Flare Gas Recovery

Flare Gas Recovery in Oil and Gas Refineries

O. Zadakbar, A. Vatani and K. Karimpour

INTRODUCTION

Worldwide, final product costs of refinery operations are becoming proportionally more dependent on processing fuel costs, particularly in the current market, where reduced demand results in disruption of the optimum energy network through slack capacity. Therefore, to achieve the most cost-beneficial plant, the recovery of hydrocarbon gases discharged to the flare relief system is vital. Heaters and steam generation fuel provision by flare gas recovery leaves more in fuel processing and thus yield increment. Advantages are also obtained from reduced flaring pollution and extended tip life. During recent years in Iran, all projects have included the collection of associated gases. Thus, flare gas recovery in oil and gas refineries are going to be neglected.

Therefore, in the present work, investigations were made into the operational conditions of 11 important refineries and petrochemical plants. After comprehensive evaluations, we devised practical methods to reduce, recover and reuse flare gases for each petroleum refinery, natural gas refinery and petrochemical plant. The list of refineries and petrochemical plants is shown in Table 1.

1. COMPREHENSIVE PROCESS INVESTIGATION

Adequate process evaluation of plants, especially the units that produce flare gases, comprehensive monitoring of flow and composition of flare gases, investigation of existing flare systems, and finding alternative choices for reusing flare gases were carried out in 11 petroleum refineries, natural gas refineries and petrochemical plants. The results of the investigation of the existing flare systems, finding alternative choices for reusing flare gases and the overall flare gas recovery system are discussed below.

1.1 Investigation into the Existing Flare Systems

Flare tips are exposed to direct flame during their service life, which can of course be quite damaging. As a result, flare tips need to periodically be taken out of service and refurbished, which adds to production costs. The life of a flare tip is related to the amount of usage. Some of the flares were revealed to burn excess gas due to tip damage. By repairing or replacing these tips, purge gas will be reduced.

Natural gases are typically used as purge gases. This use of natural gases for twenty-four hours each day is not only wasteful of a precious natural resource, it is also very expensive and can represent an expenditure of many tens of thousands of dollars per year. Since air is caused to enter the flare system from the atmosphere only when there is a decrease in the temperature of the gas contained in the pressure-tight flare system, there is a need for purge or sweep gases only when there is a decrease in the temperature of the internal gas content of the flare system. For this reason, there is no need for around-the-clock injection of purge gas for the purpose of avoiding entry of air into the flare system. However, to date, there has been no system for automated injection of purge gases into flare systems only as they are needed, due to gas system temperature decrease. Providing a pair of temperature sensors in the flare gas line is an alternative way to decrease purge gases. These two sensors are placed in close proximity. One is a fast-acting sensor, which responds rapidly to any change in temperature. The other is a slow acting sensor, which responds slowly to a change in temperature. Thus, in combination, they provide a sensor system sensitive to change in temperature in the flare gas line.

To save gas burning, it is recommended to repair or replace pilots with more reliable ones. Also, in some refineries, many ignition systems are fitted, but never in use, because they simply do not work when they are needed. As a result, purge gas is increased to enhance the reliability of the flare. Multi-pilot gas conservation systems are recommended for ignition of waste gas from a flare burner, provisions being made to reduce or limit the pilots for ignition at the most effective location as determined by the wind direction and further, if desired by the wind velocity, a reduction in the number of pilots, effecting substantial savings of combustible gas. A pilotless flare ignitor is an alternative choice too. A pilotless flare ignitor is capable of igniting waste gas issuing continually or sporadically from a flare stack and includes an ignitor housing with an open end which extends into the flare stack.

In some areas a large number of flare stacks have been installed. Global studies recommended optimizing the existing number of flares. In some areas the maximum design capacity of the equipment was reached. Hence, surplus gas is being flared. The studies identified equipment that can be debottlenecked; otherwise, additional new units need to be installed.

1.2 Finding Local Alternative Choices for Reducing and Reusing Flare Gases

Some storage tanks are fixed roof types that require positive pressure set at a certain point. The tank is directly connected to a flare. One of the studies recommended a flow suction tank gas recovery system to be installed at each fixed roof storage tank. The vapor jet system is an alternative to conventional vapor recovery technology for the recovery of hydrocarbon vapors from oil production facilities’ storage tanks. The process utilizes a pump to pressurize a stream of produced water to serve as the operating medium for a jet pump.

In particular, the refinery o.gases from a FCCU contain olefin components, up to about 20 percent by volume ethylene and up to about 11 percent by volume propylene, which components normally are not recovered from the o.gases, but which components may have value to warrant recovery and use in other petrochemical processes or uses in downstream processing.

Delayed coking operations increase the volume of byproduct non-condensable hydrocarbons generated and typically flared. A local flare gas recovery system on a delayed cocker unit is capable of recovering a huge amount of flare gases from the delayed cocker. Using some new environmentally friendly technologies reduces flare emissions and the loss of salable liquid petroleum products to the fuel gas system. New waste

heat refrigeration units are useful for using low temperature waste heat to achieve sub-zero refrigeration temperatures with the capability of dual temperature loads in a refinery setting. These systems are applied to the refinery’s fuel gas makeup streams to condense salable liquid hydrocarbon products .

