For Industry

presented by The Energy Utilization Department of the

Texas Industrial Commission 410 East Fifth Street Austin, Texas (512) 472-5059 Gerald R. Brown, Executive Director Lance E. dePlante, Manager Energy Utilization Department

The information presented herein is intended to enhance knowledge of industrial energy conservation and to provide the necessary tools to implement an energy conservation program in an industrial plant. References to specific products or ideas should not be considered endorsements of said products or ideas by the Texas Industrial Commission.

This workbook and other projects of the Industrial Energy Utilization Department are funded through a U.S. Department of Energy grant administered by the Governor's Office of Energy Resources.

TEXAS ENERGY CONSERVATION PROGRAM DISTILLATION COLUMN OPERATIONS

Prepared By J. E. SIRRINE COMPANY Houston, Texas For

TEXAS INDUSTRIAL COMMISSION

FUNDED BY GRANT FROM THE GOVERNOR'S OFFICE OF ENERGY RESOURCES THROUGH THE DEPARTMENT OF ENERGY

1978

DISCLAIMER

These materials were prepared as a result of work sponsored by the Governor's Office of Energy Resources through funds provided by the Department Energy. Neither the Texas Industrial Commission, nor the sponsoring agencies, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, expressed or implied, or assumes any legal liability for the successfulness of the implementation of energy conservation techniques described. References to specific ideas, products, and services should not be construed as endorsements. It is hoped that the information provided through these materials will be useful in your efforts to explore opportunities available for energy conservation.

TABLE OF CONTENTS

ENERGY CONSERVATION MANUAL DISTILLATION COLUMN OPERATIONS

PAGE

TITLE

DISCLAIMER

TABLE OF CONTENTS

ABSTRACT

LIST OF TABLES

LIST OF FIGURES

SECTION 1 - INTRODUCTION 1 - 1

SECTION 2 - DESIGN REVIEW, AUDIT OF ENERGY AND MATERIAL BALANCE 2 - 1

A. REVIEW OF PLANT DESIGN 2 - 1

B. AUDIT OF ACTUAL PLANT OPERATION 2 - 1

C. DATA COLLECTION DURING PLANT OPERATION 2 - 3 SECTION 3 - ENERGY SAVING IMPROVEMENTS WITH MINIMAL CAPITAL INVESTMENTS 3 - 1

