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WASTE MINIMIZATION THROUGH IMPROVED PROCESS THERMODYNAMICS:

CRUDE OIL FRACTIONATIONby

David B. ManleyThe University of Missouri, Rolla, Missouri

INTRODUCTION

Crude oil distillation is the first unit encountered invirtually every refinery. Figure 1 shows a typical refinery flowdiagram including a crude unit containing a atmospheric section, avacuum section, and a stabilizer section

3. Figure 2 shows the

crude fractionation unit in more detail including in thisparticular case two stages of vacuum distillation

6.

Fourteen million barrels per day of crude oil are currentlyprocessed in the United States1. About seven million barrels per

day are imported1. About seventy million barrels per day are

processed worldwide31. Conventional atmospheric and vacuum crude

distillation units require about 100,000 BTUs (British ThermalUnits) per barrel processed for furnace energy to accomplish theseseparations

2. Since a barrel of oil contains about 6,200,000 BTUs

of energy3, this required furnace energy amounts to the energy

content of about 1.6 percent of the processed oil. At current oil

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prices of about $15 per barrel1the energy cost of processing is

then $0.24 per barrel. As shown in table 1 the annual energy costfor the United States for this purpose is about $1.24 billion, andworldwide the annual cost is about $6.18 billion.

Furnace effluents are a major contribution to refinery wasteand consequent environmental pollution

6depending primarily on the

type of fuel used (high sulfur fuel oil, low sulfur fuel oil,natural gas, etc.). There are a number of ways to minimize this

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waste by incremental process improvements and post process wastetreatment and disposal such as stack gas scrubbing

7. Even when

using relatively clean fuels and post process waste treatmentfurnace effluents are still significant

7. Regulatory efforts to

quantify the environmental impact of these effluents have led tothe concept of "cost of waste

5,8,9." Waste costs from Nevada

regulations

9

for furnace effluents (CO2, SOx, NOx, TSP, VOCs, andCO) from conventional and "clean" crude distillation units7

aregiven in table 2 resulting in national costs ranging from amaximum of $2.698 billion per year (conventional process with highsulfur fuel oil) to a potential minimum of $0.808 billion per year("clean" process with natural gas fuel). The sulfur content ofthe fuel is a major contribution which can be reduced bypreprocessing or changing the fuel. The major contribution fromcarbon dioxide is a consequence from using fossil energy.

Since about one half of the crude oil processed in the UnitedStates is imported, the $1.24 billion per year energy cost forcrude oil distillation significantly effects the national balanceof payments, with important consequences to the national economiccompetitiveness in the world economy. Obviously, a significantincrease in oil price such as occurred in 1973 and 1979 would have

a major impact on this cost. In addition, the strategic militaryimplications of reliance on importation of a critical commoditysuch as crude oil are virtually incalculable as evidenced by therecent Gulf war. The energy efficiency of crude oil processing isconsequently a significant factor in the United States' internaleconomy, environmental cleanliness, international industrialcompetitiveness, and national security.

The first and second laws of thermodynamics allow the minimum

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possible furnace energy required for crude oil distillation to becalculated using the concepts of "reversible thermodynamics

10" and

a computer simulation of a typical crude unit including"pseudocomponents" to estimate thermodynamic properties

11. As

shown if figure 3 the relevant equations yield a theoretical valueof about 3,000 BTUs per barrel for a minimum furnace temperature

of 700

o

F. About 3% of the conventional process value. Thismaximum energy efficiency can never be completely achieved inpractice, but it serves as a target for potential processimprovements. It has been shown

12,13theoretically that any

multicomponent distillation process, such as crude oildistillation, can be designed to approach thermodynamicreversibility by using the modern technological concepts ofdistributed distillation, pinch technology, and high efficiencycolumn internals to address mass, heat, and momentum transferinefficiencies respectively

14,15. If a large reduction in furnace

energy requirements could be commercially implemented throughoutthe United States, then significant improvements in the worthwhileobjectives of economic competitiveness, environmental protection,

and national security could be achieved.

