Flowsheets heat transfers

The heat transfer coefficient for tube flow will be proportional to the mass flow raised to the power 0.8. Hence if the flowsheet mass flow is W,o and the flowsheet heat transfer coefficient is Kn,-,o< the heat transfer coefficient at any other tube-side flow, W, will be given by ... 

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

Indirect heat transfer with the reactor. Although indirect heat transfer with the reactor tends to bring about the most complex reactor design options, it is often preferable to the use of a heat carrier. A heat carrier creates complications elsewhere in the flowsheet. A number of options for indirect heat transfer were discussed earlier in Chap. 2. 

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. 

This is the fun (and frustration) of chemical reaction engineering. While thermodynamics, mass and heat transfer, and separations can be said to be finished subjects for many engineering apphcations, we have to reexamine every new reaction system from first principles. You can find data and construct process flowsheets for separation units using sophisticated computer programs such as ASPEN, but for the chemical reactors in a process these programs are not much help unless you give the program the kinetics or assume equihhrium yields. 

We assume that the reactor is constructed from fairly exotic material and has heat transfer equipment (jacket or coil) and an agitator. So the capital cost is estimated at 10 times the basic vessel cost. The total capital costs of the reactors in the three different flowsheets are 7,429,000, 4,043,000, and 3,657,000 for the 1-CSTR, 2-CSTR, and 3-CSTR processes, respectively. You can see that the reduction in cost between the 2-CSTR and the 3-CSTR processes is quite small. The cost of a 4-CSTR process could be somewhat higher because of having more vessels, even if each is somewhat smaller. 

The flowsheet was developed by sequentially guessing various unknown stream conditions in and out of heat exchangers and reactors. The flowrates, compositions, and temperatures of the recycle stream and the hot stream into the FEHE were guessed. The FEHE was sized by specifying an exit temperature of the cold stream of 122°C, as shown in Figure 6.84. An overall heat transfer coefficient of 144kcalh 1 m-2 °C 1 is specified (Fig. 6.85) in this gas-gas exchanger. 

Fig. 21.1. Heat transfer flowsheet for single contact, sulfur burning sulfuric acid plant. It is simpler than industrial plants, which nearly always have 4 catalyst beds rather than 3. The gaseous product is cool, S03 rich gas, ready for H2S04 making. The heat transfer product is superheated steam. All calculations in this chapter are based on this figure s feed gas composition and catalyst bed input gas temperatures. All bed pressures are 1.2 bar. The catalyst bed output gas temperatures are the intercept temperatures calculated in Sections 12.2, 15.2 and 16.3.

In addition to recycle streams returned back to upstream units, thermal integration is also frequently done. Energy integration can link units together in locations anywhere in the flowsheet where the temperature levels permit heat transfer to occur. The reaction and separation sections are thus often intimately connected. If conditions are altered in the reaction section, the resulting changes in flowrates, compositions, and temperatures affect the separation section and vice versa. 

Pure component physical property data for the five species in our simulation of the HDA process were obtained from Chemical Engineering (1975) (liquid densities, heat capacities, vapor pressures, etc.). Vapor-liquid equilibrium behavior was assumed to be ideal. Much of the flowsheet and equipment design information was extracted from Douglas (1988). We have also determined certain design and control variables (e.g., column feed locations, temperature control trays, overhead receiver and column base liquid holdups.) that are not specified by Douglas. Tables 10.1 to 10.4 contain data for selected process streams. These data come from our TMODS dynamic simulation and not from a commercial steady-state simulation package. The corresponding stream numbers are shown in Fig. 10.1. In our simulation, the stabilizer column is modeled as a component splitter and tank. A heater is used to raise the temperature of the liquid feed stream to the product column. Table 10.5 presents equipment data and Table 10.6 compiles the heat transfer rates within process equipment. 

There are P I diagrams for individual utilities such as steam, steam condensate, cooling water, heat transfer media in general, compressed air, fuel, refrigerants, and inert blanketing gases, and how they are piped up to the process equipment. Connections for utility streams are shown on the mechanical flowsheet, and their conditions and flow quantities usually appear on the process flowsheet. 

The accompanying figure represents the schematic flowsheet of a distillation tower used to recover gasoline from the products of catalytic cracker. Is the problem completely specified, that is, is the number of degrees of freedom equal to zero for the purpose of calculating the heat transfer to the cooling water in the condenser ... 

For cobalt, this method has been commercialized in the Kuhlmann process, using amphiphilic HCo(CO)4 [5b]. The Kuhlmann process (now the Exxon process) involves cobalt-catalyzed hydroformylation of higher alkenes, for which the flowsheet - a liquid/liquid separation - is shown in Figure 2. In this process the hydroformylation is done in the organic phase consisting of alkene and aldehyde. A loop reactor, or a reactor with an external loop to facilitate heat transfer, is often used. 

Pressure drop unit are used to simulate hydraulic operations, as for example the pressure drop in a pipeline or in a distribution network. Adiabatic operation or heat transfer with the surroundings can be treated. In flowsheeting the module is generally tailored to simulate the transport of fluids in process plants. It can cover a large variety of physical conditions, as for example three-phase calculations in different flow regimes. However, special applications are better modelled by dedicated software, as the network of utilities (water and steam), or pipeline systems in oil gas production. 

Pinch Point Analysis starts with the input of data. The first step is the extraction of stream data from a flowsheet simulation, which describes typically the material balance envelope (Reactors and Separators). Proper selection and treatment of streams by segmentation is a key factor for efficient heat integration. The next step is the selection of utilities. Additional information regards the partial heat transfer coefficients of the different streams and segments of streams, and of utilities, as well as the cost of utilities and the cost laws for heat exchangers. 

F to 500°F in a 1-2 parallel-counterflow heat exchanger with a mean overall heat transfer coefficient of 75 Btu/hr ft T.lt is converted to C by the exothermic reaction, A -(- B C, in an adiabatic plug-flow tubular reactor (Figure 4.30). For a process simulator, prepare a simulation flowsheet and show the calcula-1 tion sequence to determine ... 

This, of course, is not what we want since the pressure (and temperature) must vary to change the heat-transfer rate and the steam flow rate. It is necessary to use Flowsheet Equations in Aspen Dynamics to change the specification to have an exit hot stream with a vapor fraction of zero. Figure 13.5 shows the equation used. It is also necessary to change the pressure of the hot exit stream fxom fixed to free so that the system is not over specified. 

The fouling factor has to be determined from actual heat exchanger performance based on online measurements taken from a process unit test run. Heat exchanger clean performance is obtained from process flowsheet simulation software (e.g., Hysys by Aspen Tech or Unisim by Honeywell), while dirty performance from exchanger rating software (e.g., HTRI by Heat Transfer Research Institute). 

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