Tower Pressure - Ejectors

The steam ejectors pump away the remaining vapor pressure of water, hydrocarbons and inerts. Ejector systems typically have two stages or three by 50% ejectors. Because of the criticality for tower operation most systems are overdesigned and it may be possible for the tower to operate with one 50% ejector in each stage. Intercondensers (1st stage) and after condensers (2nd stage) condense the steam from the ejectors, tower steam and condensable hydrocarbons. Motive steam flow must be maintained for best operation. 

Viscosity Yield on Grade Viscosity Yield on Crude 

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The bottoms of the CD, also known as atmospheric residue, are charged to a second fired heater where the typical outlet temperature is about 750-775°F. From the second heater, the atmospheric residue is sent to a vacuum tower. Steam ejectors are used to create the vacuum so that the absolute pressure can be as low as 30-40 mm Hg (about 7.0 psia). The vacuum permits hydrocarbons to be vaporized at temperatures below their normal boiling point. Thus, the fractions with normal boiling points above 650°F can be separated by vacuum distillation without causing thermal cracking. In this example (Fig. 18.14), the distillate is condensed into two sections and withdrawn as two sidestreams. The two side-streams are combined to form cracking feedstocks vacuum gas oil (VGO) and asphalt base stock. 

If an ejector is not overloaded at a normal gas rate, reducing the gas load will not result in greatly improved vacuum. The ejector is simply oversized at the lower charge rate and wastes steam without obtaining any appreciable benefit in lower vacuum tower pressure (see Chapter 13). To save this wasted steam, new ejector internals are needed. The internals... 

The key tool in troubleshooting flash-zone pressure problems is a vacuum-tower pressure survey. The time to initiate this survey is just after start-up when the trays, demister, and ejector system are clean and in good condition. Pressures are best measured with a portable mercury-filled vacuum manometer. Using a vacuum pressure gauge will reduce the accuracy of observed pressure drops. Relying on permanently installed gauges for pressure drop data will not give reliable results. 

Tower pressure survey Normalized pressure survey Coke buildup on demister Flooding of PAR trays Ejector deficient Excess thermal cracking... 

Tower pressure cycling may be due to steam ejector underload and high ejector discharge pressure. 

Gas ejectors (an ejector is a vent for removing non-condensable gases from the condensing unit located downstream from the turbine. Condensate from the cooling towers is pumped to the condenser unit to condense the steam leaving the turbine. In this way, low pressure is created downstream from the turbine, which leads to improved efficiency to generate electric... 

The vapors leaving the primary barometric condenser proceed to a steam ejector that is followed by another barometric. Pressures at the tops of the towers are maintained at 50mmHg absolute. Pressure drop is 2mmHg per tray. Bottom temperatures of the three towers are 450, 500, and 540°F, respectively. Tower overhead temperatures are 200°F. Pitch and roan go to storage at 350°F and the other products at 125°F. The steam generated in the pitch and rosin coolers is at 20 psig. Process steam is at 150 psig. 

The principles and main units for vacuum rectification resemble those for atmospheric rectification. The major exceptions are that larger-diameter towers are used to maintain comparable vapor velocities at reduced operating pressures. A vacuum of 50 to 100 mm of Hg absolute is produced by a vacuum pump or steam ejector. The capacity of modern vacuum rectification units is about 3.5 million tons per annum. 

The process gases are treated differently. All chemical vessels are vented under negative pressure up to 10 in. of water by steam-jet ejectors or stainless-steel blowers. The discharge from these blowers is generally sent to scrubbing or filter-bed towers to remove certain gases and/or entrained liquid and solid particles. The filter is made up of Fiberglas mats which have a 99.9 per cent efficiency for removal of particles over... 

A rather sudden loss in vacuum or failure to reestablish normal tower top pressure after a unit turnaround is most likely a consequence of an air leak. Air or oxygen compounds (CO, CO2) in the ejector tail gas are the usual indication. 

If CO or CO2 is present, the air leak is probably in the hot part of the vacuum tower or the transfer line. Look for a leak in the ejector or vacuum-tower overhead piping if oxygen is present in the tail gas. On several units, leaks in the seal leg piping have proven troublesome. A sketch of a single-stage ejector system (Fig. 13-6) shows the iocation of the seal leg piping. Use aerosol shave cream to test for vacuum leaks. When all else fails, pressure the tower to 3-5 psig. The leak can then be located by the noise it will emit. 

Regardless of ejector capacity, if the vacuum tower uses stripping steam, the pressure at the top of the tower cannot be lower than the vapor pressure of water at the precondenser outlet temperature. 

The production of cracked gas in the heater is largely a function of the peak temperature developed inside the heater coils. When the peak temperature is suppressed, the load of cracked gas to the vacuum tower overhead steam ejectors is reduced. The ejectors can, therefore, pull a deeper vacuum, lowering the tower flash-zone pressure and increasing gas oil recovery. 

Vacuum tower wash oil, 165 Vacuum lowers, 281—300 bottoms-pump suction pressure loss, 281— 285 high flash-zone pressure, 285-288 ejector problems, 288-292 black gas oil, 292-293 trim gas oil production, 293 pumparound draw temperatures, 293 light resid, 294 steam-to-healer passes, 295-297 gas-oil recovery improvement, 297-298 transfer-line failures, 298-299 troubleshooting problems, 299-300... 

A high vacuum-tower top pressure is a result of air leaks, excessive production of hydrocarbon gases due to thermal cracking, or a host of ejector deficiencies. 

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