Simulation calculations of thermal siphon reboilers in TASC
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In TASC, within the Thermosyphon Calculation for heat syphon reboilers, what is the difference between fixed flow and find flow? My understanding is as follows: You can specify what type of thermosyphon calculation you want to carry out. a) TASC determines the flow rate of the thermosyphon stream, taking into account the driving head of the liquid as well as the pressure losses in the exchanger and related piping. b) By specifying a fixed flow rate, TASC calculates the pressure losses in the exchanger and associated piping. Option a) is the default setting. The mass flow rate of the cold (thermosyphon) stream that you specify is considered only as an estimate. Using option b) with a fixed flow rate generally results in a physically unrealistic pressure imbalance within the thermosyphon circuit; this imbalance is reported along with the results of the thermosyphon calculation. Calculations using option b) can be useful for sizing the piping for a thermosyphon system or for designing such a system. However, the stability results related to the thermosyphon operation should be viewed with caution when using option b), especially when there is a significant pressure imbalance. The calculation type applies only to the flow rate of the cold (boiling) stream. If the heating stream is condensing – which it usually is – its flow rate is adjusted so as to ensure complete condensation. My understanding of reboiler design is that the amount of vaporization (the heat load) required by the reboiler is determined by the distillation column; if the vaporization rate at the reboiler outlet is specified, then the circulation volume can be determined, and from that, it’s possible to determine whether the pressure drops are balanced (in other words, the circulation volume, the length of the sensible heat heating section, and various pressure drops can all be determined based on the vaporization rate). It’s a process of iterative adjustments, and the goal is to find an appropriate vaporization rate that meets the requirements regarding vaporization amount and circulation volume. This is what I understand by a fixed flow rate. \"Find flow\" refers to the process by which the system itself determines the appropriate vaporization rate and amount of vaporization. Let me give an example: the vaporization rate can reach over 90%. In such cases, the heat flux often exceeds the critical heat flux, which is clearly not appropriate.
Since a specific design for a thermal siphon reboiler has not been developed, I agree with you literally – a 90% vaporization rate is indeed much higher than that of conventional thermal siphon reboilers. In the simulated design of a distillation column, once the feed rate of the material, as well as the withdrawal rates from the top and bottom of the column and the reflux rates are determined, the vaporization rate can be calculated accordingly. I also ask experienced friends to provide a detailed answer. This post was last edited by LTY on 2008-1-4 09:47]
The vaporization rate is mainly related to the medium, with other factors also affecting it.
For a vertical ammonia evaporator with thermosiphon action, where liquid ammonia flows in the tube side, what value should be chosen for the vaporization rate?
May I ask, under what operating conditions should a thermosyphon reboiler be chosen? What are the advantages of choosing it over a conventional reboiler? Why is a thermal siphon generally used for the reboiler at the bottom of the desulfurization tower? Sorry, a bunch of questions – I’m a newbie!
Your questions seem numerous, but they are actually easy to answer; the key point is that the thermosyphon reboiler is the conventional type of reboiler – perhaps 80% of reboilers are of the thermosyphon type. On the contrary, it is the other types of reboilers that are relatively special: 1. Forced-circulation falling-film reboilers, which are generally used in situations where the material is dirty, the temperature difference for heat exchange is small, or the material is highly thermosensitive ； 2. Falling film reboilers are generally used in industries involving high vacuum conditions or heat-sensitive materials ； 3. Horizontal thermosiphon, generally used in conditions with high vacuum ； .......There are many more types; I can’t remember them all at the moment; those who come later can add them.
Generally, the vaporization rate at the tube-side outlet of a thermal siphon reboiler is 20%
Many contact thermal siphon reboilers use 20%