A. OPERATING PROCEDURE REVISIONS 3 - 1

(1) Reducing the Reflux Ratio of Columns 3 - 1

(2) Lowering Product Specifications 3 - 3

(3) Lowering Pumping Costs 3 - 4

(4) Lowering Steam Usage 3 - 9

(5) Process Heaters 3 -11

B. SCHEDULING SHUTDOWNS TO MAXIMIZE ENERGY RECOVERY

AND PROFITS 3 -13

SECTION 4 - ENERGY SAVING IMPROVEMENTS WITH CAPITAL INVESTMENTS

A. OPTIMIZATION OF HEAT RECOVERY - HEAT EXCHANGERS

B. COLUMN REVISIONS

(1) Additional or More Efficient Trays

(2) Additional Column Draw

C. OPTIMIZATION OF RECOVERY AND USE OF ENERGY

(1) Introduction

(2) Column Heat Utilization

2.1 Bottoms Product

2.2 Distillate Product

2.3 Condenser Duty

2.4 Reboiler Duty

2.5 Feed Preheating

(3) Changing the Column's Temperature

(4) Two-Stage Condensation

(5) Waste Heat Boilers

(6) Multiple Effect Heat Cascading For Distillation Columns

(7) Split Tower

(8) Interreboilers, Intercondensers, and Feed Preheating

(9) Feed Preheating

(10) InterreboiLER

(11) Intercoolers and Feed Precoolers

(12) Circulating Refluxes

D. USE OF VAPOR RECOMPRESSION AND HEAT PUMPS FOR

DISTILLATION 4

(1) Introduction 4

(2) Distillation Column's Reflux and Heat Balance 4

(3) Vapor Recompression 4

(5) Theory Behind Vapor-Recompression and Heat Pumps 4 -

28 28 33 37

38 40

5.1 The Carnot Cycle 4

5.2 The Refrigeration Cycle 4

(6) Vapor Recompression 4

6.1 Situations 4

6.2 Auxiliary Heat Transfer Equipment 4

6.3 Compressor Drives and Their Energy Costs 4

6.4 Insulation of Columns Using Vapor Recompression or Heat Pumps 4 -41

6.5 Vapor Recompression for Interreboilers,

Other Columns 4 -41

(7) Reasons For Conversion of an Existing Column 4 -42

(8) Conversion of an Existing Column 4 -43

(9) Advantages of Vapor Recompression 4 -44

(10) Disadvantages of Vapor Recompression 4 -46

(11) Advantages and Disadvantages of the Heat Pump 4 -49

(12) Guidelines for Considering Vapor Recompression 4 -50

(13) Procedure for Vapor Recompression Evaluation 4 -51

(14) Example, Propane-Propylene Splitter 4 -54

14.1 Situation Statement 4 -54

14.2 Solution 4 -55

(15) Work Problem Propane-Propylene Splitter With

Bottoms Vapor Recompression 4 -60

E. IMPROVING CONTROL OF DISTILLATION COLUMNS 4 -61

F. REDUCING HEAT LOSSES USING INSULATION 4 -64 SECTION 5 - ECONOMICS 5 - 1

A. DEFINITION OF ECONOMIC TERMS 5 - 2

(4) Investment Tax Credit 5 - 5

(8) Discounted Cash Flow 5 - 6

B. CONCEPT OF INVESTMENT EQUIVALENCE TO SAVE ENERGY 5 - 8

C. ECONOMIC INTERPRETATIONS FOR ENERGY SAVINGS 5 - 9

D. STEAM ECONOMICS 5 -11

E. COOLING WATER 5 -13

F. COMPRESSED AIR 5 -14

G. VACUUM PUMPS AND STEAM EJECTORS 5 -14

H. EXCHANGERS USED FOR HEAT RECOVERY 5 -15

I. CONCLUSION 5 -15 SECTION 6 - BIBLIOGRAPHY WITH ABSTRACTS 6 - 1

SECTION 7 - APPENDICES 7 - 1

A. ENERGY SAVINGS CHECLKIST - GENERAL 7 - 1

B. PROCESS ENERGY CHECKLIST 7 - 6

C. REFERENCES - TECHNICAL ARTICLES 7 -10

D. SOLUTION TO WORK PROBLEM 4-F-15 7 -14

ABSTRACT

Distillation operations have been branded as high energy users. An estimate 3% of the total energy used in the United States in 1976 was for distillation. Energy conservation is indicated. This manual is addressed to the small or medium sized chemical or refining company. It is structured to guide these people on how to analyze and reduce energy requirements. The criteria of no reduction or increased profitability of the process are stressed in analyzing any energy saving proposals.

Information for writing the sections came from technical articles, design and operating experience, and seminars on energy conservation.

This manual is divided into seven sections. The contents of the sections are discussed in the following paragraphs.

Before any energy conservation steps can be logically taken, a knowledge of energy usage of the existing facility must be known. Section 2 of this manual describes a procedure for reviewing the original plant design, auditing the energy usage as presently operated, and collecting plant data if required for the audit.

After the distillation process is analyzed for energy usage, the first step is to study energy saving improvements needing minimal capital investments and quickly implementable. Section 3 covers this, giving ideas on changing the operating procedure and scheduling shutdowns to maximize profits and minimize energy usage.

Capital investments to save energy are generally longer term projects. These projects include the optimization of heat recovery and revisions of the column. Capital intensive and complex systems using vapor recompression or heat pumps are possible energy savers. These are covered in Section 4 along with heat losses and column control.

For distillation processes, the energy used per pound of product is a simple ratio for evaluating the performance of the program to reduce energy usage. Similarly, an economic guideline is helpful in requesting management to make decisions concerning capital investments. In Section 5, the concept of investment equivalence to save a unit of energy is developed for use as an economic guideline. The economic interpretations of several energy savings proposals are discussed. Potential conflicts in placing a cost value on various steam pressures by accountants compared to its value from a thermodynamic or energy level viewpoint are discussed.

The appendices include reprints of technical articles pertinent to distillation columns, a general energy savings checklist, a process energy checklist, and the results of a sample work problem on vapor recommendation.

