Pressure column tray

A Hquid-phase isophorone process is depicted ia Figure 4 (83). A mixture of acetone, water, and potassium hydroxide (0.1%) are fed to a pressure column which operates at head conditions of 205°C and 3.5 MPa (- 500 psi). Acetone condensation reactions occur on the upper trays, high boiling products move down the column, and unreacted acetone is distilled overhead ia a water—acetone a2eotrope which is recycled to the column as reflux. In the lower section of the column, water and alkaH promote hydrolysis of reaction by-products to produce both isophorone and recyclable acetone. Acetone conversion is typically ia the range 6—10% and about 70% yield of isophorone is obtained. Condensation—hydrolysis technology (195—198), and other Hquid-phase production processes have been reported (199—205). 

Distillation column, tray absorber Diameter, height, internal pressure, material of construction, tray type, number of trays, condenser, reboiler (see item 3)... 

The column contains a total of N-p theoretical trays. The liquid holdup on each tray including the downcomer is M . The liquid on each tray is assumed to be perfectly mixed with composition x,. The holdup of the vapor is assumed to be negligible throughout the system. Although the vapor volume is large, the number of moles is usually small because the vapor density is so much smaller than the liquid density. This assumption breaks down, of course, in high-pressure columns. 

For vacuum columns, where both absolute pressure and tray pressure drops vary significantly, a rigorous vapor-hydraulic model may have to be used. The modeling and simulation are easy. The numerical integration is quite difficult. This is because the ODEs become very, very stilT when vapor hydraulics are included in the model. 

In high pressure distillation, tray operation is usually in the emulsion regime. In small diameter (less than 1.5 m) columns, or at low liquid loads, or the low end of the pressure range (towards 10 bar), however, the froth-and spray regimes can be found. 

Low-pressure-drop applications. By virtue of their low pressure drop compared to trays (above), packings are favored in any application where it is economical to minimize pressure drop. A typical example is an atmospheric or low-pressure column whose overheads are compressed. Every pound per square inch of pressure drop here translates into greater compression ratio requirement and higher compressor capital and energy costs. 

FIG URE 8.6 F factor as a function of column pressure and tray spacing. 

Because of the low liquid rates in vacuum systems, downcomers will usually be oversized, and specific flow rates across the weir will be low. However, liquid rates in high-pressure columns may exceed values recommended for optimum tray performance across a single weir. The maximum specific flow is 70 gal/(min ft)[53 m3/(h m)] for a straight segmental weir and 80 gal/(min ft)[60 m3/(h m)] for a weir with relief wings. Above 80 gal/(min ft), a multiple downcomer arrangement should be considered. 

All columns have 16 trays, but with different spacings at different pressures, so that the column heights range from 7.6 m for the pressure columns to 12.4 m for the vacuum columns. The water produced by this plant has an acid content of less than 50 ppm. 

Nonideal behavior is manifested, for instance, in azeotropic and extractive distillation, discussed in this chapter. Another case of nonideality is the condition of vapor-liquid-liquid equilibrium (Sections 1.3.5 and 2.3.3). If the composition, temperature, and pressure on any column tray or number of trays put the mixture in the vapor-liquid-liquid region, three-phase distillation ensues. Sections 10.1.2 and... 

For preliminary design, column operating pressure and type condenser can be established by the procedure shown in Fig. 12.4, which is formulated to achieve, if possible, reflux drum pressures Pp between 0 and 415 psia (2.86 MPa) at a minimum temperature of 120°F (49°C) (corresponding to the use of water as the coolant in the overhead condenser). The pressure and temperature limits are representative only and depend on economic factors. Both column and condenser pressure drops of 5 psia are assumed. However, when column tray requirements are known, more refined computations should allow at least 0.1 psi/tray for atmospheric or superatmospheric column operation and 0.05 psi/tray pressure drop for vacuum column operation together with a 5 to 2 psia condenser pressure drop. Column bottom temperature must not result in bottoms decomposition or correspond to a near-critical condition. A total condenser is used for reflux drum pressures to 215 psia. A partial condenser is used from 215 psia to 365 psia. A refrigerant is used for overhead condenser coolant if pressure tends to exceed 365 psia. 

