Information about hides and skins

From Efficiency Finder
Revision as of 09:07, 23 August 2016 by AEE (Talk | contribs) (Energy recovery from process fluids)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

Back to Subsection DC leather


Applied Processes and techniques

The possible steps in the production of leather are shown schematically in Figure 1. There is considerable variation between tanneries, depending on the type of leather being produced.


Flow chart leather making.PNG

Figure 1: Process steps in leather making (chromium tanning)


Operations carried out in the beamhouse, the tanyard, and the post-tanning areas are performed in water or 'float'. After post-tanning, the leather is dried and subsequent operations are dry processes.

Wet processing was in the past carried out in fixed vessels or pits, with the hides or skins being moved from one pit to the next. This method is still used by some tanneries, particularly for vegetable tanning, but now mechanical agitation of either the hides and skins or the liquid is used [50, Sharphouse 1983]. The pits are not emptied between each batch of hides or skins. Several pits containing increasing concentrations of the same solution may be used for a single process step.

In most tanneries to which this BREF applies, rotating vessels are used. These may be drums (closed vessels rotating on horizontal axles), or mixers (open at one end and operating at an angle). In both types, several process steps may be carried out while the hides or skins remain in the same vessel, with the liquid being changed or recirculated.

Degreasing as a separate process step is applied only to sheepskins and pigskins and an effluent stream containing surfactants may be produced. Sheepskins may be processed without unhairing, to produce 'wool on' sheepskins.

All of the process steps might not be performed by a single entity or within one site. Some intermediate products are more stable than others, and where delays are possible, a transfer between processors is carried out at one of the stages shown with dotted lines in Figure 1. It is usual for sheepskins to be traded in the pickled state, whereas bovine hides are usually traded at the wet blue stage. Either may be traded as crust leather. Greater subdivision of processing is possible within a tannery cluster.

A person or company carrying out the processing of sheepskins up to the pickled stage is known as a fellmonger.

There is no standard pattern within the European Union of trading in hides and skins or in the form in which they are delivered to a tannery for processing. They may be:

  • delivered directly from local slaughterhouses
  • obtained from hide dealers
  • obtained from hide markets
  • traded internationally.


Energy consumption

Energy consumption in tanneries depends mainly on the following factors:

  • the production methods, and the capacity and size of equipment
  • the age and sophistication of electric motor controls
  • the amount of mechanical handling used to move hides and skins
  • the drying methods used
  • heat losses from process vessels and from buildings
  • air exchange rates to meet workplace safety conditions
  • types of waste water treatment on site
  • types of waste treatment and recovery of energy from waste on site.


Heat losses may be mitigated by thermal insulation, but may be exacerbated by a low external temperature. A high moisture content in the air may increase the energy consumed in drying. Energy use data from one climatic zone may not be an accurate guide to what may be achieved in another.

The age and efficiency of the combustion equipment and boiler plant will determine the proportion of the energy of fuel which is made available as thermal energy in the tannery. A larger central boiler may be more efficient, but if operations are dispersed on a large site, heat losses from pipework may eliminate the gains. Such considerations are not confined to tanneries and are covered more fully in the BREF document for energy efficiency (ENE) [109, EC 2008].


Table 1: Consumption of thermal and electrical energy

Table energy consumption.PNG


Table 1 gives an indication of energy consumption by type of energy input. The proportions can vary greatly between tanneries. For example, mechanical handling can be undertaken by fork lift trucks powered by internal combustion engines or by electrically powered conveyors. For this reason, the total energy input to the leather making process is a better comparator. It is necessary that data be compared for the same stages of the leather making process. Ideally, energy use should be monitored and reported separately for each process stage, and it is known that some of the most energy-efficient tanneries do so.

Where more detailed data for energy use are available it is important that comparisons between tanneries be made on the same basis. For example, 'effluent treatment' may or may not include biological treatment, which can account for more than 50 % of the total energy consumption in the treatment of tannery effluent.