LIST OF TABLES

TABLE

NO. TITLE

2 - 1 Electric Motor Study

4 - 1 Process Data for Column Shown in Figure 4-13

4 - 2 Process Results for Column in Figure 4-14

4 - 3 Process Data for Column in Figure 4-15

4 - 4 Process Data for Splitter in Figure 4-16

4 - 5 Nomenclature of Symbols Used in Section 4

7 - 1 Results for Splitter in Figure 2-1

PAGE

LIST OF FIGURES

FIGURE NO.

NO. TITLE

2 - 1 Feed Fractionator with Preheat

2 - 2 Depropanizer Unit

3 - 1 Centrifugal Pump Characteristics and System Curve

3 - 2 Expansion of Pumping System

4 - 1 Heat Availability and Requirements For Crude Tower 4 - 2 Heat Cascading Distillation Train

4 - 3 Split Tower Arrangement

4 - 4 McCabe - Thiele Diagram for System with Intermediate Condenser and Reboiler

4 - 5 Recirculating Reflux or Pumparound Tower

4 - 6 Example of Conventional Distillation Column, No Side Draw

4 - 7 Vapor Recompression Examples

4 - 8 Example of Heat Pump System

4 - 9 The Refrigeration and Carnot Cycles

4 -10 Column Using Vapor Recompression

4 -11 Hot Columns with Vapor Recompression

4 -12 Refrigerated Columns with Vapor Recompression

4 -13 Propane Propylene Splitter

4 -14 Results of Example of Propane Propylene Splitter

4 -15 Splitter with Bottoms Vapor Compression

4 -16 Splitter of Figure 4 - 15 with Data

PAGE

4 -17 Vapor Pressure of Olefin Hydrocarbons

4 -18 Vapor Pressure of Normal Paraffin Hydrocarbons

4 -19 Enthalpy Temperature Diagram for Propylene

4 -20 Enthalpy Temperature Diagram for Propane

4 -21 Control of Column Reflux to Maximize Profit and

Energy Conservation

5 - 1 Revenue and Expense Variation with Production -

Ideal Case

5 - 2 Variation of Profit with Production

5 - 3 Revenue and Expense Variation With Production -

Real Economic Case 7 - 1 Work Problem

SECTION 1 INTRODUCTION

Many words and phrases may have more than one meaning. In energy discussions, the expression "energy conservation" is presently spoken with two meanings. The original meaning is related to the first law of thermodynamics, which states that energy is always conserved, never destroyed, but changes from one form and level to another. Now that the United States is no longer endowed with new sources of low cost energy fuels, energy conservation has taken on the meaning of reducing the amount of energy used either increasing the efficiency of performing a certain task, or using a substitute requiring less energy. Examples of conservation are the use of higher efficiency air conditioning units, lighter weight automobiles, and handwashing dishes.

In the chemical industry, the meaning of energy conservation includes conserving the temperature level of the energy and in consequent the availability of the energy to produce work. Since distillation processes require large amounts of work and heat energy to perform the required separations, these processes are prime areas for better energy utilization.

Many Americans are skeptical about the United States being in an energy crisis. They say that energy is plentiful, but have they considered the cost to produce it? Russell E. Train, formerly administrator of the EPA, made the following comment in an address upon receiving the $150,000 Tyler Ecology Award:

"...the artificially low prices for more conventional energy maintained by subsidy and regulation. In 1976 the average weighted price of the industrial use of energy per million Btu was $2.55, whereas the average replacement cost---the cost of finding and producing new energy resources---was $3.74. Thus, the replacement cost of natural gas is now more than 70% above the average price, that of oil about 45% above, and that of electricity nearly 40% above. Only in the case of coal did replacement cost approximate actual price. Since our political processes have so far proven unequal to the task of achieving more economically realistic prices for energy, whether by taxes, pricing policy, or by deregulation, or any combination of these, ..."

If his costs are realistic, then the United States is living on previously developed resources. When they are depleted, the cost of energy will soar.

If the decision is made by management to reduce the energy requirements of the processes, it implies that long term profits or return on the company's investment must not decrease. This economic viewpoint is a prerequisite to the writing of any energy conservation manual.

