Side reboilers

Water Hammer and Condensate Backup Steam-Side Reboiler Control... 

In case 2, the shell-side reboiler temperature rises from 240 to 280°F (one reason for such a rise in temperature could be an increase in tower pressure). Now AT = (320°F - 280°F) = 40°F. Looking at the equation above, it looks as if Q will drop in half to 5000 lb/h (which is about the same as 5,000,000 Btu/h). Thus, the flow of steam to the reboiler has been cut in half, even though the control valve position has not moved. 

Side reboiler 1, column 2 Floating head 180.0 2.0 Carbon steel 600 294000... 

Side reboiler 2, column 2 Total for column 2 Floating head 50.0 2.0 Carbon steel 170 115000 409000... 

The way the operating lines are actually constructed depends on how the particular column is specified. Figure 5.7 shows a demonstration column with two feeds, a liquid draw, a vapor draw, a partial condenser and a reboiler, a side cooler, and a side reboiler. Also shown is the Y-X diagram and operating lines for this column. If we assume the distillate rate and composition and the reflux ratio are known, the calculations can be started at the top. A vertical line drawn through the distillate composition intersects the diagonal at a point that defines the upper end of the top operating... 

The next section operating line has a smaller slope since the vapor flow is reduced above the draw tray j by the vapor draw rate SJ and the liquid flow is assumed constant. The operating line slope is determined such that the number of stepped off stages matches the number of stages between the vapor draw and the side reboiler. 

The effect of the side reboiler on the operating line slope is determined as described above for the side cooler, taking into account that the heat duty here is positive. 

If the number of stages between the side reboiler and bottom reboiler is known,... 

The effect of redistributing the reboiler duty between the bottom reboiler and a side reboiler a few trays up the column can also be studied on the Y-X diagram. As discussed in Section 5.2.2, a side reboiler in this case has the effect of increasing the slope of the operating line below the side reboiler. Given the other problem constraints, the side reboiler tends to move the operating line closer to the equilibrium curve. One result of this is the redistribution of the separation power per tray above and below the side reboiler. Moreover, the proximity of the operating line to the equilibrium curve in the vicinity of the side reboiler may have the potential to cause a pinch point in that area. 

Distillation columns and other multistage columns generally require heat addition and/or removal, which normally take place at the reboiler and/or condenser. For a variety of reasons, it may be desirable in certain types of columns to add or remove heat at intermediate trays aside from the condenser and reboiler. Examples of such columns include demethanizers that utilize a side reboiler alongside the bottom reboiler, multi-product columns where intermediate condensers are associated with some of the side products, and absorber intercoolers used for partial removal of the heat of absorption. The method most commonly employed for exchanging heat between a column tray and a heat source or sink is the pumparound, where a fluid is drawn from the tray, sent to a heat exchanger, then pumped back to the column. The following sections pertain to the various applications of side heaters and coolers and the different types of pumparounds. 

The use of process streams to supplement the reboiler duty using side reboilers is similar in principle. In demethanizers, for example, one process stream may be used as a heat source for the bottom reboiler, while another stream, at a lower temperature, may be used as a side reboiler a few trays above the bottom reboiler, where the temperature level is low enough to allow heat exchange with the process stream. 

Principles and design of tube-side reboilers are discussed in several texts and papers (e.g., 41, 81, 113-114, 124, 174, 267, 282, 311) and are outside the scope of this distillation text. A brief overview of some of the main principles is presented below. 

Injecting lift gas into the outlet liquid line. This reduces the density in this line and initiates flow. This technique is commonly used in pressure fractionators and when the reboiler is located well below the reboiler return nozzle (e.g., in cryogenic gas plant demethanizer side reboilers) (170, 394,421). The injection nozzle should be downstream of the reboiler outlet temperature indicator (421). 

Minh, L.Q., Husnil, Y.A., Long, N.V.D. and Lee, M. (2014) Retrofit and debottlenecking of naphtha splitter process to thermally coupled distillation sequence with a side reboiler. Journal of Chemical Engineering of Japan, 47, 687-692. 

Fig-5 refers the BAHE-2 Gas/Liquid exchanger streams temperature trends. Most important point here hot stream (i.e. feed gas) outiet temperamre is almost constant —65degC even when hot inlet stream temperature is varying. This is mainly because 1. Cold stream-2 (i.e. side reboiler) is driven by thermosiphon hence self-adjusts the flow thorough exchanger as heat load varies. 2. Cold stream-1 (i.e. expander feed separator liquid) flow impacted due to loss of level in drum when feed gas C3+ content reduces. 

Now look at Cold stream-1 (i.e. expander feed separator hquid), heat duty for this stream is reducing by 1.38MW. Reduction in C3+ HC is causing loss in V401 (Expander inlet drum) level which in turn reduce mass flow of cold stream-1 and results into sudden shift of thermal load to cold stream-1 (i.e. side reboiler). Shift of thermal load is so rapid that it causes cold stream 1 outlet stream temperature RoC of 3-4degC/min. 

Preheater (HEX), column 1 Side condenser, column 1 Total for column 1 Side reboiler 1, column 2 Side reboiler 2, column 2... 

There are several key opportunities for column optimization, which include reflux ratio improvement, pressure changes, feed preheating, side reboiling/condensing, and feed stage location. A pinch tool called a column grand composite curve (CGCC) (Dhole and Linnhoff, 1993) was developed to provide an aid for the evaluation of these improvements. 

For complex towers with pump-arounds, side condensers or side reboilers, and side product draws, a tower can be divided more than two sections, and the... 

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