Best available technologies (BATs)

1. In order to reduce energy consumed in drying, BAT is to optimise the preparation for drying by samming or any other mechanical dewatering.


2. In order to reduce energy consumption for wet processes, BAT is to use short floats.


Description

Reducing the energy used to heat water by reducing hot water use.


Applicability

The technique cannot be applied in the dyeing process step and for the processing of calfskins.

Applicability is also limited to:

  • new processing vessels;
  • existing processing vessels that allow the use of, or can be modified to use, short floats.


Table 2: BAT-associated energy consumption rates

BAT energy consumption.PNG


Techniques to consider in the determination of BAT

Use of short floats

Description

The use of short floats entails the starting and rotation of less balanced process vessels. There is an increase in the rate of use of electrical energy, but this is balanced by the shorter process times involved.


Achieved environmental benefits

Because it reduces process water heating the use of short floats can make an obvious difference to the energy use in a tannery.


Driving force for implementation

This technique is used to achieve a reduction in the use of water and process chemicals, as well as energy.


Example plants

Short floats are now widely used in European plants. Reference literature [84, Ludvik J. 2000].


Energy recovery from process fluids

Description

Energy savings can be achieved by heat pumps incorporating recovery systems. Waste heat can be used from and for other processes.

By means of heat exchangers, energy can be recovered from the waste process water, from condensate from vacuum dryers, from evaporated water from high frequency drying, or from exhaust air from drying.

The cooling water from the vacuum dryer, which is not polluted, can be used in the hot water supply.


Achieved environmental benefits

Reduced energy use may be achieved. Environmental performance and operational data Up to 75 % of waste heat from drying may be recovered. About 10 – 20 % of the energy consumption of vacuum drying may be recovered for hot water supply needs.

See the section below for the heat pump technique in the drying process.


Economics

Recovery of thermal energy by the means of heat pumps is normally only economical at a price relation of:

Economic formular.PNG


Example plants

Tanneries in France have been reported to be implementing the heat pump technique. Reference literature [16, Frendrup 1999].

Improved drying techniques

Description

Low temperature drying (LTD) machines are available with reduced energy consumption, although in some cases they can lengthen the drying process (e.g. LTD drying tunnels may require all night to dry leathers, compared with 4 hours in conventional hang drying tunnels, but may have three times the capacity).

Considerable reductions in energy consumption can be achieved by optimising the mechanical dewatering processes prior to drying.

Temperature and humidity during the drying need to be carefully controlled. Elimination of the greatest possible amount of water in samming may mean energy savings of 0.5 – 1 GJ/t raw hide in drying. Keeping drying temperature low and drying time and amount of exhaust air at the necessary minimum will keep heat losses to a minimum (although, consideration of leather properties will have priority).

In order to avoid energy losses for reheating, drying installations should be run as continuously as possible. The heat capacity and heat transmission of new installations are as low as possible. A system for the use of heat pumps in drying exists, developed in France (see Section 4.12.2). Without the use of a heat pump, the energy consumed is mainly thermal energy. The only exception is high frequency drying, which uses electrical energy exclusively. Due to the high costs of electrical energy and to the high investment cost, this method has only gained a limited acceptance.

It is obvious the natural drying of the leather is the method with the lowest energy consumption but it is impractical for much of the year in many parts of Europe due to climatic conditions. These include low temperatures, high rainfall and the associated humidity, combined with unpredictable variations in all these factors.

For finish drying, infrared heating is an energy-saving method.

Reduced energy use may be achieved.


Environmental performance and operational data

For example, a study carried out on a paste drying unit and on a hang drying unit showed that the overall thermal efficiency of the first machine was approximately 2.9 kg of steam per unit of water evaporated, whereas the second machine required approximately 2.5 kg of steam per unit of water evaporated. The poorer performance of the paste drying unit was found to be related to 30 % heat losses due to leaks, and insufficient insulation of the unit. In this case, energy savings were achieved by improving the insulation of the unit, reducing heat losses and optimising the operating procedures.

