Saturday, December 1, 2012

Flare Gas Recovery Ejector

Problem
To prevent gas emissions to atmosphere, a simple flare gas recovery system was required which could safely and economically compress low pressure waste gas back into the production process.

Solution
Transvac Flare Gas Recovery Ejectors are ideally  suited to this application because they can employe high-pressure gas energy; from an existing source, to entrain and compress waste to a pressure where the gas can be used in the production process.

For this application the Transvac Ejector was designed to use high pressure motive gas from the 2nd stage Compressor discharge to entrain low pressure flare gas and to discharge into the inlet of the 1st stage Compressor.

The Gas Ejector was designed to handle the maximum flare gas flow conditions, but in order to maintain the desired pressure conditions on the low pressure side a simple control system is used to recycle make-up gas from the discharge of the Ejector. 



Benefits
      Emission of greenhouse gases to atmosphere eliminated
      Potential reduction in tax liability
      Waste gas is recovered and added to production
      No running costs because existing energy used to motivate the Ejector
      Ejector has no moving parts
      No maintenance (very attractive for remote installations)
      Simple to install as part of existing pipework system
      Low cost option
      Safe, reliable operation
      Easy to control using standard techniques

Design
Transvac Flare Gas Recovery Systems are designed to meet site specific requirements and to recognised codes including ASME B31.3 etc. with full non-destructive testing.

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Tuesday, October 2, 2012

Fluid Flow : Velocity Head

Two of the most useful and basic equations are
where
Dh = Head loss in feet of flowing fluid
u = Velocity in ft/sec
g = 32.2 ft/sec2
P = Pressure in lb/ft2
V = Specific volume in ft3/lb
Z = Elevation in feet
E = Head loss due to friction in feet of flowing fluid

In Equation 1 Dh is called the “velocity head.” This expression has a wide range of utility not appreciated by many. It is used “as is” for
1.     Sizing the holes in a sparger
2.     Calculating leakage through a small hole
3.     Sizing a restriction orifice
4.     Calculating the flow with a pitot tube

With a coefficient it is used for
1.     Orifice calculations
2.     Relating fitting losses, etc.

For a sparger consisting of a large pipe having small holes drilled along its length Equation 1 applies directly. This is because the hole diameter and the length of fluid travel passing through the hole are similar dimensions.

An orifice on the other hand needs a coefficient in Equation 1 because hole diameter is a much larger dimension than length of travel (say 1/8 in. for many orifices). Orifices will be discussed under “Metering” in this chapter.

For compressible fluids one must be careful that when sonic or “choking” velocity is reached, further decreases in downstream pressure do not produce additional flow. This occurs at an upstream to downstream absolute pressure ratio of about  2 : 1. Critical flow due to sonic velocity has practically no application to liquids. The speed of sound in liquids is very high. See “Sonic Velocity’‘ later in this chapter.

Still more mileage can be gotten out of Ah = u2/2g when using it with Equation 2, which is the famous
Bernoulli equation. The terms are
1.     The PV change
2.     The kinetic energy change or “velocity head”
3.     The elevation change
4.     The friction loss

These contribute to the flowing head loss in a pipe. However, there are many situations where by chance, or on purpose, u2/2g head is converted to PV or vice versa. We purposely change u2/2g to PV gradually in the following situations:
1.     Entering phase separator drums to cut down turbu lence and promote separation
2.     Entering vacuum condensers to cut down pressuredrop

We build up PV and convert it in a controlled manner to u2/2g in a form of tank blender.

Here are various recommended flows, velocities, and pressure drops for various piping services.






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Wednesday, September 26, 2012

Failure Mode and Effects Analysis

Description of the Method
A FMEA is used to examine each potential failure mode of a process to determine the effects of the failure on the system. A failure mode is the symptom, condition, or fashion in which hardware fails. It may be identified as a loss of function, a premature function (function without demand), an out-of-tolerance condition, or a physical characteristic, such as a leak, observed during inspection. The effect of a failure mode is determined by the system's response to the failure.

Analysis Procedure
A FMEA has three steps: (1) defining the process, (2) performing the analysis, and (3) documenting the results. Defining the process for study and documenting the results can be performed by a single person. The analysis itself must be performed by a team.

DEFINING THE PROCESS. This step identifies the specific vessels, equipment, and instrumentation to be included in the FMEA and the conditions under which they are analyzed. Defining the problem involves establishing an appropriate level of resolution for the study and defining the boundary conditions for the analysis.

