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Jan-2012

Energy recovery with compact 
heat exchangers

Compact heat exchangers are used for heat recovery applications where high efficiency is vital, space constraints apply, or exotic materials are required

Marcos Matsufugi, Alfa Laval
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Article Summary
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 straightforward 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.3

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 All refineries do this to some extent, but most still use outdated shell-and-tube heat exchangers with low thermal efficiency.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 (α1 and α2). Thin plates (small δ) 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-and-tube heat exchanger:

1 - 1 + 1 + δ
k    α1   α2      λ   

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

Another important feature of compact heat exchangers is the capability to operate with a counter-current 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).
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