1.3 Flare Gas Recovery System

Environmental and economic considerations have increased the use of gas recovery systems to reclaim gases from vent header systems for other uses. Typically, the gas is recovered from a vent header feeding a flare. Depending on vent gas composition, the recovered gas may be recycled back into the process for its material value or used as fuel gas. Vent gas recovery systems are commonly used in refineries to recover flammable gas for reuse as fuel for process heaters. The Tabriz petroleum refinery and Shahid Hashemi-Nejad (Khangiran) gas refinery are the most important parts of our work. The results of these case studies are discussed.

1.3.1 Flare Gas Recovery for the Tabriz Petroleum Refinery

The Tabriz petroleum refinery consists of 14 refining units and 10 units related to other services. The nominal capacity of the Tabriz refinery is 80000 barrels per day, but by executing the authorities’ augmenting schemes, nominal capacity has been increased to 115000 barrels per day. The crude oil, up to 115000 barrels in a day, is brought from crude oil preserving tanks to a distillation unit in order to be separated into oil cuts. The necessary crude oil is supplied from the Ahwaz oil fields via a 16-inch pipeline. The Tabriz petroleum refinery normally burns off 630 kg/h gas in flare stacks. The average quantity and quality of flare gas are shown in Table 2.

Having investigated the operational conditions of the Tabriz petroleum refinery, especially the units which produced flare gases, we proposed practical methods to reduce, recover and reuse flare gases for the Tabriz petroleum refinery.

There are some alternative choices for using recovered gases. The most important choices are: using flare gases as fuel gas, for electricity generation and as feed gas. In the next step, we tried to find the best choice for using recovered flare gases. Regarding the operational and economic evaluation, recovery of hydrocarbon gases discharged to the flare relief system is probably the most cost-beneficial plant retrofit available to the refinery. Use of flare gases to provide fuel for process heaters and steam generation leaves more in fuel processing, thus increasing yields. Regarding the results of data analyses, the mean value of molecular weight of the gas is 19.9, and the flow discharge rate is modulated between 0 and a maximum of 800 kg/h. The average temperature is 80C and the average pressure is 1 bar.

1.3.2 Flare Gas Recovery for the Shahid Hashemi-Nejad (Khangiran) Gas Refinery

The Shahid Hashemi-Nejad (Khangiran) is one of the most important gas refineries in Iran. The necessary natural gas is supplied from the Mozdouran gas fields. The Shahid Hashemi-Nejad (Khangiran) Gas Company consists of 5 sour gas refineries, 3 dehydration units, 3 sulfur recovery units, 2 distillation units, 2 stabilizer units and 14 additional units related to other services. The Shahid Hashemi-Nejad (Khangiran) gas refinery normally burns off 25000 m3/h gas in flare stacks. The analysis of operational conditions shows that some units normally produce flare gases more than other units. The compositions of flare gases produced by these units are shown in Table 3. These streams make the main flare stream. In addition, the process specifications of flare gases in the Shahid Hashemi-Nejad (Khangiran) gas refinery are shown in Table 4.

After a comprehensive process evaluation, we devised practical methods to reduce, recover and reuse flare gases for the Shahid Hashemi-Nejad (Khangiran) gas refinery. In addition, the flame igniter system, the flame safeguards and the existing flare tip have to be replaced.

The fuel gas of the Shahid Hashemi-Nejad (Khangiran) gas refinery is supplied by sweet gas treated in the gas treating unit (GTU). Due to a pressure drop in the gas distribution network in Mashhad city in the northeast of Iran, during cold seasons, they encourage using flare gases as an alternative fuel gas resource and eliminating the use of sweet gas produced in a GTU. Regarding the Shahid Hashemi-Nejad (Khangiran) gas refinery recommendations and the operational evaluations, recovery of hydrocarbon gases discharged to the flare relief system is probably the most cost-beneficial plant retrofit available to the refinery.

2. FGRS DESIGN

2.1 Flare Gas Design for the Tabriz Petroleum Refinery

The design considerations include: the flare relief operation and liquid seal drum, the flow and composition of flare gases and the refinery fuel system. The considerations led to a unit design for normal capacity up to 630 kg/h. Our proposed flare gas recovery system is a skid-mounted package which is located downstream of the knockout drum, as all flare gases from various units in the refinery are available at this single point. It is located upstream of the liquid seal drum as pressure control at the suction to the compressor will be maintained precisely, by keeping the height of the water column in the drum. The compressor selection and

design depends on the system capacity and turndown capability. The most appropriate type and number of compressors for the application are selected during the design phase of the project. Liquid ring compressor technology is commonly used because of its rugged construction and resistance to liquid slugs and dirty gas fouling. A number of characteristics which must be taken into account when compressing flare gas are as follows:

The amount of gas is not constant, the composition of the gas varies over a wide range, the gas contains components which condense during compression, and the gas contains corrosive components. A modular design which includes two separate and parallel trains capable of handling various gas loads and compositions is recommended for the Tabriz petroleum refinery.

The recommended system consists of compressors which take suction from the flare gas header upstream of the liquid seal drum, compress the gas and cool it for reuse in the refinery fuel gas system. It includes two LR compressors, two horizontal 3-phase separators, two water coolers, piping and instruments. The compressed gas is routed to the amine treatment system for H2S removal. The e.ect of the devised FGRS on flaring in Tabriz petroleum refinery is shown in Figure 1.

The FGR system with LR compressor for the Tabriz petroleum refinery is shown in Figure 2.