REVIEW OF PROCESSES

Many crude distillation processes were proposed in the early1900s

19before the industry settled on the currently used

conventional pipestill unit20. Conventional sharp split

distillation of crude oil has been commercialized4

in both a"direct" sequence of distillation columns as shown in figure 4 andin an "indirect" sequence of distillation columns as shown infigure 5. The direct sequence or "shell still" process has the

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advantage that each product is vaporized from the crude oil at theminimum temperature thus minimizing heating; but, because theproducts must be fractionated to meet specifications, considerablerefluxing and remixing with the crude oil occurs thus requiringrevaporization at higher temperatures.

The indirect sequence or "pipe still" process virtually eliminatesthe refluxing and remixing but requires heating the entire charge

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to the maximum temperature in order to vaporize all the productsbefore fractionation. The entire charge must then be cooled fromthe maximum process temperature. Although combinations of theseflowsheets are used, they all retain some of the disadvantages ofeach because conventional "sharp split" distillation is used. Useof a preflash tower

22to bypass a light fraction around the

furnace helps to improve efficiency. Another inefficiency isremoval of all the vacuum gas oils at the lowest pressure (highestvacuum). Use of two vacuum towers

23to remove the light vacuum

gas oils at an intermediate vacuum as shown in figure 2 helps toreduce this inefficiency. Each of these innovations increases thecomplexity and cost of the process, but incrementally improves theenergy efficiency.

Figure 6 shows a flowsheet of a modern atmospheric pipestill, indirect sequence, which has been mechanically integratedinto two column vessels

4. In figure 6 the cold crude oil is

preheated by heat exchange with the product streams and with theproducts

4from the vacuum unit as shown in figure 7. The heat

integration of the crude charge preheat train can be optimizedusing "PINCH" technology

14,17,21and typically can reduce furnace

heat by about 35% to around 65 MBTU/BBL at the expense ofincreased heat exchanger area.

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Theoretically12,13

a completely distributed distillationdesign as shown in figure 8 for ethylene recovery

24will achieve

the highest energy efficiency and the smallest size unit. In acompletely distributed distillation sequence the most volatile andleast volatile products entering each column are separated. Asthis strategy propagates through the process the "christmas tree"of distillation columns in figure 8 is generated. In contrast tothe conventional direct sequence of figure 4, in the distributeddesign refluxing and remixing of vaporized products into the heavycrude is minimized; and, in contrast to the conventional indirectsequence of figure 5, in the distributed sequence each product isheated to the minimum possible temperature. Appropriate

procedures for the design of a distributed distillation unit havebeen developed by the author

15and others

24,25, but are not

published in general.

One key element in distributed distillation design is theefficient implementation of interstage heat exchange through theuse of dephlegmators

26such as used in ethylene recovery

28.

Conventional interexchangers can also be used. A second keyelement in distributed distillation design is the opportunity forthe mechanical consolidation of columns through the use ofinternal partitions

29which have been commercialized in Europe

30.

Using these technologies the typical process composite heating and

cooling curves (from PINCH technology

14

) can be respectivelylowered and raised in temperature by significant amounts. By thenredesigning the heat exchanger network large energy and capitalreductions can be achieved because more energy is reused as itcascades through the process. Capital savings are enhanced by themechanical consolidations of equipment.

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The design procedures for thermodynamically reversiblemulticomponent distillation processes have been developed forcryogenic processes because of the strong economic incentivegenerated by the high cost of refrigeration utilities. These same

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procedures can be productively applied to above ambienttemperature processes such as encountered in the refining industryand specifically to crude oil distillation to save furnace energyand capital costs. In ethylene recovery, for example, the mainprocessing problem is the removal of valuable condensable productssuch as ethylene and propylene from noncondensable gases such as

hydrogen and methane. Because the noncondensable gasesdramatically lower the condensation temperature of ethylene andpropylene, high pressures (550 psia) and low temperatures (-215

oF)

are required for recovery with associated high capital and utilitycosts. In crude oil distillation, by analogy, the main processingproblem is the removal of valuable volatile products such asgasoline and naphtha from nonvolatile liquids such as gas oils andasphalt. Because the nonvolatile liquids dramatically raise theboiling temperature of gasoline and naphtha, low pressures (25mmhg) and high temperatures (850

oF) are required for recovery with

associated high capital and utility costs. The two processes arethermodynamically mirror images of each other, and the sameprocessing concepts can be used to improve both.

