Environmental Engineering Reference
In-Depth Information
Similar to biodiesel plants, a pretreatment step is typically required to remove insoluble materials
and trace metals, and two reaction stages are used. Unlike base-catalyzed transesterification,
hydroprocessing is robust to high concentrations of FFAs, thus allowing other lower-cost materials
such as tallow oil and waste greases to also be used as feedstock. Advanced hydroprocessing
also produces a low-cloud-point product that can be used neat or blended with petroleum diesel
without limitation.
In the Ecofining process, pressurized feedstock is mixed with recycled hydrogen and then sent
to a multistage adiabatic catalytic hydrodeoxygenation reactor (R1) where the feedstock is saturated
and completely deoxygenated. A vapor-liquid separator is used to recover excess hydrogen, and gas
recycle to R1 is set to achieve a minimum hydrogen partial pressure. Conversion of feed is complete,
and the volumetric yield of deoxygenated hydrocarbon products is greater than 100%. Selectivity
to diesel boiling-range paraffin is very high. The primary deoxygenation reaction byproducts are
propane, water, and carbon dioxide.
The effluent from R1 is separated to remove carbon dioxide, water, and low-molecular-weight
hydrocarbons. The resultant diesel is mixed with additional recycle gas and then routed to a catalytic
hydroisomerization reactor (R2) wherein a branched paraffin-rich diesel fuel is produced. In this
manner, the cold flow properties of the diesel are adjusted to meet required specifications. The
isomerization reaction is selective and consumes very little hydrogen.
Isomerized product is separated from excess hydrogen in a conventional gas/liquid separator.
After purification, the excess hydrogen is recycled back to both reactors to maintain the minimum
required hydrogen partial pressure. Make-up hydrogen is added to the process to balance
chemical consumption and solution losses. The liquid product is sent to the product recovery
section of the process where conventional distillation steps are used to separate co-products such
as propane and naphtha.
5
Table 8.1 contrasts Ecofining™ inputs and outputs to those of conventional biodiesel production
(Kalnes et al. 2007). The Ecofining process for producing HRD (green diesel) operates at conditions
similar to a petroleum hydrodesulfurization unit and integrates well within existing petroleum
refineries. If required, a portion of the light fuel co-product can be steam-reformed to generate all
of the hydrogen consumed in the process (Kalnes et al. 2007). Estimates of fossil energy demand
(FED), cumulative energy demand (CED), and life-cycle greenhouse gas (GHG) emissions for
biodiesel and HRD produced in the UOP/ENI Ecofining process, green diesel (GD), are compared
to petroleum diesel in Table 8.2.
taBle 8.1
comparison of Biodiesel and hrd Process Inputs and outputs
ecofining Green diesel
Biodiesel
wt%
vol%
wt%
vol%
Feeds
Feeds
Vegetable oil
100
100
Vegetable oil
100
100
Hydrogen
1.5-3.8
Methanol
10
11
Chemicals
a
4
Products
Products
Propane
5
9
FAME
96
100
Butane
0-2
0-3
Glycerol
10
7
Naphtha
<1-7
1-10
Green diesel
75-85
88-99
a
Chemicals include sodium hydroxide (NaOH) and an acid to neutralize the products.