This manual is divided into seven sections, it is assumed that the reader has sufficient technical knowledge to understand the principles of heat transfer, separation operations, and thermodynamics. After information is presented on how to conduct an energy audit of the distillation process, energy saving ideas that require minimal capital investments are given. Similarly, ideas for long term capital investments are discussed. Finally, economics and the concept of investment equivalence to save a unit of energy are detailed.

The appendices include copies of technical articles pertinent to distillation processes. It also lists ideas on energy savings in general and specific to distillation operations. It is the purpose of this manual to aid the chemical company in reducing the energy requirements of the distillation units without a reduction in profitability of the process.

SECTION 2

DESIGN REVIEW AND AUDIT OF ENERGY AND MATERIAL BALANCE

Before proceeding with a detailed energy analysis on your distillation unit as presently operated, you should find out the energy consumption of the same type of separation by the industry. Your sources of information are: (1) similar distillation columns within the company, (2) contact with the original engineering design company, (3) contact with technical people from your professional groups or college or professional friends, and (4) the technical literature. For example, Mix, Dweck, and Weinberg estimated and reported specific consumptions in Btu/lb of product for various product separations in the CEP April, 1978 issue (see Appendix 7-C). They believe that a large percentage of the columns in operation can be retrofitted for energy conservation with attractive economic benefits. 2-A. REVIEW OF PLANT DESIGN

Your plant engineering files should contain all the design information for the process. If it is not available, this information should be requested from the original design company. In particular, process flow sheets, design calculations, piping and instrumentation drawings, specifications of the equipment purchased, performance characteristics of the equipment, utility usage tabulations, and revisions since the original installation are very valuable for the analysis. Examples of process flow sheets are found in Figures 2-1 and 2-2.

Design values for fuel, steam, and electrical usage should be found on the utility summary forms. Calculated values for specific operating conditions should be in the process calculations. Values for fuel and steam usage should be indicated on the process flow sheet. For example, if the design values showed 30,000 lbs per hr. of 75 psig saturated steam to produce 6000 lbs per hr of product, the ratio of the pounds of 75 psig saturated steam to pounds of product is 5. If the condensate is not recovered, the energy usage is (1185 - 48)5 or 5685 Btu per lb. If a competitor operated with the same ratio of steam to product, but recovered the condensate at 200° F, his energy usage is (1185 - 188)5 = 4985 Btu per lb. This is an energy saving of 12%.

Specifications of purchased equipment and their performance are valuable for any plant study. They must be used with caution because revisions may have been made since the original installation. If the changes were not documented (not uncommon in small plants) or simply given verbally to the present unit supervisor, you may not know that revisions occurred. 2-B. AUDIT OF ACTUAL PLANT OPERATION

After the background information is compiled and the energy information extracted, the present energy usage of the unit should be determined. Plant accounting records should be checked for present and past usage of steam, fuel, electricity, etc. This information may be reported on a monthly basis on "value added" sheets or "production cost" sheets. All values reported by accounting should be considered questionable until they can be verified for accuracy. Instruments may be broken. Flow meters may measure usage for more than one unit, and the flow split guesstimated. If the guess was wrong, the estimated values recorded by accounting are in error and could incorrectly bias your decision on a proposed energy conservation project.

Plant inspections should be made of the measuring instruments. An orifice meter may have been calibrated for 100 psig line pressure, but the actual gas pressure found in the plant is 150. The meter's conversion factor and reported usage will be incorrect.

Production rates reported by accounting should be confirmed. Production figures are based upon meter readings and/or product shipments plus storage tank content changes. A level indicator on a storage tank may be based upon a 0.800 gravity liquid, but the actual gravity is 0.750. The production figure is not correct.

A heat and material balance can be made of the existing operation after the plant instrumentation has been corrected. This information will be compared with the original design balance and other energy figures found. 2-C. DATA COLLECTION DURING PLANT OPERATION

When developing a heat and material balance for the existing operation, you may have insufficient information recorded on daily operating and laboratory logs to compile the balance. Since distillation units are generally well instrumented, the only expense burdens for a plant data collection test are the manpower to collect the data and laboratory charges to perform the analyses on the special samples. Of course, if one flow meter measures steam usage to two different units, an additional meter must be added to separate the units.

