PARAMETRIC STUDY ON LNG VAPORIZER WITH AIR, WATER AND STEAM AS HEATING MEDIA

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1 IJAAT International Journal of Advances on Automotive and Technology Promech Corp. Press, Istanbul, Turkey Vol. 1, No. 1, pp , January, Manuscript Received May 7, 2016; Accepted June 14, This paper was recommended for publication in revised form by Co-Editor Yasin Karagoz PARAMETRIC STUDY ON LNG VAPORIZER WITH AIR, WATER AND STEAM AS HEATING MEDIA *Neha Patil Department of Mechanical Engineering, Lokmanya Tilak College of Engineering, Koparkhairne. (Navi Mumbai, Maharashtra, India) Keywords: Liquefied Natural Gas (LNG), Vaporizer, Re-gasification, Kettle Reboiler, Air, Water, Steam * Corresponding author id- nehapatil9337@gmail.com ABSTRACT Natural gas, in the liquid form takes up about 1/600th the volume of its gaseous state, which provides a cost-effective means to transport natural gas from the sources of supply to the available markets. This Liquefied Natural Gas (LNG) is used in medium and heavy-duty vehicles. The liquefied natural gas is pumped out of the storage tanks and the system includes means for transferring heat from the heating fluid to the LNG so that it returns to its natural gaseous state. The natural gas is then pumped in to the natural gas transmission system, and supplied to the market. This vaporizer for LNG comprises of a Kettle reboiler for total vaporization and a Shell and Tube heat exchanger as super heater to attain the delivery pressure for natural gas. The heat transfer coefficients were maximized for the process fluids so that the shell side and tube side pressure drops are kept within permissible limits. The optimum design of LNG vaporizers is a tedious job. To overcome this, a Visual Basic program was developed using various correlations and equations, and the results obtained were validated with HTRI. Design was carried out for all the three fluids; air, water and steam. For a given heat duty of vaporizer, characteristics curves for both tube side and shell side were generated for various tube materials and tube sizes. On basis of these characteristics, the variation of tube side and shell side could be easily compared and hence the design can be finalized for particular requirement or application. INTRODUCTION Liquefied Natural Gas (LNG) is the fastest growing hydrocarbon fuel; while gas as a primary fuel source is forecast to grow at 3% in the coming two decades, LNG as a subset is forecast to grow at double that rate over the same period [2]. Conventional fuels like coal, oil and petroleum are composed of complex molecules, with a higher carbon, nitrogen and sulfur contents. When combusted, release higher ratio of carbon emissions, pollutants and ash particles into the environment. There is a long-standing interest in advancing the use of alternative fuels for conserving energy and reducing the nation s dependence on foreign oil supplies. The combustion of natural gas, on the other hand, releases very small amounts of sulfur dioxide and nitrogen oxides, virtually no ash or particulate matter, and lower levels of carbon dioxide, carbon monoxide, and other reactive hydrocarbons. Natural gas is odorless, colorless, nontoxic and noncorrosive gas, composed primarily of methane. When natural gas is cooled to C at slightly above atmospheric pressure, it liquefies and is termed as Liquefied Natural Gas. In the liquid form, natural gas takes up about 1/600 th the volume of its gaseous state, which provides a cost-effective means to transport natural gas from the sources of supply to the available markets. LNG is used in medium and heavy-duty vehicles. Although the energy density of LNG is about 60% compared to diesel the fuel, costs are much lower and LNG should give lower operating costs. LNG is usually transported overseas by a cryogenic tanker to LNG receiving terminal, where it is unloaded and stored in double-walled, vacuum-insulated pressure vessels known as, cryogenic storage tanks. With help of a heat exchanger this liquid form is converted to natural gas and sent through pipelines to utilization place of natural gas as an energy source. This re-gasification activity is significant in terms of operating costs and possible impact on environment. 19