A "first order" distributed distillation process as shown infigure 9 is appropriate for crude oil processing. To reducecomplexity with minimal impact on thermodynamic reversibility thelarge number of prefractionators present in the completelydistributed process of figure 8 is truncated. Theprefractionation is "first order" because only one product isdistributed in each column of the prefractionation train. Forexample, the first column which separates LPG from naphthadistributes gasoline between the overhead and bottoms (a gasolinedistributor column). The overhead from the gasoline distributorcolumn is further distilled in a stabilizer (LPG column) to removeLPG from gasoline. The bottoms from the gasoline distributor

column is further distilled in a prefractionator which separatesgasoline from kerosine and distributes naphtha (a naphthadistributor column). The procedure is then repeated as theheavier fractions are removed at progressively lower pressures.Because the prefractionation columns are designed to separaterelatively wide boiling fractions they are very lightly refluxedand are consequently relatively thermodynamically efficient. Thefirst order distributed distillation process for crude oilfractionation is still relatively complex and requires a largenumber of columns and heat exchangers.

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However, if the hot products are used to supply heat to the

prefractionator reboilers, then this process is relativelyefficient and requires about 50,000 BTU/BBL processed. It isquite similar to that proposed by Elf

18and shown in figure 10.

Elf Aquitaine of Paris, France has proposed a "ProgressiveSeparation Scheme for Crude Oil Distillation

16,17,18" which uses

some of the concepts of distributed distillation and whichachieves a furnace energy requirement of about 50,000 BTUs perbarrel of oil. About one half of the value for a conventional

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process. In addition, the Elf authors claim some reduction in thecapital cost of the plant. However, the Elf process as publisheddoes not include many of the above mentioned modern technologicalconcepts for reversible multicomponent distillation. Using theseconcepts and practical design practices

3,4the author has simulated

further improved processes achieving furnace energy requirements

as low as 20,000 BTUs per barrel which approaches the theoreticallimit of 3,000 BTUs per barrel.

As discussed above furnace energy for a conventional processis about 100 MBTU/BBL of capacity. The 1993 capital cost for aunit

1,2of this kind is about $750 per barrel per day of capacity.

Improvements in heat integration of the crude preheat train can

reduce the furnace energy to about 65 MBTU/BBL of capacity7,14,17,21

.The major inefficiency of this conventional process is the heatingof all the crude charge including the LPG and gasoline to amaximum temperature of about 700

oF. The Elf process

18achieves

its energy efficiency at the expense of greater complexity byusing more prefractionation and more pressure levels. However,the estimated 1993 capital cost of a new Elf progressivedistillation crude unit

1,16is about $600 per barrel per day of

capacity. This reduction in capital cost over a conventional unit

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is debatable, but ostensibly the reductions in equipment sizeoutweigh the increased complexity costs.

The first order distributed distillation process can befurther improved through thermal integration as shown in figure11. To reduce pressure drop and the number of heat exchangers,

the prefractionation columns can be refluxed from the finalfractionators; and two pairs of final fractionators can bethermally coupled in a multieffect arrangement. This directlyreuses the thermal energy in the bottom column to reboil the uppercolumn and also contributes to the thermal efficiency of theprocess. Interreboilers can also be used to improve the heatintegration.

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Results from thermally integrating a distributed distillationprocess for crude oil fractionation are shown by the before andafter process composite heating and cooling curves in figure 12which shows how the heating and cooling curves are moved apartthrough improved process design.