The figures given in Table 1 are for the energy consumption of various drying methods, without and with the use of heat pumps.

Table 1: Energy consumption of various drying methods

Drying methods.PNG


Driving force for implementation

The driving forces are to reduce energy costs and to meet energy use targets.


Example plants

Tanneries in France have been reported to be implementing the heat pump technique. Reference literature [56, Pearson et al. 1999] [105, Pfisterer 1986].


Energy recovery from waste by digestion

Description

Anaerobic digestion of organic waste fractions to produce a fuel gas.


Technical description

Anaerobic treatment of wastes is a well known technique which can be used to produce energy from waste and by-products from the leather industry. Green fleshings are suitable for biogas production, but are subject to appropriate animal health controls.


Achieved environmental benefits

A reduction of fossil fuel use, a reduction of CO2 emissions and a reduction of the volumes of waste for disposal.


Environmental performance and operational data

It is possible to recover around 3 GJ per tonne of raw hide through digesting limed fleshings and waste water sludge.


Cross-media effects

The formation of hydrogen sulphide can occur during the process. The final disposal routes for digestates from tannery wastes may be more restricted than those from anaerobic digestion of other wastes.


Technical considerations relevant to applicability

The technique is applicable to both new and existing installations. In practice, the technique will normally use waste material from several different sources (e.g. manure from agriculture).


Economics

The cost-benefit calculation depends on many parameters such as the amount of waste, cost of disposing of fleshings, gas or electricity price.


Driving force for implementation

Increasing energy prices and possible CO2 emissions trading contribute to the case for energy recovery by biogas. Another driving force for the installation of a biogas plant is the increasing costs for the disposal of waste.


Example plants Elmo Sweden AB. Reference literature [ 85, Hauber and Knödler 2008 ] [16, Frendrup 1999] [4, Andres 1997].


Energy recovery from waste by combustion

Description

Fat recovered from wastes is burnt as a fuel.


Technical description

Fleshings (and other fatty wastes) are minced to approximately 5 – 10 mm, heated to 75 – 85 ºC and separated e.g. using tricanters into tallow (10 – 20 %), solids 'greaves' (35 – 55 %), and a water fraction (35 – 55 %). The tallow contains up to 99 % fat and can be used in an appropriate burner as a direct substitute for oil fuel; the calorific value is about 85 – 90 %.


Achieved environmental benefits

A reduction of fossil fuel use and a reduction of the volumes of waste for disposal can be achieved.


Environmental performance and operational data

The emissions from burning the substitute fuel generated from fleshings do not show significant differences regarding CO2, or CO compared to gas oil. Combustion of fat from the fleshings can cover 50 – 70 % of the total demand for thermal energy.


Cross-media effects

The technique also generates two different waste fractions which have to be disposed of. The water fraction represents an additional load on effluent treatment facilities, while the greaves have to be disposed of as waste.


Technical considerations relevant to applicability

The method can be used on or off site. For on-site operation, the heat generated can be used in the tannery. Depending on the status of the material being burnt, compliance with waste incineration legislation will require a more complex plant.


Economics

The cost-benefit analyses depend on local conditions. Important factors are the amount and the composition of the fleshings, the cost for disposal of the fleshings, the cost saved on the fuel which is replaced and the possibilities and costs for the disposal of the residue. The capital cost of the plant can be high, especially if compliance with waste incineration legislation is required.


Driving force for implementation

The drivers are increasing energy prices and costs of disposal.


Example plants

One tannery in Germany and several other tanneries in Europe run installations for the generation of substitute fuel on-site. An English sheepskin tannery gets 20 % of its thermal energy from the incineration of sheepskin fat. Reference literature [ 85, Hauber and Knödler 2008 ].


Source: Joint Research Centre, Best Available Techniques (BAT) Reference Document for the Tanning of Hides and Skins, 2013, p.13-14


Back to Subsection DC leather