Level column bottom

Configuration number Benzene product composition Distillate rate Overhead accumulator level Column bottom level Method for composition controlb Comment ... 

Base temperature Column feed rate Column base level Column bottom-product rate... 

The aqueous layer from the ester column distillate, the raffinate from washing the ester, and the aqueous phase from the dehydration step are combined and distilled in the alcohol stripper. The wet alcohol distillate containing a low level of acrylate is recycled to the esterification reactor. The aqueous column bottoms are incinerated or sent to biological treatment. Biological treatment is common. 

The column bottom level is sometimes controlled by bottoms draw. Varying reboiler heating medium is another possibility. For some cases, bottoms draw level control works better and for others, heating medium level control. BojnowskF gives a case where heating medium level control was desired for two columns in a plant. One... 

The best designs provide for the percentage vaporization per pass to have been completed by the time the fluid mixture reaches the upper end of the tube and the mixture is leaving to enter the bottom chamber of the distillation column. In order to assist in accomplishing this, the initial reboiler elevation should be set to have the top tubesheet at the same level as the liquid in the column bottom section. A liquid-level control adjustment capability to raise or lower this bottoms level must exist to optimize the recirculation. Sometimes, the level in the bottom of the column may need to be 25-30% of the reboiler tube length above the elevation of the tubesheet. Therefore, the vapor nozzle return from the reboiler must enter at sufficient elevation to allow for this possibility. 

Cold Spots in the Reactor Inadequate heat>up High Main Column Bottoms Level... 

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

The magnitudes of various flowrates also come into consideration. For example, temperature (or bottoms product purity) in a distillation column is typically controlled by manipulating steam flow to the reboiler (column boilup) and base level is controlled with bottoms product flowrate. However, in columns with a large boilup ratio and small bottoms flowrate, these loops should be reversed because boilup has a larger effect on base level than bottoms flow (Richardson rule). However, inverse response problems in some columns may occur when base level is controlled by heat input. High reflux ratios at the top of a column require similar analysis in selecting reflux or distillate to control overhead product purity. 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

Seven liquid levels are in the process separator and two (base and overhead receiver) in each column. The most direct way to control separator level is with the liquid flow to the stabilizer column. Then stabilizer column overhead receiver level is controlled with cooling water flow and base level is controlled with bottoms flow. In the product column, distillate flow controls overhead receiver level and bottoms flow controls base level. 

In the multiloop controller strategy each manipulated variable controls one variable in a feedback proportional integral derivative (PID) control loop. Taking a single-feed, two-product distillation column with a total condenser and a reboiler as an example, a basic list of possible controlled variables includes the distillate and bottoms compositions, the liquid levels in the reflux accumulator and the column bottom, and the column pressure. The main manipulated variables are the reflux, distillate, and bottoms flow rates and the condenser and reboiler heat duties. 

Eollowing are two examples (16.1 and 16.2) of a distillation column that demonstrate the effect of applying different pairing strategies. In both examples the control loops for the column pressure and the liquid levels in the condenser accumulator and the column bottom are determined independently based on practical considerations. Thus, the column pressure is controlled by various techniques that may involve the condenser coolant rate, and the liquid levels are controlled by the product flow rates. What remains to be decided is how to pair the distillate and bottoms compositions with the reflux rate and the reboiler heat duty. The same distillation column is used in both examples, having a total condenser and a reboiler, one feed and two products. The column is designed to separate a benzene-toluene mixture into benzene and toluene products with specified purities. 

A pilot-scale distillation column located at the University of Sydney, Australia is used as the case study [60]. The 12-tray distillation column separates a 36% mixture of ethanol and water. The following process variables are monitored temperatures at trays 12, 10, 8, 6, 4, and the reflux stream, bottom and top levels (condenser), and the flow rates of bottoms, feed, steam, distillate and reflux streams. The column is operated at atmospheric pressure using feedback control. Three variables are controlled during the operation top product temperature, condenser level, and bottom level. Temperature at tray 8 is considered as the inferential variable for top product composition. To maintain a desired product composition, PI controllers cascaded on flow were used to manipulate the reflux, top product and bottom product streams. 

The alcohol loss levels (column 2) for various levels of bottoms concentration (even at a bottoms concentration of only 0.1 percent) is over 1 percent of the total. Adding only three stages from 9 to 12 in the stripping column cut the alcohol loss more than half. Losses in the bottoms are very important from a profit standpoint. The key question is, "How much can I afford to lose "... 

Long time response (2) long time response and increasing complexity (3) V/F control is not desirable in general (4) column bottom level control by steam flow is desirable. (Comments are due toT. Umeda). b Direct M.B. = direct material balance control Indirect M.B. indirect material balance control V/F = vapor to feed ratio control... 

