Flowsheets

2 FLOWSHEETS plant design consists of words, numbers, and pictures. An engineer thinks in terms of sketches and drawings that are his or her "'pictures." To solve a material balance problem, the engineer will start with a block to represent the equipment and then will show the entering and leaving streams with their amounts and properties. When asked to describe a process, an engineer will begin to sketch equipment, show how it is interconnected, and show the process flows and operating conditions.

Such sketches develop into flow sheets, which are more elaborate diagrammatic representations of the equipment, the sequence of operations, and the expected performance of a proposed plant or the actual performance of an already operating one. For clarity and to meet the needs of the various persons engaged in design, cost estimating, purchasing, fabrication, operation, maintenance, and management, several different kinds of flowsheets are necessary. Four of the main kinds will be described and illustrated.

with a study of the modification of an existing petroleum refinery. The three feed stocks are separated into more than 20 products. Another representative petroleum refinery block diagram, in Figure 13.20 identifies the various streams but not their amounts or conditions.

2.1. BLOCK FLOWSHEETS At an early stage or to provide an overview of a complex process or plant, a drawing is made with rectangular blocks to represent individual processes or groups of operations, together with quantities and other pertinent properties of key streams between the blocks and into and from the process as a whole. Such block flowsheets are made at the beginning of a process design for orientation purposes or later as a summary of the material balance of the process. For example, the coal carbonization process of Figure 2.1 starts with 100,000 lb/hr of coal and process air, involves six main process units, and makes the indicated quantities of ten different products. When it is of particular interest, amounts of utilities also may be shown; in this example the use of steam is indicated at one point. The block diagram of Figure 2.2 was prepared in connection

2.2. PROCESS FLOWSHEETS Process flowsheets embody the material and energy balances and include the sizes of major equipment of the plant. They include all vessels, such as reactors, separators, and drums; special processing equipment; heat exchangers; pumps; and so on. Numerical data include flow quantities, compositions, pressures, and temperatures. Major instrumentation essential for process control and the complete understanding of the flowsheet without reference to other

,..._I Sulfur -'-[ Recovery

,.._~-Phenols "'-[Recovery

[ 1

I

Air

7183

Sulfur

1070

Phenols

25

22.500

1

ibm. y

I=== y

y

Steam

Net Waste Liquids

2380 y

Primary Fractionator

Carbonizer Coal 100.000

I

Net Fuel Gas

Light Aromatics

Oils Recovery

770 y

_

,,_] Pitch "~] Distillation

!

Middle Oils (diesel, etc.)

12575

Tar Acids

3320

Heavy Oils (creosote. etc.)

2380

Pitch

3000

Char

77500

t=,=

y

Figure 2.1. Coal carbonization block flowsheet. Quantities are in lb/hr.

17

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(REPAIR) (& REVAMP) "~ 2O83 SIGNAL ~ NATURAL GASOLINE

NATURAL GASOLINE STABILIZER

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19.8 TBNS/DAY SULFUR

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54 C43 & LIGHTER ,

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4TMOSPHERIC TOPPING

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STABILIZER

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OVERHEAD =

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REFERENCE DRAWINGS

F

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DATE

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Figure 2.2. Block flowsheet of the revamp of a 30,000 Bbl/day refinery with supplementary light stocks. (The C. W. Nofsinger Co.)

THE C.W. NOFSINGER COMPANY K A N S A S CITY. M I S S O U R I

ALTERNATE- D REFINERY BLOCK FLOW DIAGRA~

SSUED FOR COMS"

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DATE DSM

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SCALE

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2.5. DRAWING OF FLOWSHEETS

information is required, particularly in the early stages of a design, since the process flowsheet is drawn first and is the only diagram available that represents the process. As the design develops and a mechanical flowsheet is prepared, instrumentation may be removed to minimize clutter. A checklist of information usually included on a process flowsheet is found in Table 2.1. Working flowsheets are necessarily elaborate and difficult to represent on the page of a book. Figure 2.3 originally was 30 in. wide. In this process, ammonia is made from available hydrogen supplemented by hydrogen from the air oxidation of natural gas in a two-stage reactor F-3 and V-5. A large part of the plant is devoted to the purification of the feed gases--namely, the removal of carbon dioxide and unconverted methane before they enter the converter CV-1. Both commercial and refrigeration grade ammonia are made in this plant. Compositions of 13 key streams are summarized in the tabulation. Characteristics of streams, such as temperature, pressure, enthalpy, volumetric flow rates, and so on, sometimes are conveniently included in the tabulation, as in Figure 2.3. In the interest of clarity, it may be preferable to have a separate sheet if the material balance and related stream information is voluminous. A process flowsheet of the dealkylation of toluene to benzene is in Figure 2.4; the material and enthalpy flows and temperature and pressures are tabulated conveniently, and basic instrumentation is represented. 2.3. PROCESS AND I N S T R U M E N T A T I O N DIAGRAMS (P&ID)

Piping and instrument (P&ID) diagrams emphasize two major characteristics. They do not show operating conditions or compositions or flow quantities, but they do show all major as well as minor equipment more realistically than on the process flowsheet.

