A B C
ACHE CAD, PLASTICATING SCREWS CAF, COMPRESSED ASBESTOS FIBRE JOINTING CALANDRIA CALCIUM CALCULATING TIME CHARACTERISTICS OF IGNITION OF HYBRID GAS SUSPENSIONS CALDER HALL CALORIE CALORIFIC VALUE OF FUEL CALORIMETRY CANDU NUCLEAR POWER REACTORS CANONICAL PARTITION FUNCTION CAP BUBBLES CAPACATIVE HEAT EXCHANGERS CAPILLARITY Capillary action CAPILLARY CONVECTION CARBOLIC ACID CARBON CARBON ARC CARBON DIOXIDE CARBON DIOXIDE POLLUTION CARBON DIOXIDE, AS A POLLUTANT CARBON DISULFIDE COMBUSTION CARBON MONOXIDE CARBON STEELS CARBON SUBNITRIDE COMBUSTION CARBON THERMOMETERS CARBONACEOUS FUELS CARBONTETRACHLORIDE CARNOT CYCLE CARNOT, NLS CARRYUNDER CARTESIAN COORDINATES CASTING OF METALS CATALYSIS CATALYSTS CATALYTIC ACTIVITY CATALYTIC CONVERSION CATALYTIC CONVERTERS CATALYTIC CRACKING OF PALM OIL CATALYTIC RICH GAS PROCESS, CRG CATHODE CAUCHY SURFACE CAUCHY'S CONVEYENCE PRINCIPLE CAUCHY'S THEOREM CAUSTIC SODA CAVITATING FLOWS CAVITATION CAVITIES, FOR NUCLEATION CAVITY, SQUARE CEA CEC CELL GROWTH CELL POTENTIAL CELLULOSIC FIRES CELSIUS TEMPERATURE SCALE CENTIGRADE TEMPERATURE SCALE CENTRIFUGAL FILTERS CENTRIFUGAL FLOWMETERS CENTRIFUGAL FLUIDIZED BED CENTRIFUGAL SCRUBBER CENTRIFUGAL SEPARATORS CENTRIFUGES CENTRIPETAL BUOYANCY CENTRIPETAL FORCE CERAMIC CRUCIBLE PLASMA FURNACE CERAMICS CERENKOV RADIATION CERMETS CFCS, CHLOROFLUOROCARBON CFD CFD MODELS CHAIN REACTION CHANG-LIN TIEN CHANNEL CONTROL Channel Flow CHANNEL INSTABILITY CHANNEL IRREGULARLY HEATED CHANNELING EFFECT CHAOS CHAR CHARACTERISTIC DRYING CURVE CHARACTERISTIC EQUATIONS, FOR SUPERSONIC FLOW CHARACTERISTICS, METHOD OF CHARACTERISTICS, OF DIFFERENTIAL EQUATIONS CHARCOAL CHARGE CARRIERS CHARGE COUPLED DEVICES, CCD CHARLES LAW CHEBYSHEV EQUATION CHEBYSHEV POLYNOMIAL EXPANSION CHEBYSHEV POLYNOMIALS CHELATION CHEMICAL COMPLEXITY CHEMICAL EQUILIBRIUM CHEMICAL KINETICS CHEMICAL LASERS CHEMICAL POTENTIAL CHEMICAL REACTION CHEMICAL REACTION FOULING CHEMICAL THEORIES, FOR CATALYSIS CHEMICAL THERMODYNAMICS CHEMISORPTION CHEN CORRELATION CHEVRON SEPARATORS Chezy Formula CHF CORRELATIONS CHF, CRITICAL HEAT FLUX CHILTON-COLBURN ANALOGY CHIMNEY PLUMES CHIMNEYS CHLOR-ALKALI ELECTROLYSIS CHLORINE CHLOROFLUOROCARBON, CFC CHLOROFORM CHOKED FLOW CHROMATIC DISPERSION CHROMATOGRAPHY CHUGGING INSTABILITIES Churn Flow CIRCUIT BREAKER CIRCULATION RATIO CISE CORRELATIONS CLADDING CLAPEYRON EQUATION CLAPEYRON-CLAUSIUS EQUATION CLARIFICATION CLARIFIERS Classification of foam structures CLASSIFICATION OF HEAT EXCHANGERS CLASSIFIERS CLAUSIUS CLAUSIUS NUMBER CLAUSIUS-CLAPEYRON EQUATION CLAUSIUS-MOSOTTI EQUATION CLEANING TECHNIQUES, HEAT EXCHANGERS Climate study CLIMATIZATION CLIMBING FILM EVAPORATOR Closed cell foam CLOSED CYCLE GAS TURBINE CLOSED CYCLE MHD GENERATORS CLOSED SYSTEM CLOSURE LAWS CLOUD POINT SPECIFICATION CNEN CO-GENERATION SYSTEMS CO-ORDINATE TRANSFORMATION METHODS COAGULATION COAGULATION, OF AEROSOLS COAGULATION, OF DROPS COAL COAL BURNERS COAL CARBONIZATION COAL COMBUSTION COAL GAS COAL GASIFICATION COAL RESEARCH ESTABLISHMENT, CRE COAL SLURRY COALESCENCE Coanda Effect COARSE VARIABLES FOR DYNAMICS COARSE-GRAINED APPROXIMATION COATINGS COAXIAL TWISTING FLOW COEFFICIENT OF PERFORMANCE, COP