Synthesis reactions heat removal

Figure 5.16 A tubular turbulent divergent-convergent device for alkylbenzene synthesis. 1 - reactor vessel 2 - convergent element 3 - divergent element 4 -cylindrical element 5 - jacket for heating of the initial reaction mixture 6 - jacket for reaction heat removal 7, 8, 9,10, 11 - tubes for the introduction of dry benzene, polyalkylbenzenes, recycled benzene, ethylene, and catalj ic complex, respectively ...

The Fischer-Tropsch reaction is highly exothermic. Therefore, adequate heat removal is critical. High temperatures residt in high yields of methane, as well as coking and sintering of the catalyst. Three types of reac tors (tubular fixed bed, fluidized bed, and slurry) provide good temperature control, and all three types are being used for synthesis gas conversion. The first plants used tubular or plate-type fixed-bed reactors. Later, SASOL, in South Africa, used fluidized-bed reactors, and most recently, slurry reactors have come into use. 

Methanol synthesis will be used many times as an example to explain some concepts, largely because the stoichiometry of methanol synthesis is simple. The physical properties of all compounds are well known, details of many competing technologies have been published and methanol is an important industrial chemical. In addition to its relative simplicity, methanol synthesis offers an opportunity to show how to handle reversible reactions, the change in mole numbers, removal of reaction heat, and other engineering problems. 

The compressed synthesis gas is dried, mixed with a recycle stream, and introduced into the synthesis reactor after the recycle compressor. The gas mixture is chilled and liquid ammonia is removed from the secondary separator. The vapor is heated and passed into the ammonia converter. The feed is preheated inside the converter prior to entering the catalyst bed. The reaction occurs at 450-600°C over an iron oxide catalyst. The ammonia synthesis reaction between nitrogen, N2, and hydrogen, Hj, is... 

A low-pressure process has been developed by ICl operating at about 50 atm (700 psi) using a new active copper-based catalyst at 240°C. The synthesis reaction occurs over a bed of heterogeneous catalyst arranged in either sequential adiabatic beds or placed within heat transfer tubes. The reaction is limited by equilibrium, and methanol concentration at the converter s exit rarely exceeds 7%. The converter effluent is cooled to 40°C to condense product methanol, and the unreacted gases are recycled. Crude methanol from the separator contains water and low levels of by-products, which are removed using a two-column distillation system. Figure 5-5 shows the ICl methanol synthesis process. 

This mode is used industrially for exothermic reactions such as NH3 oxidation and in CH3OH synthesis, where exothermic and reversible reactions need to operate at temperatures where the rate is high but not so high that the equilibrium conversion is low. Interstage cooling is frequently accomplished along with separation of reactants from products in units such as water quenchers or distillation columns, where the cooled reactant can be recycled back into the reactor. In these operations the heat of water vaporization and the heat removed from the top of the distillation column provides the energy to cool the reactant back to the proper feed temperature. 

SYNTHESIS To a solution of 5.0 g 4-hydroxyindole in 20 mL pyridine there was added 10 mL acetic anhydride and the reaction heated on the steam bath for 10 min. The reaction was quenched by pouring over chipped ice to which was added an excess of NaHC03. After being stirred for 0.5 h the product was extracted with ethyl acetate and the extracts washed with brine and the solvent removed under vacuum. The residue weighed 6.3 g (95%) which, after crystallization from cyclohexane, had a melting point of 98-100 °C. IR (in cm-1) 1750 for the carbonyl absorbtion. 

Figure 6.110 gives responses to changes in the setpoint of the methane composition controller. At 0.1 h, it is reduced from 24 to 20 mol% methane. The vent flowrate increases sharply and ends up at a higher steady-state value. Synthesis gas feed and product flows both increase. The temperature of the coolant in the first reactor decreases because the increase in the reaction rate requires more heat removal. 

In many olefmic reactions activation of the C=C double bond occurs, although in many reactions at least one C-H bond is transformed. Established processes are summarized in Table 3. Examples of liquid-phase reactions are the synthesis of ethers, especially methyl tert-butyl ether by reacting olefins (isobutene) and alcohol (methanol) in the liquid phase at slightly elevated temperature and pressure (Table 3, entry 22). Different processes developed differ only slightly in feed composition and design, which is optimized for heat removal [2]. 

Another reactor system which has several attractive features for heat removal is the tubular, heat-exchange reactor. Good temperature control can be achieved in the tubular reactor if the coolant approximates an isothermal heat sink. Light gas recycle can be reduced significantly compared to fixed-bed systems. Tubular reactors have been used for Fischer-Tropsch reactions and for synthesis of methanol and phthalic anhydride, for example. 

