PRELIMINARY PROJECT PRESETATION BY D R J P EVALUATION AND OPTIMIZATION OF THE NITROGEN PLANT AT A TRINIDAD FACILITY NOVEMBER 2003
OBJECTIVE To evaluate the performance of the Nitrogen Plant at the Trinidad facility. To make recommendations to address the underperformance of the plant by means of maintenance and generation of a shutdown job list. To model and optimize the pre-treatment section also using the Hysys Process engineering software.
INRTODUCTIONS RATIONALE The reliability of the supply of N 2 is a key production utility requirement. Presently the N 2 Plant cannot achieve the design rates in any of its operating modes. Over 70% of the outages in the eight year history of the Plant are related to the reliability of the main air compressor train. Nitrogen is also sold to other plants in the Point Lisas area.
LITERATURE REVIEW TECHNICAL ASPECTS AIR COMPRESSION OIL AND MOISTURE REMOVAL COOLING REFRIDGERATION ADSORPTION AIR SEPARATION STORAGE
LITERATURE REVEIW 2.2 PERFORMANCE CHARACTERISTICS OF A BASIC ROTARY SCREW COMPRESSOR A built-in volume ratio ν i. = volume in cavity when suction port closes volume in cavity when discharged port uncovers Pressure ratio = ν i k = Charging Loss- The equation for the charging loss as a percent of suction pressure is θ i = 2.5m (U 2 ) /T (10 5 ) Discharge Loss-The discharge loss equation as a percent of discharge pressure is θ e = θ i / 2R c σ B = (1.0 + θ e )/(1.0 - θ i )
LITERATURE REVIEW REFRIDGERATION UNIT The refrigeration compressor is a reciprocating semi-hermetic, high speed, four cylinder suction gas cooled machine. The refrigeration unit utilizes a standard vapour compression expansion cycle, using refrigerant R-134A. COP = = = Where Q L is the duty of the evaporator Q h is the duty of the condenser The COP is also given by (Perry)as: COP = Refrigeration Capacity, KW =Net refrigeration effect Compression Power, KW Heat of compression
INFLUENCE OF EVAPORATING AND CONDENSING TEMPERATURES ON REFRIGERATION CAPACITY LITERATURE REVEIW The effect of evaporating temperature on refrigeration capacity
INFLUENCE OF EVAPORATING AND CONDENSING TEMPERATURES O REFRIGERATION CAPACITY Effect of condensing temperature on refrigeration capacity
INFLUENCE OF EVAPORATING AND CONDENSING TEMPERATURES O REFRIGERATION CAPACITY
EFFECT OF EVAPORATING AND CONDENSING TEMPERATURE ON COMPRESSOR POWER
EFFECT OF EVAPORATING AND CONDENSING TEMPERATURE ON COP
LITERATURE REVIEW ADSORPTION All the residual water and carbon dioxide contained in the feed air stream are removed on the silica gel/molecular sieve bed of the on- stream adsorber. The adsorber bed is a split bed arrangement comprising silica gel and molecular sieve. The off-stream adsorber is thermally regenerated by heating a stream of low pressure waste gas to approximately 110 0 C in the regeneration heater, L163-E05, and passing it through the adsorber in the opposite flow direction to the process air steam when on line. After 3.5 hours the adsorber bed reaches the required regeneration temperature and the heater is automatically switched off. At this point all the carbon dioxide and water impurities should have been driven off the adsorber bed.
FACTORS INFLUENCING ADSORPTION Mass Transfer Regeneration Adsorbate concentration Pressure drop ΔP =A μ V + B ρ V 2 Where: ΔP/L is pressure drop/bed depth μ is fluid viscosity, V is superficial fluid velocity A & B are dimensional constants ρ is fluid density BLANKED
PROJECT SCOPE 1.Determination of the root cause of the inefficiency of the Plant in specific areas. 2.Optimization of the Plant performance. 3.Better tuned process controllers. 4.Complete modeling of the entire Nitrogen Plant using Hysys. 5.Verification of all Nitrogen Plant process control systems. 6.Establishment of the Operator training systems when the Distributed Control System (DCS) is used with Hysys.
SELECTED APPROACH MAIN AIR COMPRESSOR To evaluate the efficiency of the screw compressor adiabatic and volumetric efficiencies will be used. This will be the basis for the evaluation of the single stage air compressor Methodology for calculations Where: R c = Compression ratio B = Correction factor σ = (k-1) / k Note: k for air is 1.40, σ = (1.4-1)/1.4 = 0.286 k = Ratio of specific heats, C p /C v Volumetric efficiency E vr = 100 - (θ i + W s R c σ ) Where θ i is charge loss W s is slip leakage
SELECTED APPROACH REFIDGERATION UNIT Methodology for calculations Condenser duty-The duty of the condenser will be taken to be the latent heat of cooling of the refrigerant at the condensing temperature (thermophysical properties). Q H = m * ΔH cond Where m is the mass flow rate R134a Kg/h ΔH cond is the latent heat of condensation KJ/Kg Evaporator Duty-The duty of the evaporator will be taken to be the heat removed from the incoming air to change its temperature from say, 40 o C to 5 o C. The mass flow of air can be calculated using a Mass Balance around the distillation column in the cold box. (cryogenic section of the Plant). Q L = m air C p air (T in – T out ) Where m air is the mass flow rate of air Kg/h C p air is the specific heat capacity of air KJ/Kg o C T in is the inlet air temperature to the refrigeration unit T out is the outlet air temperature from the refrigeration unit The heat load on the unit will be taken as the evaporator duty.
