Pfd Sheet.docx

Mixing tanks (Vessels)  for viscous, non-Newtonian fluids or streams with suspended solids  abbreviated V In Aspen, multiple streams of the same composition cannot be sent to units or multiple streams cannot be drawn out from units; so, mixers and splitters are used; high-pressure gases and low viscosity liquids mix easily and hence do not need a mixer tank Splitter  output streams have the same temperature, pressure, and composition as the input stream; only flow rates change  pipe and valve systems Mixer:  changes composition, temperature of the output stream  T pipe joints Pumps: increase pressure of liquids  Pumping vapors/gases leads to cavitation and pump damage (NPSH = fluid pressure @ suction – vapor pressure of fluid; if pressure at suction side < vapor pressure, fluid boils and forms bubbles that spoil the impeller over time; NPSHA(available) > NPSHR (required); NPSHR is the minimum inlet pressure to prevent cavitation at a given flow rate  To avoid cavitation o keep inlet pipes short and of small dia i.e keep pipe losses to the minimum – minimal use of pipe fittings such as valves, tees o increase the static head  Pump type (centrifugal/reciprocating/axial/rotary) chosen based on flow rate, horse power required, inlet & outlet pressure of process streams and price o Centrifugal pumps convert mechanical energy of the impeller to kinetic energy (fluid velocity and pressure)  have shut-off head – when net flow through the pump stops while the pump is working – happens when gravity prevents the water from rising in the discharge pipe; liquid rises to a level and then quits rising when it cannot overcome gravity o Reciprocating or Positive displacement pumps directly displace fluid from pump inlet to outlet in discrete volumes; expand the cavity on suction side to take in fluid and decrease the cavity on discharge side for fluid to leave  do not have shut-off head; cannot operate at all against a closed discharge valve; will cause pressure in discharge line increase leading to line burst or pump damage  Abbreviated P  Drive provided by o Electric motor – power < 100 hp; high efficiency o Steam turbine - >100 hp; power outage; less efficient than electric; create motion by sending steam through turbine which generates rotation which is used to run pumps/compressors; converts thermal and pressure energy to mechanical energy; pressure and temperature energy together is enthalpy o Gas turbine – instead of steam through expander, gas/vapor is used o IC Engine – remote areas, combust fuels, highly inefficient o Efficiency: Electric > Steam> Gas> IC Engine

Compressors: increase pressure of vapors, gases  Fluid is accelerated in the turbine by rotors and then compressed by sending them through tapered channel  Abbreviated C  Drive provided by o Electrical – power < 100 hp; high efficiency o Steam turbine - >100 hp; power outage; less efficiency o Gas turbine – o IC Engine – remote areas, combust fuels, highly inefficient

Turbines (expanders): depressurize liquids, gases and vapors  Drive – none required; fluid does work on the turbine, loses its heat energy, depressurized and expands  Has negative shaft power as energy is removed from fluid  Overall efficiency = turbine efficiency as no drive  Abbreviated C (since turbines are essentially compressors in reverse) Valves  depressurize liquids, gases  ball, gate – designed to be kept all the way open or closed; using them to vary flow rates damages them  butterfly, globe – both manual valve and control valve – used to throttle or control flow  control valve - control flow rates via feedback or feed forward control mechanisms  not labelled or included in equipment summary table; not abbreviated; ancillary equipment Storage Tanks (Tk)  not accumulators or surge or flash  to store raw materials, products  heuristic: should not hold more than one month’s worth of material Control Loops/philosophy – for safety, efficiency and efficacy – predictable, consistent performance  To avoid runaway reactions, unwanted side reactions, unexpected concentration changes – feedback control loop – vary power supply or valves to open/close  Place control valves on streams that can be independently adjusted (except for level control) eg; utility streams  Avoid cv on process streams unless at the beginning or the very end of process or on a vessel controlling liquid level  Minimize dead time and lag time  Don’t adjust streams if not related to the variable being controlled  Streams with same compositions need not have two control valves on them!  Parallel heat exchangers, compressors or pumps must both be controlled to maintain similar stream conditions when those parallel streams are joined at one point  Solid line – capillary (liquid) signal; dotted line – electric (voltage) signal; # - pneumatic (air) signal  Temperature o Increase or decrease cooling supply or steam supply to heat exchanger o vary utility to reactor cooling jackets

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distillate temperature controlled by feed flow rate and composition, reboiler/condenser duties and column dimensions; distillate temp related more to concentration coming out of the top of column; more common to control reboiler heat duty but it effects distillate temperature indirectly Pressure o Best controlled by pumps, compressors, depressurization valves o power supply to pumps/compressors motors; no need of control valves as power is varied Level o Important in flash units, towers, vessels o control valve to control flow out of the equipment Flow rate o Like level control; only difference is flow rate is measured as opposed to level o Process equipment is not one size fits all; they are custom made and cannot arbitrarily adjust to any flowrate, temp, pr, concentration; o Flow rate of product control – cannot send feedback to feed due to large dead time and there may have been problems anywhere in the process and not feed flow rates Isothermal reactor - inlet temperature, inlet pressure and temperature in the interior of reactor are all controlled