The required level of resolution determines the extent of detail needed in a FMEA. The choices for the level of resolution range from the subcomponent level to the system level. To satisfy PSM Rule requirements, most FMEAs should be performed at the major component level. This level provides the best trade-off between the time necessary to perform the analysis and the usefulness of the information gained from it.

Defining the analysis boundary conditions requires the following.
1.     Identifying the system or process to be analyzed.
2.     Establishing the physical boundaries of the system or process.
3.     Establishing the analytical boundaries of the system or process.
4.     Documenting the internal and interface functions.
5.     Documenting the expected performance of the system, process, or equipment item; the system or process restraints; and the failure definitions of the equipment items, the process, or the system.
6.     Collecting up-to-date information identifying the process equipment and its functional relationship to the system.

Functional narratives about the system or process should include descriptions of the expected behavior of the system or process and the equipment components for each operational mode. Narratives should describe the operational profiles of the components and the functions and outputs of each.

To assist in the review, block diagrams should be constructed which illustrate the operation, interrelationships, and interdependencies of functional components for each equipment item. All interfaces should be indicated in these block diagrams.

PERFORMING THE ANALYSIS. The FMEA should be performed in a deliberate, systematic manner to reduce the possibility of omissions and to enhance completeness. All failure modes for one component should be addressed before proceeding to the next component. A tabular format is recommended for recording results. A FMEA worksheet is produced by beginning at a system boundary on a reference drawing and systematically evaluating the components in the order in which they appear in the process flow path. A worksheet such as that shown in Fig.1 should be completed for each equipment item, as follows.

Failure Mode. The PrHA team should list all of the equipment item and interface failure modes. Given the equipment's normal operating condition, the team should consider all conceivable malfunctions.

Cause(s). If desired, the root causes of the failure mode should be identified. Identification of root causes provides information helpful for ranking hazards.

Operational Mode. If the equipment being analyzed is subject to different modes of operation, each operational mode should be identified and analyzed separately.

Effects. For each identified failure mode, the PrHA team should describe the anticipated effects of the failure on the overall system or process. The key to performing a consistent FMEA is to assure that all equipment failures are analyzed using a common basis. Typically, analysts evaluate effects on a worst-case basis, assuming that existing safety levels do not work. However, more optimistic assumptions may be satisfactory as long as all equipment failure modes are analyzed on the same basis.
Failure Detection Method. The means of failure detection should be identified, such as visual or warning devices, automatic sensing devices, sensing instrumentation, or other indicators. The main purpose of identifying failure detection methods is to determine whether the failure mode is "hidden," i.e., not detectable for some period of time. If there is no means to detect failure, "none" should be entered into the worksheet.

Compensating Provisions. For each identified failure mode, the PrHA team should describe any design provisions, safety or relief devices, or operator actions that can reduce the likelihood of a specific failure or mitigate the consequences.

Severity Class. The severity of the worst consequence should be specified as follows.
Category I -  Catastrophic - May cause death or loss of system or process.
Category II - Critical - May cause severe injury, major property damage, or major system damage.
Category III - Marginal - May cause minor injury, minor property damage, or minor system damage.
Category IV - Minor - Is not serious enough to cause injury, property damage, or system damage, but may result in unscheduled maintenance or repair.

Remarks/Actions. For each identified failure mode, the PrHA team should suggest actions for reducing its likelihood or mitigating its effects. The actions suggested for a particular piece of equipment may focus on the causes or effects of specific failure modes or may apply to all of the failure modes collectively.

If the team discovers that a single item failure is not detectable, the FMEA should be extended to determine if the effects of a second failure in combination with the first could have catastrophic consequences. When a safety, redundant, or back-up component is evaluated, the analysis should consider the conditions that generated the need for the component.

DOCUMENTING THE RESULTS. A FMEA generates a qualitative, systematic reference list of equipment, failure modes, and effects. The results of a FMEA are usually listed in tabular format, by equipment item. Fig.1 shows a typical worksheet used in performing a FMEA.
Fig.1 Example FMEA Worksheet

For each equipment item, the failure modes for that item and, if desired, the root causes for that failure mode are identified. For each failure mode, a worst-case estimate of the consequences is identified. This worst-case estimate assumes the failure of all protection against both the failure itself and the undesired consequences of the failure. The method by which the failure is detected is specified along with any compensating provisions. Finally, any suggestions for improving safety are listed in the table.

