Staged columns valve trays

The purification unit consists of three valve-tray columns. The production medium for AA in the purification stage at 130-200 °C contains up to 16% water, 26% methyl iodide, and other components, such as methyl acetate (MA), methanol (MeOH), hydrogen iodide (HI), formic acid (FA), and propionic acid (PA) (PEP Report, 1994). The fractionation column removes the light components and portions of water in the mixture, and the dehydration column treats both water and FA. The last column, which is an SSC, produces the final AA product from the side draw by cutting off the remaining light and heavy components from... 

The present column contains 60 actual valve trays on 20-in. spacing that develop 42 theoretical stages of separation. The feed to the column is 61,750 Ib/h, which produces 18,580 Ib/h of distillate containing 0.40 wt % total xylenes, and 43,170 Ib/h of bottoms containing 0.25 wt% toluene. The feed is a superheated liquid that undergoes a 12 wt % flash on entering the column. 

The liquid from the expander is fed to the top of the demethanizer column as external reflux, and the valve outlet stream is fed to an intermediate tray. In this example, the column has 12 theoretical trays and a reboiler (a total of 13 theoretical stages). As shown in Figure 9.1, one stream goes to tray 1 and the other to tray 5, counting trays from the top of the column. Streams 3 and 4 are the overhead and bottoms products, respectively. The column and the reboiler operate at a constant pressure of 2900 kPa. The compositions, flow rates, and thermal conditions of the streams are given in Table 9.1. The process requirement is that the bottoms product should have a maximum of 0.001 mole fraction methane. 

After cooling at an appropriate temperature, the mixture is separated in gas and liquid, both operations simulated by a single flash unit FLASH. The gas phase contains most of methane, with a significant amount of C2, and small C3+. The liquid phase collects most of ethane and C3+ components. The gas is submitted to expansion in the unit EXP, in order to get low temperature and eventually the condensation of C2 and C3+ components. The liquid from the flash is depressurised by means of a valve and sent to separation in a reboiled absorption unit COL, together with the stream issued from expander. The top product is a methane gas, while the bottom product collects ethane and heavier components. Note that the unit COL has 8 trays, with the feeds on top and above the third stage. The column pressure is 22.5 bar. Finally the top gas product is compressed by the unit COMP and sent to the consumer. 

The number of stages in column Cl is set at 31. Notice that because there is no condenser, the top tray is Stage 1. Organic reflux is fed at the top. Recycle is fed at Stage 10. Fresh feed is fed at Stage 15. Column pressure is set at 2 atm because we need a control valve on the overhead vapor Une. 

Next, use a valve (Pressure ChangelValve) to flash the liquid product from the drum to a specified pressure of 3.44 bar. Add a distillation column block (via Col-umnlDistillation) to facilitate water removal from the product leaving the flash valve. Use 12 theoretical stages, a condenser, and a reboiler. Note that when you first create the block, the popup window asks for a number of theoretical trays. However, this is a misnomer, because it counts the condenser and reboiler as theoretical trays. Once you double-click the column to open the specifications form, you can visually verify that there are 12 stages identified. Once you have made the block, add mass streams for the bottoms and distillate and be sure to coimect the distillate stream to the vapor port (so you will have a partial condenser). Specify the top tray as 3.44 bar with no pressure drop in the column in the Pressure Profile tab. Be sure to increase the number of iterations from the default 15 to about 100 (15 is not likely to be enough). Next, in the Feeds and Products tab, select the feed to tray 5 with the feed convention sent to Flash the feed adiabatically..This means that, because the feed is a mixture of vapor and liquid, the vapor portion rises to the tray above (4) and the liquid... 

The most important control loop in this operation is a tray temperature controller at the middle stage of the column. The temperature is controlled at the average temperature of the minimum-boUing azeotropic temperature of the entrainer-water mixture and the normal boiling-point temperature of pure acetic acid. This will ensure that the top vapor of this colurrm will approach the binary azeotrope and the bottoms will approach pure acetic acid. The manipulated variable of this important temperature loop is the organic reflux valve. Thus the accumulation of the organic phase will be automatically adjusted throughout the batch mn. 

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