TABLE 2.1. Checklist of Data Normally Included on a Process Flowsheet 1. Process lines, but including only those bypasses essential to an understanding of the process 2. All process equipment. Spares are indicated by letter symbols or notes 3. Major instrumentation essential to process control and to understanding of the flowsheet 4. Valves essential to an understanding of the flowsheet 5. Design basis, including stream factor 6. Temperatures, pressures, flow quantities 7. Weight and/or mol balance, showing compositions, amounts, and other properties of the principal streams 8. Utilities requirements summary 9. Data included for particular equipment a. Compressors: SCFM (60~ 14.7 psia); AP psi; HHP; number of stages; details of stages if important b. Drives: type; connected HP; utilities such as kW, Ib steam/hr, or Btu/hr c. Drums and tanks: ID or OD, seam to seam length, important internals d. Exchangers: Sqft, kBtu/hr, temperatures, and flow quantities in and out; shell side and tube side indicated e. Furnaces: kBtu/hr, temperatures in and out, fuel f. Pumps: GPM (60~ AP psi, HHP, type, drive g. Towers: Number and type of plates or height and type of packing identification of all plates at which streams enter or leave; ID or OD; seam to seam length; skirt height h. Other equipment: Sufficient data for identification of duty and size

19

Included are sizes and specification classes of all pipe lines, all valves, and all instruments. In fact, every mechanical aspect of the plant regarding the process equipment and their interconnections is represented except for supporting structures and foundations. The equipment is shown in greater detail than on the process flowsheet, notably with respect to external piping connections, internal details, and resemblance to the actual appearance. Many chemical and petroleum companies are now using Process Industry Practices (PIP) criteria for the development of P&IDs. These criteria include symbols and nomenclature for typical equipment, instrumentation, and piping. They are compatible with industry codes of the American National Standards Institute (ANSI), American Society of Mechanical Engineers (ASME), Instrument Society of America (ISA), and Tubular Exchanger Manufacturers Association (TEMA). The PIP criteria can be applied irrespective of whatever Computer Assisted Design (CAD) system is used to develop P&IDs. Process Industries Practice (1998) may be obtained from the Construction Industry Institute mentioned in the References. Catena et al. (1992) showed how "intelligently" created P&IDs that are prepared on a CAD system can be electronically linked to a relational database that is helpful in meeting OSHA regulations for accurate piping and instrumentation diagrams. Since every detail of a plant design is recorded on electronic media and paper, many other kinds of flowsheets are also required: for example, electrical flow, piping isometrics, and piping tie-ins to existing facilities, instrument lines, plans, and elevations, and individual equipment drawings in detail. Models and three-dimensional representations by computer software are standard practice in design offices. The P&ID flowsheet of the reaction section of a toluene dealkylation unit in Figure 2.5 shows all instrumentation, including indicators and transmitters. The clutter on the diagram is minimized by tabulating the design and operating conditions of the major equipment below the diagram. The P&I diagram of Figure 2.6 represents a gas treating plant that consists of an amine absorber and a regenerator and their immediate auxiliaries. Internals of the towers are shown with exact locations of inlet and outlet connections. The amount of instrumentation for such a comparatively simple process may be surprising. On a completely finished diagram, every line will carry a code designation identifying the size, the kind of fluid handled, the pressure rating, and material specification. Complete information about each line--its length, size, elevation, pressure drop, fittings, etc.--is recorded in a separate line summary. On Figure 2.6, which is of an early stage of construction, only the sizes of the lines are shown. Although instrumentation symbols are fairly well standardized, they are often tabulated on the P&I diagram as in this example. 2.4. UTILITY FLOWSHEETS

There are P&I diagrams for individual utilities such as steam, steam condensate, cooling water, heat transfer media in general, compressed air, fuel, refrigerants, and inert blanketing gases, and how they are piped up to the process equipment. Connections for utility streams are shown on the mechanical flowsheet, and their conditions and flow quantities usually appear on the process flowsheet. 2.5. DRAWING OF FLOWsHEETS

Flowsheets may be drawn by hand at preliminary stages of a project, but with process simulators and CAD software packages, it is a simple matter to develop flowsheets with a consistent set of

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F i g u r e 2.3. P r o c e s s f l o w s h e e t o f a p l a n t m a k i n g 47 t o n s / d a y o f a m m o n i a f r o m a v a i l a b l e h y d r o g e n a n d h y d r o g e n m a d e f r o m n a t u r a l gas. ( T h e

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2.5. DRAWING OF FLOWSHEETS 2 1 TABLE 2.2 Flowsheet Equipment Symbols Heat Transfer

Fluid Handling

HEAT TRANSFER

FLUID HANDLING Centrifugal pump or blower, motor driven

. • Uhbesi d e ~

Shell-and-tube heat exchanger

ellside

rocess

Condenser

Centrifugal pump or blower, turbine driven

._1~~

~

Reboiler Rotary pump or blower

4 F

Reciprocating pump or compressor

Vertical thermosiphon reboiler

Ilside

Centrifugal compressor Kettle reboiler Centrifugal compressor, alternate symbol

Air cooler with finned tubes

Steam ejector

Pr~

Z~

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Fired heater

Fuel

Coil in tank

,G

Fired heater with radiant and convective coils

Rotary dryer or kiln

Evaporator

. Heat Proces'~~~

rce

Tray dryer Cooling tower, forced draft

Spray condenser with steam ejector

r

Water (continued)

22

FLOWSHEETS

TABLE 2.2.--(continued) Vessels

Mass Transfer

VESSELS

MASS TRANSFER

Drum or tank

(

)

14 L

N

Drum or tank

Tray

Packed

column

column

Storage tank

35 I_.

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Jacketed vessel with agitator

spray

column Process

Ext ract

1 Raffinate

Vessel with heat transfer coil

Bin for solids Mixer-settler extraction battery

symbols for equipment, piping, and operating conditions contained in the software packages. There is no generally accepted set of standards, although attempts have been made with little success. Every large engineering office has its own internal standards. Some information appears in the ANSI (American National Standards Institute) and British Standards publications with respect to flowsheets and piping. Useful compilations of symbols appear in books by Austin (1979), Sinnott et al. (1983) and Ulrich (1984). Many

Q

flowsheets that appear in journals such as Chemical Engineering and Hydrocarbon Processing use a fairly consistent set of symbols. As mentioned earlier, PIP (1998) is being used for flowsheets and P&IDs by many companies. Equipment symbols are a compromise between a schematic representation of the equipment and simplicity and ease of drawing. A selection for the more common kinds of equipment appears in Table 2.2. Less common equipment or any with especially

2.5. DRAWINGOF FLOWSHEETS 23 TABLE 2.2.---(continued) Separators

Conveyors and Feeders

SEPARATORS

CONVEYORS&FEEDERS Conveyor

Belt conveyor

Plate-and-frame filter

()

Rotary vacuum filter

~ "vv~l

Screw conveyor

I i . .].

