Tower Wash Section

The wash section cleans entrained liquids from the flash zone vapor phase. Vapor in excess of the amount needed to meet distillate requirements is referred to as overflash. The wash section condenses the overflash. It also provides some fractionation between the heavy lube sidestream and the vacuum resid stream. 

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To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

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 there is a question of losing a product such as an amine chemical, say diethanolamine, in an amine gas absorber overhead KO vessel, use this recommended 150-pm liquid particle sizing in the gas phase. I have personally witnessed hundreds of thousands of dollars of annual amine chemical losses in numerous amine gas-treating plants due to poor overhead KO drum design. Spend a few more very well justified dollars at design time and realize a payout of only a few weeks for this added expense I have witnessed 1.5 lb amine loss per million scf gas processed in an amine absorber overhead KO drum. For a 150-mmscfd gas plant absorber, 355 days per year production, at 1.20 per pound amine chemical cost, this computes to a 95,750 yearly loss. This is not a new discovery, as many an amine absorber installed in the 1950s had several trays in the tower top section dedicated to a water-wash section. I am confident that equal losses can be computed for other chemicals or petroleum products in similar fractionation overheads. 

Qince the first large sulfur dioxide control system was installed at the Battersea plant in London, it has taken almost 50 yrs for calcium-based scrubbing technology to become commercially acceptable. In 1926, the 125 MW coal-fired Battersea power plant was equipped with a spray packed tower and final alkaline wash section which removed more than 90% of the sulfur dioxide and particulate (I). Thames River water provided most of the alkali for absorption, and about 20% was made up from lime addition. The process operated in an open-loop manner, returning spent reagent to the Thames. 

Sight glasses Caustic-wash-tower top-section spray arrestor Submerged glandless circulating pump... 

The recently replaced gas purification section was already operating outside its design parameters because of the extra heat load placed on the system as a result of increasing the overbed water usage in the roaster. The plate heat exchangers of the humidifying tower/wash... 

Table 8-4 summarizes the product distillations for the FCCU fractionator. These distillations are consistent with the vapor and liquid flows presented in Table 8-1. The degree of separation between the tower overhead and LCO products is represented by the ASTM 5% to 95% gap of —2°F. Based on the Packie method, calculations indicate that there are 10 effective trays above the LCO draw tray. This is equivalent to roughly six theoretical separation stages. Since each pumparound section is usually represented as a single stage, we can see by referring to Figure 8-2 that the naphtha wash section is equivalent to four theoretical separation stages.

When troubleshooting a carry-over problem for a wash tower, first calculate the vertical velocity of the discontinuous phase, propane in this case. Divide the volume of propane in cubic feet per minute by the tower cross-sectional area. For velocities of less than 1.0 ft/min, entrainment should be negligible. For 1.2-1.6 ft/min, entrainment may be marginal. For more than 1.8 ft/min, excessive entrainment can be expected. 

The gas stream leaving the quench tower mainly consists of ethylene. The gas is compressed in a multistage compressor with intercooUng and then fed to the caustic tower, where CO is absorbed with a sodium hydroxide (NaOH) solution. NaOH in the gas is removed in a water wash section in the tower. The remaining water in the gas stream is removed in a molecular sieve dryer before the gas enters the final purification stage. 

In the feed preparation section, those materials are removed from the reactor feed which would either poison the catalyst or which would give rise to compounds detrimental to product quality. Hydrogen sulfide is removed in the DBA tower, and mercaptans are taken out in the caustic wash. The water wash removes traces of caustic and DBA, both of which are serious catalyst poisons. Also, the water wash is used to control the water content of the reactor feed (which has to be kept at a predetermined level to keep the polymerization catalyst properly hydrated) and remove NH3, which would poison the catalyst. Diolefins and oxygen should also be kept out of poly feed for good operation. 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

All streams leaving the extractive distillation sections are water washed to remove entrained ACN, and the ACN is recovered by distillation. Spent Cj s from the first stage distillation tower overhead may be recycled to a steam cracking unit. This material gives excellent butadiene yields. 

This is the first of the coffee decaffeination patents that describe a continuous, counter-current liquid-liquid extraction. A brief description of the process is provided here. A water extract of roasted coffee beans, called coffee liquor, which contains aromas and caffeine and other water soluble components such as carbohydrate and protein materials is fed to a vacuum suipper. The extract is concentrated to about 30-50% in an evaporator-condenser and is fed to a sieve tray tower. The liquor passes across the hays in the tower downward through downspouts countercurrent to supercritical CO2 which enters the tower at the bottom and passes upward through the holes in the sieve trays. CO2 extracts caffeine from the liquor, and the decaffeinated liquor leaves the near the bottom of tower. The condensate water from the vacuum stripper prior to the tray tower is fed to the sieve trays in the top section of the tower. The water washes the caffeine from the supercritical CO2 passing upward. The caffeine-free CO2 is recycled to the bottom of the column. 

The methanol/water solution leaving the washing tower is fed to the alcohol recovery section (5), where high-quality methanol is recovered. 

The author is familiar with a case where caustic from a wash tower backflowed into an entirely different section of the plant. The other section was normally at a higher pressure, but not at startup. The caustic attacked an aluminum heat exchanger in that section, resulting in a fire. Piping was later changed to prevent recurrence. 

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