Reactors (R) – CSTR, Plug flow, PBR, Fluidized Bed Reactor  PFR – non-agitated reactors; no axial mixing; well mixed radially  PBR – large sized catalysts  Fluidized – small catalysts  Moving bed – reactants and catalysts either move counter or co-current; when catalysts prone to fouling, decay and needs regeneration  All reactors can be designed to run under isothermal or adiabatic conditions in Aspen  Isothermal reactors: cooling water, high/medium/low pressure steam, refrigerated water, electric heat are used

Towers-distillation, adsorption, absorption, stripping, liquid-liquid extraction (T)  single feed, 2 streams out - Tray or packed columns  2 stream continuous flow separators – tray or packed; adsorption, absorption, stripping, liquidliquid extraction; separating chemicals from a stream using another stream  Distillation – separating chemicals using heat – boiling point differences and volatilities; energetically expensive but no needed to dispose or regenerate spent chemicals unlike in other types of towers – widely used for homogeneous mixtures; cheap and efficient if disposal/regeneration costs considered; boiling occurs on every tray; always a vertical column as vapor rises and liquid falls – so always a tower  Extractive/azeotropic distillation – chemicals are used to facilitate separating close boiling mixtures  Absorption – liquid used to clean a gas; based on solubility; components being removed must be more soluble in the liquid phase compared to gas; spent liquid; water used commonly; Tray or packed columns  Stripping – gas used to clean a liquid; contaminants transferred to gas from liquid (spent gas); steam and air used commonly; Tray or packed columns  Adsorption – solids to clean liquid or gas; packed column/bed; activated carbon, silica, clays are adsorbates

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Liquid-liquid extraction - One liquid washes another; immiscible liquids of varying density; based on solubility; water and hexane – common solvents Distillation – condensers and boilers usually at ground level; reflux pumped to top of tower; bottoms flow by gravity to heat exchangers and vapors rise to the base of the column

Vessels (V) – accumulation (surge tanks/ blow down drums) - flash tanks (vapor/liquid separator/knock out)  process units and are not storage tanks -– not for longer time storage

eg: Reflux Drum

Flash / knock out/ v-l separator (separation by boiling) separate a homogenous stream into vapor and liquid with different compositions not splitters where composition remains same and only flow rates change single stage distillation without condenser or reboiler

one stream coming in and going out can be horizontal or vertical

one stream in and 2 or more streams out always vertical

Accumulators/blowdown/surge temporary storage hold overflow, process streams short-term

Heat Exchangers (E)  

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Conductive or convective heat transfer across a surface and not combustion General representation of heat exchanger - Black lines zig zagging through the heater/cooler symbol carries the utilities stream to heat up or cool down the process stream (grey line) – more of a temporary place holder i.e. do not connect any utility streams to them; no way to specify the type and properties of utility streams in this type; carry out a rough calculation with this simple exchanger when designing reactors and separators; then replace generic with a rigorous, realistic exchanger such as shell and tube Shell & tube – most common type – 2 stream heat exchanger – temperature of stream not deciding factor for fluid placement in shell/tube o Tube side – high pressure fluids, vacuum pressure fluids, corrosive fouling fluids – eg: hps through tube and process stream through shell o Shell side – condensing vapors, viscous fluids – eg: vapor from top of distillation column through shell side of condenser and cooling water through tubes; vapor process stream condenses on the surface of the tubes o If both streams are high or vacuum pressure, corrosive, fouling – put the more extreme one on tube and less extreme one on shell o If both streams viscous – more viscous on shell o Neither stream under above conditions – under more extreme pressure on tube  At exact same pressure – doesn’t matter which fluid goes in tube/shell o Fixed head – tubes are fixed to tube sheets inside a shell o Floating head – tubes which are on a floating head that allows tubes to expand/contract to accommodate large temp differences between the streams and between the inlet and outlet streams o U-tube – u-shaped tubes which allow for some expansion/contraction and allow fluid flow easily through exchangers – for viscous fluids

