Energy-efficient Vacuum Systems

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Energy-efficient vacuum systems

Vacuum technology isused in petroleum ren-eries to facilitate the

distillation of heavy ends at lowtemperatures, to prevent cokingand degradation of productsand for other applications.Currently, steam jet ejectors andsteam ejector-liquid ringvacuum pump (LRVP) combi-nations are the most commonmethods for vacuum generationin petroleum reneries.

Although steam jet ejectors arevery reliable, they are highlyinefcient. Due to increasingenergy costs and environmentalconcerns, it is essential toreduce the energy required forvacuum generation. Petroleumreneries discard a lot of wasteheat to the environment, whichcould be used to reduce energyconsumption for vacuum gener-

ation. There are publicationsillustrating the merits and limi-tations of vacuum generationmethods1,3,4,6 and chilled/refrig-erated water generation.2 However, there is a need for anintegrated approach coveringall aspects of vacuum genera-tion and its energy reductionpossibilities.

The main objective of this

Case analysis of the techniques available to reduce energy consumption invacuum systems reveals the potential for cost savings

C CHANDRA SEKHARA REDDY and S V NAIDU Andhra University

G P RANGAIAH National University of Singapore

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study is to analyse variousmethods for developing ener-gy-efcient vacuum generationsystems in petroleum rener-ies. Three case studies arepresented to enhance under-standing in the selectionprocess.

Vacuum generation in refineriesSteam ejectors and LRVPs aregenerally used in petroleumreneries. A review of various

vacuum generation equipmentcapacities, operating ranges andefciencies is available.6 Steamejectors may have one or morestages in series or a series-parallel combination, with pre-and/or interstage condensers,depending on the level ofvacuum required and the utilityoptimisation and operationalexibility sought for various

plant loads. Steam ejectors arehighly reliable, and the availa-

bility of steam in petroleumreneries makes ejectors thenatural choice. However, theyare highly inefcient6 (<10%),mainly due to a lack of movingparts to convert uid velocity topressure efciently.4

LRVP most commonly useswater as a seal liquid since it

can be separated and reusedsafely. They are generally moreexpensive compared to steamejectors. However, they do notrequire large heat exchangersto condense the vapour at theiroutlet, and the operating costsof LRVPs are generally lowerthan steam jet ejectors. For

better operating cost savings, asteam ejector–LRVP combina-tion is sometimes used toreplace the last one or two

stages of a multistage steamejector system.

Design principles and utilityrequirementsThis section presents usefuldesign principles and tools forestimating the utility require-ment for steam jet ejectors andLRVP. Use of pre- and inter-stage condensers can reduce

both capital and operatingcosts for the vacuum system.The vacuum produced islimited by the temperature ofthe cooling water; the colderthe temperature of the coolingwater, the lower the vacuumproduced.

Steam requirement for ejec-tors can be estimated based onthe dry air equivalent (DAE) of

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2 PTQ Q2 2013 www.digitalrefining.com/article/1000789

LRVPs is dictated by thetemperature of seal water.With normal cooling watertemperatures of ~30°C, LRVPsare used to replace steam ejec-tors operating at suctionpressures >150 torr (usually the

last one or two stages of ejec-tors for vacuum distillationcolumns). Power required by aLRVP can be estimated usingequation 8:3

P =

Pa * Vs * ln Pd

Pa

27 000 * ηe (8)

Chilled water generationChilled water can be generatedeconomically using absorptionheat pumps and mechanicalrefrigeration. Absorption heatpumps use waste heat (such aslow-pressure steam or a hotprocess stream) rather thanmechanical/shaft energy foroperation. Lithium bromideabsorption pumps arefrequently used due to theirlower cost and application

range up to the freezing pointof water. Compared tomechanical chillers, absorptionchillers have a low co-efcientof performance (COP).Nonetheless, their operatingcosts can be substantially lower

because they use waste heat,while vapour compressionchillers must be motor driven.At lower electricity prices, a

mechanical chiller can beattractive for chilled watergeneration.7

Optimisation of vacuum systemoperating costA design strategy and proce-dure for minimising theoperating cost of a multistagesteam jet ejector system isdiscussed in this section and

suction gases (including air,water vapour and other gases).As per the HEI (Heat ExchangeInstitute) procedure for calcu-lating DAE8 , water vapour inthe suction gases is consideredseparately and all other gases

(including air) are treated as amixture, in accordance withthis mixture’s molecularweight. HEI has publishedcurves to convert suction gasstreams to DAE using molecu-lar weight and temperatureentrainment ratios. Molecularweight entrainment ratio(MW

c) is dened as the ratio of

the weight of suction gas to theequivalent weight of air.Temperature entrainment ratiois dened as the ratio of theweight of air (or water vapour)at actual suction temperatureto the weight of air (or watervapour) at 21.1°C.

