Increasing FCC Output by OxygenEnrichment

MAXIMUM CAPACITY

The objective is to maximize feed rate using O2

enrichment. Starting at the base operating conditionslimited by air availability, GOX is added to theregenerator. At the same time FCC feed is increased toproduce more coke and consume excess O2. As feed isadded, the reactor temperature is maintained byincreasing catalyst circulation rate. The increasedregenerator dense bed temperature results in a lowercat/oil ratio and consequently lower conversion than thebase case. To offset this, feed preheat is reduced andcatalyst circulation is increased to hold reactortemperature constant. All these moves are balancedwhen the CO2/CO ratio, excess O2, and reactortemperature are the same as the base case, resulting innominally the same regenerator dense bed temperature,regenerator gas velocity, and feed conversion. The feedrate increase is nearly proportional to the increase in O2

available for coke combustion. The ultimate limit oncapacity is usually product recovery; specifically, the wetgas product compressor.

The effect of O2 enrichment for maximum feed rate withthe regenerator in partial CO combustion is shown inCases 1 and 2. At 23% O2, wet gas compressorcapacity limits further feed rate increases.

The Case 2 results show a small loss in conversion andan increase in regenerator temperature compared toCase 1. Both of these can be offset by slightly reducingfeed rate (200–300 BPD) and increasing catalystcirculation. This would raise the reactor temperature andincrease the CO2/CO ratio to the base case. At 23% O2,no increase in CO afterburning was observed, and thecoke on regenerated catalyst (CRC) remained constant.

OPTIMIZE CONVERSION

The objective is to optimize conversion by maximizingcatalyst/oil ratio at the optimum reactor temperaturewhile maintaining constant feed rate. Starting at thebase operation conditions, GOX is added to theregenerator and feed preheat is reduced. At the sametime, catalyst circulation is increased to control reactortemperature, decreasing the regenerator temperature.The coke yield and conversion increase as reactortemperature and cat/oil ratio are raised. As more cokeis burned, the CO2/CO ratio in the flue gas falls and theregenerator temperature rises. All of these moves arebalanced when the CO2/CO ratio, excess O2, and reactortemperature are the same as the base case, resulting innominally the same regenerator temperature and gasvelocity and increased conversion at constant feed rate.

The coke yield increase is nearly proportional to theincrease in available O2; and conversion can be related tocoke yield. The ultimate limit on conversion (frommaximizing cat/oil ratio) is usually product recovery(specifically the wet gas product compressor) orminimum feed preheat temperature (resulting in highregenerator temperatures).

The effect of O2 enrichment for maximizing conversionwith the regenerator in partial CO combustion is shownin Cases 3 and 4. At 23% O2, the wet gas compressorlimits further conversion increases, since no more gasmake can be handled. No increase in CO afterburn wasobserved, and the CRC remained constant.

The optimal unit operation probably lies in the middle ofthe maximum feed and conversion tests. That is, moreproduct barrels can probably be made at less than maxfeed and conversion using oxygen enrichment.

FCC O2 ENRICHMENT TESTSDECEMBER 1982

Partial Combustion OperationStraight Run Gas Oil Feed

Case 1 2 3 4Max. Capacity 7/8 Max. Yield

Base O2 Enrich. Base O2 Enrich.

Control ParametersReactor • Fresh Feed Rate M BPOD 78.0 85.0 78.4 78.2 • Temperature °F 915 911 919 911 • Feed Preheat Duty MMBH 125.3 81.3 113 45.4

Regenerator • Air Rate MSCFM 130 128 132 133 • O2 Rate MSCFM – 3.7 – 3.3

Dependent ParametersReactor • Coke Yield WT% 4.3 4.5 4.5 4.9 • 430°F EP Conversion WT% 69.4 67.9 68.9 72.2 • Cat/Oil Ratio W/W 5.8 5.8 6.1 6.6