,,,tor

Elevator

Dust collector

!

Feeder

i

Cyclone separator

I

Star feeder Centrifuge

Screw feeder

Weighing feeder

1 !

Mesh entrainment separator

I _L

Tank car

Liquid-liquid

Freight car

tank

settling

L~ ~J

Raked thickener

Heavy

Light

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separator

Drum with water

Conical

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(continued)

intricate configuration often is represented simply by a circle or rectangle. Since a symbol does not usually speak entirely for itself but also may carry a name and a letter-number identification, the flowsheet can be made clear even with the roughest of equipment symbols. The letter-number designation consists of a letter or com-

bination to designate the class of the equipment and a number to distinguish it from others of the same class, as two heat exchangers by E-112 and E-215. Table 2.3 is a typical set of letter designations. Operating conditions such as flow rate, temperature, pressure, enthalpy, heat transfer rate, and also stream numbers are identified

24

FLOWSHEETS

TABLE 2.2.---(continued)

Mixing and Comminution

Drivers

MIXING & COMMINUTION

DRIVERS

Liquid mixing impellers: basic, propeller,turbine, anchor

Motor

|

ii it I

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DC motor Ribbon blender AC motor, 3-phase

Double cone blender

Turbine

I Crusher Turbines:

steam, hydraulic, gas

.~~

Roll crusher

I Pebble or rod mill

i with symbols called flags, of which Table 2.4 is a commonly used set. Particular units are identified on each flowsheet, as in Figure 2.3. Letter designations and symbols for instrumentation have been thoroughly standardized by the Instrument Society of America (ISA). An abbreviated set that may be adequate for the usual flowsketch appears on Figure 2.7. The P&I diagram of Figure 2.6 affords many examples. For clarity as well as aesthetic purposes, equipment should be represented with some indication of relative sizes. True scale is not always feasible because a tower may be 150 feet high while a process drum may be only 3 feet high and this would not be possible to represent on a drawing. An engineer or draftperson will arrange the flowsheet as artistically as possible consistent with clarity, logic, and economy of space on a drawing. A fundamental rule is that there should be no large gaps. Flow is predominantly from the left to the right. On a process flowsheet, distillation towers, furnaces, reactors, and large vertical vessels often are arranged at one level, condenser and accumulator drums on another level, reboilers on

still another level, and pumps more or less on one level but sometimes near the equipment they serve in order to minimize excessive crossing of lines. Streams enter the flowsheet from the left edge and leave at the right edge. Stream numbers are assigned to key process lines. Stream compositions and other desired properties are gathered into a table that may be on a separate sheet if it is especially elaborate. A listing of flags with the units is desirable on the flowsheet. Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually are shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P&I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. Appendix 2.1 provides the reader with material for the construction of flowsheets with the symbols of this chapter and possibly with some reference to Chapter 3.

2.5. DRAWING OF FLOWSHEETS

25

TABLE 2.3. Letter Designations of Equipment Equipment

Letters

Agitator Air filter Bin Blender Blower Centrifuge Classifying equipment Colloid mill Compressor Condenser Conveyor Cooling tower Crusher Crystallizer Cyclone separator (gas) Cyclone separator (liquid) Decanter Disperser Drum Dryer (thermal) Dust collector Elevator Electrostatic separator Engine Evaporator Fan Feeder Filter (liquid) Furnace

Equipment

M FG TT M JB FF S SR JC E C TE SR K FG F FL M D DE FG C FG PM FE jj C p B

Letters

Grinder Heat exchanger Homogenizer Kettle Kiln (rotary) Materials handling equipment Miscellane~ Mixer Motor Oven Packaging machinery Precipitator (dust or mist) Prime mover Pulverizer Pump (liquid) Reboiler Reactor Refrigeration system Rotameter Screen Separator (entrainment) Shaker Spray disk Spray nozzle Tank Thickener Tower Vacuum equipment Weigh scale

SR E M R DD G L M PM B L FG PM SR J E R G RM S FG M SR SR TT F T VE L

aNote: The letter L is used for unclassified equipment when only a few items are of this type; otherwise, individual letter designations are assigned.

TABLE 2.4. Flowsheet Flags of Operating Conditions in Typical Units 13,028

Mass flow rate, Ibs/hr Molal flow rate, Ibmols/hr

(

217 )

Temperature, ~

!

510

Pressure, psig (or indicate if psia or Torr or bar

O

I

psia

Volumetric liquid flow rate, gal/min.

65.3

>

Volumetric liquid flow rate, bbls/day

8,500

<~

Kilo Btu/hr, at heat transfer equipment

)

En,.a,py

/

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:>

9,700

Figure 2.4. Process flowsheet of the manufacture of benzene by dealkylation of toluene. (Wells,

E-107 RECYCLE COOLER 0.19 GCAL/H

TK-101 TOLUENE STORAGE 7.6M I.D. X 8.0M HT.

P-101 A/B TOLUENE FEED PUMPS 18M3/HR 312 M.L.H.

E-101 FEED PREHEATER 5.26 GCAL/H.

H-101 HEATER 4.96 GCAL/H

R-101 REACTOR 2.4M I.D. X 5.5M T/T

C-101 RECYCLE GAS COMPRESSOR 1070 AM3/H AP 3.6 BAR

D-102 RECYCLE GAS KNOCKOUT POT 0.6M I.D. X 1.8M T/T

LEGEND

1980).