Table 3 shows resulting furnace duty, total heat exchange,and total column volume for a conventional process, a conventionalprocess with maximum heat integration, a distributed distillationprocess, and a distributed distillation process with maximumthermal integration. For the conventional process significantreductions in furnace duty and total heat exchange can beaccomplished through increased heat integration, but the economicbenefits are mitigated by the reduced temperature driving forcefor heat exchange which increases heat exchange area. The total

volume of the columns is unchanged. For the advanced distillationdesign using distributed distillation alone the furnace duty,total heat exchange, and column volume are all significantlyreduced in comparison with the conventional case withoutdecreasing the temperature driving force. If in addition thetemperature driving forces are reduced through increased heatintegration a large further reduction in furnace duty can beachieved at the expense of increased heat exchanger area.

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The thermally coupled first order distributed distillation

process of figure 11 requires less capital and energy than thethermally unintegrated process of figure 9, but still has arelatively large number of pieces of equipment requiringconsiderable pumping, piping and operating maintenance. Thethermally integrated first order distributed process can also bemechanically integrated as shown in figure 13. This retains thehigh thermal efficiency of the thermally integrated process, butreduces capital and operating expense by consolidating columns ofequal pressure in single shells through the use of partitions

30.

Mechanically integrated columns such as these are in use inEurope

31, and have been observed by the author; but they are not

yet used in the United States. The thermally and mechanicallyintegrated first order distributed distillation process is not

unique as additional prefraction ("second order" distributeddistillation), additional pressure levels, and additional heatintegration could have been used. As additional complexity isintroduced the thermal efficiency improves to approach thetheoretical limit of 3,000 BTU/BBL, but at some point the capitaland operating cost of the complexity outweighs the savings due tothermodynamic efficiency. Having simulated and analyzed a numberof these configurations, it is the author's opinion that theoptimum design for a grassroots unit will achieve about 35,000BTU/BBL furnace energy at about $500/BBL 1993 capital investment.This is a 65% reduction in energy and a 33% reduction in capitalas compared to a conventional design.

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DISCUSSION

There are a number of technical reservations regarding thedistributed distillation processes for crude oil. First, thesimulations and designs are based on a "pseudocomponent"characterization of the crude oil which is only approximatelycorrect and does not account for decomposition reactions,unidentified components, exchanger fouling, and corrosion effects.In addition the simulations are based on the equilibrium stagesimplification for distillation which is only approximatelycorrect especially for an extremely complex mixture such as crudeoil. Second, the control strategies for distributed distillationare not well developed

32,33. In a first order distributed

distillation design each product specification must be metseparately in more than one separate distillation column. Infigure 9 for example, gasoline loss must be controlled not only inthe top of the LPG column and the bottom of the gasoline column,but also in the bottom of the second prefractionation (naphthadistributor) column. While making control more difficult thisflexibility improves the ability of the process to handledifferent feedstocks and also increases the degree offractionation between the products. The improved fractionation

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can be used to increase product yields23

which significantlyimproves plant profitability. Finally, the mechanicalpartitioning while conceptually reasonable requires considerablehardware development and testing.

There are also economic reservations regarding the

distributed distillation processes for crude oil. Because crudeoil processing is not increasing in the United States there isvery little demand for new units, and replacement of an existingunit no matter how inefficient is very difficult to justify basedon projected energy savings alone. Net present values forreplacement of a conventional process with either the Elf or a newadvanced process are given in table 4 as calculated from theformula:

where I = capital cost

S = operating savingsr = interest rate / cost of capitalt = project life

In both cases negative net present values are calculated,although the new process results are marginal. However, ifenvironmental effects (cost of waste) are included, then theeconomics are considerably improved and yield respectably large

))1(

(1

1t

rr

S

r

S

rINPV

+

+

+=

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positive net present values. Incorporation of these incentiveswill require government legislation as they are not native in afree market economy. Also, significant economic incentive mustexist to mitigate the risk of commercializing a new process. Ofcourse, any increase in energy prices would have a dramatic anddirect impact on the economic evaluations. Also, if the existing

unit were so physically deteriorated that it must be replacedanyway, then the proposed new process would be the economicchoice.