2.2 Flare Gas Design for the Shahid Hashemi-Nejad (Khangiran) Gas Refinery

In this case, the considerations led to a unit design for normal capacity up to 25000 m3/h. The process specifications of the outlet must be similar to refinery fuel gas. The proposed flare gas recovery system is like the proposed system for the Tabriz petroleum refinery. It has a modular design and comprises three separate and parallel trains capable of handling various gas loads and compositions.

3. SAFETY AND CONTROL

The principal potential safety risk involved in integrating a flare gas recovery system is from ingression of air into the flare header, which can be induced by the compressor suction. This could result in a flammable gas mixture being flashed o. inside the system from flare pilots. It should be noted that the FGR unit does not interrupt the flare system and should be able to handle sudden increases in load. Therefore, no modification to the existing flare system will be attempted, but with two exceptions. The connections through which the compressors will take suction on the system, and additional seal drums which will provide extra safety against air leakage into the flare system and allow the buildup of flare header pressure, during compressor shutdown or flare gas overload. Also, the compressor control system does not a.ect the flare system pressure and thus its design will be able to avoid low pressure suction in the flare system during normal operation. When the compressors are not functioning properly, automatic or manual shutdown should result. The flare system will operate as it does now with no compressors. Meanwhile, if the volume of flare gases relieved into the flare system exceeds the capacity of the FGR unit, the excess gases will flow to the flare stack.

If this volume is less than the full capacity of the FGR unit, a spillback valve will divert the discharged gases back to the suction zone to keep the capacity of the flare gas recovery unit constant. Other safeguards to the flare system against air leakage are:

• the fail-safe shutdown of the FGR unit compressors on low pressure in the flare system.
• the shutdown of the FGR unit compressors upon high inlet and/or outlet temperatures.
• adequate purge connections in the downstream of the seal drum.
• low flow switches in the purge line to the main flare header downstream of the seal drum, to cut in fuel gas as purge gas.

4. ECONOMICS AND EMISSION CONTROL

In this section, the results of economic evaluations and the results of emission control are presented. These results were obtained based on 0.11 $/m3 for fuel gas, 6 $/ton for steam and 5 cent/kWh for electricity.

4.1 Economic Evaluations for the Tabriz Petroleum Refinery

The recommended system includes two LR compressors, two horizontal 3-phase separators, two water coolers, piping and instruments. Capital investment to install the FGR system is $0.7 million which, includingmaintenance, amortization and taxes, corresponds to a payback period of approximately 20 months. Another essential e.ect of using the FGRS is gas emission reduction. By using the FGRS in the Tabriz petroleum refinery, we can decrease up to 85% of the gas emission including CO2, CO, NOx, SOx, etc.

4.2 Economic Evaluations for the Shahid Hashemi-Nejad (Khangiran) Gas Refinery

The proposed system for the Shahid Hashemi-Nejad (Khangiran) gas refinery has three LR compressors, three horizontal 3-phase separators, three water coolers, piping and instruments. Capital investment to install the FGR system is $1.4 million, which includes maintenance, amortization and taxes, with a payback period of approximately 4 months. We can decrease up to 70% of the gas emission by using the FGRS in the Shahid Hashemi-Nejad (Khangiran) gas refinery.

CONCLUSION

It is well known that there are many economical ways to achieve flaring minimization and gas conservation in oil and gas refineries. In order to find these ways, a comprehensive process evaluation of plants, especially units that produce flare gases, comprehensive monitoring of flow and composition of flare gases, investigation of existing flare systems and finding alternative choices for reusing flare gases was carried out in 11 petroleum refineries, natural gas refineries and petrochemical plants. Based on our comprehensive process evaluation, we devised alternatives to reduce gas flaring.

Recovery of hydrocarbon gases discharged to the flare relief system is probably the most cost-beneficial plant retrofit available to the Shahid Hashemi-Nejad (Khangiran) gas refinery and the Tabriz petroleum refinery. Use of flare gas to provide fuel for process heaters and steam generation leaves more in fuel processing, thus increasing yields.

Advantages are also obtained from reduced flaring pollution and extended tip life. In the Tabriz petroleum refinery, 630 kg/h flare gas will be used as fuel gas by $0.7 million capital investment corresponds to a payback period of approximately 20 months, and also 85% of gas emissions will be decreased.

In the Shahid Hashemi-Nejad (Khangiran) gas recovery, 25000 m3/h flare gas will be used as fuel gas by $1.4 million capital investment corresponds to a payback period of approximately 4 months, and 70% of gas emissions will be decreased.

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Pipe Sizing

Pipe Sizing

Friction Factor

Fluid flowing through pipes experiences resistance due to viscosity, turbulence and roughness of the pipe surface. The Darcy-Weisbach Equation (1) is commonly used for the analysis of steady-state, Newtonian-fluid flow inside pipes. It summarizes the relations between frictional head loss, fluid properties, pipe geometry and discharge.

For laminar flow (Re < 2,100), the friction factor is a function of Reynolds number only.

In turbulent flow (Re > 4,000), f depends upon Reynolds number and pipe roughness.

Hydraulically smooth pipes. In this case, the friction factor is solely a function of Re. For the determination of friction factor, Von Kármán and Prandtl developed Equation (3).

This correlation must be solved by iterative procedures, but simpler correlations given by Colebrook and Blasius are written as Equations (4) and (5), respectively.

Commercial pipe. In this case, f is governed by both Re and relative roughness, expressed as ε / D. The Colebrook-White’s Equation (6) is used to calculate f .
As this equation requires trial-and-error solution, Altshul has developed Equation (7), a computationally simpler choice.