The following equations arederived from HEI curves8 fortemperature entrainment ratios(TC

a and TC

w) and MW

c. These

are convenient for use in

computer programs:

TCa = -4 * 10-10 T3 + 3 * 10-7 T2 - 0.0005 T +

1.0131 (1)

TCw = -1 * 10-13 T4 - 7 * 10-12 T3 + 8 * 10-8 T2 -

0.0006 T + 1.015 (2)

For M = 0 to 60, MWc = 1 * 10-5 M3 - 0.00013

M2 + 0.0642 M + 0.016 (3.1)

For M = 60 to 150, MWc = -2 * 10-5 M2+ 0.0077M + 0.9464 (3.2)

Water vapour and othercomponents in the suction gascan be converted to DAE usingthe correction factors fromequations 1, 2, 3.1, 3.2 and thefollowing equation:

DAE of suction WOG

+

Ww

gas or vapour=

TCa * MW

COGTC

W * MW

CWV(4)

The amount of motive steamrequired to compress (fromsuction pressure to dischargepressure) the unit DAE mass of

suction gas/vapour in a steamejector is dened as Ra (kg of

motive steam/kg of DAEequivalent of load gas). Valuesof R

a are available4 as curves,

with suction pressure on theabscissa and discharge pres-sure on the ordinate. For roughestimation of an ejector’s steamconsumption, one can simplyuse the following equation:9

Ra =

>Pd(0.434 -

1.338+ 0.000475 Pa) - 0.187H

Pa Pa

(1.2 - P

v - 10.2)

20 (5)

Steam requirement for ejec-tors can be estimated bymultiplying DAE and R

a

values.

For condenser calculations,involving air and watermixtures, the overall heattransfer coefcient, U W/(m2K), can be estimated usingthe following developed equa-tions.4 For a gas vapourmixture with non-condensablevapour mole percentage, NC,from 1% to 50%:

U = 5.678 (220.0417 + 1.6919 ln(NC) -2.67975 [ln(NC)]2 - 1.5465 [ln(NC)]3) (6)

For a gas vapour mixturewith NC from 50% to 95%:

U = 5.678 (-245 896 + 233 845.3 ln(NC) -

83 300.5 [ln(NC)]2 + 13 183.62 [ln(NC)]3

- 782.58 [ln(NC)]4) (7)

The suction pressure of





illustrated with two case stud-ies. There are several ways toreduce the operating costs of avacuum system. Engineersoften nd it difcult to takeoptimisation decisions as mostof the information is vendorspecic. The present work


illustrates the use of simpletechniques to optimise vacuumsystem operating costs easilyand quickly. A design strategyfor optimising a new vacuumsystem is presented in Figure 1.Process simulators such asAspen Hysys can be used to

estimate a condensing temper-ature at which the majority(~90%) of vapour condenses, atvarious stages of the vacuumsystem.

Once a vacuum systemdesign is selected, optimumdischarge pressure and steam

Evaluate the type of gasesto be evacuated

Mostly non-condensablegases?

Pre-condenser usingcooling water

Is cheap source of mediumpressure steam available?

Install multistage steam ejectorsystem with optimum design

Install pre-condenser using chilledwater (generated by mechanical or

single-stage absorption chiller)

Install two-stage steam ejectorand LRVP combination

No Yes

YesNo

YesNo

YesNo

Is condensing temperatureof vapour greater thancooling water return

temperature?

Is cheap electric power orwaste LLP steam available?

Figure 1 Design strategy for a new vacuum system





consumption for each stage ofthe multistage steam ejectorsystem (see Figure 2) can beobtained by solving the follow-

ing optimisation problem; forexample, using the Solver toolin Microsoft Excel. Denitionsof parameters and variables inthis optimisation problem aregiven in the nomenclature.

The following objective func-tion is to minimise overallsteam consumption for themultistage steam ejectorsystem:

Wv = ∑ni =1

Wvi =∑n

i =1

Pdi (0.434 - 1.338 +

Pai

Pai

0.000475 Pai) - 0.187] (1.2 -

Pv-10.2

)20

(Wa +

Wwi ) (9.1)

MW

CWV

The quantities in the aboveequation are as follows.