COHERENCE FUNCTION COHERENCE STRICTURES, IN TURBULENT FLOW COHERENCE, OF RADIATION COHERENT SYSTEM OF UNITS COIL IN TANK COILED TUBE BOILERS Coiled Tube, Flow and Pressure Drop in Coiled Tubes, Heat Transfer in COILED WIRE INSERTS COKE COKE OVENS COKE-OVEN GAS COLBURN CORRELATION COLBURN FACTOR COLBURN HEAT TRANSFER FACTOR COLBURN J-FACTOR COLBURN, ALLAN PHILIP (1904-1955) COLBURN-CHILTON ANALOGY COLD ROD EFFECTS COLEBROOK-WHITE EQUATION, FOR FRICTION FACTOR COLEBROOK-WHITE FORMULA COLLECTION EFFICIENCY COLLIGATIVE PROPERTIES COLLIGEND COLLOCATION COLLOIDAL DISPERSIONS COLOR SEGREGATION IN METAL-HALIDE LAMPS COLUMN CHROMATOGRAPHY COLUMNS COMBINATORIAL MODELING COMBINED BRINKMAN-ELECTRIC BOUNDARY LAYER COMBINED CYCLES COMBINED HEAT AND MASS TRANSFER Combined heat transfer by radiation, conduction, and convection COMBINED RADIATION AND COMBUSTION COMBUSTION COMBUSTION CHAMBER COMBUSTION PRODUCTS COMFORT CONDITIONS COMITATO NAZIONALE PER LA RICERCA E PER LO SVILUPPO DELL'ENERGIA NUCLEARE E DELLE ENERGIE ALTERNATIVE, ENEA COMMERCIAL PLASMATRON COMMISSARIAT A L'ENERGIE ATOMIQUE, CEA COMMISSION OF THE EUROPEAN COMMUNITY, CEC COMMON MODE FAILURE COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION, CSIRO COMPACT HEAT EXCHANGERS COMPILER COMPLEX COMPOUND CATALYSIS COMPLEXIFICATION COMPLEXING IONS COMPLEXITY COMPOSITE FLOW COMPOSITE MATERIALS COMPOSITE MATERIALS, ABLATION OF COMPOSITE MATERIALS, COMPUTATION OF COMPOSITE POROUS LAYER COMPOSITES, THERMAL CONDUCTIVITY OF COMPOUND AUGMENTATION COMPRESSED ASBESTOS FIBER JOINTING, CAF COMPRESSIBILITY EFFECTS COMPRESSIBILITY FACTOR Compressible Flow COMPRESSION PLASMA FLOWS COMPRESSION POINT COMPRESSION ZONE COMPRESSION-IGNITION ENGINES COMPRESSORS COMPTON SCATTERING COMPUTATIONAL FLUID DYNAMIC MODELS Computational fluid dynamics Computational methods Computational methods for radiative transfer in disperse systems COMPUTER AIDED DESIGN, CAD COMPUTER PROGRAMMES COMPUTERS CONCAVE SURFACE, FLOW OVER CONCENTRATING COLLECTOR CONCENTRATION-DEPENDENT CHLORIDE DIFFUSIVITY Concept of regularization CONCRETE CONCURRENT MULTISCALE PROBLEMS CONDENSATE INUNDATION CONDENSATION COEFFICIENT CONDENSATION CURVE CONDENSATION IN ENCLOSURES CONDENSATION IN TUBE BANKS CONDENSATION IN TUBES CONDENSATION OF A PURE VAPOR CONDENSATION OF MOVING VAPOR INSIDE VERTICAL TUBES CONDENSATION OF MULTICOMPONENT VAPORS CONDENSATION ON OUTSIDE OF TUBES IN CROSSFLOW CONDENSATION RELAXATION OF SUPERSATURATED VAPOR CONDENSATION SHOCKS CONDENSATION, OF DROPS CONDENSATION, OVERVIEW CONDENSERS CONDUCTANCE PROBES, FOR LOCAL VOID FRACTION CONDUCTANCE, ELECTRICAL CONDUCTION CONDUCTION AND CONVECTION COMBINED CONDUCTION COMBINED WITH RADIATION CONDUCTION DRYING CONDUCTION EQUATION CONDUCTION IN HEAT EXCHANGER WALLS CONDUCTIVE HEAT FLUX CONDUCTIVITY CONDUCTIVITY RATIO CONDUCTIVITY, ELECTRICAL CONDUCTIVITY, OF PLASMA CONE CLASSIFIER Configuration factors for radiation transfer between diffuse surfaces CONFINED SPRAY FLAME CONFORMAL MAPPING CONFORMAL POTENTIALS CONICAL SHOCK WAVE CONJUGATE HEAT TRANSFER CONSERVATION EQUATIONS CONSERVATION EQUATIONS, SINGLE-PHASE Conservation equations, Two-phase Conservation Laws CONSERVATIVE SYSTEMS CONSTANT RATE