2 LNG PROCESS LAYOUT The processing of LNG consists of exploration and production of natural gas, liquefaction process, transportation of LNG, storing of LNG and re-gasification process. Following Fig. 1 shows the block diagram of LNG process. Exploration of natural gas To market Production of natural gas Re-gasification process Liquefaction process LNG transportation Storing LNG Fig. 1: Block diagram of LNG process The required operating conditions for the LNG vaporizer are given in below Table 1. Table 1. Required Operating Condition For LNG Name of cold fluid LNG Heat Duty (MW) 10 Inlet temperature; o K 113 Outlet temperature; o K 188 Saturated temperature of LNG; o K 193 Inlet rate; kg/s 15.6 Outlet vaporization rate; kg/s 15.4 Outlet condensate rate; kg/s 0.2 Operating pressure; KPa 1000 Latent heat of vaporization; KJ/kgK 410 To obtain a 10 MW heat duty exchanger, designing of vaporizer is held out with HTRI (Heat Transfer Research Inc.) ver.6. This software provides the required design parameters by providing the operating conditions and some basic parameters like length, diameter of tube, number of tubes, etc. Few of the co-relations used in the designing procedure are mentioned in the point below. OUTPUTS Design outputs were obtained in HTRI software. The obtained outputs are listed below in Table 2. TEMA type BKU is the kettle type reboiler with U-tubes and, BKM is the kettle type reboiler with single tube pass. For air, higher diameter tube is used to maintain the velocity of air within accepted limit (i.e. ρv 2 < 8928). From above outputs, it was observed that, with air as heating medium, total area required for vaporizer to attain 10MW heat duty is very high. The arrangement of 7 shells in parallel is to be designed for vaporization process; which is not at all area wise and cost wise efficient. Since steam as heating medium requires least area, we can say that steam is the best heating medium for LNG vaporization. Table 2. Comparison Chart of Output Designs Property Water Steam Air TEMA type BKU BKU BKM Shellside fluid LNG LNG LNG Tubeside fluid water steam air Process Parameters Inlet hot fluid temp; o K Outlet hot fluid temp; o K Hot fluid inlet pr; KPa Geometrical Parameters Tube OD; mm Wall thickness; mm Length; m Shell ID; mm Kettle ID; mm Tube count per shell Shells in series Shells in parallel Tube passes Output Results Pd in tube; KPa Pd in shell; KPa U; W/m2K Production of natural req Over design; % gas Total Area; m EFFECTS OF VARIABLES ON DESIGNS Three designs of vaporizers were selected by changing the parameters like tube materials, tube diameters, and inlet pressures. Now, from the selected designs of all the fluids, it has to be decided that which of the fluid is to be used at the required conditions. Following are some of the points considered for obtaining the efficient fluid to be used. FLOW REGIME The HTRI Xchanger Suite components calculate and use the flow regime parameters and the homogeneous liquid volume fraction to estimate flow regime at every point in the vaporizer. Each point in a baffle space and a pass is averaged to plot the flow regime as the fluid moves down the length of the exchanger. The flow regimes are calculated only for two phase fluids, flow regime map display only for LNG and steam. The boiling regime for LNG with water as heating medium is somewhat in the slug and mostly in annular-slug transition region. 20

3 must not be in slug region as it causes hazard to the tube. In the case of steam as heating medium, the regime does not lie in slug region, hence the design is safe. TUBE SIDE SKIN TEMPERATURE It is the temperature of inside wall of the tube. As inside tube the temperature is ambient and outside temperature is C, there is problem of freezing of heating fluid. Hence tube side skin temperature is a major issue mostly in case of water. a) b) Fig. 3: Graphs for skin temperature inside the tube in vaporizer with different fluids Skin temperature is integral average temperature at surface between fluid and tube wall. From the above graphs shown in figure 3 we can state that, the temperature difference in the tube wall of steam as heating medium is higher compared to others. The tube is mm thick and the difference between temperatures of wall in contact with cold fluid and hot fluid is 53 0 C; which can cause stresses in the tube material. But this doesn t lead to failure hence this design is safe. AREA Area is the important factor on which the fluid to be used is decided. This area is the space required for erection of whole plant of regasification. Fig. 2: Flow regime on shell side for a) water, b) steam, c) air as heating medium c) With steam as heating medium, boiling regime of LNG is annular-slug transition and annular region; and with air, it is mostly in the slug and somewhat in annular-slug transition region. Flow condensation regime on tubeside is displayed only for steam which is two phase fluid. The regime is in mist, annular and annular-slug transition region. The condensation regime Fig. 4. Total area of designed vaporizers For multiple shell runs (e.g., series and/ or parallel), this value is total of all shells. From the above figure 4, we can specify 21