The PSM Rule requires that a FMEA be performed by a team, all of whose members participate in the analysis. The most practical means of performing the FMEA is to prepare blank worksheets on viewgraphs or on a large display screen. For each equipment item, the PrHA team reaches a consensus on its failure modes and their causes, effects, detection methods, compensating provisions, severity (if desired), and any remarks or action items.

Staff requirements for a FMEA vary with the size and complexity of equipment items being analyzed. The time and cost of a FMEA is proportional to the size of the process and number of components analyzed. On average, an hour is sufficient to analyze two to four equipment items. For processes or systems in which similar equipment items perform similar functions, the time requirements for completing a FMEA are reduced. Fig.2 presents estimates of the time needed to perform a PrHA using the FMEA method (CCPS, 1992).
Fig.2 Time Estimates for Using the Failure Mode and Effects Analysis Method

Limitations of Failure Mode and Effects Analysis
Human operator errors are not usually examined in a FMEA, but the effects of human error are indicated by an equipment failure mode. FMEAs rarely investigate damage or injury that could arise if the system or process operated successfully. Because FMEAs focus on single event failures, they are not efficient for identifying an exhaustive list of combinations of equipment failures that lead to accidents.

Example Failure Mode and Effects Analyses

Fig.3 Partial FMEA for Dock 8 HF Supply System

Fig.4 Partial FMEA for the Cooling Water Chlorination System


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Tuesday, September 25, 2012

Energy Recovery with Compact Heat Exchangers

Compact heat exchangers are used for heat recovery applications where high efficiency is vital, space or weight constraints apply, or exotic materials are required.
Marcos Matsufugi
Alfa Laval

The refinery business is under constant pressure to increase efficiency. A highly competitive market combined with rising energy and feedstock costs require refineries to ramp up production while cutting operating costs.

Switching from shell-and-tube to welded plate heat exchangers (also known as compact heat  exchangers) is a proven and straight forward way of solving the problem. The use of compact heat exchangers offers benefits in four areas:
  •        Energy savings
  •       Less maintenance
  •        Increased production
  •        Lower installation costs.


Energy savings
Up to 50% of a refinery’s operating budget is tied up in energy costs, making energy efficiency a top
priority. Energetics Incorporated estimates that the petroleum refining industry in the US could cut energy use by as much as 54% by incorporating best practices and new technology.

Recovering and reusing energy is a profitable and easy way to cut energy costs. All refineries do this to some extent, but most still use outdated shell-and-tube heat exchangers with low thermal efficiency.

Investing in more efficient heat exchangers is profitable for energy-intensive plants such as refineries. Payback periods are often less than six months.


Cut fuel costs
Heat recovery efficiency can be increased by up to 50% by simply switching from shell-and-tube to
welded plate heat exchangers. More energy is then put back to use, energy that would otherwise have gone to waste. Atmospheric and vacuum distillation units are typical units with a high energy consumption and they represent an enormous potential for better heat integration.

Preheating of crude oil is the process that requires the largest amount of energy and where most gains can be made by using compact heat exchangers for heat recovery. There are plenty of other units in a refinery, such as hydrotreating, reforming and FCC, where switching to compact heat exchangers can be very profitable.

Reduced fuel consumption also leads to lower emissions of CO2, NOx and SOx. If the plant operates under a cap-and-trade system this will cut operating costs even further.

Efficiency up to five times higher
The heat exchanger is a key component in heat recovery. The choice of heat exchanger is important and has a direct impact on a company’s bottom line. Figure 1 shows the heat recovery level as a function of initial cost in a compact heat exchanger and a shell-and-tube. The yield from the compact heat exchanger is up to 25% higher than for the shell-and-tube at a comparable cost. Shell-and-tube solutions with the same level of heat recovery are often several times more expensive than a compact heat exchanger.



Turbulence and counter-current flow
The superior thermal efficiency of a compact heat exchanger is a result of its highly turbulent flow (see Figure 2). The corrugated heat exchanger plates cause much higher turbulence in the fluid than in a shell-and-tube at the same flow velocity.


