savings 56% 50% 70% Pay out time [year] Gas price 0,25 /m 3 (2005) 10 ECN-RX

11 5. Discussion and conclusions The equipment cost for the bare storage tank/container filled with concrete or PCM is in the range between 15 to 30% of the total cost of installation of the thermal storage system. Other important cost contributions are from heat exchangers, pumps and the installation of the piping. For the specific case of coupling a heat storage system with an existing production facility, the existing systems for steam and cooling water are already available as backup sources. On the other hand, connecting a heat storage to this existing facility puts additional technical constraints that contribute to an increase in installation cost. It is therefore recommended to evaluate the potential benefits of a thermal storage system combined with an industrial batch process also for a grass-roots situation. Only the direct savings on natural gas consumption for steam production were considered in the calculation of the pay out time. The savings on cooling water and electrical consumption for the fans of the cooling towers were not taken into consideration. These latter savings can contribute to a reduction of the pay out time to a marginal extent, and where therefore not worked out in this evaluation. However, these additional savings should be taken into account in a more detailed evaluation. The present case has shown that heat storage can be very effective in reducing the steam demand and thus the primary energy demand in batch processes. Energy savings up to 70% can be achieved without any modification of the process conditions. The possibility for heat storage was evident in the present case from the point of view of an excess of exothermic heat of reaction at a temperature level well above the level of heat demand. Heat integration between batches or heat recovery from a hot batch output to the next cold batch feed is not applied for reasons of process control. The application of a thermal storage system enables a much better heat integration of the on-site processes, without making the processes directly dependent from each other. For such situations a thermal storage system could also enable energy savings in continuous processes, where heat integration is not applied for reasons of independent process control. Beneficial for achieving short pay out times for a thermal storage system is to have it connected to a process where it is required to have many charge - discharge cycles during a day. A doubling of the amount of charge - discharge cycles with equal storage capacity will lead to doubling of the primary energy cost savings. However, short charge-discharge cycles necessitate high rates of thermal energy uptake and release. Especially for the PCM option this needs extra attention in designing the storage in such a way that the heat transfer associated with melting and solidification of the PCM matches with the required heat transfer rates. In the present case, steel balls of 10 cm diameter filled with PCM were evaluated. However the storage capacity and the heat transfer rate of these balls did not match very well with the process requirements. A much smaller storage capacity could be used, so less PCM, if the heat transfer was higher. Structures like a flat plate to contain PCM could be a better alternative for this application. The sensible heat storage system based on concrete mass has still some freedom in designing the stacking distance of the heat transfer tubes in order to achieve a better matching of the storage capacity and the heat transfer rate with the process requirements. In this evaluation of thermal storage systems for a high temperature industrial application, storage technologies were considered that are not (yet) commercially available. Some field-test systems are in development for storage of sensible heat in concrete mass but tests are still ongoing. For PCM systems the availability of materials with phase change temperatures in the ECN-RX

12 range between 120 to 200 C is very limited. Further materials research and development on new PCM's is necessary to obtain materials with melting temperatures in the desired range of 120 to 200 C. 12 ECN-RX

13 6. Nomenclature PJ = Petajoule = 1*10 15 J PCM= Phase Change Material k = kilo-euro = 1000 Euro ECN-RX

14 References Ashton, G. (1993): Design of energy efficient batch processes. In: P.A.Pilavachi (ed.), Energy Efficiency in Process Technology, pp , Elsevier Applied Science. Beer, J.G. de, M.T. van Wees, E. Worrell & K. Blok (1994): ICARUS-3, The potential of energy efficiency imporvement in the Netherlands up to 2000 and , NW&S. Herder, P., H. Roeterink & Z. Verwater-Lukszo (1999): Inventory of batch producing industry in the Netherlands, Delft University, Interduct. Kiesewetter, J. (2001): Restwarmte uit batchprocessen. (in Dutch) GR 01/3, ECN. Spoelstra, S. (2005): Nederlandse en Industriële Energiehuishouding. ECN-I , (in Dutch). Steen, D. & R.J.M. Kee (1997): Energiebesparing door Korte Termijn Opslag van Warmte/Koude. NOVEM(in Dutch). 14 ECN-RX