Another alternative is to retrofit existing units forimproved thermal efficiency at minimal capital investment. Thismay prove viable especially if one unit can be incrementallyexpanded to replace two existing smaller units. The distributeddistillation process is especially adaptable to this servicebecause the additional columns can be added incrementally to theexisting process. Partitions can also be installed in existingcolumns with minimal modifications to the superstructure. Andexisting exchangers can be reused in the modified process.

However, each revamp situation is unique and requires anindividual analysis to evaluate.

Outside the United States there is a significant market forcrude oil distillation technology, and this may be of value to theengineering and construction industry in addition to theinternational oil companies. Crude oil consumption is projectedto increase by about one and one half percent per year for severaldecades

31, and this will generate demand for about 1000 MBPD of

new refining capacity worldwide each year. At $600/BPD capitalcost this amounts to $600MM capital investment each year for crudeand vacuum units alone. If United States technology is used inthese refineries then the royalties on this technology can

potentially contribute to the national economy.

The author has informally attempted to interest a number ofoil companies and engineering companies in developing the proposedprocess with little success. Typical reactions are "How do I knowit will work?" or "Nobody will buy it." or "The (perceived)capital cost is too high." or numerous technical reservationsregarding engineering decisions. Fundamentally, withoutconsidering environmental or strategic costs, the economicincentive in the United States is just not sufficient to justifythe risk and cost of development and demonstration; and at presentthere is no accepted mechanism for incorporating the relevantenvironmental or strategic costs into the economic equations. Inaddition, unless a technology is proven, it is usually not evenconsidered as an available alternative for use in possibleregulation

7.

CONCLUSIONS

Crude oil distillation is a large energy consumer and a largewaste generator, but through the use of improved processthermodynamics significant reductions in energy consumption and

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waste generation can be achieved. However, the improved processdesigns are unproven, projected capital cost reductions areuncertain, and energy savings at current prices will not justifythe investment needed for process development andcommercialization in the United States where refining capacity isnot increasing. However, depending on the accepted economic cost

of waste generation, combined energy and waste savings may beshown to provide sufficient incentive for implementation.

REFERENCES

1Statistics, Oil & Gas Journal, Pages 75-77, December 6, 1993.

2Foster Wheeler USA Corp., Crude Distillation and Vacuum

Flasher, Hydrocarbon Processing, Page 111, September, 1986.

3Gary, J. H. and G. E. Handwerk, Petroleum Refining Technology

and Economics, 2nd Edition, Marcel Dekker Inc., Appendix B.3,Pages 317-318, 1984.

4Nelson, W. L., Petroleum Refinery Engineering, 4th Edition,

McGraw-Hill Book Company, Page 423, 1958.

5Iyengar, S., Air Emissions: Vermont Utility to Consider Cost

of Pollution, Burlington Free Press, November 30, 1993.

6Sittig, M., Petroleum Refining Industry Energy Saving and

Environmental Control, Noyes Data Corporation, 1978.

7Cindric, D. T., B. Klein, A. R. Gentry, and H. M. Gomaa,

Reduce Crude Unit Pollution With These Technologies, Hydrocarbon

Processing, Pages 45-54, August, 1993.

8Opting Into the Acid Rain Program: Proposed Rule, Federal

Register, Environmental Protection Agency, Part II, 40 CFR Part72, et al., Friday, September 24, 1993.

9Final Rule for Docket No. 89-752, Public Service Commission

of Nevada, Table 2, January 22, 1991.

10Smith, J. M., and H. C. Van Ness, Introduction to Chemical

Engineering Thermodynamics, 4th Edition, McGraw-Hill PublishingCompany, Chapter 16, Pages 548-568, 1987.

11 Atmospheric Crude and Vacuum Towers, HYSIM Special Featuresand Applications Guide, Version C2.10, Hyprotech Ltd., Calgary,Canada, Pages R1-R19, January, 1993.

