Recovery of reaction heat

Theoretical minimum (full recovery of reaction heat) 18.9... 

The recovery of reaction heat at synthesis loop includes both the quantity and the quality of the heat. The former is determined by Eq. (9.3), i.e., the net value of ammonia and the temperature difference loss of the recovery system. For example, in Fig. 9.25, inlet temperature of converter is 141°C, the outlet temperature is 284°C, so the adiabatic temperature rise of the converter is 143°C, the temperature difference lost at the cool side of the preheater front-converter is 19°C, and the rest heat can be applied to the feed water of the boiler. The recovery ratio of the reaction heat reaches 86.7%. 

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

The nitrous gas absorption step (reactions 9.13 and 9.14) is slow, especially if concentrated HN03 is required, since cooling to 2°C is then necessary. Consequently, large countercurrent towers of stainless steel are needed, with associated high capital cost. The recovery of the heat of reaction of this step is inefficient because of the low temperature of the source gases that must be maintained. It has been suggested that the energy of reaction 9.12 could be more effectively recovered if it is run in a fuel cell (see Exercise 15.8). 

Nonetheless, the continuous feeding of solid biomass is indispensable. For an energy process using hydrothermal or supercritical reactions, the feedstock must be heated to bring it into a hydrothermal reaction field. The heat requirement for bringing the inlet flow to the reaction tenq>erature is rather large. For example, assuming an inlet flow of pure water, as much as 2579 kJ kg must be added to raise the inlet flow temperature to 400°C under 25 MPa. This is almost the same as the enthalpy of evaporation of the same amount of water, which is 2676 kJ kg. Thus, the process efficiency is very low if there is no recovery of this heat from the outlet flow. 

Nowadays, Asahi, BASF, Bayer, Invista, Rhodia, Radici and Solutia use catalytic or thermal processes to destroy N2O. Recovery of waste heat from the exothermic abatement reactions is more effective with thermal systems due to their higher... 

Since the delay found in references [4] and [12] concerned the start-up of the whole oxidation plant that included a multi-pass catalytic reactor, the simulations were carried out using the model of the whole plant comprising not only the reactor itself, but also the installations for the recovery of the heat of reaction. For the assumed values of the startup was simulated from the state in which the installation was thoroughly cooled down. The SO2 concentration transients at the outlet of the reactor are shown in Fig. 3. 

Reddy and Wilhite [59] investigated application of membrane reactors in diesel reformate mixture purification isothermal two-dimensional model. The typical reformate mixture contains 9% CO, 3% CO2, 28% H2 and 15% H2O. Simulations indicate that apparent CO H2 selectivities of 90 1 to >200 1 at H2 recoveries of 20% to upwards of 40% may be achieved through appropriate design of the catalytic membrane and selection of operating cmiditions. Comparison of adiabatic and isothermal simulations indicates that accumulation of reaction heat reduces apparent perm-selectivities however, this may be mitigated by external imposition of a cotmtering thermal gradient... 

The reaction heat of the ammonia formation is large. For example, producing one kmol of ammonia at 450°C will give out 54.5 MJ of reaction heat which is equal to the heat of low-pressure saturated steam of 1.44t/t. Actually, the reaction heat cannot be recovered fully because of the temperatme difference losses among the heat-recovery equipments where only 50%-90% of reaction heat can usually be recovered. 

The recovery process of reaction heat is composed of converter, boiler or heater for feed water of boilers and interchangers (Preheater before converter) (Fig. 9.24). 

The gasifier product is scrubbed with water to cool the gas and remove any ash particles. A portion of the gas stream is sent to a water-gas shift reaction to increase the hydrogen content. Finally, low-pressure steam is generated as the product gas is cooled before additional purification. The full recovery of this heat increases the thermal efficiency of the process and is important for economical operation of the chemical processes that utilize the steam. 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

Into a 1-litre beaker, provided with a mechanical stirrer, place 36 - 8 g. (36 ml.) of aniline, 50 g. of sodium bicarbonate and 350 ml. of water cool to 12-15° by the addition of a little crushed ice. Stir the mixture, and introduce 85 g. of powdered, resublimed iodine in portions of 5-6 g, at intervals of 2-3 minutes so that all the iodine is added during 30 minutes. Continue stirring for 20-30 minutes, by which time the colour of the free iodine in the solution has practically disappeared and the reaction is complete. Filter the crude p-iodoaniline with suction on a Buchner funnel, drain as completely as possible, and dry it in the air. Save the filtrate for the recovery of the iodine (1). Place the crude product in a 750 ml. round-bottomed flask fitted with a reflux double surface condenser add 325 ml. of light petroleum, b.p. 60-80°, and heat in a water bath maintained at 75-80°. Shake the flask frequently and after about 15 minutes, slowly decant the clear hot solution into a beaker set in a freezing mixture of ice and salt, and stir constantly. The p-iodoaniline crystallises almost immediately in almost colourless needles filter and dry the crystals in the air. Return the filtrate to the flask for use in a second extraction as before (2). The yield of p-iodoaniline, m.p. 62-63°, is 60 g. 

In a 500 ml. three-necked flask, fitted with a reflux condenser and mechanical stirrer, place 121 g. (126-5 ml.) of dimethylaniline, 45 g. of 40 per cent, formaldehyde solution and 0 -5 g. of sulphanilic acid. Heat the mixture under reflux with vigorous stirring for 8 hours. No visible change in the reaction mixture occurs. After 8 hours, remove a test portion of the pale yellow emulsion with a pipette or dropper and allow it to cool. The oil should solidify completely and upon boiling it should not smell appreciably of dimethylaniline if this is not the case, heat for a longer period. When the reaction is complete, steam distil (Fig. II, 41, i) the mixture until no more formaldehyde and dimethylaniline passes over only a few drops of dimethylaniline should distil. As soon as the distillate is free from dimethylaniline, pour the residue into excess of cold water when the base immediately solidifies. Decant the water and wash the crystalline solid thoroughly with water to remove the residual formaldehyde. Finally melt the solid under water and allow it to solidify. A hard yellowish-white crystalline cake of crude base, m,p. 80-90°, is obtained in almost quantitative yield. RecrystaUise from 250 ml. of alcohol the recovery of pure pp -tetramethyldiaminodiphenylmethane, m.p. 89-90°, is about 90 per cent. 

Recycle and Polymer Collection. Due to the incomplete conversion of monomer to polymer, it is necessary to incorporate a system for the recovery and recycling of the unreacted monomer. Both tubular and autoclave reactors have similar recycle systems (Fig. 1). The high pressure separator partitions most of the polymers from the unreacted monomer. The separator overhead stream, composed of monomer and a trace of low molecular weight polymer, enters a series of coolers and separators where both the reaction heat and waxy polymers are removed. Subsequendy, this stream is combined with fresh as well as recycled monomers from the low pressure separator together they supply feed to the secondary compressor. 

Alkanolamines in aqueous soludon react widi carbon dioxide and hydrogen sulfide to yield salts, important to gas condidoiiing reactions. Tlie dissociation of die salts upon heating results in recovery of the original starting material. Tliese reactions fomi the basis of an important industrial apphcadon, ie, die "sweetening" of natural gas. 

Reaction and Heat-Transfer Solvents. Many industrial production processes use solvents as reaction media. Ethylene and propylene are polymerized in hydrocarbon solvents, which dissolves the gaseous reactant and also removes the heat of reaction. Because the polymer is not soluble in the hydrocarbon solvent, polymer recovery is a simple physical operation. Ethylene oxide production is exothermic and the catalyst-filled reaction tubes are surrounded by hydrocarbon heat-transfer duid. 

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