PERIOD, DRYING CURVE CONSTITUTIVE EQUATIONS CONSTITUTIVE RELATION, THERMODYNAMICS Contact angle CONTACT CONDUCTANCE CONTACT DISCONTINUITIES CONTACT RESISTANCE CONTAINMENT CONTINUITY EQUATION CONTINUITY SHOCKS CONTINUITY WAVES CONTINUOUS CASTING CONTINUOUS CRYSTALLIZERS CONTINUOUS FILTERS CONTINUOUS WAVE LASERS Continuum Continuum Hypothesis CONTINUUM MECHANICS CONTINUUM MODELS Contraction, Flow and Pressure Loss in CONTRACTORS CONTROL THEORY CONVECTION CONDENSATION CONVECTION DRYING CONVECTION RADIATION CONVECTIVE BOILING CONVECTIVE HEAT FLUX CONVECTIVE HEAT TRANSFER CONVECTIVE HEAT TRANSFER ENHANCEMENT CONVECTIVE MASS TRANSFER CONVERGENCE FACTORS CONVERGENCE OF SERIES CONVERGING BOUNDARIES CONVERSION FACTORS COOL FLAME EVAPORATION COOLANTS, REACTOR COOPER CORRELATION, FOR NUCLEATE BOILING COORDINATE SYSTEM COPPER CORE, NUCLEAR REACTOR CORED BRICK HEAT EXCHANGERS CORIOLIS EFFECT CORIOLIS EFFECT, IN ATMOSPHERIC CIRCULATION CORIOLIS MASS FLOWMETER CORLISS VALVE CORONA DISCHARGE, ELECTROSTATIC PRECIPITATION CORONARY ARTERIES Correlated k-models CORRELATION CORRELATION ANALYSIS CORRELATION COEFFICIENT CORRELATION, FOR CONVECTIVE HEAT TRANSFER CORRELATIONS FOR NOx EMISSIONS FROM A SWIRL BURNER CONCEPT CORRESPONDING STATES, PRINCIPLE OF CORROSION FOULING CORROSION, PREDICTION METHODS FOR CORRUGATED CONDENSED TUBES CORRUGATIONS, PLAIN, PERFORATED, AND SERATED COUETTE VISCOMETER COULTER COUNTER COUNTER CURRENT FLOW LIMITATION, CCFL COUNTER CURRENT TWO-PHASE FLOW COUNTERIONIC ATTRACTION Coupled (combined) radiation and conduction COUPLED AUTOREGULATED OSCILLATING CELLS COUPLED CONDUCTION AND CONVECTION COUPLED HEAT AND MASS FLUXES Coupled radiation and convection Coupled radiation, convection and conduction COVALENT BONDING COWPER STOVES CRACKING CRAMER'S RULE CRE CREAGER-OFITSEROV PROFILE CREEPING FLOW CRITICAL CHOKING CRITICAL CONCENTRATION CRITICAL DEPOSITION VELOCITY Critical Flow CRITICAL FLOW RATE, IN ORIFICES CRITICAL HEAT FLUX IN BOILING LIQUID METALS CRITICAL HEAT FLUX IN COILS CRITICAL HEAT FLUX, CHF CRITICAL POINT, DRYING CURVE CRITICAL POINT, THERMODYNAMICS CRITICAL PRESSURE CRITICAL PRESSURE RATIO Critical Reynolds number CRITICAL SEDIMENTATION POINT CRITICAL STATE CRITICAL SURFACE TENSION CRITICAL TEMPERATURE CRITICAL TEMPERATURE, FOR SUPERCONDUCTIVITY CRITICAL TRANSITION VELOCITY CRITICAL ZONE CRITICALITY CROCCO TRANSFORMATION CROCCO'S THEOREM CROSS CORRELATION CROSS FLOW HEAT TRANSFER CROSS FLUXES CROSS SECTIONS CROSS SPECTRUM Crossflow CRUDE OIL CRYOGENIC FLUIDS CRYOGENIC PLANT CRYOGENIC PUMP CRYOGENIC USE OF STEEL CRYOSCOPIC CONSTANT CRYOSTATS CRYSTAL GROWTH CRYSTAL STRUCTURE ASYMMETRY CRYSTAL SUBLIMATION AND GROWTH CRYSTALLIZATION CRYSTALLIZATION FOULING CRYSTALLIZERS CRYSTALS CSIRO CUBIC LATTICES CUNNINGHAM COEFFICIENT CURRENT VOLTAGE CHARACTERISTICS CURRENTS, NEARSHORE CURVED FLOW CURVILINEAR CHANNELS CYANOGEN COMBUSTION CYCLIC HYDROCARBONS CYCLOHEXANOL CYCLONE FURNACES CYCLONE REYNOLDS NUMBER CYCLONE SEPARATOR CYCLONE STOKES NUMBER CYCLONES CYLINDER, INVISCID FLOW AROUND CYLINDERS, FLOW OVER CYLINDRICAL COORDINATES CYLINDRICAL FINS CYLINDRICAL POLAR COORDINATES
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CRYOGENIC PLANT

