Eoin Lynch writes an analysis of the 'Assessment of alternatives to incineration for vent gas emission abatement to meet sustainability and net zero targets on a pharma manufacturing plant'.
Summary/abstract
Global warming and climate change are commonly cited as being the most severe threat of the 21st century [1]. CO2 emissions have steadily risen by more than 90% since 1970 (EPA). One of the main contributors to the rise in CO2 emissions is industrial processes and fossil fuel combustion, which in total combine for roughly 78% of greenhouse gas emissions over the past half-century.
As such, pharmaceutical companies are increasing their efforts to reduce their carbon footprints in a variety of ways. In this paper, the benefits of moving towards more sustainable vent emission abatement techniques, such as activated adsorption, will be outlined with the aim of increasing awareness of the available measures for carbon footprint reduction for a typical medium to large-sized pharmaceutical company in Ireland.
Introduction
In line with carbon net zero targets by 2050, Irish pharma companies are considering alternative technologies to help reduce their carbon footprint. These companies promote process development and cGMP which extends to CO2 emissions associated with waste treatment, with vent emission abatement being a major contributor to carbon dioxide emissions to the atmosphere.
Carbon dependent technologies such as thermal oxidation via an incinerator is currently one of the most commonly employed vent abatement systems [2], operating under the basic principle of converting volatile organic compounds (VOCs) and other pollutants into CO2 and H2O, utilising excessive temperatures before ultimately exhausting to the atmosphere [3].
A typical waste incinerator treating VOCs produces about 10,000 tonnes CO2/year, an output equivalent to that of 3,500 cars in Ireland on average.
As such, this research examines the potential carbon dioxide emissions reduction that may be achieved with the implementation of alternative technologies for vent emission abatement, given the integration of sustainability is becoming a core value of companies aiming for low carbon manufacturing.
Adsorption overview
Adsorption is a physical process wherein adhesion of gas molecules to the surface of a solid substance occurs [4]. The pharmaceutical industry benefits from the use of a variety of adsorbent materials for VOC removal, including synthetic zeolites and activated carbon, the latter being the more commonly used.
Adsorbent materials typically have high adsorption capacity for VOCs due to their high degree of porosity, large surface area, and versatility for multi-component removal [5]. A typical fixed bed activated carbon adsorber can achieve a removal efficiency of about 97% for common VOCs such as toluene, ethanol, and heptane [6].
The gas stream carrying the VOCs is typically passed through a bed of adsorbent material as part of the VOC adsorption process [7]. The VOCs are adsorbed onto the surface of the adsorbent material as the emission stream passes through the bed. The quantity of VOCs in the gas stream that enters the adsorber, the temperature and humidity of the gas, and the properties of the adsorbent material are all major factors that impact the degree of adsorption [5].
Adsorbers do not produce carbon dioxide emissions while in the adsorption phase, which is a considerable benefit compared to incineration. The adsorbent material withholds the pollutants that have been absorbed, with this material then available to be removed from the adsorber for recycling or suitable disposal.
The reuse of recovered solvent from adsorption also provides an economic benefit as this will reduce the cost associated with raw material purchasing.
Alternatively, these recovered solvents can be sold and reused by a number of waste treatment companies in Ireland that generate solvent blends as raw materials in other processes such as auxiliary fuel production [8].
In addition, the adsorbent material be recycled and reused, which can reduce the environmental impact of such a system by reducing the waste associated with the disposal of activated carbon. By heating the adsorbent material to high temperatures, the captured pollutants can be released, restoring the adsorption capacity of the system for the next cycle.
Adsorber configuration
The three main configurations of adsorption systems used to treat VOC emission streams are fixed-bed, cannister and fluid-bed adsorbers. However, for a medium to large sized pharmaceutical company fixed bed adsorbers provide the most feasible solution [8] to vent emission abatement as outlined below.
Fixed bed adsorbers regenerate adsorbent material by manipulating the environment such that pollutants are desorbed from the surface of the activated carbon. This is an energy-intensive process at large scales but recent developments in the scalability of electrochemical and solvent regeneration may allow for more sustainable regeneration in the future [9].
Design scenario
Consider a medium to large-sized pharma company in Ireland in which an incinerator is the primary waste abatement technique employed on site. Typical large incinerators of this size have an energy demand of about 5.44 MW.
The three contributing factors of the incinerator’s total energy demand are the natural gas supply, liquid waste, and the vent gases which contribute to about 24% of the total incinerator energy demand. The data for this scenario is based on typical energy demands and emissions associated with a pharmaceutical company employing incineration.
Incinerator Carbon Intensity Factor:
In order to determine the potential CO2 emissions reduction by implementing an activated carbon adsorber, the CO2 intensity factor of the incinerator must first be calculated. This was determined using the following equation:
(1)
With an energy demand of 5.44 MW and assuming continuous operation for the year (8,760 hours), the total energy requirement is as follows:
(2)
The CO2 emissions associated with an incinerator of this size is 9,840 tonnes/year. Therefore:
(3)
CO2 Emissions Reduction with Adsorption Implementation:
Single production area
The average vent gases flow from all production areas was 2750 m3/hr and a calorific value of 34.8 MJ/m3 was assumed given the vents are designed for 5% VOCs, which is assumed to have the same calorific value of natural gas.
A production area produces a toluene-air mixture at 400 m3/hr with a toluene concentration of 5%. By treating these vent gases with a fixed bed activated carbon adsorber, a removal efficiency of 97% can be achieved, thus reducing the incinerator load by 388 m3/hr. The energy demand associated with vent gas abatement was determined as follows:
(4)
(5)
The total incinerator energy demand therefore reduces from 5.44 MW to 5.25 MW, which equates to 45.99 x 106 kWh. The total carbon dioxide emissions from the incinerator can then be calculated using the carbon intensity factor as follows:
(6)
/year (7)
The implementation of an activated carbon adsorber vent gas abatement for this production area only can therefore theoretically result in a reduction of 350.19 tonnes of CO2 emissions/year.
Site wide:
As previously mentioned, the average vent gas flow rate was 2,750 m3/hr from all production buildings. A 97% removal efficiency would result in 2,669.44 m3/hr being removed from the vent gas headers and thus reducing the incinerator load. The revised average energy demand associated with vent gas abatement can then be calculated as previous:
(8)
This results in a 97% reduction in energy demand associated with vent gas abatement, a 1.29 MW reduction in total average energy demand for the entire system to 4.15 MW. This equates to 36.354 x 106 kWh and results in total CO2 emissions as follows:
(9)
Site-wide implementation of fixed bed adsorbers for vent emission abatement can therefore theoretically reduce carbon dioxide emissions associated with an incinerator by about 2,333 tonnes/year, a percentage reduction of 23.7%. This equates to €70,000/year of savings in carbon credits @ €30/tonne [10].
Conclusions
Fixed-bed activated carbon adsorbers provide a cost-effective solution as an alternative to vent emission abatement technique to incineration.
While the regeneration step has added utility and energy demands compared to other adsorption techniques, their simplistic operation, waste minimisation and low energy intensity in comparison to incineration align with overriding industry efforts to reduce carbon emissions and to continue the effort to shift towards net carbon zero.
Author: Eoin Lynch, fifth year ME Process & Chemical Engineering, University College Cork; supervisors: Denis Ring, John Linehan, University College Cork.
References
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