Regenerator • O2 Enrichment % 21 23.2 21 23.0 • Dilute Phase Temp. °F 1275 1265 1250 1290 • Dense Bed Temp. °F 1263 1282 1260 1275 • Flue Gas XS O2 % 0.2 0.1 0.2 0.2 • Flue Gas CO2/CO V/V 4.7 3.4 2.7 4.8 • Velocity fps 3.2 3.3 3.2 3.4

Limit Air Wet GasCompressor

Air Wet GasCompressor

Feed QualityStraight Run Vacuum Gas OilBoiling Range: 350–1000°FCon Carb.: 0.3–0.4 WT%

RESULTS FROM ARCO’S EXPERIENCE IN ITSHOUSTON REFINERY SHOW HOW TO STRETCH THECATALYTIC CRACKING UNIT'S THROUGHPUT ORHELP THE UNIT HANDLE HEAVIER FEED*

D. P. Bhasin and M. S. Liebelson,Air Products and Chemicals, Inc., Allentown, PA,and G. J. Chapman, Arco Petroleum Products Co.,Houston

To increase the capacity of fluid catalytic cracking(FCC), refiners must contend with process limitations,such as insufficient coke burning in the catalystregenerator. Coke burning can be limited by: air blowercapacity, regenerator superficial velocity, waste heatboiler velocity, electrostatic precipitator loading orregenerator temperature. Oxygen enrichment of theregenerator air provides a way around the blower andvelocity limits while remaining within regeneratortemperature constraints.

This article reviews oxygen enrichment techniques,operational procedures and associated economicbenefits. Concepts are illustrated via reference to asuccessful oxygen enrichment demonstration program atArco's Houston, Texas, refinery. Since regeneratortemperature is a common limitation, the effects of O2

enrichment on the FCCU heat balance are analyzedusing computer model simulations.

OXYGEN ENRICHMENT

Before discussing a specific commercial unit, let usreview the operational and safety issues associated withFCC O2 enrichment. Oxygen enrichment is suitable:

• When the investment in an air blower cannot bejustified because of the temporary nature of theadditional air requirement (e.g., for summer months orshort-term peak loading),

• On an interim basis to increase capacity until a largerblower can be installed during a scheduled turnaround,

• When an increase in the FCC's throughput wouldrequire substantial investment because of increasedregenerator velocity, inadequate heat recovery orincreased particulate emissions.

The effects of FCC O2 enrichment on regenerator air flowfor two different process limitations will be considered. Inthe first case (Fig. 1) where air blower capacity is abottleneck, pure oxygen can be injected into the blowerdischarge without modifying the air blower or shuttingdown the FCC unit. The total gas flow entering theregenerator increases somewhat. However, since nonitrogen is added with the oxygen, the increase is only

one-fifth of that when additional air blower capacity isinstalled.

In the other case (Fig. 2), where flue gas or superficialvelocity in the regenerator is a limitation, one can reducethe inlet air flow and inject pure oxygen into the blowerdischarge without shutting down the unit. Again, noblower modifications are necessary. In this case, part ofthe nitrogen in the original air flow is replaced by pureoxygen. Therefore, more oxygen is available for cokecombustion without increasing the flue gas volume. Inthis way, oxygen enrichment reduces catalyst carryoverand maintains velocities in the regenerator vessel anddownstream heat recovery system.

21% O2

78% N2

5.3 Mscfm O2

25% O2

74% N2

1% Ar 1% Ar

Enriched air105.3 Mscfm

Air100 Mscfm

Fig. 1 – For a blower limitation, adding oxygen to 100 Mscfmof air blower output (total 105.3 Mscfm) has oxygenequivalent to 125.2 Mscfm of unenriched air.

21% O2

78% N2

5.1 Mscfm O2

25% O2

74% N2

1% Ar 1% Ar

Enriched air100 Mscfm

Air94.4 Mscfm

Fig. 2 – For a regenerator velocity limitation, reducing airblower volume so that air plus oxygen remains 100 Mscfmhas the oxygen equivalent of 119 Mscfm of unenriched air.