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A common feature of a cryogenic plant is the separation and/or Liquefaction of Gases at process conditions which may be at elevated pressures, but always involving very low temperatures. There are many industrial cryogenic processes which operate at temperatures in the region of –165°C to –195°C at their coldest point, with some operating as low as –269°C. Consequently, the conservation of cold becomes a dominant feature in the design of such processes, which focuses on highly efficient heat exchange. However, a typical cryogenic process has many elements to it, the cryogenic section being only a part of the whole flowscheme. Figure 1 illustrates a typical cryogenic plant which consists of a pretreatment section, a cryogenic section and a compressor/ expander section which provides refrigeration for the process. In many instances, feed compression may be required as in the case of air separation. Another aspect of cryogenic plant is the use of aluminum and stainless steel for the cold sections of the plant to avoid embrittlement problems encountered with carbon steel.

A flow sheet of a hydrogen recovery unit treating refinery off-gas.

Figure 1. A flow sheet of a hydrogen recovery unit treating refinery off-gas.

The prepurification section is usually needed upstream of a cryogenic plant because most feed gases will contain constituents that may freeze inside or even corrode the cryogenic equipment and will therefore require removal. Wet hydrocarbons can form hydrates at temperatures above 0°C (typically 5-15°C) and in such cases, water removal may be necessary. Freezable components include H2O and CO2 and NH3. These are usually removed from the feed gas by absorption, chilling, permeation, reversing heat exchangers, or combinations of these in a prepurification section.

