MAIN COLUMN (12-C-201)

PROCESS DISCRIPTION, PRINCIPLES FLOWS AND CONTROLS &RELATED EQUIPMENT

The Main Column is the first step in the product separation sequence. The superheated reactor vapors need to be cooled so that fractionation can be conducted. In large measure, operation of the Main Column (12-C-201) becomes an exercise in controlled heat removal coupled with sufficient liquid-vapor contacting to effect the desired degree of fractionation into desired product streams, namely main column bottoms (MCB), heavy cycle oil (HCO), light cycle oil (LCO), heavy naphtha (HCN), unstabilized gasoline and wet gas. MCB, HCO and HCN are taken as products directly from the main column, although on many FCC units HCO and HCN are not removed from the unit as discreet net product streams. Main column sidecut products are often steam-stripped in sidecut strippers for flash point adjustment. The unstabilized gasoline and wet gas are subjected to further separation in the gas concentration section. The arrangement and integration of heat exchange from FCC main column varies from refinery to refinery based on the specific requirements and economics of a given installation. The following discussion describes typical heat exchange circuits used on a FCC unit. Figure 13 shows a simplified FCC main column schematic.

MAIN COLUMN BOTTOMS PUMPAROUND CIRCUIT

The main column bottoms system is designed to de-superheat the reactor vapors, condense the bottoms product and scrub entrained catalyst particle fines from the reactor product gases. Main column bottoms (MCB) is removed from the bottom of the main column and sent to a circulating bottoms/raw oil exchanger and two steam generators. The vapors are de-superheated by circulating a large stream of cooled column bottoms over the disk and donut trays where it acts to de-superheat the reactor vapors as well as flush catalyst fines out of the column. This is shown in Figure 13. The cooled bottoms circulation rate should be set at not less than 150% of the design feed rate. The bottoms temperature is maintained at 350-360oC to minimize coking in the slurry circuit. Coking rate is a function of time and temperature, thus the pumparound rate and heat removal must be high enough to keep the bottoms temperature below the 360oC maximum. A cooled stream of circulating main column bottoms, taken from the outlet of the steam generators can be returned as quench to the main column bottom to maintain this temperature when necessary. The minimum spillback is provided for minimum slurry circulation during turndown operation. This ensures enough oil is returned to the main column adequately covering the disc and donut trays, thereby cleansing the reactor vapors of catalyst fines and preventing coke formation on the disk and donut trays due to insufficient liquid flow over the trays.

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During normal operations the heat input from the circulating main column bottoms to the feed preheat exchangers is set by product and process considerations. Heat input to the steam generators is the only variable available to the operator for adjustment. Bottoms flow to the steam generators can be adjusted to change the amount of heat rising up the column thus increasing or decreasing the column top reflux rate which acts to heat balance the column.Each exchanger in the main column bottoms pumparound circuit is designed for oil containing catalyst particles. Main column bottoms circulation flows through the exchangers on the tube side and the velocity must be kept between 1.14 m/s and 2.13 m/s (3.75 ft/s and 7.0 ft/s) for straight tubes and between 1.14 m/s and 1.75 m/s (3.75 ft/s and 5.75 ft/s) for straight tubes and between 1.14 m/s and 1.75 m/s (3.75 ft/s and 5.75 ft/s) for U-tubes. If the oil velocity falls below the minimum, catalyst will begin to accumulate on the tube walls and slowly plug the tube while greatly reducing heat transfer. If the velocity is above 2.13 m/s (7 ft/s), the tube walls may experience erosion leading to premature failure. The rates required to meet these velocities must be calculated for each exchanger before startup.

Figure 13Main Column Bottoms Circulation and Product

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Bottoms product is withdrawn to maintain a steady bottoms level in the main

column. The product stream is cooled by heat exchange with a charge preheat

exchanger and a water cooler. Bottoms product flows through the tubes in the feed

preheat exchanger with the above mentioned velocity constraints. Removal of

catalyst fines from the bottoms product is an option which may be included

depending on the end use of the bottoms product.

HCO PUMPAROUND CIRCUIT

HCO is withdrawn from the column and used to provide heat to the Debutanizer

Reboiler (13-E-312). HCO flow to this exchanger is regulated by a flow controller.

The controller is set by the operator to achieve desired product or process

specification.

The HCO circulation rates also provide proper reflux in the HCO section of the

column. A decrease in heat removal from the HCO circuit will require an increase in

heat removal elsewhere in the column. For example, if less heat is required by the

debutanizer reboiler, HCO circulation will be decreased. Lower circulation reduces

reflux to the HCO section of the main column, increasing the amount of vapor rising

up the column. If the excess heat is not removed in another pumparound circuit, the

reflux to the top of the main column will increase to remove this heat.