The safety issues involved with oxygen in refineryenvironments, and FCC O2 enrichment in particular, areof great concern due to the consequences of anuncontrolled oxygen injection into the regenerator air. Inaddition to providing monitoring and control of oxygenflow during normal operation, an oxygen flow controlsystem should ensure automatic shutoff of oxygen in theevent of an upset condition. Fig. 3 illustrates a typicaloxygen flow control system for FCC enrichment.Redundant safety interlocks are used to ensure that O2

concentration in the regen air is always below 27percent. For O2 concentrations above 27 percent, ahazards risk analysis should be conducted to ensurematerials and process compatibility.

QUESTIONS WITH ANSWERS

A common concern of refiners evaluating the use ofoxygen for FCC enrichment is what effects oxygen will

have on regenerator temperature. There are three partsto this question:

• Will regenerator temperature rise if oxygen is used toexpand gas oil capacity?

• Will regenerator temperature rise if oxygen is used toexpand resid capacity?

• Will individual catalyst particles see temperatureshigher than measured bed temperatures?

In a heat balanced FCCU which is not air blower orvelocity limited, an increase in gas oil charge at constantconversion is accompanied by an increase in catalystcirculation rate, an increase in air rate, and little or nochange in regenerator temperature. The heat evolved bythe additional coke combustion is removed by theincreased catalyst and air flows. Most of the heat istransferred to the catalyst with only 15–30% leaving withthe flue gas. When oxygen enrichment is used, theincremental nitrogen which would normally be present inthe regenerator gas is absent. The loss of this additionalheat sink produces a small increase in regeneratortemperature. The magnitude of the increase isdependent on the extent to which nitrogen diluent is

reduced. The temperature increase is larger when airblower flow is cut back to maintain or lower regeneratorvelocities. In order to calculate the rise in regeneratortemperature, an iterative procedure must be used sincetemperature rises are accompanied by decreasedcatalyst circulation, less coke make and less heatevolved. The three temperature issues are discussed inthe examples that follow.

GAS OIL

Table 1 compares operation with O2 enrichment versus asupplemental blower for an increased gas oil operation atconstant conversion and preheat. The results wereobtained using the Air Products–Houdry FCC computermodel and base case data from Arco's Houston, Texas,FCCU. The computer model was first calibrated with fourof the detailed yield analyses performed during the Arcotest program. Compared to a supplemental blower,the dense and dilute phase temperatures increase by6-8°F and 5–6°F, respectively, when oxygen enrichmentis used to increase gas oil throughput from 78,000 bpd to88,000 bpd, a 13% increase in capacity. Temperatureincrease is slightly higher for the velocity limited casedue to the requirement that total regenerator gas flowrate be held constant at 130,000 scfm.

GaseousOxygen

50–600 psig

PCPSL

FIC

FSH

(1)

FIC

FCCUAir Blower

ProprietaryOxygenDiffuser

O2System

S/DFCC

ProcessAlarms(2)

FCCURegenerator

TSH

Notes:1. Safety pressure setting based on FCC air line design pressure.2. Two typical process alarms are low air line pressure and high regenerator dense bed temperature

Table 1Model Prediction: Oxygen enrichment vs.

supplemental blower for increased gas oil feed

O2 enrichment

Basecase

Supple-mentalblower

Airblowerlimit

Velocitylimit

Gas oil feed (bpd) 78,000 88,000 88,000 88,000Temperatures (°F) Reactor 915 915 915 915 Rgn. dense 1,263 1,262 1,268 1,270 Rgn. dilute 1,275 1,280 1,285 1,286Cat. circ. (tpm) 49.6 56.1 55.2 55.0Cat/oil (lb/ lb) 5.8 5.8 5.7 5.7Conversion (v%) 71.1 71.1 71.1 71.1Rgn. orsat (v%) O2 0.2 0.4 0.4 0.4 CO2/CO 4.7 4.9 4.9 4.8Air rate (Mscfm) 130.0 144.1 130.0 126.9