Suction pressure for (i+1)thstage:

Pai+1

= Pdi - Dp

i (9.2)

Water vapour ow rate tothe inlet of (i+1)th stage:

Wwi+1

= Wwi - 18 * Pv

i

29 * (Pai+1

- Pvi) (9.3)

Saturation pressure of watervapour corresponding to thevent temperature of the ithstage condenser is a function ofthe ith stage condenser venttemperature:

Pvi = f(Tvo

i) (9.4)

Vent temperature for the ithstage condenser:

Tvo

i= T

wco + TAP

i(9.5)

Mole fraction of watervapour at the ith stagecondenser inlet:

nwi =

Wwi + Wv

i / [

Wa+

Wwi + Wv

i] (9.6)

18 29 18

Mole percent of non-conden-sable gases at the ith stage

condenser:

NCi = (1 - nw

i) * 100 (9.7)

Partial pressure of watervapour at the ith stagecondenser inlet:

Ppwi = Pd

i * nw

i (9.8)

Saturation temperature of

water vapour corresponding toits partial pressure at the ithstage condenser inlet is a func-tion of its partial pressure atthe ith stage condenser inlet:

Tvi= f(Ppw

i) (9.9)

Log mean temperature differ-ence for the ith stagecondenser:


Processgas/vapour First stage inlet

gas/vapour

Steam injector Steam injector Steam injector

Water-oilseparation

tank

Water-oilseparation

tank

MP steam

Cooling watersupply

Slop oil pump

Sour water pump

First stagecondenser

Second stagecondenser

nth stagecondenser or

after condenser

WV 1

Pv

WV 2

Pv

Pa1

Pa2

Pan

Tv1

Pd1

Tvo1

Tv2

Pd2

Tvo2

Tvn

Pdn

Tvon

A 1

WCW1

A 2

WCW2

A n

WCWn

Ww1

+ Wa

Ww2

+ Wa

Wwn

+ Wa

To firedheater

Coolingwaterreturn

WV n

Pv

Figure 2 Schematic of a multi-effect steam ejector system





LMTD

i =

(Tvi - T

wco) - (Tvo

i- T

wc)

ln

(Tvi - T

wco)

(Tvoi- T

wc) (9.10)

Overall heat transfer coef-cient for the ith stagecondenser, U, is given by equa-

tions 6 and 7.Required area for the ithstage condenser:

Ai=

Li * (Ww

i + Wv

i - Ww

i+1) * 1000 (9.11)

Ui * LMTD

i * 3600

Cooling water required forthe ith stage condenser:

Wcwi=

Li * (Ww

i + Wv

i - Ww

i+1) * 0.239 (9.12)

(Twc

- Twco

)

Decision variables: Pdi for i=1,

2,... n-1Bounds and constraints:Pa

1 < Pd

i < Pd

n and Wv

i > 0

Steam properties in equa-tions 9.4 and 9.9 can becalculated using the Excelspreadsheet, freely available atwww.x-eng.com.

The above optimisation prob-

lem (Equation 9) is applicableto systems involving air andwater only. It ignores vapoursuperheat at the ejector inlet,and vapour sub-cooling andliquid sub-cooling in condenser


system with different approachtemperatures are shown inTable 1. The vent pressurestally well with those given inthe reference,4 with error in therange +14% to -8%. For three-and four-stage steam ejector

systems, steam ow rates esti-mated by the above procedureare greater by up to 25%,whereas, for a two-stage steamejector system, the estimatedsteam ow rate was lower by~20% compared to the reportedvalues.4 The main reason is thedifference in Ra values fromthe reference graphs4 andEquation 9.1, used in the opti-misation procedure. Foraccurate estimation of steamow rates, one can use theoptimised vent pressures andthe reference graphs.4 Coolingwater consumption shown inTable 1 can be reduced furtherwith some increase incondenser area by using aseries ow arrangement, wherewater from the rst interstagecondenser ows through the

other interstage andafter-condensers.

For relatively higher suctionpressures (~40 torr), use of apre-condenser, with chilledwater cooling, can further

area calculations. Also, itassumes simple LMTD withoutany correction factor. Hence,condenser area calculations areapproximate. However, theoptimisation problem inEquation 9 is very useful to

arrive at a preliminary designconcept and/or to verifyvendors’ proposals. For largesystems involving othervapours, the optimisation prob-lem can be solved by includingsimulation data from processsimulators such as AspenHysys, Aspen Plus and Pro/II.

The following case study(case study 1), based on thedetails from the manual opti-misation of a vacuum systempresented by Power,4 illustratesthe effectiveness of the optimi-sation procedure in Equation 9.

Load/suction gas: 45.36 kg/hof air + 97.52 kg/h of watervapour at suction pressure of15 torr and suction tempera-ture of 21.1°C; dischargepressure: 815 torr; motivesteam: 10.6 barg.

