Design and analysis of poly-3-hydroxybutyrate production processes from crude glycerol

Abstract

The growing biodiesel production has generated a glycerol surplus and consequently a drop of its sale price. The development of biorefineries to convert crude glycerol to higher value products is an urgent need and an opportunity to overcome the negative impact of low glycerol prices in the biodiesel industry. Glycerol fermentation by microorganisms to useful chemicals as poly-3-hydroxybutyrate (PHB) is an interesting alternative. In this article a techno-economical analysis for PHB production from crude glycerol is presented. The glycerol fermentation process was carried out using two qualities of glycerol, either 88 or 98 wt%. Thus, the glycerol purification process was simulated and economically analyzed using Aspen Plus and Aspen Icarus, respectively. The fermentation process is carried out in two stages in which mass cell growth and PHB accumulation occurs, respectively. Also, three downstream processes to isolate and purify the PHB were considered. The process steps, namely mass cell pretreatment, PHB isolation and purification were considered in each scenario and six technological schemes were analyzed. Economical assessment results show that the most appropriate technological scheme requires purifying the crude glycerol until 98 wt%, with a downstream process involving heat pretreatment, enzymatic-alkaline digestion, centrifugation, washing, evaporation, and spray drying.

Introduction

Currently petroleum and its derivatives represent almost 35% of the total primary energy supply in the world and about 60% is employed in the transport sector [1]. The increase of oil prices and limited reserves of fossil fuels have lead to the development on renewable energy, such as liquid biofuels. One of the most important biofuels is biodiesel since its emissions from combustion engines are lower than those obtained from conventional petroleum diesel fuels (up to 100% sulfur dioxide; 48% carbon monoxide; 47% particulate matter; 67% total unburned hydrocarbons; and up to 90% in mutagenicity) [2]. Biodiesel is produced by oil or fat transesterification with a short chain alcohol, but glycerol is also obtained as a by-product in a weight ratio of 1/10 (glycerol/biodiesel). Crude glycerol (a co-product stream from biodiesel production) is mainly composed by glycerol, free fatty acids (FFA), fatty acid methyl esters (FAME), and some traces of salts. The crude glycerol composition often depends on the feedstock material, the transesterification process (catalytic way) and the recovery technology employed [3].

The growth on biodiesel industry has carried out a glycerol surplus and its consequent price decrease, which affects directly the biodiesel economy. Producers and refining companies as Dow Chemical, Procter and Gamble Chemicals were shut down due to low glycerol prices [4]. Pure glycerol is an important industrial feedstock used in food, drug, cosmetics, pharmaceutical, pulp and paper, leather, textile and tobacco industries [4]. On the other hand, crude glycerol is refined by larger scale biodiesel producers by means of a traditional separation process used to remove impurities. Some of these refining processes are filtration, chemical additions, and fractional vacuum distillation. These processes are expensive and economically unfeasible for small and medium scale plants. Thus, the development of biorefineries based on crude glycerol to produce higher value compounds is necessary in the biodiesel industry to overcome the economic glycerol drawback. An interesting application of crude glycerol is as a carbon source for some fermentation processes using microorganisms to obtain useful chemicals such as: hydroxypropionaldehyde, 2,3-butanediol, 1,3-propanediol, succinic acid, and polyhydroxyalkanoates (PHAs) [5].

Polyhydroxyalkanoates are attractive substitute biopolymers for conventional petrochemical plastics which have similar physical properties to thermoplastics and elastomers. PHAs are homo or heteropolyesters synthesized and stored intracellularly by several Gram Negative bacteria [6]. PHAs can be produced from renewable resources through a fermentation process under restricted growth conditions for nitrogen, phosphorus, sulfurs and/or oxygen in the presence of an excess carbon source, and they can also be completely biodegraded by many microorganisms [7]. PHAs are stored in form of granules by bacteria and can account for up to 80% of the total bacterial dry weight [8]. On the other hand, polyhydroxybutyrates (PHBs) were the first type of PHAs discovered and the most widely studied. PHB has similar properties to conventional plastics like polypropylene or polyethylene, and it can be extruded, molded, spun into fibers, made into films, and used to make heteropolymers with other synthetic polymers [9], [10].

As shown in Table 1, several companies around the world are PHB producers but a main drawback compared to the petrochemical plastics, is its high production costs [8]. Carbon source represents the major share of the total production costs (up to 45%). Thus, many studies have been conducted in order to find cheap carbon sources and as a result some strains of microorganisms are able to produce PHB using crude glycerol as substrate, some of these strains are shown in Table 2 [11], [12], [13], [14], [15], [16].

The goal of this article is to economically evaluate the PHB production process using crude glycerol from the biodiesel industry as a cheaper raw material by three different downstream processes distinguished by their pretreatment steps such as: (i) a chemical and enzymatic digestion, (ii) a high pressure homogenizer with a solvent extraction step using DES, and (iii) an alkaline digestion helped by a surfactant. These proposed biorefineries would be able to produce added value compounds from glycerol and also to increase the economic performance of biodiesel production.