Pressure Drop

To determine pressure drop, discharge and diameter must be known. Hydraulically smooth pipes. Using Equation (1) and the friction factor correlation for smooth pipe, Equation (8) is found.

Commercial pipes. Using Equation (1) and the friction factor correlation for smooth pipe, Equation (9) is found.

Discharge

To determine discharge, pressure drop and diameter must be known. Hydraulically smooth pipes. Equations (1) and (3) allow us to find an expression for the discharge of a smooth pipe.

Commercial pipes. Equations (1) and (6) allow us to find an expression for the discharge of a commercial pipe.

Pipe Diameter

Rearranging Equation (1) to yield an expression for pipe diameter gives Equation (13).

Smooth pipes. Substituting Equation (5) for f yields a correlation for pipe diameter.

Commercial pipes. Determining the diameter of a rough pipe requires the use of Gu, the dynamic roughness.

Manipulating Equation (7) to reflect Gu and substituting into the expression for pipe diameter gives Equation (17), commercial pipe diameter. Several design parameters can be condensed into a constant, named λ.

The range of Gu is: 0 <>6, based on the known ranges of Re and ε/ D for all pipe and flow conditions. Substituting these two extreme values of Gu into Equation (15) gives the following extreme cases, which a pipe diameter must fall between.

Case 1: Extremely smooth pipe. Gu = 0.

Case 2: Extremely rough pipe. Gu = 10 ⁶

Here, we see that even for very rough pipe (ε/ D = 0.01, Re = 10 ⁸), the diameter estimate will be
only about five thirds of that for smooth pipe.

Graphical Sizing Method

To avoid lengthy calculations, a graphical method can be used to approximate pipe diameter. Dividing Equation (17) by Equation (18), we get the diameter multiplier, Ψ.

A graphical method using Ψ can help to quickly estimate the degree of roughness the chosen pipe can withstand.

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Improving Heat Recovery

Compact Heat Exchangers

These units offer distinct advantages over shell-and-tube heat exchangers, as quantified by the example presented here

Johan Gunnarsson Alfa Laval Lund AB, Iain Sinclair and Francisco J. Alanis AspenTech UK Ltd.

Global warming is of major concern today. There is increasing pressure on industry to reduce both energy usage and the associated CO₂ emissions. An important and profitable action that industry can take is to recover more process energy and thus improve the efficient use of that energy. This not only reduces the cost of primary energy supply and lowers CO₂ emissions, but also provides benefits in terms of reductions in heat rejection and in the associated equipment and operating costs. While making such investments, it is also important that financial returns are maximized and that further opportunities for saving energy and reducing emissions are not missed. This article considers the use of compact heat exchangers (CHEs) for improved heat recovery, as they often achieve higher levels of savings with a better payout rate than more conventional alternatives.

Compact heat exchangers

The dominant type of heat exchanger in process plants today is the shell and tube. In many cases, it is an appropriate selection for the service required. However, because engineers are familiar with shell-and-tube varieties, they tend to select them “by default,” without considering alternatives. If engineers’ minds were opened to alternative technologies, such as compact heat exchangers, many heat-exchanger specifications might look different.

There are many different kinds of compact heat exchangers. The most common is the gasketed plate-and-frame heat exchanger. All CHEs use corrugated plates between the heating and cooling media. The design provides the advantages of high turbulence, high heat-transfer coefficients and high fouling resistance. High heat-transfer coefficients allow smaller heat-transfer areas compared to traditional shell-and-tube heat exchangers used for the same duty. This ultimately results in significant size reductions and weight savings as less material is needed to construct the unit. This is especially important when working with expensive corrosion-resistant metals such as titanium and Hastelloys, for example.

The gasketed plate heat exchanger is often the most efficient solution. In petrochemical and petroleum-refinery applications, however, gaskets frequently cannot be used because aggressive media result in a short lifetime for the gaskets or because a potential risk of leakage is unacceptable. In these cases, all-welded compact heat exchangers without inter-plate gaskets should be considered. There are several different kinds available in the market today. In the case presented in this article, a unit with overall fully counter-current flow is used to enable the required heat recovery, while also allowing mechanical cleaning. In addition, all welds are accessible for repair purposes if this type of maintenance becomes necessary during the life of the exchanger.

When to use CHEs

CHEs can be used in most industrial applications as long as design temperature and pressure are within the accepted range, which normally is up to 450°C and 40 barg. CHEs are often the best alternative when the application allows gasketed or fully welded plate heat exchangers, when a high-grade, expensive construction material is required for the heat exchanger, when plot space is a problem or when enhanced energy recovery is important.

When the application allows shell-and-tube heat exchangers to be manufactured completely of carbon steel, such design normally provides the most cost-efficient solution. However, even in those cases, CHEs can have advantages, such as space savings, superior heat recovery and a higher resistance to fouling, which make them well worth considering.

If you do not know if your application can be handled by compact heat exchangers, ask a vendor. Suppliers are normally willing to give you a quick budget quote when their equipment is appropriate for your application so that you can compare solutions and determine which would be best for you. As part of the vendor enquiry, design options for enhanced heat recovery can be quantified and additional energy saving benefits and capital cost changes can be defined. At this stage, in some circumstances, it may be favorable to respecify the heat-exchanger performance requirements to take advantage of the improved heat recovery that can be achieved with a CHE.