Cooling water supply andreturn temperatures are 32.2°Cand 37.8°C. Solutions to theoptimisation problem, obtainedusing Excel’s Solver, for a two-,three- and four-stage ejector

Approach Values from optimisation

temperature Utilities Inter-condensers Total Number at 1st Cooling Vent Condenser heatInter- condenser, MP steam, water, temperature, inlet pressure, Inter- and after- transfer

Case Stages condensers °C kg/h kg/h °C mmHg condenser area, m2 area, m2

1A 2 1 11.1 878 97 254 43.3 134 23.78/5.27 29.01B 3 2 16.7 840 93 300 48.9/54.4 130/284.2 18.1/4.54/2.05 24.71C 3 2 11.1 726 82 261 43.3/48.9 106.5/253.6 21.64/4.96/2.31 28.910 3 2 5.6 647 74 407 37.8/48.9 88.9/254.6 31.24/4.61/2.3 38.21E 3 2 2.8 598 69 558 35/43.3 79.5/229.8 44.84/5.5/2.28 52.61F 4 3 5.6 584 68 423 37.8/40.6/48.9 78.4/138.6/263 31.2/7.3/2.5/1.5 42.51G 4 3 2.8 557 65 680 35/43.3/48.9 71.8/148/259.3 47.4/5.74/2.6/1.5 57.21H 4 3 1.7 539 63 948 33.9/41.7/48.9 68.4/141.3/255 63.2/6.25/2.6/1.5 73.5

Optimisation results for case study 1: a vacuum system involving air and water vapour mixture

Table 1





reduce the steam requirementand the ejector system’s capitalcost. This is illustrated by thefollowing case study (case

study 2), solved using the opti-misation procedure in Equation9. Different quantities of watervapour and possible use ofchilled water are considered,and the results are summarisedin Table 2.Load gas (Cases 2A and 2B):45.36 kg/h of air + 97.52 kg/hof water vapourLoad gas (Cases 2C and 2D):

45.36 kg/h of air + 453.6 kg/hof water vapourSuction pressure and tempera-ture: 40 torr and 33.3°CMotive steam: 10.6 bargCooling water supply andreturn temperatures: 32.2°Cand 37.8°CChilled water supply andreturn temperatures: 7°C and13°C

Approach temperature at thepre-condenser and rst-stagecondenser: 1.67°CDischarge pressure: 815 torr

Water vapour in the load gasis increased by 365% for Cases2C and 2D, compared to Cases2A and 2B. No pre-condenser(using chilled water) is used inCases 2A and 2C, whereaspre-condenser, cooled withchilled water, is used in Cases2B and 2D. For all the cases,inter-stage and after-condens-ers use cooling water. From

Table 2, it can be seen that theuse of chilled water in thepre-condenser reduced thesteam consumption by 28.5%in Case 2B compared to Case2A, and by 60.3% in Case 2Dcompared to Case 2C. It alsoreduced the total heat transferarea for pre-, inter- andafter-condensers by 30.8% inCase 2B compared to Case 2A,

and by 69.0% in Case 2Dcompared to Case 2C. Moresteam and system capital costreduction can be achieved by

using chilled water, if morewater vapour is present in theload gas.

Case study 3Case study 3 is the vacuumsystem optimisation of avacuum distillation column inan existing Asian renery.Vacuum required at the top ofthe column is 35 torr, the

vacuum system suction owrate is 10 070 kg/hr and thedischarge pressure is 895 torr.Suction gases contain 5641 kg/hr of water vapour, 3730 kg/hrof HC vapour (molecularweight = 172) and 699 kg/hr ofnon-condensable gases (molec-ular weight = 29). Details of theexisting vacuum system withfour steam ejector stages are


VDU overheadvapour

10070 kg/hr

35.2 torr

80ºC

Steam injector100% capacity

Water-oilseparation

tank

Water-oilseparation

tank

4 off

482 m2

126 m2

MP steam

4525 kg/hr

10.5 barg

230ºC

Cooling watersupply

1098833 kg/hr

5 barg

32ºC

14596 kg/hr

75 torr

38ºC

Slop oil pump

Sour water pump



754 kg/hr

10.5 barg

230ºC

Steam injectors(x2)

50% capacity

784.4 kg/hr

472.3 torr

47ºC

1538 kg/hr

915 torr

41ºC

170066 kg/hr

38ºC 45.6 m2

Fourth stagecondenser

1400 kg/hr

10.5 barg

230ºC

Steam injectors(x2)

50% capacity

1314 kg/hr

89 torr

38ºC

2714 kg/hr

191.3 torr

41ºC

374333 kg/hr

38ºC

1160 kg/hr

10.5 barg

230ºC

Steam injectors(x2)

50% capacity

953.3 kg/hr

176.3 torr

44ºC

2113 kg/hr

491.3 torr

41ºC

283400 kg/hr

38ºC 46.4 m2

Third stagecondenser

747.8 kg/hr

894.8 torr

50ºC

To firedheater

Coolingwaterreturn

Figure 3 Existing vacuum system for a vacuum distillation column (base case – case 3A)





shown in Figure 3. Note that

there are two ejectors in paral-lel in each of stages 2, 3 and 4.