4 that the area required for construction of vaporizer with steam and water as heating medium is far less than the area required for construction of vaporizer with air as heating medium. With air as heating medium, 6 shells of reboiler and 4 shells of superheater are required. Heat transfer coefficient of air is very low as compared to water and steam, hence, large area is required for transferring specific amount of heat from LNG to air. PRESSURE DROP The total pressure drop value includes the nozzle pressure drops and the pressure drop inside tube (i.e. from inlet to outlet). Fig. 6: Total power required graph for vaporizers Fig. 6 shows that the power required for operation of vaporizer with water and steam as heating medium is high than the power required for operation of vaporizer with air as heating medium. Power depends on fluid flow rate, density, pressure drop and efficiency of pump. For flow of air, an air blower of 5hp (i.e KW) is used. Hence less power is required for operating vaporizer with air as fluid than the power required to operate vaporizer with water and steam as fluid. a) COSTING The total cost of vaporizer depends on manufacturing cost and operating cost. If both of these costs are within the acceptable limit then that design is to be selected. The manufacturing cost comprises of mostly material cost, labour cost and miscellaneous major heads. The labour cost and miscellaneous major head are almost same for all the three fluids; hence the manufacturing cost majorly differs according to the material cost of the vaporizer. The operating cost comprises of mostly of cost required to purchase the heating fluid, power required to circulate both the hot and cold fluids in the vaporizer and maintenance. b) Fig. 5:. Tube side pressure difference in vaporizer with a) different fluids, and b) relative comparison graph. For TEMA K shells, total pressure drop is the sum of nozzle pressure drops. The bundle has no pressure drop because it is a natural circulation cell. The pressure drop in the U-tubes is not calculated by the HTRI software. From the above graphs shown in figure 5 we can state that, the pressure drop in tube side is higher in case of water as heating medium. High flow velocities result in larger pressure drop across the section of pipe. Pressure is least in case of air. As all the pressure drops are within the allowable range, all designs are to be considered safe for working purpose. POWER REQUIRED The total power required comprises of both, power required to operate reboiler and superheater. Fig. 7: Cost of vaporizers for different fluids 22

5 From Fig. 7 shown below, final conclusions can be drawn that readily obtained waste steam is the most cost effective way to vaporize LNG and operating cost of generated steam to be used is comparatively high. Air is not at all proposed, since the cost to construct kettle reboiler using air as heating medium is very expensive. CONCLUSION 1. On studding various parameters affecting the three designs, firstly it is found that pressure drop in the tubes is higher in water than the steam and air; but as it is within the allowable range, the design is accepted. 2. The second important parameter to be optimized is area. The requirement of area for steam as heating fluid is very less as compared to others. However re-gasification unit with air as heating medium requires huge area. Hence area wise steam is the best option suggested. 