CHE versus shell-and-tube

All-welded CHEs consist of plates that are welded together (Figure 1). Among the many models available on the market today, all have one thing in common: they do not have inter-plate gaskets. This feature is what makes them suitable for processes involving aggressive media or high temperatures where gaskets cannot be used. Figure 1. All-welded compact heat exchangers are very compact compared to shell-and-tube heat exchangers.

On the other hand, some of these all-welded heat exchangers are sealed and cannot be opened for inspection and mechanical cleaning. Others can be opened, allowing the entire heat-transfer area and all welds to be reached, cleaned and repaired if necessary.

Because all-welded heat-exchanger plates cannot be pressed in carbon steel, plate packs are available only in stainless steel or higher-grade metals. The cost of an all-welded compact heat exchanger is higher than that of a gasketed plate heat exchanger. Nevertheless, in cases where gaskets cannot be used, all-welded compact plate heat exchangers are still often a strong alternative to shell-and-tube heat exchangers.

The most-efficient, compact, plate-heat-exchanger designs have counter-current flows or an “overall counter-current flow” created by multi-pass arrangements on both the hot and cold sides. Such units can be designed to work with crossing temperatures and with temperature approaches (the difference between the outlet temperature of one stream and the inlet temperature of the other stream) as close as 3°C.

As mentioned before, all-welded CHEs are very compact in comparison to shell-and-tube heat exchangers. CHEs have this advantage due to their higher heat-transfer coefficient and the resulting much smaller heat-transfer area. The units typically occupy only a fraction of the space needed for a shell-and-tube exchanger. Space savings are accompanied by savings on foundations and constructional steel work, and so on. The space needed for maintenance is also much smaller as no tube-bundle access and withdrawal space is required.

Due to the short path through the heat exchanger, the pressure drop can be kept relatively low, although this depends on the number of passes and the phase of the fluid. For most liquid-to-liquid duties, a 70 – 100 kPa pressure drop is normal, while for a two-phase flow, the pressure drop can be as low as 2 – 5 kPa.

Regarding heat recovery, the main advantage of the CHE is that it operates efficiently with crossing temperatures and close temperature approaches. This makes it possible to transfer more heat from one stream to another or to use a heating medium that is just a few degrees warmer than the cold medium.

There are two main reasons why all-welded CHEs are more thermally efficient than shell-and-tube heat exchangers:

• All-welded CHEs have high heat-transfer coefficients. This is due to the high turbulence created in the corrugated plate channels. The high turbulence results in thin laminar films on the surface of the heat-transfer area. These have a much lower resistance to heat transfer compared to the thicker film found in a shell-and-tube heat exchanger
• Counter-current flows (or overall counter-current flows) can be achieved in all-welded compact heat exchangers. This means that a single heat exchanger, operating with crossing temperatures and a close temperature approach can replace several shell-and-tube heat exchangers placed in a serial one-pass arrangement, to emulate the counter-current flow of the compact heat exchanger design

As a result, CHEs may be more cost-effective and may present a more practical alternative to shell-and-tube heat exchangers. In addition to the financial benefits, space savings can also be an important factor for upgrading existing plants as well as for new plant designs.

The advantages of CHEs over shell-and-tube heat exchangers will become clear with the following example taken from an actual application.

A real application example

In a recent feasibility study for improving the energy efficiency of a European ethylene plant, a number of opportunities to increase the export of high-pressure (HP) steam to the site’s utility system were identified. The changes included unloading the refrigerant compressors and increasing heat recovery from the quench water loop.

One such opportunity was the replacement of an existing quench water/polished water shell-and-tube heat exchanger that was limiting heat recovery. From an energy point of view, it was desirable to maximize heat transfer between these streams. This would reduce both the low-pressure (LP) steam required for boiler feed water (BFW) deaeration (due to an increase in deaerator BFW feed temperature) and would also reduce the heat-duty load on the cooling water tower (a site bottleneck), due to a reduction in quench water cooling against cooling water.

The required minimum performance of the replacement heat exchanger is detailed in Table 1.

A preliminary assessment of the suitability of a shell-and-tube heat exchanger indicated that two shells in series (468 m²) would be an economical compromise, achieving a heat recovery of 10 MW with an 11.6˚C temperature approach at the hot end.

At this stage, a compact heat exchanger was compared with the shell-and-tube alternative. An all-welded rather than a gasketed plate heat exchanger was chosen because of limited gasket lifetime when there is contact with quench water. Additionally, because of potential quench-water side fouling, an all-welded heat exchanger that could be mechanically cleaned was preferred.

As mentioned previously, selecting an all-welded CHE instead of a shell-and-tube heat exchanger makes it possible to further increase energy savings, by reducing temperature approach. In this case, the hot-end temperature approach determines the duty and thus the size and design of the heat exchanger. For a compact heat exchanger with counter-current flows it is normally possible (and economical) to decrease the temperature approach to 3 – 5°C. To take advantage of this potential, various improved heat recovery designs were investigated.

A summary of alternative heat-exchanger designs is shown in Table 2. There, it can be seen that the heat-transfer coefficient for the compact heat exchanger is much higher than for the shell-and-tube heat exchanger. This is due to the highly turbulent flow created by the corrugated plates in the CHE. As a result, a much smaller heat-transfer area is required. When comparing the cost of the all-welded CHE and the shell-and-tube heat exchanger, it should be remembered that the plate material in the CHE is stainless steel (ANSI 316L), while carbon steel is used in the shell-and-tube heat exchanger.

