Assessing the Feasibility of a Progressive Distillation Scheme

Assessing the Feasibility of a Progressive Distillation Scheme

Presented by: Brian Howard

Submitted: April 30, 2009

Topics for Discussion

Background of crude oil refining

Conventional method of crude distillation

Direct sequence vs. indirect sequence

Progressive crude distillation

Current applications of progressive distillation

Crude Oil Refining - BasicsCrude oil is a complex mixture of

hydrocarbons ◦ Compounds in crude oil contain anywhere from

one carbon to chains in excess of fifty carbons

In general, the higher the molecular weight of a component, the higher the normal boiling point

The purpose of refining is to separate and manipulate (i.e. via chemical reactions) these compounds to produce usable products

Crude Oil Refining – Box Flow Diagram

Focus of the Project

Crude Oil Refining – Process OverviewAtmospheric distillation is the primary

separation process

Downstream processes convert distillation effluents into more useful products◦ Reforming – converts naphtha to gasoline◦ Cracking – produces more gasoline from

gas oil◦ Coking – produces more gasoline from

residue and petroleum coke

Distillation – Basic TheoryDefinition: a separation process which

separates components in a mixture on the basis of differing volatilities

Volatility of a component is expressed by its vapor-liquid equilibrium ratio, K

An “at first glance” approximation of the ease with which two components can be separated is given by the relative volatility, a

The larger the value of a, the more readily the components may be separated

Distillation – Apparatus

Trays provide the medium for liquid vapor contact and mass transfer

As vapor rises through the column, it becomes enriched in the more volatile components

As liquid falls through the column, it becomes enriched in the less volatile components

Trays

Tem

pera

ture







“Unconventional” Distillation

Failed Capstone Student

Conventional Crude Distillation – Process Flow Diagram

Furnace preheats crude to temperature of feed tray

Steam fed side strippers strip lighter components from side draws improving sharpness of cuts

Pumparounds remove excess heat from the column and use it to pre-heat the feed stream

Conventional Crude Distillation – Product StreamsProducts of distillation are not pure

components◦ Contain a range of molecular weights and

normal boiling points

5 major product streams◦ Naphtha –critical component of gasoline◦ Kerosene – heating fuel, jet fuel◦ Diesel – transportation fuel◦ Gas oil – transportation fuel◦ Residue –further refined to produce more

useful products

Vola

tility

Mole

cula

r W

eig

ht

Defining Product Streams – ASTM D86The ASTM D86

distillation test measures the boiling point ranges of distillation products

The D86 criteria were used to define the products of distillation

Indirect Distillation Sequence

Conventional distillation is an indirect sequence

Least volatile (heavier) components are removed first

More volatile (lighter) components proceed to next column in the sequence

Process is repeated until desired degree of separation achieved

Mole

cula

r W

eig

ht

Conventional Crude Distillation –Envisioning the Indirect Sequence

How is this… …related to this?

Conventional Crude Distillation –Envisioning the Indirect SequenceAnswer:

Each side stripper and the corresponding rectifying section of the main column can be treated as an independent column in an indirect sequence. Pumparounds serve as condensers.

Direct Distillation Sequence

Mole

cula

r W

eig

ht

Reversal of the indirect sequenceMore volatile (lighter) components are

removed firstLess volatile (heavier) components proceed

to next column in the sequenceNo industrial applications of a purely direct

sequence







Direct vs. Indirect – So What?

Up to this point, the distinction between a direct and indirect sequence has been purely academic

Two important questions:◦Can the differences between the two

sequences increase revenues or decrease expenses?

◦If a direct sequence can produce savings, how can it be applied?

Direct vs. Indirect – Question of Savings

Distillation is one of the most energy intensive aspects of processing.

Over 2% of the energy content in a crude stream is used in distillation.

Distillation accounts for about 40% of energy use in a refinery.

