(a) to obtain the final temerature for the resulting solution i.e. 40% NaOH, we
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Question
(a) to obtain the final temerature for the resulting solution i.e. 40% NaOH, we mark one point representing the 10% NaOH at 21deg C and other point representing the 70% NaOH at 93 deg C, then draw the strainght line conecting thesetowo points and this line must contian the point representing the final solution i.e. 40%NaOH. THe resulting enthalpy for this solution was obtained from the relevant y axis which was found to be around 447 kJ/kg. we also found that the isotherm for 104 deg C passes through this point. Thus the final temperature ontained graphically through this point is 104 deg C.
(b) As the totla process can not be represented by a strainght line so, we considered that the given process occurs in two steps, first is adiabatic mixing and then the simple cooling of the resulting solution to final temperature. THe forst step reslts in a solution at 104 deg C having an enthapy of 447 kJ/kg (as obtained in part (a). when this solution is cooled to 21 deg C, the resulting enthalpy obtained from fig. 12.19 was around 163 kJ/kg. So, heat removed during this process was obtained to be 447 kJ/kg- 163 kJ/kg=284 kJ/kh. so, hest removed is 284 kJ for each kg of soluttion formed.
Explanation / Answer
Example 4.5 An aqueous solution of 10% NaOH at 21°C is mixcd with another aqucous solution of 70% NaOH al 93°C lo form a solution that contains 40% NaOH (a) If the mixing process is done adiabatically, what is the final temperature for the resulting solutlon? (b) If the final temperature in part (a) is reduced to 21°C, how much heat must be removed during the process? Refer to the Hx diagram for the NaOH/HzO system to solve the above tasks. 1200 Datum: Enthalpy tor pure liquid H at '- and vary dilute NaOH okti 20 C 1100 700 500 400 85 C 200 36 C 10 C 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Mass % NaOH Figure 12 19: // r diagram for NaO!11-0(reproduced by permission of John Wiley & Sons. Inc., USA from J.L KroschwitzRelated Questions
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