O2 rate (Mscfm) 0 0 2.5 3.1Total regen gas(Mscfm) 130.0 144.1 132.5 130.0Coke (wt % tf) 4.49 4.36 4.31 4.30CRC (base = 100) 100.0 95.0 93.0 100.0

RESID

When injecting oxygen to increase resid crackingcapacity, regenerator temperature will increasesignificantly beyond the rise seen with gas oil. Thisincrease is primarily due to the high levels of catalyticand feed coke in the resid feedstock and not due tooxygen injection per se. Based upon isothermal pilotplant tests, it is a mistake to conclude that adding residto a heat balanced FCCU will raise overall coke yield.Actually, in an FCCU with no external means of heatremoval such as steam coils or catalyst coolers, overallcoke yield is fixed by the unit heat balance at a constantreactor temperature. If the enthalpy required to crackresid is equal to that required for gas oil, overall cokeyield per barrel will remain approximately constant,regardless of feedstock. The effect of adding resid is anincrease in feed coke and a decrease in catalystcirculation coke. The net effect of this change is areduction in catalyst-to-oil ratio, which at constant overallcoke yield and reactor temperature, raises regeneratortemperature. The increased regenerator temperatureincreases the coke burning rate, thus reducing carbon onregenerated catalyst (CRC). The conversion decreasecaused by lower catalyst-to-oil ratio may be offset by theselectivity increase due to reduced CRC. Whether or notthese opposing effects allow for a constant conversionoperation depends on specific unit characteristics andthe CRC level before adding resid.Due to the interaction of many FCC variables, thecomputer model is used again to compare the same

base case with an operation where 10,000 bpd ofincremental resid is added to the feed. The resid has a70% higher laboratory (i.e., isothermal) coke yield andslightly lower laboratory conversion than the gas oil.Table 2 illustrates the output of the computer simulationmodel at constant conversion for an air blower limitedcase where first, supplemental air blower capacity isadded to burn the additional coke and second, whereoxygen enrichment is used. Regenerator temperaturerise is greater than 50°F for both cases. The reduction ofthe nitrogen heat sink with oxygen enrichment causesan additional 8°F increase in dense phase temperaturewhen compared to a supplemental blower. In thisanalysis, conversion is maintained by the increasedselectivity of the FCC catalyst at lower CRC levels.

Table 2Model Prediction: Oxygen enrichment vs.

supplemental blower for increased resid feed

Air blower limit

Basecase

Supple-mentalblower

O2

enrichmentGas oil feed (bpd) 78,000 78,000 78,000

Resid feed (bpd) 0 10,000 10,000 API gravity 10.0 10.0 10.0

Total feed 78,000 88,000 88,000Temperatures (°F) Reactor 915 915 915 Rgn. Dense 1,263 1,318 1,326 Rgn. dilute 1,275 1,330 1,337Cat. circ. (tpm) 49.6 48.7 47.9Cat/oil (lb/ lb) 5.8 5.0 4.9Conversion (v%) 71.1 71.1 71.1Rgn. orsat (v%) O2 0.2 0.2 0.1 CO2/CO 4.7 4.6 4.6Air rate (Mscfm) 130.0 146.5 133.0

O2 rate (Mscfm) 0 0 3.0Total gas (Mscfm) 130.0 146.5 133.0Coke (wt % tf) 4.49 4.45 4.39CRC (base = 100) 100.0 67.0 65.0

CATALYST PARTICLE TEMPERATURE

The last temperature issue to be addressed is the effectthat oxygen enrichment has on catalyst particletemperatures. This question has been raised primarilywhere enrichment is contemplated on FCC units of aside-by-side design where combustion air and spentcatalyst are mixed in the regen riser. The concern isthat the increased oxygen partial pressure in the riserwill cause excessive catalyst temperatures before thespent catalyst enters the regenerator bed. Once thecatalyst enters the bed, the heat of combustion isabsorbed in the larger quantity of catalyst that is present

in the dense phase. Whether or not excessive catalysttemperatures will result from enriching side-by-side unitsis a site-specific question which can best be answeredby a well-instrumented test program, such as the one atArco, that closely monitors regen riser and regen gridtemperatures. At Arco, no significant increase in regenriser temperature was observed.