641.6 TeraBTU!!!

Direct vs. Indirect – Energy Savings

In June of 2008, Natural Gas was trading at $10.82/MMBTU

As energy becomes an increasingly important factor in the operational costs of process plants, so does energy efficiency

Where does the direct sequence provide potential energy savings when compared to an indirect sequence?


In conventional distillation, the furnace must preheat crude to the feed tray temperature

This heats lighter components to higher temperatures than necessary to vaporize them

The furnace wastes energy superheating light end components in an indirect sequence


Because the direct sequence separates the most volatile (lightest) components first, superheating of lightends is avoided

Consequently, the opportunity for energy savings exists

How can the indirect sequence be applied?







Progressive DistillationDefinition: “The process consists in successively

separating increasingly heavy petroleum cuts at the head of a plurality of columns…of a first series of columns which feed individually each column of the second series.”

Naphtha

Kerosene

Diesel

Gas Oil

Residue

Steam

Progressive Distillation – A Direct Sequence?

Not a purely direct sequenceSeparations are not sharpSeparations occur in series

Progressive Distillation – JustificationProgressive distillation is not truly

a direct sequence – why bother?

Best approximation of a direct sequence provided the limitations of a complex hydrocarbon mixture

Patent claims energy savings of up to 16%Progressive scheme has been

implemented at a refinery in Germany






Comparison of conventional and progressive distillation

Conventional vs. Progressive – Comparing Apples to ApplesTo enable comparison between the two

distillation schemes, the following factors were fixed between both models:

•Crude composition profile

•D86 95% points

•Product gaps

Crude Composition Profile

Light Crude

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

-164 -42 13.2 50 73 100 128 155 183 211 239 266 294 322 350 378 405 439 495 549 597 663 781

Barr

els/

day

Normal Boiling Point (NBP) of Component (°C)

Composition is weighted more heavily towards the lower normal boiling point components

Crude Composition Profile

Heavy Crude

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Barr

els/

Day

Normal Boiling Point (NBP) of Component (°C)

Composition is weighted more heavily towards the higher normal boiling point components

D86 95% Points and Product Gaps

0

30

60

90

120

150

180

210

240

270

300

0

50

100

150

200

250

300

350

400

Naphtha

Kerosene

Normal Boiling Point (⁰C)

LB

-MO

L/H

R Temperature at which 95% of lighter compo-nent will boil

The product gap is the difference be-tween these values

Temperature at which 5% of heavier component will boil

Conventional vs. Progressive – Comparing the Simulations

Progressive

Conventional

Conventional vs. Progressive – Determining Utility Usage

In-program “Pinch” calculator determines utility consumption from inputs in the process simulator.

Additionally, Excel calculator is available but requires user input of the same process parameters

Cross-checking outputs from both calculators verifies results

R7 R8 R9 R10 R11 R12 R13 R3 R4 R5 R6 Hot Streamsnapth kero dies ago resid cond PA1 PA2 PA3 Minimum Utility

Fcp 0.1 0.07 0.04 0.0971 0.1169 0.33 0.49697 0.489991 0.539406 Hot 52.43729Tin 28.3 163.6 217 312.09 340.16 131 149.3 224.2 279.7 Cold 33.79079

ΔTmin 20 Tout 25 25 25 25 250 28.3 104.4 171.1 232.2 Pinch from to

R1 R2 Cold Streams 270 259.7 280e1 seq-1 seq-2 seq-3 seq-4 seq-5 seq-6 seq-7 seq-8 seq-9 seq-10 seq-11 seq-12 seq-13

Fcp 0.47 0.413 0.45 0.4799 0.5114 0.54 0.56752 0.585985 0.600028 0.612 0.622 0.631 0.6406 0.6499Tin 21.1 104.4 124 143.6 163.19 183 202.348 221.921 241.511 261.1 280.7 300.2 319.84 339.42Tout 104 124 144 163.19 182.77 202 221.921 241.511 261.092 280.7 300.3 319.8 339.42 359

