Hysys Blend Oil Crude

7/27/2019 Hysys Blend Oil Crude

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OPTIMIZATION OF A

REFINERY CRUDE

DISTILLATION UNIT IN THE

CONTEXT OF TOTAL ENERGY

REQUIREMENT

E. O. Okeke & A. A. Osakwe-Akofe

NNPC R&D Division, Port Harcour t,

Nigeria

APACT03, York, 2830, April, 2003


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INTRODUCTION

The Nigerian National Petroleum Corporation,has 4 refineries, in its downstream operations,

The primary goal of this refiner is to achieve and

maintain high gasoline production,Hence, the main objective of this study is tooptimize gasoline production in all the refineries,

The strategy being to first target the CDUs in

these refineries. Maximizing the yield of gasolineand its intermediates will directly impactpositively on total pool gasoline production,


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PROGRAMME FOR MAXIM IZING

GASOLINE PRODUCTION

Maximizing gasoline and itsintermediates production from therefinries has been planned to beaccomplished in phases, viz-

Phase ICDU 1 (the first refinerys CDU)

Phase I ICDU 2,3,4, 5 (the other 3 ref iner ies),

Phase I I ICatalytic plants - CRU, FCC & HF Alky

Phase Ibegan with CDU 1 as a basis toascertain plant suitability to process differentcrude oil.


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CDU 1 FEED & MAIN

COLUMN SUBSYSTEM

The CDU 1of the first of these refineries,

the object of our presentation, was

installed in the 1960s to processnaphthenic crude of API 40.3 at first and

another of API 35.4 afterward,

It has a main fractionator with 44 traysand 4 side strippers, and a stabilizer

column.


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CDU 1 DISTI LLATES

The intermediate distillates are as inconventional CDUs,

Unstabilized gasoline from the main

fractionator is further processed in thestabilizer column,

Straight run naphtha and other distillates fromthe main fractionator are routed further

downstream for processing and upgrading,Stabilizer produces an intermediate gasoline asbottoms and LPG as overhead


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CDU 1 MAIN DESIGN & HARDWARE

FEATURES

Licensed by SHELL and designed as aconventional crude distillation unit,

Crude oil characteristics and productrequirements as applicable inestablishing hardware design,

Hardware performance evaluation,

maintenance and upgrading of facilityundertaken periodically.


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MAIN FOCUS AREAS TO ACHIEVE

MAXIMUM GASOLINE IN CDU 1

Main areas are:

efficient operation of the CDU,review of configuration of CDU

to determine opportunity for

further increase in gasoline yield,


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GENERAL I ZED STRUCTURE

OF THE CDU 1

The CDU can be decomposed in stages as

follows:

Stage 1, the main f ractionator producingfeed for Stage 2 (i .e. the stabi l izer)

Achievement and sustenance of increase

yield must be progressivefrom Stage 1through Stage 2


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METHODOLOGYSTEADY STATE

SIMULATION TO OPTIM IZATION

The main stages are as follows:

Compare the crude assays for the twonaphthenic crudes,

Configure, specification and steady statesimulation of the CDU using HYSYS.Plant,

Match HYSYS.Plantsimulation results withoriginal design requirements,

Carry out optimization of the CDU

Results obtained showed good opportunity.


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COMPARISION OF THE TWO

CRUDESLight ends Crude properties

BL TNP

LV% LV% BL TNP

Methan, C1 0 0 Density, Kg/m3 847 823.6

Ethane, C2 0.02 0 API Gravity 35.4 40.3

Propane, C3 0.24 0.6 Barrel/Tonne 7.426

iso-butane, 0.36 0.7 Kinematic viscosity at 40 3.34

n-butane 0.75 1.4 Kinematic viscosity at 60 2.24 3.418

iso-pentane 1.01 1.6 Sulphur content (wt %) 0.14 0.06

n-pentane 0.77 1.6 Pour Point C 12cyclopentane 0.16 0.2

n-hexanes 0.72

other C6 2.47

Benzene 0.12

6.62 6.1

Heavy Ends ASTM D86 Properties

BL TNP BL TNP

IBP vac 363 IBP 57 39

5 400 391 5 100 91

10 404 393 10 125 114

30 418 406 30 215 188

50 439 421 50 280 259

70 463 438 70 334

90 503 460 90 446

EBP 564 482 EBP 529

Parameters

ParametersParameters

Parameters

Total


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COMPARISION OF PRODUCTS

DERIVED FROM ON THE TWO CRUDES

1.00E-06

1.00E-02

1.00E+02

1.00E+06

Crude LPG Gasoline Kerosene LGO HGO AR

Products

F

lowrates(kg

per

hour)

