refining india 2025
Making your green HVO/HEFA project greener
Jay JEONG ALFA LAVAL
the recovery of low-grade energy, using it to generate both low-pressure steam and hot water. The steam can then be used to evaporate the wastewater (as described in the effluents from the pretreatment unit [PTU] section) or to produce electricity by means of Organic Rankine Cycle (ORC) sys- tems. The hot water can be used as boiler feed water or for tank or plant heating, and it can even be supplied to district heating networks. Recovering otherwise wasted heat in this way turns process cooling from a cost generator into a profit generator. Fractionator optimisation: final product and vapour trim coolers and condensers The final optimisation step within the scope of this article is to minimise the fractionator’s cooling water requirement in all final product and vapour trim coolers and condensers. Conventional water-cooled shell-and-tube heat exchangers are designed to avoid a temperature cross, so that the cooling water return temperature is the same as or lower than the outlet product or vapour/conden- sate temperature. As a result, the difference between the cooling water supply and return temperatures is sometimes less than 10°C. This means a large amount of cooling water needs to circulate between the HVO com- plex and the cooling water plant. When WPHEs are first used to maximise the waste heat recovery (WHR) from pro- cess streams, the remaining cooling duty is minimised. In addition, for the final trim cooling or condensing duty, the cooling water return temperature can be maximised in a single heat exchanger with a minimal flooded weight and plot space requirement, thereby reducing the amount of cooling water in circulation by up to 50%. This will reduce the piping and pump cost of a new cooling water system (or minimise load on an existing system), as well as the energy consumption of the circulation pump. Summary and conclusions HVO processing has clearly made its way into petroleum refineries, where many facil- ities are already on-stream or under con- struction. The quest for the future is to maximise the use of wastes and non-edible oils as renewable feedstocks and to design the pretreatment facilities for maximum flexibility to handle such feedstocks. At the same time, there is increased focus on sav- ing energy, reducing emissions, and mole- cule management. Optimising the product fractionator for maximal performance – using high-efficiency WPHEs whose capa- bilities exceed those of conventional heat exchangers – is fully in line with these goals. Alfa Laval can be instrumental in both regards. The company has delivered more than 1,000 plants to the food and biofuel markets and is a leading supplier of pre- treatment plants for HVO processing. It has also supplied close to 3,000 WPHEs (Compabloc) for different refinery pro- cesses since the mid-1990s. It can help select the best pretreatment scheme for HVO feedstock, and its WPHEs can boost HVO processing profitability. Contact: jay.jeong@alfalaval.com
The hydrotreated vegetable oil (HVO) pro- cess is a series of processes. First, hydro- treatment (HDT) removes oxygen and splits the triglycerides into three chains of hydrocarbons. Next, the paraffins are converted into a mixture of hydrocarbons with the right cold flow properties, either through isomerisation for maximum diesel yield or mild hydrocracking (HCK) for max- imum sustainable aviation fuel (SAF) yield. Byproducts from these reactions, such as propane, CO₂, and sour water, are removed in a stripper, while the final liquid products are separated in a downstream fractiona- tion section. Figure 1 shows all HVO pro- cesses in a simplified flowchart. The following sections focus on the fractionation process and the potential to optimise the fractionator for maximal performance. Most fractionator designs are based on old rules of thumb that limit energy efficiency and product recovery while increasing project Capex. Instead, the fractionator can be optimised with high- efficiency heat exchanger solutions that have been on the market since the early 1990s. Such optimisations can drastically improve efficiency in this part of the HVO process. Fractionator optimisation: feed/bottoms interchanger The first heat exchanger position to con- sider is the feed/bottoms interchanger. In this position, the aim is to maximise energy recovery from the fractionator bottom stream for use in preheating the feed. Doing so will maximise both the final product cool- ing and the feed heating, which will reduce the load on both the final product cooler and the fractionator reboiler. The amount of energy that can be recov- ered is limited by the heat exchanger technology selected. When conventional shell-and-tube technology is selected, maximising the energy recovery requires a series of several large heat exchangers. This is often too costly or practically infea- sible to install in the plant. The alternative is to use a high-efficiency welded plate heat exchanger (WPHE). This technology enables a tight tempera- ture approach down to 3°C, which can be achieved in a single heat exchanger with a minimal flooded weight and plot space requirement. Thus, it becomes economi- cally favourable and practically feasible to maximise energy recovery. Often, at least 25% more energy can be recovered with a WPHE. This reduces the reboiler duty by an equivalent amount and may even eliminate the need for an air cooler upstream of the final trim cooler, as outlined in Figure 2 . Fractionator optimisation: overhead condenser Minimising the column operating pressure is another opportunity to maximise the energy efficiency of the fractionator. Often, this can also improve separation efficiency in regard to fractions with similar or even overlapping boiling ranges, such as naphtha and SAF, thus making it possible to maxim- ise the yield of the most high-value product.
Propane, CO , H O
Renewable products
Pretreated renewable feed
Hydrocarbon mix
n-para ns
LPG & naphtha Kero/jet (SAF) Diesel
HDT (de-oxidation)
Isomerisation or mild HCK
Fractionation
Hydrogen
Figure 1 A simplified flow chart summarising the HVO process
Renewable diesel
O gas
M
Green naphtha
Q min
Product fractionator
M
SAF
T min
M
Q max
Q min
T max
Stripper bottoms
Figure 2 Improved product fractionator design using WPHE to maximise energy recovery in the feed/bottoms interchanger
Cooling water
P min
Renewable diesel
O gas
T & dP min
m opt . Green naphtha
dT max
Product fractionator
M
SAF
m opt .
M
Q min
P min
Stripper bottoms
Figure 3 Improved product fractionator design using WPHE to minimise column pressure
Depending on the supply temperature of the cooling media, the operating pressure in the column can sometimes be reduced by 2 bar or more when using a WPHE. This reduces the fractionator reboiler duty and increases the difference between the naph- tha and SAF boiling temperatures (see Figure 3 ). Fractionator optimisation: condenser and run-down coolers Several process streams, including over- head vapour and run-down streams, are usually cooled utilising either cooling water or air, as recovering low-grade energy from these streams is difficult and expen- sive with conventional shell-and-tube heat exchangers. Such coolers require a large amount of either cooling water or electri- cal power, which puts high demands on the utility system of the HVO complex. In this way, the coolers increase both the project investment and the plant operating cost. With WPHEs, it is possible to maximise
Because the pressure in the column is decided by the overhead vapour condenser, optimal condenser design and technology are key parameters in minimising the col- umn pressure. When conventional shell-and-tube or air heat exchangers are used as overhead vapour condensers, a higher temperature approach to the supply temperature of the cooling media is required. Hence, a higher pressure in the column is needed to achieve a certain liquid yield at the condenser outlet. When a WPHE is used as an overhead condenser, it is possible to operate with only a 3°C temperature approach to the cooling media. As a result, the same liq- uid yield can be achieved at the condenser outlet at a much lower operating pressure. Moreover, thanks to the multiple short, parallel channels in the WPHE design, the condenser pressure drop can be reduced compared to conventional heat exchangers. Together, these factors minimise the neces- sary column operating pressure.
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