Startup pressure columns

Horizontal shell-and-tube condenser with liquid coolant in the tubes and vapor on the shell side (Figure 3.1). This is probably the most popular type in petroleum refineries. By comparison with the vertical design discussed below, it is much better suited to partially flooded operation. In addition, at startup time, column inerts are usually vented more easily (i.e., with less pressure drop) through condensers of this design. 

An air pocket in the pump can cause low or no pressure or flow, erratic pressure, and changes in retention time data. It may be necessary to bleed air from the pump or prime the pump according to system startup procedures. Air pockets in the column will mean decreased contact with the stationary phase and thus shorter retention times and decreased resolution. Tailing and peak splitting on the chromatogram may also occur due to air in the column. Air bubbles in the detector flow cell are usually manifested on... 

The use of packing instead of trays in multistage separation columns is common for column diameters 3-4 ft or smaller. More recently, packing has been used for larger columns because of its low pressure drops, favorable efficiencies, and high vapor capacity. Packed columns are also the preferred choice where corrosion is a potential problem. The packing material used in these situations is ceramic or polymeric. Another characteristic of packed columns is their low liquid holdup, which could reduce the amounts of off-specification products at startup and shutdown. 

Once inert gas purging and leak testing are completed (in either a pressure or a vacuum tower), the column is usually depressxired to a small positive pressime (5 to 10 psig) until the plant is ready to proceed with the startup. 

Once steaming is completed, the column should be opened to let air or inert gas in on shutdown, or placed under gas pressure (inert gas such as nitrogen or fuel gas, or process gas) at startup. The alternative of leaving small steam purges to keep the column pressure up at the conclusion of steaming is often unsatisfactory and resulted in creation of vacuum in some instances (3). Unless the column is fully open to the atmosphere, so that no vacuum can be created, a pressure of at least 5 to 10 psig should be kept in it (3). 

When the column initially contains an inert gas, the flash temperature is considerably higher when the column is pressured than when the column is depressured (compare Fig. 12.26 and c). On the other hand, overchilling at an elevated pressure can at times be more detrimental to the metal than overchilling at atmospheric pressure, because the minimum safe working temperature of the metal increases with column pressure. Both effects should be considered when devising the startup strategy and the only available gas is inert in most cases, it is still better to pressure up. 

Problems of establishing a liquid seal in the column downcomers are common in low-liquid-rate services during startup. In such services, the downcomer liquid seal may also occasionally be lost during operation. Symptoms of these problems are excessive entrainment and poor separation. As distinct from flooding, column pressure drop may remain low. These problems, and cures that can be implemented at the design stage, are described in Sec. 6.18. 

Rgur 12.9 Effect of changing column pressure on the startup stability region (From Henry Z. Kister, excerpted by special permission from Chemical Engineering, April 6,1981 copyright by McGraw-Hill, Inc., New York, N Y10020.)... 

When the column feed passes through a heater (e.g., a refinery fractionator or vacuum tower), any water lying at low points in the coils must be blown out prior to startup. In multipass coils, water must be separately blown out of each pass block valves are sometimes installed on each pass to permit this (7). If blowing into the tower, it must be performed when the tower can still tolerate water. The coils must be kept hot and/or purged from then on to prevent condensation. One pressure surge incident (7) occurred when water accumulated in one heater pass entered a refinery vacuum tower which was under full vacuum and circulating 280°F oil. 

A good dryout procedure, which includes draining all water pockets from the system, should be followed during startup. This is described in detail in Sec. 11.10. This is most important if the column was steamed out beforehand (e.g., to drive the air out). Several pressure surge incidents could have been avoided if such a procedure was adequately carried out (4, 5, 7, 23, 237, 296). 

Late one night, a brand new distillation column was wrecked by vacuum and toppled as a water filled flooded column was being pumped out. The column was designed and equipped with sufficient under/over pressure protection during normal operations, but the system was ill equipped to deal with flooded columns at startup. This was a newly constmcted plant, which had been in the pre-startup phases. 

The cold and the Demethanizer column section are cooled down and pressurized. Deethanizer vapors are flared at the overhead of Deethanizer reflux drum until Acetylene content is zero in the Acetylene reactor effluent. Effluents are diverted to C2 splitter column and flaring is gradually discontinued. The flaring operation at startup and shut-down typically lasts 7-8 days at QChem s process plants. This is typical of most processing plants in Qatar and elsewhere. 

Apply heat to the flask at such a rate that vapors reach the top of the column between 20 and 50 min after startup. Adjust heat input so as to achieve a pressure drop of less than 0.13 kPa/m (1.0 mm Hg/m) in packed columns or less than 0.065 kPa (0.5 mm Hg) in r plate columns. Program automated equipment according to the above directions. Turn on the stirring device if used. 

If further cuts at higher temperatures are required, distillation can be continued at reduced pressures, subject to the maximum temperature that the boiling liquid will stand without significant cracking. This is about 310°C in most cases. Notable exceptions are crude oils containing heat-sensitive sulfur compounds. In any case, do not make a cut within 5 C of the temperature at startup because the column will not be at equilibrium. 

---