SELECTED APPROACH REFRIDGERATION UNIT Calculation of the coefficient of performance using the Perry approach COP = Net refrigeration effect / Heat of compression Net refrigeration effect = h g – h f Where: h g - the enthalpy of vapour leaving the evaporator h f - the enthalpy of liquid leaving the condenser Heat of compression = h d – h g Where: h d - the enthalpy of vapour leaving the compressor h a = h g - the enthalpy of vapour entering the compressor
SELECTED APPROACH ADSORBERS Since the adsorbers remove moisture and carbon dioxide, a good indication for ascertaining the performance is to check for the levels of the moisture and carbon dioxide at both the inlet and outlet of the adsorbers. A pressure survey must also be conducted to determine the pressure drop across the adsorber beds. The pressure drop can be calculated using the equation below. ΔP =A μ V + B ρ V 2 L A full adsorber evaluation must be done using the curve of CO 2 pressure (mmHg) vs. CO 2 capacity (lbs/100lbs 13x) and the curve of H 2 O partial pressure (mmHg) vs. H 2 O capacity (lbs/100lbs 13x). The results should lead to a value for the maximum adsorption time, where maximum adsorption time is equal to Max CO 2 per bed (kg)/ CO 2 loading (kg/hr).
ADSORBER EVALUATION Operating ConditionsFEEDPURGE GAS HEATING PURGE GAS COOLING Flow rate (Nm3/hr)2735650 Inlet temperature (oC)511010.1 Pressure (barg)7.70.1 Phase mol% vapour100 Vapour density (kg/m3)11.01.041.41 Average (mol.wt)28.9629.25 FLUID ANALYSIS Nitrogen (mol%) Argon (mol%) Oxygen (mol%) Water (ppm v/v) Carbon dioxide 78.12 0.93 20.95 997 350 71.41 1.22 27.37 - <1 Same as heating
Heating Period – 3.5 hours Cooling Period – 4 hours Changeover Period – 0.5 hour Electric Regen Heater – 30.0 KW Number of beds : Two, one in adsorption and one in regeneration Vessel I.D. = 0.875m Bed Height = 2100 mm MS loading density = 44.5 lbs/ cuft 1500lbs Mol sieve 13x 415 lbs Micro silica gel
220 lbs Macro silica gel FOR 13X Equilibrium water capacity = 29.5 %WT Nominal pore size = 10 angstroms THERMODYNAMICS PROPERTIES Heat of adsorption max. 1800 BTU/lb H2O Specific heat approx. 0.23 BTU/lb/oF There are curves of co2 pressure (mmHg) vs co2 capacity lbs/100 lbs 13x h20 partial pressure (mmHg) vs h2o capacity lbs/100 lbs 13x
Calculations Done 1) VAPOUR PRESSURE based on temperature 0.12 psia from graph 2) WATER CONTENT IN AIR TO TSA(Temperature switch Adsorber) Vapour pressure (psia)/ TSA inlet pressure (psia) – Vapour pressure (psia) 0.12/(126.4-0.12) = 0.00095 mol/mol 3) WATER LOADING Air flow (Nm3/hr) * Water in air (mol/mol)*18*0.454/10.167 2735*.00095*18*0.454/10.167 = 2.0 kg/hr 4) CARBON DIOXIDE LOADING CO2 in air (ppm)*Air Flow to TSA (Nm3/hr) *44*0.454 / (100000*10.167) 350*2735*44*0.454 /(1000000*10.167) = 1.88 kg/hr
5) MAXIMUM WATER LOADING PER BED Equilibrium water capacity (%wt) * Mol sieve per bed (kg) 0.295*681 = 200.89 kg 6) PARTIAL PRESSURE OF CO2 No.moles of CO2 * TSA inlet pressure (barg) * 760/ (Total no of moles*1.0135) 0.035*7.7*760/(100.1347*1.0135) = 2.019mmHg 7) MAX CO2 LOADING PER BED From co2 graph : at 2.019 mmHg at 0oc (closest isotherm to 5oC) capacity of 100 lbs 13X = approx 7lbs. Amount of 13X in bed = 1500 lbs = 681 kg 1lbs 13X removes 0.07 lbs CO2 Therefore 1500 lbs will remove 105 lbs CO2=47.67kg 8) MAX ADSORPTION TIME Max CO2 per bed (kg)/ CO2 loading (kg/hr) 47.67/1.88 = 25.4 hrs
SELECTED APPROACH OTHER METHODS This Project will also employ process and thermodynamic analysis that includes the following-: I. A Material Balance of the Plant II. Turbo expander adiabatic efficiency III. Steady state and dynamic modeling using the Hysys Process. Software IV. Comparison with the Nitrogen Plant at another facility. V. Follow-up and evaluation after a possible Plant outage.