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4-D-15. Work Problem Propane-Propylene Splitter with Bottoms

Vapor Compression. In our example of the propane-propylene splitter we worked on the case of compression of the overhead bottom. Work the same column using the same assumptions but with bottoms vapor compression. Note that a desuperheater is added in the vapor return. This is used merely for mathematical purposes, in a real column the superheated vapors would be returned directly to the column. The starting point is shown by figure 4-15 and Table 4-3.

To save time, the obvious relationships and enthalpies have been included on Figure 4-16 and Table 4-4. These values are the simple result of stream equalities, and the enthalpies which are read from the chart.

The problem will be to find the work required for this case, and compare overall results with that of the overhead recompression example. The solution is found in Appendix 7-D.

Data needed k = 1.22, Z = 0.96 Figures 4-17, 4-18, 4-19 and 4-20.

### 4-E. IMPROVING CONTROL OF DISTILLATION COLUMNS

A column may be equipped with simple conventional instruments or equipped with instruments that are computer controlled. The additional cost of sophisticated computers control systems must be economically justified to management for installation on new columns or retrofitted on existing columns. Savings may be from lower energy costs, higher production rates, lower capital requirements for intermediate storage between columns, less off specification product, etc.

Conventional instruments cannot control at optimum conditions because they cannot take corrective action until the variable being controlled has moved from its setpoint. Also, the optimum operating conditions depend upon the feed rate and feed composition. Since the control points must be changed to new optimum conditions, the operator needs assistance in deciding these changes. A computer can calculate the new setpoints and adjust the controllers automatically. Management is generally reluctant to make major expenditures to retrofit process units with these process control systems, based upon economic guesstimates by engineers on the pay back.

An alternate to this reluctance is to use the Distributed Process Control Systems (DPCS). The distributed control concept means that a single failure of a control cannot affect more than a limited area of the process. Thus, it is possible to install a DPCS by installing the system in steps, each step being justified economically.

The benefits of microprocessor control of columns are discussed in a recent article by M.R. Skrokov (Appendix 7-C). In addition to design engineering savings in manpower, the operating costs of the plants are reduced by the better control. However, the availability of control systems using microprocessors is claimed to be currently limited and appears aimed at total plant operation. Thus, microprocessing equipment for controlling single column may not be commercially available. Mr. Skrokov recommends that the instrument manufacturers supplement their large multiprocessing systems with small dedicated microprocessors. This observation could be the reason why small companies will be unable to use the economic benefits of microprocessors until small systems are available.

Let us assume a distillation column with distillates and bottom products. Component A has a value of \$.30 per pound in the distillate, but no value in bottom product. Similarly, component B has a value of \$.20 per pound in the bottoms, but no value in the distillate. How do you operate the column assuming a restraint of minimum purity? Dr. Latour, in his paper presented at the ISA Conference in Houston, May 23, 1978, developed this problem. Figure 4-21 shows a plot of the value of the products versus column reflux ratio. The maximum profit occurs at a reflux flow rate where the net recovered value peaks on 24. The lowest energy cost is not at this point, but at the specification restraint point a reflux flow of 13. When considering both profit and energy usage, the column should be controlled at a reflux flow rate between the maximum profit point and the specification restraint. Exceeding the reflux flow at the maximum profit point wastes energy.

If the value of the products from the column is fixed, the only restraint being the minimum specification, then the maximum profit and minimum energy usage are both at the reflux flow rate where the specification restraint is located.

At the ISA meeting in Houston, in May 1978, Mr. D.E. Lupfer presented a paper on manipulating the distillation column pressure to increase production and save energy. Operation of columns at the lowest pressure without flooding the column or overloading the condenser, had been practiced by Mr. Lupfer on hundreds of columns. Shinskey (Appendix 7-C), Skrokov (Appendix 7-C), and Fauth and Shinskey (Appendix 7-C) have also discussed the benefits of operating the columns pressure as low as feasible. In the Fauth article, the floating pressure control was part of an advanced control system for a typical gas plant depropanizer. Of the total cost reduction of \$1269 per day by using the control system, \$345 was attributed to energy savings by the floating pressure control systems.