In the first approach, we may consider a classical standalone control structures, as displayed in Fig. 13.5. Reactor feed is on flow control (PI), and outlet stream on level control (PI). For the distillation column a classical inventory control is column pressure with condenser cooling (PI), base level with bottom product (P), and reflux drum level with distillate (P). Quality control loops are top composition with reflux and bottom composition with reboiler duty, both as PI controllers. 

Column pressure is controlled by adjusting the speed of the column compressor through a steam flow control-speed control-pressure control cascade system. Reflux is flow controlled. Reflux drum level sets distillate flow. Base level sets bottoms flow. 

In the column, reflux is flow controlled, reflux drum level is controlled by distillate, base level by bottoms, pressure by vent vapor, and temperature by steam to the auxiliary reboilcr or vapor from the evaporator. 

Additional guidelines for differential-pressure level transmitters on column-bottoms sumps have been presented by Buckley (67). 

Low liquid levels can be as troublesome as high liquid levels. When bottom level is lost, vapor can flow out of the column bottom. In one incident (210), such vapor flow ruptured the bottom product storage tank. Low bottom levels can also cause cavitation and overhehting of bottom pumps. In some services, a low bottom level can excessively cottCentrate some chemicals, inducing an undesirable reaction. If these chemicals are unstable (e.g., peroxides, acetylenic compounds), an explosion may result. Some reported accidents (97, 275) were initiated by low liquid levels at the bottom sump. 

As with column bottom level, when an excessively high or low level can be hazardous, it is a good practice to install high-reliability level instrumentation including alarms, trips, and indicators, and to mount these as per Figs. 5.4 and 15.16. The trips usually switch off rotating machinery downstream of the accumulator and/or cut off product flow. 

A bottom temperature indicator measuring liquid temperature may read vapor temperature (which may be considerably lower) when the level drops. The consequences are similar to those of a fouled thermocouple. A malfunctioning level indicator can thus lead to a dangerously misleading, but apparently consistent, indication of both level and bottom temperature (Fig. 13.8). In one case (275), this led to overheating and an exothermic reaction at the column base, which in turn caused residue discharge from a column vent. 

Keeping the liquid level constant is important. Installing a preferential baffle in the column bottom compartment (Sec. 4.5) heis been recommended (178, 357). This baffle separates the bottom and reboiler sumps and ensures a constant head to the reboiler. 

Installing a valved dump line connecting the column bottom outlet line with the reboiler inlet line (Fig. 15.4a). This technique is only needed when the column reboiler sump is separated from the column bottom sump by a baffle or when the reboiler liquid comes from a trapout pan. The valve remains shut during normal operation, but is opened during startup to lower the level and inspire thermosiphon action during startup. One case where this technique was successfully used has been described (237) the author has had several similar experiences. 

The reboiler may also experience inverse response, often referred to as reboiler surge or reboiler swell. An increase in heat input may increase the volume of vapor in the reboiler or the pressure drop in the reboiler and its outlet piping. This will temporarily back up liquid into the column bottom, causing liquid level to rise. 

When a steady bottom flow is desired, scheme 16.4e is at a major advantage and is usually preferred (258, 259, 309, 332, 362) unless inverse response is likely. When inverse flow is troublesome, an unconventional direct MB control scheme (Fig. 16.6a) can provide the cure. Buckley et al. (65, 66) reported satisfactory performance of this type of scheme in a column where scheme 16.4e failed due to inverse response. In that column, bottom flow was too small to effectively control bottom level, and column base holdup was to be kept at a minimum to avoid material degradation (i.e., schemes 16.4a, b, and d were unsuitable). 

Reboiler vapor should enter above liquid levd. Level indication should be provided in columns. Radioactive scans are useful for liquid level detection at column bottoms. 

Column bottoms, drawn from the weir compartmrat of a kettle rdxrilo, preheated column feed. Preheat was not controlled. A rising bottom level increased bottom flow and feed preheat. The greater preheat reduced column downflow, bottom level, and bottom flow. This in turn reduced preheat and raised bottam level A cycle developed. 

Distillate and bottoms were controlled by accumulator and sump levels, respectively, feed and reflux on flow control and boilup was temperature-controUed Tower pall rings were replaced by higher-capadfy rings (bottom) uid wire-mesh structured packing (top) to increase c )acity and reduce reflux. The column was sensitive to ambient disturbances (e.g., rainstorms). The reflux reductions escalated this sensitivity to an extent that annulled the revamp benefits. The temperature control was ineffective due to its narrow range of variation. Problems were solved by controlling boilup on sump level and bottom product on flow control. 

Level float in a column bottom sump bounced and finals broke due to impingement of entering steam. Problem fixed by installing a shielding baffle over the level connection. 

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