化工程序设计.ppt

化工程序設計儀器裝置設計與選擇化工程序裝置的設計與選擇 選擇可解決問題之技術與設備 經濟上可行及省錢 合乎環保要求 易於操作及合乎安全規範 易於購置且有可信的供應廠商Examples and solutions Removal of acid gases (CO2 and H2S) May choose a gas scrubber What is the proper solvent? Henrys law is applicable for solvent selectionExamples and solutions Consider a large-size compressor Should we used a steam turbine-driven or motor-driven compressor? The answer is a steam turbine-driven compressor The reason is for lower noise pollution Rules of Thumb（經驗規則） Fans are used to raise pressure about 3%. Blowers raise to less than 40 psig. Compressors to higher pressures. Heat transfer coefficients (Btu/(hr)(sqft)(F) for estimating purposes: water to liquid, 150. liquid to liquid, 50. liquid to gas, 5. condenser, 150. reboiler, 200. Rules of Thumb Batch reactions are conducted in stirred tanks for small daily production rates. Tubular reactors are suitable to high production rates at short residence times and when substantial heat transfer is needed. Equipments for Transportation To transport materials from one equipment to the next To supply energy in the form of pressure Transportation of gases (Woods, 1995)Select a device for air flow rate of 103dm3/s and a pressure raise of 1000 kPa? Check the previous Figure for the general selection purpose A centrifugal compressor, or a 2- or 3-stage reciprocating compressor is appropriateSelection of a pump-First consideration is the viscositySelection of a pump for a fluid with viscosity of 100 mPa s, flow rate of 20 L/s (or 0.02 m3/s), and a head of 30 m From the above Figure, a single-stage centrifugal radial flow pump is appropriateThe Net Positive Suction Head (NPSH) The NPSH is the pressure or head, required at the inlet or suction flange in excess of the vapor pressure of the liquid at the temperature of the pumping condition. NPSH = (The pressure supplied at the flange) (The vapor pressure of the liquid at the pumping temperature)Example and solutionFor pumping a liquid, we have the following conditionsVapor pressure of liquid at operating temperature is 5 kPaAtmospheric pressure is 90 kPaFluid density is 1 Mg/m3The pump is located 5 m above the liquidThe friction loss in the suction line is 1.2 mIf the required NPSH of the pump is 4 m, will the pump work without cavitating?Solution to the problem To avoid cavitation, we need to supplied a NPSH at least 0.5 m greater than the required NPSH Since the supplied NPSH is only 2.47 m, the pump cavitatesmNPSHlossfrictionzppNPSH47. 22 . 1) 50()8 . 90 . 1590(2121Transportation of solidsProblem and solution We need to transport 15.5 dm3/s of wheat horizontally a distance of 15.2 m. What is the appropriate equipment? Check with the design data book A crew conveyor is appropriate For a trough loading of 45%, find the screw diameter as 0.32 mHeat Exchange Equipments: General Classification Furnaces Exchangers Boilers Condensers Cryogenic units General applications for heating and cooling (see the next figure)Rough Sizing of Furnace Example and solution Select a furnace to heat up an oil The heating load is 2 x 107 kJ/h Consider a wider heat flux range What will be the total area of tubes? Select a A-furnace From the previous figure, estimate the total tube area as 300 m2Heat Exchanger Design: General Steps Determine the heat load Select the heating/cooling medium Make an overall selection Decide the inlet and outlet temperatures Estimate the heat transfer coefficient Determine the heat transfer areaCirculation Systems for Heat ExchangeDetermination of the Heat Load Sensible heat Latent heat Heat of reaction The total heat load is the summation of all above terms It is easily obtained from the energy balance of process flowsheetSelection of the Medium High Temperature Combustion gases Hot mineral oil Usual Temperature Condensing steam Air or water Very Low Temperature Ammonia EthyleneExample and solution We wish to heat a system to 2000C, what medium may be used? From the last figure, we can choose Dowthern J (alkylated aromatic) if the pressure is about 2 - 10 atm Use steam at higher pressure (20 atm) Choose Dowthern A at a lower pressureSelection of Equipment Based on the General Range (Some figures are shown below)Selection of the Inlet and Outlet Temperatures Normally, use countercurrent configurations If water is used as the cooling medium, its outlet temperature rarely exceeds 500C (to prevent erosion and corrosion) For air as the cooling medium, its exit temperature is usually less than 500C The “approach temperature” is usually 10 (heat exchange, condensation) or 20 0C (boiling) Use the log mean temperature difference (LMTD) as the driving force in the countercurrent heat exchanger For many cases, the mean temperature difference is 0.75(LMTD) for practical design purposeEstimation of the overall heat transfer coefficient The overall heat transfer coefficient U is determined from the heat transfer coefficients for the fluid on the inside and outside of the tubes (resistance effects are added to h) Approximate