The formula below describes the overall heat transfer coefficient. High turbulence increases the film heat transfer coefficients ( a 1 and  a 2). Thin plates (small  d ) also have a  positive effect on heat transfer. The result is an overall heat transfer coefficient (k) that is three to five times higher than for a shell-andtube heat exchanger:

k = Overall heat transfer coefficient, W/m2°C
= Film heat transfer coefficient, W/m2°C
= Wall thickness, m
= Wall conductivity, W/m°C

Another important feature of compact heat exchangers is the capability to operate with a countercurrent flow; hot fluid enters the heat exchanger at the end where the cold fluid exits. This makes it possible to handle crossing-temperature programmes in a single heat exchanger (that is, to heat the cold fluid to a temperature that is higher than the outlet temperature of the hot fluid). This is especially important in heat recovery, since the maximum amount of energy is recovered when the cold fluid is heated to a temperature very close to that of the hot fluid.

The high efficiency means compact heat exchangers can exploit temperature differences as low as 3°C. This makes it possible to recover heat from sources that have previously been deemed worthless.

Case study: feed/effluent exchanger
A refinery in the US replaced two shell-and-tubes with a single compact heat exchanger as a feed/effluent exchanger in an isomerisation plant. The result was a 43% increase in heat recovery, from 5.8 MW to 8.3 MW. As an added bonus, the new solution also allowed the refinery to eliminate a downstream air cooler (see Table 1).


Case study: overhead condensers
A refinery in Italy replaced old air coolers on the atmospheric distillation column with two compact heat exchangers. The heat that was previously cooled off into the air is now recovered and used for preheating crude oil. The result is additional heat recovery of 11.5 MW (39.3 MMBtu/h) and an annual saving in fuel of €2.5 million (see Table 2).





Profitable energy recovery
Energy-saving investments often have short payback periods, even at much lower energy price levels than today’s. In the future, energy efficiency will most likely be a prerequisite for staying in business.

In its World Energy Outlook 2008 report, the International Energy Agency (IEA) predicts world energy demand to increase by 45% over the next 20 years. It also predicts that the supply of fossil fuels will not be able to meet this demand, even when taking new, undiscovered fields into account.

More and more governments around the world will probably start charging industries for emitting CO2, with emission credits becoming more and more expensive. The result of all this will undoubtedly be increasing energy prices; just how much is hard to predict. In 2007, the IEA predicted oil prices to stay at $50–55 per barrel until 2030. A year later, in June 2008, they peaked at $147 dollars.

There are many ways to fight the energy challenge. Consulting firm McKinsey made a thorough investigation of future energy needs and supply, comparing the benefits of different alternatives. It came to the following conclusion: “McKinsey has looked long and hard to obtain an affordable, secure energy supply while controlling climate change. Energy efficiency stands out as the single most attractive and affordable component of the necessary shift in energy consumption.”

Obviously, the first step towards lower energy costs is to start using less energy. Increasing efficiency is the least costly and most easily implemented solution to the energy challenge for most refineries.

Less maintenance
One of the key features of a compact heat exchanger is the highly turbulent flow. Apart from improving heat transfer, it also makes heat exchangers less susceptible to fouling problems. The high turbulence means fouling deposits are not deposited on the heat transfer areas. This results in longer service intervals, more operating time and more recovered heat than with a shell-and-tube design. Less fouling also leads to lower cleaning costs.

This self-cleaning effect is especially large in spiral heat exchangers (SHE). These are compact heat exchangers with a single channel design. This design causes fouling deposits to be flushed away wherever they start to build up. Spiral heat exchangers are the correct choice for heavy-fouling duties and can handle solids, slurries and fibres. Typical duties for spiral heat exchangers in refineries are cooling fluid catalytic cracking (FCC) bottom products or visbroken residues.

The smaller heat transfer area compared to a shell-and-tube heat exchanger means cleaning will be both quicker and require fewer cleaning chemicals. The small heat transfer area also leads to a smaller hold-up volume, which means compact heat exchangers respond faster to process changes. The equipment can therefore be shut down and restarted more quickly when serviced.

Energy savings
Fouling leads to higher energy consumption. Heat transfer efficiency drops as fouling builds up, meaning the boiler or burner has to provide more heat. Pumping the fluid through a fouled heat exchanger also requires more power to compensate for the increasing pressure drop. Reduced fouling will also have a positive effect on energy bills.