Design

HYSYS


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I NCREASING GASOLINE YIELD

For a given CDU, yield of gasoline derivatives

depends on,

Feed characteristics,Process requirements/operating conditions.

From the above therefore, since feed is

constant, optimizing gasoline yield will

depend on process requirements/operating

conditions.


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FRONT-END CDU 1 EVALUATION

FOR HYSYS IMPLEMENTATION

The evaluation of the CDU is as follows:

Establish a reliable CDU configuration, determine

process conditions using HYSYS and match these

with the original plant design basis andrequirements,

Properly decompose the structure of the CDU and

determine boundary conditions for optimization,

Achieve a reliable process optimization in the

context of total energy requirements.


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OPTIM IZATION PARAMETERS

The parameters for optimization are derived fromprocess/hardware environments, viz,

The main fractionator and the stabilizer are

linked together: stabilizer feed comes from themain fractionator,

The other gasoline blending stock, SRN, aderivative from the main fractionator is routed

for further processing,Four side strippers in the main fractionator,

The stabilizer has a condenser and a reboiler


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PLANT ARRANGEMENT FOR

OPTIMIZATION


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HEAT LOAD DISTRIBUTION

CDU has an integrated heat exchangernetwork for heat recovery which sharesloads, viz, Q1,,Q7, where Q4 and Q5 are

utilities,

Heat loads in the network are assumed tobe efficiently shared,

Heat supplied through the crude chargeand for the various steam strippingsupplies are constant.


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HYSYS FLOWSHETET

CONFIGURATIONOveral l CDU


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HYSYS FLOWSHEET CONF IGURATION

Main Column Subsystem


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MODELLING PROCEDURE

Stage-wise approach was adopted, viz,

Evaluate CDU configuration and steady state simulation

data to determine opportunity for optimization,

Based on the structure of CDU process and hardwarerequirements, evolve an optimization algorithm and define

boundary conditions to be solved by HYSYS.Plant,

Define steady state parameters from HYSYS.Plant

simulation as first level data, and referenced as baseor

designvalues,

Optimize the overall gasoline yield in the context of total

energy requirement.


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OPPORTUNITI ES FOR

OPTIM IZING GASOLINE YIELD

We observed the following:

The columns are linked in sequential arrangement,

Possibility of enhanced recoveries of gasoline in the

nearest distillates below and above SRG, ie SRK andLPG, and in the stabilizer overhead,

To maintain high quality gasoline to meet baseor

designspecification, the path to solution must be

constrained,Problem is non-linear.

Based on these conditions an algorithm was developed


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THE ALGORITHMHeat Loads

Heat load differential at steady state

Qibase = Q1base + Q2base+ + Q7base 1Heat load at any level of optimizationQiopt = Q1opt + Q2opt+ + Q7opt 2And the differential

Qdifferential = Qiopt - Qibase 3


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THE ALGORITHMGasoline Yields

Gasoline yield at steady state

yibase = y1base + y2base+ + y7base 4Gasoline yield at any level ofoptimization

yiopt = y1opt + y2opt+ + y7opt 5And the differentialydifferential = yiopt + yibase 6


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THE ALGORITHMObjective Function

Incorporating the various energy and gasoline costs, theresultant differential becomes,

INB = y*differential - Q*differential 7 The objective function becomes

Max [f(X1,X2,X3) = y*differential-Q*differential] 8Where,

y*differential & Q*differential are gasoline and energy costs,X1, main column naphtha stripper reboiler return temp,

X2, main column kero stripper reboiler return temp,X3, stabilizer reboiler return temp,

Subject to RON and RVP of gasoline being within baseordesignvalues.