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For efficient heat transfer, wide range of areas but expensive, have high pressure drops; inappropriate for corrosive, fouling, viscous fluids o Plate and frame – maximum heat transfer using minimum volume; hot and cold fluids enter and exit on same side of exchanger o Plate and fin/ compact – industrial radiators; highest maximum surface area heat exchanger o Either stream can be on shell/tube Smaller heat exchangers, for viscous fluids o Double pipe – tube inside a tube o Multiple pipe – multiple tubes inside a tube Exchangers when stream undergoes phase change – evaporation, condensation, boiling o Kettle Reboiler – like a shell and tube; has unique shape to accommodate overhead vapor space; good for boiling fluids on the shell side; eg: boiling liquid in a distillation column o Bayonet – like a shell and tube without shell; can be installed inside a vessel; tubes fixed to a tube plate only on one end; each tube has a smaller one within it creating a inner concentric cylinder and annulus; high pressure fluid enters through inner concentric tube, moves to the opposite end, reverses direction and exits via the annulus; inlet fluid is kept separate from the outlet fluid through the use of 2 tube plates; process stream inside the vessel is like shell side and exchanges heat with the fluid in the tube Air-cooled exchanger – for large areas, low maintenance and operating costs o Uses blown air to cool down a liquid or gas o Motor-driven fan unit with rotating fan blades to circulate air and cool down the ht fluid in the tube bundle mounted on top of the fan unit by air convection o Air – shell side; fluid – tube side (no shell exists actually) Temp change across a heat exchanger be greater than 5 C but not more than about 15-20c to avoid energy burden on equipment supplying utility streams to process (>20c) or excessively high utility flowrate (<5c) which requires large steam boiler or cooling tower; adjust flowrate of utility stream to get the desired temperature change For utility streams that undergo phase change, a temperature difference of 5 – 10c should be enough Vary flowrate or type of utility to achieve desired temperature of process streams and to keep safe operating temperatures

Furnaces or Fired Heaters (H)  To heat streams to 400 – 1500 C by using flammable gases (methane, ethane, propane leftover streams that need to be disposed) natural gas – ng; fuel gas – fg; fuel oil – fo; liquified petroleum gas – lpg; flammable liquid hydrocarbons – gasoline, diesel, flammable oil; solids- coal, coke, anthracites  Most fuel sources have same adiabatic flame temperature – exiting gases are at same temp for almost all the fuels for a well-insulated system  Very expensive, furnace insides lined with ceramic bricks for insulation  Very tall with high vent stacks;  vertically aligned tubes have process streams sent through them to be heated by surrounding hot gases; flammable gas on shell side  Radiant section – tubes in bottom section that receive most heat from gases being burnt  Convective section – lower in temperature than radiant section and above radiant section Stream tables/ Flow summary table – physical properties of process streams  Temp, pr, vapor fractions, flow rates, compositions - mass/mole fractions, enthalpy, heat capacity, density for each stream number

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List reactants first and then products for each stream Align numbers to the right – right justify for readability

Equipment Summary Table Provide critical physical characteristics of each piece of equipment that has a letter and number designation on PFD; help estimate the initial and ongoing cost of each equipment and profitability of the project  Compressors/Pumps/Turbines o Type, drive, materials of construction (MOC), properties of stream – flow rate, inlet/outlet temp, inlet/outlet pressure, fluid density, vapor pressure, Shaft power (power required to run a pump/compressor or generated in a turbine) o Shaft power = theoretical power/ (shaft efficiency * drive efficiency); theoretical power is by assuming 100% efficiency of shaft and drive; overall efficiency = shaft efficiency * drive efficiency for pumps, compressors; overall efficiency = turbine efficiency for turbines as no drive  Reactors/Vessels/Towers o Orientation – vertical or horizontal; plug flow/pbr – horizontal; fluidized, moving bed, cstr – vertical; towers/flashes – vertical; accumulators – vertical/horizontal; o MOC – based on temp, chemical composition – durable, cost o Stream properties – inlet flowrates, temp, pr, vapor fraction; outlet properties not listed to avoid clutter; from Aspen o Dimensions  height or length of reactor, diameter - calculated based on residence time, pressure drop. Minimum fluidizing velocity, package placement, density of packaging etc;  Height of flash– from Aspen/hand calculations  Depend on flow rate, physical properties of stream, desired residence time of liquid, volatility  height of distillation column – based on number of trays and their spacing (default: 0.6 m or 2 ft spacing in Aspen; not to be used for towers with <1ft diameter); number of theoretical stages assumes 100% separation and is < number of trays with incomplete separation  Number of trays = (theoretical stages – partial condenser- partial reboiler)/ /tray efficiency  Tower height = [(number of trays-1) *tray spacing] + overhead vapor space + bottoms liquid holdup space  total condenser completely condenses the vapor to liquid , vapor fraction = 0 and is not considered an equilibrium stage; partial condenser has vle and so is a stage; partial reboilers – heat exchanger partially vaporizes liquid; total reboilers are rare o internals  reactors – inert packing, catalyst, trays – weight, size and type of catalyst and type of reactor; number of trays and their MOC  distillation columns – number of trays, MOC, type of trays  vessels – single tray, MOC, type of tray o Condenser and reboiler heating duty of distillation – not designated explicitly as heat exchangers o Reflux ratio – distillation column – must be included to help size reflux drum, reflux pump; higher reflux ratio means large diameter column and high condenser duty o heating jacket duty – isothermal reactors; zero for adiabatic

