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March

2017

HYDROCARBON

ENGINEERING

87

contributes to corrosion, and is relatively high in oxygen

content. There are many uses for the higher alcohols and

hydrocarbon products derived from ethanol, including

biojet fuel and potential gasoline components, which are

more favourable than ethanol itself.

In the oligomerisation work, a proprietary catalyst had

shown good conversion and promising selectivity. After

laboratory development, a sizeable amount of product

was required for testing. The catalyst was ultrasensitive to

poisons, such as metals and sulfur, thus requiring

feedstock pretreatment. Associated water production

during synthesis required a three phase separation step

that is not present in a hydrogenation pilot plant for

petroleum hydrocarbons. Accordingly, SwRI’s alternative

fuels centre 8 l pilot plant was adapted to separate the

three reactor beds to allow different conditions for

hydrodemetalisation, sulfur removal by a sorbent, and

oligomerisation in the main reactor. Other modifications

were required for online sampling and for water removal.

The extensive modifications required a new hazard

and operability study (HAZOP) review, as it regarded the

different hazard profile of ethanol in large quantities and

the extensive revised flow diagram. Receiving neat

ethanol without denaturant required upgrading the

federal license to possess and dispense ethanol. Sampling

and maintenance (mostly line breaking) necessitated

improved procedures for the unit.

While the processing itself went well, the sampling

and analyses for assessing selectivity of the catalyst were

challenging. The oxygenates among the products had

dissimilar affinities for the sample lines normally used for

hydrocarbons, requiring fluorcarbon sample lines to be

heated enough to prevent condensation, or adherence of

the potential analytes of the highest molecular weights.

Three phase separation was achieved by raising the

level in the usual gas-oil separator following the synthesis

reactor. This ensured that the organic product went

forward to the distillation column while produced water

was periodically released to collection manually.

Maintaining the level in the separator seemed a challenge

at first and resulted in valve stem replacement. The

corrosive nature of some products may have contributed

to the erosion of the control surfaces.

Continuous production of biodiesel

fuel

Producing esters for blending into diesel fuel has become

widespread, mostly in batch operations employing methyl

alcohol solutions with a strong base in order to allow the

methyl groups to produce the esters and catalyse the

reaction. Starting with whole crop oils and restaurant waste

oil, glycerol is produced in the aqueous layer that may be

recovered as wet glycerol in addition to the primary ester

product in the organic layer. While this conversion now

consumes mostly food crop oils, many other oil crops,

including those that grow on arid land, are suitable for

making biodiesel. The aqueous waste remains a problem for

disposal, often eliminated by deep-well injection as an

added expense that can be ripe for abuse by improper

disposal.

An improvement to the batch technology used a

proprietary catalyst developed at SwRI, which

immobilised on the catalyst surface the basic function

that is normally provided by soluble hydroxide. The

catalyst allowed the esterification to be conducted in a

continuous flow reactor with high selectivity for the

methyl ester of the fatty acids that made up the

feedstocks that were tested, including ones from plant

and animal sources.

During a baseline test in which excess methanol was

being used, unexpectedly, no glycerol was detected in the

product. Instead, a variety of oxygenates were produced,

the most abundant of which were commodity chemicals.

This is a fortunate feature of the continuous process, as

the proliferation of the batch biodiesel production

drastically lowered the market value of wet glycerol.

Conclusion

The three elements of processing biofeedstocks

presented in this article have all possessed an element of

the unexpected. Process modelling has become so reliable

that many results for whole plant performance and

economics have been accepted, without customary

validation at the semiworks scale or even the pilot-scale.

Business models have previously not budgeted for

intermediate-scale testing before taking the first step

towards commercialisation.

Not only is pilot-scale testing a form of insurance

against pitfalls during commercialisation, it is an

opportunity for further discovery of the peculiarities of

a technology and for process enhancement, giving future

added value. Pilot testing avoids the miscalculation of

rates and physical parameters, tests assumptions,

validates models, and reveals unexpected phenomena.

For cases in which a process guarantee is offered by a

process licensor, independent pilot-scale testing can

avoid costly delays while the technology originator

attempts to rectify erroneous estimates of process

operating variables. Pilot processing and its associated

costs may be regarded as a form of insurance against the

unknown, at worst, and an investment in process

improvement in any case.

Figure 3.

The laboratory scale pilot plant for

biodiesel production.