Energy Analysis

In the previous chapters we prepared a material and energy balances. The focus of this chapter is on an energy analysis of operations and their integration.

TASK: Analyze Energy Requirements

Objective

To analyze auxiliary and utility systems for design parameters.

Subtasks

1. Calculate and check the material and energy flows of the main process, auxiliaries, and utilities in sufficient detail to determine the following:
   • Energy consumed for raw materials and utilities.
   • Energy consumed in waste disposal.
   • Material and energy credits for by-products.
   • Net energy charged to main products.
   • Energy dissipated or wasted.
2. Determine energy performance parameters for each unit operation (yields, efficiencies, coefficients of performance, etc.).
3. Identify all specifications, design parameters, and assumptions that are necessary to analyze the energy requirements for the system under study.
4. Catalog and analyze the energy needs for each step of the process. This can be determined from the previous material and energy balances.
5. Prepare a Sankey diagram of the energy flows for the process.
6. Note the design operating conditions and note what other conditions or ranges of conditions might be considered plausible. Where exiting operating conditions are not available, assume reasonable operating conditions
7. Analyze all process energy and material balances to determine possible improvements in the process.
8. Recommend changes in the process to reflect the changes that would be necessary from the integration of utilities. Some process designs will have dramatic changes such as the deletion/addition of sections or rearrangements of processing units. All process designs will undoubtedly have changes in material and energy balances. Also include any recommendations deemed necessary to correct bad designs or errors.
9. Prepare the energy analysis report.

Comments

• The emphasis of the energy analysis is on determining the performance - not on identifying equipment hardware specifications.
• Where appropriate use recommended factors for preliminary designs.
• All calculations should be prepared as per the project calculation file format guidelines.

Energy Analysis Guide

After the process flow diagram has been developed to the point that a material and energy balance has been calculated, there are analysis that will reveal additional ways to reduce energy consumption but will require modifications of the flow diagram.

The Energy Balance

The basic data needed for an energy analysis is an energy balance of each process section. The study should be done by an engineer who is thoroughly familiar with the process flow diagram. The objective is to define in detail the energy input, energy utilized, and the energy dissipated or wasted. In some areas this will necessitate additional equipment.

The additional costs associated with this improvement in energy efficiency must be weighed against the potential reduction in energy requirements. Having identified the individual energy uses, the engineer can then determine methods for reducing or using these energy wastes. The next step is to evaluate the alternate methods and recommend the best one.

Energy Surplus

After the energy balances have been completed, some coordination between the engineers responsible for the various areas is required. The energy analyst should look at the areas or sections of the proposed process that wastes energy and determine if these energy wastes could be recovered economically. Possibly there is no use for the steam in the area that it is generated, but could be used in some other section of the plant.

Let's say that there is potential for recovering waste heat from furnace flue gases by using it to Suppose further that you determine that it is impractical to preheat combustion air and there is no use for low pressure steam within this section. The process engineer in charge of this section should check with the engineers in charge of designing other sections of the plant if they can perhaps find a use for low pressure steam.

Steam System

Use Lower Pressure Steam

Search for situations where use of high pressure steam can be switched over feasibly to lower pressure steam. It is advantageous to use the lowest pressure when high pressure steam is not required for say providing high temperatures.

This is particularly true when the lower pressure steam is being supplied from extraction or back-pressure turbines or a low pressure boiler separate from the high pressure boiler. Of course, lowering pressure by a pressure reducing valve offers no savings in energy.

Consider Cascading

Possibly the condensate from one heating operation can be used used as the heating media for another process step before returning the condensate to the boiler. Possibly the feed stream to a reactor can be pre-headed by exchanging heat with the reactor effluent stream for exothermic reactions.

Be sure verify that the heat duties match. If there is an excess duty on one side, possibly you will need to add a trim heater/cooler to meet these needs. Make sure that you are not having heat flow from lower to higher temperatures. This is particularly a problem trap when matching vaporization and condensation operations. Look at the costs of these duties to see how important it is to effectively integrate heating/cooling loads by proper cascading.

Increase Condensate Return to Boilers

Loss of condensate is a waste of heat and of valuable high purity water. Identify all sources of condensate and evaluate economic feasibility of installing pumps and insulated piping to return condensate to the boiler feedwater tank.

If the condensate is contaminated, evaluate possible clean_up.