The degree of success of a plant data collection test is influenced by the preparation and planning stages. Step one is to list the data required for calculating the heat and material balance. Measuring locations are marked on the engineering flow diagram. Step two requires a tour of the unit, confirming and having calibration checks made of critical measuring instruments. Dial thermometers, pressure gauges, and dp cells are examples of these instruments. Table 2-1 is an example of a data collection sheet for electric motors in the unit. When reading pressure drops across an exchanger, it is preferable to use the same pressure gauge to read up stream and downstream pressures. A three way selector valve such as made by D/A Manufacturing Co., Tulia, Texas is a very convenient option for making two readings with the same pressure gauge. A more expensive option is to use a pressure differential transmitter.

The accuracy of flow meters can be checked by the use of a prover, if the necessary piping manifold is in place or installed. Otherwise, the meter design calculations and test results made by the instrument department should be studied and checked. If an orifice meter is in use, you can visually confirm that the upstream side of the orifice plate is inserted in the line correctly and that the orifice size stamped on the plate agrees with specifications. The condition of the orifice opening cannot be checked unless it is removed.

After all instruments are checked, you can take one data set of readings, noting time to make readings, and problems in collecting readings or samples. A heat and material balance can be calculated and inconsistencies noted. For example, in making an energy balance across an exchanger, the heat transferred to the colder stream is found higher than the cold stream. An incorrect temperature reading or flow rate may be the reason. When this "dry run" is completed and changes made, the plant test and evaluation are performed.

A data collection run for the electrical usage is determined by reading amperage loads on each motor and reading the wattmeter for the unit over the test period. Electric motors connected to instrument air and plant air compressors should be included in the energy audit.

TABLE 2-1 MECHANICAL EQUIPMENT STUDY ELECTRIC MOTORS

UNIT STUDIED _ KW=KWH/TEST HRS DATE

PURPOSE OF STUDY __PRODUCT

TIME ELECTRICAL METER READ AT END OF TEST _ METER READING _ PRODUCTION RATE

TIME ELECTRICAL METER READ AT START OF TEST _ METER READING _ TEST NUMBER

LENGTH OF TEST __USAGE _ TEST DATA ALSO FOUND ON

PROCESS FLOWSHEETS #

EQUIPMENT

MOTOR NAMEPLATE

ACTUAL OPERATING DATA

RATIO OF ACTUAL TO NAMEPLATE

DESIGN

COMM.

NUMBER

NAME

HP

FLA

KWH

AMPS*

VOLTS

POWER FACTOR

KW

HP

AMPS

HP

HP

TOTALS

X

X

X

X

X

KW BY METER **

X

X

X

X

X

X

X

X

X

* AVERAGE READING ON 3 LEGS.

** DIFFERENT BETWEEN METERA USAGE AND MOTOR USAGE MUST BE FOR LIGHTING AND HAVC, IF UNREALISTIC, FIND OUT WHY.

* AVERAGE READING ON 3 LEGS.

** DIFFERENT BETWEEN METERA USAGE AND MOTOR USAGE MUST BE FOR LIGHTING AND HAVC, IF UNREALISTIC, FIND OUT WHY.

FIGURE 2-1

FIGURE 2-1

2 - 6

FIGURE 2-2

FIGURE 2-2

SECTION 3

ENERGY SAVINGS IMPROVEMENTS WITH MINIMAL CAPITAL INVESTMENTS

Process units built prior to 1973, the year of the drastic rise in energy costs, were generally designed on a low capital cost investment basis for maximum rates of return. Energy saving equipment was included in the investment if it obviously improved the return on investment. No extensive engineering was directed at energy in the design phase.

In the current period of high energy costs, economics still dictates how much energy a new plant design can conserve. But the incentive to expend more engineering time in the design phase to optimize the process with maximum energy conservation has increased. Likewise, there is the economic incentive to return to older operating plants and retrofit them with additional energy saving equipment.

Similarly, years ago, plant operators had been instructed to minimize off specification production. They achieved this and reduced the amount of scrutiny and effort needed to operate the unit by producing a purer product than necessary. This results in an increase in energy usage. This section of the manual will cover changes in plant operation with minimal capital investments to reduce the energy required to produce one pound of product. 3-A. OPERATING PROCEDURE REVISIONS

Your operating procedures were probably written before the large increase in energy cost drew attention to energy conservation as one primary objective. In addition, the operators are probably using the procedures only as a guide and have developed their own procedures based upon ease of operation.

