Bottom temperature

The extract is vacuum-distilled ia the solvent recovery column, which is operated at low bottom temperatures to minimise the formation of polymer and dimer and is designed to provide acryUc acid-free overheads for recycle as the extraction solvent. A small aqueous phase in the overheads is mixed with the raffinate from the extraction step. This aqueous material is stripped before disposal both to recover extraction solvent values and minimise waste organic disposal loads. 

Conventional polymeri2ation inhibitors are fed to each of the distillation columns. The columns are operated under reduced pressure to give low bottom temperatures and minimi2e polymeri2ation. 

Intermediate Reboiler. Inclusion of an intermediate reboder moves the heat-input location up the column to a slightly colder point. It can permit the use of waste heat for rebod when the bottoms temperature is too hot for the waste heat. 

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

An additional series of process tests and plots can be helpful. The Delta-P over each section should be monitored and the reflux increased/decreased at constant bottom temperature. The composition of the heavy key in the overhead should be monitored. Plots should then be made of both Delta-P and of composition vs. reflux. Additional information concerning these tests can be found in Norman Lieberman s book entitled Troubleshooting Process Operations. ... 

Here, we refer to small amounts of water rather than large slugs that could damage the trays. Often the water will boil overhead and be drawn off in the overhead accumulator bootleg (water drawoff pot). However, if the column top temperature is too low, the water is prevented from coming overhead. This plus too hot a bottom temperature for water to remain a liquid will trap and accumulate water within the column. The water can often make the tower appear to be in flood. 

Many columns have water removal trays designed into the column. Top or bottom temperatures may have to be changed to expel the water if the column isn t provided with water removal trays. In some instances, the water can be expelled by venting the column through the safety relief system. 

In one process the crude styrene is first passed through a pot containing elemental sulphur, enough of which dissolves to become a polymerisation inhibitor. The benzene and toluene are then removed by distillation. The elthylbenzene is then separated from the styrene and tar by passing this through two distillation columns, each with top temperatures of about 50°C and bottom temperatures of 90°C under a vacuum of about 35 mmHg. The tar and sulphur are... 

Two objectives are immediately clear. If the top temperature can be raised and the bottom temperature lowered, then the ratio t= (Tjnin/Tjnax) decreased and, as with a Carnot cycle, thermal efficiency will be increased (for given /a,). The limit on top temperature is likely to be metallurgical while that on the bottom temperature is of the surrounding atmosphere. 

The temperature or number of trays can then be varied until the calculated outlet liquid composition equals the assumed composition, and the vapor pressure of the liquid is equal to or less than that assumed. If the vapor pressure of the liquid is too high, the bottoms temperature must be increased. 

The bottoms temperature of the tower can be approximated rt t ie desired vapor pressure of the liquid is known. The vapor pressure of a mixture is given by ... 

The bottoms temperature can then be determined by calculating the bubble point of the liquid described by the previous iteration at the clio-sen operating pressure in the tower. This is done by choosing a tempei a-ture, determining equilibrium constants from Chapter 3. Volume I, and computing ... 

Typically, bottoms temperatures will range from 200-400°F depending on operating pressure, bottoms composition, and vapor pressure requirements. Temperatures should be kept to a minimum to decrease the heat requirements, limit salt build-up, and prevent corrosion problems. 

An LTX unit is not a very efficient stabilizer because the absence of trays or packing keeps the two phases from approaching equilibrium at the various temperatures that exist in the vessel. In addition, it is difficult to control the process. Typically, for a 100-psi to 200-psi operating pressure, a 300°F to 400°F bottoms temperature is required to stabilize completely the condensate. The heating coil in an LTX separator is more Uke-... 

Absorber oil then flows to a still where it is heated to a high enough temperature to drive the propanes, butanes, pentanes and other natural gas liquid components to the overhead. The still is similar to a crude oil stabilizer with reflux. The closer the bottom temperature approaches the boiling temperature of the lean oil the purer the lean oil which will be recirculated to the absorber. Temperature control on the condenser keeps lean oil from being lost with the overhead. 

By decreasing the pressure and increasing the bottoms temperature more methane and ethane can be boiled off the bottoms liquid and the RVP of the liquid stream decreased to meet requirements for sales or further processing. Typical liquid recovery levels are ... 

The design of any of the distillation processes discussed requires choosing an operating pressure, bottoms temperature, reflux condenser temperature and number of trays. This is normally done using any one of several commercially available process simulation programs which can perform the iterative calculations discussed in Chapter 6. 

It is essential to realistically establish the condensing conditions of the distillation overhead vapors, and any limitations on bottoms temperature at an estimated pressure drop through the system. Preliminary calculations for the number of trays or amoimt of packing must be performed to develop a fairly reasonable system pressure drop. With this accomplished, the top and bottom column conditions can be established, and more detailed calculations performed. For trays this can be 0.1 psi/actu-al tray to be installed [149] whether atmospheric or above, and use 0.05 psi/tray equivalent for low vacuum (not low absolute pressure). 

The requirement of bottoms temperature to avoid overheating heat sensitive materials may become controlling. 

Wdien bix and Pik/hk e constants at a fixed or constant pressure, but evaluated for the light (1) and heavy (h) keys at top and bottom temperatures, their relationship is [94] ... 

This compares quite well with the selected 1,800 mm bottoms pressure. Bottoms temperature is 320°F. 

Determine bottoms temperature by bubble point on liquid Xfi. 

Assuming the tower pressure as set and an average of top and bottom temperatures can be selected (these may become variables for study), read equilibrium Kj values from charts for each component in gas. 

Lower pressure drop for vacuum systems Allows for better low pressure and vacuum system operation and lower bottoms temperature, with less degradation of bottoms product. 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

Low Reactor temperature Long Residence Time in the Reactor and Main Column High Bottoms temperature Low Bottoms Pumparound Rate Cold exchanger tube wail V temperature ... 

The unit operating philosophy and its apparent operating limits often dictate unit constraints. For example, limitations on the main column bottoms temperature, the flue gas excess oxygen, and the slide valve delta P often constrain the unit feed rate and/or conversion. Unfortunately, some of these limits may no longer be applicable and should be reexamined. Some of them may have resulted from one bad experience and should not have become part of the operating procedure. 

High bottoms temperature leading to fouling or high LCO endpoint... 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

If the main fractionator bottoms temperature is limited to 690°F, adding a pool quench can provide additional LCO product recovery. Assuming there is no penalty for the bottoms product quality and there is available cooling capacity in the upper section of the fractionator, this incremental LCO yield is valuable. 

Fig. 2.17 Temperature distribution in the streamwise direction. The solid circles ( ) represent the bottom temperature, and the empty circles (o) represent the fluid temperature. Reprinted from Mishan et al. (2007) with permission...

For the operating conditions, the set-points of EC and DMC compositions at the top and bottom are 0.01 and 0.2996, respectively, and the bottom temperature should not exceed 140°C to prevent the decomposition of reactants. From these plots, it can be concluded that the MPC outperforms the PI controller in terms of response speed in disturbance rejection, maintaining the variables at set points, and optimization capability. Especially, the PI controller failed to maintain the DMC composition set-point due to the slow long-term dynamics caused by the interaction between the RD column and azeotropic recovery column. 

Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analysers are available they can be incorporated in the control loop, but more complex control equipment will be needed. 

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