Compact plate heat exchangers used for refinery heat recovery

15th February 2008

The use of compact-plate heat exchangers (CPHEs) enables refiners to tackle the increasing emphasis on reducing energy consumption while maintaining a positive environmental profile, indicates a report by industrial process technologies supplier Alfa Laval.

CPHEs properties make them more efficient than, and provide a compact solution to, traditional shell-and-tube (S and T) heat exchangers.

CPHEs, which are much smaller in size than traditional heat exchangers and can be fitted in the footprint of existing installations, can increase capacity and recover more heat by using fewer heat exchangers.

Savings in fuel consumption and reductions in emissions, owing to greater energy recovery, give attractive investment payback periods.

Operators also say that using CPHEs lead to a faster response to changes in the process, including plant start-up and shutdown, and leads to longer intervals between services.

When service is needed, the unrestricted access to the heat transfer surfaces makes it is easier to restore full-heat transfer efficiency.

HEAT TRANSFER EFFICIENCY
CPHEs are differentiated from traditional shell-and-tube heat exchangers in their use of corrugated plates to form heat exchanger channels.

The many contact points achieved when these plates are stacked, force the fluid to spiral through the channels, which in turn, induces high turbulence.

For the same flow velocity through the channels, a CPHE achieves a higher turbulence than a S and T, thus giving rise to thermal efficiency that is between three and five times higher.

Increased turbulence leads to a higher wall-shear stress, which allows the CPHE to operate for longer intervals without the need for maintenance, owing to the wall shear stress’ cleaning effect that reduces any fouling inside the heat exchanger.

HEAT RECOVERY POSSIBILITIES
The CPHE can operate with a countercurrent flow. In such a flow, the hot fluid enters the heat exchanger at the end where the cold fluid exits.

This makes it possible to handle crossing temperature programmes, where the cold fluid is heated to a temperature that is higher than the outlet temperature of the hot fluid, in a single heat exchanger.

This is especially important in heat recovery, where the cold fluid can be heated to temperatures very close to those of the hot fluid, recovering as much energy as possible.

Mean temperature difference (MTD), the temperature difference between the hot and the cold fluids, is the driving force for heat transfer. The larger the MTD, the more effortless the heat transfer, and vice versa.

The effort needed to carry out a certain heat transfer duty is often measured in terms of its theta value or thermal length.

The temperature differences between the two fluids is very small. The resulting large theta value means that the driving force for the heat recovery duty is low, and the two fluids must remain in contact for a long time to be able to exchange heat.

This is tackled by making the tubes longer, in S and T, by arranging the tubes with many passes or connecting several tubes in series. This can result in hydraulic problems, because the channel velocity through the large units is reduced, and thus increasingly lowers the heat exchanger’s thermal efficiency, and increases fouling problems.

In contrast, the high thermal efficiency of the CPHE, combined with opportunities for operating with a counter-current flow, allows the CPHE to deal with long temperature programmes with a small MTD. Usually only one CPHE is needed to tackle the required heat recovery duties.

INSTALLATION CONCERNS
A smaller heat transfer area (HTA) is not the CPHE’s only source of savings. The CPHE compact basic design and the HTA required for each specific duty, also assembled effectively, contribute to savings.

Thus, the CPHE with around 320 m2 of HTA needs less than 1,5 m2 of floor space for the installation and around 10m2 of total floor space, including the service area, with one m added all around the equipment.

The corresponding S and T heat exchangers with six-m-long tubes would need around 15 m2 of floor space for the installation, and 60 m2 of floor space including service area, in order to take into consideration the room for removal of the tube bundle.

The CPHE’s compact design also accounts for reduced weight, which reduces installation costs, especially in construction or foundation work.

In estimating installation costs, a factor of three to three-and-a-half times the initial investment cost is often used for S and Ts, while the corresponding factor is normally less than two for CPHEs.

Additionally, the reduced hold-up volume means that the CPHE responds faster to changes in the process operating parameters, such as at start up and shut down.

SERVICE NEEDS

The increased turbulence and elimination of hydraulic problems extend the operating intervals between services.

The increased service work interval saves money on both maintenance costs and production downtime. With a better response to process changes, the plant can be shut down and restarted more quickly.

If chemical cleaning is used, the lower hold-up volume speeds the process up, with less chemicals to dispose of once cleaning is completed.

For mechanical cleaning, unbolting the frame provides complete access to the heat transfer surface for cleaning, using a hydro jet of up to 500 bar.

In a refinery, preheating crude oil uses the largest amount of energy, and the maximum gains from the CPHE for heat recovery can be made during this process.

However, the CPHE also benefits other parts of refineries where heat recovery is an issue.

EMISSION SAVINGS KYOTO PROTOCOL
Substantial savings on emissions can also be made. For every t of natural gas burned, about 2,6 t of carbon dioxide (CO2) are released. Kyoto Protocol terms state that one CO2 emission allowance, also called a permit or credit, gives the right to emit one t of CO2, or CO2 equivalent.

Companies can buy and sell CO2 credits, at prices of traded credits determined by market forces. Some US emission traders forecast market prices of around $20 a credit before 2008, the first commitment year of the Kyoto Protocol.

This is about one-half of the shortfall fines, at €40 a credit, proposed in Europe. Using $20 a credit, an additional $50 000 can be saved on reduced CO2 emissions for each 1 MW of energy reduction in the heater, yearly.

Other emissions that can be traded are sulphur oxides (SOx) and nitrogen oxides (NOx) permits. Refiners with emissions below the permitted cap values can sell their surplus permits, and vice versa.

The normal trading price for SOx and NOx is around $1 000/t, and for every t of natural gas burned, about 15 kg of SOx are released. This means that savings of over $30 000, owing to reduced SOx and NOx emissions for each 1 MW of energy reduction in the heater, can be achieved every year.