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PRESSURE BAR

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TEMPERATUREO C

r~

STREAM NUMBER

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I. STREAM NUMBER HYDROGEN METHANE BENZENE TOLUENE TOTAL MOLAR FLOW TOTAL MASS FLOW TEMPERATURE PRESSURE TOTAL HEAT FLOW

KMOL/H KMOL/H KMOL/H KMOL/H KMOL/H KG H ~ BAR GCALS/H

MATERIALBALANCE 1

2

3

4

0.0 0.0 0.0 108.70 108.70 10.000 15 14 1.33

0.0 0.0 0.40 143.90 144.30 13.270 18 1.1 1.77

470.80 329.40 6.10 0.43 806.73 6727 38 20.4 -3.36

285.00 15.00 0.00 0.00 300.00 810 38 24.0 0.40

5

6

730.80 326.90 5.80 0.41 1063.91 7.132 38 24.0 -257

7

730.80 326.90 6.20 144.31 1208.21 20452 320 23.7 4.46

730.80 326.90 6.20 144.31 1208.21 20452 600 230 942

Figure 2.5. Engineering (P&I) flowsheet of the reaction section of plant for dealkylation of benzene. (Wells,

1980).

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TX-101 TOLUENE STORAGE 7.6 x 8.O HT

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REACTOR

26

ITEM NO

DUTY G CAUH

E-101NB FEED PREHEATER 5.26

SHELLSIDEAP BAR : TUBE SIDE ~P BAR

0.3 0.3

TITLE

8 25.00 17.50 0.30 0.02 42.82 355 58 240 -017

9

10

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E-102 REACTOR EFFLUENT CONDENSER 3-74 G C A L / H

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T-101 BENZENE COLUMN 1.5 MID X 2 0 0 M T/I"

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Figure 2.6. Engineering flowsheet of a gas treating plant. Note the tabulation of instrumentation flags at upper right. (Fluor Engineers, by

way of Rase and Barrow, 1957).

30

FLOWSHEETS

@@ @@ @@ @@ @@ @@ @

REFERENCES Analysis (composition) controller, transmitter

Differential pressure controller, transmitter

Flow rate controller, transmitter

Liquid level controller, transmitter

Pressure, controller, transmitter

Temperature controller, transmitter

General symbol for transmitter

Control valve

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\ Point of detection Figure 2.7. Symbols for control elements to be used on flowsheets. Instrument Society of America (ISA) publication no. $51.5 is devoted to process instrumentation terminology.

D.G. Austin, Chemical Engineering Drawing Symbols, George Godwin, London, 1979. D. Catena, et al., Creating Intelligent P&IDs, Hydrocarbon Processing, 65-68, (November 1992). Graphic Symbols for Piping Systems and Plant, British Standard 1553, Part 1: 1977. Graphic Symbols for Process Flow Diagrams, ASA Y32.11.1961, American Society of Mechanical Engineers, New York. E.E. Ludwig, Applied Process Design for Chemical and Petrochemical Plants, Gulf, Houston, Vol. 1, 1995. Process Industry Practices, Construction Industry Institute, University of Texas, Austin, 1998. H.F. Rase, and M.H. Barrow, Project Engineering of Process Plants, Wiley, New York, 1957. R.K. Sinnott, J.M. Coulson, and J.F. Richardson, Chemical Engineering, Vol. 6, Design, Pergamon, New York, 1983. G.D. Ulrich, A Guide to Chemical Engineering Process Design and Economics, Wiley, NewYork, 1984. G.L. Wells, Safety in Process Design, George Godwin, London, 1980.

Appendix 2.1 Descriptions of Example Process Flowsheets vacuum gas oil (HVGO) is charged to the top plate of zone 2, removed at the bottom tray and charged to furnace no. 2 that operates at 500 psig and 925~ Effluents from both furnaces are combined and enter the soaker; this is a large vertical drum designed to provide additional residence time for conversion under adiabatic conditions. Effluent at 500 psig and 915~ enters the bottom zone of the main fractionator. Bottoms from zone 1 goes to a stripping column (5 psig). Overhead from that tower is condensed, returned partly as reflux and partly to zone 3 after being cooled in the first condenser of the stripping column. This condensing train consists of the preheater for the stream being returned to the main fractionator and an air cooler. The cracked residuum from the bottom of the stripper is cooled to 170~ in a steam generator and an air cooler in series. Live steam is introduced below the bottom tray for stripping. All of the oil from the bottom of zone 3 (at 700~ other than the portion that serves as feed to furnace no. 1, is withdrawn through a cooler (500~ and pumped partly to the top tray of zone 2 and partly as spray quench to zone 1. Some of the bottoms of zone 1 likewise is pumped through a filter and an exchanger and to the same spray nozzle. Part of the liquid from the bottom tray of zone 4 (at 590~ is pumped to a hydrogenation unit beyond the battery limits. Some light material is returned at 400~ from the hydrogenation unit to the middle of zone 4, together with some steam. Overhead from the top of the column (zone 4) goes to a partial condenser at 400~ Part of the condensate is returned to the top tray as reflux; the rest of it is product naphtha and proceeds beyond the battery limits. The uncondensed gas also goes beyond the battery limits. Condensed water is sewered.

The following are examples of process descriptions from which flowsheets may be prepared for practice. Necessary auxiliaries such as drums and pumps are to be included even when they are not mentioned. Essential control instrumentation also is to be provided. Chapter 3 has examples. The processes are as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

visbreaker operation, cracking of gas oil, olefin production from naptha and gas oil, propylene oxide synthesis, manufacture of butadiene sulfone, detergent manufacture, natural gas absorption, tall oil distillation, recovery of isoprene, vacuum distillation.