The floating control systems operation is discussed in the Shinskey and Fauth article. Although the cost of the instrumentation for floating the pressure is low, column temperatures can no longer be used for control because they will vary with pressure. Thus, other control devices such as analyzers, or pressure compensated temperature measurements are required. Shinskey made the following comment in his article on "Control Systems Can Save Energy":

"At first, operators are skeptical of floating-pressure control -they feel more comfortable with constant pressures and temperatures. When its contribution to energy savings is pointed out, they are generally willing to try it. After a brief trial period, they learn that it does not interfere with quality control, and even increases production capacity; soon it becomes accepted. Yet at each installation and with each new application, the concept of floating specifications needs to be sold again."

Benefits as high as a 30% reduction in energy usage are reported by Shinskey, so the floating pressure control system deserves serious consideration.

### 4-F. REDUCING HEAT LOSSES USING INSULATION

The desired amount of insulation on a distillation column depends on the individual situation and varies at parts of the column. For example, suppose we have a distillation column with a top temperature 180°F, bottoms 230°F, and are using cooling water for the condenser and 40 psig steam for the reboiler. Then the insulation required on the reboiler and bottom section should be based on the value of 40 psig steam, as a Btu lost will have to be replaced by more steam. On the other hand, insulation on the condenser will save no energy, and in fact cost money as the Btu's saved by insulation must be removed by cooling water.

A different situation could occur for a column that operates at 400°F bottom, 300°F top, using 600 psig steam in the reboiler, producing 25 psig steam in the condenser. For this case, insulating the column bottoms and reboiler will save valuable 600 psig steam so much is needed. Insulation on the upper section of the column is also valuable as saved heat generates useful 25 psig steam in the condenser. So this entire column needs insulation.

In the case of a column operating in the refrigerated condition, insulation must be used on the condenser and top portion of the column to prevent heat flowing into the column, which would then have to be removed by expensive refrigeration. If the reboiler and bottom sections of the column are also cold insulation will also be required as these sections are part of the coolant cycle.

Insulation may be required for reasons other than energy savings. Insulation on the column will prevent the column from being affected by swings in the weather, changing the heat transfer rate at the tower surface. For cold columns, prevention of ice condensation may be desired. There are OSHA limits on the maximum permissible bare metal temperature for personnel protection. Also, if located indoors in a small specialty operation, insulation could improve the general workplace conditions.