The HCO circuit also provides flush oil to the Main Column Bottoms Pumps (12-

TP-211 A/B). A spillback to the main column is provided for control of internal

reflux. The spillback operates off a level controller on the HCO drawoff tray.

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Figure 14Heavy Cycle Oil Circulation

LCO PUMPAROUND AND PRODUCT CIRCUIT

Circulating LCO provides heat to Stripper Reboiler (13-E-309) and Debutanizer Feed Exchanger (13-E-311). Flow to the exchanger is regulated by a flow controller that is set according to process requirements. A stream of LCO, after heat exchange at the circulating Stripper Reboiler is sent to the Sponge Absorber (13-C-302) as “lean” oil to absorb light gasoline range components from the light gas in the gas

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concentration section. The “rich” oil from the sponge absorber is returned to the main column with the cooled LCO circulation stream, providing internal reflux for the main column LCO section.

This unit contains an LCO stripper for LCO product flash point adjustment. The light ends in the LCO are removed by steam stripping. The LCO product is cooled in an air-fin-exchanger, Light Cycle Oil Product Cooler (12-EA-202) and Light Cycle Oil Product Trim Cooler (12-E-207) and sent to storage (82-G-204). Flow to storage is set by a flow controller (12-FIC-226). The cooled LCO product can also be used as cutter stock to the heavy fuel oil static mixer. The amount of LCO produced depends upon operating conditions, feed quality and catalyst type. Stripped LCO is available for use as instrument flush, flushing oil, and pump flush.

Figure 15Light Cycle Oil Circulation and Product

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NAPHTHA PUMPAROUND AND PRODUCT CIRCUIT

Naphtha product is pumped and used for heat exchange with the feed coming in from the Raw Oil Feed Surge Drum (12-V-201) on flow control and returned to the main column as internal reflux for the main column naphtha section. Figure 16 shows a flow scheme for the naphtha pumparound circuit.

Main Column Overhead System

Reactor product vapors contain large quantities of light gas and gasoline vapors, which pass through the entire main column as saturated gases. These products are condensed in the Main Column Condenser (12-EA-203), the Main Column Trim Condenser (12-E-208) and separated in the Main Column Receiver (12-V-202). A quantity of the condensed hydrocarbon liquid (unstabilized gasoline) is pumped to the main column as reflux (see Figure 17).

Reflux to the main column controls the overhead vapor temperature. This temperature determines the endpoint of the gasoline product. The reflux also heat balances the column. If heat removal from one section is changed the reflux rate will respond in the opposite direction to maintain a constant top temperature. For example, if more heat is required for feed heat up, heavy naphtha circulation will be increased to provide the heat. The increase in heavy naphtha circulation will condense more vapor rising up the column. Less heat will reach the top of the column. The top temperature controller will then reduce the reflux flow to maintain the column top temperature, thus heat balancing the column.

The unstabilized gasoline liquid in the receiver not used as reflux is pumped to the gas concentration unit on receiver level control. Gas flows to the suction drum of the wet gas compressor in the gas concentration unit. Water from the overhead receiver is pumped to the sour water treating unit.

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Figure 16Heavy Naphtha Circulation and Product

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Figure 17Main Column Overhead System

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Figure 2FCC Main Column Flow and Control

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RAW OIL CHARGE SYSTEM

The Raw Oil Charge System provides feed to the unit, removes heat from the

fractionation section, and heats the charge to the desired reactor riser inlet temperature.

Feed enters the charge drum directly from one or more hot or cold feed sources. The

charge drum acts as a surge control to dampen process flow variations from upstream

units. Drum pressure floats on the main column pressure through a line connected to the

tower in the vapor space above the HCO draw tray. Drum level control is maintained by a

level controller that signals valves on the feed streams.

The raw oil is pumped on flow control through a series of heat exchangers, called the

feed preheat train, prior to entering the reactor riser through the feed nozzles. The raw is

heated by exchanging with the circulating Heavy Naphtha, net MCB, and circulating

MCB. Usually all streams except the circulating main column bottoms are flow

controlled by the operator. The final raw oil temperature is controlled by passing some oil

around the circulating main column bottoms exchanger. Figure 3 shows a typical feed

preheat train.