1. VISBREAKER OPERATION Visbreaking is a mild thermal pyrolysis of heavy petroleum fractions whose object is to reduce fuel production in a refinery and to make some gasoline. The oil of 7.2 API and 700~ is supplied from beyond the battery limits to a surge drum F-1. From there it is pumped with J-1A&B to parallel furnaces B-1A&B from which it comes out at 890~ and 200 psig. Each of the split streams enters at the bottom of its own evaporator T-1A&B that has five trays. Overheads from the evaporators combine and enter at the bottom of a 30-tray fractionator T-2. A portion of the bottoms from the fractionator is fed to the top trays of T-1A&B; the remainder goes through exchanger E-5 and is pumped with J-2A&B back to the furnaces B-1A&B. The bottoms of the evaporators are pumped with J-4A&B through exchangers E-5, E-3A (on crude), and E-3B (on cooling water) before proceeding to storage as the fuel product. A side stream is withdrawn at the tenth tray from the top of T-2 and proceeds to steam stripper T-3 equipped with five trays. Steam is fed below the bottom tray. The combined steam and oil vapors return to T-2 at the eight tray. Stripper bottoms are pumped with J-6 through E-2A (on crude) and E-2B (on cooling water) and to storage as "heavy gasoline." Overhead of the fractionator T-2 is partially condensed in E-1A (on crude) and E-1B (on cooling water). A gas product is withdrawn overhead of the reflux drum which operates at 15 psig. The "light gasoline" is pumped with J-5 to storage and as reflux. Oil feed is 122,480 pph, gas is 3370, light gasoline 5470, heavy gasoline is 9940, and fuel oil is 103,700 pph. Include suitable control equipment for the main fractionator T-2.

3. OLEFIN PRODUCTION A gaseous product rich in ethylene and propylene is made by pyrolysis of crude oil fractions according to the following description. Construct a flowsheet for the process. Use standard symbols for equipment and operating conditions. Space the symbols and proportion them in such a way that the sketch will have a pleasing appearance. Crude oil is pumped from storage through a steam heated exchanger and into an electric desalter. Dilute caustic is injected into the line just before the desalting drum. The aqueous phase collects at the bottom of this vessel and is drained away to the sewer. The oil leaves the desalter at 190~ and goes through heat exchanger E-2 and into a furnace coil. From the furnace, which it leaves at 600~ the oil proceeds to a distillation tower. After serving to preheat the feed in exchanger E-2, the bottoms proceeds to storage; no bottoms pump is necessary because the tower operates with 65 psig at the top. A gas oil is taken off as a sidestream some distance above the feed plate, and naphtha is taken off overhead. Part of the overhead is returned as reflux to the tower, and the remainder proceeds to a cracking furnace. The gas oil also is charged to the same cracking furnace but into a separate coil. Superheated steam at 800~ is injected into both cracking coils at their inlets. Effluents from the naphtha and gas oil cracking coils are at 1300~ and 1200~ respectively. They are combined in the line just before discharge into a quench tower that operates at 5 psig and

2. CRACKING OF GAS OIL A gas oil cracking plant consists of two cracking furnaces, a soaker, a main fractionator, and auxiliary strippers, exchangers, pumps, and drums. The main fractionator (150 psig) consists of four zones, the bottom zone being no. 1. A light vacuum gas oil (LVGO) is charged to the top plate of zone 3, removed from the bottom tray of this zone and pumped to furnace no. 1 that operates at 1000 psig and 1000~ A heavy

31

32

APPENDIX 2.1

235~ at the top. Water is sprayed into the top of this tower. The bottoms is pumped to storage. The overhead is cooled in a water exchanger and proceeds to a separating drum. Condensed water and an aromatic oil separate out there. The water is sewered whereas the oil is sent to another part of the plant for further treating. The uncondensed gas from the separator is compressed to 300 psig in a reciprocating unit of three stages and then cooled to 100~ Condensed water and more aromatic distillate separate out. Then the gas is dried in a system of two desiccant-filled vessels that are used alternately for drying and regeneration. Subsequently the gas is precooled in exchanger E-6 and charged to a low temperature fractionator. This tower has a reboiler and a top refluxing system. At the top the conditions are 280 psig and -75~ Freon refrigerant at - 9 0 ~ is used in the condenser. The bottoms is recycled to the pyrolysis coil. The uncondensed vapor leaving the reflux accumulator constitutes the product of this plant. It is used to precool the feed to the fractionator in E-6 and then leaves this part of the plant for further purification.

4. PROPYLENE OXIDE SYNTHESIS Draw a process flowsheet for the manufacture of propylene oxide according to the following description. Propylene oxide in the amount of 5000 tons/yr will be made by the chlorohydrin process. The basic feed material is a hydrocarbon mixture containing 90% propylene and the balance propane which does not react. This material is diluted with spent gas from the process to provide a net feed to chlorination which contains 40 mol % propylene. Chlorine gas contains 3% each of air and carbon dioxide as contaminants. Chlorination is accomplished in a packed tower in which the hydrocarbon steam is contacted with a saturated aqueous solution of chlorine. The chlorine solution is made in another packed tower. Because of the limited solubility of chlorine, chlorohydrin solution from the chlorinator is recirculated through the solution tower at a rate high enough to supplement the fresh water needed for the process. Solubility of chlorine in the chlorohydrin solution is approximately the same as in fresh water. Concentration of the effluent from the chlorinator is 8 lb organics/100 lb of water. The organics have the composition Propylene chlorohydrin Propylene dichloride Propionaldehyde