12Petyluk, F. B., V. M. Platonov and D. M. Slavinskii,

Thermodynamically Optimal Method for Separating MulticomponentMixtures, International Chemical Engineering, Volume 5, Number 3,Pages 55-561, July, 1965.

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13Franklin, N. L. and M. B. Wilkinson, Reversibility in the

Separation of Multicomponent Mixtures, Trans. Inst. Chem. Engr.,Volume 60, Pages 276-282, 1982.

14Tjoe, T. N. and B. Linnhoff, Using Pinch Technology for

Process Retrofit, Chemical Engineering, Pages 47-59, April 28,

1986.15

Manley, D. B., P. S. Chan and D. B. Crawford, ThermodynamicAnalysis of Ethylene Plant Distillation Columns, Proceedings ofthe 4th Ethylene Producers Conference, A.I.Ch.E, Pages 1-25, March31-April 1, 1992.

16Crude Distillation, Refining Handbook 1990, Hydrocarbon

Processing, Page 88, November 1990.

17Krueger, A. W., J. P. Gourlia and M. Fromager, Progressive

Separation Scheme for Crude Oil Distillation, Presented at 1989National Petroleum Refiners Association Annual Meeting, March 19-

21, San Francisco, California, 1989.

18Devos, A., J. P. Gourlia and H. Paradowski, Process for

Distillation of Petroleum by Progressive Separations, U. S. PatentNo. 4664785, May 12, 1987.

19Process Section, I - Distillation, Refiner and Natural

Gasoline Manufacturer, Volume 11, Number 2, Pages 60-97, February,1932.

20Kutler, A. A., Crude Distillation, Petro/Chem Engineer, Pages

9-11, May, 1969.

21Huang, F. and R. Elshout, Optimizing the Heat Recovery of

Crude Units, Chemical Engineering Progress, Pages 68-74, July,1976.

22Potts, M. F., Crude Oil Fractionation Method, U. S. Patent

Number 3320158, May 16, 1967.

23Wharton, G. W. and E. P. Hardin, Three Stage Unit Improves

Crude Split, Petroleum Refiner, Pages 105-108, October, 1958.

24Kaiser, V. and M. Picciotti, Better Ethylene Separation Unit,

Hydrocarbon Processing, Pages 57-61, November, 1988.

25Kaibel, G., Energy Integration in Thermal Process-

Engineering, International Chemical Engineering, Volume 32, Number4, Pages 631-641, October, 1992.

26Yearout, J. D., R. R. Provin and J. S. Browne, Fractionating

Apparatus, U. S. Patent Number 4574007, March 4, 1986.

27Crawford, D. B., T. M. O'Connor and R. R. Tarakad, Method for

7/30/2019 oilfract

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Partial Condensation of Hydrocarbon Gas Mixtures, U. S. PatentNumber 4726826, February 23, 1988.

28McCue, R. H. Jr., Cryogenic Separation of Gaseous Mixtures,

U. S. Patent Number 5035732, July 30, 1991.

29

Giroux, V. A., Fractionation Method and Apparatus, U. S.Patent Number 4230533, October 28, 1980.

30Kaibel, G., E. Blass and J. Kohler, Thermodynamics -

Guideline for the Development of Distillation Column Arrangements,Gas Separation & Purification, Volume 4, Pages 109-114, June 4,1990.

31World's Reliance on OPEC oil increasing, says IEA,

Hydrocarbon Processing, Page 31, July, 1993.

32Elaahi, A. and W. L. Luyben, Control of an Energy-

Conservative Complex Configuration of Distillation Columns for

Four-Component Separations, Ind. Eng. Chem. Process Des. Dev.,Volume 24, Pages 368-376, 1985.

33Buckley, P. S., W. L. Luyben, and J. P. Shunta, "Design of

Distillation Column Control Systems", Instrument Society ofAmerica, Research Triangle Park, North Carolina, Section 7.6,Pages 176-180, 1985.