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Figure 3Feed Preheat System

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FEED BYPASS CONTROL

Many emergency situations encountered on FCC units call for quick diversion of the feed from the riser. Figure 4 shows the normal arrangement of the feed to riser bypass system. When required, a control board mounted switch is activated which trips a solenoid valve controlling the pneumatic signals to the feed bypass valves, causing these valves to move to their failure positions, i.e. the valve in the line to the riser closes and the valve in the bypass line to the main column opens. Normally, the next course of action is to open the emergency steam to the riser wye to either maintain catalyst circulation if the regenerated catalyst slide valve remains open or to clear the riser of catalyst if the regenerated catalyst slide valve is closed.

Figure 4Feed to Riser Bypass System

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MAIN COLUMN AND RELATED EQUIPMENT The column as shown in Figure 23 may be divided into two sections: the regular fractionator and the lower disc and doughnut trays.

Figure 23Main Column (12-C-201)

The bottom six trays are designed for high vapor and liquid flow rates. The trays are shown schematically in Figure 24. A coke trap, shown in Figure 25, prevents large pieces of coke or other debris from entering the bottom line.

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A steam ring may be used on some units to steam strip the bottoms material of any lighter hydrocarbons. The column is fitted with three sample lines that are normally called try lines; the bottom try line is also used as a sample line. These are used to check the bottoms level because the level glass and indicator are subject to occasional plugging by fines or coke particles. Both level instruments are protected with an LCO flush that flows into the level instrument from the top.

The FCC main column is mounted on a table top instead of the skirt mounting seen on most columns. Tabletop mounting simplifies maintenance work on the bottoms lines, which are subject to plugging from coke or catalyst fines. A separate suction line to each pump allows operation to continue through one line while the other is cleaned. As an additional note, every valve in the bottoms circuit should be installed with the stem up to keep catalyst fines out of the bonnet. If plant layout does not allow a stem to be straight up, it should be as close to the vertical position as possible.

The upper part of the tower is similar to any other fractionator. Sidecut streams must be stripped to remove absorbed light material. Stacked heavy cycle oil and light cycle oil strippers are shown in Figure 26. Steam is the most common stripping method, although reboiler strippers are sometimes used. Reboiled strippers decrease the amount of sour water that must be treated. The overhead vent from each stripper returns to the main column in a vapor space. The overhead from the column is condensed by fin-fan air coolers or tube-and-shell water coolers, normally a combination of the two. Water from the gas concentration section washes the ammonia chloride and some other salts from the condensers. The sour water is separated in the overhead receiver and sent to a sour water stripper. Gasoline and gas go to the gas concentration section, with some gasoline returned to the tower as reflux.

The main column is made of regular or killed carbon steel, with a 1/8 inch (3 mm) thick Type 405 or 410 (12 Cr) cladding from below the naphtha or LCO draw tray to the bottom outlet. Any nozzle or manways in this clad section should also be alloy lined. All the trays and caps in the column are Type 410 stainless.

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Figure 24Typical Disc And Donut Pans

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Figure 25Typical Coke Trap

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Figure 26(Stripper Arrangement)

Heavy Cycle Oil Stripper (Bottom)Light Cycle Oil Stripper (Top)

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MAIN COLUMN BOTTOMS

The main column bottoms pumps are detailed in Figure 27. The minimum distances shown are important to prevent catalyst fines from setting in al line that is not in service, allowing it to plug.

Figure 27Main Column Bottoms Pumps

The suction and discharge lines of the main column bottoms pumps range from 5 Cr, 1/2 Mo to 9 Cr, 1 Mo. The exchanger metallurgy ranges from carbon steel to 5 Cr, 1/2 Mo. Carbon steel does not last long if it is exposed to hot main column bottoms so the slurry service side of the tubes, tube sheets, and heads should be clad with a chrome alloy, such as Type 410. After the first exchanger, most of the piping is carbon steel.

The recommended main column bottoms velocities through the tubes are 1.14 to 2.13 m/s (3.75 to 7.0 ft/sec) for straight tubes and 1.14 to 1.75 m/s (3.75 to 5.75 ft/sec) for U-tubes. Deviations outside of these ranges will show up as excessive amounts of settled catalyst fines or erosion to the tubes.

The Main Column Bottoms Circulation Pumps (12-TP-211 A/B) are turbine driven. Speed is limited to 2000 rpm to minimize erosion. The shaft is tungsten carbide or

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stellite coated, with a light cycle oil flush to the throat bushing and wear rings and HCO to the mechanical seal. SKEC recommends aluminum foil packing with light cycle oil flush for cooling and lubrication. Graphite or asbestos have occasionally been used in this service. Many refiners have installed mechanical seals with good success.

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