It should also be noted that the pressure drop is higher for the compact heat exchanger than for the shell-and-tube heat exchanger. This will, of course, increase the fluid-pumping cost. A true comparison must take these costs into account. However, since the pumping costs are usually small when compared to the overall energy savings achieved, the financial outcome for this example is unlikely to change.

The installation cost of shell-and-tube heat exchangers will be higher, especially for a multi-shell design. In this case, the total installed cost comparison would therefore be significantly more favorable for compact heat exchangers than the purchase cost comparison given above.

For the heat exchangers considered in this example, Table 3 shows how energy and emissions reductions improve as the cold-side outlet temperature is increased to reduce the hot-end temperature approach from 11.6°C to 3.9˚C. To achieve this, 50% more compact heat exchanger surface area is required. This increases the cost of the unit by only 26%; however, on the other hand, two shell-and-tube heat exchangers in series would be required to achieve the same performance, which would require 85% more heat-transfer area, at a 69% higher cost.

All design options offer reasonable monetary savings. Heat exchanger selection is therefore primarily driven by capital cost. A compact heat exchanger design allows improved heat recovery with only a marginally longer payback time, and therefore, is a strong candidate for selection.

Figure 2. For a compact heat exchanger with counter-current flow, as shown in Figure 3, it is normally possible to decrease the temperature approach to 3–5°C.

Figure 3. Counter-current flows can be achieved in all-welded compact heat exchangers. This means that a single heat exchanger, operating with crossing temperatures and close temperature approach, can replace several shell-and-tube heat exchangers placed in a serial, one-pass arrangement.

The all-welded compact heat exchanger in Case 3a provides maximum energy savings and CO₂ credits at a lower size, cost and payback time than the corresponding shell-and-tube heat exchanger in Case 3b. With 17% additional monetary saving, the payback time for the compact heat exchanger is only 8% longer, whilst the payback time for the shell-and – tube heat exchanger design is 44% longer.

The following two points should also be noted:

· The installation cost of the all-welded CHE should be lower than for a shell-and-tube, especially when the shell-and-tube design is a multi-shell arrangement, as in this comparrison

· All-welded CHEs often provide better lifecycle performance and lower maintenance costs than shell-and-tube designs, because there is less fouling. Less fouling means less-frequent cleaning, which in turn reduces downtime (or at least the maintenance work). Compact all-welded heat exchangers are also very easy to clean. Their panels can simply be removed to allow mechanical cleaning with high-pressure water. Shell-and-tube heat exchangers, on the other hand, take longer to clean

Final remarks

There is increasing pressure on industry today to reduce CO₂ emissions. Reducing energy use by improving process heat recovery, is an effective way for companies to respond to this pressure.

Reducing energy use lowers costs for primary energy supply and thus reduces operating costs. Also if primary energy supply is reduced, heat rejection must also reduce. Overall, the capital investment cost for all heat transfer equipment is often lower.

It is our experience that opportunities for improved heat recovery and reduced CO₂ emissions exist in most chemical process industries (CPI) plants, and that some of these opportunities can be realized with short payback times. This allows companies to contribute to CO₂ reduction initiatives and to reap financial benefits.

Effective feasibility studies for reducing energy use should follow a systematic approach and involve equipment vendors, to ensure that all potential opportunities are fully exploited.

Finally, all-welded compact heat exchangers can often improve heat recovery, while achieving greater savings with a better payback rate than more conventional alternatives such as shell-and-tube heat exchangers.

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Indonesia Greenhouse Gas Emissions

Government Prepares Energy Programs to Reduce Greenhouse Gas Emissions

According to studies, fossil fuels account for 287 million tons of greenhouse gasses. The industrial sector is the major source of pollutant, contributing to 29% of the total amount, followed by the transportation sector (27%), power plants (14%), flaring (11%), manufacturing process (10%), commercial and other minor sectors (9%).

The government is planning on an integrated program in the energy sector to reduce greenhouse gas emissions. The program includes increasing fuel diversification by means of oil and gas optimization including the development of biofuel, fuel subsidy reduction, increasing coal utilization through renewable energy development (coal-powered plants, liquefied coal, geothermal power > 6000 MW in 2020), and energy conservation programs which includes energy efficiency and implementation of Demand Side Management (DSM) technology.

From a production point of view, the program is carried out to increase efficiency and reduce leakage during extraction, processing, and transportation of fossil fuels. Further developments of thermal energy technology is also needed, which is supplemented by reducing carbon fuel use (use of non-carbon / low-carbon fuels, increasing efficiency, de-carbonization of fuels and acids), and implementation of CO2 storage and separation for recycling.

Indonesian energy users are encouraged to implement DSM technology as a form of participation to reduce greenhouse gas emissions. On a household scale, DSM may be carried out by reducing fuel inputs and increasing technology efficiency, including such simple steps as changing light bulbs (use of CFL, FTL, and LED), and peak shaving / peak shifting steps.

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Energy Efficiency

MANAGE YOUR ENERGY COSTS

Conducting an energy assessment provides a business plan for improvements

Imagine trying to construct a chemical plant without a piping and instrumentation diagram (P&ID). Just get a bunch of people together at a site and tell them to “make it happen.” Could the plant be built? Well, maybe, but the task would certainly take longer and the result would be a confusing maze of systems, some incomplete and many working at cross purposes. Once the plant is finally done, the operations staff would probably spend all their working hours simply untangling the mess made during construction.

