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www.petrofed.org coefficient). Therefore prediction of boiling nuclear boiling diminishes resulting in its incipience heat transfer and circulation rates are decreasing the overall of heat transfer the primary requirements for the design and coefficient as the film coefficient is greater in efficient operation of the thermosiphon reboiler. Vertical tube thermosiphon reboiler is generally the nucleate boiling. operated either as heat transfer equipment or as a (v) The lower tower pressure results in a lower separation system. When the thermosiphon percent vaporization, therefore, for pressure reboiler is operated as the heat transfer unit, it less than 5.0 psig, 15% may be the highest generates the vapour phase for counter current two-phase vapour-liquid flow required for vaporization for a thermosiphon reboiler. components separation in a rectification or (vi) The vertical tube length is usually 2.5 to 5.0 m stripping column. Performance specification for (8 to 16 ft), although lengths upto 6.0 meter the reboiler is given by a heat duty and (20 ft is quite common). Longer tubes are optimization aims to increase the overall heat possible, but the whole distillation column transfer coefficient and/or reduce the driving would have to be raised to accommodate temperature difference. However when them. Tube length less than about 1.8 meter thermosiphon reboiler is operated as separation (6 ft.) leads to uneconomical designs. unit, it will be specified through a concentration of volatile product in the concentrate. Typical (vii) The outlet nozzle of the reboiler and the return applications are: water– ethylene glycol, acetone- pipe to the distillation column may be water, ethanol-water etc. designed in a number of ways, however, the side – outlet type is the simplest and the most While considering a vertical tube thermosiphon commonly used. Generally the flow area in reboiler for a duty, following guidelines must be the outlet nozzle is equal to that in the tubes to strictly observed: avoid instability. The inlet nozzle may enter the lower header at the bottom or side, (i) Most commonly the thermosiphon reboiler is whichever is most convenient depending operated/designed in the region of slug flow. upon the header type and ground clearance. As the two-phase move in the upward The diameter of the inlet nozzle and pipe work direction the percent vapor flow increases and is less than that of the outlet nozzle, as it is only the flow type becomes an annular or mist flow. required to carry single phase liquid and a The mist flow pattern decreases the value of high pressure drop here will usually improve heat transfer coefficient considerably. The percent vaporization therefore should not be the stability of the unit. more than about 30% by weight. (viii)For low pressure operation (less than 0.2 bar) the problem is of achieving steady nucleate (ii) The viscosity of the feed should be less than boiling because of the relatively large volume approximately 0.50 cp otherwise the flow of vapour produced by a small amount of difficulty may occur. vaporization. At low pressure the sensible heat duty is large and this can lead to a low (iii) The tower level for the thermosiphon reboiler average heat transfer coefficient. High (natural circulation) should be great enough pressure operation is made difficult because it to overcome the pressure drop to inlet flow is easy to exceed the critical heat flux and line, exchanger and return line. Hence the phase density differences that drives the unit should be designed considering a large liquid level for safety. For natural circulation, circulation is small. vertical exchanger is to set the top of the tube (ix) Vertical tube thermosiphon reboiler performs at the lowest design liquid level as too high a more efficiently at the liquid submergence level may be detrimental to the exchanger level of 50 to 75%. Though the value of heat operation. transfer coefficient was very high for some systems whose viscosities were less, when the (iv) The IPB has clearly indicated the two distinct submergence level was maintained at 30%. regions which depend strongly on the liquid The performance of reboiler got adversely head. When the level head increases for affected for system having high viscosity like vertical exchangers, the liquid head forces ethylene glycol as the flow became very low liquid to higher levels in the tubes. Thus and intermittent. Thus it is recommended that convective transfer is predominant and while considering vertical tube thermosiphon 24
25 www.petrofed.org reboiler for any operation the viscosity of the entering into the reboiler. feed should be less then approximately 0.5 cp. Note: The article contains information from the open literature and the work carried out by the author on a number of organic (x) The development of oscillations is higher at liquids and their binary mixtures. low liquid submergence and more with organic liquids than water therefore a valve must be incorporated in the pipeline before Obituary He was an independent consultant for the past 15 years rendering service to institutes, corporates and Government in India and aborad. He also found time in between to deliver lectures. He prepared a policy paper on India's Road to Hydrogen Economy for the Indian National Academy of Engineering along with Dr. T. K. Roy in 2006 and delivered the Dr. P. K. Mukhopadhyay keynote address in use and supply of hydrogen in petroleum refining, petrochemicals and After over 50 years of dedicated contribution to fertilizers at the Lovraj Kumar Memorial Trust the oil & gas industry, the brilliant chemical workshop. engineer Dr. Pranab Kumar Mukhopadhyay, 74, passed away on March 15, 2009 after a brief illness. He is survived by his wife and two He was a recipient of the Lala Shriram National daughters. Award for Leadership in Chemical Industry by the Indian Institute of Chemical Engineers and the NRDC Award for Invention of Solvent A gold medalist in chemical engineering from Extraction Process for production of pure Jadavpur University and Ph.D. in Technology aromatics. He was a Fellow of the Indian from Gubkin Petroleum Institute, Moscow, his Institute of Chemical Engineers and the Indian five decade professional experience covered National Academy of Engineering. technical service, teaching, R&D, consultancy and working in close association with research & academic institutes, operating companies Dr. Mukhopadhyay had been a key speaker at and commercialising a number of processes prestigious conferences and seminars. His last and products. With 30 publications and 50 lecture, probably, was on the PetroFed platform papers presented in various national and to invitees on Energy Saving in Pumps, Fans international fora, Dr. Mukhopadhyay was a and Compressors on December 30, 2008. recipient of the Petrotech 2005 Life-time Achievement Award. Our heartfelt condolences to the bereaved family. Beginning his career with the Stanvac Refinery (now HPCL Refinery) at Mumbai in 1955 he later taught chemical engineering at IIT, Delhi before heading the R&D of Engineers India Limited for 13 years. In 1983 he became the first full time Director on the Board for R&D at IndianOil. He also served IndianOil as its Chairman.
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