Table 1 gives a list of typical gases treated by cryogenic units. Typical impurities are listed for each of these gases. In general, these impurities are removed upstream to less than 1 ppm. However, in some cases the liquids formed in the process dissolve the freezable components to a significant extent, making it possible to allow several hundred ppm to enter the cryogenic unit. Each case has to be considered carefully by the cryogenic unit designer, who uses reliable solubility and equilibrium data to determine the levels acceptable in the cryogenic unit. The impurity concentration in the feed gas determines the nature of the pretreatment process used. Table 2 shows a very general guide to pretreatment processes used for various impurity levels; each case must be carefully analyzed to select the proper scheme. Impurities such as HCl, NH4Cl, Hg and arsenic compounds are generally found in very low concentrations and catalytic methods are often used to remove them.

Table 1. Freezable impurities present in cryogenic plant feed gas

Table 2. Pretreatment methods used to treat freezable components

The clean, dry gas is admitted to the cryogenic part of the plant, which cools, condenses and separates the desired components in the gas at low temperatures. In order to produce low temperatures, it is imperative that all parts of the cryogenic unit are designed as efficiently as possible (see Liquefaction of Gases). Exergy analysis [Tomlinson et al. (1990)] shows how important process efficiency is in reducing power consumption and improving product yields.

Both reciprocating and rotating compression equipment is used in feed compression and refrigeration cycles in a cryogenic plant. The major difference between compressor equipment for cryogenic plant compared with noncryogenic plant is in the lubrication system. Many gas compressor preceding cryogenic processes are oil-free on the process side. If oil-lubricated compressors are used, then oil-removal systems are included after the machines. For these types of reciprocating compressors, piston rings are made of composite materials, which include graphite, thereby promoting lubrication. Packing glands are purged with dry gases to form a seal between the oil-lubricated crankcase side and the process end. In centrifugal machinery, a dry-seal system is provided to prevent process gas being contaminated with oil so that the metal bearings are allowed thorough lubrication while the process gas is kept dry.

Expansion turbines are used extensively to provide efficient refrigeration. These require dry operation for the process gas in a similar manner to 'dry' centrifugal compressors, and therefore employ similar lubrication and seal systems.

Reciprocating expanders are also used today in helium liquefiers, where flows are relatively small. Also, design of rotary expanders for low molecular weight gases to achieve a high efficiency is difficult without using very high speeds.

One of the most important aspects of a good cryogenic plant design is the effective use of heat exchangers. This is clearly brought out by exergy analysis during the conceptual design stage. Cryogenic plants generally utilize three types of heat exchangers: shell-and-tube, wound-coil, and plate-fin. The most widely used is the plate-fin. To appreciate why this is so, the three types of heat exchanger are compared in Table 3 in terms of area per unit volume, maximum design pressure, practical approach temperatures, number of streams handled in one unit, and materials of construction. The materials of construction have cryogenic service in mind. Another effective heat exchanger that can be applied in cryogenic service is the etched plate compact heat exchanger. This is particularly suitable where pressures exceed 10 MPa.

Table 3. Types of heat exchangers used in cryogenic service

A cutaway drawing of a plate-fin heat exchanger is shown in Figure 2.

Figure 2. 

Distillation is extensively used in cryogenics to produce pure components from mixtures of gases. There are some significant differences in cryogenic plant mass transfer equipment which aim to make the units as compact as possible. Consequently, sieve trays with very low tray spacing (80 to 100 mm) are extensively used in air separation and other areas. Structured packing is also seeing a significant increase in applications in cryogenic service. Table 4 gives a summary of mass transfer equipment used in cryogenic plant.