3. The third important parameter is the cost, which has to be least for the construction of any of the unit. From the study it is clear that if waste steam is used as source of heat, than steam has the minimum cost. 4. Hence it can be concluded that, steam is the best option as heating medium for vaporization of LNG in efficient and cost effective way as it requires less space and material. NOMENCLATURE Symbol Name of term SI Units A Total area m 2 dt in Inside diameter of tube M dt out Outside diameter of tube M DT coldend Temperature difference at cold end o C DT hotend Temperature difference at hot end o C F Friction factor for tube - F t Correction factor for LMTD - h i Heat transfer coefficient on tubeside W/m 2 K h o Heat transfer coefficient on shellside W/m 2 K K m Thermal conductivity of tube material W/mK L Tube length M LMTD Log mean temperature difference o C m Mass flow rate of fluid kg/s MTD Mean temperature difference o C N p Number of passes - N t Number of tubes - Pd Pressure drop Pa Q Heat duty W Rfc Fouling resistance of cold fluid - Rfh Fouling resistance of hot fluid - U o Overall Heat Transfer Coefficient W/m 2 K v Velocity of fluid m/s ρ Density of fluid kg/m 3 APPENDIX Design Procedure-The usual pool type (kettle) reboilers have submerged type bundles and a vapor disengaging space. Here the vapor leaves the reboiler at the saturation temperature and may have some entrained liquid. If a dry or superheated vapor is required then additional separators are used or tubes are placed in the vapor space to dry and superheat the vapor. In this case, an additional separator (superheater) is used. The basic design equation to be used is, Q A (1) U o MTD MTD- Mean Temperature Difference MTD is given by, MTD Ft LMTD (2) Where, ( DThotend DTcoldend ) (3) LMTD DThotend ln DTcoldend The velocity (v) in m/s can be calculated by following formula, m v (4) N N A t The calculated velocity should be within accepted limit (i.e. ρv 2 < 8928). Pressure drop (Pd) in bar on shell side in pool boiling case is almost negligible. Hence tube side pressure drop is calculated with help of Darcy s equation in bar. It should be within allowable range. 2 Flv 5 Pd 10 (5) 2 dt in Overall heat transfer coefficient is calculated by following formula, 1 (6) U dt out dt out ln 1 1 dt dt out in dtout Rfc Rfh h o hi dtin 2Km dtin Since boiling heat transfer coefficient is dependent upon temperature difference across the boiling film, the differential fouling resistance will alter the boiling heat transfer coefficient. Hence, care should be taken to limit the differential fouling resistance to a reasonable value so that the boiling heat transfer coefficient and, therefore, the thermal designs are authentic [9]. For this purpose, it is suggested that the overdesign (excess area) be maintained at around 10%. p 23

6 ACKNOWLEDGEMENT I would like to take this opportunity to express my deep gratitude and genuine regards to Dr. A. T. Pise Dy. Director of Technical Education, Mumbai and Mr. Arvind Kaushik, Head of Thermal Engineering Group, PRDH, Larsen & Toubro, Mumbai for their valuable guidance and Dr. Amit Patil National Institute of Technology, Suratkal for being a constant source of inspiration and motivation. I would also like to thank all my family and friends who were always besides me and maintained my spirits throughout the course of study. Lastly I am thankful to those whose names cannot be included due to limits of space and have contributed to complete my work. REFERENCES 1. K. Kennelley, P. Patterson, Method for Producing, Transporting, Offloading, Storing and Distributing Natural Gas to a Market Place BP Amoco Corporation, Chicago, IL(US) J. Baan, M. Krekel, R. Leeuwenburgh, M. M. McCall, Offshore Transfer, Re-Gasification and Salt Dome Storage of LNG Proceeding of Offshore Technology Conference, Houston, Texas, USA C. C. Yang and Z. Huang, Lower Emission LNG Vaporization LNG journal, Foster Wheeler North America Corporation, USA. 2004, pg Kim H, Jung H, Design and Construction of LNG Regasification Vessel GASTECH, Bilbao, Spain H. Jeong, H. Chung, S. Chul, S. Chung, Optimum Design of Vaporizer Fin with Liquefied Natural Gas by Numerical Analysis Gyeongsang National University A. Smith, J. Mak, LNG Re-gasification and Utilization Annual Atlantic Canada Oil and Gas Halifax, Nova Scotia D. Q. Kern, Process Heat Transfer Palen, Small, Heat exchanger design handbook Hemispher publications. 9. R. Mukherjee, Practical thermal design of Shell-and- Tube Heat Exchangers Begell house Inc., Tubular Exchanger Manufacturers Association (TEMA) Standard. 11. G. Radhakrishna. Design, Selection and Operation of Heat Exchangers Workshop held by Larsen & Toubro limited. Mumbai, India Heat Transfer Research Incorporation (HTRI) Design Manual. 13. R. Agarwal, LNG Regasification - technology evaluation and cold energy utilisation Meisam Babaie, Queensland University of Technology, Australia. Retrieved from: 7-proceedings/Process-8-Randeep_Agarwal.pdf 14. Wikipedia: Regasification of Liquefied Natural Gas Retrieved from: 24