PRELIMINARY PROJECT PRESETATION

BY

D R J P

EVALUATION AND OPTIMIZATION OF THE NITROGEN PLANT AT A TRINIDAD

FACILITY

NOVEMBER 2003

OBJECTIVEOBJECTIVE

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.

INRTODUCTIONSINRTODUCTIONS

RATIONALE

The reliability of the supply of N2 is a key production utility requirement.

Presently the N2 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 REVIEWLITERATURE REVIEW

TECHNICAL ASPECTS

AIR COMPRESSION

OIL AND MOISTURE REMOVAL

COOLING

REFRIDGERATION

ADSORPTION

AIR SEPARATION

STORAGE

LITERATURE REVEIWLITERATURE REVEIW

2.2 PERFORMANCE CHARACTERISTICS OF A BASIC ROTARY SCREW COMPRESSOR

A built-in volume ratio νi. = volume in cavity when suction port closes

volume in cavity when discharged port uncovers

Pressure ratio = νik =

1/1 ccad BRR

k

volumedischarge

lumesuction vo

Charging Loss- The equation for the charging loss as a percent of suction pressure is

θi = 2.5m (U2) /T (105)

Discharge Loss-The discharge loss equation as a percent of discharge pressure is

θe = θi / 2Rcσ

B = (1.0 + θe)/(1.0 - θi)

LITERATURE REVIEWLITERATURE 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 QL is the duty of the evaporator

Qh 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

 

innet

in

W

Q

, inout

in

QQ

Q

Lh

L

QQ

Q

INFLUENCE OF EVAPORATING AND CONDENSING TEMPERATURES ON

REFRIGERATION CAPACITY

LITERATURE REVEIWLITERATURE 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 REVIEWLITERATURE REVIEWADSORPTIONADSORPTION

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 1100C 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 = AμV + BρV2

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 SCOPEPROJECT 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 APPROACHSELECTED 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

1/1 ccad BRR

Where:Rc = 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, Cp/Cv

Volumetric efficiency Evr = 100 - (θi + Ws Rcσ)

Where θi is charge loss

Ws is slip leakage

SELECTED APPROACHSELECTED APPROACH

REFIDGERATION UNITREFIDGERATION 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). QH = m * ΔHcond

Where m is the mass flow rate R134a Kg/h

ΔHcond 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, 40oC to 5oC. 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).

QL = mair Cp air (Tin – Tout)

Where mair is the mass flow rate of air Kg/h

Cp air is the specific heat capacity of air KJ/Kg oC

Tin is the inlet air temperature to the refrigeration unit

Tout 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 = hg – hf

Where: hg - the enthalpy of vapour leaving the evaporator

hf - the enthalpy of liquid leaving the condenser

Heat of compression = hd – hg

Where: hd - the enthalpy of vapour leaving the compressor

ha = hg - the enthalpy of vapour entering the compressor

SELECTED APPROACHSELECTED APPROACH

ADSORBERSADSORBERS

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 = AμV + BρV2

L

 

A full adsorber evaluation must be done using the curve of CO2 pressure (mmHg) vs. CO2

capacity (lbs/100lbs 13x) and the curve of H2O partial pressure (mmHg) vs. H2O capacity

(lbs/100lbs 13x). The results should lead to a value for the maximum adsorption time, where maximum adsorption time is equal to Max CO2 per bed (kg)/ CO2 loading (kg/hr).

ADSORBER EVALUATIONADSORBER EVALUATION

Operating Conditions FEED PURGE GASHEATING

PURGE GAS COOLING

Flow rate (Nm3/hr) 2735 650 650

Inlet temperature (oC) 5 110 10.1

Pressure (barg) 7.7 0.1 0.1

Phase mol% vapour 100 100 100

Vapour density (kg/m3) 11.0 1.04 1.41

Average (mol.wt) 28.96 29.25 29.25

       

FLUID ANALYSISNitrogen (mol%)Argon (mol%)Oxygen (mol%)Water (ppm v/v)Carbon dioxide  

 78.120.9320.95997350

 71.411.2227.37-<1

 Same as heating

Heating Period – 3.5 hoursCooling Period – 4 hoursChangeover 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 13x415 lbs Micro silica gel

 

220 lbs Macro silica gel 

FOR 13XEquilibrium water capacity = 29.5 %WTNominal pore size = 10 angstroms 

THERMODYNAMICS PROPERTIESHeat of adsorption max. 1800 BTU/lb H2OSpecific 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 APPROACHSELECTED APPROACH

OTHER METHODSOTHER 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.

 

THE ENDTHE END

THANK YOUTHANK YOU