Warning: Do not use negative or zero temperatures (shift scales) Use F*Cp larger than 1 Cannot use more than 40 different temperatures

Conventional vs. Progressive – Utility for a Light Crude Hot Utility (MW)

Conventional 58.43

Progressive 61.49

Percent Change 5.24%

Light Crude

Annual Energy Cost

Conventional $97,830,000

Progressive $106,888,000

Difference $9,058,000

Light Crude

Cold Utility (MW)

Conventional 46.06

Progressive 103.55


5% increase in hot utility consumption for progressive model

$9 million increase in operational costs for progressive model

125% increase in cool utility consumption for progressive model

Conventional vs. Progressive – Utility for a Heavy CrudeHeavy Crude

Hot Utility (MW)

Conventional 73.40

Progressive 68.31

Percent Change -6.93%

Heavy Crude

Annual Energy Cost

Conventional $130,462,000

Progressive $119,347,000

Difference -$11,115,000

Heavy Crude

Cold Utility (MW)

Conventional 11.56

Progressive 37.46


7% decrease in hot utility consumption for progressive model

$11 million decrease operational costs for utility consumption224% increase in hot utility consumption for progressive model

Conventional vs. Progressive - ResultsResults are mixed for hot utility

◦Heavy Crude: 7% reduction◦Light Crude: 6% increase

Does the hot utility savings produced in processing a heavy crude justify implementing a progressive scheme?Heavy Crude

Cold Utility (MW)

Conventional 11.56

Progressive 37.46


Analyze the cost of an increased cold utility and capital costs of new columns

Conventional vs. Progressive – Costs of Cold UtilityDramatic

increase in cold utility requires cooling water capacity increase

Capital Costs for expansion◦ Piping◦ Cooling Tower◦ Heat Exchanger

AreaCost: $1.35

million

Conventional vs. Progressive – Capital Costs of Installing Column Network

Implementing the progressive scheme would require the addition of 6 columns

Estimated capital cost:

$3.65 million

Conventional vs. Progressive – Pay Out Time

Cold Utility Expansion $1,350,000

Column Expansion $3,648,000

Annual Energy Savings $11,115,000

Pay Out Time (years) 0.45

Pay out time is slightly over 5 months, indicating an excellent return on investment and sustained energetic savings.

Conventional vs. Progressive – Hold on a Minute… A progressive scheme that has produced energy

savings of up to 16% is already working in Germany

Conventional vs. Progressive – Implemented Progressive Scheme

Progressive Scheme in Germany is drastically different. Involves a pre-flash unit placed before the furnace, which knocks off lightest components before the main column.

Conventional vs. Progressive – Conclusions

The progressive distillation scheme studied in this report showed mixed results with respect to reducing utility consumption. For a heavy crude, a progressive scheme would ultimately produce annual savings of $11 million after a payout time of 5 months. For a lighter crude, no such savings were observed

ReferencesBagajewicz M. and S. Ji. “On the Energy Efficiency of Stripping-Type Crude Distillation.” Ind. Eng. Chem. Res. 2002, 41, 12, 3003‐3011. Bagajewicz, Miguel and Ji, Shuncheng. “Rigorous Procedure for the Design of Conventional Atmospheric Crude Fractionation Units. Part I: Targeting.” Ind. Eng. Chem. Res. 2001, 40, 617-626. Bagajewicz M. and S. Ji. “Rigorous Targeting Procedure for the Design of Crude Fractionation Units with Pre‐Flashing or Pre‐Fractionation.” Ind. Eng. Chem. Res. 2002, 41, 12, 3003‐3011. Devos et al. United States Patent No. 4,664,785. May 12, 2987. Dobesh, Dan et al. “Evaluation of the Energy Savings Claims of Progressive Distillation.” Unpublished. 2008. Perry, Robert H. et al. Perry’s Chemical Engineers’ Handbook. 7th ed. McGraw Hill, New York: 1997.

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Assessing the Feasibility of a Progressive Distillation Scheme