Table 4. Mass transfer equipment summary

To improve efficiency of cryogenic processes, process intensification is always seriously examined during process selection. An excellent example where this has succeeded is in the use of plate-fin heat exchangers to carry out simultaneous heat and mass transfer. Figure 3 shows a section of plate-fin heat exchanger used for distillation purposes. This is particularly suitable for a situation where a lower molecular weight gas contains a few percent of heavier components. The partly-cooled vapor may enter at point A, any condensate is knocked out in V1, the vapor then enters the reflux exchanger, E1, at B, and is cooled by refrigerant DE and product XY. The exchanger is always mounted vertically so that the feed stream cools in an upward direction to C. As it cools, condensate forms. Instead of the condensate passing up with the main gas flow, it runs back down the heat exchanger to the bottom and B. As it runs back, it is warmed by the upcoming vapor stream which tends to boil-off "light ends". Therefore, the liquid that emanates from the base B contains only small amounts of "light ends" and most of "the heavy ends". This liquid is approaching the equilibrium condition of the vapor entering at B and not the condition at the cold end of the exchanger C. This is the important factor of the reflux exchanger, which makes it more efficient than straight cooling and partial condensation. Several equilibrium stages can be catered for in a single long core, at the same time providing refrigeration at each theoretical stage.

A plate-fin heat exchanger used as a mass transfer device.

Figure 3. A plate-fin heat exchanger used as a mass transfer device.

The designer has to cater for heat transfer and distribution problems for plate-fins exchangers, as well as mass transfer. In counter-flow of vapor and liquid an appropriate flooding correlation is required.

REFERENCES

Blakey, P. (1981) 'Cryogenic gases' Chemical Engineering (Oct 5) 113-24.

Clarke, M. E. and Gardener, J. B. (1976) 'Refrigeration with expansion turbines', Contemp Phys., 17(6), 507-28.

Diehl, J. E. and Koppany, C. R. (1969) 'Flooding velocity correlation for gas liquid counterflow in vertical tubes'. Chem. Eng. Prog. Symp. Series 65(92), 77.

Diery, W. (1984) 'The manufacture of plate-fin heat exchangers at Linde'. Linde Reports Sci. Tech. 37.3.

Gregory, E. J. (1987) 'Heat Exchangers'. In Cryogenics Engineering (ed. B. A. Hands), Chap. 8, 193 Academic Press, London.

Haselden, G. G. (1971) Cryogenic Fundamentals. Academic Press, London.

Holm, J. (1985) 'Energy recovery with turboexpander processes'. C.E.P (July), 63.

Isalski, W. H. (1989) Separation of Gases, Oxford Science Publications.

Isalski, W. H. (1993) Refrigeration B/l, Kempe's Engineers Yearbook, Benn.

Keens, D. (1980) ‘Technology of LNG plants still undergoing a steady evolution'. Oil and Gas Journal (Apr. 7) 84.

Tomlinson, T. R. et al. (1990) Exergy Analysis in Process Development, The Chemical Engineer, 18 and 25 October.

References

  1. Blakey, P. (1981) 'Cryogenic gases' Chemical Engineering (Oct 5) 113-24.
  2. Clarke, M. E. and Gardener, J. B. (1976) 'Refrigeration with expansion turbines', Contemp Phys., 17(6), 507-28. DOI: 10.1080/00107517608219054
  3. Diehl, J. E. and Koppany, C. R. (1969) 'Flooding velocity correlation for gas liquid counterflow in vertical tubes'. Chem. Eng. Prog. Symp. Series 65(92), 77.
  4. Diery, W. (1984) 'The manufacture of plate-fin heat exchangers at Linde'. Linde Reports Sci. Tech. 37.3.
  5. Gregory, E. J. (1987) 'Heat Exchangers'. In Cryogenics Engineering (ed. B. A. Hands), Chap. 8, 193 Academic Press, London.
  6. Haselden, G. G. (1971) Cryogenic Fundamentals. Academic Press, London.
  7. Holm, J. (1985) 'Energy recovery with turboexpander processes'. C.E.P (July), 63.
  8. Isalski, W. H. (1989) Separation of Gases, Oxford Science Publications.
  9. Isalski, W. H. (1993) Refrigeration B/l, Kempe's Engineers Yearbook, Benn.
  10. Keens, D. (1980) ‘Technology of LNG plants still undergoing a steady evolution'. Oil and Gas Journal (Apr. 7) 84.
  11. Tomlinson, T. R. et al. (1990) Exergy Analysis in Process Development, The Chemical Engineer, 18 and 25 October.

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CRYOGENIC FLUIDS
LIQUEFACTION OF GASES

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