Case study: feed/effluent
exchangers I
One of the largest refineries in the US had severe fouling problems in a desalter unit where two shell-and-tubes were used for cooling the desalter effluent. The problem was solved by substituting the two shell-and-tube exchangers for one spiral heat exchanger. The higher thermal efficiency meant a heat transfer area in the new heat exchanger could be half the size of the shell-and-tubes.

One of the main problems with shell-and-tube exchangers was the fast-increasing pressure drop caused by fouling. After the spiral heat exchanger was installed, the pressure drop was stable and thermal performance was much better over time.

The old shell-and-tubes had to be cleaned every month. The new spiral heat exchanger was cleaned for the first time after 14 months.

No heavy fouling was observed, only a thin layer of grease on the effluent side and minor scaling on the feed-water side (see Table 3). The compact nature of the spiral heat exchanger means it is easier to perform maintenance.


Increased production
Many refineries have bottlenecks related to heating or cooling. It is often impossible or very costly to increase heating or cooling capacity, meaning they are left unresolved. Investing in more efficient heat exchangers is often the best way to overcome these limitations. The higher the efficiency of the heat exchanger, the more heat can flow through it. This means the process fluid is heated or cooled with the extra degrees needed to resolve the bottleneck, simply by raising heat exchanger efficiency. The result is higher production capacity at a low investment cost.

Compact heat exchangers resolve bottlenecks without adding any additional investment or operating costs for more heating.

More performance per square metre
Thanks to the smaller heat transfer area required, compact heat exchangers offer significantly higher capacity per square metre of floor space than shell-and-tube exchangers offer. As restrictions in pace and building structures often apply, switching to compact heat exchangers is a straightforward way to boost production without having to rebuild the plant. Using the same support structures, you get the required capacity boost simply by substituting the old equipment with new.

More uptime
Compact heat exchangers require less downtime for maintenance than do shell-and-tube exchangers, since service intervals are longer and the cleaning process is faster. Increased uptime also leads to higher production output over time.

In the desalter example above, shell-and-tubes had to be cleaned 12 times per year and the compact heat exchanger less than once a year. The increase in uptime is substantial and leads to higher production output.

Case study: feed/effluent
exchangers II
To improve overall performance in its semi-regenerative catalytic reforming process, a refinery in France replaced 12 shell-and-tube feed/effluent heat exchangers with a single, large-scale compact heat exchanger. This resulted in a 33% increase in capacity and reduced pressure drop from 4 to 1.5 bar.

Improved heat recovery also led to lower energy consumption by 5.6 MW (19.1 MMBtu/h) and lower emissions for the fired heater. The payback time was 12 months.

Lower investment costs
Total investment costs are usually significantly lower for compact heat exchangers than for shell-and-tube  exchangers. This is because the costs for the heat exchanger and installation are often lower, and because the utility systems can be used more efficiently.

Lower costs for the heat exchanger
Since the required heat transfer area is three-to-five times smaller for a compact heat exchanger than for a corresponding shell-and-tube design, much less material is needed to build the unit. This has a positive effect on price, especially when tough conditions call for exotic materials such as high-alloy steel or titanium.

Case study: atmospheric distillation
unit I
Petrobras compared the costs for shell-and-tubes and compact heat exchangers. Heat exchangers were to be used in an atmospheric distillation unit, to preheat crude using heat recovered from kerosene and HVGO streams. The comparison showed that the costs for shell-and-tube exchangers were 3.8 and 5.6 times higher for the respective positions.

Lower installation costs
Installation costs can be cut considerably by using welded plate heat exchangers instead of shell-and-tube exchangers when expanding plant capacity. The foundations can be made smaller and the heat exchangers are easier to fit into existing structures thanks to their compact nature and lighter weight.

When estimating the total installed cost, a factor of 3.0–3.5 times the cost of the heat exchangers is often used for shell-and-tubes, compared to less than two for compact heat exchangers.

Case study: atmospheric distillation
unit II
A refinery in Asia analysed different options for heat recovery on the atmospheric distillation column. Special alloys had to be used in the heat exchangers due to the aggressive media. Since the shell-and-tube solution would require a larger heat transfer area, the cost became 2.3 times higher than for a compact heat exchanger setup (see Table 4).


A shell-and-tube installation (including space for extracting the tubes) would also occupy a 20 times larger volume on-site, 840 m3 (12 x 14 x 5 m) compared to 37.8 m3 (1.8 x 6 x 3.5 m).