The following alternativesare considered for retrottingto minimise the operating costsof the existing vacuum systemin the base case (Case 3A).

Cases 3B and 3CFour-stage steam ejector systemwith one pre-condenser (at the

inlet of the rst-stage steamejector), cooled by chilledwater. A single-stage absorp-tion chiller is used in Case 3Bfor generation of chilled waterusing very low-pressure ashsteam at 1.5 barg and 130°C. Amechanical chiller, using R143arefrigerant, is used for thegeneration of chilled water inCase 3C. COP of the chiller is

5.68. Details of Cases 3B and

3C are shown in Figure 4.

Case 3DThe rst two stages are steamejectors, and the last two stagesare replaced by one LRVP. Themodied vacuum system isshown in Figure 5.

Cases 3E and 3FFor a pre-condenser using

chilled water generated by asingle-stage absorption chiller(Case 3E) and a mechanicalchiller (Case 3F), the rst twostages are steam ejectors andthe last two stages are replacedwith one LRVP. This improvedsystem is shown in Figure 6.

Equipment costsCapital costs of steam ejectors

are derived from an available

reference chart.4 Costs forcondensers are estimated usingthe Capcost program (based onan Excel spreadsheet).10 Fixedcosts for absorption, mechanicalchillers and LRVP are based onvendor quotations. The installedcost for a mechanical chiller andLRVP were taken as twice thepurchase cost, whereas theinstalled cost for an absorption

(single-stage-LiBr) chiller wastaken as 1.5 times the purchasecost since the absorption chilleris a packaged unit and involvesless expensive installation.Costs for foul water treatmentare used from reference 5.Utility costs assumed are: MPsteam = $32.14/ton; LLP steam= $31.65/ton; cooling water =$0.05/ton; and electric power =


Chiller

VDU overheadvapour

10070 kg/hr

35.2 torr

80ºC

Steam injectors(x2 )50% capacity

Pre-condenser

Chilled waterreturn13ºC

Water-oil

separationtank

Water-oil

separationtank

71333 kg/hr

292.6 m2

120 m2

1008 kg/hr

24.75 torr

14ºC

13ºC

MP steam

792 kg/hr

10.5 barg

230ºC

Cooling watersupply

5 barg

32ºC

Chilled watersupply642833 kg/hr

5 barg

7ºC

Cooling waterreturn

1800 kg/hr

75 torr

38ºC

1400 m2

Slop oil pump

Sour water pump



705 kg/hr

10.5 barg

230ºC

Steam injectors(x2)50% capacity

769 kg/hr

472.3 torr

44ºC

1474 kg/hr

915 torr

41ºC

160300 kg/hr

38ºC 40 m2

Fourth stagecondenser

1090 kg/hr

10.5 barg

230ºC


1272 kg/hr

69 torr

36ºC

2362 kg/hr

191.3 torr

41ºC

290900 kg/hr

38ºC

1048 kg/hr

10.5 barg

230ºC


945 kg/hr

176.3 torr

44ºC

1993 kg/hr

495 torr

41ºC

266100 kg/hr

38ºC 40 m2

Third stagecondenser

748 kg/hr

894.8 torr

50ºC

To firedheater

Coolingwaterreturn

Figure 4 Four-stage steam ejector system with one pre-condenser (at the inlet of first-stage steam ejector), cooled bychilled water (Cases 3B and 3C)





$0.15/KW. Payback period isdened as:

Payback period =

Capital cost for new

case -

capital cost for base case

Operating cost for base

case -

(10) Operating cost for

base case

The results for all the cases


VDU overheadvapour

10070 kg/hr

35.2 torr

80ºC

Steam injector100% capacity

Water-oilseparation

tank

Water-oilseparation

tank

4 off

482 m2

125 m2

MP steam

4525 kg/hr

10.5 barg

230ºC

Cooling watersupply

1128833 kg/hr

5 barg

32ºC

14595 kg/hr

75 torr

38ºC

Slop oil pump

Sour water pump



1400 kg/hr

10.5 barg

230ºC

Steam injectors(x2)

50% capacity

1314 kg/hr

89 torr

38ºC

2714 kg/hr

191.3 torr

41ºC

374333 kg/hr

38ºC

953.3 kg/hr

176.3 torr

44ºC

41ºC

174 kW

30000 kg/hr

32ºC

Seal watercooler

733 kg/hr

894.8 torr

44ºC

To firedheater

Coolingwaterreturn

M

Figure 5 Modified vacuum system with steam ejectors for the first two stages and last two stages replaced with oneLRVP (Case 3D)

