Side Coolers, Heaters

Heat sources or sinks other than the condenser and reboiler may exist at different points in the column such as in the case of pumparounds. As an example, a column with a partial condenser may have a side cooler, a few trays down the column. For trays above the side cooler, the energy balance around passing vapor and liquid streams and the overhead is unchanged. Therefore, the graphical construction of trays starting at the top is not affected by the side cooler. When the side cooler is crossed, the energy balance becomes 

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Taking the specific net power output (net output power per unit total heat exchanger surface area) as the design objective function, optimize the warm-side (heater or high-temperature side heat exchanger) and cold-side (cooler or low-temperature side heat exchanger) working fluid temperatures. 

This equation relates the liquid and vapor profiles above and below the heater/ cooler tray to the heat duty qj. Referring to Figure 5.7, if the slope of the operating line below the side cooler stage is known or can be calculated, the line is constructed directly. If the number of stages between the side cooler and the upper feed is known, the slope of the operating line must be varied to match the number of stages. 

Side coolers or heaters are used for redistributing the heat load along the column in order to control liquid and vapor traffic or L/V ratios in the various sections of the column. Pumparounds may be used as the means of transferring heat between the side heaters or coolers and the column. 

Each side cooler or heater placed on the column adds one degree of freedom to the column. By varying the side cooler or heater duties, it is possible to meet certain performance specifications such as liquid or vapor profiles or side product purities. The side condensers can enhance the side product purities because better fractionation may be achieved by manipulating the LIV ratio below the side draw. 

A distillation column side draw constitutes the main product of the column, where lighter impurities are removed in the distillate and heavier impurities are removed in the bottoms. Because of anticipated variations in the feed composition the column is equipped with a side cooler just below, and a side heater just above the side draw to improve operating flexibility. What potential operating problems could the side cooler and heater help alleviate What specifications would you use to define the column operation ... 

The diagram below (left) shows a device formed by a saies of three materials A, B, and C for transferring heat from one side to another, called Peltier effect converter or Peltier cooler/heater. The metal plates made of material C ensure a good mechanical behavior and a good contact with external heat sources/sinks and with the materials A and B, while allowing optimal thermal and electrical conduction. Between these plates, three in number, are the materials A and B to form two sequences of thermocouples ACC and CBC in series (from the electrical circuit viewpoint), each of these pairs using two... 

Build the cycle as shown in Fig.4.30. Assume that the compressor is isothermal, the heater is isobaric, the turbine is isothermal, the cooler is isobaric, and the hot and cold sides of the regenerator are isobaric. 

Assume a process each for the devices (1) pumps as adiabatic with 100% efficiency, (2) turbines as adiabatic with 100% efficiency, (3) heat exchanger as isobaric on both cold and hot sides, and (4) heater and coolers as isobaric. 

Assume compressors are adiabatic with 100% efficiency, heater and cooler are isobaric, and both hot and cold sides of the heat exchanger are isobaric. 

Figures 17.10 and 17.11 illustrate the color and severity of ABS formation and fouling. This is an SCR on a refinery heater. The ABS precipitated on the edge of the access door probably due to cooler temperatures created because of a seal leak. The interior side and top edges of the door showed ABS corrosion once the maintenance personnel performed their inspection. If the seal problem went uncorrected, eventually ABS would have destroyed the door requiring it to be replaced.

Subsequently the pure extraction gas flows through a heat exchanger where it is indirectly cooled with water to rise its density. When possible, it is intended to get a liquid phase which is collected in a connected buffer vessel (KP). The installed membrane compressor is able to pump gas as well as highly compressible liquids, which are withdrawn from the buffer vessel To achieve a maximum flexibility with respect to extraction medium and operation parameters a cooler (WT6, for gas phase recompression) or a heater (WT4, for liquid phase recompression) can alternatively be used to bring the recompressed solvent back to extraction temperature Before it is entering the main column a side stream is removed for the feed presaturation. 

All of the commercial simulators include models for heaters, coolers, heat exchangers, fired heaters, and air coolers. The models are easy to configure, and the only inputs that are usually required on the process side are the estimated pressure drop and either the outlet temperature or the duty. A good initial estimate of pressure drop is 0.3 to 0.7 bar (5 to lOpsi). 

The G.P. chamber is equipped with an air cooler of about 5500 BTU size (Item 3) is located about 1.8 meters off the floor. An evaporator control valve (Item 2) on the refrigerate line allows temperature control of the air cooler condenser coils. Temperature of the coils is maintained just above the dew point for 50% relative humidity. An electric baseboard heater (Item 6) is equipped with a hydrastatic thermostat control (Item 11). The base board heater is placed near the floor and opposite of the chamber door. A humidifier (Item 4) is located approximately 1.8 meters above the floor and just to the right of the air cooler. The nozzle of the humidifier is pointed slightly toward the back of the chamber. A humidistat (Item 10) is centered between the floor and end walls on the same side as the humidifier. A strategically located recording hygrothermograph is used to monitor both temperature and humidity. 

Horizontal SCF heater (when the liquefied fiuid is pumped) or cooler (when the fiuid is compressed in gas phase), with one pass on the shell side and two passes on tube side, U-tubes being acceptable... 

A total column differs from a column section in that the former may have side feeds and products and coolers and heaters. Specifically, a conventional distillation column has one feed stream, an overhead (distillate) product and a bottoms product, a condenser, and a reboiler, as shown schematically in Figure 5.3. 

Columns with Multiple Feeds, Side Draws, and Side Heaters/Coolers... 

FIGURE 5,7 Column with multiple feeds, side draws, and side heaters/coolers. 

Side Heaters/Coolers The effect of a side heater or cooler on the L/V ratio is determined from an energy balance at stage j, the stage where heat duty per unit time, Qj, is transferred. If the side heater/cooler is placed at stagethe streams directly affected by the heat transfer are Lj and Vj (compositions Xj and Yp. The role of the side heater/cooler in modifying the vapor and liquid profiles above it and below it is analyzed based on energy and material balances. The energy balance is written as... 

The temperature variation in an absorption or stripping column is a side effect and is not a required contributor to separation, as in distillation. In fact, in situations where the temperature variation is excessive, the column may have to be cooled or heated in order to counter the absorption/stripping thermal effects. The column may be operated isothermally, with all stages maintained at the same temperature, by applying the right amounts of heater/cooler duties. 

Complex distillation may be defined as a multistage vapor-liquid separation process that includes one or more of the following features multiple feeds, side draws, pumparounds, and side heaters or coolers. 

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 purposes of side heaters and coolers may be categorized as follows utilization of heat sources or sinks at different temperature levels, removal of heat of absorption, and control of vapor and liquid flows in the column. 

The implications of side heaters or coolers on the column performance are dis-... 

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