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 scrubberWhat is the proper solvent?Henrys law is applicable for solvent selection,Examples and solutions,Consider a large-size compressorShould we used a steam turbine-driven or motor-driven compressor?The answer is a steam turbine-driven compressorThe 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 nextTo 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 purposeA centrifugal compressor,or a 2-or 3-stage reciprocating compressor is appropriate,Selection of a pump-First consideration is the viscosity,Selection 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 appropriate,The 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 solution,For 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 NPSHSince the supplied NPSH is only 2.47 m,the pump cavitates,Transportation of solids,Problem 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 bookA crew conveyor is appropriateFor a trough loading of 45%,find the screw diameter as 0.32 m,Heat Exchange Equipments:General Classification,FurnacesExchangersBoilersCondensersCryogenic unitsGeneral applications for heating and cooling(see the next figure),Rough Sizing of Furnace,Example and solution,Select a furnace to heat up an oilThe heating load is 2 x 107 kJ/hConsider a wider heat flux rangeWhat will be the total area of tubes?Select a A-furnaceFrom the previous figure,estimate the total tube area as 300 m2,Heat Exchanger Design:General Steps,Determine the heat loadSelect the heating/cooling mediumMake an overall selection Decide the inlet and outlet temperaturesEstimate the heat transfer coefficientDetermine the heat transfer area,Circulation Systems for Heat Exchange,Determination of the Heat Load,Sensible heatLatent heatHeat of reactionThe total heat load is the summation of all above termsIt is easily obtained from the energy balance of process flowsheet,Selection of the Medium,High TemperatureCombustion gasesHot mineral oilUsual TemperatureCondensing steamAir or waterVery Low TemperatureAmmoniaEthylene,Example 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 atmUse steam at higher pressure(20 atm)Choose Dowthern A at a lower pressure,Selection of Equipment Based on the General Range(Some figures are shown below),Selection of the Inlet and Outlet Temperatures,Normally,use countercurrent configurationsIf 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 500CThe“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 exchangerFor many cases,the mean temperature difference is 0.75(LMTD)for practical design purpose,Estimation 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 values for U are available in many literature The total area is finally calculatedQ(heat duty)=U x Area x DT,Direct contact Heat Exchange Equipments,Example and solution,Fatty acid drops are cooled down from 255 to 140 0C,at a flow rate of 0.8 kg/s and heat capacity of 2.8 kJ/kg0CA direct contact countercurrent heat exchanger(spray tower)is chosen.Water inlet is at 60 0C,heat capacity is assumed as 4.2 kJ/kg0CThe volumetric heat transfer coefficient for a spray tower is from 2 to 70 kW/m3K.Take it as 20 kW/m3K in this problemIf the diameter is 0.9 m,estimate the height of heat transfer region,Example and solution,The cooling load is:Q=0.8 x 2.8 x(255-140)=257 kWThe outlet water temperature is:257=0.35 x 4.2 x(T-60)Hence T=235 0CThe LMTD is:LMTD=(140-60)(255-235)/ln(80/20)=43 0CThe volume of tower is:Q(257 kW)=U(20 kW/m3K)x V x 43 0CV=0.3 m3The height of heat transfer region is found as 0.5 mLiterature has shown a result of 0.9 to 1.2 m,Cooling Tower,Rough Sizing of Cooling Water,Liquid loading:1.7 L/m2 for fully packed cross section,and 2.4 L/m2 for cross flow configurationVolumetric gas flow rate(per cross sectional area)is 1800 dm3/m2 sRatio of liquid and gas flow rate is around 1.0Height of cooling tower:H=4.6 to 6 m,if approach T is 8.3 to 11 0CH=7 to 9 m,if approach T is 4.5 to 8.3 0CH=10.7 to 12 m,if approach T is 2.2 to 4.5 0C,Example and solution,To cool down 312 kg/s of water from 43 to 29.3 0C,a cooling tower is selectedThe wet bulb temperature of air is 23.4 0CEstimate the size of a fully packed tower,Example and solution,The cross sectional are is:A=312 kg/s/(1 kg/L)(1.7 L/m2s)A=183 m2 The gas flow rate is:V=183 m2 x 1800 dm3/m2s=330,000 dm3/s This is about 330 m3/s x 1.2 kg/m3=396 kg/sThe liquid to gas flow rate is:312/396=0.79Less air is needed.Let it be 312 kg/s,or 260 m3/sThe cooling range is:Approach T=29.3 23.4=5.9 0CThe height of tower is:H=8 mLiterature example gives a result of 6.4 m,Problem to design a heat exchanger,Methanol at a flow rate of 100,000 kg/h is cooled from 95 to 40 0C.Water is used as the coolant with a temperature rise of 25 to 40 0CDesign a shell and tube heat exchanger:Number of tubes,Shell diameter,Pressure drop,etc.,Reactor design,PFTR configurations,STR configurations,Problem 1,Consider the dehydration of toluene to make benzeneGas phase reaction plus solid catalystTemperature is 590 to 650 oCPressure is 6 to 11 MPaReaction is first order with respect to toluene and 0.5 order to hydrogenExothermic reaction:50 MJ/kmol,Solution to Problem 1,Relatively high pressure and temperatureSelect a PFTR reactor with catalyst packed in tubesMay operate adiabatically or remove some of the heat,Problem 2,Consider the alkylation of isobutane to produce alkyllateLiquid phase reaction with immiscible sulfuric acid catalystTemperature 7 to 13 oCPressure 580 to 650 kPaVery rapid reaction,very complex chain mechanism,slightly exothermic,Solution to Problem 2,This is liquid reaction at room temperature and high pressureUse a CFSTR reactorThis is a relatively clean reaction.Use the internal heat exchanger,Problem 3,Consider the suspension polymerization of vinyl chloride to make PVCLiquid monomer drops suspended in water with water soluble initiatorTemperature is 25 oC,pressure is 0.85 MPaExothermic reaction:115 MJ/kmol monomer convertedNo catalyst,complex reaction mechanism,Solution to Problem 3,Use a batch STR,or a series of CFSTRWater is added to control the removal of heat,and thus control the temperatureTemperature variation must be minimized to keep a good molar mass distribution of the productKeep a narrow RTD,