Column tray 10 temperature is controlled by adjusting the pressure in the propane vaporizer, which is controlled by refrigeration compressor speed. 

LPG Lean oil stripper Column was pressured up throu a connection in the overhead system while liquid circulated throu its valve tr. The gas could not travel downward, causing mechanical damage to top 12 trays. This later resulted in premature flooding. Always pressure columns fn>m the bottom up, espedaUy when column contains valve trays. 

In order to evaluate the developed model and to its limitations, laboratory experiments and accompanying simulations were carried out. The laboratory runs were performed using a pressure column having a diameter of 80 mm and up to 25 bubble cap trays. The reactive zone was equipped with Sulzer Katapak-S in heights of 20-40 cm. The column set-up is shown in Fig. 3.8. The catalyst pockets of the packing were filled with the ion-exchange resin described above. The column was operated at a pressure of 500 kPa. Feed rates were set between 2 and 5 kg/h and the reflux ratio was varied in the range 5-10. The feed was introduced above the reactive section. Further experiments were performed with the feed position below the reactive section. 

In order to keep the reboilers down to a reasonable size, the column has to be heated either with steam at a pressure of not less than 3.5 to 5 bar or wiA waste heat at a temperature level above 140-150°C. Unlike the prerun column, the pressurized column is a genuine distillation column as the overhead product has to meet the purity requirements of US Grade AA methanol. The reflux rate, the number of trays and the heat input can be varied within certain limits, and the most favourable design of the column and its economical operation have to be established by optimizing calculations. A column with the above-mentioned number of trays reaches its operating optimum with a reflux ratio of approximately 3.0 and a heat input of about 2.0 GJ per ton of total methanol produced. As the overhead product ftom the pressurized column is used to heat the atmospheric column, either of the two coliunns has to be used to distill some 50 % of the total methanol produced, except for slight differences in the reflux ratio. 

In view of the fact that the water content in the bottoms product of the pressurized column is twice that of the prerun column, the hydrocarbons transferred ftom the prerun column into the pressurized column will reliably be found in the bottoms product, i.e. they are transferred to the atmospheric distillation column. Thus, ethanol becomes the key component for the pressurized column. Since the bottom product of the pressurized column - unlike that of the atmospheric column - does nOt have to meet certain purity requirements, this column need not have a side outlet for ethanol, but the ethanol is quantitatively transferred to the atmospheric columit. The high methanol content in the bottom of the pressurized column facilitates ethanol separation. Nevertheless, for the same number of trays, the pressurized operation of this column leads to a higher reflux than in the atmospheric column. The bottoms product ftom the pressurized column is transferred to the atmospheric column at approximately 125-35°C. The overhead product is obtained at approximately 115-125°C, condensed in the reboiler of the atmospheric column, and fed to the reflux drum of the pressurized column. From there, some of the overhead product is withdrawn by way of an after-cooler as on-spec methanol while the rest is pumped back uncooled as reflux to the column head. 

The raw methanol considered in Fig.4.5 contained ll.Owt. % of water, 0.08 wt. % of ethanol and 10 ppm of n-decane. These figures were roughly doubled in the feed to the atmospheric column as some 50% of the methanol content of the raw methanol had been boiled off in the pressurized column. As shown by the diagram, ethanol is withdrawn from the 41st theoretical tray, i.e. below the feed tray, while the hydrocarbon is taken off the 27th tray, i.e. above the feed tray. These are the points where the concentrations of the two substances clearly reach their peaks. The quantity of methanol lost with the Cio is negligible it amounts to less than 0.1 wt. %. Some 0.42 wt. % of methanol go to the fuel system together with the ethanol. 

EF 1693 kgmol/h (48 056 kg/h) at 2056 kfti and -24.4 °C Nf =86 Methane (0.003), Ethylene (0.8388), Ethane (0.1604), Propene (0.0005) Ethylene mole fraction in Overheads = 0.999 Ethane mole fraction in bottoms = 0.995 Column top pressure = 1997 kRa Pressure drop/tray= 0.7 kPla No. of ideal trays = 125 Total condenser Peng-Robinson model... 

Towers, columns- trayed/packed Vacuum system equipment Vessels- pressure, storage Crushers.flakers.mills.stock Evaporators, dryers, cryst. 

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