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HYSYS OPTM IZER

Primary variables (X1, X2, X3) are manipulated to

maximizeINB. Primary variables must have

upper & lower limits, and these are used to

normalize the primary variables, viz,

Xinorm= [(XiXilower)/(XiupperXilower)] . Where Xi= X1,

X2, X3

Objective functionas defined by INB

,

Constraints as defined for RON & RVP,


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OPTIM IZATION BY SEQUENTI AL

QUADRATIC PROGRAMMING

Sequential Quadratic Programming (SQP) was

applied for solution.

SQP minimizes a quadratic approximation of

the Lagrangian function subject to linear

approximations of the constraints. The second

derivative matrix of the Lagrangian function is

estimated automatically. A line searchprocedure utilizing the watchdog technique

(Chamberlain & Powel) is used.


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PROBLEM SOLUTION

Sequential quadratic programming

was found to be ideal for solution,

Solution was found for all casesstudied,

General increase in yield of stabilizer

feed and SRN from the main column,

Gasoline yield was increased by 8 %


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BASE & OPTIM IZED VALUES

Base Optimi zed Dif ference

NAPHTHA Side Stripper Reboiler Temperature, C, X1 174 176 2

KERO Side Stripper Reboiler Temperature, C, X2

245 260 15

STABILIZER Reboiler Temperature, C, X3 155 153.05 -1.95

STABLIZER Feed, m3/hr 45 47.97 2.97

Straight Run NAPHTHA product, m3/hr, y1 63.57 64.27 0.7

GASOLINE, m3/hr, y2 41.3 44.24 2.94

Total Gasoline (Straight-run Naphtha+Gasoline) 104.87 108.51 3.64

HYSYS Resul t


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TESTING ALGORITHM ROBUSTNESS &

RELATIONSHIP OF KEY PARAMETERS

Some optimization test runs were done using sameHYSYS.Plantto

Test the robustness and reliability of the algorithm

at achieving early convergence,Determine the variation of key parameters, thatimpact on the structure of the CDU and theinteraction of the main fractionator and thestabilizer. These parameters are the naphthastr ipper reboi ler return temp, the kero str ipper

reboiler return temp, and the stabil izer gasoline.


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VARIATION OF GASOLINE WITH NAPHTHA

STRIPPER REBOILER RETURN TEMP

174

176

178

180

182

184

43.52

43.88

44.15

44.4

44.45

44.72

45.02

Gasoline (m3/hr)

Naphthas

tripper

reboilerreturn

Tempera

tureC


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VARIATION OF GASOLINE WITH KERO

STRIPPER REBOILER RETRUN TEMP

235

240245

250

255

43.52

43.88

44.15

44.4

44.45

44.72

45.02

Gasoline (m3/hr)

Kerostrippe

rreboiler

returnTemp

eratureC


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VARIATION OF GASOLINE WITH

STABI L I ZER REBOILER RETRUN TEMP

146

148150

152

154

156

43.52

43.88

44.15

44.4

44.45

44.72

45.02

Gasoline (m3/hr)

Stabilizer

reboiler

returnTemp

eratureC


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OBSERVATIONS FROM THE

OPTIMIZATION

The optimization based on this algorithmachieves earlyconvergence,

As expected, the naphtha stripper (X1) and

kero stripper reboiler (X2) temperatures haveindirect impact on the stabilizer gasoline, whilethe stabilizer reboiler (X3) temperature has adirect impact on the same gasoline yield,

The 3 parametersX1, X2 & X3 aremanipulated as appropriate to optimize thegasoline produced.


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CONCLUSION

Sequential quadratic programme technique ideal for

solution,

Solution ofthe algor ithm isreliable, achieving early

convergence in the cases studied,Objective of obtaining increased gasoline yield in the

context of reduced energy requirement achieved,

Since the configuration of the refinery CDUs are

similar, this algorithmcan be applied to optimize theCDU 2,3,4,5in the other 3 refineries


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