Figure 6 Improved vacuum system with a pre-condenser, first two stages with steam ejector and last two stages

replaced by one LRVP (Cases 3E and 3F)

Chiller

VDU overheadvapour

10070 kg/hr

35.2 torr

80ºC



Pre-condenser

Chilled waterreturn13ºC

71333 kg/hr

292.6 m2

120 m2

1008 kg/hr

24.75 torr

14ºC

13ºC

MP steam

792 kg/hr

10.5 barg230ºC

Cooling watersupply

Chilled watersupply642833 kg/hr

5 barg

7ºC

Cooling waterreturn

1800 kg/hr

75 torr

38ºC

1400 m2



1090 kg/hr

10.5 barg230ºC

1272 kg/hr

69 torr

36ºC

2362 kg/hr

191.3 torr

41ºC

290900 kg/hr

38ºC

Water-oilseparation

tank Slop oil pump

Sour water pump

945 kg/hr

176.3 torr

44ºC

41ºC

174 kW

30000 kg/hr

32ºC

Seal watercooler

733 kg/hr

894.8 torr

44ºC

To firedheater

Coolingwaterreturn

M

266100 kg/hr

38ºC






coolers and product coolers,which can also be used togenerate low-pressure steam. IfMP steam cost is lower (~75%compared to the base case, dueto credit from power genera-tion at steam turbines), amechanical chiller will not beeconomical. For this case, asingle-stage absorption chiller(Case 3B) will be feasible(payback ~3.3 years) only if thelow-pressure steam is availableat zero cost (see Table 3). Theoption involving a pre-condenser (cooled by chilledwater), rst two stages with a

steam ejector and the last twostages replaced by one LRVP(Cases 3F and 3E), is a veryattractive investment with apayback period of 1.8 to threeyears (with an incrementalcapital cost of $1.75 million to$3.25 million). Thus, replace-ment of the last two stages of amulti-stage steam ejectorsystem with one LRVP is

highly benecial.

ConclusionsThis article analysed the tech-niques available to reduce theenergy consumption of vacuumsystems used in petroleumreneries. Key requirements,

benets and constraints forimplementation of these tech-niques are highlighted, and

availability of waste low-pres-sure steam or low electricityprices.

Accurate estimation of avacuum system’s suction loadfor various plant operatingscenarios is difcult. Hence,considerable safety margin withrespect to suction load is oftenallowed in the system design.In this case study, the rst-stagesteam ejector is a single unit,hence it is very difcult toreduce the steam consumptionif the operating suction gas loadis lower than the design rate.However, for the case of a

steam ejector system with achilled water pre-condenser,electrical power required for thechiller can be reduced by capac-ity control or by operating afew chillers arranged in paralleloperation. Thus, operating cost

benets can be furtherincreased. Availability of plotspace and maintenance costsare the other critical issues for

installing a chiller. Installationof a mechanical chiller andLRVP may require modicationcosts at a power intakesubstation.

Waste LLP steam can berecovered economically fromthe steam condensate system.Petroleum reneries oftendiscard a lot of waste heatthrough furnace stacks, n-fan

are presented in Table 5. Forthe cases where operating costsincreased compared to the basecase, payback is shown as “noteconomical” in this table.

Analysis of the resultsAnnual operating costs of avacuum system are very signif-icant compared to the installedcosts. Hence, optimum vacuumsystem conguration is essen-tial to minimise operatingcosts. Even for the optimumdesign, the majority of theoperating cost arises from therst stage, which handles the

maximum ow rate of gases/vapours. The only way toreduce this cost is to condensethe vapours before they reachthe rst-stage ejector.Depending on the suction pres-sure of the rst stage, chilledwater may be required. For therenery case study, themechanical chiller option (Case3C) has a payback period of

5.28 years, which will reduce to2.25 years if the power cost islower at 75% compared to the

base case (see Table 3). If wastelow-pressure steam (~1.5 barg)is available at zero cost, asingle-stage absorption chiller(Case 3B) will have a paybackperiod of 2.45 years. Thus, the

benets of pre-condensersusing chilled water depends on

Values from optimisation

Utilities Pre-condenser Inter-condensers Total Number of Chilled Cooling Vent Vent Vent heat Inter-stage Pre- MP steam, water, water, pressure, Pre-condenser, temperature pressure, Inter- and after- transfer Case Stages condensers condenser kg/h kg/h kg/h mmHg area, m2 °C mmHg condenser area, m2 area, m2