75 mol % 19 6

Operating pressure of the chlorinator is 30 psig, and the temperature is 125~ Water and the fresh gas stream are at 80~ Heat of reaction is 2000 Btu/lb chlorine reacted. Percentage conversion of total propylene fed to the chlorinator is 95% (including the recycled material). Overhead from the chlorinator is scrubbed to remove excess chlorine in two vessels in succession which employ water and 5% caustic solution, respectively. The water from the first scrubber is used in the chlorine solution tower. The caustic is recirculated in order to provide adequate wetting of the packing in the caustic scrubber; fresh material is charged in at the same rate as spent material is purged. Following the second scrubber, propylene dichloride is recovered from the gas by chilling it. The spent gas is recycled to the chlorinator in the required amount, and the excess is flared. Chlorohydrin solution is pumped from the chlorinator to the saponifier. It is mixed in the feed line with a 10% lime slurry and preheated by injection of live 25 psig steam to a temperature of

200~ Stripping steam is injected at the bottom of the saponifier, which has six perforated trays without downcomers. Propylene oxide and other organic materials go overhead; the bottoms contain unreacted lime, water, and some other reaction products, all of which can be dumped. Operating pressure is substantially atmospheric. Bubblepoint of the overhead is 60~ Separation of the oxide and the organic byproducts is accomplished by distillation in two towers. Feed from the saponifier contains oxide, aldehyde, dichloride, and water. In the first tower, oxide and aldehyde go overhead together with only small amounts of the other substances; the dichloride and water go to the bottom and also contain small amounts of contaminants. Two phases will form in the lower section of this tower; this is taken off as a partial side stream and separated into a dichloride phase which is sent to storage and a water phase which is sent to the saponifier as recycle near the top of that vessel. The bottoms are a waste product. Tower pressure is 20 psig. Live steam provides heat at the bottom of this column. Overhead from the first fractionator is condensed and charged to the second tower. There substantially pure propylene oxide is taken overhead. The bottoms is dumped. Tower pressure is 15 psig, and the overhead bubblepoint is 100~ Reactions are C12 + H20 --~ C1OH + HCL C3H6 + C12 + H20 ~ C3H6CIOH + HC1 [ 2C3H6C1OH + Ca(OH)2 C3H6 + C12 ~ C3H6C12 I ~ 2C3H60 + CaCI2 + 2H20 C3H6C1OH ~ CzHsCHO + HCI I Show all necessary major equipment, pumps, compressors, refrigerant lines. Show the major instrumentation required to make this process continuous and automatic.

5. MANUFACTURE OF BUTADIENE SULFONE A plant is to manufacture butadiene sulfone at the rate of 1250 lb/hr from liquid sulfur dioxide and butadiene to be recovered from a crude Ca mixture as starting materials. Construct a flowsheet for the process according to the following description. The crude Ca mixture is charged to a 70 tray extractive distillation column T-1 that employs acetonitrile as solvent. Trays are numbered from the bottom. Feed enters on tray 20, solvent enters on tray 60, and reflux is returned to the top tray. Net overhead product goes beyond the battery limits. Butadiene dissolved in acetonitrile leaves at the bottom. This stream is pumped to a 25-tray solvent recovery column T-2 which it enters on tray 20. Butadiene is recovered overhead as liquid and proceeds to the BDS reactor. Acetonitrile is the bottom product which is cooled to 100~ and returned to T-1. Both columns have the usual condensing and reboiling provisions. Butadiene from the recovery plant, liquid sulfur dioxide from storage, and a recycle stream (also liquified) are pumped through a preheater to a high temperature reactor R-1 which is of shell-andtube construction with cooling water on the shell side. Operating conditions are 100~ and 300 psig. The combined feed contains equimolal proportions of the reactants, and 80% conversion is attained in this vessel. The effluent is cooled to 70~ then enters a low temperature reactor R-2 (maintained at 70~ and 50 psig with cooling water) where the conversion becomes 92%. The effluent is flashed at 70~ and atmospheric pressure in D-1. Vapor product is compressed, condensed and recycled to the reactor R-1. The liquid is pumped to a storage tank where 24 hr holdup at 70~ is provided to ensure chemical equilibrium between sulfur dioxide, butadiene, and butadiene sulfone. Cooling water is available at 32~

7. NATURAL GAS ABSORPTION 3 3

6. DETERGENT MANUFACTURE The process of making synthetic detergents consists of several operations that will be described consecutively. ALKYLATION

Toluene and olefinic stock from storage are pumped (at 80~ separately through individual driers and filters into the alkylation reactor. The streams combine just before they enter the reactor. The reactor is batch operated 4 hr/cycle; it is equipped with a single impeller agitator and a feed hopper for solid aluminum chloride which is charged manually from small drums. The alkylation mixture is pumped during the course of the reaction through an external heat exchanger (entering at - 1 0 ~ and leaving at -15~ which is cooled with ammonia refrigerant (at -25~ from an absorption refrigeration system (this may be represented by a block on the FS); the exchanger is of the kettle type. HC1 gas is injected into the recirculating stream just beyond the exit from the heat exchanger; it is supplied from a cylinder mounted in a weigh scale. The aluminum chloride forms an alkylation complex with the toluene. When the reaction is complete, this complex is pumped away from the reactor into a storage tank with a complex transfer pump. To a certain extent, this complex is reused; it is injected with its pump into the reactor recirculation line before the suction to the recirculation pump. There is a steam heater in the complex line, between the reactor and the complex pump. The reaction mixture is pumped away from the reactor with an alkymer transfer pump, through a steam heater and an orifice mixer into the alkymer wash and surge tank. Dilute caustic solution is recirculated from the a.w.s, tank through the orifice mixer. Makeup of caustic is from a dilute caustic storage tank. Spent caustic is intermittently drained off to the sewer. The a.w.s, tank has an internal weir. The caustic solution settles and is removed at the left of the weir; the alkymer overflows the weir and is stored in the right-hand portion of the tank until amount sufficient for charging the still has accumulated. DISTILLATION

