Heat transfer stream

Heat and mass transfer have an influence on each other, which can be taken into account by the Ackermann [5] correction factor E for the heat transfer stream... 

Specifying the hot utility or cold utility or AT m fixes the relative position of the two curves. As with the simple problem in Fig. 6.2, the relative position of the two curves is a degree of freedom at our disposal. Again, the relative position of the two curves can be changed by moving them horizontally relative to each other. Clearly, to consider heat recovery from hot streams into cold, the hot composite must be in a position such that everywhere it is above the cold composite for feasible heat transfer. Thereafter, the relative position of the curves can be chosen. Figure 6.56 shows the curves set to ATn,in = 20°C. The hot and cold utility targets are now increased to 11.5 and 14 MW, respectively. 

Find a way to overcome the constraint while still maintaining the areas. This is often possible by using indirect heat transfer between the two areas. The simplest option is via the existing utility system. For example, rather than have a direct match between two streams, one can perhaps generate steam to be fed into the steam mains and the other use steam from the same mains. The utility system then acts as a buffer between the two areas. Another possibility might be to use a heat transfer medium such as a hot oil which circulates between the two streams being matched. To maintain operational independence, a standby heater and cooler supplied by utilities is needed in the hot oil circuit such that if either area is not operational, utilities could substitute heat recovery for short periods. 

By constrast, Fig. 7.46 shows a diflFerent arrangement. Hot stream A with a low coefficient is matched with cold stream D, which also has a low coefficient but uses temperature diflferences greater than vertical separation. Hot stream B is matched with cold stream C, both with high heat transfer coefficients but with temperature differences less than vertical. This arrangement requires 1250 m of area overall, less than the vertical arrangement. 

Stream Supply temp. T, rc) Target temp. Tr rC) AH (MW) Heat capacity flow rate CP (WN C- ) Heat transfer coefficient h(MW... 

The thermal profile through the reactor will in most circumstances be carefully optimized to maximize selectivity, extend catalyst life, etc. Because of this, direct heat integration with other process streams is almost never carried out. The heat transfer to or from the reactor is instead usually carried out by a heat transfer intermediate. For example, in exothermic reactions, cooling might occur by boiling water to raise steam, which, in turn, can be used to heat cold streams elsewhere in the process. 

If indirect heat transfer is used with a large temperature difference to promote high rates of cooling, then the cooling fluid (e.g., boiling water) is fixed by process requirements. In this case, the heat of reaction is not available at the temperature of the reactor effluent. Rather, the heat of reaction becomes available at the temperature of the quench fluid. Thus the feed stream to the reactor is a cold stream, the quench fluid is a hot stream, and the reactor effluent after the quench is also a hot stream. 

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. 

Following the pinch rules, there should be no heat transfer across either the process pinch or the utility pinch by process-to-process heat exchange. Also, there must be no use of inappropriate utilities. This means that above the utility pinch in Fig. 16.17a, high-pressure steam should be used and no low-pressure steam or cooling water. Between the utility pinch and the process pinch, low-pressure steam should be used and no high-pressure steam or cooling water. Below the process pinch in Fig. 16.17, only cooling water should be used. The appropriate utility streams have been included with the process streams in Fig. 16.17a. 

Thus loops, utility paths, and stream splits offer the degrees of freedom for manipulating the network cost. The problem is one of multivariable nonlinear optimization. The constraints are only those of feasible heat transfer positive temperature difference and nonnegative heat duty for each exchanger. Furthermore, if stream splits exist, then positive bremch flow rates are additional constraints. 

Uij = overall heat transfer coefficient between hot stream i and cold stream j... 

Equation (F.l) shows that each stream makes a contribution to total heat transfer area defined only by its duty, position in the composite curves, and its h value. This contribution to area means also a contribution to capital cost. If, for example, a corrosive stream requires special materials of construction, it will have a greater contribution to capital cost than a similar noncorrosive stream. If only one cost law is to be used for a network comprising mixed materials of construction, the area contribution of streams requiring special materials must somehow increase. One way this may be done is by weighting the heat transfer coefficients to reflect the cost of the material the stream requires. 

If condensation requires gas stream cooling of more than 40—50°C, the rate of heat transfer may appreciably exceed the rate of mass transfer and a condensate fog may form. Fog seldom occurs in direct-contact condensers because of the close proximity of the bulk of the gas to the cold-Hquid droplets. When fog formation is unavoidable, it may be removed with a high efficiency mist collector designed for 0.5—5-p.m droplets. Collectors using Brownian diffusion are usually quite economical. If atmospheric condensation and a visible plume are to be avoided, the condenser must cool the gas sufftciendy to preclude further condensation in the atmosphere. 

Hot product char carries heat into the entrained bed to obtain the high heat-transfer rates required. Feed coal must be dried and pulverized. A portion of the char recovered from the reactor product stream is cooled and discharged as product. The remainder is reheated to 650—870°C in a char heater blown with air. Gases from the reactor are cooled and scmbbed free of product tar. Hydrogen sulfide is removed from the gas, and a portion is recycled to serve as the entrainment medium. 

Basic Heat-Transfer Equations. Consider a simple, single-pass, parallel-flow heat exchanger in which both hot (heating) and cold (heated) fluids are flowing in the same direction. The temperature profiles of the fluid streams in such a heat exchanger are shown in Figure 2a. 

The importance of equations 37—39 is that once the heat-exchanger effectiveness, S, is known for a given heat exchanger, one can compute the actual heat-transfer rate and outlet stream temperatures from specified inlet conditions. This process is known as rating a given heat exchanger. 

Optimum heat-transfer results when the thermal capacity rates of the two fluid streams are balanced, ie, where or = 1.0. 

Translate the heat-transfer area deterrnined above into corresponding tube bundle dimensions. If different from those assumed in step (2), repeat steps (2) through (8) until satisfactory agreement is reached. The method caimot be appHed to cases in which U varies along the tube length or the stream... 

Direct Contact Heat Exchangers. In a direct contact exchanger, two fluid streams come into direct contact, exchange heat and maybe also mass, and then separate. Very high heat-transfer rates, practically no fouling, lower capital costs, and lower approach temperatures are the principal advantages. 

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