In the two-stage scheme, the unconverted material from the first stage becomes feed to a second hydrocracker unit. In this case, the feed is already purified by the removal of sulfur, nitrogen, and other impurities; and the second-stage can convert a larger percentage of feed with better product quality.
A heavy gas oil feed contains some very high-boiling aromatic molecules. These are difficult to crack and, in feed recycling operation, tend to concentrate in the recycle itself. High concentration of these molecules increase the catalyst fouling rate. In a two-stage operation, the first stage is a once-through operation; hence, the aromatic molecules get no chance to concentrate, since there is no recycle. The first stage also reduces the concentration of these molecules in the feed to the second stage; therefore, the second stage also sees lower concentration of these high-boiling aromatic molecules.
The two-stage operation produces less light gases and consumes less hydrogen per barrel of feed. Generally, the best product qualities (lowest mercaptans, highest smoke point, and lowest pour point) are produced from the second stage of the two-stage process. The poorest qualities are from the first stage. The combined product from the two stages is similar to that from a single stage with recycling for the same feed quality.
The two-stage scheme allows more flexible adjustment of operating conditions, and the distribution between the naphtha and middle distillate is more flexible. Compared to partial and total recycling schemes, the two-stage scheme requires a higher investment but is overall more economical.
The oil feed to the reactor section consists of two or more streams (see Figure 3-3). One stream is a vacuum gas oil (VGO) feed from the storage tank and the other stream may be the VGO direct from the vacuum distillation unit. Also, there may be an optional recycling stream consisting of unconverted material from fractionator bottom. The combined feed is filtered in filters F-01 to remove most of the particulate matter that could plug the catalyst beds and cause pressure drop problems in the reactor. After the oil has passed through surge drum V-02, it is pumped to the reactor system pressure by feed pump P-01.
Hydrogen-rich recycled gas from the recycling compressor is combined with oil feed upstream of effluent/feed exchangers E-01/02. The oil gas stream than flows through the tube side of exchanger 02A and 02B, where it is heated by exchange with hot reactor effluent. Downstream of the feed effluent exchangers, the mixture is further heated in parallel passes through reactor feed heater H-01. The reactor inlet temperature is controlled by the Temperature Recorder and Controller (TRC) by controlling the burner fuel flow to the furnace.
A portion of the oil feed is by passed around the feed effluent exchanger. This bypass reduces the exchanger duty while maintaining the duty of reactor feed heater H-01 at a level high enough for good control of reactor inlet temperature. For good control, a minimum of 50-75°F temperature rise across the heater is required.
Makeup hydrogen is heated on the tube side of exchanger E-01 by the reactor effluent. This makeup hydrogen then flows to the reactor.
Hydrocracker reactor V-01 is generally a bottle-type reactor. The makeup hydrogen after preheating in exchangers E-01 flows up through the reactor in the annular space between the reactor outside shell and an inside bottle. The hydrogen acts as a purge to prevent H2S from accumulating in the annular space between the bottle and outside shell. It also insulates the reactor shell.
After the makeup hydrogen has passed upward through the reactor, it combines with the recycled gas and the heated oil feed from the feed heater in the top head of the reactor. The hot, vaporized reaction mixture then passes down the reactor. Cold quenching gas from the recycling compressor is injected to the reactor between the catalyst beds to limit the temperature rise produced by exothermic reactions.
The reactor is divided among a number of unequal catalyst beds. This is done to give approximately the same temperature rise in each catalyst
bed and limit the temperature rise to 50°F. Thus, the first and second beds may contain 10 and 15% of the total catalyst, while the third and fourth beds contain 30 and 45% of the total catalyst.
Reactor internals are provided between the catalyst beds to ensure thorough mixing of the reactants with quench and ensure good distribution of vapor and liquid flowing to each bed. Good distribution of reactants is of utmost importance to prevent hot spots and maximize catalyst life.
Directly under the reactor inlet nozzle is a feed distributor cone inside a screened inlet basket. These internal elements initiate feed stream distribution and catch debris entering the reactor. Below the inlet basket, the feed stream passes through a perforated plate and distributor tray for further distribution before entering the first catalyst bed.
Interbed internal equipment consists of the following:
• A catalyst support grid, which supports the catalyst in the first bed, covered with a wire screen.
• A quench ring, which disperses quenching gas into hot reactants from the bed above.
• A perforated plate for gross distribution of quenched reaction mixture.
• A distribution tray for final distribution of quenched reaction mixture before it enters the next catalyst bed.
• A catalyst drain pipe, which passes through interbed elements and connects each catalyst bed with the one below it.
To unload the catalyst charge, the catalyst from the bottom bed is drained through a catalyst drain nozzle, provided in the bottom head of the reactor. Each bed then drains into the next lower bed through the bed drain pipe, so that nearly all the catalyst charge can be removed with a minimum of effort.
Differential pressure indicators are provided to continuously measure pressure drop across top reactor beds and the entire reactor.
The reactor is provided with thermocouples located to allow observation of catalyst temperature both axially and circumferentially. Thermocouples are located at the top and bottom of each bed. The temperature measured at the same elevation but different circumferential positions in the bed indicate the location and extent of channeling through the beds.
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