Consider Cogeneration

Consider generating the steam at the highest steam pressure and dropping the pressure through a steam turbine. This will allow the highly efficient generation of electrical power for use in the process.

Electric Motors and Equipment

Electric motors and equipment, such as centrifugal pumps, operate with best efficiency at rated load. If they are operating at reduced load, efficiency suffers.

Compressed Air

Survey all uses of plant air to find the minimum pressure levels required. Lowering compressor discharge pressure saves energy. If all but one or two users can be satisfied with a lower pressure, an evaluation of the feasibility of installing a separate compressor or a booster to supply these higher pressure users should be made.

Insulation

Check potential energy losses for equipment operating at high temperatures. Make a determination if energy losses can be reduced economically by specifying better thermal insulation.

Worksheets

The same worksheets as are presented in the material and energy balance chapter are used here for material and energy balance presentations. Also, several special worksheets have been developed to provide a convenient/systematic way for collecting, summarizing, and presenting the energy analysis. They also provide a checklist for identifying information which is necessary.

Energy Equivalents Guide

A uniform method for calculating the energy requirements for a process. The accounting procedure is based on the KJ of energy usage. This requires a consistent basis for evaluating the energy of all utilities and raw materials consumed.

The equivalent energy is the factor to be used to put all energy requirement estimates on a common basis. For this purpose, an equivalent energy is defined as the KJ of fuel or energy that is consumed in generating a given utility.

Fuels

For fuel energy consumption, the definition is the energy equivalent is defined as the heat of combustion.

Electrical Power

For purchased electrical power from a public utility, the accounting for KJ is more involved. While one KWH of electrical energy is capable of producing 3600 KJ of heat, this is not the number to use.

Rather, the amount of energy required to produce the KWH of electrical energy should be used. Typical delivered public utility generation efficiencies (including line losses) are less than 35 percent. Therefore, if the local utility has an efficiency of say 30 percent, the equivalent energy usage per KWH is nominally 12,000 KJ.

Steam

The fuel used in generating steam is what should be used as the energy equivalent. This means that the energy equivalents are based on the boiler efficiencies.

Condensate

Collecting condensate and returning it to the boilers, reduces the boiler make-up water requirement and save both the heat content difference between the hot condensate and fresh water as well as reduce the pre-treatment energy requirements. This depends on the quality of the returned condensate as well as the temperatures of the condensate and fresh makeup water. 

Other Utilities

For other utilities such as water and compressed air, the equivalent energy requirements are the energy equivalents used for

Energy Equivalents Worksheet

Entries

UTILITIES

Identify all utilities required by the process.

Unit

Units of the quantity of the utility referenced.

KJ/Unit

Cost of one unit of utility used.

Remarks

Note the basis and any special conditions regarding the energy equivalents given such as assumed efficiencies of production.

STEAM

Steam energy equivalents. Enter the pressure and super heat conditions of the steam.

CONDENSATE

Condensate return energy equivalents. Enter the temperature and quality of the condensate.

ELECTRICAL

Electrical energy equivalents for: purchased power and cogenerated power.

FUEL

Fuel energy equivalents for: fuel gas, LPG, oil and coal.

WATER

Water energy equivalents for: process water, boiler feed water, cooling water and potable water.

GASES

Compressed gas energy equivalents for: instrument air and nitrogen.

WASTE DISPOSAL

Waste disposal energy equivalents for: solid waste (define types), process waste water, sanitary waste water, and vent gases.

Instructions

• List all utilities entering or leaving the process and indicate their units and energy equivalents per unit.
• Denote any special conditions for the equivalent energy value given in the far right column.

Sankey Diagram Guide

Instructions are given for the preparation of Sankey diagrams. Sankey diagrams are used for displaying flows through a system. They are especially useful for displaying mass, energy, and cash flows.

Preparation of Diagram

• The first consideration is to determine the basis of the diagram. Typically a basis is selected as the flow rate of the quantity under consideration, such as the fuel input or investor cash flow.
• Then, draw the flow streams. In a Sankey diagram, a stream is represented by two parallel lines whose distance apart is proportional to the flow rate.

Example

To illustrate the concept of a Sankey flow diagram, a simple example is presented.

System

An oil fired heater is used to heat a heat transfer medium. The heat transfer medium is pumped to an evaporator in which steam is generated. Then the heat transfer medium is returned to the fired heater. A block diagram of the example process is shown in Figure.