Separation of the reactor product is effected in a ten-plate batch distillation column equipped with a water-cooled condenser and a Dowtherm-heated (650~ 53 psig) still. During a portion of the distillation cycle, operation is under vacuum, which is produced by a two-stage steam jet ejector equipped with barometric condensers. The Dowtherm heating system may be represented by a block. Product receiver drums are supplied individually for a slop cut, for toluene, light alkymer, heart alkymer, and a heavy alkymer distillate. Tar is drained from the still at the end of the operation through a water cooler into a bottoms receiver drum which is supplied with a steam coil. From this receiver, the tar is loaded at intervals into 50 gal drums, which are trucked away. In addition to the drums which serve to receive the distillation products during the operation of the column, storage tanks are provided for all except the slop cut which is returned to the still by means of the still feed pump; this pump transfers the mixture from the alkymer wash and surge tank into the still. The recycle toluene is not stored with the fresh toluene but has its own storage tank. The heavy alkymer distillate tank connects to the olefinic stock feed pump and is recycled to the reactor. SULFoNATION

Heart alkymer from storage and 100% sulfuric acid from the sulfuric acid system (which can be represented by a block) are pumped

by the reactor feed pump through the sulfonation reactor. The feed pump is a positive displacement proportioning device with a single driver but with separate heads for the two fluids. The reactor is operated continuously; it has a single shell with three stages which are partially separated from each other with horizontal doughnut shaped plates. Each zone is agitated with its individual impeller; all three impellers are mounted on a single shaft. On leaving the reactor, the sulfonation mixture goes by gravity through a water cooler (leaving at 130~ into a centrifuge. Spent acid from the centrifuge goes to storage (in the sulfuric acid system block); the sulfonic acids go to a small surge drum or can bypass this drum and go directly to a large surge tank which is equipped with an agitator and a steam jacket. From the surge drum, the material is sent by an extraction feed pump through a water cooler, then a "flomix," then another water cooler, then another "flomix" (leaving at 150~ and then through a centrifuge and into the sulfonic acid surge tank. Fresh water is also fed to each of the "flomixers." Wash acid is rejected by the centrifuge and is sent to the sulfuric acid system. The "flomix" is a small vertical vessel which has two compartments and an agitator with a separate impeller for each compartment. NEUTRALIZATION

Neutralization of the sulfonic acid and building up with sodium sulfate and tetrasodium pyrophosphate (TSPP) is accomplished in two batch reactors (5hr cycle) operated alternately. The sodium sulfate is pumped in solution with its transfer pump from the sodium sulfate system (which can be represented by a block). The TSPP is supplied as a solid and is fed by means of a Redler conveyor which discharges into a weigh hopper running on a track above the two reactors. Each reactor is agitated with a propeller and a turbine blade in a single shaft. Sodium hydroxide of 50% and 1% concentrations is used for neutralization. The 50% solution discharges by gravity into the reactor; the 1% solution is injected gradually into the suction side of the reactor slurry circulating pump. As the caustic is added to the reactor, the contents are recirculated through a water-cooled external heat exchanger (exit at 160~ which is common to both reactors. When the reaction is completed in one vessel, the product is fed gradually by means of a slurry transfer pump to two double drum dryers which are steam-heated and are supplied with individual vapor hoods. The dry material is carried away from the dryers on a belt conveyor and is taken to a flaker equipped with an air classifier. The fines are returned to the trough between the dryer drums. From the classifier, the material is taken with another belt conveyor to four storage bins. These storage bins in turn discharge onto a belt feeder which discharges into drums which are weighed automatically on a live portion of a roller conveyor. The roller conveyor takes the drums to storage and shipping. Notes." All water cooled exchangers operate with water in at 75~ and out at 100~ All pumps are centrifugal except the complex transfer, and the sulfonation reactor feed, which are both piston type; the neutralization reactor recirculation pump and the transfer pumps are gear pumps. Show all storage tanks mentioned in the text.

7. NATURAL GAS ABSORPTION A gas mixture has the composition by volume: Component Mol fraction

N2 0.05

CH4 0.65

C2H6 0.20

C3H8 0.10

34

APPENDIX 2.1

It is fed to an absorber where 75% of the propane is recovered. The total amount absorbed is 50 mol/hr. The absorber has four theoretical plates and operates at 135 psig and 100~ All of the absorbed material is recovered in a steam stripper that has a large number of plates and operates at 25 psig and 230~ Water is condensed out of the stripped gas at 100~ After compression to 50 psig, that gas is combined with a recycle stream. The mixture is diluted with an equal volume of steam and charged to a reactor where pyrolysis of the propane occurs at a temperature of 1300~ For present purposes the reaction may be assumed to be simply C3H8 ~ C2H4 + C H 4 with a specific rate k - 0.28/sec. Conversion of propane is 60%. Pressure drop in the reactor is 20 psi. Reactor effluent is cooled to remove the steam, compressed to 285 psig, passed through an activated alumina drying system to remove further amounts of water, and then fed to the first fractionator. In that vessel, 95% of the unconverted propane is recovered as a bottoms product. This stream also contains 3% ethane as an impurity. It is throttled to 50 psig and recycled to the reactor. In two subsequent towers, ethylene is separated from light and heavy impurities. Those separations may be taken as complete. Construct a flow diagram of this plant. Show such auxiliary equipment as drums, heat exchangers, pumps, and compressors. Show operating conditions and flow quantities where calculable with the given data.