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temperature decrease in a flash is because of phase change and not because of any heating or cooling jacket  distillation columns and vessels do not have any jackets Heat Exchangers o Type – based on cost, area, stream properties, stream conditions o Area – A = Q/U*LMTD; U = overall heat transfer coefficient (depend on moc, phase of the stream); lmtd = temp driving force; A = heat transfer surface area o Heat duty o Lmtd o Shell-side properties o Tube-side properties o MOC –extreme temperatures of streams going through exchanger, corrosive/reactive chemicals and cost; can choose different materials for shell and tube; cladded materials – fluids are in contact with less reactive surface but the exchanger has the thickness of the cheaper material below o Designate the phase of the fluid going through heat exchanger based on the vapor fractions Fired Heaters or Furnaces o Type – gas/liquid/solid o Shell and tube side MOC – SS, Nickel Alloy, Ceramics (up to 3000 C) o Heat Duty o Radiant, Convective Area o Process stream physical properties – shell side pressure need not be specified as most gases rise through the furnace and are vented out at the top of furnace

Materials of Construction – temperature, concentration breakdown, chemical property data  Durability of equipment o resistance to very high or low temperature, reactivity, corrosion o pressure not an important variable in MOC as most materials can be manufactured with appropriate wall thickness to withstand high pressures  Cost of material  Carbon steel – can’t stand corrosive fluids - -45 C to 480 C  Low Alloy steel  Stainless Steel – 304 SS, 316 SS – resist corrosion and reactivity with many acids, bases; can with stand very low and high temp reactions  Aluminum and alloys, copper and alloys, nickel and alloys, titanium and alloys  Cladded materials – one material (costly) layered on top of a less expensive material  Chemical compatibility charts – design books (estimation of capital costs chapter) Utility Summary Table – physical properties of utilities streams  Utility streams are numbered sequentially at the last after process streams to organize them in one block for easier readability  Can incorporate them in stream tables (includes unnecessary information such as heat capacity, density, flow rates of reactants/ products which only add bulk to the table) or separately (fewer rows, neat without extraneous information)  All kinds of steam, cooling water, refrigerated water are all listed as water; can deduce the type of utility used based on temperature, pressure and vapor fraction  Cooling water – CW – water that comes from cooling tower and enters exchanger at 30C and exits at 40 C

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Refrigerated Water – rw, enters at 5C Refrigerated Brine – rb – dissolved salt in water, -45C in Lps – Low Pressure Steam – saturated steam at 140 – 160C in Mps – medium pressure steam – saturated steam at 180 – 200 C in Hps – high pressure steam – saturated steam at 250 -270 C in Electric heat – el – 220/440/660 volts electricity to resistively heat up a stream Htm- heat transfer medium – high boiling point hydrocarbon to heat up a stream up to 400 C Freon/ethane/propane/butane/ammonia/glycol – other utility streams without any letter abbreviation

Miscellaneous  For liquid flow, assume pipeline pressure drop of 2psi/100 ft of pipe and a control valve pressure drop of at least 10 psi; for each 10-ft rise in elevation, assume pressure drop of 4 psi  Use an expander to reduce pressure of a gas or a turbine to reduce pressure of a liquid (instead of using a simple valve) when more than 20 hp and 150 hp can be recovered respectively o Theoretical adiabatic horsepower equation  Line sizing hydraulics – liquid, gas, 2 phase  Tracing PFDs  Process Conditions Load Sheets – Pump, compressor, heat exchanger, fired heater, column, tanks, vessels Purpose is to transmit process data for equipment specification, hydraulics calculation or for planning utility systems Shut off pressure – Centrifugal pumps Discharge pressure of the pump at zero flow where maximum differential pressure occurs Pump run out End-of curve point whose flow corresponds to choked condition with minimal differential pressure In a Newtonian fluid the shear stress is proportional to the shear rate and the proportionality constant is called the viscosity incompressible flow or that density variations along the flow path are negligible Reynolds number (Re) – laminar or turbulent Dimensionless parameter called the Reynolds number can be used to define the flow region – laminar or turbulent Laminar flow is always encountered at Re < 2000. Generally turbulent flow is encountered at Re > 4000. Between 2000 and 4000 a transition region is found where the type of flow may be either laminar or turbulent and the fluid exerts properties somewhere in between the two flow regions. Friction factors are involved in pressure drop calculations. The friction factor is a function of two main parameters: Re number and relative roughness For laminar flow, the Darcy friction factor depends only on Re and not on relative roughness and is given by the following equation: f = 64/Re In fully developed turbulent flow the friction factor is much less dependent on Re number and almost completely dependent on relative roughness.