3-A-1. Reducing the Reflux Ratio of Columns

The optimization of the reflux ratio of the distillation column can produce significant energy savings. The investigation can start by checking the operating manual and column performance specifications for the design conditions, including the reflux ratio. If the design conditions are no longer valid due to changes in feed composition or product requirements, it is recommended that a vigorous distillation calculation be made. If the calculations are very difficult, you can make use of commercial computer programs made available through various computing service bureaus (see section 4-B). The design reflux should be compared with the actual ratios controlled by each shift operator. The daily laboratory analyses of the column products are compiled and compared with the design specifications. If the column is operated at a reduced production rate, the design reflux rate should be calculated for this reduced rate.

It is extremely difficult to change people, even more difficult when it requires more work effort without visually seeing the results. If one operator was found who operated the column at a lower reflux ratio than the others, you might get the confidence of the operators by getting all the operators to maintain this ratio. If you merely write a note in the unit's operating log leaving instructions, you will probably not be successful in lowering the reflux ratio. You must work closely with the superintendent, foremen, and operators instilling confidence as you show the energy savings resulting from their efforts. If the operating department has monthly meetings for the supervisory people, you can use it as a forum to present your objectives, how you plan to approach them, and request their support and assistance. Later you can report progress and discuss problem areas.

Steam or fuel usage per pound of product can be tabulated daily along with reflux ratio, product purity, etc. and compared with column performance before the change. The savings in energy can be converted to a monetary value and reported to the operating people. As an alternate you might represent the energy savings as barrels of imported oil per year.

As the reflux ratio is reduced, a point will be reached at which the operators are overworked and having difficulty in maintaining product purity. This is the opportunity to show your concern to the operators by backing off on the ratio.

3-A-2. Lowering Product Specifications

Sometimes, product specifications can be lowered. Who decided on the present product specifications? Are they justifiable? For example, the sales group may have had the product purity increased to justify selling more product and beating the competitors. The buyer may require a purity in excess of his real needs. Higher purity product requires more energy to be consumed per pound of product. Since the sales department has probably expressed an optimistic opinion as to the value of higher product specifications in the market place, an economic analysis based upon their opinions would most likely say to make no specification changes. A better approach may be to analyze the specification requirements for each type of user of the chemicals and determine if the higher specification is required. A different selling technique may retain the customer even if product specifications are lowered to save energy.

If the product from the column is feed to another unit in the plant, then the effect of lowering the purity on the other unit must be determined. Thus, the energy conservation project requires the additional collection and tabulation of operating data. A statistical approach may be required to fully interpret the results of changes due to the variability of the processes by changes in other parameters. 3-A-3. Lowering Pumping Costs.

When making an inspection of the unit for an energy audit, you should note any operation of two centrifugal pumps in parallel. Within the distillation unit, you can have reflux pumps, product pumps, feed pumps, pumpa-rounds, etc. with spares. Other examples are cooling water pumps in the water cooling tower and cooling pond systems.

If the pumping system was designed for one pump and the operator places the spare pump in service, too, he has not doubled the flow rate. Instead, each pump provides one half of the developed system flow rate and each operates at the identical head. To understand this, let us assume a centrifugal pump characteristic curve as shown in Figure 3-1. At 100 gpm of flow, one pump produces 130 ft of head. If identical pumps are on stream, the flow is 100 + 100 or 200 gpm at 130 ft of head. The characteristic curve for two pumps was developed this way and is also shown in Figure 3-1. The actual flow rate through the piping system is set by the intersection of the pump curve with the system head curve. Referring to Figure 3-1, the flow rate is 160 gpm with one pump operating and 172 gpm with two pumps on stream. In the latter case, each pump is handling one half the flow or 86 gpm.

The efficiency of centrifugal pumps varies with flow rate. Thus, pumps are selected in the design phase to operate at or near their highest efficiency. As seen in Figure 3-1, the pumping efficiency decreased from 46.5% at 160 gpm to 34% at 86 gpm. Assuming an electric motor efficiency of 95%, the energy used in both cases is determined as follows:

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