Heat transfer mixed vapor

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Heat transfer in the furnace is mainly by radiation, from the incandescent particles in the flame and from hot radiating gases such as carbon dioxide and water vapor. The detailed theoretical prediction of overall radiation exchange is complicated by a number of factors such as carbon particle and dust distributions, and temperature variations in three-dimensional mixing. This is overcome by the use of simplified mathematical models or empirical relationships in various fields of application. 

Hot water boilers are potentially more susceptible to gas-side corrosion than steam boilers due to the lower temperatures and pressures encountered on low- and medium-temperature hot water boilers. With low-temperature hot water especially, the water-return temperature may drop below the water dewpoint of 50°C, causing vapor in the products of combustion to condense. This, in turn, leads to corrosion if it persists for long periods. The remedy is to ensure that adequate mixing of the return water maintains the water in the shell above 65°C at all times. Also, if medium or heavy fuel oil is to be used for low- or medium-temperature applications it is desirable to keep the heat transfer surfaces above 130°C, this being the approximate acid dewpoint temperature of the combustion gases. It may be seen, therefore, how important it is to match the unit or range of unit sizes to the expected load. 

For the smelt-water case. Nelson suggested the water in contact with the very hot smelt was, initially, separated by a thin vapor film. Either because the smelt cooled—or because of some outside disturbance— there was a collapse of the vapor film to allow direct liquid-liquid contact. The water was then heated to the superheat-limit temperature and underwent homogeneous nucleation with an explosive formation of vapor. The localized shocks either led to other superheat-limit explosions elsewhere in the smelt-water mass or caused intense local mixing of the smelt and water to allow steam formation by normal heat transfer modes. 

A quantitative understanding of certain primary combustion phenomena, e.g., liquid fuel-droplet vaporization and burning, gas phase chemical reaction kinetics, radiation heat transfer from combustion products, and mixing of reactants and combustion products, is required to develop a rational approach for the effective utilization of synfuels in industrial boiler/furnace systems. Those processes are defined by the interaction of a number of mechanisms which are conveniently described in terms of physical and chemical related processes. The physical processes are ... 

A vapor mixture ofmethanoK 1) and water(2) containing 56-mol-% methanol enters a condenser at 101.33 kPa at its dew point of 82.85 C. It is completely condensed and leaves the condenser at its bubble point of T2.05°C. How much heat must be transferred in the condenser for each mole of mixture condensed The latent heat of vaporization of methanol at its normal boiling point of 64.7°C is 35,228 J mol-1. The heat of mixing of a liquid mixture containing 56-mol-% methanol at 72°C is estimated as -500 J mol-1. 

Condensation of mixed vapors of immiscible liquids is not well understood. The conservative approach is to assume that two condensate films are present and all the heat must be transferred through both films in series. Another approach is to use a mass fraction average thermal conductivity and calculate the heat-transfer coefficient using the viscosity of the film-forming component (the organic component for water-organic mixtures). 

---