Clearly, the P&ID makes the job easier and leads to a better plant. Similarly, such a blueprint — an energy assessment or audit — is crucial for successfully controlling energy costs. It will underscore the value of systemwide improvements, pursued in stages so that process interruptions are minimized, and of using early savings to pay for later projects.

Trying to manage energy without an assessment will yield results that at best fall short of expectations or at worst interfere with the plant’s core mission.

An energy assessment provides a summary of how much energy is input to a plant and its distribution to various departments and systems. It summarizes fuel, power and water use required by process activities as well as by site heating, ventilation, lighting, sanitation, etc. The assessment summarizes energy inputs over a period of time (usually a year) and expresses consumption relative to production levels and weather conditions.

Plant managers often resist the suggestion of conducting an energy assessment. Perhaps the most common objection is: “We don’t have the money to pay for an assessment.” Let’s say we’re talking about a typical medium-sized plant that spends perhaps $2 million on energy each year. Studies by the U.S. Department of Energy and others suggest that the average facility can cut 10% to 20% of its energy consumption. Having a 10% potential savings equates to admitting that $200,000 is being wasted annually. A very good energy assessment might take a few days and cost about $20,000. So, what we’re really being told is: “We don’t have the money for an energy assessment because we need to pay for the fuel that we’re going to waste.” Too bad, because it only gets worse: energy prices are likely to rise faster than the price of an assessment.

Plus, energy assessments often can be had for free through utilities, state energy offices and university-based industrial assistance programs (see www.oit.doe.gov/iac). The knowledge gained from an assessment will return value in many ways:

• The audit itself probably will reveal anumber of low- or no-cost adjustments that immediately pay for themselves. One good example is shutting off steam mains that serve abandoned process lines.

• Armed with knowledge of its energy consumption, a manufacturer has a lot more leverage to negotiate contract terms with marketers through whom fuel commodities are purchased. Marketers earn fees based on the amount of fuel they broker; an uninformed energy consumer gives the marketer a blank check.

• Energy consumption information provides a baseline for quantifying the actual impacts of energy improvements. Managers can’t claim victory if they don’t know where they started.

• Baseline energy data help decision-makers prioritize improvement opportunities by targeting the prime movers that consume the most fuel.

• Knowing fuel consumption is essential foraccurately determining the operating costs of individual pieces of equipment — and thus for understanding the need for upgrades, replacement or fuel-switching.

• The assessment also provides an inventory of emissions sources. It will present and prioritize opportunities to reduce the risk of noncompliance with emissions regulations.

The energy assessment is a blueprint — a business plan — for improving plant performance through smart energy choices. As the term “business plan” implies, outcomes aren’t accomplished all at once but as a part of a measured process. A business plan will identify resources, milestones and planned outcomes. Perhaps most importantly, an energy assessment describes how a plant manager can make energy decisions that contribute directly to business goals.

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Selecting TEMA Type HE

Selecting TEMA Type Heat Exchangers

TEMA is a set of standards developed by leading heat exchanger manufacturers that defines the heat exchanger style and the machining and assembly tolerances to be employed in the manufacture of a given exchanger. TEMA stands for Tubular Exchanger Manufactures Association. An advantage of TEMA standards is that end customers recognize that the specifications set forth comprise industry standards that directly relate to recognized quality practices for manufacturing. Vendors who build to TEMA standards can be competitively compared because tolerances and construction methods should be very similar for a given design.

The general description of the three major TEMA classes are:

• TEMA C - General Service
• TEMA B - Chemical Service
• TEMA R - Refinery Service

Tema R is the most restrictive and TEMA C is the least stringent. TEMA B and TEMA R are very similar in scope. TEMA R includes the requirement for confined gasket joints where recesses must be machined in the flanges and tubesheets. Spiral wound gaskets with a ring construction also meets this TEMA R requirement. TEMA R also requires a greater minimum thickness for some components.

This paper has the purpose of defining the major TEMA constructions and identify the advantages, limitations and applications suitable for each type.

TEMA designations refer to the front head design, the shell design and the rear head design. For example, a TEMA type BEM has a type B front head, a type E shell and a type M rear head design.

There are special conditions such as with high vapor flows, high pressure and temperature crossing where a combination of special TEMA features is advantageous. For example, K type shells allow for proper liquid disengagement for reboilers and J and H type shells accomodate high vapor flow.

Straight Tube, Fixed Tubesheet, Type BEM, AEM, NEN, Etc.

This TEMA type is the simplest design and is constructed without packed or gasketed joints on the shell side. The tubesheet is welded to the shell and the heads are bolted to the tubesheet. On the NEN heat exchanger, the shell and the head is welded to the tubesheet. Typically, a cover plate design is provided to facilitate tube cleaning. This TEMA category, especially the NEN, it is the lowest cost TEMA design per square foot of heat transfer surface.

Advantages

• Less costly than removable bundle designs
• Provides maximum amount of surface for a given shell and tube diameter
• Provides for single and multiple tube passes to assure proper velocity
• May be interchangeable with other manufacturers of the same TEMA type

Limitations

• Shell side can be cleaned only by chemical methods
• No provision to allow for differential thermal expansion between the outer shell and the tubes. Must use an expansion joint

Applications

• Oil Coolers, Liquid to Liquid, Vapor condensers, reboilers, gas coolers
• Generally, more viscous and warmer fluids flow through the shell
• Corrosive or high fouling fluids should flow inside the tubes

Removable Bundle, Externally Sealed Floating Tubesheet, Type OP, AEW, BEW.