Lower costs for utility systems
Before investing in new utility systems such as cooling towers and boilers, it is wise to see if the same result can be achieved by increasing heat recovery. Recovering more energy in the process often leads to reduced heating and cooling needs. Switching to compact heat exchangers often means production can be increased while still using the same utility systems.

Reliability
Compact heat exchangers offer the best of two worlds and combine the benefi ts of traditional plate heat  Exchangers with those of shell-and-tube exchangers. The all-welded design ensures trouble-free performance that does not change over time.

Many of the compact heat exchangers that are in use in refi neries have been operating for decades and are still delivering top results. Apart from reliable sturdiness, compact heat exchangers bring you high efficiency, compact size, minimum maintenance, low pressure drop and the ability to operate at high pressures and temperatures.

They can be used in many positions in a refi nery. The installations are often for heat recovery applications, where high efficiency is essential (crude preheating, feed/effluent heat recovery and boiler feed water preheating). Compact heat exchangers are often being used where space or weight constraints apply (overhead condensers and reboilers) and where exotic materials are required due to corrosion (desalter water, naphtha toppings, sour water, amines and alkylation).

The self-cleaning effect in spiral heat exchangers also makes them very suitable for heavy fouling applications (cooling FCC bottoms or visbroken residues).

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Preheat Train Network Modifications

In this case history, a crude distillation unit (CDU) preheat train network in a Saudi Aramco refinery was simulated and analyzed for anticipated modifications to the network. This analysis helped eliminate inefficiencies in the network, and, based on the insights from the analysis, various options were generated and the existing network was reconfigured. The reconfiguration allowed the temperature of the crude preheat network, which processes Arab Light crude oil, to be increased to the maximum of 277°C from a previous temperature of 261°C.

Existing configuration.

Desalted crude from the tank is heated by the crude column top pumparound, light gasoil (LGO) product, heavy gasoil (HGO) product, LGO pumparound (LGO PA), HGO pumparound (HGO PA), heavy vacuum gasoil (HVGO) pumparound and vacuum residue (VR) product, as shown in Fig. 1 in exchangers E1 to E7, respectively. The current crude preheat temperature entering the CDU furnace is around 261°C. This exchanger network is validated using heat exchanger design software and by adjusting the foulingcoefficients.
 

Modifications required.

The base-case network was altered for anticipated modifications in the future. The reasons for the modifications are listed below:
Vacuum slop circuit. In the current configuration (Fig. 2), the vacuum slop is recycled to the vacuum tower through the vacuum furnace. The purpose of this recycle is to recover the VGO components and send the VGO to the hydrocracker; however, this is not achieved in the current operation due to vacuum furnace limitations and insufficient separation in the wash section. As a result, this vacuum slop stream (which is lower in viscosity) goes with the vacuum tower bottoms. The mingling of streams deteriorates the feed to the asphalt oxidizer and creates operational problems in meeting the penetration property of the asphalt.
 
To address this concern, the vacuum slop stream from the vacuum tower is available at a temperature of 380°C, which is withdrawn as a separate cut and is used to increase the preheat temperature of the crude. This proposed new exchanger is configured to be in parallel with the existing heat exchanger E4 in Fig. 1. Fig. 3 shows the rerouting of the vacuum slop.
 


Future splitter configuration. To meet the clean gasoline specification of 1% benzene in gasoline, the existing naphtha splitter must remove the benzene precursors in the catalytic reformer feed by increasing the initial boiling point of the heavy naphtha. This process requires a higher reboiler duty. In addition, the heavy naphtha from the hydrocracker needs to be processed in the naphtha splitter, as this feed also contains benzene precursors.
Currently, hydrocracker heavy naphtha is not part of the naphtha splitter feed. The hydrocracker heavy naphtha feed volume is 12,500 barrels per day (bpd), and the existing naphtha splitter capacity is 23,000 bpd. Figs. 4 and 5 show the naphtha system’s current and planned configurations, respectively. As the current naphtha splitter cannot handle this higher throughput with higher reboiler requirement, the existing naphtha splitter will be mothballed. The existing reboiler, which uses HGO PA flow and gives a duty of 10.4 million kilocalories per hour (MMkcal/hr), will also be mothballed.
High-pressure steam will be used in the reboiler of the new naphtha splitter to meet the higher reboiler requirements. For the column to be in heat balance, this 10.4 MMkcal/hr of heat removal is required. In the proposed exchanger network, this stream (HGO CR) will be used to preheat the crude. 

