Adiabatic operations Recycle reactor

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

Heat transfer in the radial direction is poor, so large reactors are best suited to adiabatic operation. If the temperature rise is significant, it may be controlled in some processes by recycling the liquid product or introducing a quench stream at an intermediate point. 

Since the thermal effect of the reactions is very high (see the process parameters in Tables 7.1 and 7.2), the adiabatic operation of the reactor is not possible. In order to control the reactor temperature, the reaction mass is recycled at a high rate through the heat exchanger. 

The H-Oil reactor (Fig. 21) is rather unique and is called an ebullated bed catalytic reactor. A recycle pump, located either internally or externally, circulates the reactor fluids down through a central downcomer and then upward through a distributor plate and into the ebullated catalyst bed. The reactor is usually well insulated and operated adiabatically. Frequently, the reactor-mixing pattern is defined as backmixed, but this is not strictly true. A better description of the flow pattern is dispersed plug flow with recycle. Thus, the reactor equations for the axial dispersion model are modified appropriately to account for recycle conditions. 

For comparison, for adiabatic plug-flow reactor (R = 0) of the same volume, the outlet extent is Zout — 0.742, and 9out =1.135. The production rate of product B is 1,131 mol/min. For adiabatic CSTR R = oo) of the same volume, the outlet extent is Zout = 0.841, and 9out= 1.132. The production rate of product B is 1260 mol/min. Note that for both isothermal and adiabatic operation, a recycle reactor provides a higher production rate of product B than a corresponding plug-flow reactor and a CSTR. 

Reactor Product Column Purge/Separator Recycle Column Stabilizer Column Compressor 5 1 H2 TL ratio production rate of 265 Ibmole/hr 80% conversion TL reactor effluent temperature of 1150°F reactor inlet temperature of 1150°F reactor pressure of 500 psia reactor outlet temperature 1300°F flexibility adiabatic operation Production rate of 265 Ibmole/hr product purity flexibility Minimize Hz losses flexibility eeonomics Minimize TL and benzene losses flexibility economics Flexibility economics Flexibility economics ... 

Four pilot plant experiments were conducted at 300 psig and up to 475°C maximum temperature in a 3.07-in. i.d. adiabatic hot gas recycle methanation reactor. Two catalysts were used parallel plates coated with Raney nickel and precipitated nickel pellets. Pressure drop across the parallel plates was about 1/15 that across the bed of pellets. Fresh feed gas containing 75% H2 and 24% CO was fed at up to 3000/hr space velocity. CO concentrations in the product gas ranged from less than 0.1% to 4%. Best performance was achieved with the Raney-nickel-coated plates which yielded 32 mscf CHh/lb Raney nickel during 2307 hrs of operation. Carbon and iron deposition and nickel carbide formation were suspected causes of catalyst deactivation. 

Nickel catalysts were used in most of the methanation catalytic studies they have a rather wide range of operating temperatures, approximately 260°-538°C. Operation of the catalytic reactors at 482°-538°C will ultimately result in carbon deposition and rapid deactivation of the catalysts (10). Reactions below 260°C will usually result in formation of nickel carbonyl and also in rapid deactivation of the catalysts. The best operating range for most fixed-bed nickel catalysts is 288°-482 °C. Several schemes have been proposed to limit the maximum temperature in adiabatic catalytic reactors to 482°C, and IGT has developed a cold-gas recycle process that utilizes a series of fixed-bed adiabatic catalytic reactors to maintain this temperature control. 

There is no separate shift conversion system and no recycle of product gas for temperature control (see Figure 1). Rather, this system is designed to operate adiabatically at elevated temperatures with sufficient steam addition to cause the shift reaction to occur over a nickel catalyst while avoiding carbon formation. The refractory lined reactors contain fixed catalyst beds and are of conventional design. The reactors can be of the minimum diameter for a given plant capacity since the process gas passes through once only with no recycle. Less steam is used than is conventional for shift conversion alone, and the catalyst is of standard ring size (% X %= in). 

Dr. Moeller I think to answer this question now is a bit difficult. It s just a mechanical problem of the maximum temperature the recycle compressor can handle. So, in the end, we will go to the inlet temperature to the compressor in the range of the inlet temperature to the reactor. So what we are endeavoring to attain is a simple reaction system consisting of an adiabatic reactor in series with waste heat boilers and nothing more than one recycle compressor. These compressors are used in the chemical industry with no problem in operation. So, in the end, you can go to hot recycle with an inlet compressor temperature the same as the inlet reactor temperature. All the heat from... 

One common method for managing the adiabatic temperature rise in fixed-bed reactors is to dilute the reactant stream with a gas that would provide the mass to absorb the heat of reaction. Light hydrocarbon reaction products (methane, ethane, propane) can be recycled for this purpose. However, the costs associated with the recycle operation could be substantial if the reaction is strongly exothermic. Special attention must be given to reduce the recycle ratio. 

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