Minimum approach temperature

Once the minimum utility cost has been identified, tradeoffs between operating and fixed costs must be established. This step is undertaken iteratively. For given values of minimum approach temperatures, the pinch diagram is used to obtain minimum cooling cost and outlet gas temperature. By ccmducting enthalpy balance around each unit, intermediate temperatures and exchanger sizing can be determined. Hence, one can evaluate the fixed cost of the system. Next, the minimum approach temperatures are altered, until the minimum TAC is identified. 

The temperature difference between the recooled water temperature and the inlet air wet bulb temperature is called the approach. The lower the ap proach, the more complex the tower s design becomes. The normally used minimum approach temperature is 2 °C. 

Determine the pinch temperature and the minimum utility requirements for the process set out below. Take the minimum approach temperature as 15 °C. Devise a heat exchanger network to achieve maximum energy recovery. 

Find the minimum utility requirements for this process, for a minimum approach temperature of 10 °C. 

A minimum approach temperature to the solvent freezing point is defined. 

Feasible refers to a process (HEN structure) which satisfies all physical constraints (nonnegative exchanger loads) and performance specifications (target temperatures, minimum approach temperature, specified energy recovery). 

Most chemical processing plants include pure or multicomponent streams which change phase or which have strongly temperature-dependent heat capacities. Under these conditions the minimum approach temperature in a network can occur anywhere inside an exchanger. Therefore integral or differential equations are generally required to locate A Tm. 

Minimum approach temperature allowed in a HEN, K Smallest approach temperature actually occurring in a HEN, K A8 Expected uncertainty (in any uncertain variable) (12)... 

A Tmm or EM AT, Minimum approach temperature, specifies the minimum temperature difference between two streams exchanging heat within an exchanger. 

Remark 1 The problem statement is identical to the problem statement of section 8.5.3.1 for the synthesis of HENs without decomposition (Ciric and Floudas, 1991). Note that as in section 8.5.3.1, there is no specification of any parameters so as to simplify or decompose the original problem into subproblems. In other words, the level of energy recovery (specified by fixing H RAT), the minimum approach temperature (EM AT), and the number of matches are not specified a priori. As a result, there is no decomposition into subnetworks based on the location of the pinch point(s), but instead the pinch point(s) are optimized simultaneously with the matches and the network topology. The approach presented for this problem is from Yee and Grossmann (1990) and is an alternative approach to the one of HEN synthesis without decomposition proposed by Ciric and Floudas (1991), and which was presented in section 8.5.3. 

To design the heat exchanger, the heuristic is used that the minimum approach temperature differential ATH = Tont — TCout is 25 K, which is reasonable for the temperature level in this process. This pinch temperature differential occurs at the hot end of the heat exchanger. An overall heat transfer coefficient U = 0.142 kJ s 1 m-2 K-1 is used in this gas-gas system. 

The evolutionary development method of Linnhoff and Flower (2 ) utilizes the temperature interval diagram to represent an initially created network and also the concept of the freedom (F) of an exchanger. The latter has the physical dimension as a heat load (kW) and is related to the larger heat capacity flow rate of the two streams matched in an exchanger (CPL), the smallest actual temperature difference within the exchanger (AT ) and the minimum approach temperature of the exchanger (AT ) according to the expression... 

A process has four streams with the characteristics given below. Devise a heat-exchange network to maximize the annual savings as compared to no heat exchange. Use a minimum approach temperature ATmin = WC. 

Inlet temperature = Temperature rise in condensers = Minimum approach temperature =... 

At what value of the minimum approach temperature does the problem in Example 3.16 become a threshold problem Design a heat exchanger network for the resulting threshold problem. What insights does this give into the design proposed in Example 3.16 ... 

Wen, Y. and Shonnard, D. R (2003). Environmental and economic assessments of heat exchanger networks for optimum minimum approach temperature, Comput. Chem. Eng., 27, pp. 1577-1590. 

Assume that a heating medium is available at 331.5°K and that cooling media are available to give minimum approach temperatures of 5.6°K. 

The latter is obviously obtained independently (and for all M) from the definition of the over-all volumetric transfer coefficient, based on apparent exterior temperatures. The obvious and common technique of studying column performance is to keep G, and T (in) constant and vary G until the desired minimum approach temperature is obtained, corresponding to (Gc)min> Fig- 25(a). Now, since T 2 (out) ideally approaches T, only asymptotic-... 

Initial observations are that the TID graphically shows the possible matches between the hot and the cold streams that will not violate the minimum approach temperature. This is noted by ensuring the thermodynamic constraint that heat is only passed from a hot stream at a temperature interval equivalent to or higher than a cold stream temperature interval. Thus, the TID provides an overall quick, qualitative view of potential stream matches without an excess of information. 

The portion of the hot composite curve and the cold composite curve shows the minimum cooling and heating utilities, respectively, that must be supplied. Increasing the minimum approach temperature will shift both curves farther apart, reducing the total heat integration... 

---