Stream Number

Vapor, lb moles 0 460 0 0 460 60 0

Liquid, lb moles 100 0 500 400 0 0 40

Temperature, deg F

Pressure, psig 43 40 45 40 45 40 45

Enthalpy, Propylene, Btu/lb

Enthalpy, Propane, Btu/lb

Enthalpy, Propylene, Btu/lb mole

Enthalpy, Propane, Btu/lb mole

Enthalpy, Average, Btu/lb mole o

Stream Number

1

2

3

4

5

6

7

8

Vapor, lb moles

0

460

0

0

460

60

0

0

Liquid, lb moles

100

0

500

400

0

0

40

434.63

Temperature, deg F

0

-9

2.8

-9

2.8

-9

2.8

22.8

Pressure, psig

43

40

45

40

45

40

45

72.4

Propylene, Mole Fraction

0.8

1

0.5

1

0.5

1

0.5

1

Propane, Mole Fraction

0.2

0

0.5

0

0.5

0

0.5

0

Enthalpy, Propylene, Btu/lb

97

268.2

97.1

93.1

270.5

268.8

97.1

107.1

Enthalpy, Propane, Btu/lb

101

0

102.2

0

27 6.1

0

102.2

0

Enthalpy, Propylene, Btu/lb mole

4082

11,311

4086

3918

11,383

11,311

4086

4507

Enthalpy, Propane, Btu/lb mole

4454

0

4507

0

12,132

0

4507

0

Enthalpy, Average, Btu/lb mole

4156

11,311

4296

3918

11,757

11,311

4296

4507

Stream Number

9

10

11

12

13

14

15

Vapor, lb moles

0

34.63

433.76

434.63

0

434.63

26.24

Liquid, lb moles

433.76

0

0

0

26.24

0

0

Temperature, deg F

2.8

-9

2.8

-9

2.8

51

2.8

Pressure, psig

45

40

45

40

45

72.4

45

Propylene, Mole Fraction

0.5

1

0.5

1

0.5

1

0.5

Propane, Mole Fraction

0.5

0

0.5

0

0.5

0

0.5

Enthalpy, Propylene, Btu/lb

97.1

268.8

270.6

258.8

97.1

284

270.5

Enthalpy, Propane, Btu/lb

102.2

0

275.1

0

102.2

0

270.5

Enthalpy, Propylene, Btu/lb mole

4086

11,311

11,383

11,311

4086

11,953

11,757

Enthalpy, Propane, Btu/lb mole

4507

0

12,132

0

4507

0

0

Enthalpy, Average, Btu/lb mole

4296

11,311

11,757

11,311

4295

11,953

11,757

Vapor, lb moles 0 460 0 0 460 60 0

Liquid, lb moles 100 0 500 400 0 0 40

Pressure, psig 43 40 45 40 45 40 45

Enthalpy, Propylene, Btu/lb

Enthalpy, Propane, Btu/lb

Enthalpy, Propylene, Btu/lb mole

Enthalpy, Propane, Btu/lb mole

Enthalpy, Average, Btu/lb mole o

 Stream Number 1 2 3 4 5 6 7 8 Vapor, lb moles 0 460 0 0 460 60 0 400 Liquid, lb moles 100 0 500 400 0 0 40 0 Temperature, deg F 0 -9 2.8 -9 2.8 -9 2.8 -9 Pressure, psig 43 40 45 40 45 40 45 40 Propylene, Mole Fraction 0.8 1 0.5 1 0.5 1 0.5 1 Propane, Mole Fraction 0.2 0 0.5 0 0.5 0 0.5 0 Enthalpy, Propylene, Btu/lb 97 268.8 97.1 93.1 270.5 268.8 97.1 268.8 Enthalpy, Propane, Btu/lb 101 0 102.2 0 275.1 0 102.2 0 Enthalpy, Propylene, Btu/lb mole 4082 11,311 4086 3918 11,383 11,311 4086 11,311 Enthalpy, Propane, Btu/lb mole 4454 0 4507 0 12,132 0 4507 0 Enthalpy, Average, Btu/lb mole 4156 11,311 4295 3918 11,757 11,311 4296 11,311 Stream Number 9 10 11 12 13 14 Vapor, lb moles 0 0 0 Liquid, lb moles 0 0 0 Temperature, deg F 2.8 2.8 2.8 2.8 Pressure, psig 45 45 45 45 45 Propylene, Mole Fraction 0.5 0.5 0.5 0.5 0.5 0.5 Propane, Mole Fraction 0.5 0.5 0.5 0.5 0.5 0.5 Enthalpy, Propylene, Btu/lb 97.1 97.1 97.1 270.5 Enthalpy, Propane, Btu/lb 102.2 102.2 102.2 275.1 Enthalpy, Propylene, Btu/lb mole 4086 4086 4086 11,383 Enthalpy, Propane, Btu/lb mole 4507 4507 4507 12,132 Enthalpy, Average, Btu/lb mole 4296 4296 4296 11,757

NOMENCLATURE OF SYMBOLS USED IN SECTION 4 NOMENCLATURE FOR SECTION 4-B

N - Number of trays in the column.

P - Column pressure in ATM.

R - Reflux rate

Rm - Minimum reflux rate

S - Separation factor as given by

- Concentration, mole fraction

- Murphee plate efficiency

- Relative volatility (light key to heavy key)

Subscripts

D - Distillate

B - Bottoms Sub-subscripts

LK - Light key

HK - Heavy key NOMENCLATURE FOR SECTION 4-C

B - Bottoms product, flowrae

BV - Bottoms product flowate, taken as a vapor cp - Heat capacity at constant pressure cv - Heat capacity at constant volume

D - Distillate, overhead product, flowrate.

DL - Distillate product flowrate, taken as a liquid 6 - Efficiency, Carnot cycle.