2A 4 3 No 351 0 44 581 NA NA 33.9/41.7/48.9 76.5/157.6/280.9 31.4/4/2/1.4 3928 4 3 Yes 251 9011 25 346 36 11 33.9/41.7/48.1 110.6/203.4/329.6 10.97/2.4/1.5/1.2 27

2C 4 3 No 632 0 110 202 NA NA 33.9/41.7/48.1 55.9/119.1/226 150/6.9/2.4/1.35 16120 4 3 Yes 251 42 901 25 346 36 34.70 33.9/41.7/48.1 110.6/203.4/329.6 10.97/1.9/1.2/1 50

Effect of pre-condenser with chilled water cooling on steam consumption in the multi-stage steam ejector system(case study 2)

Table 2






plant by considering site-spe-

cic factors such as energy cost,plot size, capital cost, acceptablepayback period, operationalreliability, maintenance andsafety issues.

Nomenclature

Ai Area for the ith stage condenser,

m2

Dpi Process gas-side pressure drop

across the condenser at the outlet of the

ith ejector, torr

ation, thus improving energy

efciency and also reducingcarbon emissions. The econom-ics of such an optimisation varyfrom one site to another as thecosts of steam, power andextent of waste heat recoveryvary greatly. A detailedeconomic study similar to theone shown in the present studycan be conducted to decide the

best strategy for a particular

strategies for selection and

implementation of a suitablemethod are outlined. It can beconcluded from the analysisthat use of chilled water at thepre-condenser reduces theenergy costs of vacuumsystems. As reneries operatemany steam ejector vacuumsystems, considerable potentialexists for reducing energyconsumption for vacuum gener-

Details of the vacuum system Case 3A Case 3B Case 3C Case 3D Case 3E Case3FSteam ejectors MP steam, ton/hr 7.814 3.635 3.635 5.9 1.882 1.882Condenser area Total area, m2 2145 1893 1893 2020 1813 1813LRVP Power consumption, KW 0 0 0 174 174 174 Cooling water, ton/hr 0 0 0 30 30 30Single-stage Cooling water, ton/hr 0 1126 0 0 1126 0

absorption chiller LLP steam, ton/hr 0 10.58 0 0 10.58 0 Power consumption (includes 0 160 0 0 160 0 power for chilled water pumps), KWMechanical chiller Power consumption, KW 0 0 1010 0 0 1010 Cooling water, ton/hr 0 0 618 0 0 618Total utility requirements MP steam, ton/hr 7.814 3.635 3.635 5.9 1.882 1.882 LLP steam, ton/hr 0 10.58 0 0 10.58 0 Chilled water, ton/hr 0 642.8 642.8 0 642.8 642.8 Cooling water, ton/hr 1099 1197 689 1129 1227 719 Power requirement, KW 0 160 1010 174 334 1184Savings at foul water stripper LP steam saved at foul water stripper, ton/hr 0 0.72 0.72 0.33 1.02 1.02 Power saved at foul water stripper, KW/hr 0 11.41 11.41 5.23 16.19 16.19 Cooling water saved at foul water stripper, ton/hr 0 2.56 2.56 1.17 3.64 3.64Operating cost, million $/year Base case 2.68 4.48 2.44 2.29 4.13 2.10 Considering LLP steam for 2.68 1.54 2.44 2.29 1.20 2.10 absorption chiller is zero cost

Considering electric power cost is 2.68 4.43 2.11 2.23 4.03 1.71 75% of the cost considered If MP steam cost is 75% of that previously due 2.13 4.22 2.18 1.87 4.00 1.96 to credit from power generation at steam turbines If MP steam cost is 75% of that previously due 2.13 1.29 2.18 1.87 1.07 1.96 to credit from power generation at steam turbines and LLP steam to absorption chiller is at zero costCapital cost, million $ Steam ejectors 0.371 0.159 0.159 0.330 0.118 0.118 Surface condensers 1.691 1.691 1.691 1.497 1.497 1.497 LRVP 0.000 0.000 0.000 0.692 0.692 0.692 Single-stage absorption chiller 0.000 3.000 0.000 0.000 3.000 0.000 Mechanical chiller 0.000 0.000 1.500 0.000 0.000 1.500 Total installation cost 2.062 4.850 3.350 2.519 5.307 3.807Payback period Base case - Not 5.28 1.16 Not 2.98 economical economical If LLP steam for absorption chiller is - 2.45 5.28 1.16 2.19 2.98

available at zero cost If electricity cost is 75% of that - Not 2.25 1.01 Not 1.80 considered for the above cases economical economical If MP steam cost is 75% of that - Not Not 1.75 Not 10.37 considered in the base case (due to economical economical economical credit from power generation at steam turbines) If MP steam cost is 75% of that - 3.30 Not 1.75 3.05 10.37 considered in base case and LLP economical steam to absorption chiller is available at zero cost