In the majority of fixed-bed reactors for industrial synthesis reactions, direct or indirect supply or removal of heat in the catalyst bed is utilized to adapt the temperature profile over the flow path as far as possible to the requirements of an optimal reaction pathway. Here a clear developmental trend can be observed. 

The Fischer-Tropsch synthesis of hydrocarbons is used on a large scale for fuel production in South Africa [78, 79]. Synthesis gas generated from coal in Lurgi fixed-bed gasifiers enters the Synthol reactor (Fig 18), where it is reacted over an iron catalyst at 340°C. The reactor works on the principle of the circulating fluidized bed. The mean porosity in the riser is 85%, and the gas velocity varies between 3 and 12ms1 [2]. Reaction heat is removed by way of heat-exchanger tube bundles placed inside the riser. 

In the Haldor Topsoe process several reactors are used, arranged in series. The heat of reaction is removed by intermediate coolers. The synthesis gas flows radially through the catalyst beds. 

After final cooling by air or cooling water, the synthesis gas is compressed (6) and sent to the synthesis loop (7). The synthesis loop is comprised of a straight-tubed boiling water reactor, which is more efficient than adiabatic reactors. Reaction heat is removed from the reactor by generating MP steam. This steam is used for stripping of process condensate and thereafter as process steam. Preheating the... 

The synthesis loop consists of a recycle compressor, feed/effluent exchanger, methanol reactor, final cooler and crude methanol separator. Uhde s methanol reactor is an isothermal tubular reactor with a copper catalyst contained in vertical tubes and boiling water on the shell side. The heat of methanol reaction is removed by partial evaporation of the boiler feedwater, thus generating 1-1.4 metric tons of MP steam per metric ton of methanol. Advantages of this reactor type are low byproduct formation due to almost isothermal reaction conditions, high level heat of reaction recovery, and easy temperature control by... 

The earlier plants operated at deficit, and needed an auxiliary boiler, which was integrated in the flue gas duct. Auxiliary burners in tunnels or flue gas duet were additionally used in some instances. This situation was partially caused by inadequate waste heat recovery and low efficiency in some energy consumers. Typically, the furnace flue gas was discharged in the stack at rather high temperature because there was no air preheating and too much of the reaction heat in the synthesis loop was rejected to the cooling media (water or air). In addition, efficiency of the mechanical drivers was low and the heat demand for regenerating the solvent from the C02 removal unit (at... 

Synthesis of Siloxane-Polyimide Thermoplastics. In a typical siloxane-polyimide thermoplastic preparation, a 2-L, three-neck flask equipped with an overhead mechanical stirrer, Dean-Stark trap with condenser and nitrogen inlet, and a thermometer was charged with 106.41 g (0.230 mol) of DiSiAn, 119.71 g (0.230 mol) of BPADA, 49.74 g (0.460 mol) of mPD, 8.55 g (3 wt %) of 2-hydroxypyridine, and 635 mL of o-dichlorobenzene. The mixture was warmed to 100 °C for 1 h to dissolve the monomers and catalyst. Polyamic acids precipitated and redissolved when the mixture was heated to 150 °C for 1 h. The solution was then warmed to reflux, and 15 mL of water of reaction was removed by azeotropic distillation. The mixture was maintained at 180 °C for 16 h. The solution became noticeably more viscous. The polymer was isolated and purified as described previously to obtain 232 g (90%) of polymer with an IV of 0.54 dL/g. The isolated polymer was characterized spectroscopically. DSC indicated a Tg (glass transition temperature) of 196 °C. 

The industrial scale reaction of synthesis gas to ammonia in pressure reactors takes place in a cyclic process in which the ammonia formed is removed from the reaction gas and the unreacted synthesis gas returned to the reactor. In addition to the ammonia formed, inert gases and the liberated reaction heat have to be continuously removed from the cyclic process. The excess heat of the product gas is used to heat the feed synthesis gas to the reaction temperature in a heat exchanger integrated into the reactor. Additional waste heat can be utilized for steam generation. The pressure loss in the synthesis gas due to its passage through the synthesis loop is compensated for and the fraction of synthesis gas converted replaced by fresh compressed synthesis gas ( fresh gas ). 

There is another problem in chemical synthesis. If large amounts of molecules are activated and allowed to collide with other molecules to react in a very short period, there may be the problem of heat removal in the case of exothermic reactions. Many synthetically useful reactions are highly exothermic. However, if most of the starting molecules react in a very short period, a significant amount of heat should be generated in that... 

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