This design allows for the removal, inspection and cleaning of the shell circuit and shell interior. Special floating tubesheet prevents intermixing of fluids. In most cases, straight tube design is more economical than U-tube designs.

Advantages

• Floating tubesheet allows for differential thermal expansion between the shell and the tube bundle.
• Shell circuit can be inspected and steam or mechanically cleaned
• The tube bundle can be repaired or replaced without disturbing shell pipe
• Less costly than TEMA type BEP or BES which has internal floating head
• Maximum surface for a given shell diameter for removable bundle design
• Tubes can be cleaned in AEW models without removing piping.

Limitations

• Fluids in both the shell and tube circuits must be non-volatile, non-toxic
• Tube side passes limited to single or two pass design
• All tubes are attached to two tubesheets. Tubes cannot expand independently so that large temperature differential applications should be avoided
• Packing materials produce limits on design pressure and temperature

Applications

• Intercoolers and aftercoolers, air inside the tubes
• Coolers with water inside the tubes
• Jacket water coolers, oil coolers or other high differential temperature duty
• Place hot side fluid through the shell with entry nearest the front end

Removable Bundle, Outside Packed Head, Type BEP, AEP, Etc

This design allows for the easy removal, inspection and cleaning of the shell circuit and shell interior without removing the floating head cover. Special floating tubesheet prevents intermixing of fluids. In most cases, straight tube removable design is more costly than U-tube designs.

Advantages

• Floating tubesheet allows for differential thermal expansion between the shell and the tube bundle.
• Shell circuit can be inspected and steam cleaned. If the tube bundle has a square tube pitch, tubes can be mechanically cleaned by passing a brush between rows of tubes.
• The tube bundle can be repaired or replaced without disturbing shell piping
• On AEP design, tubes can be serviced without disturbing tubeside piping
• Less costly than TEMA type BES or BET designs
• Only shell fluids are exposed to packing. Toxic or volatile fluids can be cooled in the tubeside circuit
• Provides large bundle entrance area, reducing the need for entrance domes for proper fluid distribution

Limitations

• Shell fluids limited to non volatile, non toxic materials
• Packing limits shell side design temperature and pressure
• All tubes are attached to two tubesheets. Tubes cannot expand independently so that large thermal shock applications should be avoided
• Less surface per given shell and tube diameter than AEW or BEW

Applications

• Flammable or toxic liquids in the tube circuit
• Good for high fouling liquids in the tube circuit

Removable Bundle, Internal Split Ring Floating Head, Type AES, BES, Etc.

Ideal for applications requiring frequent tube bundle removal for inspection and cleaning. Uses straight-tube design suitable for large differential temperatures between the shell and tube fluids. More forgiving to thermal shock than AEW or BEW designs. Suitable for cooling volatile or toxic fluids.

Advantages

• Floating head design allows for differential thermal expansion between the shell and the tube bundle.
• Shell circuit can be inspected and steam cleaned. If it has a square tube layout, tubes can be mechanically cleaned
• Higher surface per given shell and tube diameter than .pull-through. Designs such as AET, BET, etc.
• Provides multi-pass tube circuit arrangement.

Limitations

• Shell cover, split ring and floating head cover must be removed to remove the tube bundle, results in higher maintenance cost than pull-through
• ore costly per square foot of surface than fixed tube sheet or U-tube designs

Applications

• Chemical processing applications for toxic fluids
• Special intercoolers and aftercoolers
• General industrial applications

Removable Bundle, Pull-Through Floating Head, Type AET, BET, etc.

Ideal for applications requiring frequent tube bundle removal for inspection and cleaning as the the floating head is bolted directly to the floating tubesheet. This prevents having to remove the floating head in order to pull the tube bundle.

Advantages

• Floating head design allows for differential thermal expansion between the shell and the tube bundle.
• Shell circuit can be inspected and steam or mechanically cleaned
• Provides large bundle entrance area for proper fluid distribution
• Provides multi-pass tube circuit arrangement.
• Suitable for toxic or volatile fluid cooling

Limitations

• For a given set of conditions, this TEMA style is the most expensive design
• Less surface per given shell and tube diameter than other removable designs

Applications

• Chemical processing applications for toxic fluids
• Hydrocarbon fluid condensers
• General industrial applications requiring frequent cleaning

Removable Bundle, U-Tube, Type BEU, AEU, Etc.

Especially suitable for severe performance requirements with maximum thermal expansion capability. Because each tube can expand and contract independently, this design is suitable for larger thermal shock applications. While the AEM and AEW are the least expensive, U-tube bundles are an economical TEMA design.

Advantages

• U-tube design allows for differential thermal expansion between the shell and the tube bundle as well as for individual tubes.
• Shell circuit can be inspected and steam or mechanically cleaned
• Less costly than floating head or packed floating head designs
• Provides multi-pass tube circuit arrangement.
• Capable of withstanding thermal shock applications.
• Bundle can be removed from one end for cleaning or replacement

Limitations

• Because of u-bend, tubes can be cleaned only by chemical means
• Because of U-tube nesting, individual tubes are difficult to replace
• No single tube pass or true countercurrent flow is possible
• Tube wall thickness at the U-bend is thinner than at straight portion of tubes
• Draining of tube circuit is difficult when mounted with the vertical position with the head side up.

Applications

• Oil, chemical and water heating applications
• Excellent in steam to liquid applications

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