ECONOMICS

The incentive for using FCC O2 enrichment is to obtainan incremental gas oil or resid charge to the FCCU byincreasing regenerator coke burning capacity.Enrichment is economical if:

• The value added gained from additional throughput islarger than the cost of oxygen;

• Alternative modifications to regeneration equipment orthe air blower are either more expensive than the presentvalue cost of oxygen or require substantially longer leadtime or downtime when compared with O2.

For example, in a situation where the regenerator vessel,electrostatic precipitator or the waste heat boiler is at avelocity limit, additional capacity obtained with increasedair flow must be accompanied by expensive equipmentmodifications. Oxygen enrichment allows increasedcapacity at constant velocities and makes economicsense on a long-term basis. On the other hand, if an airblower limit exists with no other equipment limitations,installing supplemental blower capacity will, in general,be less expensive than oxygen enrichment on a long-term basis. However, in this latter situation, oxygencould be used economically on an interim basis until anew blower is installed.

The incremental gross profit obtained from 10,000 bpdof incremental FCC feed for combinations of productvalue added and oxygen price is shown inFig. 4. The chart is based on a fixed 5% overall cokeyield on feed and therefore assumes either additional gasoil feed or resid cracking with no external source of heatremoval. For example, if the value added in upgrading anincremental 10,000 bpd of gas oil feed to FCC productsis $5/bbl, the incremental gross profit after deductingoxygen cost is between $9MM and $14MM per year.

The spread in incremental gross profits is a function ofthe oxygen supply mode. For relatively small volumes orfor short-term requirements, oxygen is moreeconomically supplied as a liquefied gas. Liquid oxygen(LOX) is transported by truck to the refinery where it isstored in insulated tanks. The LOX is then vaporizedwith steam or air for injection into the blower dischargeline. For larger requirements, gaseous oxygen (GOX) isgenerated at an Air Products owned and operated

oxygen plant and then transported via pipeline to therefinery. Energy and transport costs make LOX moreexpensive than GOX on a unit cost basis. However, asillustrated in Table 3, this price disadvantage may beoffset by a shorter required supply period and a quickerresponse time.

Value added, $/bbl

Incr

emen

tal g

ross

pro

fit,

$MM

/yea

r

Gaseou

s oxyg

en (G

OX)

40

20

10

0

50

30

1 2 3 4 5 6 7 8 9 1011 12 13

Liquid

oxyge

n (LOX)

Gas Oil ResidFig. 4 – These incremental gross profit data are based onbeing able to add 10 Mbpd additional feed.

COMMERCIAL RESULTS

An air blower limitation at the 75,000 bpd Sinclairdesigned FCC unit at Arco's Houston, Texas, refineryprevented feed rate increases. The unit is scheduled fora turnaround in 1985 that will allow for debottleneckingthe blower, but the incentive to increase productionduring the interim period was strong enough to consideroxygen enrichment as a short-term alternative. The ArcoFCCU is a side-by-side design where all of thecombustion air is combined with catalyst in theregenerator riser line. A major question was whether ornot higher oxygen partial pressure would cause anincreased burning rate in the regen riser resulting inexcessive catalyst particle temperatures, Because ofthis concern, it was decided that a well-instrumentedtest program be undertaken before installing oxygenenrichment on a permanent basis. The test run wasdesigned to safely answer the following questions:

• Will oxygen enrichment cause excessive regen andregen riser temperatures?

• Will oxygen enrichment improve FCC yields?

• How much extra feed can be processed using oxygenenrichment?

