By Peter Rose, Alfa Laval Limited

It is somewhat disconcerting to think that, almost 10 years into a new century, we are still trying to resolve the energy problems that dogged the process industries throughout the last three decades of the previous one.
Energy crunches have come and gone and even the onset of the current downturn has not significantly disturbed that trend. Prices have dropped in line with reduced demand but, when the larger global economies start to recover, they will, inevitably, start to suck up significant amounts of oil again and prices will rapidly resume their upward spiral. This spiral will be given an added twist by the recent news that all of the globe’s major oil fields have passed peak production. From now on, we will all be running on empty. Meanwhile, energy demand is predicted to soar by 60% by the year 2030.
As if that weren’t enough, ever tougher environmental legislation is being implemented to reduce C02 emissions and ward off the perceived threat of global warming.

These twin threats place a huge responsibility on the process industries in general and process engineers in particular. Once, their principal responsibility would have been simply to maintain or improve the efficiency of a particular process. Now, they must, simultaneously, find ways to reduce energy consumption and lessen environmental impact.
That is why, despite their inherent conservatism, the processing industries are showing growing enthusiasm for the benefits of compact heat exchangers, such as Plate Heat Exchangers, in all of their various guises.  Compact heat exchangers are used to recover and recycle heat, act as condensers, evaporators or re-boilers and provide levels of efficiency and dependability on a par with traditional shell & tube (S&T) units but without the additional size, weight, installation and operational costs that accompany the more traditional exchangers. Expressed in the most basic terms, a compact plate type heat exchanger provides more heat transfer capacity within a much smaller footprint.

For instance, you would need a Shell &Tube exchanger with a capacity of roughly 1000m2 to obtain the same heat transfer capacity provided by a compact heat exchanger measuring just 3 m3.
One sector that is looking for ways to downsize its investment but not its efficiency is the oil & gas industry. In a typical refinery, the job of pre-heating crude oil is the most energy-intensive process. A 100,000 barrels per day (BPD) refinery, typically, uses approximately 120 MW of energy to pre-heat its crude oil from 250C to 3500C. Although energy for this process is provided by a fired heater such as a boiler, it is also increasingly recovered in one form or another from various stages of the refining process.
Consequently, the more energy recovered from the overall refining process and applied to the crude heating stage, the lower the dependency on the fired heater and the lower the consumption of fuel.  For instance, a reduction of 1MW in heater energy input will correspond to a saving of almost 1000 tonnes of natural gas consumption: Matched, of course, by a similar reduction in carbon emissions.

The traditional exchanger for this duty has been the Shell & Tube unit.  S&T heat exchangers have been around for a very long time – and for a very good reason. They are extremely good at what they do.
The S&T exchanger is simple to design, mechanically uncomplicated and can be engineered in a wide range of materials. As such, its performance and life costs are both fairly predictable and easy to calculate. This performance can also be enhanced by the addition of finned tubes or tube inserts. The downside to this particular kind of exchanger is its relative inflexibility.  Once engineered for a specific duty it is difficult, subsequently, to alter or adapt the exchanger to meet changing process needs.

The sheer scale of the average S&T means substantial materials costs and this has to be taken into serious consideration when it comes to exchanger selection. They provide a relatively poor return for their enormous size in terms of efficiency or heat transfer coefficient. Thanks to their cross-flow method of heat transfer, large, bulky units, or several exchangers connected in series, are necessary if a process demands temperature crossing, where the cold fluid is heated to a higher temperature than the outlet temperature of the hot medium.

This raises practical as well as operational space issues. Increasing the size of an S&T, multiplying the number of passes or connecting a number of tubes in series not only adds to the installation and service space required but also reduces thermal efficiency and increases the risk of fouling; thanks to the lower velocity of the fluids through the tubes. Regular serious fouling can lead to the need for more frequent and lengthy Downtime for cleaning.

By contrast, compact units such as the plate heat exchanger operate in true counter-current flow, which enables temperature crossing to be achieved in a single, compact exchanger. Counter current flow means that the hot fluid enters the heat exchanger at the same end as the cold fluid exits. In practice, this enables the exchanger to recover and recycle energy more efficiently than with an S&T since the cold fluid can be heated to temperatures very close to that of the hot fluid, thereby recovering as much of the available heat as possible.

