Heat Exchangers and Furnaces

As discussed in the previous two sections, heat exchange is commonly used in conjunction with separation and reaction steps to change the temperature and/or phase condition of a process stream. When using a process simulator to perform heat exchange calculations, it is necessary to select a method of heat exchange from the following six possibilities, where all but the last two involve the separation, by a solid wall, of the two streams exchanging heat 

Heat exchange between two process fluids using a double-pipe, shell-and-tube, or compact heat exchanger. 

High-temperature heating of a process fluid using heat from the products of combustion in a furnace (also called a fired heater). 

Heat exchange within a reactor or separator, rather than in an external heat-exchange device such as a shell-and-tube heat exchanger or furnace, as described in Section 5.5. 

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Atmospheric Distillation - The desalted crude oil is then heated in a heat exchanger and furnace to about 750°F and fed to a vertical, distillation column at atmospheric pressure where most of the feed is vaporized and separated into its... 

Equipment Failure pumps, tubes in heat exchangers and furnaces, turbine drivers and governor, compressor cylinder valves are examples of equipment which might fail and cause overpressure in the process. If an exchanger tube splits or develops a leak, high pressure fluid will enter the low side, overpressuring either the shell or the channels and associated system as the case may be. 

Heat exchangers and furnaces. Manufacturers are usually supplied with the duty, corrected log mean-temperature difference, percent vaporized, pressure drop desired, and materials of construction. 

For example, in a distillation column we are usually interested in controlling the purity of the distillate and bottoms product streams. In chemical reactors, heat exchangers, and furnaces the usual controlled variable is temperature. In most cases these choices are fairly obvious. It should be remembered that controlled variables need not be simple, directly measured variables. They can also be computed from a number of sensor inputs. Common examples are heat removal rates, mass flow rates, and ratios of flow rates. 

Pressure drops of fluids flowing through heat exchangers and furnaces may be estimated with the following heuristics. 

Preheating and heating system—heat exchangers and furnace... 

As the world crude oil supply dictates, most refiners are cutting crude runs. Too often, one result is an increase in the BTUs required to process a barrel through the crude unit. For the average crude unit, 90% of its energy input is provided by furnaces. Theoretically, both heat exchanger and furnace efficiency increase at lower loads. Why, then, does overall heat economy suffer at lower charge rates A few items that the troubleshooter may ponder are ... 

The most effective phosphoms production technology uses a submerged arc furnace. The submerged arc furnace performs three functions chemical reactor, heat-exchanger, and gas—soHd filter, respectively, each of which requires a significant amount of preparation for the soHd furnace feed materials. 

A typical ethane cracker has several identical pyrolysis furnaces in which fresh ethane feed and recycled ethane are cracked with steam as a diluent. Figure 3-12 is a block diagram for ethylene from ethane. The outlet temperature is usually in the 800°C range. The furnace effluent is quenched in a heat exchanger and further cooled by direct contact in a water quench tower where steam is condensed and recycled to the pyrolysis furnace. After the cracked gas is treated to remove acid gases, hydrogen and methane are separated from the pyrolysis products in the demethanizer. The effluent is then treated to remove acetylene, and ethylene is separated from ethane and heavier in the ethylene fractionator. The bottom fraction is separated in the deethanizer into ethane and fraction. Ethane is then recycled to the pyrolysis furnace. 

It was noticed that the order of process items in the layout spacing recommendations is almost identical. The furnaces and fired heaters are on the top of the list (see Table 18). The next group is formed by compressors and high hazard reactors. Air coolers, ordinary reactors and high hazard pumps appear next. After that come towers, process drums, heat exchangers and pumps. The last and safest group is formed of equipment handling nonflammable and nontoxic materials. 

Flow of Two Fluids. The major applications are in absorption, extraction, and distillation, with and without reaction. Other applications, also quite important, are for shell-and-tube or double-pipe heat exchangers, and noncatalytic fluid-solid reactors (blast furnace and ore-reduction processes). 

Crude oil is pumped from storage through a steam heated exchanger and into an electric desalter. Dilute caustic is injected into the line just before the desalting drum. The aqueous phase collects at the bottom of this vessel and is drained away to the sewer. The oil leaves the desalter at 190°F, and goes through heat exchanger E-2 and into a furnace coil. From the furnace, which it leaves at 600°F, the oil proceeds to a distillation tower. 

If the system consists of a series of adiabatic reactors, there are two basic configurations. The first has heat exchangers or furnaces between each of the reactors to cool or heat the reactor effluent before it enters the next reactor. The second configuration uses cold shot cooling. Some of the cold reactor feed is bypassed around the upstream reac-tor(s) and mixed with the hot effluent from the reactor to lower the inlet temperature to the downstream catalyst bed. 

We start by studying the steady-state design and economics of a process with a single adiabatic reactor. The design considers the entire plantwide process reactor, heat exchangers, gas recycle compressor, preheat furnace, condenser, and separator. The economic objective function is total annual cost, which includes annual capital cost (reactor, catalyst, compressor, and heat exchangers) and energy cost (compressor work and furnace fuel). 

Calculate the temperatures in and out of the furnace and heat exchanger, and calculate the heat transfer area. 

This chapter has two alternative structures for feed preheating. Both use a feed effluent heat exchanger, but one also uses a furnace. Steady-state economics favor use of only a heat exchanger. Dynamic controllability favors the use of both a heat exchanger and a furnace. 

The crossover temperature must satisfy an overall heat balance around the furnace system, including transfer line heat exchangers and feed preheaters, and must not exceed certain limits to avoid incipient cracking. 

The HDA process (Fig. 1.1) contains nine basic unit operations reactor, furnace, vapor-liquid separator, recycle compressor, two heat exchangers, and three distillation columns. Two vapor-phase reactions are considered to generate benzene, methane, and diphenyl from reactants toluene and hydrogen. 

The bottoms from the DIB contains most of the nC4, along with some iC4 impurity and all of the heavy isopentane impurity. Since this heavy component will build up in the process unless it is removed, a second distillation column is used to purge out a small stream that contains the isopentane. Some n C4 is lost in this purge stream. The purge column has 20 trays and is 6 ft in diameter. The distillate product from the second column is the recycle stream to the reactor, which is pumped up to the required pressure and sent through a feed-effluent heat exchanger and a furnace before entering the reactor in the vapor phase. 

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