Table 3Liquid vs. gaseous oxygen supply tradeoffs

Hauled inliquid

oxygen (LOX)

Pipelinedgaseous

oxygen (GOX)Length of supply

commitment1 day–5 years 5 years–15 years

Construction period 4–8 weeks 9–24 monthsPrice Higher Lower

A thermocouple was hot-tapped in the regen riser andthis temperature was closely monitored along with pointsunder the grid. The yield benefit was evaluated byattempting to burn more coke at constant feed rate andcat/oil ratio, thereby reducing carbon on regeneratedcatalyst (CRC). The feed benefit was obtained byburning more coke at constant CRC and cat/oil ratio,thereby raising throughput.

Oxygen enrichment into the FCCU was implementedwithin four weeks from the time of a go-ahead decision.Liquid oxygen for the test was transported to the refineryat a rate of ten tanker trucks per day. The LOX waspumped into two mobile customer stations which actedas surge tanks for the varying flow rates. Each mobilecustomer station contained a 3,000-gallon tank and asteam vaporizer capable of gasifying 125 Mscfh ofoxygen. Oxygen flow was controlled via a flow controland instrumentation system incorporating redundantsafety interlocks to prevent high oxygen concentrationsin the blower discharge line. Oxygen injection wasaccomplished by hot-tapping a 6-in. nozzle on the airblower discharge line and inserting a 4-in. diffuserdesigned to prevent impingement of unmixed oxygen onthe carbon steel blower line and downstream flowcomponents.

A safety and operational training session was given to allunit operators to ensure that correct operatingprocedures were followed.

The test program demonstrated that with oxygen,7,000 bpd of additional gas oil feed could be charged tothe FCCU at constant conversion and without excessiveregenerator temperature. Eight detailed yield tests wereperformed over a two-week period. The FCCU was lined-out for 24 hours before each unit evaluation and datacomparison were made. Analyses were performed withina one-day span in order to minimize any unnoticedfeedstock or unit changes. Table 4 compares results fortest runs before and after oxygen injection,demonstrating increased yield (Trial 1) and increasedcapacity (Trial 2). In Trial 1, CRC could not be reducedwith oxygen enrichment due to catalyst flow problemswhich were unrelated to oxygen injection. There was a1.5% increase in gasoline yield from the higher cat/oil

and conversion, but this was not economically significantat the time due to the small differential price betweengasoline and heating oil. In Trial 2, additional gas oilcapacity of 6,950 bpd was obtained with the addition of222 Mscfh of oxygen at a 23.3% enrichment level. Theconversion decrease from 69.4 wt% to 67.9 wt% wasdue to the lower reactor temperature and weight balancecorrections to the data and not due to O2 enrichment. InTrial 2 regenerator temperature increased by 19°F andpreheater duty was cut by 44 MMBtu/h.

Because of the test success, Arco will use oxygen,when needed, until the next turnaround when blowermodifications might be implemented.

Table 4Arco test results

Increased yieldTrial #1

Increased capacityTrial #2

Test run BaseO2

Enrichment BaseO2

Enrichment

Date 12/08/82 12/09/82 12/11/82 12/10/82

Total feed (bpd) 78,360 78,216 77,966 84,967Feed with b.p. of 430°F + (bpd) 76,774 77,881 77,681 84,635Main air blower (Mscfm) 131.5 132.3 130.0 128.0

Oxygen enrichment ( Mscfm) 0 3.4 0 3.7Oxygen concentration (vol %) 21.0 23.0 21.0 23.2

Conversion ( wt%) 68.9 72.2 69.4 67.9Average reactor temp. (°F) 919 911 915 911Regen riser temp. (°F) 918 914 940 920Regen bed temp. (°F) 1260 1275 1263 1282Regen dilute phase temp. (°F) 1284 1302 1275 1291

Preheater duty ( MMBtu/h) 113 45.4 125.3 81.3

*Reprinted from HYDROCARBON PROCESSING, September 1983Copyright © 1983 by Gulf Publishing Co., Houston, Texas.

All rights reserved. Used with permission.