The temperature difference between the hot and cold sides is expressed as the Mean Temperature Difference (MTD) and it is this that drives the actual transfer of heat. The higher the MTD figure, the quicker and easier will be the process of transferring heat from one stream to the other.
One very practical example of this occurs in a typical gas sweetening process, where the interchanger (Fig.1) uses solvent from the bottom of the stripper to raise the temperature of the solvent. The more efficient this process, the less steam (or other energy source) is required to be put into the stripper column reboiler. As a consequence, the volume of coolant required for the lean solvent cooler leading back to the absorption tower is also reduced.

This ability to work with minor temperature differences is what contributes so significantly to the compact PHE’s thermal efficiency. It is a function of design as well as fluid thermodynamics.  Turbulence plays an important role in transferring heat. The smooth internal surfaces of a standard S&T do little to promote or improve turbulent flow. However, the characteristic corrugated patterns pressed into the surface of PHE plate are designed to do just that. When the plates are pressed together, the contact points form channels, which encourage the fluids into a spiral flow, which creates turbulence. The net effect of this turbulent flow is to provide thermal efficiency that is up to five times as high as that of a comparable S&T. The other benefit of high turbulent flow is a lower tendency to fouling. Turbulence scours the heat transfer surfaces of the exchanger, keeping the walls clear of fouling. Any fouling that does occur tends to increase the turbulence of the flow, which, in turn, increases the scouring action.
This ability to recover low-grade heat that might otherwise go to waste is assuming more significance. It is possible, for instance, to design compact heat exchangers into existing and new condenser systems to enhance a plant’s energy efficiency. 

In distillation and stripping systems, overhead gases are collected and returned to the condensing column via condensation.  (Fig. 2) Generally, either air coolers or heat exchangers (condensers) using cooling medium are used to obtain the desired condensate. As in most refinery processes, steam is generally used to provide the necessary heat.  Equally, any of the heating of boiler feed water that can be carried out using recovered heat will reduce the fuel load and the environmental impact.

One way to achieve (Fig. 3) this is to split the column condenser into two separate units; one that uses boiler feed water for the condensation process and the other using normal cooling water. This configuration enables heat already available in the process to be recovered to pre-heat boiler feed, reducing the volume of fuel required for steam generation.
An alternative approach is shown in Fig 4. whereby the feed into the column is the cooling medium rather than boiler feed water. This arrangement again reduces the volume of steam consumed in the reboiler, which translates into significant energy savings.

How much are these savings worth?
Obviously, the answer to this lies in the cost of energy at any given time. But, based on current natural gas prices and the value of emission credits, it is entirely reasonable to assume that every megawatt of energy saved represents an annual saving in excess of $250,000. Taking as an example, for a refinery in North America this means a substantial saving since they replaced eight S&T exchangers with a similar number of Compabloc all-welded PHEs to act as condensers and recover heat from overhead vapour steam to pre-heat boiler feed. 

The new condensers – which are installed in the fluidized catalytic cracking unit (FCCU) – were needed because the original units were suffering badly from corrosion in the tube bundles caused by bisulphides, chlorides and cyanide trapped in the overhead vapour. However, the cost of replacing existing components with new versions made from corrosion-resistant materials was greater than replacing the units with eight compact condensers manufactured in Hastelloy C276®.

What was even more of a factor in the decision to opt for the compact condensers was the potential for energy savings. The installation was designed primarily to transfer a significant proportion of the heat - 13.5MW - in the FCCU to pre-heat water for the steam plant. Recovering this heat released more steam capacity to allow for future expansion without the need for additional boiler plant.

The system consists of eight Compabloc compact condensers arranged in four parallel series of two. The four units at the top recover the heat to pre-heat the boiler feed while the bottom four are used as trim coolers employing process cooling water. The heating medium is overhead vapour, which enters the condensers at 1410C and is condensed and then cooled to exit at 290C. It is cooled by boiler feed, which is in turn heated to 1280C.
Each condenser is vertically mounted and condensation occurs in horizontal channels arranged in two passes on the condensing side. It is this multi-pass arrangement that enables efficient cooling and condensing to take place despite relatively close temperature approach between the two media. It is also the key to the efficient recovery of large volumes of energy from the condensing steam that has made the project economically and environmentally successful.

The total heat recovered from the plant is around 14MW, which translates to savings of at least $4.3 million in respect of the fuel and around $1.1 million for emissions. Given the capital cost of the installation was around $7 million, this meant that the compact condensers paid for themselves inside 15 months.

Summing up, in an era of declining energy resources and continuous pressure to reduce carbon emissions, the solution for many heat transfer problems lies with the compact heat exchanger in all its forms.  

Alfa Laval Ltd
Camberley
Surrey

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