Synthesis of crude preheat train.

A new, preliminary heat exchanger network (Fig. 6) was synthesized to accommodate the above modifications. While modifying the crude preheat train network, the following impact on the equipment was kept in mind:
  • Prevention of vaporizations in the furnace pass-control valves, as it is difficult to control two-phase flows across pass-control valves. Inadequate flow in the furnace pass flows will also lead to coking
  • Column heat balance.
  • Column hydraulics.
  • Impact of hot streams going directly to the other unit.



The changes made in the base-case network are listed below:
·      Exchanger N1 was added parallel to E4 (see Fig. 6) using vacuum slop (vacslop) and vacuum residue ex-E7 as the hot fluid. This modification is required to improve the viscosity of the vacuum residue to the asphalt oxidizer. The current viscosity of the feed to the asphalt oxidizer is 1,500 centistokes (cst), and the required viscosity is 2,000 cst.
·     Another exchanger N2 (E5-2, similar to E2) was added parallel to E2 using HGO PA fluid ex-E5 (hereafter referred to as E5-1) as the hot fluid. This modification is performed to accommodate the 10.4-MMkcal/hr duty in the HGO PA circuit.
·     Increased area in E4 from the 2-parallel-1-series arrangement to a 2-parallel-2-series design and added cooler N3 downstream of E4.
Due to the first two modifications, the inlet temperature to E4 has increased, which decreases the logarithmic mean temperature difference (LMTD) available across the unit. Since E4 is the LGO PA exchanger, the column will not be in heat balance if the required heat removal is not performed. The required duty was 18.8 MMkcal/hr, and the available duty was 12.7 MMkcal/hr (see Table 1). Therefore, additional area and a cooler were added in the LGO PA circuit to meet the duty requirement of the column.
The required HGO PA duty is 26.8 MMkcal/hr, and the available duty is 29.8 MMkcal/hr. As the heat removed in HGO PA is higher by 3 MMkcal/hr, the requirement of LGO PA duty will come down by 3 MMkcal/hr. As both LGO and HGO are mixed outside of the column and go to the diesel hydrotreater (DHT), the splitting of the duty between LGO and HGO pumparound is not a concern from a separation point of view. However, it does impact the column draw temperature, which will slightly reduce the LMTD across E3 (HGO product/crude exchanger) and E5 (HGO PA/crude exchanger).

Results of network modification.

In the modified network, the obtained preheat temperature was 266°C. The duty, LMTD and area of each exchanger in the network are presented in Table 1. From Table 1, it can be observed that:
·    Exchanger E6, which has a higher area, is experiencing the lowest LMTD; therefore, any modification that increases the LMTD will significantly increase the heat recovered from E6.
·     The exchanger preceding exchanger E6 is heated by HGO circulating reflux (CR), which is at 337°C; this is higher than the hot stream (HVGO CR) temperature of E6, which has decreased the LMTD in E6.
This preliminary network was analyzed for possible improvement in the preheat temperature. The analysis indicated that heat recovery can be increased by 45% by boosting the area by 56% (see Table 2).
The analysis also indicated that the driving force across exchanger E7 further limited the heat recovery. Fig. 7 displays the driving-force plot. The figure indicates that the driving force in E7 can be increased by decreasing the inlet temperature in E7. This temperature adjustment can be achieved by operating E5 in parallel with E7.


Case 1. Based on the insights derived from Table 1 and Fig. 7, to improve the heat recovery, the crude stream in E7 and E5 was split by operating E5 in parallel with E7. The objective of this modification is to increase the LMTD across E7 and E6. However, it also decreases the LMTD across E5-1. The net effect is shown in Table 3, and the modified network is shown in Fig. 8. With this arrangement, the preheat temperature has increased from 266°C to 269°C.

Case 2. From LMTD and approach data in Table 3, it can be inferred that heat recovery in E5-1 can still be improved by increasing the area. Hence, another case study was performed by adding two similar exchangers in a series in E5-1. The results are tabulated in Table 4. The preheat was found to be increased to 277°C.
 


The HGO PA is now providing an extra 4.2 MMkcal/hr more than required, which will reduce the LGO PA duty requirement by the same amount for the column to be in heat balance. Then, the required LGO PA cooler duty comes down to 2.6 MMkcal/hr.
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