6m , 6p - Efficiencies of real process as compared to the Carnot cycle,

6p causes inefficiencies to result as heat in the process, 6 causes no heat addition.

h - Enthalpy, Btu per pound or pound mole.

PBL - Pressure of the bottoms liquid . PC - Pressure at which the cold side boils. Ph - Pressure at which the hot side condenses. POV - Pressure of the overhead vapor, q - Heat flow, between two substances.

qc - Heat flow out or to a cold, constant temperature source or the total condensing duty for the distillation column. qc' - Heat flow from the condensing side of the condenser-reboiler in a vapor recompression system. qh - The heat flow to or from a hot source, or the heat duty of the reboiler in a distillation column. qh' - The heat flow from the boiling side of the condenser-reboiler in a vapor recompression system. R - Reflux ratio of the column, reflux liquid ratio to distillate product.

PV - The reflux vapor from the column bottom to provide R reflux at the top. Tbl - Bubble point temperature of the bottoms liquid. Tc - Temperature of the cold source. Th - Temperature of the hot source.

TOV - Bubble point temperature of the overload vapor. V - Volume of gas, ft3.

W - Work, work energy input into process.

WIDEAL - Work needed by a Carnot cycle to move a heat flow qc to the hot sink.

Z - Compressibility factor.

DHvap - Heat of vaporization

ATexÄTex - Temperature difference driving force across the condenser-

reboiler.

HEAT AVAILABILITY AND REQUIREMENTS FOR CRUDE TOWER

Temperature, F 700

0 100 200 300 400 500 600 700 Enthalpy X Mass Rate, M Btu/hr

-Residuum -Reflux

-Crude

FIGURE 4-2 HEAT CASCADING DISTILLATION TRAIN

FIGURE 4-2 HEAT CASCADING DISTILLATION TRAIN

FIGURE 4-3 SPLIT TOWER ARRANGEMENT

FIGURE 4-3 SPLIT TOWER ARRANGEMENT

McCABE-THIELE DIAGRAM FOR SYSTEM WITH INTERMEDIATE CONDENSER AND REBOILER

0.0 Light Component in Liquid, mole fraction 1.0

FIGURE 4-6 EXAMPLE OF CONVENTIONAL DISTILLATION COLUMN, NO SIDE DRAW
FIGURE 4-7 VAPOR RECOMPRESSION
FIGURE 4-8 EXAMPLE OF HEAT PUMP SYSTEM

Te Th

Temperature

Te Th

Temperature

Te Th

Temperature

Te Th

Temperature

FIGURE 4-10 COLUMN USING VAPOR RECOMPRESSION

FIGURE 4-10 COLUMN USING VAPOR RECOMPRESSION

FIGURE 4-11 HOT COLUMNS WITH VAPOR RECOMPRESSION CASE IA, EXCESS CONDENSER DUTY, HEAT IS EXPENSIVE, COOLING IS CHEAP

CASE IB EXCESS REBOILER DUTY, HEAT IS EXPENSIVE, COOLING IS CHEAP

REFRIGERATED COLUMNS WITH VAPOR RECOMPRESSION CASE IIA EXCESS CONDENSER DUTY, COOLING IS EXPENSIVE, HEAT IS CHEAP

CASE IIB, EXCESS REDOILER DUTY, COOLING IS EXPENSIVE, HEAT IS CHEAP

di sti llate product Liquid

I Main

Compressor

I Main

Compressor

bottoms product Liquid

FIGURE 4-13 PROPANE PROPYLENE SPLITTER

4 - 85
4 - 86
4 - 87

FIGURE 4 - 17 VAPOR PRESSURE OF OLEFIN HYDROCARBONS

4 - 88

1000 800

400 300

100 80

AO 30

 A C/ D / E / F/ B / G/ /w/ /l/ ' 1 / // / / / // ' I / //

A Ethane B Propane C n -butane D n-pentane E n - hexane F n-heptane G n -octane H n-nonane I n - decane J n-undecane K n-dodcane

100 150 200 250 300 400 500

Temperature, deg F

FIGURE 4-19 ENTHALPY TEMPERATURE DIAGRAM FOR PROPYLENE