8. TALL OIL DISTILLATION Tall oil is a byproduct obtained from the manufacture of paper pulp from pine trees. It is separated by vacuum distillation (50mm Hg) in the presence of steam into four primary products. In the order of decreasing volatility these are unsaponifiables (US), fatty acid (FA), rosin acids (RA), and pitch (P). Heat exchangers and reboilers are heated with Dowtherm condensing vapors. Some coolers operate with water and others generate steam. Live steam is charged to the inlet of every reboiler along with the process material. Trays are numbered from the bottom of each tower. Tall oil is pumped from storage through a preheater onto tray 10 of the pitch stripper T-1. Liquid is withdrawn from tray 7 and pumped through a reboiler where partial vaporization occurs in the presence of steam. The bottom 6 trays are smaller in diameter and serve as stripping trays. Steam is fed below tray 1. Pitch is pumped from the bottom through steam generator and to storage. Overhead vapors are condensed in two units E-1 and E-2. From the accumulator, condensate is pumped partly as reflux to tray 15 and partly through condenser E-1 where it is preheated on its way as feed to the next tower T-2. Steam is not condensed in E-2. It flows from the accumulator to a barometric condenser that is connected to a steam jet ejector. Feed enters T-2 at tray 5. There is a pump-through reboiler. Another pump withdraws material from the bottom and sends it to tower T-3. Liquid is pumped from tray 18 through a cooler and returned in part to the top tray 20 for temperature and reflux control. A portion of this pumparound is withdrawn after cooling as unsaps product. Steam leaves the top of the tower and is condensed in the barometric. Tray 5 ofT-3 is the feed position. This tower has two reboilers. One of them is a pumparound from the bottom, and the other is gravity feed from the bottom tray. Another pump withdraws material from the bottom, and then sends it through a steam generator and to storage as rosin acid product. A slop cut is withdrawn from tray 20 and pumped through a cooler to storage. Fatty acid product is pumped from tray 40 through a cooler to storage. Another stream is pumped around from tray 48 to the top tray 50 through

a cooler. A portion of the cooled pumparound is sent to storage as another unsaps product. A portion of the overhead steam proceeds to the barometric condenser. The rest of it is boosted in pressure with high pressure steam in a jet compressor. The boosted steam is fed to the inlets of the two reboilers associated with T-3 and also directly into the column below the bottom tray. The vapors leaving the primary barometric condenser proceed to a steam ejector that is followed by another barometric. Pressures at the tops of the towers are maintained at 50mm Hg absolute. Pressure drop is 2 mm Hg per tray. Bottom temperatures of the three towers are 450, 500, and 540~ respectively. Tower overhead temperatures are 200~ Pitch and rosin go to storage at 350~ and the other products at 125~ The steam generated in the pitch and rosin coolers is at 20 psig. Process steam is at 150 psig.

9. RECOVERY OF ISOPRENE Draw carefully a flowsheet for the recovery of isoprene from a mixture of C5 hydrocarbons by extractive distillation with aqueous acetonitrile according to the following description. A hydrocarbon stream containing 60mol % isoprene is charged at the rate of 10,000 pph to the main fractionator D-1 at tray 40 from the top. The solvent is acetonitrile with 10 wt % water; it is charged at the rate of 70,000pph on tray 11 of D-1. This column has a total of 70 trays, operates at 10psig and 100~ at the top and about 220~ at the bottom. It has the usual provisions for reboiling and top reflux. The extract is pumped from the bottom of D-1 to a stripper D-2 with 35 trays. The stripped solvent is cooled with water and returned to D-1. An isoprene-acetonitrile azeotrope goes overhead, condenses, and is partly returned as top tray reflux. The net overhead proceeds to an extract wash column D-3 with 20 trays where the solvent is recovered by countercurrent washing with water. The overhead from D-3 is the finished product isoprene. The bottoms is combined with the bottoms from the raffinate wash column D-4 (20 trays) and sent to the solvent recovery column D-5 with 15 trays. Overhead from D-1 is called the raffinate. It is washed countercurrently with water in D-4 for the recovery of the solvent, and then proceeds beyond the battery limits for further conversion to isoprene. Both wash columns operate at substantially atmospheric pressure and 100~ The product streams are delivered to the battery limits at 100 psig. Solvent recovery column D-5 is operated at 50 mm Hg absolute, so as to avoid the formation of an azeotrope overhead. The required overhead condensing temperature of about 55~ is provided with a propane compression refrigeration system; suction condition is 40~ and 80 psig, and discharge condition is 200 psig. Vacuum is maintained on the reflux accumulator with a two-stage steam ejector, with a surface interstage condenser and a direct water spray after-condenser. The stripped bottoms of D-5 is cooled to 100~ and returned to the wash columns. Some water makeup is necessary because of leakages and losses to process streams. The solvent recovered overhead in D-5 is returned to the main column D- 1. Solvent makeup of about 20 pph is needed because of losses in the system. Steam is adequate for all reboiling needs in this plant.

10. V A C U U M DISTILLATION This plant is for the distillation of a heavy petroleum oil. The principal equipment is a vacuum tower with 12 trays. The top tray is numbered 1. Trays 1, 2, 10, 11, and 12 are one-half the diameter of the other trays. The tower operates at 50 mm Hg.

10. VACUUM DISTILLATION 3 5

Oil is charged with pump J-1 through an exchanger E-I, through a fired heater from which it proceeds at 800~ onto tray 10 of the tower. Live steam is fed below the bottom tray. Bottoms product is removed with pump J-3 through a steam generator and a water cooled exchanger E-3 beyond the battery limits. A side stream is taken off tray 6, pumped with J-2 through E-l, and returned onto tray 3 of the tower. Another stream is removed from tray 2 with pump J-4 and cooled in water exchanger

E-2; part of this stream is returned to tray 1, and the rest of it leaves the plant as product gas oil. Uncondensed vapors are removed at the top of the column with a one-stage steam jet ejector equipped with a barometric condenser. Show the principal controls required to make this plant operate automatically.