Analysis of alternatives for the vacuum system of a vacuum distillation column in a petroleum refinery

Table 3






P Power for LRVP, KW

Pa, Pd Suction and discharge pressures,

torr

Ra Ratio of motive steam flow rate

to DAE flow rate of steam ejector suction

gas/vapour

T Temperature of suction gas/

vapour, °C

TCa

Temperature entrainment ratio

for air

TCw Temperature entrainment ratio

for water vapour

U Overall heat transfer coefficient

for the condenser, W/m2K

Vs Suction volumetric flow for LRVP,

m3/h

WOG

Flow rate of gases/vapours other

than water vapour, kg/hr

Ww Water vapour flow rate, kg/h

ηe Efficiency of LRVP

References1 Aliasso J, Choose the right vacuum

pump, Chemical Engineering, March

1999, www.graham-mfg.com/usr/pdf/

TechLibVacuum/ 222.PDF, accessed in Jan

2012.

2 ASHRAE Handbook, Refrigeration (I-

P) edition, American Society of Heating,

Refrigerating and Air-Conditioning

Engineers, 2010.

3 Bannwarth H, Liquid Ring Vacuum

Pumps, Compressors and Systems, Wiley-

VCH Verlag GmbH & Co KGaA, Weinheim,2005.

4 Power R B, Steam Jet Ejectors for the

Process Industries, 2nd ed, McGraw-Hill,

2005.

5 Prakash S, Refining Processes

Handbook, Gulf Professional Publishing,

2003.

6 Ryans J, Bays J, Run clean with dry

vacuum pumps, Chemical Engineering

Progress, 32-41, Oct 2001.

7 Reddy C C S, Rangaiah G P, Naidu S

V, Waste heat recovery methods andtechnologies, Chemical Engineering, 28-

Li Latent heat of vapour in the ith

stage condenser, kJ/kg

LMTDi Logarithmic mean temperature

for the ith stage condenser, °C

MWCWV

Molecular weight entrainment

ratio for water vapour

n Number of ejector stages

NCi Mole percentage of non-

condensable gases in the ith stage

condenser inlet

nwi Mole fraction of water vapour in

the inlet of the ith stage condenser

Pai, Pd

i Suction and discharge gas

pressures for the ith stage ejector, torr

Ppwi Partial pressure of water vapour in

the ith stage condenser inlet, torr

Pv Motive steam pressure, barg

Pvi Saturation pressure of water

corresponding to vent temperature of the

ith stage condenser, torr

TAPi Approach temperature for the ith

stage condenser vent ,°CTv

i Saturation temperature of water

vapour at the ith stage condenser inlet,

°C

Tvoi Gas temperature at the ith stage

condenser vent, °C

Twc

, Twco

Cooling water supply and return

temperatures, °C

Ui Overall heat transfer coefficient

for the ith stage condenser, W/m2K

Wa Air flow rate in load gas, kg/h

Wcwi Cooling water flow rate to the ith

stage condenser, kg/hWv

i Steam flow rate for the ith stage

ejector, kg/h

Wwi Water vapour flow rate at the

inlet of the ith stage ejector, kg/h

General

M Molecular weight

MWc Molecular weight entrainment

ratio

MWCOG

Molecular weight entrainment

ratio for the gases other than water

vapour

NC Mole percentage of non-condensable vapour/gas

38, Jan 2013.

8 Standards for Steam Jet Vacuum

Systems, Heat Exchange Institute, 6th ed,

2007.

9 Trambouze B, Petroleum Refining,

Vol. 4, Materials and Equipment, Editions

Technip, Paris, 1999.

10 Turton R, Bailie R C, Whiting W B,

Shaeiwitz J A, Analysis, Synthesis, and

Design of Chemical Processes, 3rd ed,

New Jersey, Prentice Hall, 2009.

C Chandra Sekhara Reddy is the Lead

Process Design Engineer with Singapore

Refining Company and a PhD scholar

at Andhra University, Visakhapatnam,

India. He holds bachelor’s and master’s

degrees in chemical engineering from

Andhra University and IIT Kanpur.

S V Naidu is a Professor in the

Department of Chemical Engineeringand Dean, Planning and Resource

Mobilisation with Andhra University’s

College of Engineering. He holds

bachelor and doctoral degrees in

chemical engineering from Andhra

University, and a master’s from R.E.C.,

Warangal.

G P Rangaiah is Professor and Deputy

Head in the Department of Chemical

& Biomolecular Engineering with the

National University of Singapore. He

holds bachelor’s, master’s and doctoraldegrees in chemical engineering, from

Andhra University, IIT Kanpur and

Monash University, respectively.

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