Basis

The basis of the associated Sankey diagram is 100 units of heat in the fuel per unit of time.

Fired Heater Balance

In addition to the fuel energy input to the heater, are 20 units per unit time of thermal energy of the combustion air. And, the recycled oil has a heat flow rate of 50 units per unit time. Radiant energy losses from the heater are 15% of the fuel energy input and the flue gas effluent heat is 35% of the fuel energy input. Hence, the heater efficiency is 50% based on fuel input.

Evaporator Balance

In addition to the hot transfer medium entering the evaporator, thermal energy of the feed water is 15 units per unit of time. Thermal energy losses from the evaporator via of radiation and blowdown are 15 units per unit of time.

Analysis

As may be noted, 75 units of the heat appears in the steam. This gives an apparent net thermal efficiency of 75% for the overall system. The large recycle stream of 50 units per unit time must be questioned. Should be necessary to recycle so much heat? Or, could some of the this thermal energy be used elsewhere before the heating medium is returned to the heater?

REPORT 15-4: Energy Analysis

The energy analysis report provides insight as to the detail and overall energy requirements are for the system. 15-4.1

Summary

Introduction

Scope

A brief statement of the facilities covered by the energy analysis. This can usually be accomplished in one or two short sentences.

Basis

Clearly define the premises upon which the specification is based as well as any limitations imposed.

Production Rates

The annual rate of production should be stated in units commonly used for the product. This figure should agree with that used in the project scope. The product quality should be clearly defined, e.g., "dry or wet basis," "refined," etc.

Operating Factors

The annual operating hours, ratio of maximum to average flows, etc., should be recorded. These critical items must be determined carefully. The factors which combine to make up these items should be explained in detail in the supporting calculations.

This allows the design engineer to provide adequately for operating requirements without compounding safety factors.

Other Factors

Almost every energy analysis includes factors in addition to the foregoing. For example, a purchased crude material may contain a variable amount of impurity, whose removal requires distillation facilities and related utilities.

If the material and energy balance is based upon the maximum percentage of such impurity, this qualification should be stated.

Engineering Assumptions

Almost every energy analysis includes assumptions to supplement the known facts. Some assumptions are easily obtained from similar unit operations of commercial processes. Other assumptions must be regarded as speculative and subject to engineering judgements.

All assumptions and operating practices should be fully documented. This will allow an assessment of their impact on the overall economics.

Energy Analysis

Utility Overall

Auxiliary and utility overall configuration.

Utility-Resource Requirements

Discussion

Complicated qualifications should be expanded in a process description and the supporting calculations. A discussion of how rigorous or how inflexible the assumption might be and which factors are limiting permits the design engineer to take advantage of research and operating know-how.

Conclusions and Recommendations

Provide a list of conclusions and recommendations.

Acknowledgements

Special mention of assistance from individuals, groups, and departments, who are not preparing the report.

References

The list of references should include all references pertaining to data and information included in the report.

Appendices

Include any accompanying data pertinent to the report that has not been included above.

Supporting Calculations

Calculations are included to clarify the basis of the numbers developed for the material and energy balances. Generally, only those calculations need be included which involve assumptions or which cannot be readily reproduced by an experienced engineer. Thus, routine calculations may be omitted.

Utility Requirements Checklist

The following checklist gives questions typically asked in the review of utility service requirements:

Cooling Tower Water

• Could the heat be exchanged or recovered?
• Could air fin coolers be used?

River Water

• For what purpose is it being used?
• Could clarified or recycled water be used?

Clarified Water

• For what purpose is it being used?
• Could another source be used?

Well Water

• Is this boiler feed makeup?
• Could another source of water be used, including waste well water?
• Is the water to be recycled?

Fuel Gas

• What is the BTU range?
• What is the composition, including sulfur content?

Natural Gas

• For what purpose is it being used?
• Could another gas or source of fuel be used?

High Pressure Steam

• Could low pressure steam be used?
• Any exhausting or venting?
• If steam is for driver use, should an electric motor be used?
• Is condensate recovered?

Low Pressure Steam

• What is the source of the steam?
• Any exhaust or venting?
• Is condensate recovered?

Condensate

• For what purpose is it being used?
• What is its quality?
• What is the source?

Nitrogen

• For what purpose is it being used?
• What is the pressure level?